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Advanex EGST
Advanex EGST
 
    Extension Spring Design
With Spring Expert, Mark Hayes
   
 
 
    Introduction

When our engineers receive an enquiry for a new extension spring, their attention is immediately focused on the end fixation, which is usually a hook. Nearly always, unless specifically designed not to be, the end hook is the position of maximum stress in service. Our engineers know that the maximum design stresses advised in EN 13906-2 are wrong, and they have their own design rules drawn up with the assistance of the author. So long as the end hook has been carefully considered, extension springs are precise engineering components that store energy with extraordinary efficiency.

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For a new extension spring our engineers also check that the initial tension load is within the range that is relatively easy to produce. Almost all extension springs are manufactured close coiled, as shown in figure 1, and there is an initial load to be applied before coils start to separate. It is extremely difficult to make extension springs with zero initial tension, and this design option is strongly discouraged unless the spring is open coiled. Equally, there is a level above which it is difficult to keep the initial tension load consistent from spring to spring. Hence, we will offer extension springs with some initial tension - not too little, and not too much, within the recommended range so that our products have as good a tolerance control as can be achieved with extension springs.

Static Applications

Many extension springs are used for applications that are essentially static - defined as having a design life of less than 10,000 operations. For these, the operating stresses are only important in that they define the maximum body extension allowable before the spring is permanently deformed. And for extension springs with hooks the same diameter as the spring body (by far the most usual arrangement), it is only the torsional stresses in the spring body that matter. Guidance is given on the maximum possible deflection from the free length in a research paper published in 2013 (1) - a paper for which Advanex supplied the test springs. If an extension spring is deflected too far, some of its initial tension will be pulled out reducing the load output of the spring. It will not fracture.

Assuming the end hooks are the same diameter as the body coils, as shown in figure 1, it is also important that the hooks are positioned accurately as the spring body will stay straight as the spring is deflected if the hooks are directly over the geometric axis of the spring. When the body coils stay in-line then the risk of contact with nearby components is minimised and high spring efficiency is ensured. The straightness of the spring under load is independent of the initial tension load so long as this parameter is within the recommended range.

Diagnosing The Cause Of Failure

When the design life of an extension spring is more than 10,000 cycles, the (un)bending stress at the inside surface of the end hook may give rise to a risk of fatigue failure. Figure 1 shows an extension spring with a German hook directly over the axis of the spring.

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Figure 1 - Extension spring with a German hook over the axis

If a load is applied to the hook in figure 1 the position at which there could be a risk of fatigue failure is marked with an "X". We have the most advanced software in the world for predicting the risk of a fatigue failure being initiated at "X", and know this risk can be reduced by making the hook smaller in diameter or shot peening it (2). A surprising result given in reference 2 is that the fatigue performance of extension springs is almost independent of the initial tension, and it is the applied load range (hence stress range) that determines the risk of fatigue. Figure 1 shows a German hook, but could equally have shown many other hook shapes including the English hook or D hook, the same principles apply, and Advanex's software allows prediction of fatigue and relaxation (loss of load in service).

References

(1) "On the effects of heat treatment on the properties of extension springs" Mark Hayes, Cabwire Conference, Milan, Nov. 2013
(2) www.tech-spring.eu Reports 2 and 3
   
 
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