Prosthetic Arm Force Reducer Team 1 – Halliday's ... - Ohio University

Prosthetic Arm Force Reducer
Team 1 – Halliday’s Heroes
Team Members:
Dan Cole
Jay Duffy
Greg Harvey
Josh Hlebak
Michael Massey
Lisa Molitoris
Lou Monnier
Lena Richards
Monday, June 9 th , 2008
Abstract:
There is a need for an assistive technology device that focuses on extremity loss above the elbow
within the field of agriculture. Our focus is with farmers who have physically demanding jobs to
be able to perform tasks which require reliable simulated usage of the appendage and therefore,
continue to run a profitable farm operation. The main goals of this device are to increase the
potential grip strength of the prosthetic, reduce the necessary input force and therefore physical
strain, and make the device serviceable enough that any maintenance can be done by the
customer. A prosthetic arm force reducer was manufactured by designing a pulley mechanical
advantage system housed within the hollow forearm section of the prosthetic. The system
reduces the input force required by the user by 47% from 18 lbs to 9.5 lbs and costs only 9.6% or
an additional $575 dollars of the total prosthetic arm.

Table of Contents Page Page
1.0 Introduction………………………………………………………………………………....3
1.1 Initial Needs Statement…………………………………………………………..…3
2.0 Customer Needs Assessment………………………………………………………….…....3
2.1 Weighting of Customer Needs……………………………………….......................5
3.0 Revised Need Statement and Target Specifications…………………………………..……5
4.0 External Search……………………………………………………………….…………….7
4.1 Benchmarking……………………………………………………………………....8
4.2 Application Patents………………………………………………………………..14
4.3 Application Standards……………………………………………………………..15
4.4 Application Constraints…………………………………………………………...16
5.0 Concept Generation……………………………………………………………………….17
5.1 Concept Generation……………………………………………………………….17
5.2 Concept Development, Scoring and Selection……………………………….……21
6.0 Concept Selection…………………………………………………………………………23
6.1 Data and Calculations for Feasibility and Effectiveness Analysis………………..23
6.2 Concept Development, Scoring and Selection…………………………………….28
7.0 Final Design…………………………………………………………………………….....29
7.1 System Personalization and Operation…………………………………………....40
7.2 How is it Manufactured...........................................................................................40
7.3 Cost Analysis & Bill of Materials…………………………………………………45
7.4 Design Validation…………………………………………………………………47
8.0 Conclusion………………………………………………………………………...………48
Appendix A – Split Hook Sample Calculation…………………………………………………..51
Appendix B – Interview Summaries……………………………………………………………..52
Appendix C – Tim Lang’s Forearm Dimensions & Data………………………………………..55
References………………………………………………………………………………………..56
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1.0 Introduction
Limb loss generally refers to the absence of any part of an extremity due to surgical or traumatic
amputation (Amputee Coalition of America 2008). Upper limb loss accounts for about 30% of
the 350,000 persons with amputations in the United States (Kulley 2008). There are also multiple
causes for upper limb loss such as disease, traumatic accidents, infections, and tumors.
The field of agriculture is a very dangerous work environment with heavy equipment and many
chances for accidents. Farming accidents are a common cause of upper limb loss that can put
many farmers out of work. We would like to work with a farmer to develop an upgrade to the
current prosthetic systems in use which would be able to assist that farmer in their daily work.
Important objectives of the design project involve:
• Full customer research to determine all the positives and negatives of the current system
• Full benchmarking research to determine what has been attempted already and what has
or has not worked
• Solving the customers main needs with as little manufacturing and complexity as possible
Through research, feedback, and further design our goal is to meet our objectives and design an
assistive device for people with upper arm loss, specifically in the field of agriculture. A high
standard of excellence will be put forth on all our efforts reflecting our strong team work,
willingness, enthusiasm, and genuine interest to improve workplace conditions for people with
disabilities.
1.1 Initial Needs Statement
There is a need for assistive technology devices that reduce barriers that prevent persons with
severe disabilities from entering or advancing in the workplace. Devices are needed to address
environmental accommodation, functional assistance, and mobility issues for people with
cognitive disabilities, developmental disabilities, and physical impairments (vision, hearing and
mobility) (NISH National Scholar Award for Workplace Innovation & Design 2007/2008).
2.0 Customer Needs Assessment
A customer has been identified who has upper limb loss above the elbow of his right arm. Our
customer, Tim Lang, operates a dairy farm in Marietta, Ohio and lost his right arm
approximately two years ago. Now that Tim has a prosthetic arm he is able to continue operating
his farm, but with physical discomfort and difficulties. Through interviews (See Appendix A)
and meetings with him as well as rehabilitation engineers, vocational supervisors, and prosthetic
suppliers we have compiled a list of design features that would aid him in his daily farming tasks
(See Table 2.0.1).
Our customer has identified physical discomfort as his biggest problem with his prosthetic
followed by a desire to have increased grip strength and lastly to make the device easy enough to
fix that he himself could make repairs to it without having to send it in to a specialized
prosthetics manufacturer. In order to weight our needs, the Analytical Hierarchy Process was
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used for three main categories. These categories consist of user friendly, reliable, and affordable.
The contents of these categories can be found in Table 2.0.2 below.
Table 2.0.1 - Initial Customer Needs List Obtained
From Interviews and Observations
Reduced user input force
Increased grip strength
Reliable
Durable
Servicable
Light weight
Corrosion resistant
Simple to use
Affordable
Minimal maintance
Rugged
Adaptable to current prosthetic
Optimal size for best operation
Doesn't add too much cable travel
Safe
Table 2.0.2 - Hierarchal Customer Needs List
1. User friendly
1.1 Reduced user input force
1.2 Increased grip strength
1.3 Simple to use
1.4 Doesn't add too much cable travel
1.5 Light weight
1.6 Safe
1.7 Adaptable to current prosthetic
1.8 Optimal size for best operation
2. Reliable
2.1 Durable
2.2 Servicable
2.3 Minimal maintance
2.4 Corrosion resistant
2.5 Rugged
3. Affordable
3.1 Low manufacturing costs
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2.1 Weighting of Customer Needs
Weighting the customer needs is a key part of engineering before the design process. By
weighting the needs, one can see the most important aspects of the project which will make
further design decisions easier and more consistent. The weighting should also be directly
related to the customer’s needs and desires as well. This assures that the customer will be happy
with the final result.
After weighting our project, it was determined that the reliable and user friendly categories were
the most important needs of our project and customer. The breakdown of these needs can be
found in Table 2.1.1 below.
Table 2.1.1 - Analytical Hierarchy Process Breakdown
User Friendly Reliable Affordable
User Friendly 1.00 1.00 1.00
Reliable 1.00 1.00 1.00
Affordable 3.00 3.00 1.00
Total 6.00 6.00 4.00
Weight 0.333 0.333 0.222
3.0 Revised Need Statement and Target Specifications
There is a need for an assistive technology device that specifically focuses on upper-extremity
loss above the elbow within the field of agriculture. Our particular focus will be farmers who
have physically demanding jobs to be able to perform tasks which require reliable simulated
usage of the appendage and therefore, continue to run a profitable farm operation. “Health
professionals and others who have contact with farmers with disabilities need to be cognizant of
the strong desire and continued ability to farm after severe injury. More attention should be
given to farm-specific occupational rehabilitation programs, such as AgrAbility, and in the
engineering of prostheses and other assistive technology.” (Reed & Claunch 1998)
This need exists due to the prevalence of disabling injuries in agriculture due to farming being an
incredibly dangerous profession. Theses persons with upper extremity loss within the field of
agriculture need a specific prosthetic to continue working properly and efficiently in their daily
farming tasks. In order for farmers to continue their livelihood, increasing the ease of use, grip
strength, and serviceability of a standard prosthetic will be our main focus.
To reduce to the physical impact on the user’s body and the amount of input force required to
open the hooks, improved comfort and ease of use will be the main focal point. Also, many
current available prosthetics are not robust enough nor have a sufficient gripping force to meet
the demands of users who work in physically intensive occupations. Increasing the grip force of
a standard split hook at the contact point(s) of the grippers will address this need for increased
grip strength. Serviceability will be addressed by the maintenance being able to be performed
easily by the customer.
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Table 3.0.1 – Customer Needs List
Need # Need (In Order of Importance)
1 Manageable input force
2 Increased grip force
3 Serviceability
4 Reliability
5 Affordability (under $700 US)
6 Corrosion Resistance
7 Light weight (Less than 5 lbs)
8 Simplicity of operation (No electrical components)
Table 3.0.2 – Needs Metric List
Metric Need # Metric Importance # Units
#
(3 High, 1 Low)
1 1,2,8 Input force 3 lbs
2 1,2 Closing force 3 lbs
3 all Unit price 2 $
4 3,8 Unit dimensions 1 in
5 1,2,3,4,6,8 Unit life 3 #cycles
6 7,8 Unit weight 2 lbs
USpecification quantities include:
1. UInput forceU: A one-half decrease in input force is necessary. The input force to operate
the prosthetic is human input: the user’s shrugs their shoulders, moving the cable. This
cable is attached to the back harness and runs down the arm to the terminal device.
This one-half decrease is necessary because the user has complained of nightly
discomfort from using his current split-hook loaded with many rubber bands (up to 7).
2. UClosing forceU: Enough closing force must be provided for the user to hold a nail steady
and nail it into a wall. The closing force must be greater than or equal to a traditional
spilt hook utilizing 7 rubber bands of tension.
3. Unit priceU: A typical sized split hook prosthetic device made of stainless steel costs about
$565 - 1,100 dollars. The entire mechanical advantage system will ideally cost under
$700 which corresponds to bottom quarter of the price range.
4. Unit dimensionsU: Our system must fit into the user’s existing forearm prosthetic. In
order to ensure safe operation, there shall be no extraneous parts attached to the forearm.
5. Unit lifeU: The unit life will be based on stainless steel being the favorable material for the
mechanical advantage components because of its strength and corrosion resistant
characteristics. An operating temperature range of -30 to 120˚F has also been specified,
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which could limit the use of rubber bands. The life specified should be at least 3 years or
10,800 cycles (360 working days a year with 10 cycles per day).
6. Unit weightU: A typical split-hook terminal device plus forearm section weighs about 2
pounds. Since this project is concerned with enhancing functionality, some extra amount
of weight will likely be incurred by the modifications. Our maximum weight is to stay
under 5 pounds (approximately the weight of an average adult forearm and hand). The
customer has added that extra weight will not act as a hindrance. The internally located
mechanical advantage system will instead increase the strength of his currently hollow
forearm section.
4.0 External Search
The Disability Act of 1990 was monumental in creating equal opportunities for people with
disabilities. Though this act has made significant progress, only 25% of people between the ages
of 16 to 64 (commonly considered as the working age demographic) with disabilities are
employed. Of the remaining 75%, two-thirds of them wish they were employed (Health
Progress, May/June, 2000).
Our research began by getting in contact with the Athens County Bureau of Vocational
Rehabilitation (BVR). The BVR is a branch of the Ohio Rehabilitation Services Commission
(ORSC) which is a state agency that is annually responsible for vocational rehabilitation of
55,000 Ohioan’s with disabilities. On average in a single year, the RSC aids 8,000 Ohioans with
disabilities into obtaining and retaining a job. Since 1990, the agency has assisted nearly
100,000 citizens of Ohio with disabilities. The Athens County BVR provides rehabilitation
services in ten counties of Ohio to individuals whose primary impairments range from physical,
to emotional, and to mental. Many of their clients have orthopedic and mental health issues, and
on average, there are 850 to 1,000 active customers with a variety of disabilities.
Our main contact at the BVR was George Platounaris. George is the rehabilitation vocational
supervisor at the BVR with over 30 years of experience. Upon meeting with him for the first
time on October 10 th 2007, (see Appendix B for details) George was able to reference us to two
rehabilitation design engineers who work in the field and mentioned he could be a contact point
for information from those two individuals as well as help us obtain a potential customer.
George extended the invitation to attend weekly meetings at BVR where feedback from industry
professionals could be obtained.
Contact with George lead to a phone interview (conducted later that same day) with
rehabilitation industrial designer Mark Ficocelli (see Appendix B for details) who works on
special projects for the BVR. Mark emphasized the fact that most of his work is done as a oneon-one
interaction with a specific customer. He emphasized that each person required a different
solution and that it is very difficult to create an adaptable solution. One area that he suggested
we pursue is the field of prosthetic attachments, and referenced a recent case where he had
worked with a farmer to create an attachment that could hold different sized wrenches.
The solution to this problem resulted in the farmer with a type of hook-hand that could be pulled
open by flexing the muscles of the back. Mark suggested buying burlap feed sack and
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attempting to create an attachment that could open and then hold the feed sack. Another
suggestion he made was to attempt to put ourselves in the shoes of the person with a disability
and spend the day trying to replicate the disability that is to be addressed. This would allow the
team members to have first-hand experience with the particular disability. Other ideas he
mentioned were excursion amplifiers, back harnesses, page turners, devices to help people get
dressed and to feed themselves, home modifications, work site ergonomics, rural and farming
work, computer adaptation, weight distribution, body position, and seating for people in to
wheelchairs.
After analyzing and debating the external research that was gathered, the decision was made to
focus on the area of upper extremity loss with emphasis on agricultural solutions. Patent
research was done to find solutions that might involve prosthetics or adaptations to equipment
that farmers must use. In the area of prosthetics, emphasis was given to the fact that the arm
would be used for physical labor and must be able to withstand and perform such heavy labor but
yet reduce the amount of physical impact on his or her body.
4.1 Benchmarking
In order to better understand what might assist an agricultural worker who had lost an upper
extremity, currently existing solutions were researched. What was found is that a limited number
of workable solutions currently exist. A variety of devices were researched even if they did not
cater strictly to farming. Hosmer sells a Model 7 Work Hook. (Fig. 4.1.1) On their website, the
hook is described as having a 1-1/8” opening to accommodate a broom or shovel handle. The
tines of the hook are serrated and it includes a knife holding, nail holding, and screw-driver
holding device in the guards. The hook is made from stainless steel and each rubber band
provides approximately 1.5lbs of applied tension. Springs are also available from the same
manufacturer and can be used in place of the rubber bands. The springs are stainless steel and
apply approximately 9lbs of tension per spring.
Fig. 4.1.1 – Hosmer Model 7 Work Hook
Otto Bock, one of the global leaders of health care products, produces a vector hook terminal
device with two force settings. (Fig. 4.1.2) On their website, the hook is described as a unique
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design of the two load hook allows users to easily switch between two spring tension settings for
different activities. Instead of using rubber bands the Otto Bock model uses a smaller diameter
spring encircled inside a larger diameter spring on both sides of the hook. The name Vector
Hook comes from the user’s ability the flip the moment arm in which the springs are attached
from a 45° degree angle (relative to the tines of the hook) making it much easier to open to a
more difficult 90° setting. The 10A60 model comes in either stainless steel or aluminum with a
standard ½”-20 threading on the stud.
Fig. 4.1.2 – Otto Bock Vector Hook Model 10A60
The i-LIMB Hand is the first commercially available multi-articulating bionic hand. Since it has
five independently powered digits, many different grips can be achieved (key grip, power grip,
precision grip, and index point). The precision grip is shown in Figure 4.1.3. The i-LIMB Hand
and patient interact in a symbiotic way. The hand is controlled by sensing electrical impulses in
the forearm, and is powered by internal batteries. The batteries drive the five motors for a
complete day and recharge overnight. Cosmesis is a covering for the i-LIMB Hand that provides
a more realistic appearance and protects the internal mechanisms. This device costs two to three
times more than traditional myoelectric devices. The product is robust, is made from extremely
strong plastics, yet it cannot be exposed to water.
Fig. 4.1.3 – Touch Bionics: The i-LIMB System
TRS’s Adult Grip Prehensors are high performance hand replacements. The 1/2 inch diameter
threaded stud will attach to any U.S. made wrist unit. They are high in strength, constructed
reliably, and require low maintenance. The Adult Grip Prehensors are body powered and are to
9

