Calculation elements

The following arguments concern calculation elements for the sizing of recirculating ball screws.

In the design of any mechanism including a recirculating ball screw the following values are of significant importance:

• combined bending and compressive load

• nominal life

• axial load

• critical rotation speed

• rigidity

This measure depends on the different nature of the surfaces which come into contact with each other, rotation speed of the screw and, in particular, inclination of the helix and other parameters. It is therefore extremely variable and difficult to calculate accurately.

To allow approximate calculation of the necessary torques in the various movements it is advisable to refer to the diagram below: this gives approximate curves from which both direct efficiency h_d (which refers to transformation of rotary movement in translation motion) and indirect efficiency h_i (when, vice versa, it is a translation motion being transformed into a rotary movement) can be calculated.

_ _ _ h_d

direct efficiency

____ h_i

indirect efficiency

d₀

nominal screw diameter (mm)

P_h

screw pitch (mm)