Spring Essentials – Torsion Springs – a little primer
In the world of springs, one type stands out as the answer to many applications. As with any mechanical device, it has its uses — and its limits. I’m talking about the one and only “torsion” spring.
The word “torsion” implies something that twists or rotates — circular motion. Unlike a compression spring that is pushed, or an extension spring that is pulled, a torsion spring rotates as forces are applied.
Most torsion spring applications require the spring to be preloaded in some sort of assembly, and then rotate to create a torque that will provide resistance (such as with an accelerator pedal) or provide a return force (as with the deck guard on your riding mower). They are also used to control and curb the forces of gravity for things such as trailer doors, dishwasher and oven doors, or to bring that hinged door back in place after you grab the next to last newspaper from the dispenser. Another popular use for torsion springs is the constant force needed to gently raise and lower garage type doors of all sizes and weights. Although extension springs can be used for this use, torsion springs provide low spring rates and do a better job of keeping heavy doors from rocking back and forth during their travel up and down.
Torsion springs consist of a coiled body with legs extending from either end. The legs can be straight or formed into any number of shapes to accommodate the assembly. Torsion springs can be formed with the coils either closed, or open. If the coils are closed, there is the chance that initial tension (the force that holds coils together) may cause some binding and rubbing. For this reason, torsion springs are many times coiled with the coils open.
The typical application of torsion springs is for one leg to be held in place and the other will move to provide the force. But, when torsion springs are engaged, they are changing their shape. As the spring rotates, the body diameter decreases and the free length increases. The farther the spring is rotated, the I.D. continues to get smaller and the free length continues to get longer. This means the movement of the spring must be known completely for an engineer to calculate what I.D. and free length will result. If the free length grows too much, the spring could bind and cause the assembly to fail, or break other parts of the mechanism. The same applies to the I.D. If the torsion spring has a long free length, it may need to be guided over a shaft or stud to keep it in line. If the I.D. decreases too much, it will wrap around the shaft and lock up. This can destroy the mechanism and total failure of the product.
However, there are also applications where the binding of a torsion spring is a good thing. This is the case for “clutch” springs. Clutch springs are torsion springs that purposely grab and hold a shaft to keep it from moving. This is useful to engage drive shafts and put machinery in motion. Another clutch application is for devices such as aircraft and automobile seating that require precision torsion clutch springs to allow seats to come forward or backward, and then stay in place when the spring wraps its I.D. tightly around the shaft. In many cases, the wire is square instead of round on clutch springs to increase the “gripping” area. If round wire is used, only a small surface is touching the shaft, whereas square wire greatly increases the gripping area and force. ◆