Fatigue Stress:
All springs have finite fatigue limits, the limit depending fatigue stress and the degree of fluctuating loads. The four most common fatigue stress conditions include constant deflection, constant load, unidirectional stress and reversed stress. A spring inside a valve assembly is an example of a constant deflection where the spring is cycled through a specified deflection range. An example of a constant load spring is the use of vibration springs under a dead weight where the load applied to the spring does not change during operation but the deflection will. A unidirectional stress is one where the stress is always applied in the same direction such as used in the return spring of an actuator. A reversed stress is applied first in one direction then in the opposite direction such as used in a regulator valve. The three stages to a fatigue failure include crack initiation, crack propagation and finally fracture of the spring material.
Static springs can be used in constant deflection or constant load applications. A constant deflection spring is cycled through a specified deflection range, the loads on the spring causing some set or relaxation which in turn lowers the applied stress. The spring may relax with time and reduce the applied load. Under constant load conditions, the load applied to the spring does not change during operation. Constant load springs may set or creep, but the applied stress is constant. The constant stress may result in fatigue lives shorter than those found in constant deflection applications.
A corrosive environment may accelerate the time to fatigue failure, corrosion reducing the load-carrying capability of a spring and its life. The precise quantitative effect of a corrosive environment on spring performance is difficult to predict. Springs are almost always in contact with other metal parts. If a spring is to be subjected to a corrosive environment, the use of inert materials provides the best defense against corrosion. Protective coatings can also be applied. In special situations, shot peening can be used to prevent stress corrosion and cathodic protection systems can be used to prevent general corrosion. The spring material is normally more noble (chemically resistant to corrosion) than the structural components in contact with it because the lesser noble alloy will be attacked by the electrolyte. The effects of corrosion on spring reliability must be based on experience data considering the extent of a corrosive environment.
Surging (resonant frequency response) can occur in high-speed cyclic applications if axial operating frequencies approach the axial natural frequency of the helical-coil spring. If the material and geometry of the axially reciprocating spring are such that its axial natural frequency is close to the operating frequency, a traveling displacement wave front is propagated and reflected along the spring with about the same frequency as the exciting force. This condition results in local compressions and rarefactions producing high stresses and/or erratic forces locally, with consequent loss of control of the spring-loaded object. Surging of a valve spring, for example may allow the valve to open erratically when it should be closed or vice versa.
Spring Relaxation:
Springs of all types are expected to operate over long periods of time without significant changes in dimension, displacement, or spring rates, often under fluctuating loads. If a spring is deflected under full load and the stresses induced exceed the yield strength of the material, the resulting permanent deformation may prevent the spring from providing the required force or to deliver stored energy for subsequent operations. Most springs are subject to some amount of relaxation during their life span even under benign conditions. The amount of spring relaxation is a function of the spring material and the amount of time the spring is exposed to the higher stresses and/or temperatures.
Static springs can be used in constant deflection or constant load applications. A constant deflection spring is cycled through a specified deflection range, the loads on the spring causing some set or relaxation which in turn lowers the applied stress. The spring may relax with time and reduce the applied load. Elevated temperatures can cause thermal relaxation, excess changes in spring dimension or reduced load supporting capability. Under constant load conditions, the load applied to the spring does not change during operation. Constant load springs may set or creep, but the applied stress is constant. The constant stress may result in fatigue lives shorter than those found in constant deflection applications.
In many applications, compression and extension springs are subjected to elevated temperatures at high stresses which can result in relaxation or loss of load. This condition is often referred to as "set". After the operating conditions are determined, set can be predicted and allowances made in the spring design. When no set is allowed in the application, the spring manufacturer may be able to preset the spring at temperatures and stresses higher than those to be encountered in the operating environment.
A highly stressed spring will set the first several times it is pressed. Relaxation is a function of a fairly high stress (but usually lower then that required to cause set) over a period of time. Creep in the spring may lead to unacceptable dimensional changes even under static loading (set). A spring held at a certain stress will actually relax more in a given time than a spring cycled between that stress and a lower stress because it spends more time at the higher stress. The amount of spring relaxation over a certain period of time is estimated by first determining the operating temperature, the maximum amount of stress the spring sees and how long the spring will be exposed to the maximum stress and the elevated temperature over its lifetime.
Miscellaneous Failure Modes:
Most extension spring failures occur in the area of the spring end. Extension springs are designed to become longer under load and their maximum length must be controlled for long life. Their turns are normally touching in the unloaded position and they have a hook, eye or some other means of attachment. For maximum reliability, the spring wire must be smooth with a gradual flow into the end without tool marks, sharp corners or other stress risers. The spring ends should be made as an integral part of the coil winding operation and the bend radius should be at least one and one-half times the wire diameter. Other failure mechanisms and causes may be identified for a specific application to assure that all considerations of reliability are included in the prediction.
Cited:
Mechanical-Spring-Failure-Modes
