Ball screw bearings are used where linear motion is required to carry medium/high loads with relatively high accuracy. Reliability, accuracy and smooth operation are paramount in most ball screw applications. Ball screw bearings arte often required to have high stiffness and low axial play which conventional screw nut assemblies usually fail to achieve. Precision ball nut assemblies can achieve this by simultaneously preloading the nut on the screw whilst still retaining a smooth action and high stiffness. The durability of a ball screw assembly comes down to its machining accuracy and correct selection of materials and material treatment.
Ball screw assemblies can be fundamentally broken down into two components, the lead screw itself and the nut. How the nut interacts with the screw is of key importance for a successful result. There are 3 main types of ball circulating nuts and up to four methods of preloading the nuts to provide the stiffness required. Each method will be described as follows.
Return tube nut design
The ball return tube method simply recycles the balls back to the start point by the use of an external ball tube. The balls can circulate around the screw from 1.5 up to 3.5 turns before being fed back. This design is economic and suites a fine lead and high helix designs.
Deflector nut design
The ball deflector method uses an internal groove within the nut to recirculate the balls over the land (or ridge) of the screw back to their start point. This design is suitable for fine lead ball screws. The nut outside envelope dimensions are smaller and thus suitable for close clearance assemblies.
Cap nut design
The end cap method simply lets the balance circulate for the full length of the nut then returns the balls to an end cap which is designed to feed back to the start point. This design suites high speed positioning systems.
There are many methods for preloading ball screw nuts. The double nut system simply places a spacer which is machined specifically to size between two nuts. The spacer pushes each nut against the lead screw via the balls. The same effect can be obtained by using a spring system between two nuts. The third method of preloading a nut involves offsetting the screw lead by a specific amount, thus causing preload. Finally, using oversize balls can also crate preload although this method provides the shortest nut length, only light preloads can be created.
A nuts external profile can be broken down into three basic types although special designs can give an infinate range of possibilities.
- Circular shape - type I - usually used for shaft diameters less than 16mm.
- Circular shape - type II - this design has a cut off segment on one side for shaft diameters of 20mm and over.
- Circular shape - type III - this design has a cut off segment on both sides for miniature ball screws with a fine lead.