Nature of Mechanical Machine Design - Mechanical Engineering ...

Nature of Mechanical Machine DesignDr. Yi JiaDepartment of Mechanical EngineeringUniversity of Puerto Rico at MayaguezSpring 2005UPRM –Spring 2005 INME 4012 - Lecture 1 1

What is Engineering Design• Engineering design is an iterative decision-makingprocess of applying the various techniques and scientificprinciples for the purpose of defining a device, a process,or a system in sufficient detail to permit its realization.• Mechanical design is really a broader study encompassingall disciplines of mechanical engineering, thermal andfluid sciences, solid mechanics, materials and processes,manufacturing sciences.• The three major variants of engineering design areadoptive design, development design and creative design.UPRM –Spring 2005 INME 4012 - Lecture 1 2

Adoptive Design• Adoptive design is the use of existing/knownscientific principles and technical information fordevelopment of systems or device with suitablemodifications/changes. Very often only minoralterations or modifications are made in the existingdesigns (based on the feed back from manufacturing,service or marketing departments). This type ofdesign needs no special knowledge or skill andattended by first level designers with ordinarytechnical training.UPRM –Spring 2005 INME 4012 - Lecture 1 3

Creative design• Creative design on the other hand is development ofan unusual or novel solution to meet an exiting need.Very often it results in or needs further scientificunderstanding. This type of design needs creativethinking, higher technical ability and can beattempted by only experienced designers who havepersonal qualities of sufficiently high order.UPRM –Spring 2005 INME 4012 - Lecture 1 5

What is Machine Design• Machine design is one aspect ofengineering design. The ultimate goal inmachine design is to size and shape theparts (machine elements) and chooseappropriate materials and manufacturingprocesses so that the resulting machinecan be expected to perform its intendedfunction without failure.UPRM –Spring 2005 INME 4012 - Lecture 1 6

Machine Element Design• This course on machine element design is first basic course in‘real engineering’ and primarily concerned with the basicscientific principles related to design and analysis of commonmachine elements and devices or more specifically generalpurposecomponents or elements of a machine or mechanicaldevice.• General-purpose elements: Components/elements of various(kinds) machines, which are identical in shape or geometryand carry out same or similar function. Example: Shafts,bearings, springs, fasteners, gears brakes, clutches etc.• Machine is a combination of certain general purpose andspecial purpose elements which can transmit power (ormotion) in a controlled manner and which is capable ofperforming some useful work or task.UPRM –Spring 2005 INME 4012 - Lecture 1 7

Nature of Machine Element Design• To design machine element and deices, youmust be competent in the design of individualelements that comprise the system. But youmust also be able to integrate severalcomponents and devices into a coordinated,robust system that meets your customer’s need.• Design of machine elements is an integral partof the larger and more general field ofmechanical design. Designs and designengineering create devices or systems to satisfyspecific needs.UPRM –Spring 2005 INME 4012 - Lecture 1 8

Nature of Machine Element Design - Iteration• At the initial stages of machine design, we face a dilemma. Thedesigner can not predict with accuracy what environmental loads theuser will subject the machine to. The dilemma can be resolved onlyby iteration, which means to repeat, or to return to a previous state.We must assume some trial configuration for each part; use the massproperties of that trial configuration in dynamic force analysis todetermine the forces, moments, and torques acting on the part, andthen use the cross-sectional geometry of the trial design to calculatethe resulting stresses.• Most likely, on the first trial, we will find that our design failsbecause the materials cannot stand the levels of stresses presented.We must then redesign the part by changing shapes, size, materials,manufacturing processes, or other factors in order to reach anacceptable design. It is generally not possible to achieve a successfulresult without making several iterations through this design process.UPRM –Spring 2005 INME 4012 - Lecture 1 9

A Design Process1. Identification of need– The chief objective of an enterprise that produces a product is to satisfythe customer. It is essential that you know your customers’ desiresbefore beginning a product design2. Background research– background information is necessary to fully define and understand theproblem.3. Goal statement– after background research it is possible to restate the goal in a morereasonable and realistic way than in the original problem statement.4. Task specification– task specification bound the problem and limit its scope5. Synthesis– as many alternative possible design approaches are sought, usuallywithout regard for their values or quality. In this step, the largestpossible number of creative solutions are generated.UPRM –Spring 2005 INME 4012 - Lecture 1 10

A Design Process6. Analysis– the possible solution from the previous step are analyzed and eitheraccepted, rejected, or modified.7. Selection– the most promising solution is selected.8. Detailed design– once an acceptable design is selected, the detailed design can be donein which all the loose end are tied up, complete engineering drawingmade, manufacturing specification defined etc.9. Prototyping and testing– the actual construction of the working design is first done as a prototype10. Production– finally in quantity in productin• note that iteration is require within the entire process, moving anystep back to any previous step, in all possible conbmbinations, anddoing this repeatedly.UPRM –Spring 2005 INME 4012 - Lecture 1 11

