Handig
Mechanical
- Jun 15, 2006
- 6
In one of my companies products, a steel leaf spring operates two plastic parts. The spring sometimes brakes before its required life. See attached photo's.
Description:
size between eyes is 15.0mm after production. After assembly, the size is 16.4mm. In one cycle, the spring is strained to 20.0mm, relaxed to 19.8mm, strained to 20.0 and relaxed to 16.4mm again.
Required life is 100k cycles. At this moment, about 10% of the springs break at 40k-60k cycles, all other reach 250k cycles without cracks.
Spring material is WS 1.4310 (AISI 301) with tensile strength 1700-1900MPa. After bending, the edges are rounded, spring is annealed and shot-peened.
Strange thing is that we have a similar spring in a similar product that seems to be heavier loaded, but does make the required number of cycles.
This size between eyes is 13.0mm after production. After assembly, the size is 16.4mm. In one cycle, the spring is strained to 19.8mm, relaxed to 19.0mm, strained to 19.8 and relaxed to 16.4mm again.
As far as I can figure out, mean stress in first spring is much lower than in first spring and amplitude of cyclic stress is only slightly higher. This should lead to a longer fatigue life of the first spring, but it doesn't.
I have consulted the spring manufacturer and other experts.
Options tested (all with negative result): changing shot-peening parameters, investigation of tooling on burrs and sharp edges, change in spring material.
Can anyone explain the shorter fatigue life of the first spring, or better: give advise on how to extend fatigue life?
Thanks in advance.
Description:
size between eyes is 15.0mm after production. After assembly, the size is 16.4mm. In one cycle, the spring is strained to 20.0mm, relaxed to 19.8mm, strained to 20.0 and relaxed to 16.4mm again.
Required life is 100k cycles. At this moment, about 10% of the springs break at 40k-60k cycles, all other reach 250k cycles without cracks.
Spring material is WS 1.4310 (AISI 301) with tensile strength 1700-1900MPa. After bending, the edges are rounded, spring is annealed and shot-peened.
Strange thing is that we have a similar spring in a similar product that seems to be heavier loaded, but does make the required number of cycles.
This size between eyes is 13.0mm after production. After assembly, the size is 16.4mm. In one cycle, the spring is strained to 19.8mm, relaxed to 19.0mm, strained to 19.8 and relaxed to 16.4mm again.
As far as I can figure out, mean stress in first spring is much lower than in first spring and amplitude of cyclic stress is only slightly higher. This should lead to a longer fatigue life of the first spring, but it doesn't.
I have consulted the spring manufacturer and other experts.
Options tested (all with negative result): changing shot-peening parameters, investigation of tooling on burrs and sharp edges, change in spring material.
Can anyone explain the shorter fatigue life of the first spring, or better: give advise on how to extend fatigue life?
Thanks in advance.