Preload

In all those applications where extreme accuracy of positioning is essential, it is advisable to eliminate any play between the screw and the nut. This can be done by assembling two opposed nuts separated by a spacer and by applying a determined preload to eliminate any radial (overhang) and axial loads.

This procedure gives the whole system considerable rigidity; however, it is necessary to limit the preload according to certain values (approximately 0.12 x C_a ) as higher values generate more friction and greater heating of the screw without conferring any appreciable advantage. It is also possible to obtain a preload in a single nut via a variation in pitch at the centre of the nut itself during grinding (see diagram). If application data is known it is preferable to set the screw as a function of applied load and desired rigidity.