Friction

Depending upon the spring arrangement, frictional forces arise during the compression and extension of springs between individual springs, between the springs and guide rod, and at the edges of the spring where load is applied. This results in a variation between the calculated characteristic load curve and the actual loading and unloading characteristic load curves for a given application.

Friction for a single disc spring

As shown in Fig. A, during compression frictional forces µ_R . F(µ_R) create a moment that counteracts the moment of the applied load and thus increases the required compression force F(µ_R). During extension, frictional forces create a moment acting in the same direction as the load moment and therefore reduce the required retaining force.

Fig. A: Single disc spring with edge friction: a) Overall view b) Compression c) Extension

The actual coefficients of friction depend upon the surface finish of the components through which the load is applied, the radii at edges I and III of the disc spring, and the lubricant used.

Friction in stacks of disc springs in parallel

When a spring stack consisting of n discs is compressed, radial frictional forces µ_M x F(n, µ_M) acting in opposing directions on the upper and underside of the disc occur on the surfaces of contacting disc springs (Fig. A). These frictional forces are in addition to the frictional forces created at the edges where the load is applied to the spring stack. This results in n frictional moments of friction which counteract the moment of the applied load and thus increase the required compression force. When the applied load is removed, frictional forces reduce the required retaining force.

Fig. A: Friction forces on a stack of disc springs in parallel

The percentage force deviation is independent of the compression. The use of thicker disc springs (Series A) enables greater damping effects to be achieved.

Experience shows that with a decreasing number of discs n in the pack an increasing deviation in the form and point of origin of the lower characteristic from the calculated values is recorded (Fig. B). This is the result of the accumulating deviations of the single discs from the ideal form, particularly out-of-roundness on the surface area of the cone and deviations in the overall height l_o.

Abb. B: Comparison of measured and calculated characteristic load curves for a single disc and a stack of comprising 2 to 4 disc springs arranged in parallel

These form deviations have the effect that both after interchanging the spring positions within the pack and by rotating single springs in the pack slightly different characteristic curves can occur. In general, however, over time a steady loading and unloading characteristic curve is established.

For safety reasons, even stacks of parallel springs are provided with internal or external guidance. If disc springs with a low friction design are used, the resulting frictional forces are usually negligible.

Friction in stacks of disc springs in series

In this case it is assumed that disc springs of low-friction design are used. For example, the springs are designed with a special inner edge contour that minimizes the friction between the guide rod and disc spring stack. This results in the uniform deflection of the disc springs in a spring stack consisting of individual springs arranged in series. The risk of premature spring fracture from over-stressing the springs at the moving end of the stack is thereby reduced.

Fig. A: Deflection of individual disc springs in a series stack

Fig. B: Comparison between the measured and calculated characteristic load curves for a spring stack consisting of 10 springs arranged in series

Fig. A shows the difference in deflection of the individual disc springs in both low friction and not low friction spring stacks. The characteristic load curves measured during loading and unloading for series stacks of disc springs differ slightly from one another as well as the calculated characteristic load curve (Fig B).

Deviations in the ideal geometry of the individual disc spring results in an uneven transmission of load from one spring in the parallel stack to the next. This results in lateral displacement of the springs, which are then pressed with great force against the guide element. If such laterally displaced springs are located at the moved end of the stack, the lateral forces generate a high amount of friction because of the large spring defelection.

Consequently, it should be emphasised that the use of parallel stacks of disc springs in a series stack can result in non-uniform deflection and in a higher operating temperature at high frequency. Thus, the overall fatigue life of the spring stack is reduced.