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)Guidelines to assist the designer in the “module versuspiece-part’’ consideration are presented in the paragraphsthat follow.5-5.1.1 General "ModuIe Versus Piece-Part”CriteriaThe following are general considerations in the tradeoffbetween modular and piece-part design: (A “yes”answer favors a modular design effort.)1. Can the design effort be simplified by using previouslydeveloped standard “building blocks"? (Assumethat the reliability and input-output characteristics areconsistent with the new system.)2. Can deployed equipment be upgraded with newerand better functional units that replace older assembliesof component parts?3. Does modular design take advantage of automatedmanufacturing methods?4. Could the module be procured commercially?5. Does modularity provide a more effective distributionof effort among the maintenance echelons?6. Are recognition, isolation, and replacement offaulty units facilitated by modularization—which resultsin an increase in operational readiness?7. Will modular design ease the problem of trainingmaintenance personnel? Are fewer and less skilled maintenancepersonnel required?8. Is the use of automatic diagnostics facilitated bymodularization?5-5.1.2 General Module CriteriaIf a decision has been made to “go modular”, then thebasic philosophy of the modular concept must be pursuedproviding a design that can be maintained at the leastpossible cost, with minimal downtime, and that willrequire lesser skills and less manpower at the unit level.The following guidelines apply:1. Design equipment so that a maximum number ofmodules can be fault isolated by using the instrumentationprovided as part of the equipment.2. Provide the modules with as much self-fault testingand isolation capability as possible.3. Install modules to be accessed, disconnected. andreplaced and connected without special tools or handlingequipment under ambient conditions.4. Provide modules that require minimum postmaintenanceservicing or calibration.5. Provide module designs, i.e., encapsulation, thatprotect critical parts from environmental damage duringstorage and handling in forward areas.6. Design modules to maximize the potential fordiscard-at-failure, rather than to repair, for all modulesplanned for replacement at the unit level.5-5.1.3 Throwaway (Discard-at-Failure) CriteriaThrowawy (disposable) modules are easy solutionsfor difficult and potentially time-consuming maintenanceproblems. The advantages and hazards associated withthrowaway modules are addressed in pars. 5-3.2 and 5-3.3. respectively.Disposable modules should be designed. manufactured.and installed to meet the following criteria:1. Expensive parts art not discarded because offailure of inexpensive parts.2. Long-lived parts are not scrapped because offailure of short-lived parts.3. Low-cost. noncritical, and generally availableitems are typically made disposable.4. Disposable modules are encapsulated for protectionand ease of handling but remain compatible withperformance and reliability requirements.5. Test procedures, to be applied before disposal,provide clear and unequivocal results.6. Items are clearly marked for disposal at failureand so indicated in maintenance manuals and supplycatalogs.7. Discard instructions are documented in operatorand maintenance manuals and in supply catalogs.8. The precious metal parts of disposable modulesare designed for ease of salvage.9. Parts that can become contaminated are designatedfor proper protective measures.10. Devices bearing a security classification aremarked to provide the proper channels for disposal.5-5.2 FUNCTIONAL GROUPING FORMODLUARIZATIONFunctional relationships are used to facilitate fault isolationand to provide repair parts that arc compatiblewith the logistical support plan. Thc conscious effort tolocate and package components in self-contained functionalunits facilitates both the operation and maintenanceof a system (Ref. 2). Techniques emplyed toachieve a functional relationship are1. Logical flow grouping2. Circuit grouping3. Component grouping4. Standard construction5. Frequency grouping.Each of these techniques is discussed in the paragraphsthat follow.5-5.2.1 Logical Flow GroupingLogical flow, as its name implies, is the technique ofgrouping components of the overall system to parallel theinputs of individual assemblies or subassemblies as theyfunctionally relate to the overall system. For example, thelogical grouping for an automotive vehicle would be5-4
Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)power plant, transmission, and power train---the directionof the impetus (power) is from the engine to thepower train, not the reverse.Guidelines to be followed are1. Package and locate components parallel to theirfunctional relationships as established by a flow chart.Fig. 5-1 illustrates this idea.2. Select methods and subassemblies so that only asingle input check and single output check are required toisolate a fault within an item.5-5.2.2 Circuit GroupingCircuit grouping is the technique of grouping circuits—electric, hydraulic, or power for a specific function, e.g.,the audio and video circuits of television receivers andrecorders. Guidelines to be followed are1. Locate all parts of a given circuit or logicallyrelated group of parts—e.g., an automotive transmission- -together in a common volume.2. Place each circuit within the group in a separatemodule, e.g., a printed circuit board of the plug-in typethat can easily be replaced.5-5.2.3 Component GroupingComponent grouping is the technique of groupingtogether components with similar functions or that possesscommon characteristics. Guidelines to be followedare1. Locate items together that perform similarfunctions—e.g., amplification circuits, fuses, and relays.2. Segregate resistors, capacitors, and transistorsinto a minimum number of different locations on subassembliesand terminal boards.3. Group inexpensive components on a single chassisto Facilitate throwaway at failure.4. Group gages and instrument readouts—necessaryfor the control or monitoring of a function—into aninstrument panel to facilitate operator surveillance.5. Segregate components on the basis of significantvariations in the required maintenance tasks. For example,items that are to be cleaned by different methods—steam or solvent--should be packaged so that cleaning ispossible with minimum masking.5-5.2.4 Standard ConstructionStandard construction is a technique that follows nopreconceived set of rules. This construction creates a finalproduct by balancing a number of factors- heat loss,component size, final unit size and weight, and eyeappeal—to arrive at a compromise, which varies for eachapplication of the standard item. Examples of such acompromise are the simple radio receivers available onthe civilian market—they are configured in headsets,hand-held portable radios, clock radios, etc., for consumerappeal.5-5.2.5 Frequency GroupingFrequency grouping is the technique of grouping multiple,similar parts that are likely to require replacement atthe same time. This technique is most beneficial if thedominant failure mode is wear, fatigue, or a similar agedependentmode. Periodic replacement of assembliesbecause of anticipated failure makes possible a systemthat is, for all purposes, rebuilt.5-5.2.6 Evaluation of Grouping MethodsAn empirical evaluation of the previously describedequipment grouping techniques was performed by techniciansof different skill levels on a simple system and acomplex system (Ref. 3). Based on the performance dataused to make the evaluation—troubleshooting time,amount of information gained per unit of time, technician’ssubjective performance, and engineering criteriaitwas determined that the logical flow method is superiorto the standard construction method because the logicalflow method clearly enhanced the ease of equipmentmaintenance. The other grouping techniques also werepreferred to the standard construction method; however,Figure 5-1.Design for Functional Utilization That Corresponds to Modularization5-5
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