Example of Load Capacity Selection

The following duty cycle is given for a machine:

For stiffness reasons a ballscrew with double nut should be selected, and therefore the direction of the external load is to be considered. The preload is set to 1000 N to keep all axial loads within the preload range. As a rule of thumb, the following formula can be used:

Both nuts must be calculated separately:

"Nut A"

The same applies to the second nut, but with inverted external loads:

Although the formula which takes the preload into consideration leads to a negative result, this will simply lead to unloading of the second nut. Exact calculation or crosscheck with the above mentioned formula can ensure that this will not happen.

"Nut B"

As the resulting loads in nut "A" are reasonably higher and the nut should be symmetrical ( both nuts same load capacity), the service life of the ballscrew will be determined by nut "A".

By means of the formula which takes the preload of singel nuts into consideration the equivalent dynamic axial load in nut "A" is:

F_m^* = 1583 N

The average speed is:

n_m = 1250 1/min

The desired service life should be 20000 hours (L_h) with a working percentage of 50 %. With the average speed, the number of revolutions for the desired service life can be evaluated:

By means of the formula for the dynamic axial load capacity for a double nut the required dynamic axial load capacity is:

A ballscrew with a dynamic axial load capacity of 14600 N (4/25 mm) is selected, resulting in a service life of: