How to compensate for alignment errors with profile rail guides

When an application has uneven mounting surfaces or the potential for alignment errors, the typical linear guide solution is to use round shafting and ball bushings, which can tolerate imprecise mounting and misalignment. Profiled rail guides, on the other hand, require precision mounting to ensure even load distribution among the recirculating balls. For this reason, profiled rails are most commonly used in applications that have machined mounting surfaces and tight tolerances.

But some applications require the load carrying capacity and stiffness that only profiled rail linear guides can offer, but in an area where preparation of the mounting surface and precision alignment aren’t feasible. In these cases, profiled rail linear guides with self-alignment capability can satisfy the application requirements and tolerate misalignment, without compromising the performance of the guide system.

Raceway arrangement matters (sometimes)…

One of the fundamental design principles of profiled rail guides is the raceway arrangement. This refers to how the load-carrying balls and rail raceways are arranged relative to each other, and how the resulting contact lines are positioned. The two raceway arrangements found on profiled rail guides are face-to-face (also referred to as the “X” arrangement) and back-to-back (also referred to as the “O” arrangement).

Linear Bearing Design Face-to-Face Alignment Errors

The face-to-face design is also known as the “X” arrangement because the contact lines between the balls and the raceways point inward, forming an “X” inside the rail.
Image credit: Bosch Rexroth

Linear Bearing Design Back-to-Back Alignment Errors

The back-to-back design is also known as the “O” arrangement because the contact lines between the balls and the raceways point outward, forming an “O” around the rail.
Image credit: Bosch Rexroth

The primary difference between these two designs is the length of the moment arm created by the reaction forces when a moment load is applied to the bearing. The face-to-face arrangement has a much shorter moment arm (roughly half the length) than the back-to-back arrangement. This means that its moment load rigidity is lower than that of a bearing with a back-to-back arrangement, but the lower rigidity allows the face-to-face design to compensate for misalignment.

The face-to-face (“X”) raceway arrangement can compensate for higher misalignment errors than the back-to-back (“O”) arrangement can.
Image credit: SKF

This is especially useful in applications that use two rails in parallel, where there may be a height deviation between the carriage mounting surfaces. And, because the face-to-face arrangement is used on both two-row and four-row bearings, miniature bearings—which are commonly two-row designs—also have this self-aligning capability.

…but not always

Even though profiled rail bearings that use the back-to-back (or “O”) arrangement don’t have an inherent self-aligning capability, there are ways to modify these bearings so they can compensate for misalignment. Bosch Rexroth is one manufacturer whose profiled rail bearings use the back-to-back arrangement. They’ve created a self-aligning version of this design by allowing the load plate, which is a precision steel strip that the load-bearing balls ride on, to pivot on its supporting surface inside the housing. This ensures uniform loading on the balls that are in the load zone, thus avoiding premature wear and reduced service life.

The design of Bosch Rexroth’s Super Runner Block ensures that the load on the internal balls is relatively even (shown here at 2, 5, and 10 degrees of misalignment) to avoid premature wear.

Hybrid designs also work

Another custom self-aligning profiled rail design comes from THK. In their version, the guide block (referred to as an LM block) is made of cast iron and houses a cylindrical spline nut. The nut is partially cut away in order to conform to the profiled rail, and the cylindrical cross-section allows the block to self-align. The main application for this design is to compensate for alignment errors between two rails in parallel. It is also a good solution for what THK refers to as “rough” mounting, with unmachined surfaces or lack of alignment between rails.