 Compression
Spring Key Parameters and Reference Numbers
Compression Spring Diameter:
Helical compression springs can be described by three different diametric numbers:
- outside diameter (“OD” ) is specified when the spring will function in a cavity
- inside diameter (“ID”) is specified when the spring will function over a rod or shaft
- mean diameter (“D”) is used in the stress and deflection calculations, and is equal to half the sum of the outside and inside diameters.
Compression Spring Wire Diameter:
(“d”) is the diameter of the wire used to manufacture the spring and is a factor used to calculate spring index.
Compression Spring Index:
The ratio of the mean coil diameter to wire diameter (D/d). Springs with an index higher than 12 can tangle; springs with an index lower than 4 can be difficult to form. Therefore, for ease of manufacturing and packaging, the preferred spring index range is from 4 to 12.
Compression Spring Free Length:
“Lo”; the overall length of an unloaded (“free”) spring.
Compression Spring Solid Height:
“Ls”, the minimum length of a compression spring with all of its coils closed, when no further deflection is caused by additional load. If solid height is a critical application dimension, it should be specified as a maximum—generally figured as the solid height plus an allowance equal to half of the wire diameter.
Number of Compression Spring Coils:
Active coils (“Na”) are the coils in a compression spring that are free to deflect under load. For squared-end springs, Na is equal to the total number of coils (“Nt”) minus 2 (the turns at each end that are inactive, or “dead”, and in contact with the spring seat). The number of active coils in a plain-end spring is greater and depends on the seating method. The greater the number of active coils, the lower the spring rate.
Compression Spring Pitch:
“p”, the distance between wire centers in adjacent active coils. Current recommended practice is to specify the number of active coils rather than pitch.
Compression Spring Rate:
The change in load per unit of deflection, generally expressed in pounds per inch. Spring rate is determined by the amount of force, in pounds, required to constrict a spring by one inch. Material size directly impacts spring rate. For example, increasing a wire diameter by 1 percent will result in a 4 percent stronger spring; decreasing diameter by 1 percent will result in a 4 percent weaker spring. Increasing the mean diameter by 1 percent will decrease the spring rate by 1 percent. Adding coils weakens the rate, while removing coils strengthens the rate.
For assistance download our Compression Spring Design Sheet
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