Torsion springs, whose ends are rotated in angular deflection, offer resistance to externally applied torque. The wire itself is subjected to bending stresses rather than torsional stresses, as might be expected from the name. Springs of this type are usually close wound, reduce in coil diameter, and increase in body length as they are deflected. The designer must consider the effects of friction and arm deflection on the torque.
Special types of torsion springs include double torsion springs and springs having a space between the coils to minimize friction. Double torsion springs consist of one right-hand and one left-hand coil section connected together, and working in parallel. The sections are designed separately with the total torque exerted being the sum of the two.
Type of Ends
The type of ends on torsion springs should be carefully considered. While there is a good deal of flexibility in specifying special ends and end forming, the cost may be increased and a tool charge incurred. The designer should check nominal free angle tolerances in Table 2, this section, with respect to application requirements.
In addition to supplying the information requested on the torsion spring specification form on page 26, it is important that a drawing be provided detailing the end configurations.
The type of ends and installation affect spring load and deflection as shown in Figure 1. All torsion springs have three or more points of contact with at least two at the ends and one at the arbor. For clarity, the designer should specify the position of the contact points on the spring and their position relative to one another. (Figure 1 shown) .
The basic formulas for torque or moment (M) and bending stress (S) used in designing torsion springs are shown below.
In the formulas the constants 10.8 and 6.6, while not strictly theoretical, give results closer to the actual values obtained. in the formulas the constants 10.8 and 6.6, while not strictly theoretical, give results closer to the actual values obtained.
D = Mean coil diameter, in. (mm)
E = Modulus of elasticity, psi (MPa)
d = Diameter of round wire, in. (mm)
S = Bending stress, psi (MPa)
Nt = Number of coils
M = Moment or torque, ibin. (Nmm)
T = Deflection, number of turns or revolutions of spring
b = Width, in. (mm)
t = Thickness, in. (mm)