708 MECHANICAL DESIGN AND PACKAGING 



mounted equipment is no longer effectively isolated from the vibration or 

 shocks transmitted through the airframe structure. It has been seen that 

 vibration isolators are most effective when they have large static deflections 

 and concomitant low natural frequencies. To avoid snubbing during 

 acceleration, it is necessary either to sacrifice space by providing for these 

 large isolator deflections or to sacrifice isolation effectiveness by using 

 stiffer mounts with smaller static deflections. If the duration of a high load 

 factor is expected to be short compared with the duration of severe vibra- 

 tion, it may be best to permit snubbing under certain circumstances in order 

 to obtain the best isolation during most of the time. On the other hand, 

 the effect of isolators may be so badly degraded by this type of environment 

 that a more reliable radar will be obtained with no isolators whatsoever. 

 Careful consideration of the expected environment must precede the choice 

 of a specific design. 



13-9 MOISTURE 



Moisture is classified as a natural environment and can exist whenever 

 humidity, fog, ice, snow, sleet, rain, dew, hail, or salt spray are present or 

 when a part is immersed in water. The principal problems resulting from 

 the presence of moisture are corrosion, reduction in the effectiveness of 

 insulation, and the possible growth of fungus. 



Moisture generates two major types of corrosion, chemical and electro- 

 chemical. Corrosion caused by the combined action of the atmosphere and 

 moisture on the surface of a body is termed chemical corrosion. Corrosion 

 between two different metals existing in a conductive medium is known as 

 electrochemical corrosion. The corrosion of the metals used in electrical 

 systems results in a decrease in structural strength of individual components 

 and assemblies, as well as the sticking and binding of moving parts. 



Moisture on electrical parts will affect the insulation resistance. If the 

 insulation material has a porosity that will allow absorption of the water, 

 the resistance of the insulation will be decreased and a power loss will result. 

 Insulation that allows a complete water film to form on its surface is 

 undesirable. If the insulation surface is such that water collects in drops 

 and has a tendency to run off, the effect on the insulation resistance is small. 



Protecting electrical equipment from the effects of the moisture environ- 

 ment can be accomplished by control of design techniques, material 

 selection and surface finishes. Although the exclusion of moisture from 

 parts and equipment is elementary in theory, it is surprisingly difficult in 

 practice and has required the development of many special techniques. 

 Humidity control by mechanical or chemical means may be used on 

 hermetically sealed containers, free-breathing containers, or containers 

 with controlled venting. Other techniques in moisture control include the 