e used with a harness. The website claims that the Grip Prehensors are the highest efficiency of
any body powered prosthetic device available. Applications range from peeling a banana and
slicing a tomato to using a wrench or hammer, weightlifting, or shooting a bow. The range of
gripping force can exceed 100 pounds. The models are stainless steel with titanium side plates
and polyurethane gripping surfaces as options. A few applications along with material choices
appear in Fig. 4.1.4 below.
Fig. 4.1.4 – TRS Adult Grip Prehensors and Sample Applications
Liberating Technologies, Inc. markets a device known as the RSL Steeper MultiControl Plus
prosthetic hand system. These hands can close in as little as 0.9 seconds and come in a variety of
sizes. The RSL Steeper MultiControl Plus Hand has a new power management system called
PowerForce. This electronic circuit provides additional grip force on demand and acts like an
electric automatic transmission. An image of this product can be seen below in Fig. 4.1.5.
Fig. 4.1.5 – Liberating Technologies, Inc. RSL Steeper MultiControl Plus
Motion Control Inc. provides an electronic prosthetic arm for above elbow amputations. Known
as the Utah Arm, motion control released the third version in 2004. In this particular version,
microprocessors were incorporated into the arm to allow wearers to make fine-tune adjustments
to the movement of the arm. This particular arm provides proportional control which allows the
wearer to move the arm and hand slowly or quickly in any position. The Utah Arm 3 can be
found below in Fig. 4.1.6.
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Fig. 4.1.6 – Utah Arm 3
MAGNUM Parallel Grippers are used in the robotics industry. They utilize third-generation
Zaytran technology and are made of tough, corrosive resistant materials. The MAGNUM
parallel grippers are used in a variety of environments from welding, grinding, machining, clean
room, disk fabrication, and food processing. The force to weight ratio of these grippers is in
excess of 200. The gripper consists of two independent systems; force and synchronizing double
helix. The MAGNUM Parallel Gripper mechanism is double sealed to ensure isolation from the
environment. In harsh environments the double seals protect the parallel gripper from
contamination that could lead to failure. The MAGNUM Parallel Grippers are shown in use in
Fig. 4.1.7.
Figure 4.1.7– Zaytran: MAGNUM Parallel Grippers
A vector is described by its magnitude and direction. Vector prehensors (Fig. 4.1.8) yield a
variable gripping force by varying the direction of the rubber band closing force yet keeping its
magnitude constant. When the rubber band angle is 90 degrees, grip force is maximized. As the
band angle decreases, the torque applied by the band about the pivot decreases. Currently, the
best design to date uses elastomer bands will last for more than 70,000 cycles at the highest grip
force setting. Bands are contained within the casing to shield the elements. With the current
design, the elastomer bands can be replaced by an individual with Phillips’ head and flathead
screwdrivers in about 10 minutes. At the highest setting, grip force begins at about 11 lbs and
gradually rises to nearly 20 lbs as the hooks are opened. There are 11 intermediate settings that
provide a choice of grip force levels between these two extremes.
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Figure 4.1.8- Vector Hook and Vector Grip
Custom Prosthetic Services LTD. offers a conventional body-powered prosthesis that works by
means of the operator’s body to overcome the closing force of the terminal device. (Fig 4.1.9)
The system works by the user shrugging his or her shoulder. This movement is intern captured
by the butterfly back harness system (Fig 4.1.10), which is attached to a cable that runs down the
arm and is connected to the terminal device.
Figure 4.1.9 – Custom Prosthetic
Services Body-Powered Upper
Extremity Prosthetic
Figure 4.1.10 – Body-Powered Prosthetic
Butterfly Harness
Some advantages of this system is that it is highly durable, useful in wet, dirty or dusty
environments, reduced maintenance cost compared to electrically driven prostheses, and is
relatively simple. Yet, some drawbacks include that the user have a sufficient residual limb
length, musculature, range of motion, and the overall prosthesis is not cosmetically pleasing due
to exposed cables and hooks.
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Feature
Hosmer Model 7
Work Hook
Otto Bock Vector
Hook 10A60
Touch Bionics: i-
LIMB System
TRS Adult Grip
Prehensors
RSL Steeper
MultiControl
Plus
MAGNUM
Parallel Grippers
Vector Hook &
Vector Grip
Adjustable No Yes Uncertain Yes No Uncertain Yes
Electronics No No Yes No Yes Yes No
Power Source Body Powered Body Powered Batteries Body Powered Batteries Batteries Body Powered
Cost ~$350 ~$400 +$1000 ~$400 ~$600 ~$300 ~$400
Serviceability Yes Yes No Yes No Uncertain Yes
Corrosion
Resistant
Reduces Input
Force
Table 4.1.1 – Terminal Device Product Benchmarking
Yes Yes Yes/No Yes No Yes Yes
No Yes Yes Yes/No Yes Yes Yes
Table 4.1.2 – Prosthetic Arm Product Benchmarking
Feature Utah Arm 3
CPS Body-
Powered
Prosthetic
Adjustable Yes Yes/No
Electronics Yes No
Power Source Batteries Body Powered
Cost N/a ~$5,000
Serviceability No Yes
Corrosion
Resistant
Reduces Input
Force
Yes Yes
Yes No
Through benchmarking, it has been determined that there is a diverse market of terminal devices
as well as compatible substitutes. Further up the arm, the user has fewer options to choose from.
In order to aid in concept generation and the overall project’s scope one must consider the
customer’s needs as seen in section 2.0.
The Hosmer Hook is a tried and true classic terminal device made specifically for agricultural
purposes. It does not require any electrical components, it is affordable, serviceable, and is
corrosion resistant. One of its major drawbacks is that there is no force adjustability and it would
require modifications in order to use springs as the resistive force rather than its standard rubber
bands. The Otto Bock Vector Hook 10A60 shares the same strengths as the Hosmer model but
the user can adjust the force and therefore reduce to input needed to open the hooks. Also,
springs come standard on the Vector Hook model.
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The i-LIMB Hand has taken years to develop and the overall level of detail is out of our scope.
The myoelectric power and five independent motors are a little too advanced for this project, and
the battery power it utilizes is something that we may not want to incorporate into our final
design. The TRS Adult Grip Prehensor is very durable, so we may be able to incorporate its
strength and simplicity of design components into our final design.
The RSL Steeper MultiControl Plus is not sturdy enough for an agricultural environment, yet
does have a convenient feature which is a quick closing time and automated power system. The
MAGNUM Parallel Grippers utilize a veritable design with a great strength to weight ratio that
could be used to aid us in final concept selection, but it would require an electrical power source
in order to operate. The Vector Prehensors are very useful because the gripper force is
adjustable. This is good for the farmers to use high grip force when using heavy duty jobs then
lower the force for tasks that require less force.
The two prosthetic forearms presented in this section (as seen in Table 4.1.2) represent the two
major types of prosthetic arms. The Utah Arm 3 is a typical electric powered microprocessor
prosthetic arm, and the Custom Prosthetic Services Body-Powered Upper Extremity Prosthetic is
the typical body-powered prosthetic arm. Due to its robustness, years of testing and field
experience, and simplicity the body powered prosthetic might be the ideal path for our
customer’s needs.
The benchmarking has illuminated some strong points and weak points of products that are
currently available. We will use some of the strengths and improve some of the weaknesses seen
in the above products to make our design work with our need statement, customer input, and
requirements. Durability, grip strength, and ease of use will be strong differentiators of our final
design as compared to the benchmarked products.
4.2 Applicable Patents
The following are patents that may apply to the particular focus of our project:
1. Loveless, J. H., "Prosthetic Load-Lift Hook Locking Mechanism," U. S. Patent 4,074,367,
February 21, 1978.
• Describes an electronically controlled pawl and ratchet system that would increase the
grip strength and lifting capacity of a prosthetic arm. A ratchet wheel in the elbow of the
arm is driven by a motor and pulley system located in the upper portion/shoulder of the
arm. This system is used to clamp the gripping portion of the arm, located in the position
of the hand. A nice system, however, it is fairly complicated and requires that the entire
arm be prosthetic. There would be no use for this system in the case of an amputation at
the elbow.
2. Cooper, C. M., "Harness for Control of Upper Extremity Prosthesis," U. S. Patent 3,188,655,
June 15, 1965.
• Describes a harness that can be attached to a hook at the end of a prosthetic arm. By
raising their opposite arm the user of this harness can open the hook at the end of
their prosthetic arm. When the arm is lowered to the normal position the prosthetic
14

