MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)CHAPTER 5MODULARIZATIONThe role of modularization in reducing maintenance time and improving operational availabililty isdiscussed. The advantages and disadvantages of throwaway modules are presented. Guidelines are lisled toassist the engineer in deciding on the physical attributes of a module and techniques of component grouping.Examples of the application of the modular concept to Army materiel are given. A design checklist formodularization is presented.5-1 INTRODUCTIONThe support concept for a new system ultimately consistsof a series of support requirements—one set for eachlevel of support together with the procedures, techniques,and services needed for satisfactorily meetingeach requirement. Every system must be provided with itsunique support plan designed to meet its peculiar needs.As has been mentioned in previous chapters, the determinationo! the support requirements depends to a considerableextent on the operational requirements of the systemin question. For example, the operational availabilitydesired for a system may dictate a mean downtimerequirement of such short duration that it can be met onlyby providing for a maximum repair capability at the unitlevel. Because this requirement maintenance at the userlevel usually employs personnel with limited skills forsupport work. a need for designing the system for easyidentification and isolation of faults is established. Theadditional requirement that rapid repair be facilitatedonce a fault has been identified calls in turn for modulardesign. Once these steps have been taken, general guidanceprinciples to govern the design evolve, e.g.,1. Modules will be removable by user personnelwithout the use of special tools.2. Modules will be interchangeable without therequirement for maintenance adjustments.3. Modules will cost no more than a given amount orwill not cost more than a given amount to repair. (This isto make throwaway at the user level economicallyfeasible.)4. Modules will be replaced within a specified periodof time, i.e., a maximum limit on time to replace module.(This is necessary to meet the downtime requirement forthe system as a whole.)This discussion leads to the definition of a modulei.e., “A module is a part, subassembly, assembly, or componentdesigned to be handled as a single unit to facilitatesupply and installation, operations, and or maintenance.”It can be either repairable (at the intermediateor depot level) or nonrepairable (discard-at-failure).Modularization is achieved by dividing equipment intophysically and functionally distinct parts or modules. Themodule must be functionally complete to permit testingand verification apart from interfacing items. Thus modularizationenables subsystems, assemblies, and subassembliesto be designed as removable entities that can meetthe criteria for line-replaceable units (LRUs). (An LRU isan item whose removal and replacement with a like serviceableitem is considered the optimum method of repairor restoration of a higher order system.)The modularization concept is not confined exclusivelyto hardware items; software modules also lend themselvesto this concept. Software modules are discussed in par.5-4.5-2 ADVANTAGESThe concept of modularization creates a divisible configurationthat is more easily maintained. Troubleshootingand repair of modularized assemblies, therefore, canbe performed more rapidly. Use of this technique to themaximum improves accessibility, makes possible a highdegree of standardization, provides a workable base forsimplification, and provides an optimum approach tomaintainability at all maintenance levels.To realize these advantages, the module should be1. Easily identified or determined without the useof sophisticated test equipment and procedures to havefailed or malfunctioned: failure should be recognizable atthe module level rather than at the system level of whichthe module is a part.2. Readily and easily accessible (see Chapter 4); theneed for accessibility to be in direct ratio to the specificfailure rates of the modules3. Replaceable in less time and with fewer technicianswith a lesser skill level than would be possible byrepair-in-place4. Replaceable by technicians without the need forspecial tools and instructions. Mating connections betweenmodule and end-item should be easily recognized so thatthe module may be quickly detached and replaced withoutrecourse to maintenance manuals for instructions.5. Designed to be emplaced without adjustment orcalibration in order to be compatible with the system5-1