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)among these and other disciplines.The current “remove-and-replace” philosophy is thepreferred policy; it displaces the "repair-in-place" policyfor restoring materiel to a serviceable condition in thecombat zone (Ref. 2). Thus accessibility is essentiallysynonymous with maintainability where remove-andreplaceoperations are involved. The sum of many “small”times saved due to a simple feature such as accessibilityadds up to large reductions in maintenance time andeffort, and thus increases availability.In summary, in planning for accessibility, the items thatwill require the most frequent work must be examined todetermine the type of maintenance each will require.Items that have high failure rates or require frequentservice should be given preferential positioning for access.For example, a control assembly might consist of a powersupply module and a control module. If the power supplyis expected to fail more often than the control module andboth modules are to have a common access, the powersupply should be closer to the access. However, if thecontrol module requires frequent adjustment to keep itsperformance up to par, then it should be allotted thepreferred access position. Next, types of access, covers.methods of mounting, and required dimensions aredetermined. Once the types of access have been selected,the designer—working in conjunction with the maintainabilityengineer—must make certain that each access islarge enough either for physical access or the employmentof service or test equipment. This strategy is presented inthe paragraphs that follow.4-2 FACTORS AFFECTINGACCESSIBILITYConsiderations of accessibility are influenced by, andin turn exert influence on, virtually all other maintainabilityfactors; thus its unique importance. Gaining access tocomponents is second in importance only to fault isolationas a time-consuming maintenance activity; if automaticfault-isolation is embedded in the equipment,access unquestionably will rank first.Accessibility requirements are determined by the maintenanceaction required which may be visual or physical,or both, depending on whether the task is inspecting,servicing, adjusting, repairing. or replacing. Generally,the requirements represent two needs. namely,1. Placement of items requiring frequent maintenanceattention where they can be easily seen andreached. Visibility is mandatory where a potential hazardexists.2. Design of openings to permit access to componentsand to provide space in which to perform maintenanceoperations.Guidelines for meeting these requirements are1. Locate assemblies and parts so that structuralunits and other parts do not block access to them.4-3 MAINTENANCE ACCESSThree general types of maintentance actions requireaccess, namely.1. Inspection either sight or touch2. Testing3. Part adjustment, repair. or replacement.To perform these maintenance actions, a means ofaccess is required either merely to observe or to “lay one’shands on the unit”. To determine the type, size, shape,and location of the access opening, it is necessary to havea thorough understanding of (Ref. 3)1. operational location, setting, and location of theunit within the environment2. Frequency of using the access3. Maintenance tasks performed through the access4. Time required to perform maintenance functions5. Types of tools and accessories required6. Work clearances required7. Types of clothing operator and technician arelikely to wear8. How far into the access the operator or technicianmust reach9. Visual requirements of task10. Mounting of items. units, and elements behindaccess11. Hazards in using access12. Size, shape, weight, and clearance requirementsfor logical combinations of human appendages, tools.units, etc., that must enter the access.Inspection primarily is performed by sight or touch.For example, a sight gage can be used to assure properfluid levels. Inspections that require feeling for leaks aretactile and require physical rather than visual access.When inspection or testing locates a failed part, the accessmust be large enough to permit component repair orreplacement.Table 4-1 provides criteria for decisions relative to thetypes of accesses for the three maintenance actions. In theapplication of this table. inspection that requires touchrather than sight appears in the “For Physical Access”column. The recommendations related to accesses andcovers are illustrated by Fig. 4-1 (Ref. 3). A general discussionof accesses for the three types of maintenance4-2
Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)Figure 4-1. Types of Covers and Accesses (Ref. 3)actions is presented in pars. 4-3.1, 4-3.2, and 4-3.3. Theparameters of access type, size, shape, and locationapplying the given list of considerations—are discussed indetail in pars. 4-4 and 4-5.4-3.1 ACCESS FOR VISUAL INSPECTIONONLYVisual access should be unblocked unless exposure islikely to degrade equipment or system performance. Aclear, plastic window can be used if dirt, moisture, orother foreign materials present a problem. If physicalwear or the action of solvents and the environment arelikely to reduce the transparency of the plastic cover. abreak-resistant glass window should be used. If glass willnot meet the stress or other requirements, then a quickopening,metal cover should be used. Fig. 4-1 illustratesvarious cover types.Visual inspection usually requires a light intensity of 551m/m 2 (lux) or greater. Brightness contrast between thetarget object and its background must be considered aswell as glare from both the work and light source. If anexternal light source is used, the transparency of the4-3
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