Dynamic Loading & Fatigue Life

Dynamic loading of disc springs occurs when the load continuously changes from preload to final load.

The "stress-time" curve of such disc springs which pulsate uniformly is sinusoidal. This is not true in cases of impact loading and therefore it is difficult to predict their life and behavior.

Disc spring "life" may be differentiated into 2 categories:

1. Limited life: where cycles vary without failure between 40,000 and 2,000,00 cycles.

2. "Unlimited life": cycles in excess of 2·106 without failure. For virtually indefinite life, the table below indicates the appropriate values required given in percent of travel, relating preload to final load AND considering the disc spring thickness.

  Max Deflection

in % of h

Preload

in % of h

Disc Thickness <=.039 Disc Thickness b>=.157
15 50 44
25 56 49
50 67 64

Fatigue Life

Fatigue life for a disc spring is defined by the effective number of stress cycles that can be sustained prior to failure under certain conditions. This depends on the minimum stress, maximum stress and stress range.

The diagrams presented here are for evaluating fatigue life of single disc springs or series stacks not more than 6 springs. There are three basic groups, depending on thickness (see legend under each diagram).

The horizontal border line enclosing the top portion of the graph (zone) represents the yield strength of the spring steel material.

Intersection points of min/max stress limits which fall outside the graph/zone boundaries are to be avoided as they indicate spring failure is likely at an early stage.

The graphs were developed based on empirical test data. The test loads were sinusoidally executed.

 

How to use the graphs

1) For standard catalog disc springs:

a) Determine preload stress... see Belleville Disc Spring tables for values.

b) Determine final load stress... see Belleville Disc Spring tables for values

The intersection of the stress coordinates will indicate the range of fatigue life that may be expected.

2) For non-standard or special disc springs...

a) Determine the preload stress from formulas for points S2 and S3. Use the HIGHER of the two values for preload and final load.

b. Repeat above procedure for FINAL STRESS... using again the higher of the values found.

3) EXAMPLE

M188207: .709 x .323 x .0276( See Belleville Chart)

Preload Stress at Deflection f=.5h : 124000 psi

Final Load Stress at Deflection f=.75h : 174000 psi

Intersection Point between nearby 2MIO-Cycles-Line:

Predicted Cycles: 1.5MIO

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