264 



BELL SYSTEM TECHNICAL JOURNAL 



THICKNESS OF STOCK SUBMITTED 



Fig. 7 — Test specimen for fatigue study. 

 (The piece must be free from scriber and tool marks. The 2 in. radius fillets 

 must join the tapered portion without an abrupt break. The intersection of the 

 tapered sides produced must be 3^ in. from the end.) 



Clock-Spring-Steel Springs 



These springs are used as vibrating elements in interrupters, where 

 a high fatigue endurance is required, and for springs that must be 

 worked at high pressures and rapid buildup. The material has the 

 chemical composition shown by Table 3 and mechanical properties given 

 by Table 4. This is a carbon-steel, heat treated and then cold rolled, 

 and is by nature brittle. To guard against excessive brittleness and 

 at the same time provide a high strength cold rolled steel, a bend test 

 has been developed. 



TABLE 3 



Chemical Composition of Clock Spring Steel 



Min., 

 per cent 



Carbon 0.85 



25 



Manganese 0. 



Silicon 



Sulphur 



Phosphorus 



Nickel, chromium, tungsten 



Vanadium Optional 



Iron Remainder 



TABLE 4 



Mechanical Properties of Clock Spring Steel 



Ultimate strength 250,000 to 290,000 lb. per sq. in. 



Proportional limit 160,000 to 215,000 lb. per sq. in. 



Modulus of elasticity 27.6 X 10« 



Max., 

 per cent 



1.15 



0.60 



0.22 



0.025 



0.03 



0.10 



0.25 



