Simple Spring Maintenance for Mechanics

A bolt in tension acts as a spring

A spring is an elastic body designed to store energy when deflected. A tightened bolt is a spring. A cylinder hold-down stud is also a spring. Springs are "machine elements."

Springs are springy (stiffness) for two reasons:
1. Geometry
2. Elastic modulus

The loading a spring can take before bending or breaking is affected by:
1. The material's yield (bending) point or elastic limit. Anneal the spring by over-heating it and you reduce the spring material's yield point so it bends at a lower load. It's just as springy but over a much reduced range of load. A spring damaged by exceeding the yield point is called  "stress relaxation", "creep", "load loss".
2. Material's fatigue strength or ability to work over many cycles. Often valve springs are shot peened to increase fatigue strength.
3. Hydrogen embrittlement. Early spring fracture for no apparent reason.

Impulse coupling spring

The mechanic protect the spring by:

1.  preventing excessive heat¹ and,
2. preventing anything that might reduce fatigue strength. Fatigue strength is reduced primary by corrosion causing pits that act as crack initiation points. 

The mechanic can describe a spring failure as:

1. Loaded beyond its yield point (spring collapsed or creep) possibly due to excessive heat lowering the yield point.
2. Fatigue fracture (it broke).
3. Hydrogen Embrittlement (manufacturing deficiency -- poor quality control of the first order)

Hydrogen embrittlement fracture in shroud spring

Excessive Heat
1. 475 F. can be considered as the maximum temperature for aircraft engine valve springs. Consider that cylinder head maximum is typically 500 degrees F. and this temperature is taken between the cold (intake) and hot (exhaust) portion of the cylinder. At 500 F for CHT it is conceivable that the exhaust valve spring could be at or above this temperature. Above 475 a couple of changes start to occur in the valve spring.

1. Fatigue life is reduced as the shot peened compressive residual stress is relieved.
2. Temper strength is reduced above 475 degrees.

From experience over-heated valve springs are often dark and crispy from oxidized oil. They also have collapsed meaning that their free length is reduced.