The notes on helical compression spring terminology refer to diagram at bottom of page. At Master Spring, we’ve been creating custom helical compression springs for more than 65 years. Learn more about how we can help you design and build the compression springs for your next application by calling us at
Common Compression Spring Terms:
Per Spring Manufacturers Institute Handbook of Spring Design, commonly used compression spring terminology includes:
- d wire diameter
- I.D. Inside Diameter
- O.D. Outside Diameter
- D. Mean coil diameter = (O.D. + I.D.)/2 or O.D. – d
- Lf Free Length = The overall length of a compression spring in the unloaded position.
- P Pitch = The distance from center to center of the wire in adjacent active coils. Recommended practice is to specify the number of active coils rather than pitch.
- C Spring Index = D/d = ratio of mean coil diameter (D) to wire diameter (d). The preferred index range is 4 to 12. Helical compression springs with high indexes tangle and may require special packaging. Springs with indexes lower than 4 are difficult to form.
- es Squareness = Angular deviation between the axis of a compression spring and a normal to the plane of the ends. Squareness can be measured by standing a sample spring on end on a horizontal flat plate and bringing the spring against a straightedge at right angles to the plate. The helical compression spring is rotated to produce a maximum out-of-square dimension es. Normally squared and ground springs are square within 3º when measured in the free position.
- ep Parallelism refers to the relationship of the ground ends, and is determined by placing a spring on a flat plate and measuring the maximum difference in free length around the spring circumference.
- Bearing Surface: To improve squareness and reduce buckling during operation, a bearing surface of at least 270º is required. Squared and ground compression springs are normally supplied with a bearing surface of 270º to 330º. Additional grinding results in thin sections.