MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)TABLE 10-8. VIBRATION-INDUCED DAMAGE TO ELECTRICALAND ELECTRONIC EQUIPMENT (Ref. 6)Component CategoryCabinet and framestructuresChassisCathode-ray tubesMeters and indicatorsRelaysWiringTransformersDamage ObservationsAmong some 200 equipment cabinet and frame structures subjected to shock andvibration, damage included 30 permanent deformations, 17 fractures in areas ofstress concentration, two fractures at no apparent stress concentrations, 23 fracturesin or near welds, and 26 miscellaneous undefined failures.Nearly 300 chassis subjected to shock and vibration experienced 18 permanentdeformations, eight fractures in or near welds, nine fractures at no apparent stressconcentrations, 46 fractures at points of stress concentration, and 12 miscellaneousfailures.Cathode-ray (CR) tubes are susceptible to vibration damage if they are improperlymounted and supported. CR tubes with screens larger than 5 in. are particularlysusceptible. Of 31 cathode-ray tubes subjected to shock and vibration, thedeflection plates of one tube became deformed, another had a filament failure, fivesuffered envelope fractures, and one had a glass-socket seal break.Although the moving coil type of meter comprises the majority of units in thiscategory, other indicators such as Bourdon tubes and drive-type synchros were alsotested. Of the latter group, most of the failures were either erratic performance orzero shift difficulties. Nearly 200 units were subjected to shock and vibration. Twosuffered permanent deformation of the case, one had elements loosened, 12 gaveerratic readings, one had the glass face fractured, two developed internal opencircuits, two had loose or damaged pivots, three had deformed pointers, and 10others failed from miscellaneous causes.Relays present a particularly difficult problem for dynamic conditions because of thedifficulty in balancing all of the mechanical moments. Shock generally causesfailure in the form of the armature failing to hold during the shock. A total of 300relays were subjected to shock and vibration. Armature difficulties accounted for 29defects, four relays had contacts fuse or burn because of arcing, one had the coilloosened on the pivots, two had the springs disengaged from the armature, and foursustained miscellaneous defects.Wiring failure from shock and vibration is a serious problem. A defect not onlyresults in malfunctioning of the equipment, but presents a difficult troubleshootingjob in locating the wire break. In a number of equipments subjected to shock andvibration, the failures were as follows: 10 cold solder joints opened, 14 leadsupportedcomponents had the leads fail, insufficient clearance caused three casesor arcing, and insufficient slack caused nine lead failures. In addition. three plasticcable clamps fractured, 14 solder joints or connections failed, 16 solid conductorwires broke, and 92 sustained miscellaneous failures.In electronic equipments transformers are probably the heaviest and densestcomponents found on an electronic chassis. Because of the weight and size oftransformers, shock and vibration are more likely to produce mechanical ratherthan electrical failures. Although not all mechanical failures immediately preventthe transformer from functioning properly, they eventually result in destruction ofthe transformer and damage to surrounding components. Of 80 transformerssubjected to shock and vibration, 17 had the mounting stubs break at the weld, fourhad the bottom frame fail, and two suffered broken internal leads due to motion ofthe coil in the case.10-16