hook is closed. The harness idea is one worth consideration but used in the manner
shown here it is doubtful that the gripping force needed in our application could be
achieved. A modification of this system could definitely be used in a future design.
3. Threewit, D. M., "Terminal Connection for Control Cables," U. S. Patent 2,493,841, January
10, 1950.
• Describes a means for attaching and operating a cable in order to open and close a
hook attached at the end of a prosthetic arm. A modification of this patent could be
used in conjunction with the above harness.
4. Radocy, R. and Dick, E., "Prosthetic Terminal Device," U. S. Patent 4,225,983, October 7,
1980.
• Describes a claw that is to imitate the gripping action of the thumb and forefinger.
The device is closed through the use of an attached cable and is spring-loaded to
return to the open position. The device utilizes two manual locking devices and three
gripping surfaces to provide a variety of closed positions that allow for the grasping
of objects of different sizes.
5. Landsberger, S. L., "Artificial Hand For Grasping an Object," U. S. Patent 7,087,092,
August 8, 2006.
• Describes an alternative gripping device that utilizes two “fingers” which are
connected to a “thumb.” This device resembles the human hand except in uses three
“fingers” instead of five.
6. Farquharson, R. H. and Still, D. P., "Attachment for Artificial Arm Prosthetic Device," U. S.
Patent 5,464,444, November 7, 1995.
• Describes a terminal device comprises of a first main part in operable and pivotal
combination with a second main part, the combined main parts providing on one end
a device for attaching to the end of an arm prosthesis, and on the other a device for
attaching a variety of implements, the said device for implement attachment
providing articulation capabilities that allow positioning of the implements in a
variety of positions relative to the position of the arm prosthesis.
7. Zajac, T. S., “Device for Gripping Workpieces,” U.S. Patent 4,591,199, May 27, 1986.
• Describes a device for which fluid pressure is applied to opposite pistons connected
to a gripping jaw hence moving the jaws open and closed. A rod extending along the
axis of the two cylinders which is interconnected to the pistons is the means for
synchronous movement. The interconnection affects rotation of the rod in opposite
directions upon movement of the pistons with a bearing assembly located between the
two fluid cylinders supporting the rod for rotation. The MAGNUM Parallel Grippers
(Fig. 4.1.7) utilize this patent.
4.3 Applicable Standards
There are Quality Standards set for suppliers of Durable Medical Equipment, Prosthetics,
Orthotics, and Supplies (DMEPOS). The organization recommends that the supplier of a custom
fabricated, custom fitted, custom-made prosthetic device be trained in a wide variety of treatment
15

options. The definition given for a prosthetic device is any device (other than dental) that
replaces all or part of an internal body organ (including contiguous tissue), or replace all or part
of the function of a permanently inoperative or malfunctioning internal body organ.
The prosthetic device must be in accordance with Medicare contractor policies (if applicable to
user). The supplier shall perform a diagnosis-specific clinical examination and access and
understand manufacturer guidelines prior to fitting. There should be an implementation plan also
should be consistent with the prescribing physician’s written plan of care. Appropriate
beneficiary follow-up care consistent with the items or service(s) provided, the beneficiary’s
diagnosis, specific care rendered should be provided. The beneficiary or caregiver should be
informed of the procedures for repairing, replacing, and/or adjusting the device or items.
The caregiver should review care and maintenance instructions and provide necessary supplies
(e.g. adhesives, solvents, lubricants) to attach, maintain, and clean the items, as applicable, and
provide information about how to subsequently obtain necessary supplies. Also inspection and
monitoring should be done for potential complications.
Though these standards are more applicable to a supplier of the prosthetic device, not so much
the designer, useful information could still be applied to our project. One important
consideration is to fully educate the user on how the prosthetic works, inform them of any
potential hazards or limitations of the device, and verify the compliance with the customer’s
current prosthetic. If any questions arise, a medical professional will be a great source of further
detail in each specific case. In the context of our design it is important that the end user be
properly and thoroughly trained in how to operate the system before using it as well as informed
of the procedures for repairing, replacing, and/or adjusting the device.
4.4 Applicable Constraints
Internal constraints include beginner level engineering experience, a limited budget, and a time
constraint for development. The skill level of the team is entry-level with respect to engineering
abilities meaning not all our decisions would be similar to decisions that would be made by
seasoned prosthetic engineers given the same design circumstances. We will make the best
decisions and choices with our knowledge, background research, and feedback received from
professionals and customers.
The construction of the product will be professional, yet will not be on the same level as a fullscale
manufacturing plant since budget is limited, commodity prices are on the rise, and our
manufacturing facilities are a bit small and limited. Compromise, value engineering, and advice
from professionals will be utilized to help make smart decisions. Too much redesigning will not
be possible on a limited budget or schedule. This will be accounted for by planning and
developing concepts early and often before our final design criteria is selected. The overall
product will reflect our high quality operation and level of thought and will help consumers with
upper extremity loss succeed in the field of agriculture.
There is an external constraint to our project which would be the limited market. A person with
upper extremity loss and who would like to peruse and prosper in the agricultural field would fit
our needs area. They must also have knowledge of our product and have adequate resources to
16

purchase it. Safety is another external constraint. The device must be safe which means an
ability to be released quickly so that secondary injuries do not occur in an emergency situation.
Standards, previous patents, and pre-existing products will guide our final design, yet with
creativity and ingenuity, our product will be significantly differentiable and salable to fit the
target market and help people with upper extremity loss work in agriculture.
5.0 Concept Generation
The typical approach to generating concepts used by this team has been to brainstorm
individually and then relay any ideas at the weekly meeting. During these meetings each concept
is critiqued by all members of the group and the team is able to build off ideas submitted by
other individuals in the group.
The Bureau of Vocational Rehabilitation (BVR) and their engineers have provided meaningful
feedback regarding concepts developed by the group. Additional customer input was added
during the conceptual generation phase. The National AgrAbility Project is concerned with
assisting disabled farmers and ranchers. Mark Novak is involved with the project and relayed
areas in agriculture where disabled farmers are seeking assistance. He was also able to link
information related to projects that have already been completed by engineering design groups at
the University of Wisconsin - Madison. Through the BVR direct customer contact was achieved
with Tim Lang, a dairy farmer in Marietta who has a complete arm prosthetic on his right arm.
Tim has also been instrumental in providing sound feedback on concepts as well as providing
ideas of his own.
The feasibility of each of the concepts is our main concentration. If the idea is not feasible, and
has no real-world expectations or usages, then there would be no reason to move forward with
the design aspect of the idea. Once a concept has been deemed feasible it can be allowed to
progress in the design process.
Initial feedback from Tim Lang resulted in several areas of concentration for the design of a
system that could benefit him in his everyday activities. To address these areas the following
concepts were deemed feasible:
A) Fabricating a hook of our own design that incorporates a vector hook system to allow
flexibility in grip strength and opening force required.
B) Using the Otto Bock Model 10A60’s current vector system to produce flexibility in grip
strength and thus opening force required.
C) Creating a mechanical advantage system to mount inside of the prosthetic forearm in order
to reduce the opening force required from the user.
D) Upgrading the springs used by the Otto Bock Model 10A60 to a stronger version that would
increase the grip strength of the hook.
5.1 Concept Generation
Initial brainstorming for conceptual design began in class with discussion regarding possible
routes that could be taken to solve our refined need statement. After each discussion each person
17

was given a few days to come up with some conceptual designs of their own. Once properly
prepared, we would meet once more as a whole and discuss different alternatives that each
member had thought of with regard to our project idea(s). This practice has been the most
effective process that our group has used to generate alternative concepts. These concepts were
drawn on a board in front of all members and discussion was given to each idea. No concept was
thrown out, but instead the pros and cons of each device were debated so that each member
might be able to come up with ideas for improvement or new ideas all together. Sketching and
solid edge modeling of these concepts are shown below.
5.1.1 Concept A – Fabricated Hook
Figure 5.1.1 – Sketch of Initial Hook Concept
The initial concept of a hook to be fabricated is shown in Figure 5.1.1. This hook utilized a
vector system by allowing the “screw adjustment” on the left side of the drawing to side between
the upper position and lower position, symbolized by the arrow in the drawing. An additional
feature of this first design was the implementation of an adjustable grip width by utilizing a
movable jaw. The dashed line in the bottom right of the drawing shows the “cart” that the
movable jaw would have moved through in order to accomplish this feature. Future contact with
Tim Lang cited that this feature was not needed and would have been a waste of resources.
18

Figure 5.1.2 – Solid Edge Model of Refined Hook Concept
Shown above is the second iteration of the hook concept. The movable jaw feature has been
eliminated from this design and a different approach to the vector system has been implemented.
Rather than use a “screw” to adjust the vector position of the hook, the body of the book has
been drilled out in three places to allow for three potential locations of spring placement. The
lever on the left side of the hook can be pulled outward and then slid and released into the
appropriate location.
5.1.2 Concept B – Otto Bock Model 10A60 Vector Hook
During the design of our own vector hook system it was discovered that there was a pre-existing
hook of very similar operation. The Otto Bock Model 10A60 shown below incorporates and 2setting
vector system.
Figure 5.1.3 – Otto Bock Model 10A60 Vector Hook
19

As can be seen in Figure 5.1.3 the Otto Bock hook allows the user to select an approximately
half force setting by placing the lever in the location shown in the photo. By flipping the lever to
the left (with respect to the fig.) the springs are placed in the full force orientation. Inspection of
Figure 5.1.2 and Figure 5.1.3 shows that both our concept and the existing Otto Bock design are
very similar.
5.1.3 Concept C – Mechanical Advantage System Located in Forearm Prosthetic
The concept of a mechanical advantage system was developed in order to further reduce the user
input required by the customer. The decision was made to implement the system inside of the
hollow prosthetic forearm system. It was deemed a safety hazard to try and incorporate any type
of similar concept on the exterior of the forearm as it could easily be snagged on many of the
rapidly moving parts encountered by a farmer in his day-to-day work. Below, Figure 5.1.4
displays how a 2:1 mechanical advantage could be achieved.
Figure 5.1.4 – Mechanical Advantage Concept
This concept incorporates the use of a pulley to achieve the mechanical advantage. The cable
attached to the user’s harness would wrap around the pulley and anchor inside of the arm. A
separate cable would then travel from the pulley to the user’s hook. The rings shown at either
extremity of the drawing would mount to the interior walls of the forearm. Figure 5.1.5 shows
the design incorporated inside of the forearm with the Otto Bock hook attached.
Figure 5.1.5 – Mechanical Advantage System Concept Inside Forearm
20

