Helical compression springs can be described by three different diametric numbers:
(“d”) is the diameter of the wire used to manufacture the spring and is a factor used to calculate 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.
“Lo”; the overall length of an unloaded (“free”) spring.
“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.
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.
“p”, the distance between wire centers in adjacent active coils. Current recommended practice is to specify the number of active coils rather than pitch.
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.