allowable stress for static or quasistatic loads

Static Design

Static or rarely changing loads exist when:

• Disc springs carry only static, non-changing loads.
• Disc springs are subject to occasional load changes at greater time intervals and less than 10,000 load cycles during the planned service life.

Disc springs are normally designed with an overall height l_o, so that they can be flattened under static or rarely changing loads without the overall height l_o reducing by more than the permissible tolerance. The stress σ_OM at point OM defined in formula 9 applies here. 
 
Permissible Stress

Plastic deformations occur, when the stresses in certain areas exceed the yield strength. References stress is σ_OM. Its value should not exceed the tensile strength R_m of the material used. For spring steel as per DIN EN 10132-4 and DIN 17221 the tensile strength is R_m≈1600 N/mm². For other materials, the respective applicable yield point values must be used. Disc springs as per DIN 2093 and our factory standards listed in the disc spring dimension tables were designed according to an earlier method using the stress at point l.

For this reason, some of these springs exceed the permissible stress at the point OM. As these springs have been manufactured for years with this overall height l_o, we have not changed the height. With these types of springs there is the possibility of slight setting in use.