A ball screw is a mechanical system capable of converting rotary motion to linear motion or vice versa. An example of such a system, shown in Figure 1, is composed of a ball screw threaded shaft, a ball nut body, balls, ball recirculation elements and wipers (when present).
One of the main ball screw features is a high efficiency obtained by rolling of balls between the screw shaft and the nut body. Where ball contacts shaft and nut body, the rolling friction occurs. This feature is one of main advantages compared to alternative solutions like an acme screws, where the screw thread surface slides directly on the nut thread surface, so the sliding friction occurs in the contact zone.
Ball screws can be classified as follows (in accordance with Standards ISO 3408 and DIN 69051):
■positioning ball screws,
■transport ball screws.
The difference between the two typologies is related to application requirements, where an accuracy and a position repeatability are the most important.
The positioning ball screw is used where high stiffness, high positioning accuracy and high repeatability is required. Ball screws with preloaded nut are mainly used in these applications.
The transport ball screw is used for moving a load where stiffness, accuracy and/or repeatability is not required.
The above mentioned standards ISO 3408 and DIN 6905 also define all ball screw constructive parameters.

Parameter Name	Parameter Value
Type	round-thread
Other characteristics	with nut