Common ball screw terms explained

When discussing ball screw assemblies, there is often confusion surrounding common nomenclature used to describe their structure and operation.  Circuits, turns, lead, pitch, and starts are all widely used, and misused, terms to quantify various aspects of ball screw assemblies. Although these specifications are related to one another, each has a unique meaning and significance to ball screw design and performance.

Lead and Pitch

Lead and pitch are a good example of related, but different, specifications. Lead refers to the linear distance traveled for each complete turn of the screw, while pitch is the distance between screw threads. These terms are often used interchangeably, and for single-start screws (see below for definition), lead and pitch are equivalent. However, lead and pitch are not equal for screws with multiple starts.

Circuits and Turns

Also related are the concepts of ball circuits and turns. A ball circuit is a closed path of recirculating balls. “Turns” refers to the number of trips the balls make around the screw shaft before being recirculated. The relationship between circuits and turns is influenced by the recirculation method. Ball returns that use the deflector or thread-to-thread (aka cross-over) method recirculate each turn of balls individually. Therefore, the number of turns is equal to the number of circuits.

Internal Deflector Recirculation Image credit: Bosch Rexroth

When balls are returned via an internal channel or an external tube, the recirculating balls can cross several threads, so one circuit can have multiple ball turns. Put another way, the balls will make several trips around the screw shaft before being recirculated. Multi-start ball screw assemblies typically use the internal channel method of recirculation. These can be designed to have multiple circuits, by incorporating more than one internal recirculation channel in the nut body.

Internal End Cap Recirculation Image credit: Bosch Rexroth

External Tube Recirculation Image credit: Bosch Rexroth