Effects of Friction
Disc spring friction is manifested as a frictional load acting in opposition to the external loads applied to the disc spring. It is evident as hysteresis on the load/ deflection characteristic curve of the disc spring. The frictional component is in turn positive during the loading cycle, becoming negative during the unloading cycle.
The following three major factors must be taken into consideration when considering the effects of friction on disc springs:
A) Friction between the loading elements or end-plates and the disc springs themselves.
B) Friction which occurs between the adjacent surfaces of disc springs assembled in parallel (i.e., sliding surface friction).
C) Friction between the guiding elements (whether rod or housing guides) and the disc springs.
For cases A and B (above), the frictional component on the total disc spring load may be calculated by means of the following. Note that this calculation is valid for a single disc spring assembly or unit consisting of (n) disc springs in parallel.
Figure 26:
Friction between the ends of the loading elements and the disc spring as well as between the contact surfaces of disc springs arranged in a parallel component
with the factors defined as follows:
F Disc spring load without friction
n Number of disc springs in the component
arranged in parallel
WM Friction value for friction in case B
WR Friction value for friction in case A
The sign is negative (-) during the loading cycle.
The sign is positive (+) during the un-loading cycle.
Friction Values:
Table 6
The range of these values results from variations in lubrication and surface texture. When n = 1 the equation describes the characteristics for a single disc spring between flat end-plates.
For disc spring stacks consisting of several parallel units arranged alternately in series, the friction from source A becomes increasingly insignificant. Approximately the friction is given by:
Friction from source C is dominant for all disc spring stacks in series which consist of a large number of single disc springs or parallel units with a small number in parallel.
In unfavourable situations the friction can be so high, that the ideal length of a series stack must be reduced. Disc springs with large cone angles in particular have high friction. The value of this type of friction cannot be calculated at the present state of knowledge.