Welcome to the new installment on spring design and clarification. Many of my customers want to know what is the proper call out for pitch in a coil spring. There have been many conversations regarding this and I think there needs to be some clarification.

The term pitch was derived from screw manufacturing which is the tip of the peak of a screw thread to the next peak of the screw thread. Since round wire does not have a peak or sharp edge, the pitch is measured to the center of the wire. Any time you try to measure pitch on a spring you’re going to end up with different interpretations of where the actual center of the wire is located. 

This is especially true on a compression spring. Since there are a variety of interpretations, this can result in an inaccurate measurement error that causes quality rejects for no reason. In order to find the proper and accurate call out for a pitch, there needs to be a measurement taken in the space between the coils.

The main reason this measurement needs to be accurately taken is because it will ensure that there is no rubbing (torsion springs coil to coil) and plating has the opportunity to coat the wire between the coils. The best way to do this is to put a reference dimension on the pitch or space between the coils, and control the body length (free length on compression springs). The other option is using pre-coated wire, or stainless steel wire when using extensions springs, which have negative pitch.

Pitch per spring type:

• Compression Spring: Free length and coil count control pitch.

• Extension Spring: (negative pitch) which is controlled by initial tension.

• Torsion Spring: Body length controls pitch.

You can learn more about these springs here.

Final note: Coiling of wire to have super accurate pitch has never been a consistent venture in making springs. Focus on the most important part of springs which is loads. A reference dimension on the pitch/spacing between coils is the best way to call out pitch on drawings.

That’s my best pitch on this subject. Don’t worry, and let Apex figure out the best pitch for your spring product.

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