Standards and Standardization• Once upon a time there were no standards or even uniformity among the sameproducts, for example, bolts, nuts and screw threads. Finding a replacement was notan easy task. This lack of standards and uniformity was costly and ineffective for agreat variety of reasons.• Standardization is the term applied to the obligatory norms to which variousindices of a product should correspond. These indices are types, grades, parameters(dimensions), and quality characteristics, test methods, rules of marking, packingand storage of products, raw materials and semi-manufactured goods.• Standardisation is the key to ‘cost effectiveness’ and ‘quality’. Standardisations ofspecifications for machine elements promote their quality and increase their servicelife. It curtails the time needed for their designing, quickens their entry into serviceand simplifies repairs as new standard ones easily replace worn or damaged parts.• An important feature of standardization, in present day mechanical engineering isthe principle of interchangeability without which lot and mass production isimpossible.UPRM –Spring 2005 INME 4012 - Lecture 1 12

Standards and Codes• A standard is a set of specifications for part, materials, or processesintended to achieve uniformity, efficiency and a specified quality. One ofthe important purposes of a standard is to place a limit on the number ofitems in the specifications so as to provide a reasonable inventory oftooling, sizes, shapes and varieties.• Code: A code is a set of specifications for the analysis, design,manufacture and construction of something. The purpose of a code is toachieve a specified degree of safety, efficiency, and performance or quality.• Interchangeability is the property of machine parts or units making itpossible to utilize them in assembly (or replace them during repair/service)without any additional machining (fitting) while preserving the technicalrequirements for the given unit, mechanism or machine functioning.• For parts to be interchangeable their dimensions need not ideally coincide.It is enough that the actual size of each part lies within the assigned limits.UPRM –Spring 2005 INME 4012 - Lecture 1 13

Standards and Codes• In mechanical engineering the following is standardized:– Designation of general engineering quantities, rules forconstruction of drawings; series of numbers applied to lineardimensions.– Structural elements of the majority of machine components, bar,beams, angle, channels, plates rounds etc.– Accuracy (Fits and tolerances) and surface finish of parts.– Materials, their chemical compositions, basic mechanicalproperties and heat treatment– Shape, size of most common parts and units: bolts, screws,studs, nuts, washers, cotton pins, rivets, dowels, keys, forms anddimensions of spline joints etc.– Belts chains, diameter and width of pulleys; couplings andclutches and antifriction bearings. Modules of gears and wormwheels etc.UPRM –Spring 2005 INME 4012 - Lecture 1 14

Engineer’s Responsibility• Essence of machine design is to define and calculate motions,forces, and changes in energy in order to determine the size, shape,and materials needed for each of machine element.• Engineer’s responsibility is to calculate and predict the mode andconditions of failure for each element and then design it to preventthat failure.• A good design engineer would provide a foolproof solution to aproblem, obtained at an economical cost and comprising ofminimum number of parts. To be a successful designer, theengineer must use the best available scientific understandingtogether with empirical information, a good judgement and often adegree of imagination or ingenuity. Then only best products suitableto a welfare society and marketable in a competitive economy couldbe turned out.UPRM –Spring 2005 INME 4012 - Lecture 1 15

The Criteria for Evaluating Machine Design Decisions• In approaching a design, the designer should establish criteria that will guide thedecision-making processes inherent in any project. Because each design problemhas many alternative solutions, each can be evaluated in terms of the list of criteria.There may not be a single best design, but designers should work toward anoptimum design, that is design should maximize benefits and minimizedisadvantages. The following are general criteria for machine design– Safety– Performance (the degree to which the design meets or exceeds the design objectives)– Reliability (a high probability that the device will reach or exceed its design life)– Ease of manufacture– Ease of service or replacement of components– Ease of operation– Low initial cost– Low operating and maintenance costs– Small size and low weight– Low noise and vibration, smooth operation– Use of readily available materials and purchased components– Prudent use of both uniquely designed parts and commercially available components– Appearance that is attractive and appropriate to the application.UPRM –Spring 2005 INME 4012 - Lecture 1 16

Skills Needed in Machine Design• technical drawing and CAD• properties of materials• manufacturing processes• statics, dynamics, and strength of materials• kinematics and mechanismsBuilding on these skills, and using this book, youshould now be ready to learn design of machineelement and mechanical design.UPRM –Spring 2005 INME 4012 - Lecture 1 17