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Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)fied that a diagram of the item immediately suggests themethod of assembly or disassembly; however, until atechnician is thoroughly familiar with the operation, heshould be encouraged to study the documented procedurethe “read instructions when everything else fails” is adangerous philosophy. In a design of this nature, assemblyis obvious by means of component parts that fittogether in a unique manner because of their externalconfiguration. Also since familiarity and use of a commonpart are important considerations in part design andselection, maximum effort should be made to promotethe use of standard parts and components.The assembly of parts can be simplified by part designor by support equipment designed to position parts sothat they cannot be incorrectly aligned or mounted. Acommon method is to provide locating holes for the insertionof dowel pins to position parts in the correct orientationfor mounting or for ease of insertion. Supportequipment, by proper design, can permit ease of positioningfor installation of attachments—e.g., a carrier for aweapon pod could be designed with a wing-to-carrierguidebar that could align the pod for quick attachment.Removable fasteners, e.g., screws and bolts, should beof the same size wherever possible and of a standard size.and only in the number required to secure an item properly.This simple design feature will reduce the partsinventory and lessen the requirement for special tools.2-2.6.2 Scheduling of MaintenanceScheduled and deferrable maintenance functions canoften be performed during the same time period, whichresults in reduced equipment downtime. i.e., increasedavailability. For example, a modification work order(MWO) that does not affect system safety or functioningcan be deferred until the equipment undergoes routinescheduled maintenance, or a clutch adjustment could bedeferred until an oil change was required. Also the necessaryspecial tools, repair parts, and manuals can beassembled in preparation for the deferred maintenance;this will reduce administrative downtime.2-2.6.3 Simplified Diagnostic TechniquesBecause the preponderance of repair time required forany item, subsystem, or system normally is attributable tofault isolation, it is imperative that provision be made forthe most effective diagnostic (troubleshooting) routines.Manual techniques—basically trial-and-error efforts byskilled technicians using meters, oscilloscopes, and othertest devices, as well as detailed schematics, to isolate amalfunctioning component by progressively eliminatingthose that are still functioning—should be eliminated atall levels of maintenance except depot. Semiautomaticand/or automatic techniques must be prescribed for unitlevel maintenance because unit maintenance is structuredfor quick turnaround based on repair by replacement andminor repair—check, adjust, clean, lubricate, and tightenaddition of fuel, and maintenance of liquid levels. Testequipment, if neccssary at the unit level, should be of the"go" “no go” type, which requires no interpretation ofsignal data. The vacuum tube testers, located in drugstores before the introduction of solid-state electronics.are examples.2-2.7 SOFTWARE MAINTENANCEEase of maintenance usually is associated with gooddocumentation, a small number of statements per program,and the level of language used to write the programs.Programs should be prepared in high level (compiler)language, whereby a single statement translatesseveral into many machine-language instructions. Forexample, consider three variables, A, B, and C related bythe statementVARA = VARB + VARCwhich translates into1. Load VARB2. Add VARC3. Store VARA.In most languages each variable must be uniquely declared(defined) and typed integer, character, logic, number ofbytes, and manner in which stored prior to use in anyprogramming instruction (module). Examples of higherlevel languages are ADA, PASCAL, and C. ADA is thepreferred language for Army software and, therefore,should be used.Block structuring a technique allowing programsegmentation into blocks of information or subroutinesof a total program should be used because when properlyapplied in a top-down arrangement, they are essentiallyself-documenting. This technique permits the programto be read and understood by someone who did notwrite the program; thus it lends itself to being easilychanged or maintained. In applying this technique.backward referencing (looping) should be avoided.2-3 EXAMPLES OF COMPLEXMODULES REDESIGNEDFOR SIMPLICITYThis paragraph describes five complex hardware andsoftware modules that have been redesigned by applying arange of simplification principles.2-3.1 UH-60A HELICOPTERThe UH-60A BLACK HAWK utility helicopter requiresonly 2.5 to 3 man-hours of maintenance per flighthour at the unit level (Ref. 1). This compare to 12 to 20 hfor the family of helicopters it replaces. This increase inmaintainability can be attributed largely to simplificationof the main rotor head design. Unlike previous designs.which had upper and lower plates tied together withvertical and horiziontal hinges, the BLACK HAWK main2-4
Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)rotor head is forged from a single piece of titanium. anelastomeric bearing, which is completey dry and requiresno lubrication and is said to have an average life of 3000 h,eliminates the need for hinges. In earlier designs, thehydraulic reservoir seals have been known to fail andcause loss of lubrication and wear to the main rotor head,which necessitates overhaul. Typically, mean time betweenremoval (MTBR) for older designs of such componentswas less than 500 h. Simplification has resulted indynamic component MTBR requirements of at least 1500h.2-3.2 155-mm HOWITZERFor many years, the muzzles of the 155-mm guns andhowitzers were proteccted by mechanical muzzle plugs;these plugs consisted of a metal dish that was locked inplace by turning a hand wheel which took up on athreaded link and thus spread a three-legged expandingtoggle inside the gun barrel. This assembly cost $64 andrquired maintenance. A molded neoprene plug thatrequires no maintenance was substituted at a cost of $6.This simplification also has improved safety: forgettingto remove the plug before firing no longer has disastrousresults.2-3.3 M230 GUNIt is imperative that any gun used in a combat vehicle beeasily maintainable. the current trend is toward externallypowered guns, which based upon their design, haslead to a reduction in replaceable parts. this fact translatesinto ease of maintenance. for example, the 30-mmautomatic, externally powered gun, M230, has only 149parts, including barrel, drive motor, feed string, andrecoil adapters. This compares favorably to 300 parts inthe Model 188 Gattling gun. Fig. 2-1 contrasts a Gattlinggun design with that of the M230. Fig. 2-2 illustrates thesell-powered feed design that makes part reductionpossible.Further, the new 30-mm gun has been designed forquick disassembly without the need for supplementarytools or equipment. The tools required for disassemble}are provided with the gun mechanism. Disassembly andreassembly are accomplished in three minutes or less. Thegear boxes are sealed for the life of the unit (Ret. 2).2-3.4 INFRARED SUPPRESSORTo reduce the threat from infrared homing missiles, thegas turbines used in tanks and helicopters must be cooled.This cooling was achieved with large fans which weredifficult to remove and replace. A new approach using the“ocarina” IR suppressor named after a simple musicalinstrument breaks up the hot gas plume and is said todraw in more cool air than the engine is generating as hotexhaust. There are no moving parts in this simplifieddesign (Ref. 3).2-4 SIMPLIFICATION CHECKLISTTable 2-1 should be used in evaluating the design of anitem for simplification. If an answer is “no” for any questionon the checklist, the design should be restudied todetermine whether correction is required.Figure 2-1. Comparison of the M188 20-mm Gattling Gun to the M230 30-mm Gun2-5
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