5.1.4 Concept D – Upgraded Springs for the Otto Bock Hook
Once the similarities between our concept and the existing Otto Bock hook were noted, the team
was fairly adamant about sourcing an Otto Bock hook rather than fabricate our own design. The
Otto Bock hook incorporates four springs total – two springs on either side of the hook with one
spring encompassing the other on either side of the hook (see Figure 5.1.3). In order to increase
the grip force of this hook, more rugged springs could be incorporated. Figure 5.1.6 shows what
the Otto Bock hook might look like with a stronger spring incorporated on one side of the hook.
Figure 5.1.6 – Otto Bock hook with Upgraded Spring
5.2 Concept Development, Scoring and Selection
Table 5.2.1 (as seen on the next page) was used in order to select which of the four concepts
should be pursued. It was decided by the team that our resources could be allocated effectively
to develop 2-3 of the concepts that were discussed. Two sets of criteria were used in determining
the best concepts – “user needs” and “producer” criteria. Various aspects of these categories
were the broken down and rated with what we deemed an “importance factor.” Each concept
was then objectively rated in each of these areas and the value given to each concept was
multiplied by the importance factor. In order to determine the best concepts all that had to be
done was sum the total ratings of each individual concept. Ultimately the decision was made to
progress with the two highest rated concepts.
21

User Need Criteria
Producer
Criteria
Concept
Table 5.2.1 – Concept Scoring
Fabricated
Hook
Otto Bock
Model 10A60
Mechanical Adv.
System
User Need Criteria Standards:
1) Reduced User Input – Does this concept have the potential to provide a significant
reduction in user input force required?
2) Increased Grip Strength – Does this concept have the potential to provide an increase
in grip strength over Tim’s current hook?
3) Serviceable – Could this concept be serviced by the user without the assistance of a
professional?
4) Reliable – Will this product function correctly without regular maintenance so as not
to reduce the users’ productivity?
5) Corrosion Resistant – Can this concept be made out of materials that can withstand
the agricultural environment?
6) Affordable – Will this concept provide enough value to the customer that they can
justify the cost?
7) Simplicity of Use – Will this concept be as easy to use as the customer’s current
application?
8) Light Weight – Will this concept be light enough so as not to discomfort the user with
added weight at the wrist or forearm?
UProducer Criteria:
1) Easily Manufactured – Does this team, as the producer of the product, have the skills
and tools necessary to create and build all parts?
2) Affordable – Can this product be made cheap enough that it can be sold at a profit and
still provide significant value to the customer?
3) Marketable – Would this concept provide a feature that would interest customers?
4) Original – Does this concept provide a feature that is either new or vastly improved
over current products in the market?
Otto Bock
Spring Upgrades
Importance
Factor (0-1)
Rating [( 1-exceeds spec, 0-does not meet spec)*imp. factor]
Reduced User Input 1.0 0.70 0.70 0.90 0.00
Increased Grip Strength 1.0 0.50 0.60 0.00 0.90
Serviceable 0.9 0.36 0.72 0.72 0.63
Reliable 0.9 0.36 0.72 0.63 0.54
Corrosion Resistant 0.8 0.64 0.64 0.64 0.48
Affordable 0.6 0.18 0.42 0.54 0.42
Simplicity of Use 0.5 0.25 0.45 0.40 0.35
Light Weight 0.4 0.20 0.28 0.28 0.32
Easily Manufactured 0.9 0.09 0.90 0.63 0.90
Affordable 0.8 0.24 0.40 0.72 0.40
Marketable 0.8 0.16 0.64 0.40 0.48
Original 0.5 0.40 0.25 0.40 0.25
TOTAL 4.1 6.7 6.3 5.7
Relevance (1-highest, 4-lowest) 4 1 2 3
22

6.0 Concept Selection
6.1 Data and Calculations for Feasibility and Effectiveness Analysis
6.1.1 - Static Analysis of Split-Hook Gripping Force and Necessary User Input
Two of our proposed designs resemble traditional prosthetic terminal devices. Two of the most
important parameters of a gripping device are the gripping force it provides, and the force needed
to open it. The following analyses are configured to resemble a classic “split hook” design.
Figure 6.1.1 – Classic Split Hook Design
This design uses two hooks, one is mobile (upper hook in picture), and one is fixed (lower hook).
The hooks are held together by a series of rubber bands. The user opens the hook by applying a
force to the cable (seen at the right side of the picture). The amount of rubber bands, and the
nature of the “cable post” are two things that can alter the gripping force, and the amount of user
input needed to open the mobile hook. The following analyses utilize simple static models to
formulaically simulate this design.
UDetermining Gripping ForceU:
Figure 6.1.2 shows a simple static model of the moveable hook on a split hook prosthetic
terminal device.
Fr = Spring or rubber band force Fg = Gripping force
Dr = Rubber Band Distance from Pivot L = Overall length of hook
Figure 6.1.2 – Static Model of Moveable Hook
23

Using simple static analysis, the resulting grip force (Fg) can be calculated by summing the
moments about the pivot point on the right side:
The term “Fr” in these equations is equal to k*xo (via Hooke’s Law), where “xo” is the
elongation of the rubber band at the closed position of the hooks.
UDetermining Needed User InputU:
Figure 6.1.3 shows “top-down” views of the mobile hook in the closed (left) and partly open
(right) positions
Fr = Force of rubber band Dr = Distance from rubber band to pivot
Fc = Cable force supplied by user (constant) Dp = length of cable post
Figure 6.1.3 – Top-Down FBD
“Fc” is considered a constant for this section, and it is assumed that the user is applying
maximum force throughout the entire opening range of the hook. It is also assumed that the
hook runs through a guide before it connects to the post on the hook. This keeps the user force
vector in one direction at all times. Since torque is what we are after for this section, the forces
“Fr” and “Fc” have to be resolved, so they are perpendicular with “Dr” and “Dp”, respectively.
Figure 6.1.4 (as seen on the next page) shows the components needed to resolve the user input
force (Fc).
(2)
(3)
(4)
24

Figure 6.1.4 – Input Force FBD
The interior angle between Fc and Fc’ is equal to Ѳ. Therefore, both the resolved force, and
subsequent torque can be determined easily:
(user induced torque)
Figure 6.1.5 shows the components necessary to resolve the force due to the rubber band.
Figure 6.1.5 –Rubber Band FBD
Please note that the rubber band force (Fr) will increase as a function increasing Ѳ. Therefore,
Fr must be defined functionally:
For this equation, “Fro” is equal to the rubber band force at Ѳ =0 (which is equivalent to the “Fr”
from Figure 6.1.1). Also, “x” is equal to the amount of additional elongation the rubber band
(5)
(6)
(7)
(8)
25

experiences for any Ѳ greater than 0. The interior angle between “Fr” and “Fr’” is equal to Ѳ.
The resolved rubber band force (Fr’) can then be solved similar to the resolved user input force:
Figure 5 illustrates the way that the term “x” is also a function of Ѳ.
Figure 6.1.6 – Displacement & Angle FBD
Combining these two functions, the resolved rubber band force and the resulting rubber bandinduced
torque can be determined:
Figure 6 shows the hook drawing with both forces resolved.
Figure 6.1.7 –Resolved Forces FBD
(9)
(10)
(11)
(12)
26

Since the torques of the both the rubber band (Tr) and the User (Tc) can be determined easily by
using Figure 6, the following fraction can be utilized to monitor the user’s capability to open the
hook.
(Capability Index)
By plotting the Capability Index for the hook’s entire range of angular motion, the relative ease
which the user will experience while opening the hook (assuming they apply full force for the
entire travel of the hook) can be observed. This is shown in Figure 6.1.8.
The plot labeled “GOOD” indicates that the user is capable of opening the hook throughout its
entire range of motion. The ease of opening, however, diminishes as the springs/rubber bands
are stretched increasingly. The plot labeled “BAD” shows that at some intermediate Ѳ, the
torque input from the user becomes equivalent to the torque produced by the stretched
springs/rubber bands. Since the user’s input force is assumed to be maximum, the user is not
capable of opening the hooks beyond this point.
UDetermining Minimum Opening Force:
Figure 6.1.8 – Capability Index Plots
It may also be of interest to find what minimum amount of user force (Fcm) is needed to open
the hooks to some angle Ѳ. This can be done easily, using the equations derived from the prior
sections.
(13)
(14)
(15)
(16)
27

6.1.2 - Analysis of Pulley Based Mechanical Advantage
Figure 6.1.9 – Mechanical Advantage by the Use of a Pulley
Using basic physics, a pulley can be used to theoretically cut the input force needed to move an
object in half. When a pulley is not used, the input force is equivalent to the output force.
Likewise, when a pulley is being used as shown in Figure 6.1.9, the tension in the cable around
the pulley is the same. In the system used for this project, the “W” would be the hook post. The
“W/2” on the right side is the grounded cable and the “W/2” on the left side is the cable that
connects to the customer’s back harness.
A disadvantage to the pulley system is the work required still remains the same.
Work = force * distance
Since the force is cut in half, the distance required is twice as much as when the pulley wasn’t
used. This trade off of cutting the input force in half but doubling the necessary cable travel
could be an issue in our finalized design.
6.2 Concept Screening, Development and Selection
Through our interaction with Tim, he stated that he wished for more grip force out of his current
prosthetic, but also cited pain in his back and shoulders after a long days work with his current
prosthetic’s grip force. Realizing that this was a major issue to Tim, we began discussing ways to
address this problem. We very quickly realized that a mechanical advantage system was most
likely the only way that our group would be able to achieve this feat. The system would allow
for reduced stress on his body due to the input force being cut in half, therefore allowing for the
possibility of a higher grip strength. After speaking with Tim and explaining the concept of the
mechanical advantage system, he seemed very eager to see what our design had in store for him,
as well as to test the input force required to open the hook. Therefore, we began focusing on,
and designing a mechanical advantage system to fit within Tim’s forearm.
(17)
(18)
28

Once the design of the system was underway, we realized that it would be nearly impossible to
design the system to fit within Tim’s current forearm without numerous problems. Also, we
would not be able to remove his forearm due to him needing it in his everyday life. Therefore
our next step of the project was to find and order a forearm very similar to Tim’s so that we
could put our system within that forearm instead.
Our group met with Tim numerous times throughout the designing and manufacturing processes,
where we took numerous measurements of Tim’s current forearm, cable travel, wall thickness,
etc. enabling us to properly order a similar forearm from a prosthetic manufacturer. Tim pointed
us in the direction of Yankee Bionics, a prosthetic manufacturing company in Akron, Ohio that
he worked with in the manufacturing of his current prosthetic device. The company was nice
enough to donate a slightly used forearm for the use in assisting Tim. Once we received the
forearm, we finalized all of the necessary measurements and began manufacturing the
mechanical advantage system, and completed the installation. Once our system was bolted
within the forearm, the cable travel was then established. A small hole was then drilled through
the exterior wall of the forearm, allowing the cable to be attached to the hook.
Once the manufacturing was completed, the only step left in our project was to let Tim test the
overall system and see how it performed. Where, after visiting Tim and allowing him to test the
system, he was overwhelmed with how much easier our system was to open than his original
prosthetic.
7.0 Final Design
7.0.1 Introduction
A prosthetic arm force reducer was manufactured by designing a pulley mechanical advantage
system housed within the hollow forearm section of the prosthetic with a 10A60 Otto Bock
prosthetic terminal device.
This design emerged after extensive interaction with customers, primarily Tim Lang, who
precisely met the demographic we specified at the beginning of the project. Not only did this
interaction help us to identify legitimate customer needs and eliminate unnecessary design
features, but it also allowed us to take real measurements, which we used to confidently refine
the newly-emergent subsystems.
Through application of DFMA design principles, we have designed the mechanical advantage
system to meet customer expectations, be readily manufactured, easily installed, minimally
intrusive, and reasonably simple to analyze. This section concerns the implications of our design
decisions.
7.0.2 Impacts/Effects
The first major impact concerns the choice of hook that we are using. The customer’s current
hook utilizes rubber bands to supply the closing force of the hook. As a result, the customer has
resorted to attaching a large number of rubber bands in order to achieve the grip strength that can
be accomplished by using the springs available with the Otto Bock Model A60. The two-load
29

