Home Products Extension Springs Key Parameter / Reference Numbers

Key Parameter / Reference Numbers

Extension Spring Free Length:
"Lo"; the overall length of an unloaded ("free") spring. The distance between hooks as measured from the inner surface of each hook, as shown in the illustration. 

Extension Spring Body Length:
"LBODY"; the overall length of the body (excluding hooks) of an unloaded ("free") spring. This dimension is measured as the distance between the outer surfaces of tne end coils of the body. This dimension can be calculated as d(N+1), where d= wire diameter and N=number of coils in, assuming there is no pitch in the body coils.

Extension Spring Body Diameter:
Helical Extension springs can be described by three different diametric numbers:

  • outside diameter ("OD")
  • inside diameter ("ID")
  • mean diameter ("D") is used in the stress and deflection calculations, and is equal to half the sum of the outside and inside diameters.

Extension Spring Wire Diameter:
("d") is the diameter of the wire used to manufacture the spring and is a factor used to calculate spring index. 

Extension Spring Index:
The ratio of the mean coil diameter to wire diameter (D/d). Springs with an index lower than 4 can be difficult to form.

Extension Spring Maximum Extended Length:
The maximum length that an Extension spring can be stretched before overstressing the spring. Unlike a compression spring, where the maximum compression is usually related only to the stress in the body of the spring, an extension spring has several stress points. The body of the spring can be overstressed if stretched too far. When this occurs the spring will not return to its original length following removal of the stretching load.

The hooks can also be overstressed. An extension spring may also experience excessive hook stresses, where the hook will begin to fail while the body of the spring is still within an acceptable stress range. Check with your spring designer to be sure your spring has been fully reviewed for body stress as well as hook stress.

Number of Extension Spring Coils:
Active coils ("Na") are the coils in an Extension spring that are free to deflect under load. For standard hook configurations, Na is approximately equal to the total number of coils ("Nt") in the body. For springs with threaded inserts or swivel hooks you will have "dead coils" on the ends of the body. Na will therefore be less than Nt. Conversely, hooks and loops will increase the number of active coils, usually in the range of 0.1 - 0.5 Na.

Extension Spring Pitch:
"p", the distance between wire centers in adjacent active coils. This dimension is equal to the wire diameter (d) when the coils are "close wound", where adjacent coils are contacting each other in the free state. Occasionally, extension springs may be made with pitch where the adjacent coils are not contacting each other 

Initial Tension:
P1 - The force that tends to keep coils of a close wound extension spring closed and which must be overcome before the coils start to open. When initial tension is present adjacent coils are exerting a force against each other. In order to stretch the spring a force must be applied which exceed the forces of initial tension. This is measured by applying a gradually increasing load on the spring and recording that load at the point when the coils begin to separate. Initial tension is dependent on index, material, manufacturing method and post-forming stress relief operations. Some materials are capable of higher initial tension values. Other materials may allow no initial tension at all.

Extension Spring Rate:
The change in load per unit of deflection, generally expressed in pounds force per inch. Spring rate is determined by the amount of force, in pounds, required to extend a spring by one inch. Material size directly impacts spring rate.