The pattern of change of load on the triple offset shaft of a mud pump is shown. It is shown that the loading is non-stationary. The change in the load in addition to torque variations associated with the vibrations of the pump also depends on the angular position of the rotating shaft. As a result, for the considered design of the pump during one revolution of the crank shaft, 6 different load cases of shaft bending and twisting moments were received. For each of these options, depending on the angular position of the shaft, we calculated load and graphs of change of bending and shear stresses in the dangerous section for the period of the operating cycle. The safety factor of the fatigue strength in the intermediate sections of the crank shaft has been defined. The histogram of loading was presented and the coefficient of durability was found. The issue of having to perform the calculation of low-cycle fatigue was addressed. Accounting for non-stationary loading shaft helps to clarify the definition of safety factor and make a more informed opinion on the issue of ensuring the required durability.