Recirculating Ball Screw

This type of power screw is used for high speed high efficiency applications, and is becoming more common. It has become popular because it is essential in CNC, as well as robots and aircraft actuators. The ball screw assembly is as shown below and includes a circular shaped groove cut in a helix on the shaft. The ball nut also includes an internal circular shaped groove which matches the shaft groove. The nut is retained in position on the shaft by balls moving within the groove. When the nut rotates relative to the shaft the balls move in one direction along the groove supporting any axial load. When the balls reach one end of the nut they are directed back to the other end via ball guides. The balls are therefore being continuously recirculated.

The recirculated ball screw has the following advantages and disadvantages to the conventional threaded power screws:

Advantages:

High Efficiency - Over 90%
Predictable life expectancy
Precise and repeatable movement
No tendency for slip-stick
Minimum thermal effects
Easily preloaded to eliminate backlash- with minimum friction penalty
Smoother movement over full travel range
Smaller size for same load
Can be used to create rotary motion from linear motion

Disadvantages:

More expensive of course!
Requires higher levels of lubrication
Tendency to move - (Requires additional brakes if locking is required)
Susceptible to contamination

For the same capacity ball screws are not as rigid as conventional power screw (elasticity of balls) but this is usually trivial compared to shaft elasticity.