Calculation of the equivalent Load

Ballscrews usually will be used carrying axial loads under dynamic conditions. The selection therefore has to take into consideration the load and the travel - or number of revolutions - made under this load. The normal service life expectancy is based on the fatigue of the ball material. Basically, travel made under higher load will determine the actual service life more than travel made under lower loads. As any application will give a constant load, a mean load must be calculated, which will result in the same service life. This so-called dynamic equivalent axial load F m is then to be compared with the dynamic axial load capacity C_a . For simplification, a typical work cycle of the machine under design should be described along with load and load direction, percentage of time and speed for every step.

In the simplest case - non-preloaded single nuts - these values can be converted to the dynamic equivalent axial load F_m and the average speed n_m by means of the following formulas:

F_m = Dynamic equivalent axial load [N]
F_i = Actual load [N]
n_i = Actual speed [rpm]
q_i = Time of each duty cyclus [%]
n_m = Average speed [rpm]