It is common practice to use different materials for the bearing and the guide rail; with the guide rail, it is usually harder to reduce friction. Material wear is concentrated in the contact surface of the linear bearing, which is the softer component. The guide rails, shafts, and bases (for plain linear bearings) are commonly constructed from hardened steel, ground steel, and anodized aluminum, which are all harder types. http://www.remotioner.com/bearing/rod-end-bearing/