13-15] RELIABILITY 717 



Each of these factors must be considered, controlled, and allowed for if 

 the required degree of reliability is to be attained. It is obvious that 

 regardless of how thoroughly all other factors are controlled, the neglect of 

 any one will seriously degrade the final reliability. 



Attaining High Reliability. Most manufacturers of military 

 electronic equipment have reliability organizations whose sole respon- 

 sibility is that of ensuring that the equipment they build is sufficiently 

 reliable to meet its requirements. These organization usually serve the 

 design engineers in a staff capacity, by providing them with information 

 they can use in the design process. Specifically, they provide component 

 lists and design handbooks which contain preferred components and circuits 

 and required safety factors. The information in the handbooks is usually 

 based upon data obtained on older equipments already used in the field. 

 A feedback loop is thus generated between the older equipments in the field 

 and the design of new equipments, with the result that past errors in design 

 or component choice are not repeated. 



Experience has shown that the basic design of the equipment determines 

 the maximum reliability the equipment can possibly attain. For this reason, 

 reliability must be designed into equipment; it cannot be provided by 

 superior manufacturing and testing techniques alone. Many organizations 

 have instituted a procedure known as saturation designing, in which the 

 proper designs of equipments are very thoroughly and critically reviewed, 

 in the light of past experience. 



A major area of reliability attainment efforts, as presently conducted, is 

 that of reliability testing. Such testing is normally carried out during the 

 development of the equipment and is designed to determine the equipment's 

 capability to meet its intended environment. This of course requires a 

 highly specific definition of the environmental stresses, so that they may be 

 simulated in the laboratory. Test-to-failure programs, designed to test the 

 limits of the equipment endurance capabilities, are also widely used. 

 Perhaps the basic purpose of all reliability testing is to determine the 

 equipment's weak spots so that they may be remedied before the equipment 

 is placed in large-scale production and use. 



Reliability Measurement. The numerical reliability of an existing 

 electronic equipment can be determined by actually operating the equip- 

 ment in a realistic operational environment for sufficient time to attain 

 statistically valid data. The number of hours the equipment is operated, 

 as well as the number of failures it suffers, are recorded and used in the 

 calculation of the average number of operating hours between successive 

 failures, commonly referred to as mean time to failure. This empirically 

 determined number can then be used to determine the probability of 

 successful equipment operation for any specified length of time. Before 



