Torsional Modulus and How it Affects the Stability of Springs
In a well-designed spring, torsional movement can occur 10 million times without causing any deformation to the spring. However, if there isn’t enough room to design a spring with enough steel in it, the twisting motion becomes more severe and the wire may not return to its original position. When this happens, we refer to this occurrence as “the spring took a set”.
Torsional movement or torsional modulus is the term used to describe how the increased deflection of a spring causes more of the wire to be twisted. When a spring is deflected, the action on the wire itself is that of twisting. It is this twisting motion that causes a spring to provide resistance to the force being imposed and causes the spring to return when the load is removed.
For example, if we have a spring which is overstressed and is one-inch free length, when we bring that spring all the way down to solid height (see blog on solid height) the spring may only come back to .900” rather than the full one inch. The reason that this occurs is once again, that the wire was twisted too far and did not come back all the way–this is called torsional yield.
Many times, a spring can be designed and the function is such that the spring takes a set but then is stable after the initial set. This occurs when the wire is twisted too severely and does not come all the way back but is not twisted so severely as to continue to deform. In such a case the spring company can wind the spring longer than required, bring it to solid height a few times and then be confident that the spring will be stable. However, if the twisting function is very severe, the spring may not be stable after it has been set and will continue to set each time the spring is deflected.
Spring manufacturers have tables which guarantee a minimum strength on the wire. However, each batch of wire manufactured might have a slightly different strength. Therefore, if a spring is near the maximum before yielding, one batch of wire may not yield at all and the next batch of wire may yield with the very same load as the first batch. The only surefire way to check if a spring will take a set is to try it after it has been wound and see what happens.
For a spring manufacturer, it could be really expensive to set each spring individually. If you are having 10,000 springs made and they take a set, setting each spring individually can add significantly to the cost of the spring. One way to avoid the cost of having a spring manufacturer set the spring is for the customer to bring the spring to solid in the installation of the application.
As the oldest continuous manufacturer of springs in the country, Hardware Products Company’s spring geeks are familiar with such requests and are always happy to adjust processes to better suit your needs. It is our priority to help you understand the intricacies of spring manufacturing as they relate to your spring needs. So, we’ve put together a comprehensive glossary of spring terminology, keeping you–the buyer–in mind.