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)6. Designed if not a "throwaway” (see par. 5-3)to insure that components of the module are accessible sothat a higher level of maintenance can service or repair thecomponents7. Designed for standardization and interchangeability(see Chapter 3) if the same function is performed inother subassemblies of the overall system.Additional advantages of modular design are1. New equipment design may be simplified anddesign time may be shortened by using previously developedstandard “building blocks”, i.e., modules. Of course,the building blocks must be consistent with the reliability,compatibility, and integrated system testing requirementsof the adopted system.2. Current equipment can be modified in servicewhen newer and better functional units become availableprovided the new unit does not affect input-outputcharacteristics.5-3 THROWAWAY MAINTENANCE5-3.1 GENERALModularization lends itself to throwaway maintenance.Strictly defined, throwaway maintenance is a maintenancepolicy whereby components or items of equipmentto a given level are discarded at failure rather thanrepaired. It embraces the terms “discard-at-failure” maintenanceand “nonmaintenance design”. As a policy, it isbased on the principle that every system design has a levelof repair at which it is more practical and economicallyfeasible to throw away a failed item or component than itis to repair that item or component.The level of throwaway to be selected for a given designis dependent on many factors and may be established atany point between the complete system and any of thepiece parts of its subsystems. the higher levels of throwawayobviously provide for increased system availability,but they may dictate costs of such magnitude that a lowerlevel must be chosen. Higher levels of throwaway areuniversally acceptable whenever costs are not a determiningfactor.As a means of increasing the availability of equipment,throwaway maintenance is a logical extension of the“maintenance float” concept. This maintenance concept.i.e., the need to have major items of equipment on hand toreplace or substitute for equipment removed from servicefor either scheduled or unscheduled maintenance, hasbeen used by industry and the Army. Since most of theitems in this category are expensive, costs preclude theirdiscard—hence a pool of repaired items as well as newreplacement items. On the other hand, no one wouldconsider attempting to repair an electrical fuse, a lightbulb, or a fan belt; such items would be thrown away, andnew ones substituted. Somewhere between these two5-3.2 ADVANTAGESAmong the typical advantages obtained by selecting ahigh throwaway level for a given system that is beingdesigned are the following:1. Costs of record keeping are drastically reducedbecause the number of expendable items is high.2. Because the number of line items needed to supporta system is drastically reduced, the possibility isincreased that a part needed to repair a malfunctioningsystem will be available.3. Greater possibility in packaging is affordedbecause access to the interior of throwaway modules isnot required; this makes possible both a very high packagingfactor and greatly reduced overall volume.4. System reliability is increased by the extent towhich unitized packaging makes possible encapsulationwith its protection against corrosion and humidity, andthe use of rigid assemblies with their high resistance toshock and vibration with fewer make and break connections.5. Periodic replacement of assemblies (modules)because of normal failure makes possible a system that is.for all purposes. rebuilt. This feature is not exclusive tothrowaway maintenance.)6. In-line changes that do not affect input-outputcharacteristics can be made in assemblies without additionalrepair parts and without changes in field documentationbeing required.7. Manufacturing costs per unit are minimizednot only because the volume of production is increased.but also none of the assemblies are designed to berepairable.8. Modifications and all problems associated withthem are minimized because design imperfections can becorrected by changes being made in the complete module.9. Documentation of equipment is reduced becausethe user or maintenance personnel do not need to knowthe internal makeup of individual assemblies.10. Requirements for maintenance personnel andtheir respective skill levels are reduced because assembliesare replaced, not repaired. Similarly, test equipmentrequirements are reduced. This results in considerablesavings in many areas e.g., manpower, training, facili-5-2
Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)ties. documentation, repair parts, and the test equipmentitself.5-3.3 HAZARDSApplication of the throwaway concept creates severalimportant hazards, each of which requires provisions toprevent the development of major disadvantages. Thesehazards are1. The frequent failure of a single, relatively inexpensivecomponent necessitates discard of an expensiveassembly, which results in increased maintenance dollarcosts. The obvious counteraction to be taken is the use ofcomponents of approximately equal reliability in eachassembly. This should be required even if more expensivecomponents are required to obtain an approximation ofequal reliability.2. Frequent failure of a module, resulting in discard,does not reveal the cause of failure. A postmortem analysisof the failed module may discover that (a) the samecomponent causes failure and, if replaced by a morerelitible one, could extend the life of the module considerablyor (b) the module is a victim of a false alarm thaterroneously signals failure of the module, i.e., the monitoringor diagnostic equipment is at fault.3. Procurement times for assemblies tend to begreater than those for components, which possibly leadsto excessive system downtime. To prevent this, increasedemphasis must be placed on stocking the critical items insufficient quantities at the unit level.4. The technical capability of the personnel of theunit in the area of self-support generally deterioratesbecause of the decreasing need for piece-part repair activities.Obviously, one of the objectives of throwaway maintenanceis the elimination of a requirement for high skilllevels for technicians.Accordingly. a decision to adopt a high-level throwawayconcept for the design of a system is predicated on anumber of conditions, i.e.,1. Replacement assemblies should be available inadequate quantities at the user level.2. The reliability of components within each assemblyshould be approximately equal.3. The reliability of the assemblies comprising a systemshould be sufficiently high to compensate for theirrelatively high cost.Despite these hazards and conditions, wherever considerationsof operational availability are paramount, adefinite attempt should be made to design a system foremployment of throwaway assemblies.5-4 SOFTWARE MODULESSoftware modules are similar to hardware modules inthat they may be designed to perform specific functions.Software modular design, however, cannot be applied toall types of equipment with equal advantage. Its greatestapplication is in electronic equipment to monitor ordetermine the status of a circuit, subassembly, or system;diagnose or troubleshoot a failure or malfunction; orperform a self-check on an item of test equipment. Separateprograms (modules) can be written, using the samemicroprocessor, to perform each of these functions foreach of the different hardware modules that requires it.The advantages of software modularization are (Ref. 1)1. By using tailored software programs, i.e., focusingon a specific item or function, the software is lesscomplex. Therefore, it requires less frequent maintenanceand fewer steps to locate a fault. Programs can be readilychanged to accommodate a retrofit replacement item.2. Lesser routines maybe more easily understood bythose responsible for subsequent program maintenance.3. Less likelihood that a modification to a specificprogram will affect other programs. This reduces thepossibility of maintenance-induced failures.4. The number of discrete problems possible in aspecific subroutine is often many orders of magnitude lessthan the number possible in the complete overall program.Thus testing is more manageable. Software modulescan be tailored to the demands of the system components.5. Software programs can be easily modified basedon operational experience.6. Imperfections in module software that remainedunidentified up to the time of failure–-although they wereintroduced when the software was originally written orsubsequently modified-can be corrected without impactingthe complete system.5-5 DESIGN CRITERIA5-5.1 MODULARIZATION VERSUS PIECE-PART DESIGNThe general criteria for modularization versus piecepartdesign are1. Feasibility. Feasibility criteria often lead to aquick decision; if it is not within the state of the art todevelop a modularized design as an alternative, piece-partdesign wins by default.2. Life Cycle Cost. When modularization is a feasiblealternative, its cost-effectiveness must be consideredas it applies to the life cycle of the item. The cost aspect isparticularly important if a throwaway module is beingconsidered.3. Compatibility With Logistical Support Plan.Most “module versus piece-part” decisions are made tosatisfy the demands imposed by the logistical supportplan to accommodate the operational demands of thesystem. The requirement for immediate repair at the unitlevel by unskilled personnel demands modular replacement(see par. 5-l).5-3
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