The relationship between the mean stress and the corresponding amplitude of the alternating stress, also known as stress ratio R, has to be defined in the shaft strength calculation according to DIN 743 for each stress component. In the case of rotating shafts, the bending stress is often alternating, but what does it look like with the torsion? A shaft mostly does not turn continuously, as there exist start-up processes involving variable torques. In the operation of power units the torque stays essentially constant with small fluctuations. As generally understood, and also recognized as being on the safe side, a pulsating torque with R_t = 0 is frequently assumed for the strength verification, and others perform the calculation with a virtually constant torsional moment with, for instance, a stress ratio of R_t = 1/KA. The application factor KA is then applied for the relationship between maximum and minimum stress.

Since the permissible stress amplitude according to DIN 743 depends upon the equivalent mean stress, the safety factor can be smaller either for constant or alternating torsion.

Normally, as there are different operating states with variable torques and with virtually constant torques, all this cases could be considered in a load spectrum. The MESYS Shaft Calculation enables the user to vary the stress ratio in a load spectrum. Two calculations could thereby be carried out, one with constant stress ratio and another with pulsating or alternating stress ratio. For each case, a fatigue strength verification is done and as a result the minimum safety factors for each analyzed cross section are displayed. In this way, it is possible to conduct a fast verification for the two borderline cases.

An example of application is available here.