Technical Considerations for Compression Spring Design and Specifications
The Tolerance/Cost Ratio
Some job shops are staffed and equipped to produce springs to extreme limits of precision. However, the end uses of most springs do not require more than "commercial tolerances."
These +/- allowances can result in economies while maintaining quality. Wherever commercial tolerances are acceptable, costs can be minimized. Commercial grade springs can often be produced on our automatic equipment. The following commercial tolerances are usually considered for springs like these shown.
Materials can be selected as they relate to expected (a) cycles of stress, (b) temperature, (c) corrosion, (d) chemicals, (e) water and moisture, and (f) special involvements. Commercial tolerances of materials vary with each wire diameter. For example: wire diameter .007" could be +/-.0002" while .090" diameter could be +/-.001"
Determining Free Length
The overall length of an unloaded spring is its free length. The commercial tolerance of free length is determined by the Spring Index Number, which is the relationship of the spring diameter to the diameter of the wire from which it is made, and the number of active coils per inch (pitch dimension) in relation to the overall length. For example, a spring having 12 coils per inch with a Spring Index Number "10" has a free length commercial tolerance of +/-.030".
Coil diameter tolerance is determined by the diameter of the wire and the Spring Index Number. For example, springs made from wire with .035" diameter, having a index factor of "10", carry a commercial tolerance of +/-.007" for the coil diameter.
Single and Multiple Load
Commercial tolerances of compression springs include +/- 10% of the load requirement. Springs with a multiple load carry a tolerance of +/- 10% on each load. For example, a spring with a free length of 2" working in the compressed state could have a specified load at 1 1/2" and also at 1". Commercial tolerance would be +/- 10% of each load.
The spring rate is determined by the amount of force required to constrict a compression spring by one inch. Using the "LOAD" example, if the 2" free length spring required 1 lb. to compress it to 1 1/2", and 2 lb. to compress it to 1", its spring rate would be 2 lb., and its commercial tolerance would be +/- 3.2 oz. (+/- 10%).
The grinding of compression springs provides squareness of closed ends. (Closed ends result from reducing the pitch of the end coils until they touch) The commercial tolerance for end grinding is +/- 3.
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