The Mean Coil Diameter (D) of a spring is the Outside Diameter minus one wire diameter (d).
And, the Mean Coil Diameter (D) is also the Inside Diameter (ID) plus one wire size (d).
D = OD - d
D = ID + d
In spring design, the Mean Coil Diameter is used more for calculating the rate and ultimate stress in the spring rather than for fit.
The most practical method for measuring Mean Coil Diameter is to measure the Outside Diameter and subtract one wire size.
When calculating the mean diameter of a torsion spring it is important to note changes that occur when the spring is deflected. These changes will cause the spring’s diameter to change, and need to be addressed during the design stages.
Please contact the Newcomb Spring facility nearest you for more information on calculating mean coil diameter and other considerations of spring design.
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