MAXIMUM ALLOWABLE STRESSES FOR COIL EXTENSION SPRINGS IN STATIC AND FATIGUE APPLICATIONS

Maximum Allowable Stresses (K_W corrected) for Helical Extension Springs in Static Applications
Material	In Torsion		In Bending
Material	Body	End	End
Patented, cold-drawn or hardened and tempered carbon and low-alloy steels	45 - 50 %	40 %	75 %
Austenitic stainless steel and nonferrous alloys	35 %	30 %	55 %
Source : From Shigley's Mechanical Engineering Design [Ref 2] page 529 and Standard Handbook of Machine Design [Ref-3] Chapter 24 page 33 This information is based on the following conditions: set not removed and low temperature heat treatment applied. For springs that require high initial tension, use the same percent of tensile strength as for end.

Maximum Allowable Tensile Strength for ASTM A228 and Type 302 Stainless Steel Helical Extension Springs in Cyclic Applications
Number of Cycles	Percent of Tensile Strength (In Bending)
10⁵	51 %
10⁶	47 %
10⁷	45 %
Source : From Shigley's Mechanical Engineering Design [Ref 1] page 532 and Standard Handbook of Machine Design [Ref-3] Chapter 24 page 34 This information is based on the following conditions: not shot-peened, no surging and ambient environment with a low temperature heat treatment applied. Stress ratio = 0.

Budynas.R , Nisbett.K . (2014) . Shigley's Mechanical Engineering Design . 10th edition. McGraw-Hill
Shigley J.E. , Mischke C.R. (1996), Standard Handbook of Machine Design , 2nd edition