Springs, Module 7, Certify66

Notes

Main ideas

Springs store energy by elastic deformation and are used for return forces, preload, damping and vibration isolation. The common types are compression, tension (extension) and torsion.
The key serviceability checks are FREE LENGTH, the length unloaded, and PERMANENT SET, which is a free length shorter than specification, showing the spring has been stressed past its elastic limit and can no longer produce its designed force.
A spring with permanent set is scrapped, not stretched back: the material has yielded and the fault will return.
Load testing checks the force produced at a specified compressed or extended length against the schedule, which catches a weak spring whose free length still looks acceptable.
Visual inspection covers cracks (especially at the ends and at the inside of the coils where stress is highest), corrosion and pitting, which act as stress raisers and start fatigue failures, and wear or scoring from rubbing against a guide.
Coils are checked for squareness, closeness and evenness, coils that have closed up unevenly indicate distortion.
Springs are handled and stored so they are not nicked, distorted or corroded, and are fitted with the correct end orientation where the design requires it.
⚠ Exam trap: a spring that has taken a permanent set is rejected, it is never stretched or re-set back to length.

Key formulas

Solved examples

A return spring measures 48 mm free length against a specified 52 mm minimum. What is the finding and the action?
The spring has taken a permanent set, it has been stressed beyond its elastic limit and will no longer deliver the designed return force. It is rejected and replaced; stretching it back to length does not restore the material properties.
Why is light corrosion pitting on a spring treated more seriously than on a flat bracket?
A spring works permanently in a cyclic stress regime, so a pit acts as a stress raiser and becomes the origin of a fatigue crack. What would be cosmetic on a lightly loaded bracket is a failure initiation site on a cycling spring.

Springs