How to Count the Coils on a Compression Spring

Springs are essential to life. In fact everything on the planet acts like a spring, which follow Hooke’s law (F = k *x) . When force is applied to an object, it will deflect. If more force is applied, it will deflect more.

Count the total coils of the spring starting at one end and move toward the other end. This number does not have to be an integer. For an Open spring, this is the number of active coils. For a Open and Ground spring, subtract 1. For Closed or Closed and Ground, subtract 2 to get the active coils.

Counting the number of coils on a compression spring can be tricky.  There are two variables that specify the number of coils.  The value Na is the number of active coils and Nt is the total number of coils.  Only in the case of an open spring is Na equal to Nt.

Counting the coils in compression springs are tricky because not all of the coils count. First of all, you cannot count any coils where the coils are closed or touching.  These are inactive coils which don’t allow the spring to be a spring. 

The image below has common compression spring end types. The most common spring type is closed and ground which allow for stacking and even load distribution. The disadvantage is they take up more space.

Start counting each coil from one end to the other, starting where the section first opens.  (Indicated in image above) For an open end spring, this is the first coil.  Keep counting until you get to the other end of the spring.  Generally speaking, coils are counted as full turns, but half and quarter turns are also widely used.  Many spring manufacturers will allow two digits of precision on the number of coils.  For torsion springs, like clothes pins, you will specify the end condition in degrees.   

In the examples shown, each spring has 6 coils even though the closed end spring has 8 total coils.  The table below shows the relationship between active coils, total coils, the solid height and the pitch of each spring type. 

Note: It is customary to allow a 3% tolerance on the solid height of the spring to account for the end conditions, wire diameter tolerance and helical wind waviness.

Corey Rasmussen

Corey is the Managing Director of the Mentored Engineer and owner of Rasmussen Designs. He received his BSME from Baylor University and holds a professional engineering license in North Carolina and Texas. He has been an engineer since 2002 with extensive experience in engineering design, fabrication and troubleshooting. He specializes in mobile equipment, hydraulic systems and machine design. He has two patents

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