What does a ball bush on a linear shaft give?

Frictionless motion of the linear shaft.

Let's make an axial groove on the shaft, which groove corresponds to the radius of bush balls. Now we got a frictionless linear movement, combined with two important characteristics:
 

•      rotary torque-free transmission and

•      higher load capacity.

This is a ball spline.
A linear bush (ball spile nut), that could have greater load capacity, and a shaft, which is rotatable, contrary to a rail profile.

A perfect system for both fast movement and quick rotation.

If you want to read more about the ball spline, as

• the factors that affect the proper functioning,
• the grooves and the balls how influence the traffic load and tolerance,
• the difference between the installation variations,
• and much more information

check out the following document!