hook allows the customer to operate in the full strength position only when necessary. Customer
interaction has confirmed that this full strength setting offers more grip force than his currently
modified prosthetic. Additionally, the customer can use half of the grip force and, therefore,
have only half of the input force by a simple adjustment of a lever. Immediate effects of the
upgrade to springs are that the user will be able to perform a wider variety of tasks with the
increased grip force and also use the hook at half power, similar to a standard existing one-load
hook. Long term effects could include fatigue and increased stress on the user’s body if the hook
is used at full load fairly often and possibly an increase in the stresses in the entire prosthetic
resulting in shorter product life due to fatigue.
The mechanical advantage system offers relief for this. The ideal case for this system would
give the user a 2:1 advantage in the input force required to open the spring hook. For example:
Assume the original spring at the high setting required 35 lbs. Now, with the mechanical
advantage pulley, it should ideally only require 17.5 lbs to open the hook at full load. The
impact of this system to the user would be a drastic reduction of the input force required for both
settings on the two-load hook. Our design reduces the input force required by the user by 47%
from 18 lbs to 9.5 lbs or about a 1.9:1 advantage.
The negative impacts discussed for the move to the Otto Bock hook and its stiffer springs would
be reduced significantly, because even with an increase in grip force, the user would be able to
exert less force than in an unmodified system. With respect to the mechanical advantage system,
the user would theoretically have to pull their cable twice as far as was previously required to
fully open the hook in order to balance the energy and forces for a 2:1 mechanical advantage. It
currently takes about 1.75 inches of cable travel to fully open the customer’s hook. The Otto
Bock hook, however, does not open as wide as the customer’s hook. This reduces functionality
somewhat, but is beneficial in that he will only have to pull the cable ~2.75” inches, instead of
the 3.5 inches it would take to fully open his current hook.
Short term effects of a mechanical advantage system upgrade would be an increase in user
comfort due to the ease in which he or she can use their prosthesis. It would allow for the
immediate implementation of an increased-strength two-load hook without increasing the force
needed to operate such a device.
Long term effects are an increased life for the cable operating the pulley, and an increase in the
workload the user could accomplish. Some possible negative long term effects could include
shorter lifespan of the cable post and the cable connecting it to the pulley since they would be
experiencing an increase in tension from the springs. Also, there is a possibility of eventual back
and shoulder injuries from prolonged use, though this fact exists with all current prosthetic hook
designs.
The overall impact of the production of this system on the environment or society would not be
very significant. As currently designed, this upgrade kit uses mostly pre-produced parts that
need little machining. Most of the work required is in assembly and can be done by a few
trained operators. The greatest impact or effect of the combined upgrade system would be to the
life of the customer who would be using this system on a daily basis, allowing him or her wider
range of tasks that they can complete and decreasing the wear on his or her body.
30

7.0.3 Professional/Ethical Standards
Our team’s main goal is to help our customer, Tim Lang. We believe that our decisions should
benefit him, first and foremost, and also stay within our parameters with respect to safety of our
team and customer, budget, and design specifications. Our team conducts our meetings in a
professional and productive manner, and pays close attention to our schedule to maximize
efficiency and overall benefit to Tim. This attitude is reflected in our decision making process,
customer and team interaction, and devotion that each team member has to our goals and
successful completion of our project. Also, working together as a cohesive unit is far more
productive than acting as individual with disjointed priorities.
7.0.4 Function
In order to add mechanical advantage to our system, a movable pulley was mounted in the
forearm of the prosthetic. A movable pulley allows the user to input only half of the force
required to open the hook. This is due to the fact that the forces on the pulley have to balance
when the pulley is at equilibrium. Since two ends of the input cable are supporting the pulley in
one direction, they will equally share the load pulling on the axle of the pulley. In our case, the
load on the pulley will be the force required to open the prosthetic hook. Therefore, our
customer will be able to pull on the free end of the input cable with half of the force required to
open the hook. In the ideal case this will be a 2:1 advantage. A small disadvantage to the system
is that it requires twice the input cable travel in order to obtain the mechanical advantage. This
has been talked over with the customer and has been deemed an acceptable trade-off for gaining
a 2:1 advantage.
7.0.5 FMEA & Safety
Figure 7.0.1 – Mechanical Advantage Schematic
For the hook, initial consideration was given to minimize ways in which the Otto Bock design
may fail in the agricultural setting where it would be used. This was approached by completing
FMEA of the hook design, and the results of this analysis are presented in table 7.0.1.
31

Failure Mode
Table 7.0.1 - Risk Priority Number Analysis for Split Hook (Environmental)
Potential
Severity
Likelihood of
Occurrence
Probability of
Detecting
Cable Break (at fitting) 7 3 8 168
Corrosion Over Time 2 10 5 100
Threading of Prosthetic
Connection
RPN
6 2 6 72
Environmental Deposits 2 10 1 20
Loss of Movement of
Vector Hook
3 5 1 15
These failure modes were listed in order of importance according to the risk priority number
(RPN) calculated in the far right column. The RPN is the product of potential severity,
likelihood of occurrence, and detect-ability of the particular failure mode, as ranked on a scale
from one to ten.
Since the Otto Bock hook has been left in stock, OEM condition, failures relating to the stresses
present in the hook itself were not included. The mechanical advantage, however, does allow the
user to hypothetically pull twice as hard on the hook at full opening. Although this situation is
not practical, it could lead to failures in the hook, which is not designed to experience twice as
much force as is needed to open it to its maximum width.
Corrosion over time and the hook accumulating environmental deposits are completely
unavoidable considering the working conditions that it will be used under. As such, a stainless
steel hook has been selected and all fabricated parts shall be made of stainless steel.
Additionally, any mud or other element that fills some part of the hook should be able to be
easily removed by our user.
The only other failure mode of the hook of much concern was if the threading of the hook could
be stripped in some way. The prosthetic arm provides the biggest guard against anything
happening to the threading as the hook threads into a bushing that is then inserted into the
prosthetic arm. This ensures that the threading is not constantly under torque but instead the arm
will bear this force.
FMEA was also completed for the mechanical advantage portion of the design in the same
manner as conducted for the environmental effects on the system. The FMEA results are
presented below in table 7.0.2.
32

Table 7.0.2 - Risk Priority Number Analysis for Mechanical Advantage
Potential Severity Likelihood of Occurance Probability of Detecting RPN
Threading of Rings Strips
Resulting in System Pulling
Away From Prosthetic Walls
7 (Inoperable)
3
6
168
Pulley Torques in Track
and Locks
5 (Temp. Inoperable)
4
8
160
Rope Slips From Pulley
Wheel
5 (Temp. Inoperable)
4
8
160
Bearings Seize in Track 5 (Temp. Inoperable)
4
4
80
Tracks Break Attachment
With Rings
7 (Inoperable)
1
6
42
By comparing the RPN’s of the mechanical advantage system to the RPN’s of the environmental
effects, the numbers are similar. At very worst, the user loses the ability to open or close the
hook. At minimum, the faulty system would pose significant to the user.
The worst failure with regard to mechanical advantage is if the rings that attach to the prosthetic
were to strip and the bolts pull away. This would result in the system becoming completely
inoperable and could cause damage to the prosthetic arm as well. This failure possibility is
minimized by the fact that correct design of the threads can nearly eliminate that possibility. In
addition, there would be the possibility of detecting the failure as the system should develop
some amount of “slop” or looseness before it became completely separated from the prosthetic
wall.
The possibility of the pulley seizing in the track or the rope slipping off the pulley wheel is
legitimate; however, such a failure would only leave the system temporarily inoperable. It would
be a significant inconvenience for the user, but would not leave them in any real danger.
If the bearings were to lock in the track it would leave the system inoperable but there should be
only a very small chance of this occurring. Additionally this would more than likely occur due
to build-up of some type of deposit in the track that could be removed to allow the system to
operate once again. If the tracks were to break at the ring it would leave the system totally
inoperable, but as with the bearings, once again there is a very small chance of occurring.
7.0.6 Economics/Value
The decision to fabricate the tracks for the mechanical advantage system out of square tubing
was economical. Both the manufacturing time and price are significantly less for this design
than it would be to fabricate the tracks out of solid stock. The options for buying tracks were
around the same base cost as the square tubing we chose ($7.82 per 12” length), but the quality
and material were not acceptable. The most common track was made out of extruded aluminum
which could bend and deflect relatively easily and is not compatible for welding with stainless
steel.
33

The rings are the most expensive part of the mechanical advantage system because they will be
made out of a 2-1/2” diam. stainless steel pipe. The minimum length of stock for this pipe size is
12’’ and as such a large amount of extra material had to be purchased. The high cost
(approximately $73) is hard to justify for the construction of a single system, however this stock
would be fine if multiple systems were to be constructed.
A nylon pulley with a stainless steel housing was selected. This item cost around $20 and will
require no additional modifications. The bearings were selected due to the flexibility of sizes
available and the low cost. Ball bearings are approximately one-third the cost of rubberized
wheels. The 1/4” diameter stock for the axles and spacers was very inexpensive at around $6 per
12” length. All materials, with the exception of the nylon pulley, of the mechanical advantage
system are stainless steel which makes any necessary welding possible and is good for corrosion
7.0.7 Design Analysis
For mechanical advantage system, yielding of the tracks is the most pertinent failure mode for
formal analysis. This failure mode involves potential yielding of the track’s sidewalls due to the
forces inherent to having the pulley travel centrally through the forearm. The central translation
of the pulley requires the cable leading to the hook’s cable post to be at an angle at all times.
This is illustrated below in a top-down view.
Figure 7.0.2 – Pulley Vector Analysis
The concern here is that the side-wall force “Fw” could torque the walls of track, and make them
yield. There is also a possibility of a shearing failure where the tracks are welded to the retaining
rings. Fortunately, theta decreases as the pulley is moved away from the front of the forearm
during operation. Therefore, when the hook is fully open, the pulley is at its farthest point from
the wrist, which results in the smallest value of theta to be encountered. This is desirable
because at this condition, the largest forces are applied, but the effect of Fw is (geometrically) at
its smallest.
With these considerations in mind, this failure mode was analyzed at the point of highest user
input with FEA software. FEA was chosen because of the unusual loading condition, and
because it can simultaneously display the stresses at the sidewalls of the tracks, and the welded
area.
34

The actual FEA testing involved utilizing a drawing of our current track. By using our hook
simulation spreadsheet, a user input force of 70 lbs was needed, and a side-load of 13.4 lbs
(determined by the triangular relationship pictured above) was distributed among nodes in the
inner wall of one track to simulate the effect of the bearing force “Fw/2” (only one track). The
material chosen was annealed 302 stainless steel, because it was similar to the 304 stainless steel
in which our actual tracks are made of. The following pictures give a visual summary of our
FEA results.
Figure 7.0.3 – Algor track model 1
Figure 7.0.4 – Algor track model 2
Figure 7.0.5 – Algor track model 3
35

