This is a question commonly asked by engineers and product designers looking to use energy storage devices for a particular application. Engineers will find several answers to this question online, most of which revolve around torsion’s spring durability and ability to exert rotary force or angular movement. But the answer to this question goes beyond just the spring’s function and durability. Besides, these advantages will only be felt if engineers correctly design and specify torsion springs for a particular application.

Figure 1: Torsion spring advantages will only be felt if engineers correctly specify a torsion spring for a particular application. Source: Winai Tepsuttinun/Adobe StockFigure 1: Torsion spring advantages will only be felt if engineers correctly specify a torsion spring for a particular application. Source: Winai Tepsuttinun/Adobe Stock

What is a torsion spring?

A torsion spring is a flexible device that stores mechanical energy when it is twisted. Its design is similar to compression springs. However, unlike compression springs subjected to compressive forces, torsion springs are subjected to bending stress. As a result, they store angular energy and exert rotary force.

Figure 2 is an illustration of a typical torsion spring. The legs of these springs are attached to separate components, and the spring helps to maintain rotational pressure between the surfaces of these components by allowing them to rotate along the center of the torsion spring.

Figure 2. Torsion spring illustration. Source: Yapparina/Public DomainFigure 2. Torsion spring illustration. Source: Yapparina/Public Domain

Key parameters of torsion spring design

Spring index

Spring index helps you determine whether your torsion spring design is manufacturable. Spring index can be calculated using:

As a rule, you want a torsion spring with a spring index greater than 4 and less than 12. Torsion spring designs with a spring index of less than 4 (or greater than 12) are generally not likely to be manufacturable. This is because torsion spring designs with a spring index of less than 4 are prone to tool marks, high stresses and mechanical damage during manufacturing. In contrast, torsion spring designs with a spring index greater than 12 do not behave well when coiled, forcing the manufacturer to work at a slow rate.

While a torsion spring design with a spring index ranging from 4.1 to 5.9 is manufacturable, it is associated with high manufacturing costs. The ideal torsion spring index ranges from 6 to 12.

Spring rate

Spring rate measures the constant attempt of a torsion spring to return to its normal shape after it has been subjected to bending stress. Simply, spring rate is the change in load per unit deflection. It can be calculated using:

Where:

E = modulus of elasticity (psi)

D = mean diameter (inches)

d = wire size (inches)

N = number of active coils

Consider the example of a torsion spring design with a wire diameter of 0.05 inches, a mean diameter of 0.45 inches and three active coils. If stainless steel (modulus of elasticity = 28 psi) is chosen for this torsion spring design, then the following correlations can be used to determine the spring index and spring rate.

First, the spring index is simply:

This is within the desirable spring index limit, meaning the spring is manufacturable. Second, the spring rate per 360° for this torsion spring design can be estimated using:

torque5torque5

= 12.002 lb/360°

= 0.033 lb/1°

Torsion force

The torsion force can be calculated using:

torque6torque6

So for a spring rate of 0.033 lb/1° and a distance of 90°, the torsion force (or load) would be 3 lb. Keep in mind that the wind direction of torsion force is critical to the operation of torsion springs. Engineers looking to generate torsion force in the clockwise direction must opt for a left-hand spring. In contrast, a right-hand spring is ideal for generating force in the counterclockwise direction.

Some advantages of torsion springs

Durable torsion springs are much more durable than extension springs. For example, a typical torsion spring can last up to 20,000 cycles, whereas most extension springs need to be replaced after about 10,000 cycles. This is because torsion springs use twisting motion, which causes less wear and tear.

Compact and easy-to-use: torsion springs occupy little space, making them ideal for applications with space limitations. Engineers can also adjust them easily.

[Learn more about torsion springs with Engineering360]

Conclusion

Torsion springs are important energy storage devices widely used in a broad range of industries today. While this article presents helpful information about torsion spring design, there are several other things an engineer must consider when specifying torsion springs. For instance, there is still the need to determine torsion spring material, torsion spring leg configurations and spring stress under different loading conditions, to name a few. Therefore, engineers are advised to reach out to torsion spring manufacturers to discuss their application requirements.

To contact the author of this article, email engineering360editors@globalspec.com