TELEPHONE APPARATUS SPRINGS 259 



pressure. In other words, there must be room enough to flex the 

 spring to the point where it will take a set within the space provided. 

 A spring of high proportional limit such as one of clock-spring steel 

 may be bent nearly double without being permanently deformed. 

 Obviously, such a spring could not be adjusted in the key shown by 

 Fig. 1. 



The modulus of elasticity should be within the range of 12,000,000 

 to 20,000,000 lb. per sq. in. in order that the load-deflection rate will 

 not be too steep to permit reasonable ease of adjustment by hand. 



The endurance limit must be high enough to permit satisfactory 

 operation throughout the life of the apparatus. In cases where space 

 limitation is a factor, reference must be made to the stress-cycle graph 

 made for the material to determine if the spring may be expected to 

 stand up in service. 



Creep or deformation under sustained load must not take place 

 since the material will lose tension. Brass, nickel silver, and phosphor 

 bronze may be expected on the basis of years of experience to hold 

 adjustments when stressed up to approximately their proportional 

 limits. Other materials when considered for telephone apparatus 

 springs are investigated to determine their "creep" characteristics. 



Season cracking takes place with highly stressed brass under severe 

 atmospheric conditions, and for this reason springs that are required 

 to hold their pressures without being deflected for long periods are 

 not made from this material. Nickel silver will also season crack 

 under still higher stress, and phosphor bronze least of all. In designing 

 these springs, generous fillets and easy curves are employed to prevent 

 the building up of localized high stresses that may lead to season 

 cracking under sustained load and fatigue failure under repeated 

 flexure. 



When these springs are used as wipers in electrical circuits where 

 arcing can occur, brass and nickel silver are not employed for the 

 reason that the heat of the arc breaks down the material, volatilizes 

 the zinc, and disintegrates the metal. For this reason, phosphor 

 bronze, which does not contain zinc and has superior wear resistance 

 to brass and nickel silver, is employed. 



In addition to the foregoing properties, these springs must be 

 resistant to atmospheric corrosion and capable of being readily 

 soldered with soft solder. Nickel silver, as may be seen from Table 2, 

 is superior to brass in its mechanical properties and in addition may 

 be readily spot welded. As a result of years of experience it has 

 been found that springs made of this material are capable of main- 

 taining adjustment in a satisfactory manner in service. Phosphor 



