MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

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Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)2-2.1 COORDINATION OF EQUIPMENTAND JOB DESIGNThe design of new equipment is also the design of newjobs for both the operator and the maintenance technician.The more human factors engineering is consideredin the design of new equipment, the better these two jobsof operating and maintaining can be done. In the designof equipment, effort should be directed toward simplifyingthe operator’s and maintenance technician’s tasks;this can be accomplished by the recognition of the importanceof integrating job design and equipment designduring the early stage of equipment design. The comparativesimplicity or difficulty of the maintenance task is builtinto the equipment. Thus the engineer is unconsciouslydesigning a job when he designs a piece of equipment;accordingly, he should be aware of the capabilities andlimitations of the human being on whom effective maintenancedepends. Therefore, specialists in human factorsengineering and in personnel training should assist inplanning the equipment. Even an equipment that performscomplex operations can be designed so that it iscomparatively easy to operate and maintain – a telephoneis an example—by providing standard and interchangeableparts, by using a simple method of testing and diagnostics,and by providing ready access to defective parts.These factors are touched upon in the paragraphs thatfollow and covered in detail in subsequent chapters. Thcdesign engineer must remember that regardless of theexcellence of his design, it will be no better than themaintenance of it, and the maintenance will be performedby personnel of average ability.2-2.2 REDUCTION OF PARTSReduction in the number of parts in an end-item shouldlead to a lower number of maintenance actions and thusto improved end-item availability. However, part reductionis subject to interface agreements between maintainabilityand other dsign disciplines; these other disciplinesoften benefit from increased numbers of parts. reliability,safety, and survivability specialists often promoteredundancy of parts to avoid mission aborts, accidents,and combat loss. the maintainability specialist mightalso want to add parts to increase testability and selfhealingaspects of the end-item. thus, the important considerationin part reduction is that it must involve combinedefforts and trade-offs which optimize systemcost-effectiveness.Part reduction often can be achieved by examininginterfaces between different work assignments. Forexample, the fuel system designers for the end-item andthe engine suppliers may both have supplied check valvesto perform an identical function. In special weapon adaptionkits, test interfaces should be examined. If electricalfuses are required to protect the safing and arming circuitsfrom excessive currents or voltage accidentally imposed2-2by external monitoring and or test equipment, thesefuses should be located in the external test equipment, notin both the adaption kit and the tcst set. The test set is thelogical location because of easy access for replacement.Also the inclusion of unnecessary parts in circuits reducesthe reliability of the circuits out of proportion to theirconvenience and may be the source of sneak circuits.Modularization dissussed in detail in Chapter 5contributes to simplicity by reducing the number ofexposed parts that may require replacement. If themodule is discarded or destined for repair lit the depotlevel, there is a serendipitous effect it reduces the needfor addressing the contained parts in manuals and repairparts lists. reduces the required skill level of technicians,and reduces the inventory of repair parts. Despite thebenefit, modularizaition has the potential for increasingthe cost of repair parts and complexity of repair at thedepot level.2-2.3 VALUE ENGINEERINGValue engineering may also be employed to bring aboutsimplification. Value engineering is a questioning type oftechnique; the type of approach can be characterized bythese questions:1. What is it?2. What does it do?3. What does it cost?4. What else will do the job?5. What does that cost?Cost must obviously be interpreter to include the logisticalcost. In pursuing this approach it is important that theessential product performance, reliability, and maintainabilityare locked into the value engineered item. Like anygood problem-solving technique, value engineering is achallenging and searching methodology. It is forever foreingthe value engineering practitioner to dig for fundamentals—i.e.,to determine what the part or function isreally intended to achieve and whether the part isnecessary.Part reduction need not be limited to end-item design.Life cycle cost can be reduced by critically examining theneed for every piece of support equipment—stands, tools,testers, and transport devices. this is especially applicableto the apparent need for new support equipment whenexisting standard equipment can be made compatiblewith new end-items. also the end-item may be altered,without loss of function, to be compatible with existingsupport equipment.2-2.4 CONSOLIDATION OF FUNCTIONSConsolidation of functions is probably the most importantdesign technique for simplification. In the abstractthis can be illustrated by the simple example of multiplyinga series of numbers by a common factor and summingthe result—i.e., multiply the elements b, c, d, and e by a
Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.common factor a and sum the result. This operation canbe performed byThe end results are identical; however, the first operationrequired four multiplications and one addition. and thesecond operation required only one multiplication andone addition. An analysis of the task to be performedindicates the procedure to be adopted to simplify theoperation. An analysis of the various functions to beperformed by components of weapon systems and thetypes of available hardware to do the required task oftencan lead to overall simplification. An operational orhardware approach can be employed to achieve simplification.In the operational approach, hardware performingsimilarfunctions could be conveniently grouped to facilitateoperator performance. For example, if a systemrequired a number of readouts to determine its operability,the readouts should be grouped onto a single panel tofacilitate observation. Also any adjustment required tobring a meter reading into line should be easily observableby the operator making the adjustment.In the hardware approach, multiple functions can beincorporated into a single item of hardware or controlledby or from a centralized location or operaition. For example,turning on the power switch of a personalized computernot only provides power but also activates the “poweronself-test” ( POST) that initiates software programswhich check for the presence of peripherals, clear statusflags so that the computer can be set up according to aspecified use condition. check the presence and conditionof hard disks and other subsystems, and determine theamount of access memory available and test to insure nofailures.A good example of the consolidation of relatedmechanical or electrical functions into a single activatoror control—lever, switch, component- to produce simplificationis the automobile, with which everyone isfamiliar, i.e.,1. Control of windshield wiper and washer by asingle lever2. Consolidation of ignition and starter switch into asingle assembly3. Use of braking systems that feature a self-adjustmentcapability4. Use of engine assemblies that feature a valve lashcompensator5. Use of a notched serpentine belt. with powertakeoff from crankshaft, to wrap around pulleys for everypower accessory i.e.. one pulley to perform manyfunctions.DOD-HDBK-791(AM)Logic must prevail in the practice of consolidation offunctions because, carried to the extreme, it could resultin a more complex system.2-2.5 IMPROVED ACCESS TO PARTSAccessibility is defined as a design feature that affectsthe ease of admission to an area for the performance ofvisual and manipulative actions. Thus, as a prime factorin relation to maintainability, accessibility relates to theconfiguration of hardware rather than to the physical andother limitations of personnel. Considerations of accessibilityare, and in their turn exert influence on, virtually allother maintainability factors. For simplicity in maintenance,accessibility must satisfy two needs, i.e.,1. Access to an item for inspection and testing, providingample room to attach test equipment2. Space in which to adjust, repair, or replace thefailed item.Configuration, or packaging, enhances accessibility byplacing items that are expected to require maintenancemost frequently where they will be most accessible. If thefailure rate for a particular item is high, accessibility ismandatory.A detailed discussion of accessibility and methods ofachieving it are presented in Chapter 4.2-2.6 STREAMLINED MAINTENANCEPROCEDURESThe ultimate goals of maintainability design are reductionto a minimum of the system support requirementsafter the system has been released to the user and thefacilitation of whatever maintenance work the system willrequire. The integrated logistical support plan, initiatedwhen the performance requirements of the system werebeing formulated, dictates how the system is to be maintained,e.g.,1. Malfunction isolation shall be positive to thethrowaway level and shall not require human decision orreference to manuals.2. All throwaway items shall be replaceable within“x” minutes.3. No maintenance adjustments shall be required.The task of maintenance simplification does not endwith the establishment of the support plan. Derivativeactions, which may be regarded as a subset of the basicplan, can be taken by both the designer and user to insurethe success of the support plan, e.g.,1. Part configuration2. Maintenance scheduling3. Simplified diagnostic techniques.2-2.6.1 Part ConfigurationMaintainability simplification can be compared to asuccessful “do-it-yourself’’ kit. A good design is so simpli-2-3
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