Machine Design

Rails Keep Linear Bearings on the Straight and Narrow

Combining rails with different raceway styles solves parallelism problems in linearmotion systems.

Edited by Martha K. Raymond

Andrew Cook
General Manager
Rollon Corp.
Sparta, N.J.

Linear-motion systems operate most efficiently when rails and sliders aren’t under a lot of friction and stress. The truth is, engineers must deal with some friction on most of these systems. For example, friction can be a problem when the mounting surfaces for the rails are not parallel. This pushes the sliders out of alignment which adds stress and leads to slider failure. Setting up parallel linear-motion systems is critical, and most often a timeconsuming step.

But a simple way to solve parallelism problems while maintaining the system’s original preload setting combines rails with different raceway styles in one linear system. One such system is a two-rail, self-aligning linear system, called Compact Rail High-Performance. The system can reduce assembly time and tolerate some mounting and structural errors. The design makes the system well-suited for applications handling loads up to 10,000 lb and where linear precision within ±0.003 in. is needed without spending time machining mounting surfaces.

The rail system comprises a wide range of rails and sliders with precision, high-load capacity, and fast and silent movement. The type of misalignment expected in the system, whether axial or angular, determines the rail-system design. The three different types of rails, T, U, and K, each have a particular raceway shape. For example, parallel errors in axial assemblies are eliminated with the T + U system.

The reason this combination works is because the rails of the U-series have flat raceways, offering lateral freedom to the slider. The maximum axial displacement of the slider in a U-rail is given by S1 and S2. S1 is the maximum available displacement of the slider toward the inner part of the rail, while S2 is the maximum displacement toward the outer rail, considering the nominal dimension of the rail and slider unit, Bnom.

When a T + U rail combination is used, the slider in the T-rail guides the movement and supports loads, while the slider in the U-rail compensates structural or assembly parallelism errors while still sharing its part of the radial load.

For example, a pair of T + U rails let the sliders work correctly even when the angle between the inner and outer lanes is not zero. Calculate the maximum value of the angle between the two lanes using a known rail length. The distance the slider can move in U-rails from the maximum inner position S1 to the maximum outer position S2 is:

a = arctan (S*/L),

where S* = S1 + S2 in.; and L = rail length, in.

While the T + U system solves axial alignment problems, the K + U system resolves parallelism problems in almost all directions. The K rail guides the movement, carries the load, and accommodates structural and assembly errors. The U-rail carries radial loads and allows the slider to overcome axial parallelism errors. Because the slider can tilt up to 2° in and out of the rail, two rails mounted horizontally 36 in. apart could have over 1-in. difference in mounting height along the horizontal plane without creating more friction and compromising slide quality and life.

The K-slider can tilt because of the design of one of the rail raceways and the roller. The shape of the bottom raceway guides and contains the rollers. The top raceway contains a radius which allows the roller more freedom of movement without modifying the original preload setting.

© 2010 Penton Media, Inc.

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