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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
Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)TABLE 10-9. EFFECT OF ACCELERATION ON MILITARY EQUIPMENT (Ref. 6)ItemMechanical: moving parts,structures, fastenersElectronic and electricalElectromagneticThermally activeFinishesMaterialsEffectPins may bend or shear; pins and reeds deflect; shock mounts may break awayfrom mounting base; mating surfaces and finishes may be scored.Filament windings may break; items may break away if mounted only by theirleads; normally closed pressure contacts may open; normally open pressurecontacts may close; closely spaced parts may short.Rotating or sliding devices may be displaced; hinged part may temporarily engageor disengage; windings and cores may be displaced.Heater wires may break; bimetallic strips can bend; calibration may change.Cracks and blisters may occur.Under load, materials may bend, shear, or splinter; glue lines can separate; weldscan break.tively larger amounts of deposited energy, however, producepermanent and accumulative damage.The design of material that will be exposed to a nuclearradiation environment can be extremely complex. Ofprime consideration is the radiation level for which theitem is being designed. For example, in the design ofsemiconductor devices for nuclear radiation environments,the use of gold–-which has a large absorptioncross section—is sometimes avoided. For items that aresensitive to radiation, the use of lead shielding is employed.In electronic circuits, for example, knowledge ofthe radiation sensitivity of particular circuit elements isemployed in the design to provide for continued operationof the circuit even when the properties of the sensitiveelements may vary within wide limits.In addition to blast. fire, and radiation resulting from anuclear explosion, a large electrical charge is transportedin a short period of time. This produces a large transientpulse of electromagnetic energy known as electromagneticpulse (EMP). The EMP can be considered to be verysimilar to the common phenomenon of a lightning stroke.The magnitude and extent of EMP far exceeds electromagneticfields created by any other means; its duration isless than 1 ms. The electromagnetic signal from the EMPconsists of a continuous spectrum with most of the energycentered about a median frequency of 10 to 15 kHz (Ref.6).10-3 .2.1.8 Electromagnetic Radiation Effects(Ref. 6)The following is excerpted from Ref. 6:“For electronic equipment operating within the communicationsand microwave bands, environmental electromagneticfields can be harmful in three basic ways: (1)interference, (2) overheating, and (3) electric breakdown.First, the presence of extraneous electromagnetic fieldscan produce interference, particularly in communicationchannels, but also in other electronic equipment such asnavigation, radar, and command and control units. Thisinterference to a system can be caused by (1) other systemsoperating in frequency ranges that interfere with theoperation of the desired equipment, and (2) undesiredsignals generated by the system itself. Good design practiceand proper siting of equipment are usually sufficientto eliminate problems encountered in the second category.“Electromagnetic interference is classified in a numberof ways but, for measurement purposes, it is usually classifiedaccording to its spectral characteristics. The twogeneral classifications are broadband interference, inwhich a wide range of frequencies are involved, and narrowbandinterference, which is centered about a discretefrequency. In addition, the interference is classified withrespect to its duration. That which is constant withoutinterruption is called continuous wave or CW interference.Interference that is periodic and occurs in burstswith a regular period is called pulse interference. Pulseinterference can be either narrowband or broadbanddepending upon the pulse duration. In addition, nonrepetitiveshort duration bursts of broadband noise arecalled transient interference. Lightning, for example, is atypical example of transient interference. Electromagneticinterference can be coupled into equipment either bydirect radiation or by conduction on power lines or structures.[For a thorough discussion of electromagneticinterference and compatibility, see Ref. 21.]“Through proper frequency management, many interferenceproblems can be reduced. Unfortunately, theproblem is complicated because the electromagneticenvironment contains not only the desired electromagneticradiation, but also spurious and undesired interferencefrom both natural and man-made sources. As thenumber, complexity, and output power of electronic systemsin use grow, the problem of the electromagneticenvironment and equipment compatibility becomes moreserious. For example, within the military, the density ofelectronic equipment in the field has grown to the pointthat hundreds of equipments now occupy the same operationalenvironment as did a few equipments in World War11. It is noted that, in discussing electromagnetic interference,the fields usually spoken of are not high enough to10-17
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