All torsion springs exert pressure along a path that is circular. In other words they provide a torque.
It can be confusing when you look at the stresses in these springs and compare to compression springs. This is because compression and extension springs create stresses which are torsional, while a torsion spring is subject to bending stresses.
Torque is found in other springs like clock springs. Clock springs are also called flat coil springs, or motor springs, or power springs, etc. You can learn more about this type of spring on this web page for
Power and Clock Springs.
In most cases when we are talking about torsion springs we mean a helical spring made with round wire, square or rectangular wire that is loaded by torque.
Most of the time these springs use round wire. Rectangular wire can also be used and sometimes is more efficient to this type of loading.
These springs are used in many different ways. There are no standard applications. Any inquiry to the spring manufacturer needs to have drawings or samples that clearly show the type of ends required.
It should also spell out any limiting dimensions, like maximum outside diameter, minimum inside diameter, available length, number of coils, wire size, and whether right-hand or left-hand.
If these specifications are not known, but loads need to be met, then...
- specify the torque in inch-pounds at loaded position
- show the angular relations of the ends when at this load
- show the angular relation of the ends when the spring is free (if important)
- show the degrees of travel beyond the loaded position
- note the diameter of the hub or shaft the spring will fit over when working (this will help the designer determine the inside diameter of the spring to be larger than the hub or shaft at maximum deflection.)
A lot of this type of spring is closed coiled. (No space between coils). Variations in length occur when there is variation in wire dimeter and the actual number of coils.
Keep in mind, when this spring is wound up one coil it has increased in length by one wire size. For two wire sizes it would be increased in length by two wire sizes. You need to allow for clearance of this extra length.
In all cases these springs should activate their load in a direction that decreases the diameter of coils. There are a few applications, like clutches or friction brakes, where the springs are activated by "unwinding" them. It is always done with very small travel and under confined conditions that prevent the end coils from bending outward. This is what happens if the coil is not enclosed.
When this type of spring is designed to operate by reducing the coil diameter history shows that the hub or shaft the spring operates over should be approximately 90% of the smallest inside diamteter which the spring is reduced to. Without good support on the inside diameter the spring will be prone to early failure.
The rate of a helical torsion spring is constant throughout its entire range of deflection if no friction is present. If friction is there it is normally the result of the spring binding on the supporting post or from tension between coils.
The material all the way to the end os the legs needs to be considered as part of the spring. This is because the wire is active in bending all the way to the point where the load is applied. Treat this additional length in the ends as extra coils in the rate fomrula in the equations shown here. The only time to ignore this extra length in the ends is if it is small compared to the developed length of the body coils.
Torsion springs can be difficult to test accurately because of friction. These problems are discussed in more detail on this page:
Design for Manufacture and Assembly.
Wire in a helical torsion spring is stressed in bending. The nominal stress is computed according to the formulas shown above.
What the suitable stress level is depends on the residual stress pattern.
For example, a spring made from hard drawn or pre-tempered wire has a beneficial residual stress pattern if the direction of the loading causes the spring diameter to decrease. In the case the values in the chart below can be used.
Here is a page that discusses stress levels in more detail-
Choices of Operating Stresses in Springs
If the direction of loading enlarges the diameter, the spring will set at a very low stress unless it is properly stress relieved. Proper time and temperature in the oven after forming will eliminate the detrimental residual stress in this direction.
The direction of loading of a torsion spring needs to be noted. This will help the spring manufacturer process the spring properly and avoid premature failure in the field.
Square or Rectangular Wire
Square or rectangular sections are more efficient in bending. This is why they are often used in these springs. Rectangular wire wound on edge also reduces stress in the spring ends. In any case, round wire is more available and costs less. This is why it is most often used where the stresses are not high.
SPECIAL CASE:A special type of torsion springs are those used in mechanical clutches. These are most often made from square or rectangular wire. Formulas for these cross sections are given in the graphic above on this page. Note that allowance must be made for keystoning of this wire as discussed at
Helical Compression Spring Design and Special Considerations.
Take note that in these springs with each revolution of deflection the solid length of the spring increases by one wire size and the diameter decreases. The new diameter is
. You must remember to allow for these size changes in the design of the mechanism the spring is to be installed.
Most long torsion springs must have a center support arbor to prevent buckling. The spring rate and stress will rise markedly if the torsion spring binds on this arbor when deflected. The arbor diameter should be about 20% smaller than the inside diameter of the spring at maximum deflection.
Stress concentration from bends may be estimated from the chart below. These stress concentration factors apply only to springs in fatigue service. In static service, very slight localized setting may result from the high stress concentration.
The link below will take you to a page on raw material for torsion springs.
Spring Steel Types Used in Spring Making
Now it's time to take a look at some design examples. This page on
Torsion Spring Design Techniques
goes over an example by using a logic diagram. This is a good way to help get the thinking processes in the proper order. Go take a look!
This page, Torsion Winders, Radial Formers, and 3-D Benders, will show you the various types of spring and wire forming machinery available to make torsion springs.
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