Linear slides are simple linear motion devices composed of a stationary base and a moving carriage. Linear stages are slides with a drive mechanism that provide controlled, precise positioning along a linear axis. Linear slides and linear stages use several types of bearings and guide mechanisms. Air bearings ride on a thin cushion of air and provide support in only one direction. Ball slides use a bearing system composed of two rows of balls on both sides of the base. Ball spline shafts have ground grooves for the movement of recirculating balls. Crossed rollers are enclosed in rails with machined, V-shaped grooves that are ground at 90°. Dovetail slides are interlocking, direct contact systems suitable for heavy load applications. Hardened way slides or box ways are similar to dovetail slides, but use the direct contact of the base and saddle. Linear bushing systems contain recirculating linear bushings and a shaft, shaft support, and carriage. Linear motion guides or square rail linear bearings include a guide rail and carriage unit. Needle rollers or M/V ways are housed in cages mounted on matching machined rails. One roller is machined in an M-shape. The other is machined in a V-shape. Linear slides and linear stages with plain bearings and sleeve bushings are also available.