MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)1. Provide the greatest accessibility for either theremoval of bearings for bench relubrication or their purgingand relubrication in the aircraft. This requirementstems from the need to replace bearings and change lubricantto suit the range of temperatures and conditions atwhich certain aircraft may operate.2. In using split bearings optimize accessibility bymaking the plane of the split correspond with accessports. For example, split the crankshaft bearing on anengine connecting rod to permit removal of the bearingthrough an external access without removing the crankcasecover.3. To permit changing of the bearings without disassemblyof the entire component, mount two bearings intandem rather than using a single bearing of larger size.This will permit changing one bearing while the otherbearing supports the load.4. Where accesses are located over dangerousmechanical components, design the access door so that itturns an internal light on automatically when opened.Also provide a highly visible warning label on the accessdoor.4-6.7.4.1 Drawer-Type HousingAssemblies, such as instrument panels and electricaland hydraulic units, can be made more accessible by theuse of drawer-type housings. Basically, a unit should bemade up of a housing with many cavities. Each cavityshould contain a component that would serve, as far aspractical, a separate portion of the overall system. Theoverall unit should be mounted on a rugged frame withthe front cover also serving as the control panel for theenclosed compartments. The frame should be supportedby hinges that come apart readily to permit withdrawal ofthe whole component. Rests, limit stops, guards, and/ orretaining devices should be provided as part of the basicchassis to prevent the unit from falling from the aircraft.No special tools should be required to withdraw the component.The front panel of the component should have adust seal to prevent contaminants from entering the cavityin the housing. When ventilation is required to cool thecavity, suitable filters should be installed in the ventilatingintake and exhaust ducts. Component test points, ifrequired, should terminate at an easily accessible terminalboard or strip.The design of drawer-type housings offers the followingdesirable maintenance features:1. The component may be withdrawn and rapidlycalibrated, serviced, inspected, or repaired.2. The component maybe removed completely fromthe housing, placed on a bench in front of the housing,and, with the use of an adapter cable(s), serviced andmaintained with full access to the component.3. The component may be taken to a test area andrepaired, calibrated, tested, or inspected under idealconditions.4. The component maybe replaced with an identicalunit from stock, which rapidly returns the system toavailability.4-6.7.4.2 Major Unit HousingsMajor unit housings, such as engine nacelles, shouldhave hinged or removable housings that can be rapidlyopened and closed for inspection and repair. The fastenersrequired to secure the housings should be kept to aminimum and should be removable with speed tools, suchas speed wrenches and screwdrivers.4-7 ACCESSIBILITY CHECKLISTTable 4-7 summarizes the important design recommendationsto be considered when designing for maintainability.The checklist contains several items that arenot discussed separately in the text. These items areincluded here because their necessity in the design is soobvious that they might otherwise be overlooked. If theanswer to any item on the checklist is “no”, the designshould be reexamined to determine the need forcorrection.4-28