Results Summary
• Maximum stress = 3154 psi (This value was very localized near the end of the
loading area, and may not be an accurate depiction. These areas are shown in red
in the top screenshot. Much more of the stress occurred in the green region,
which indicated ~2000 psi).
• Maximum displacement = 0.00035 in (The displacement results are shown in the
second and third screenshots).
These results instilled confidence in the design, because of the minimal stresses and
displacements involved.
The unit life was calculated based on the cable being the limiting factor. The cable breaking is
the most likely failure to occur within the specified life (3 years). The cable used is 3/32”
diameter type 304 Stainless Steel. It is constructed of 7x9 strand cord with a plain coating.
Based on the cable breaking strength of 920 lbs. (as specified by McMaster.com), the stress to
failure would be 133,277 psi. Drawing an S-N Curve and calculating the relationship between
stress, S, and unit life, N, yields the relationship, S = 229,723N (-0.0851) . When S equals the
breaking stress, the life of the cable is 13,675 cycles.
7.0.8 Customer
Much of the mechanical advantage design is based on or around customer input. Two significant
parties have contributed a great deal input that is reflected in our system’s design. The first is
from a direct link to a potential customer named Tim Lang. Tim is a young dairy farmer who is
missing his right arm from the shoulder down and needs to reduce the amount of strain on his
back and shoulders due to his body powered full arm prosthetic.
Our first time visiting Tim greatly shifted our project from modifying the terminal device to have
a moveable jaw and a compliant hook to allow for a wider grip to looking further down the arm
and implementing a mechanical advantage system. Tim was very satisfied with his opening
width (approximately 5 inches) and did not see a need to increase the width any further. Much of
our work up to our first visit with Tim had been conjectures of what a typical arm prosthetic user
might desire but by physically seeing him use his prosthetic many dilemmas with our old design
became apparent.
The second time our team visited Tim we came prepared with our refined mock-up
demonstrating the concept of the mechanical advantage system. Upon seeing this Tim suggested
using rounded head bolts so that the arm would not snag on anything. He also mentioned that one
to two extra pounds in his forearm unit would not be a problem. Another piece of feedback that
was critical was Tim agreeing that the trade off of reducing the input force needed to open the
hooks in half was worth doubling the cable travel and therefore exaggerating his shoulder shrug.
With that feedback it confirmed that a mechanical advantage system was not only needed but
wanted by the customer. Tim also led us to his prosthetic manufacturer Yankee Bionic which
happened to have a forearm piece with very similar dimensions to Tim’s forearm.
36

Another sector that has aided in our design is from the Bureau of Vocational Rehabilitation
(BVR) specifically George Platounaris. The BVR is comprised of councilors who provide
services leading to employment for people with physical and mental disabilities. Through
George and his staff engineers he has confirmed that our design is well suited for our target
customer, very marketable, and usable. This feedback is based on several customers that George
and other councilors have worked with in the past.
One other sector that has directed our design is from manufacturers of terminal devices,
especially Otto Bock. Through contact with Otto Bock we have realized that our initial vector
hook idea (having multiple resistive force settings) was not a new idea and that the very simple
yet effective system they were using was the culmination of a lot of engineering and customer
feedback.
7.0.9 DFMA
When exploring the design for manufacturing and assembly (DFMA) for the mechanical
advantage module it is important to design each component based upon its manufacturability,
dimensions, material, functionality, machinability, and other assembly considerations. By
considering all of those items it will help to ensure a simple but yet effective design. Our module
can be broken down into four parts as seen in Figure 7.0.9.
1
Ax
3
Figure 7.0.9 – Mechanical Advantage Solid Edge Model
1 = Hoops, 2 = Pulley, 3 = Tracks, 4 = Axle
Part #1 – Hoops: There will be two hoops that will encase the entire mechanical advantage
module. The hoop closer to the hook is the “wrist” hoop, and one further back is the “elbow”
hoop. Hoops were chosen over square tubing due to the circular shape of the interior of the
2
37

forearm. This gives the user the ability to push the unit into the forearm and obtain a tighter fit
due to more contact area between the hoop surface and forearm. It also increases the amount of
rigidity in the system. Furthermore, the circular shape increases the distance between the tracks,
which allows more room for the pulley’s installation. The wrist hoop is chamfered to increase the
contact area and therefore make a tighter fit and to ensure that the corner of the hoop is not
catching against the side of the forearm (this would have been much more difficult to do with
square tubing). Both hoops will be made of type 304 stainless steel due to its high corrosion
resistance, good weldability and machinability, and 30,000 psi yield strength. The only
disadvantage of using circular hoops was the need for flats milled on their outside surfaces to
ensure proper hole drilling/tapping, and on their inside surfaces to fasten down of the tracks.
Part #2 – Pulley: The entire pulley sub-assembly (the pulley wheel, pulley housing, and the
pulley axle) were purchased as one part. Our choice of pulley has a removable axle, so a
different axle (one which met our diameter requirements) could be used in its place. Another
important feature of the pulley is that it has a rigid eyelet which reduces twisting in the output
cable. The entire pulley assembly is technically a “pulley block”, which means that the wheel is
housed in a steel casing. The casing is an important feature, because it keeps the input cable
from slipping off the groove of the wheel. The pulley wheel is capable of guiding a cable of up to
1/8” diameter which is well above the 3/32” cable being used. The pulley will be somewhat
loosely mounted to the axle and no machining will be required.
Part #3 – Tracks: To make the tracks, ½” x ½” square tubing was chosen. Square tubing lent
itself to track design, requiring minimal milling and drilling operations after the stock had been
cut to length. Another critical issue is that the tracks and hoops are made of the same stainless
steel alloy (type 304), which ensures a better and more uniform weld when the track is welded to
the wrist hoop.
Part #4 – Axle: The choice of a removable-axle pulley block allowed quick removal of the stock
axle without damage to the pulley. The original axle was replaced with a stainless steel one,
which was not only more resistant to environmental effects, but was also turned to the
appropriate diameter to operate within the track openings. The ends of the axle had a slightly
larger OD than the center section, and were separated by grooves. This was done to prevent the
axle from shifting perpendicular to the motion of the pulley.
Other Considerations: The forearm prosthetic will require some machining such as drilling for
all eight of the mounting screws. Also, the output cable will require a drilled slot in the forearm
to reach the hook’s cable post. All of the machining operations on the forearm will be aided by a
template that lays out the location of each machining operation.
38

Figure 7.0.10 Cable Path for Mechanical Advantage System
No modifications were required for the input or output cable. Swage operations, however, were
used to terminate the input cable to the elbow hoop (marked “X” in Figure 7.0.10), and to
terminate the output cable at the pulley eyelet and cable post of the hook.
7.0.10 Testing (Mock-Up)
To test the feasibility and demonstrate the concept of our mechanical advantage design it was
critical to produce a mock-up. This mock-up used a 2 inch I.D. PVC pipe cut to 10 inches to
help simulate the amount of space in a typical
prosthetic forearm. All of the parts were
bought at Lowe’s and the total cost of the
mock-up was just under $28 and took four
hours to manufacture. After completing the
mock up for the mechanical advantage system,
our group then used a fish scale to
approximate the change in input force needed.
The scale was used with the original,
unmodified hook to see what the input force
was, and then this was compared to the input
force using the mechanical advantage. The
overall force value did exactly what was
expected, it was cut in half. The force without
the mechanical advantage was determined to
be approximately 35 lbs, whereas the force
while using the mechanical advantage was
approximately 17 lbs.
Figure 7.0.11 Mock Up
39

7.1 System Personalization and Operation
Before installation of our upgrade, it is important to consider the sizing of the individual
components that comprise our system, as they apply to each specific customer. The most
important dimensions, then, apply to the space available within the forearm and the operation of
the prosthetic itself. Specifically, the diameter of the forearm at the location of each ring, the
overall length of the forearm piece, and the required length of cable travel needed to fully open
the hook are the most pertinent. Knowing these critical dimensions allows us to judge whether
or not the forearm meets the minimum requisite length to use our system. This minimum
requisite length is based on the assumption that the pulley will travel through the wrist hoop
during operation. Therefore, the wrist hoop must have a large enough ID to allow for this.
The length of the track must also be long enough to allow sufficient pulley travel for the user to
open the hook completely. The overall track length also affects the location of the elbow hoop.
The elbow hoop must be placed far enough within the forearm so as not to interfere with the
mechanism within the prosthetic arm associated with forearm position setting. Assuming that
this requisite length is met by a potential customer’s forearm, the sizing of the hoops is achieved
by a simple turning operation to their OD, and the tracks can also be made using the same
milling tools and techniques. The pulley in our design was chosen specifically to be small and
strong, which should make it applicable to almost any customer, even one with a smaller
forearm. Once the system has been manufactured based on the specifications appropriate to the
customer’s, it can be installed into the forearm (please see the “Assembly” portion of Section
7.2) and operated.
During operation, when tension is applied to the input cable (see Figure 7.0.10), tension is also
applied at its termination point on the other side of the pulley. This causes the pulley wheel to
rotate, moving the entire pulley block towards the elbow. The pulley block is connected to the
output cable which is connected to the cable post of the hook. During the motion, the angled
position of the output cable (see Figure 7.0.2) will, ideally, cause intimate contact between the
pulley’s axle, and the side of the track it is touching. This contact should cause the axle to roll
along the track, rather than slide, because rolling is desirable for wear characteristics.
If any type of maintenance has to be performed by the user, the forearm must be removed from
the forearm assembly. This requires the external cable leading from the harness to the forearm to
be disconnected. From there, the forearm should be able to be completely removed from the rest
of the arm assembly.
Most maintenance issues will require partial disassembly of the mechanical advantage system.
This is most easily accomplished by removing the back ring (the one closer to the elbow). First,
the nuts used to fasten the tracks to the ring must be loosened. Next, the external screws used to
retain the ring on the outside of the forearm must be removed.
7.2 How it is Manufactured
Consult the Drawing Package, found separately from report, for more detailed CAD drawings of
all parts.
40

7.2.1 Rings (Part #’s THH-001 [Elbow] & THH-001 [Wrist])
1) Obtain 2.5 OD 2.0 ID 304 Stainless tube.
2) Face off front of tube to ensure square face.
3) Turn down to 2.28 OD for front ring and 2.45 for back ring.
4) Place 5 degree chamfer .1 inch from front.
5) Cut off ring at .5 inch long.
6) Mill .275 flat on the x axis 0 degrees relative to ring. The flats will be .01 down on the z
axis.
7) Center flat and use start drill .2 inches from front of ring (y axis). For the back ring it is
.2 inches from the back of the ring.
8) Drill hole with number 3 drill bit.
9) Tap hole with ¼ 28 threads per inch tap.
10) Repeat steps 6-9 at 90, 180, and 270 degrees relative to ring.
7.2.2 Track (2x) (Part # THH-003)
Figure 7.2.1. - Ring Manufacture
1) Cut ½ x ½ box steel to approximately 3 inches using band saw.
2) Mill sides so square and overall length in 2 29/32 inches.
3) Center box steel on y axis and start drill .2 inches from outer edge.
4) Drill mounting holes with 9/32 inch bit.
5) Repeat steps 3-4 on other side of the box steel.
6) Flip box so the holes now are facing downward.
7) Mill .45 back on x axis from face of box.
8) Repeat passes with an increase in depth until thickness of track is only .1 inch.
41

9) Center box on y axis and mill 3/16 groove through length on box.
7.2.3 Axle (Part # THH-004)
Figure 7.2.2 - Track Manufacture
1) Face off .25 in OD stock
2) Groove to .17 in diameter start .15 in from edge and continue .19 in inward.
3) Repeat groove 1.32 to 1.5 from front face with 0.17 diameter.
4) Cut off with overall length 1.66 inches
Figure 7.2.3 - Axle Manufacture
42

7.2.4 Collars
1) Using lathe turn overall length from .25 to .22
7.2.5 Assembly
1) Drill cable exit hole on right hand side (looking from the top of the arm) ¾ inch from the
front of the arm using a ½ in bit
2) Place front ring in forearm chamfer side first
3) Ensure that the mounting holes are at 0, 90, 180, and 270 degrees relative to arm.
4) Using a flash light mark were mounting holes shine though
5) Remove ring and drill holes with 3/8 inch bit
6) Bolt two tracks (vertically) with the nuts and other mounting holes inside the arm on the
front ring
7) Place back ring inside of arm and line holes so that they at 0, 90, 180, and 270 degrees.
Also ensure that the tracks are square.
8) Using flash light as before mark mounting holes
9) Drill mounting holes with 3/8 inch bit.
Fig 7.2.4 - Forearm Assembly
10) Attach bolts on the two mounting holes only.
11) Place the collar then the pulley then the other collar on axle.
12) Ensure everything is square and tighten down collars using allen wrench on set screw.
43

