Springs
Inspection and testing of compression, tension and torsion springs, free length, permanent set, load testing, cracks, corrosion and coil condition.
Springs, summary notes
- 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.
- Free length
- the unloaded length, compare with the schedule
- Permanent set
- free length below limit → reject, do not re-stretch
- Load test
- force produced at a stated length, per the maintenance data
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 concept map
Springs
Springs quiz
Springs, quiz
1. A spring whose free length is below the specified minimum has:
Taken a permanent set and must be rejectedSimply been compressedGained strength2. Free length is measured:
With the spring unloadedAt working compressionUnder a test load3. A load test on a spring checks:
The force produced at a specified lengthIts free length onlyIts corrosion resistance4. Corrosion pitting on a spring is serious because it:
Acts as a stress raiser and starts fatigue crackingOnly affects appearanceIncreases the spring rate5. The three common spring types are:
Compression, tension and torsionFlat, round and squareStatic, dynamic and thermal6. A spring with a permanent set should be:
ReplacedStretched back to lengthHeat treated and reused7. Cracks in a spring are most likely to start:
At the ends and the inside of the coilsAt the mid-point of the outer surfaceOnly where it contacts a guide8. Uneven or closed-up coils indicate:
DistortionCorrect preloadNormal wear