The Duty Cycle and its Equivalent Load

To calculate the impact of actual duty on the fatigue life of the ball screw, it is necessary to convert the varying loads Fi into a mean load Fm which will have the same effect on life as the actual duty cycle. This mean load Fm is therefore called "equivalent" load.

To simplify things, the actual duty is normally divided into steps. Then the maximum thrust (or, if the ball screw is preloaded, its modified load), speed and duration are used to calculate the equivalent load. Instead of using time and speed ni·qi, the absolute number of revolutions can be used in this calculation.

Equivalent Load

The duty cycle of machine tools is normally provided as:

x₁%  Rapid with thrust  y₁
x₂%  Roughing with thrust y₂
x₃%  Finishing with thrust y₃
x₄%  Dwell

If the duty cycle provided includes a detailed description of moves, the respective revolutions can be entered instead of q_i·n_i .