Collar
Pulley
Axel
Fig. 7.2.5 - Pulley Unit Assembly
13) Remove pulley and axle apparatus and loop and clamp cable to the front of the pulley.
14) Loop second cable around pulley and loop and clamp around mounting hole bolt. Clamp
down with a nut. ( Mount on left side for a right arm and right side for a left arm)
15) Maneuver the front cable so that it falls through the cable exit hole and axle is in the
tracks.
16) Using nuts tighten down the tracks on the ring.
17) Screw in split hook.
18) Loop cable from exit hole in cable post and clamp in place. Make sure pulley is pulled all
the way forward in the tracks.
Figure 7.2.6 - Cable Path and Final Assembly
44

7.3 Cost Analysis & Bill of Materials
It is an important factor to try to keep the overall cost of this system to a minimum. Even
though this is a new and innovative system, the people that will benefit will still need it to be in
the appropriate price range. Since there is not another example of this system on the market it is
hard to compare it to price. Tim Lang’s full arm prosthetic cost approximately 6000 dollars. So
it would seem reasonable to keep this system upgrade to 10-15% of the total full arm prosthetic.
The first initial consideration for the cost analysis is the materials for the alpha prototype as
shown in Table 7.3.1. There are a lot of wasted materials to complete the initial prototype. For
example only one of the twelve inches of the 2.5 inch stainless tubing is used. Also the bearings
that were purchased were proven to be not useful to the overall design. Even though they are not
in the current system they still have to be considered for the material cost of the alpha prototype.
There was also an excess of bolts, washers, and oval sleeves purchased. These inefficient uses of
materials lead to the high material cost of $178.17. It is expected that there will be a drastic
decrease in materials for the beta prototype and for the production of multiple units.
Table 7.3.1 - Material Costs for Alpha Prototype
Part Use Price Quantity
2-1/2" DIA. Round Piping Rings $72.89 1
1/2"x1/2" Square Piping Tracks $7.82 2
Pulley Pulley $20.46 1
Double-Shielded Bearings Bearings $8.00 2
1/4" DIA. Stock Axle, Misc. $5.95 1
3/32" Wire Rope Cable $1.34/ft 10 ft
Oval Sleeve for
3/32" Rope
Cable
Attachments
$8.43/pack 1 pack
1" OD Flat
Washer
Bolts for
Attachment
$8.64/pack 1 pack
Truss Head
Attach Rings
$10.64/pack 1 pack
Machine Screw
Set Screw
Shaft Collar
to Arm
Lock Pulley
in Place on Axle
$2.84 2
Total Cost for Alpha Materials : $178.17
The next consideration is the manufacturing costs of the system. For the alpha prototype
approximately 40 hours was spent on the completion. This was mainly due to trial and error of
certain aspects of the design. For example the tracks had to be re-manufactured because there
was not ample room to attach the nut to the bolt. A section of the track had to be milled away so
there would be enough room to secure it in place. From Table 7.3.1 the cost of labor was $15
dollars per an hour, the overhead factor was one, equipment factor was a half, and the tolerance
factor was 0.25. The reason for the generalized tolerance factor is the system had some
tolerances that are more open than others. So with a value of 0.25 both examples of the
tolerances are considered. With all this in consideration the overall cost of the alpha prototype
was $1828.17 dollars. This is 31% of the overall system cost of $6000 dollars. The majority of
this is labor costs and is assumed to decrease as more prototypes are produced.
45

Table 7.3.2 - Alpha Prototype Total Costs
Alpha Prototype
a. Total Time to Complete in Hours
Complete Manufacture and Assembly
40
b. Labor rate for the Operation ($/hr) $15 (level 2 skilled Labor)
c. Labor Cost ($) = a x b 600
d. Basic Overhead Factor 1
e. Equipment Factor 0.5
f. Special Operation/Tolerance Factor 0.25
g. Labor/Overhead/Equipment Cost ($) = c x (1+d+e+f) 1650
h. Purchased Materials/Components Cost 178.17
Total Cost for Alpha Prototype Production: $1828.17
With the final production cost of $1828.17 dollars was simply too high of a price for any person
to purchase the system. Table 7.3.3 shows the material costs if ten beta units were produced.
For ten prototypes only one section of tubing needs to be purchased which drastically reduces the
individual material costs. The total amount of material costs is $508.56 to produce ten units.
This is only $50.86 dollars in material costs to produce one beta prototype. This results in a
decrease of $127.31 in material costs for every beta prototype.
Table 7.3.3 - Material Cost for Ten Beta Units
Part Use Price Quantity
2-1/2" DIA. Round Piping Rings $72.89 1
1/2"x1/2" Square Piping Tracks $7.82 5
Pulley Pulley $20.46 10
1/4" DIA. Stock Axle, Misc. $5.95 2
3/32" Wire Rope Cable $1.34/ft 30
Oval Sleeve for
3/32" Rope
Cable
Attachments
$8.43/pack 3 packs
1" OD Flat
Washer
Bolts for
Attachment
$8.64/pack 3 packs
Truss Head
Attach Rings
$10.64/pack 3 pack
Machine Screw
Set Screw
Shaft Collar
to Arm
Lock Pulley
in Place on Axle
$2.84 20
The manufacturing for the beta types would also be reduced considerably. Using the same
method for the alpha prototype, the overall manufacturing costs for a beta prototype is shown in
Table 7.3.4. From previous experience and time trials the total time to complete the different
operations are shown. It is also assumed that the labor will be paid $15 dollars an hour for their
work. All of the other factors are the same as the alpha prototype. From this the total
46

manufacturing costs are $525.00. This is a decrease from the alpha prototype of $1125, and puts
the overall cost of the system to $575.86. This is an alpha to beta savings of $1252.31. The main
reasons for this price drop are the inefficient use of alpha materials and the errors in the
manufacturing. This price is in the 10- 15% range of the total prosthetic arm cost of $6000.
Table 7.3.4 - Manufacturing Costs for Beta Prototype
Operation Operation Operation
1 2 3 Operation 4
Cut Cut
square grooves in
tracks to pulley axle
desired to allow
Cut pipe length travel in
tubing to and drill the tracks.
1" sections bolt Insert into
and turn holes. pulley and
down on Mill lock into
lathe. opening position Bolt
Drill bolt for pulley with assembly to
holes axle spacers prosthetic
a. Total time to complete operation(s) in hours 5 3 2 3
b. Labor rate for the operation ($/hr) 15 15 15 15
c. Labor Cost ($) = a x b 75 45 30 45
d. Basic overhead factor 1 1 1 1
e. Equipment factor 0.5 0.5 0.5 0.5
f. Special operation/Tolerance factor
g. Labor/Overhead/Equipment Cost ($) = c x
0.25 0.25 0.25 0.25
(1+d+e+f) 206.25 112.50 82.50 123.75
h. Purchased materials/Components cost 12.15 7.82 22.44 8.45
Total Labor/Overhead/Equipment Cost $ 525.00
Total Purchased Material/Components Cost $ 50.86
Total Cost for Assembly $ 575.86
7.4 Design Validation
Throughout our design and manufacturing phases, interaction with our end user has shaped our
project, and allowed us to effectively address real-world considerations. Ultimately, three main
goals were set forth from analyzing our customer input: reduction of input force (to reduce strain
on the user’s body), increase in grip force (to allow the user to complete a wider variety of tasks
with the prosthetic), and to make the product both reliable and serviceable (so the customer can
fix it easily, if need be).
For the reduction of input force, we decided on a pulley-based mechanical advantage system
early on. This configuration guaranteed a 2:1 reduction in user input force, as well as twice as
much cable travel to get it open. Since this feature was most important to the user, and would
47

equire the most engineering resources for implementation, a 2:1 force reduction was deemed
appropriate, and the corresponding grip force increase would be chosen so as not to negate the
beneficial effects of the pulley. As mentioned in section 7.0, the 2:1 reduction was validated
using a simple fish scale, which indeed registered half the input force to open the hooks when
used with our mechanical advantage system. Additionally, the Otto Bock hook also has less
opening distance than the one the customer currently uses. This presents a clear trade off: if the
customer chooses to use our product, the size of objects he will be able to grasp will be more
limited. On the other hand, the smaller opening will mean less cable travel than he is used to.
His current prosthetic would have to experience 3.5 inches of cable movement with our device,
but the Otto Bock (with less opening) capacity, only requires ~2.75 inches.
Choosing a grip force that corresponded appropriately to our 2:1 mechanical advantage proved
challenging. Unlike the mechanical advantage system, where many options were available for
force reduction, the only feasible way to increase grip force was to outfit the Otto Bock with
higher stiffness springs. This was problematic because many of the springs whose dimensions
were appropriate for our application could not deflect to the necessary distance. Furthermore,
there was much variation in the stiffnesses of the dimensionally-applicable springs; this made
choosing one with an appropriate amount of grip force difficult. Fortunately, one customer
meeting led to a qualitative test of the user’s current grip force versus the Otto Bock’s. This
simple test showed that there was a marked increase in grip force by using the Otto Bock. This
was most likely due to the fact that the “pinch point” of the customer’s hook was much further
from the rubber band than the Otto Bock’s pinch point is from its springs (see Figure 6.1.2)
When the customer was finally outfitted with our prosthetic, we were confident that an increase
in the ease of use and grip force was present. Even at the Otto Bock’s higher grip force setting,
the customer commented that the force required to open the hook was less than his current
prosthetic (it would be much better to have some actual numbers to validate these claims).
8.0 Conclusion
Our project successfully met our objectives. Mr. Tim Lang, the dairy farmer from Marietta for
whom this system was designed, is extremely satisfied with our product. Though Tim Lang will
not be using our exact prototype, he will be working with the Bureau of Vocational
Rehabilitation engineers to implement our mechanical advantage system in a new prosthetic
forearm that he will be receiving from Yankee Bionics. The forearm used in our prototype was
donated by Yankee Bionics, and it is the exact size Tim needs, yet it was not equipped to be
fitted with a rotational chuck system. This system allows Tim to rotate his hook 180° to be able
to drive his stick shift tractor and quickly change to operating his milking machines.
Since the forearm we received did not have the capability of utilizing a rotating chuck system,
our team decided that we should not proceed with finding a way to rotate our hook. The 180°
rotation that Tim needed was not feasible for our system, so we thought the slight rotation we
would achieve would not add value to the overall design. Tim’s next forearm will have the
proper rotating chuck system which he will need to fully rotate his hook for everyday operations.
The BVR engineers should easily be able to move the mechanical advantage system to Tim’s
new forearm. Bolting the holes on the forearm and securing the bolts will be the only major
48

