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Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)their superiority was not as clearly demonstrated as thatof the logical flow method.The selection process is not mutually exclusive however;choosing one particular method does not excludethe subsequent use of another method as the systemevolves e.g., the logical flow method having been decidedupon as the principal technique, circuit grouping could beused to augment the grouping technique.5-5.3 TYPICAL DESIGN GUIDELINESThe following are typical design guidelines relative tomodularization that will facilitate maintenance:1. Make modules and component parts approximatelyuniform in basic size and shape for best packaging.2. Minimize interconnections between neighboringmodules.3. Emphasize modularization for unit level replacementto enhance the operational capability. Modularizationversus part replacement for intermediate and depotmaintenance can be largely determined by cost factors.4. Standardize modules and receptacles; however,particular care must be taken to prevent inadvertentplugging into the wrong receptacle.5. Use guide pins for plug-in modules to permiterror-free insertion.6. Use quick-disconnect hold-down devices to permiteasy removal.7. Design all repairable modules so that the rapid,easy removal and replacement of malfunctioning partswithin the module can be accomplished by intermediateand depot level maintenance technicians.8. Divide equipment into as many units as are electricallyand mechanically practicable while consideringthe efficient use of space.9. Use an integrated approach i.e., simultaneouslyconsider materials, design, and application toachieve cost-effectiveness.10. Optimize the components of a module for agiven single function rather than for multiple, divergentfunctions—many compromises may be necessary to accommodatemultifunction units, which will render the moduleless than optimum and will defeat the purpose ofmodularization.11. Design electronic modular units—if indicated tobe in an unserviceable condition by built-in test equipment(BITE) or monitor to permit operational testingwhen removed. The immaturity or unreliability of theBITE may have resulted in a false alarm. This is particularlyimportant if the module is expensive.12. Match the physical separation of equipment intoreplaceable units with the functional design of the equipment.This will maximize the functional independence ofunits and minimize interaction between units.13. If a major assembly can be made of two or moresubassemblies, design the major assembly so that a subassemblycan be removed independently, i.e.. without removalof the other subassemblies. This unitization isespecially vaauable when the subassemblies have varyinglife expectancies.14. Unless it is structurally infeasible, design allequipment so that rapid, easy removal and replacement ofmalfunctioning components can be accomplished by onetechnician.15. Where possible, make modules small and lightenough for one person to handle and carry. Removableunits should have a mass less than 18 kg(401b). Units witha mass greater than 4.5 kg (10 lb) should have handles.16. Consider part weight and size of a module inrelation to its installed location to assure that handlingloads are compatible with both male and female maintenancepersonnel, i.e., from the 5th percentile female to the90th percentile male.17. Where possible, make each module capable ofbeing checked independently. If adjustment is required,design so that each module may be adjusted independentlyof other units.18. Design control levers and linkages so they can beeasily disconnected from components to facilitate moduleremoval and replacement.5-6 EXAMPLESTypical examples of good modularization techniqueswill be presented for five areas, namely,1. Electronic and electrical equipment2. Sectionalization of missiles and rockets3. Tank-automotive equipment4. Modularization in armament5. Helicopter engine.5-6.1 ELECTRONIC AND ELECTRICALEQUIPMENTLogical flow and circuit packaging are both logicoriented: however, circuit packaging usually is on a levelbelow that of logic flow because, as indicated in par.5-5.2.6, the selection of one technique does not excludethe use of other grouping techniques at subordinate levels.Consider, for example, the Army's Targct Acquisitionand Designation System (TADS) (Ref. 4), which featuresthe power supply as a black box (module); one of thepower supplies for the four TADSS is shown in Fig. 5-2.In contrast, the stabilizing control system on the Army'sAdvanced Attack Helicopter (AAH) features the powersupply as a component circuit board within a black box(module). In both cases the black boxex are modulesbased on the logic that supplying power is the first logicalfunction and, at the unit level, they represent identicalmaintenance actions—i.e., replace when unserviceable.However, the AAH black box which is designed formaintenance at the intermediate or depot level—uses the5-6
Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)Figure 5-2. Modules in Target Acquisition and Designation System (TADS)circuit grouping technique for the power component containedwithin the black box. Some other modules comprisingthe TADS are illustrated in Fig. 5-3.Component grouping might subsequently be employedto segregate similar parts into modules. For example, allrelays could be located on one card within a black box.Army missiles often have their gyroscopes segregated by apackaging technique, i.e., a packaged component as amodule, because gyroscopes frequently fail if left dormantin storage (Ref. 5).The Navy has established an extensive program referredto as the “Navy Standard Electronic Module Program”(SEM) (Ref. 6). The SEM (Ref. 6) is based on the principleof limiting redundant design through the use of standardfunctions, which achieves cost benefits through largeproduction volumes and wide competition. The basicobjectives of SEM are1. To partition electronic functions so that they arecommon to a majority of equipment applications2. To document modules that have functional specifications(to preclude dependence upon a specific vendor,design, or technology). This results in long-term availabilityand cost savings through vendor innovation andcompetition.3. To achieve high reliability through stringent qualityassurance requirements for module design andproduction.4. To discard modules upon failure; this is madepossible by high reliability and low cost.5. To provide flexible, modular, mechanical packagingrequirements that employ various circuit and packagingtechnologies, and adapt to the various mechanicalconfigurations of the equipment.6. To ease the logistic support burden on the congestedsupply system by extensive intersystem commonalityof a limited number of modules.The Air Force has investigated the benefits that couldaccrue to their programs through the SEM approach(Ref. 7). Results indicated that the application of SEMconcepts to simulation equipment could prove beneficial.For small and medium production quantities of singletypes of simulators, the SEM version had the lowest life5-7
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