Rail parallelism

Posted by Emelie Neckman, 2024-10-09

Rail parallelism refers to how well two rails follow each other laterally along their length—in other words, how consistent the spacing is between them. If the rails aren’t parallel, internal stresses build up in the linear guides, leading to extra wear and shortened service life.

To avoid this, it's important to follow the recommended tolerance levels for parallelism. These tolerances depend on:

The accuracy of the parts the rails are mounted to
The tolerances of the guide’s components—such as carriage and rail classes

In high-precision applications, frames and parts are typically machined to high accuracy, reducing internal stresses. If you can machine the mounting surfaces for both rails in the same setup, without reclamping, the results are even better. The same principle applies to the surfaces where the carriages are mounted.

Take time to properly align and measure the rails and carriages—you’ll reduce stress and improve performance.

In some setups, one rail is mounted straight and locked against a reference edge (fixed rail), while the other is mounted on a flat surface (floating rail). The floating rail then self-aligns due to lateral forces, achieving near-perfect parallelism.

Linear guides are robust—they can handle heavy loads and still offer long lifespans, even with tolerances beyond what's listed in the catalog.

In simpler setups—like frames made from aluminium profiles that flex under load—rails are often mounted on flat, unreferenced surfaces. Here, the screw fasteners alone determine the rail position. The play between screw holes and screw diameters becomes the effective positioning tolerance. But even in these cases, internal construction forces tend to "pull" the rails into better alignment over time.

So, even if your precision isn’t top-tier, lifespan is usually still more than acceptable. And for the most basic setups (like aluminium profiles), there are more forgiving carriages available—like pendulum or self-aligning carriages, which help reduce built-in stress.