operation necessary. This convertibility also applies to future times when Tim receives a new
forearm from the company. Every 3 years the forearm and cable are replaced by insurance, so
Tim could do the future changeovers of the mechanical advantage system to the forearm himself
or with the help of BVR engineers. The cable should then be able to last another 3 more years
until the next changeover. As seen in Table 8.0.1, the system will last 13,675 cycles which is
approximately equivalent to 3.8 years (360 working days a year using the system 10 times per
day). This lifecycle number is based on the cable being the limiting factor.
The project met all of the specifications as outlined in Section 3.0. The following table
summarizes these specifications and relates them to the actual performance of our design.
Table 8.0.1 Performance Relative to Specifications
Specification Actual
Avg. 9.5 lbs (47%
Acceptable?
Input Force Avg. 18 lbs
Reduction)
Equivalent to 11 rubber
Yes
Closing As strong or stronger bands at hook’s highest
Force than 7 rubber bands
setting Yes
Unit Price Less than $700 Total Prototype cost $575 Yes
Unit Fit into Tim’s prosthetic Fits into Tim’s prosthetic
Dimensions forearm
forearm Yes
Unit Life 10,800 Cycles (3 years) 13,675 Cycles (3.8 years) Yes
Unit Weight Less than 5 lbs 4 lbs Yes
The true value of this design allows Tim a way to more comfortably work in the fields exerting
one-half less input force than he traditionally had to use with his former prosthetic. This will
greatly lessen the discomfort that he experiences from using the prosthetic in his daily work.
The system is easily cleanable, corrosion resistant, simple to operate, and utilizes the standard
cable and cable connectors that both Yankee Bionics and Tim are familiar with already. The
mechanical advantage system being fully contained in the forearm is extremely unique.
Currently, no product on the market like this exists. The system is very compact, yet the
materials chosen are quality and easily machineable and replaceable.
Our recommendation is to continue with our mechanical advantage project. The user would send
in his/her forearm to our company and we would install the mechanical advantage system for
them. Though maintenance is relatively simple, we would also offer maintenance and
replacement services to our customers. Our company would be in partnerships with prosthetic
forearm manufacturers such as Yankee Bionics. If we needed to modify the design to fit a larger
range of forearms, we would do so in the future to help the maximum amount of people.
The prosthetics industry is very specialized for each user. Our product can be adjusted for
different sized forearms by modifying the outer diameter dimension of the two rings. Further,
the system is bolted to the forearm using 8 rounded head bolts. The only variable tasks in our
49

design would be adjusting the ring size and bolting the system to the forearm. Since our product
has a level of standardization that many others do not in the prosthetics industry, this makes our
product unique and will help maximize the users’ benefits. As an added option to increase the
appearance, a neoprene sleeve would be available, streamlining the look of the forearm. This
was not a concern for Tim because he works in the fields all day, but others may request it.
Overall, Tim will be able to improve the quality and ease of his workday. Tim’s current
prosthetic utilizes 1.75” of cable travel, while our design requires 2.125” of cable travel. If this
becomes a working issue for Tim, we suggest the BVR engineers attach another cable to the
outside of the forearm which will allow Tim to switch the mechanical advantage system on or
off.
The closing force measurements were based on data collected directly from Tim. He said that
the mechanical advantage system with the Otto Bock hook (at its highest setting) was equivalent
in closing force to his current hook using 11 rubber bands. He also said it was much easier to
operate our system especially given the high closing force.
A full prosthetic arm typically costs around $6,000. Our system would only cost an additional
9.6% of the full prosthetic arm’s cost. This is a small price to pay for the value added with
respect to increased comfort and ease of operation. Tim says that becoming accustomed to living
with a prosthetic was extremely difficult at first, but stated, “The body’s like a chameleon; it will
adapt to whatever circumstances.” Our goal is to help other people ease the transition period to
get used to using a prosthetic arm. Tim will be reaping the rewards of our mechanical advantage
system, and we hope many other people will have this same benefit.
50

Appendix A – Split Hook Sample Calculation
Using the hook pictured at the beginning of section 6.1 for basic dimensions, an analysis of the
forces and torques present at a 30° opening will be performed.
-Spring Force at closed position (Fr): 30 lbs
-Spring constant (k): 50 lbs/in
-Length from pivot to end of hook (L): 5 in
-Distance from pivot to spring (Dr): 1 in
-Length of cable post (Dp): 1.5 in
-Opening Angle (Ѳ): 30 degrees
-Maximum force user can achieve (Fc): 70 lbs
Using Equation (3) to determine the grip force at Ѳ = 0, it is found to be 7.5 lbs
To find the torque which the user can generate at Ѳ =30, Equation (6) is used, and a torque of
90.9 in-lbs is found.
To find the torque of the spring at the same Ѳ, Equation (11) is used to get a torque of 51.7 inlbs.
The Resulting Capability Index, calculated simply by using Equation (12), gives a ratio of 1.76.
Since this value is greater than 1, the user is capable of opening the hook to this extent.
If an item was placed in the within the hooks at this angle, the resulting grip force can be
calculated using a modified version of the “Fg” formula used to find the grip force at Ѳ = 0:
Performing this calculation, the object within the hooks would feel a grip force of 12.9 lbs.
51

Appendix B – Interview Summaries
Mark Ficocelli - Phone Interview (9:30AM – 10:30AM) Oct. 10 th 2007
• Is an industrial designer that does a lot of private consulting and contract work in
conjunction with the BVR (Bureau of Vocational Rehabilitation).
• Much of his work is found through places like the BVR and from word-of-mouth and is
almost always with one specific case at a time.
• Much of his design is mechanical that focuses on a specific need for a person with a
disability.
• He suggested looking into upper extremities prosthetic devices (hook hand) and
considered that to be the best mass marketable idea.
• He mentioned a recent case that involved a farmer who had recently lost his hand and he
needed a way the hold different sized box wrenches. With his hook-hand prosthetic he
was unable to keep a firm grip on the heavy wrenches. Much of the solution to this
problem involved adding more rubber bands to the hooks hinge joint and using a thicker
cable that ran from his prosthetic to a back brace, He was able to open and close the hook
by simply flexing his back muscles.
• One suggestion he mentioned for that type of project would be to buy a burlap feedbag
and see if the prosthetic could open and hold a bag. He suggested 60lbs of closing
pressure.
• One item he stressed was to modify the man not the machine he uses. It is much more
sensible to make a universal adaptor on a prosthetic for picking up different sized objects
rather than making jigs or different attachments for each size.
• Another item he stressed was to focus of a specific function. Much of his design is
custom for the person and specific for their particular disability. It is very difficult to
create a universal device to help a lot of people because there is such a wide range of
unique disabilities.
• Another idea was to put yourself into the disabled person’s position and see what
difficulties you find. He suggested riding around in a wheelchair all day around campus
or blindfolding yourself and try to navigate a building. This might help us spawn ideas.
• He mentioned that wheelchair design was very specific but very marketable. Also it
might help to visit a design office or firm to get a feel for a typical office space.
• Patent searching is for marketing but you should understand and know about it.
• Other ideas he mentioned was excursion amplifiers, back harnesses, page turners, devices
to help people get dressed and to feed themselves, home modifications, work site
ergonomics, rural and farming work, computer adaptation, weight distribution, body
position, and seating for people in to wheelchairs, and yes even one handed chainsaws.
• Sources and products he sited where the OQO (thumb board electric device) Premobile.
• One ADA standard he mentioned was a 1 to 12 slope for wheelchair ramps.
52

George Platounaris - Interview (12:00PM – 1:00PM) Oct. 10 th 2007
• Is the rehabilitation vocational supervisor at the BVR. He has been working in
rehabilitation consulting for close to thirty years.
• The BVR is a state agency that services 10 counties out of Ohio. Annually it services
50000 people with disabilities most of which have orthopedic and metal health issues.
Typically they have 850 to 1000 active customers with a whole array of disabilities. Each
year RSC puts over 8000 individuals with severe disabilities to work.
• One good contact he mentioned was a Linda McQuistion who is typically and is
considered the main engineer for the BVR.
• He also mentioned a professor at the University of Cincinnati named Howard Baum who
is in industrial design who has done a lot of design for adaptive devices for people with
disabilities.
• He referred us to a Jerry Olsheski who is a professor in the college of education at OU
who is or was chair of the department of rehabilitation counseling.
• Another referral was a Mark Gold who is now deceased. During his life he worked
intensely with mentally disabled people and developed a program in the 1970 called Try
Another Way to help people with disabilities obtain careers.
• He suggested visiting special schools or classrooms and would gladly get us in contact
with facilities, machine shops, sheltered workshops, or special schools.
• He suggested getting in touch with WOUB to get a story line going and get the
communities interest. This could possibly lead to sponsorship or increase our funding.
• He was very interested in getting in contact with Dr. Kremer (he now has his email
address) to ask questions and facilitate any services he can offer.
• Sources he mentioned were Clovernook and Diagnostic Hybrids which is a lab that has
been recently hiring a lot of people from Personnel Plus because of the amount of manual
labor.
• He mentioned weekly meetings that we could definite sit in on. (I really don’t have much
information about these meetings).
• George seemed very eager to help us and possibly coordinate a project with us.
53

Tim Lang – 2 nd Interview (2:00 PM – 4:00 PM) February 27 th 2008
• Did not seem to think additional weight in forearm would be an issue. Agreed to place
small (1-2 lb) weight in forearm for a couple arms sometime soon as a trial.
• Had no concerns with the Otto Bock hook. Really liked the vector system and had an
overall good impression of the hook.
• Increased range of motion due to mechanical advantage is going to be an issue. We need
to find a way to address this so that he doesn’t have to double his current movement
pattern. Also, current range of travel is caused by cable post coming into contact with the
wrist of his prosthetic, an issue we should be cautious of with our design.
• We took pictures of harness operation so that it could be examined by entire team. See
pictures posted by Jay.
• Threading of Otto Bock hook was a perfect match with his arm.
• Forearm connects to elbow which connects to the upper arm (three piece system). The
gearing of the arm placement is located in the elbow and the forearm appears to be
connected by screws in a fairly simple manner. See pictures.
• Problem with the cable pinching at the elbow if we run it directly into the forearm
through the current opening. We will need to address this issue in our design by finding
an alternative way for the cable to enter the forearm. Possibility of drilling slanted holes
for entry and exit or a slot into the arm at the elbow to avoid pinching the cable.
• Current cable is guided by leather loop as well as bolted connection points. Cable also
runs through a casing at points where there is a bend. Pictures will show this.
• Details of current cable should be able to be obtained by contacting Gene at Yankee
Bionic.
• Any bolt heads on the outside of the prosthetic must be smooth and round. Additionally,
the nuts on the interior of the arm are some sort of special flat type nut.
• Cable did not seem to be very flexible. Must be cautious in looping around pulley.
54

Appendix C – Tim Lang’s Forearm Dimensions & Data
• Note: All measurements were done with a dial caliper on 2/27/08
Thickness = 0.225”
Overall Length* = 6.80”
Useable Length = 5.50”
Wrist (O.D.) = 2.10”
2” from wrist (O.D.) = 2.45”
4” from wrist (O.D.) = 2.80”
6.8” from wrist (O.D.) = 2.95”
Taper = Continuous through entire length
Cable Dia. = .086” (~3/32”)
Length of Cable Post = 1.0”
Angle of Cable Post = 45° ± 10° (Same as Otto Bock hook)
Depth of Wrist Chuck = 1.0” into forearm (threaded)
Cable Travel = 1.75”
Input Force = 4-6 kg
Flexibility of Cable = able to make 0.5” loop
* - The overall length is the shortest distance from the wrist chuck to the scooped cutout of the
forearm.
UOther Notes
• The prosthetic is comprised of three major parts (the shoulder/upper arm, the elbow, and
the forearm).
• Tim was uncertain of how to disconnect the forearm at the elbow for he has never done it
before. The pictures might yield some clues of how it is done but is can be done.
• The wrist chuck can be completely removed from the forearm via six to eight flat screws.
• The threading of the Otto Bock hook is the same as Tim’s hook
• When the forearm is all the way up, it and touches the elbow unit causing a pinch point at
the scooped cutout of the forearm.
• Tim didn’t seem to have a problem with 1 to 2 lbs of extra weight in the forearm.
• Tim claims that the tighter the harness the less shrugging necessary and the less amount
of cable travel. Also, that he is capable of a larger shrug.
55

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58

Drawing Package
1. THH‐001………………………………………………………………………..Elbow Ring
2. THH‐002………………………………………………………………………..Wrist Ring
3. THH‐003………………………………………………………………………..Track
4. THH‐004………………………………………………………………………..Axle
5. THHA‐001……………………………………………………………………...System Assembly View