We recommend calculating the stress for both points using formulas 11 and 12. Use the larger value to determine fatigue life using the applicable diagrams (figure 18 – 20).

Minimum Preload to Prevent Superficial Cracks

After head treatment all discs springs are going to be scragged or pre-stressed, which causes a plastic deformation in the region of cross-sectional point l. This results in residual tensile stress at this point in the unloaded spring. When loaded there is then a change from tensile to compressive stress which can result in the formation of cracks during dynamic loading. To avoid these the tensile stress must be balanced out by applying a suitable pre-stress. Therefore, dynamically loaded disc springs should be preloaded to at least s = 0.15 to 0.20 ho.

Permissible Stress

The stress calculated for the working range of the spring is compared with the fatigue diagrams in figure 18 – 20. These provide standard values of the permissible stress range σH for N > 2.106, N = 5.105 and N = 105 load cycles in dependency on the minimum stress σU for dynamically loaded, non-shot-peened disc springs. Intermediate values for other load cycles can be estimated.

A fatigue diagram is indicated for each of the 3 manufacturing groups as per DIN 2093. These groups are divided by the disc thickness as follows:

Group 1: t less than 1.25mm
Group 2: t = 1.25 to 6mm
Group 3: t over 6 to 14mm

These diagrams were developed from laboratory tests on test machines with an even sinusoidal load by means of statistical evaluation, whereby a survival rate of 99% was assumed. This means that for a large enough sample a failure rate of 1% can be expected due to fatigue.

The diagrams are applicable to single springs and spring stacks with up to 10 single springs stacked in series, operating at room temperature with hardened and perfectly finished inner or outer guides and minimum preload deflection of s1 = 0.15 to 0.20 ho.

Figure 18 – click to enlarge