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Tutorial Fatigue Loading & Helical Springs

By Prof S S Chauhan, IEC College of Engineering & Technology, Greater Noida
Fatigue Loading
The springs have to sustain millions of cycles of operation without failure, so it must be designed for infinite life. Helical springs are never used as both compression and extension springs. They are usually assembled with a preload so that the working load is additional. Thus, their stress-time diagram is of fluctuating nature. Now, for design we define,
Certain applications like the valve spring of an automotive engine, the springs have to sustain millions of cycles of operation without failure, so it must be designed for infinite life. Unlike other elements like shafts, helical springs are never used as both compression and extension springs. In fact they are usually assembled with a preload so that the working load is additional. Thus, their stress-time diagram is of fluctuating nature. Now, for design we define, then the stress amplitude and mean stress values are given by: if we employ the Goodman criterion, then The best data on tensional endurance limits of spring steels are those reported by Zimmerli. He discovered the surprising fact that the size, material and tensile strength have no effect on the endurance limits (infinite life only) of spring steels in sizes under 10mm(3/8 inches). For all the spring steels in table the corrected values of tensional endurance limit can be taken as: = 310 Mpa for unpeened springs= 465 Mpa for peened springs. The stress amplitude and mean stress values are given by:

If we employ the Goodman criterion, then
The design or resulting factor of safety will depend on the spring material selected and their endurance strength. In the absence of data on the endurance limit, the best data on torsional endurance limits of spring steels are those reported by Zimmerli. He discovered the surprising fact that the size, material and tensile strength have no effect on the endurance limits (infinite life only) of spring steels in sizes under 10mm(3/8 inches). For all the spring steels the corrected values of torsional endurance limit can be taken as:
= 310 MPa for unpeened springs = 465 MPa for peened springs.