MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

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Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)10-3.2.1.2 Desert RegionsDesert regions occupy approximately 19% of the landsurface of the earth. The outstanding attribute of alldeserts is dryness. A widely accepted definition of ’’desert”is an area with an annual rainfall of less than 254 mm (10in.). Hot deserts are further characterized by a clearatmosphere and intense solar radiation. both of whichresult in temperatures as high as 52°C (126°F) andambient illumination levels as high as 10,280 cd/m 2 (3000ftL) (Ref. 16). This intense solar radiation combined withterrain that has a high reflectance can create high levels ofglare. Other characteristic phenomena associated withdeserts are atmospheric boil and mirages. Design fordesert areas should also consider sand and dust, whichnearly always accompany dryness.The high daytime temperatures, solar radiation, dust,and sand combined with sudden violent winds and largedaily temperature fluctuations may create many of thefollowing maintenance problems:1. Heat can lead to difficulties with electronic andelectrical equipment, especially if they have been designedfor moderate climates.2. Materials—such as waxes—soften, lose strength,and melt.3. Materials may lose mechanical or electrical propertiesbecause of prolonged exposure.4. Fluids may lose viscosity.5. Joints that would be adequate under most otherconditions may leak.Heat can also cause the progressive deterioration of manytypes of seals found in transformers and capacitors.Capacitors of some types develop large and permanentchanges in capacity when exposed to temperatures above49°C (120°F).The temperature extremes for electronic equipmentoperating in a desert environment are shown in Table10-6. The following factors also should be considered:1. Dry cells have a short life in hot environments anddeteriorate rapidly at temperatures above 35° C (95° F).2. Wet batteries lose their charge readily.3. Tires wear out rapidly.4. Paint, varnish, and lacquer crack and blister.In the desert, relays, all types of switching equipment,and gasoline engines are susceptible to damage by sandand dust. Sand and dust hazards present severe problemsto finely machined or lubricated moving parts of light andheavy equipment. Sand and dust get into almost everynook and cranny and in engines, instruments, and armament.Desert dust becomes airborne with only slight agitationand can remain suspended for hours so that personnelhave difficulty seeing and breathing. The mostinjurious effects of sand and dust result from their adherenceto lubricated surfaces, but glass or plastic windowsand goggles can be etched by sand particles driven by highwinds.10-3.2.1.3 Arctic RegionsIn arctic regions the mean temperature for the warmestsummer month is below 10°C (50°F), and for the coldestmonth, it is below -32°C (-25°F). The extremely lowtemperatures of these regions change the physical propertiesof materials. Blowing snow, snow and ice loads, icefog, and windchill cause additional problems.Problems associated with the operation and maintenanceof equipment seem to be more numerous in arcticregions than elsewhere and are caused mainly by driftingsnow and extremely low temperatures. The temperatureextremes to which electronic equipment may be exposedare shown in Table 10-7.With the exception of inhabited areas, vehicle transportationis uncertain and hazardous because of theabsence of roads. Travel from base to base is over rugged.snow-and-ice or tundra-covered terrain. Drifting snowcan enter a piece of equipment and either impede itsTABLE 10-7. ARCTIC TEMPERATUREEXTREMES FOR ELECTRONICEQUIPMENTConditionsLow temperature, drivingsnow, ice dustTemperatureExposed arctic:-70°C (-94°F), extreme-40°C (-40°F), commonSubarctic:-25°C (-13°F), commonTABLE 10-6. TEMPERATURE EXTREMES FOR ELECTRONIC EQUIPMENTOPERATING IN A DESERT ENVIRONMENTConditionsDry heat, intense sunlight,sand dust, destructive insectsTemperatureDay high:+60°C (+140°F), air+75°C (167°F), exposed groundNight low:Relative humidity-l0°C (+14°F) 5%Large daily variation:22°C (72°F), average10-14
Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)operation or melt and then refreeze inside. Then, whenthe unit generates heat, the melted ice can cause shortcircuits, form rust, or rot organic materials.The subzero temperatures may produce the effects thatfollow:1. Volatility of fuels is reduced.2. Waxes and protective compounds stiffen andcrack.3. Rubber, rubber compounds, plastics, and metalslose their flexibility, become hard and brittle, and are lessresistant to shock.At a temperature of -34°C (-30°F) batteries are reduced incurrent capacity by 90% and will not take an adequatecharge until warmed to 2°C (35°F). The variations in thecapacitance, inductance, and resistance of electrical componentsand parts can become great enough to requirereadjustment of critical circuits.10-3.2.1.4 Vibration (Ref. 6)Vibration in the environment can degrade materiel inseveral ways, i.e.,1. Malfunction of sensitive, electric, electronic, andmechanical devices2. Mechanical and or structural damage to structuresboth stationary and mobile3. Excessive wear in rotating parts4. Frothing or sloshing of fluids in containers.Table 10-8 (Ref. 6) indicates the effects of vibration onelectrical and electronic equipment.When vibration becomes severe enough to cause malfunctionor failure, measures must be taken to permitmateriel to survive in such an environment. The processof reducing the effects of the vibration environment isknown as vibration control and consists of varying structuralproperties such as inertial, stiffness, and dampingproperties of mechanical systems to attenuate theamount of vibration transmitted to the materiel or toreduce the effects of the transmitted vibration.A variety of techniques can attenuate vibration.Obviously, a very effective method is to remove the vibrationat its source. Damping, i.e., a process of producing acontinuing decrease in the amplitude of the vibration,also may be employed, This is accomplished by employingfrictional losses that dissipate the energy of the system,i.e., an energy-absorbing mechanism. Detuning ordecoupling a member from a resonant frequent) can alsobe used.10-3.2.1.5 Shock (Ref. 6)Equipment subjected to shock loads responds in acomplex manner. The shock load can overstress anddeform the basic equipment structure or damage fragilecomponents attached to the structure. Both responsesexist together, and their relative intensities are a functionof the shape, duration, and intensity of the shock pulse;the geometrical configuration; total mass; internal massdistribution; stiffness distribution; and damping of theitem or equipment. The effects of shock include breakageof brittle or fragile components, displacement of massivecomponents, and change in geometrical relationshipamong components.To protect against shock, it is necessary to1. Isolate the equipment from the shock forcesthrough proper packaging and stowing techniques2. Design equipment in a way that will make itunsusceptible to the shock environment.Though not a shock in the classical sense, damage thatcan be introduced by electrostatic discharge during normalhandling of modern electronic devices mandates thatthe potential for damage be controlled. By proper packagingand methods of discharging static electricity fromworkers and tools, the sensitive components handled duringmanufacture, test, and repair can be protected (Ref.17).Fundamentals of package design, barrier, cushioning,and container material are discussed in Refs. 18, 19, and20.10-3.2.1.6 Acceleration (Ref. 6)Most items of materiel are designed to operate within anarrow band of accelerating forces centered on the normalgravitational force of 1G. When accelerations differappreciably from 1G, items fail to operate properly. Theeffects of large accelerations on equipment include structuraland mechanical failures, abnormal operation ofelectron tubes, characteristic changes in vibration isolators,and malfunctions due to deformation of parts. Typicaleffects of acceleration on various types of equipmentand components are listed in Table 10-9 (Ref. 6).The primary means of protecting materiel againstdamage from the acceleration environment is throughproper packaging. Other techniques include the use ofshock mounts; the selection and use of the correct types ofmaterials in terms of weight, strength, and flexibility; andproper structural mounting of component parts.10-3 .2.1.7 Nuclear RadiationThe effects of nuclear radiation are derived from theamount of energy deposited within a material by theradiation and the form that the energy depositionassumes. Thus if a material absorbs little radiation, it maybe unaffected in many applications. If the energy depositionis large, however, a material may lose its structuralintegrity. Most effects fall inbetween these two extremes.Solid-state electronic devices, for example, are extremelysensitive to nuclear radiation effects because their operationis very sensitive to the structure of the material.Absorbed radiation that ionizes an atom or displaces anatom in a semiconductor will affect the operation of adevice that uses the semiconductor.Damage to materials is classified into two categories,i.e., transient radiation damage and permanent radiationdamage. This categorization is derived from the fact thatmany incidents of nuclear radiation in the environmentare transient and produce transient effects in material.The magnitude of such effects decays with time, and theperformance of materiel exposed to a transient radiationevent often will return to its initial state. Transient effectsusually are associated with low radiation doses. Rela-10-15
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