Torsion Spring Diameter:
Helical Torsion springs can be described by three different diametric numbers:
Torsion Spring Wire Diameter:
(“d”) is the diameter of the wire used to manufacture the spring and is a factor used to calculate spring index.
Torsion 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.
Torsion Spring Body Length:
“LBODY”; the overall length of the body of an unloaded (“free”) spring. This is measured as the distance between the outer surfaces of the end coils of the body. This dimension can be calculated as d(N+1), where d= wire diameter and N=number of coils of an unloaded (“free”) spring, assuming the body coils are close wound.
Torsion Spring Maximum Deflection:
The maximum rotation (in degrees) that a torsion spring can be deflection before overstressing the spring. The body of the spring can be overstressed if deflected too far. When this occurs the spring will not return to its original position following removal of the deflecting load. The legs can also be overstressed. A torsion spring may also experience excessive leg bending stresses, where the leg 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 leg stress.
Number of Torsion Spring Coils:
Active coils (“Na”) equals the total coils in the body of the spring plus the additional contribution from the legs of the spring. This is approximately 1/3 of the moment arms expressed as an equivalent number of turns.
Torsion Spring Pitch:
“p”, the distance between wire centers in adjacent body coils. This dimension is equal to the wire diameter (d) when the body is “close wound”, where adjacent coils are contacting each other in the free state. However, it is also common to produce torsion springs with pitch in the body to avoid the effects of friction during deflection. In this case it is recommended practice is to specify the number of active coils and a body length rather than pitch.
Torsion Spring Rate:
The change in load per unit of deflection, generally expressed in pounds per degree. Spring rate is determined by the amount of force, in pounds, required to rotate a spring by one degree.