MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.4-6.5.1 Type of MaintenanceAs with other types of equipment, tank-automotivemaintenance will be either corrective or preventive. Theimpact of accessibility on each type follows:1. Preventive Maintenance:Preventive Maintenance Checks and Services(PMCS) other than daily operator and/or crew checksnormally will account for approximately 20% of totalmaintenance time. The overwhelming problem regardingPMCS—including daily checks is to reduce the timerequired for these services to the absolute minimumnecessary. A daily PMCS checklist that cannot be executedby the crew in 20 min for a truck and 45 min for atank will most likely be rushed over or ignored; therefore,optimum placement of components to be checked is critical.Assure good accessibility of fluid reservoirs, filters,and other checkpoints. Eliminate the need to removearmor plates or other obstacles in order to reach thecomponents involved in the PMCS checklist. The quarterlyPMCS checklist—which will include monthly services—-shouldtake no more than eight clock hours toperform. Semiannual and annual services naturally willtake longer, but since they are performed less frequently,their contribution to total maintenance time, and hencethe maintenance ratio. is not as significant.Accessibility of a specific component may determinewhether the part will be serviced during PMCS orallowed to fail. For example, a U-joint on a specific lightcombat vehicle was designed to be lubricated at the quarterlyservice. With this scheduled service. the reliabilitywas effectively the life of the vehicle. However. the locationof the U-joint necessitated removal of the powerpack. Thus this quarterly lubrication required over 8clock hours. The U-joint, therefore, was changed at aslight increase in cost to a permanently lubricated designwith a projected mean time between failures (MTBF) of1000 h and a replacement time—mean time to repair(MTTR)—of 9 clock hours. With an annual usage of 350h, failure could be expected approximately once everythree years. By accepting this small risk of failure-–-withapproval from the user community—design engineersreduced the maintenance ratio and increased the availabilityof the system. A small increase in unit cost wastraded off for a lower life cycle cost. If no other failuresare assumed, the inherent availability calculations areMTBMScheduled Service =MTBM + MTTR— 350/4= 0.92(350/4) + 8Run to Failure =MTBFMTBF + MTTR— 1000=0.99.1000 + 9DOD-HDBK-791(AM)2. Corrective Maintenance. Accessibility considerationsand ground rules discussed in the beginning of thischapter can be applied to tank-automotive equipment.Time spent getting to the faulty part is wasted time. If it isassumed that reliability cannot be further improved, thenthe lower the expected reliability of a part, the moreconsideration should be given to its accessibility. Refer tothe discussion on frequency of maintenance in par.4-6.5.3.4-6.5.2 Maintenance EnvironmentTank-automotive equipment is unique in that maintenancewill be required in most every imaginable environment.In peacetime, maintenance generally will be performedon a hardstand in a sheltered bay, often withpower lift capability, and a pit. No such amenities willexist under wartime conditions. Do not rely on having apit to gain access to maintenance or service points underthe vehicle. Jack stands that support a vehicle on a hardstandmay sink in soft ground or mud if not properlydesigned. Lift capability used to remove components forservice or better access will not be available unless amobile wrecker is called in. Also a component that isaccessible under “classroom conditions” may not beaccessible when caked over with hardened mud anddebris.4-6.5.3 Task FrequencyIf a power pack is removed only once a year, it really isnot important if the task requires 30 man-hours to perform.If the pack must be removed monthly, however, thecontribution of this task to the total maintenance burdenis multiplied by 12. The importance of task frequency tothe total maintenance burden is shown in Tables 4-3 and4-4. Table 4-3 lists the top ten replacement parts, byfrequency of occurrence, for two types of vehicles. Table4-4 lists the top ten replacement parts, by man-hours toreplace, for the same vehicles.TABLE 4-3. LIST OF TOP TENREPLACEMENT PARTS BY FREQUENCYOF OCCURRENCEM113 Family(Ligh Combat Vehicle)M915 Series(Heavy Tactical Truck)Engine OilGear ShiftTransmission OilGasketTrack PadsCableAntifreezeShaftTrack ShoesBreatherFuel Filter Decreasing TireRubber Cushion Frequency Fuel FilterOil FilterOil FilterRoad WheelAir FilterBatteryBattery4-23