Calculating Permissible Stresses

The allowable stresses depend upon the type of loading that the spring is subjected to.

3 types of load application are considered:

(a) Static or rarely alternating loads with fewer than 10⁴ load cycles over the required life of the spring

(b) Alternating loads with 10⁴ < N < 2 x 10⁶ load cycles over the required life of the spring.

(c) Alternating loads with N > 2 x 10⁶ load cycles over the required life of the spring.

For (a):

For applications with a static or rarely alternating load, the highest calculated stress at the upper inner edge of the single spring (cross-sectional point I) is decisive. The stress at cross-sectional point I has the highest magnitude and is thus decisive for the setting behaviour. For springs made of high grade steel as specified by DIN 17221 and DIN 17222, the compressive stress calculated for point I should not exceed the values specified in the table above in the flat position S_c = h_o.

Table: Maximum permissible stresses in the flat condition

In the case of higher compressive stresses the spring may undergo a high degree of setting. If the maximum permissible calculated comrpessive stresses are exceeded in the case of special sizes, such springs can also undergo a high degree of setting.

For case b) and c):

The maximum tensile stresses on the lower side of disc spring are critical for springs subjected to dynamic loading. Fatigue fracture always begins on the lower of the spring. Fracture will begin at cross-section II or III depending upon which position is subjected to the higher cyclical stress level.

Minimum preload

For dynamic loading, the disc spring must be installed with sufficient preload to prevent fracture at the upper inside edge (cross-sectional point I). Radial surface cracks may occur at the upper inside edge due to residual tensile stresses caused by the presetting process.

Experience shows that the minimum compressive stress, ō_l, should be about - 600 N/mm² for DIN springs. This corresponds to a preload deflection of s_u ≈ 0.15 h_o …0.20 h_o

While springs with lower stresses in the flat condition can have a lower preload deflection, a larger preload deflection is needed for springs with very high stresses in the flat condition.

Fatigue strength values

The fatigue strength graphs provided here are based upon many years of testing Mubea disc springs. They show the permissible calculated stress on the underside of the disc that is decisive for fatigue failure. The fatigue strength graphs have been calculated for different disc springs of varying thickness and varying numbers of load cycles.

Fatigue strength values

The graphs apply to Group 2 and 3 disc springs made of 50 Cr V 4 and Group 1 disc springs made of Ck 67. The maximum fatigue life can be achieved with a statistical probability of 99 % under the following conditions:

a) Spring stacks with a maximum of 10 individual discs stacked facing alternate ways

b) Sinusoidal deflection - time function with a constant stroke and at a constant frequency below the permissible thermal limit 

c) guidance on guide elements (rod or sleeve) per the requirements noted in and hardened and ground load application surfaces at the stack ends

d) proper lubrication

e) operation at room temperature and normal atmosphere (i.e. no excessive humidity, no corrosive chemicals, etc.)

Deviations from these test conditions may reduce the number of load cycles that can be achieved. This applies especially to sudden loads that can occur during operation in the case of faulty lubrication, or when corrosion and surface imperfections are present.

As the number of disc springs in a stack increases, the number of load cycles that can be achieved are reduced in comparison to a single disc spring. One reason is the carrying deflection of the individual springs within the stack. This is influenced by:

• The friction between the springs and guide rod
• The friction between the springs in parallel stacks

Mubea disc springs can accept higher dynamic stress levels, or operating cycles, compared to the requirements of the DIN 2093 standard. This is shown in the fatigue strength diagrams where the dynamic stresses and the allowable stresses are compared to the corresponding values in DIN 2093. (fig. C and D).