Ball splines provide nearly friction-free linear motion while simultaneously transmitting torsional loads. The path of the bearing balls is straight, not helical, which allows for a very efficient coupling device suitable for a variety of applications. Ball splines have grooves that have been ground into them for the recirculating ground balls to run inside. The outer shell that houses the balls is called a nut rather than a bushing. There are either four or six sets of ball retainers. By increasing the contact area of the ball bearings on the shaft to approximately 45 degrees, the side load and direct load carrying capabilities are greatly increased. Each nut can be individually pre-loaded at the factory to decrease the available radial play to ensure rigidity. This process not only increases the contact area, increasing direct loading capabilities, but it also restricts any radial movement, increasing the overhung moment capabilities. This creates a sturdier structure that can handle a very strenuous working environment. 

Specifications

Important specifications to consider when specifying ball splines include:

• overall shaft length
• inner race length
• outer diameter of the spline shaft
• overall length of the spline nut
• outer diameter of the spline nut
• static torque
• dynamic torque

The overall length of the shaft includes the length for mounting. The inner race length is the overall length that the ball nut is allowed to travel in a straight line, before returning to its original position. Inner race is the term used to describe the groove the bearing balls ride in.  The outer diameter of the spline shaft is the largest overall diameter of the shaft. The outer diameter of the spline nut is the maximum outer diameter of the ball spline nut. The static torque is the maximum thrust load that can be applied to the ball nut in a stationary position.  The dynamic torque is the maximum thrust load that can be applied to the assembly, which will result in a predetermined minimum life of travel.

Features

Common mounting options for ball splines include:

• pin or set screw
• key and pin
• square spline nut
• geared flange spline nut  

A pin or set screw is suitable for applications with low torque requirements. A key and pin is applicable for applications with high torque requirements. On a square spline nut, the spline nut can be directly installed on a machine body. On a geared flange spline nut, the gear teeth are cut into the outer flange of the spline nut. Common features for ball splines include hollow shaft and hollow shaft inner bore diameter. Hollow shaft is useful for piping, wiring, or relieving compressed air. The hollow shaft inner bore diameter is the inside diameter of the shaft.  An important environmental parameter to consider when searching for ball splines is the operating temperature.

Image credit:

NB Corporation of America