714 MECHANICAL DESIGN AND PACKAGING 



easy, in most cases, to isolate the trouble in a pa.rt\cu\a.r function. This 

 suggests that each plug-in subassembly should provide a complete function. 

 The breakdown of a major assembly into subassemblies can be made in the 

 same manner that a block diagram is drawn to illustrate the functions 

 within a major assembly. Each block corresponds to a plug-in subassembly. 

 When trouble is isolated in a certain function, the corresponding plug-in 

 subassembly can be replaced. 



Plug-in assemblies may have a modular design. That is, the general design 

 of the various subassemblies is similar, and the linear exterior dimensions 

 are integral multiples of some lowest common denominator, such as the 

 smallest plug-in unit. Often two linear dimensions are the same for all units 

 of a modular design, the third dimension being the only one to vary. The 

 use of modular design simplifies and accelerates the actual design effort as 

 well as the manufacturing processes. Modular design also lends itself well 

 to automatic manufacturing. 



Plug-in subassemblies can be repairable, or they can be designed for 

 discard after failure. The relative advantages of these two design philos- 

 ophies depend upon the type of service expected, duration of service life, 

 logistics, and maintenance facilities. 



13-13 TRANSPORTATION AND SUPPLY 



During transportation of airborne electronic equipment, severe environ- 

 mental conditions may occur that may result in damage to the structural 

 and electrical component parts of the system. The most serious environ- 

 ment encountered during transportation is the dynamic loading induced by 

 vibration and shock energy input. The magnitudes of the loads depend 

 upon the mode of transportation — airplane, railroad, truck, or ship. A 

 good method for protecting equipment from the damaging effects of 

 transportation is to provide shipping containers incorporating shock or 

 vibration isolators. The equipment should have lifting lugs clearly labeled 

 to help prevent damage during handling. 



In maintaining an adequate supply of radar systems and parts, problems 

 are encountered in the prevention of deterioration in storage. Moisture, 

 dust, and temperature are among the principal items responsible for this 

 deterioration. Although the equipment has presumably been designed to 

 operate in spite of these environmental factors, it is good practice to protect 

 the equipment as much as possible by providing clean dry storage areas 

 which are not subject to extremes of temperature. Since the shipping 

 containers should, in any event, be designed to protect the equipment from 

 the transportation environment, they can, with little or no additional 

 expense, be made suitable to act as storage containers. 



