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Introduction
When our engineers receive an enquiry for a new compression, extension or torsion spring, there are a number of parameters that they consider to make sure that the spring will give the service performance that their customers expect. For instance, they check that spring dimensions are consistent with specified load or rate, and that the stresses are within accepted limits. However, their ability do this accurately for torsion springs is less precise than with other spring types, and this article will enumerate some of the problems that occur when checking torsion springs.
Torque
Not all torsion springs have a specified torque output at a given angular deflection, but for those that have, it is easier for our engineers to measure torque (or moment) accurately if it is specified from a datum, rather than simply as an angle between legs. This is because it is often difficult to set a torsion spring on a torque tester with contact on each leg, its mandrel (if there is one), and a zero deflection.
An even greater obstacle to obtaining accurate torque results is the influence of the mandrel. Sometimes torsion springs don’t operate on a mandrel, but most do, and this article will just consider those for which the mandrel keeps the active coils in line with each other. It will also consider only windup torsion springs because most are used in this direction, and the performance is very much better than those that are unwound in use. There will be a frictional interaction between the inside surface of the active coils of a torsion spring and the outside surface of its mandrel. Hence the lubrication conditions need to be specified for us to give torque results that are representative of their customers’ service conditions.
It is also imperative that the customer specifies the direction of loading in which the torque applies as the torque in the windup direction will always be higher than in the unwinding direction. Torsion springs exhibit hysteresis, and hence a loss of energy on each cycle whereas their compression and extension counterparts show less than 1% hysteresis in most cases. The material used for the mandrel will also have an influence, particularly if it is made from a material softer than spring wire because the mandrel will wear quickly and this will affect the torque.
Finally, and least known, the diameter of the mandrel will influence the torque. The larger the mandrel, the higher the torque, so long as the mandrel is not too large – avoiding the torsion spring binding on its mandrel. The formula for the maximum mandrel size is given in BS EN 139063, and our engineers will use this formula. If the maximum mandrel size is used, and it is made of a material with a similar hardness to the spring wire, and is lubricated, the formula for calculating torque will be reasonably accurate. If these conditions do not apply the actual torque output of the spring will be less than calculated, but the friction might compensate in the windup direction. Our engineers will not be able to calculate the friction, but if they know the actual service conditions, they will be able to measure the total moment (torque + friction) on their torque testers.
Stress
The result of winding up a torsion spring is that there will be a bending stress in the outside surface of each of the active coils. The torque output, at a given angle from the specified datum, can be calculated, and torque is proportional to stress. EN 139063, or any international standard for spring design (they all have the same formula) will enable the calculation of this bending stress. We will compare this stress with the tensile strength of the wire used, and if the applied stress is less than 70%, then the torsion spring should work without problem.
Unfortunately, the difficulty that our engineers will have is that they know the stress formula is, at best, approximate. During use, the formula assumes the coils of the spring will remain the same round shape that they coil, as is the case with compression or extension springs. The active coils of a torsion spring in service become oval in shape, meaning there is a distribution of stress around each coil (1), and there are no published formulae to enable precise calculation of the maximum bending stress.
Conclusion
Torsion springs are less precise spring types than any other, and if it is possible to use a compression or extension spring, even working over an arc, then that design solution is sometimes preferable. Reading this article, you might suppose that designing torsion springs is impossible, and whilst precision is not possible, it would be quite wrong to suggest that torsion springs won’t give a satisfactory performance – most do, but designing them to the limits of what is possible should be done with great caution and lots of testing. With generous safety margins torsion springs give good reliable performance.














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