Combinations of shafts and linear bushings (or oil-less bushings) as rails and sliders are the simplest form of linear guide mechanism.

（1）Shortcomings of linear guide with shafts and bushings.

Following shortcomings can be noted.

（2）Advances in linear motion guide devices

Simplified depiction of the advancements is as follows.

Explained by simplified pictorial

	a）	A ball spline has ball rolling grooves on the shaft, and free to move only in linear direction. (See [Fig.1])
	b）	A guide rail support is provided under the entire length of the shaft, increasing the shaft rigidity. Mating linear bushing has a lower section removed to account for the guide rail. (See [Fig.2])
	c）	Since a free moving ball retainer is the bearing element, no movement restrictions in either rotation or linear.
	d）	Principally similar in construction to b), but ball rolling grooves are provided on the rail to increase bearing contact surfaces for high load capability.