The Concept of Counting Coils on Different Types of Springs

Counting coils, especially the number of active coils versus the number of total coils, remains to be one of the most misinterpreted concepts of spring manufacturing. Despite being the simplest feature to measure, the process of counting coils has resulted in a surprising number of errors in drawings and in new product development.

What Are Active Coils?

Active coils are pretty much what the term implies – they must be active. In other words, they must be able to either deflect or extend or bend in the case of a torsion spring. For example, when we have a compression spring that stands nice and straight on the table, it can do so because the end coil has been closed to touch the adjacent coil and then the last coil is ground so that half of the material is removed and the spring stands nice and flat.

However, because that end coil is sitting flat on the table, it is not allowed to deflect in any way. That renders the coil inactive. And since the other side of the spring is the same way, there is also an inactive coil on the other end. It is therefore said that on a compression spring with “closed and ground ends” there are two inactive coils added to the active coils to come up with the “total coils”.

If the end coils on a compression spring are closed such that they touch the adjacent coils but are not ground, they are still considered inactive and the active coils plus two inactive coils still constitute the total coils.

Sometimes there is a need for the compression spring to have a higher than normal height at which all the coils touch each other and the spring is considered “at solid”. If this is the case, the spring manufacturer may put two or three coils touching each other at each end of the spring. In that case, all the coils touching each other are deemed inactive.

Counting Coils on Extension Springs

The process of counting coils on extension springs is slightly different from that on compression springs. On an extension spring, the spring engineer typically figures out the number of active coils needed and then the spring winder adds two additional coils for the hooks or loops. One additional coil on each end of the spring is then bent out to form the hook or the loop. Once that is done, the active coils are reduced by two. Therefore, unlike on a compression spring, only the coils that are on the helix are considered active on an extension spring.

If the extension spring is to have an extended hook or loop, the spring is wound with the correct number of active coils and then a straight end of the correct length is added for later bending as a secondary operation.

From calculating spring rate to counting coils, at Hardware Products, our aim is to make your spring ordering process as easy as possible. Contact one of our spring geeks today to help you find answers to your spring-related questions.