Mounting linear guides and the limits of screw fastening
Published by Emelie Neckman, 2024-09-25
In previous posts, we looked at linear guide reference edges when mounting rails and carriages. That was mainly about improving accuracy by aligning rails and carriages against reference edges.
But there’s another important reason to include reference edges.
When you fasten a rail to the base with force, friction between the rail and the base creates a resistance that prevents the rail from shifting sideways when lateral forces are applied.
As long as this friction force is greater than the lateral load, the rail won’t slide, and your precision is preserved. But if the lateral load exceeds the friction force, the rail will start to slip—until the mounting screws hit the sides of their holes. This can cause a loss in precision and, if the force is high enough, may even shear the screws and cause the rail to detach.
In applications where there’s a risk of rail slippage, you need a reference edge to absorb those forces. If the load acts in both directions, you need reference edges on both sides.
When should you start thinking about a reference edge against the rail or carriage?
It depends on several factors, including:
- The strength class of the screws you're using
- Whether the screws are tightened with the correct torque
- The mounting direction of the carriage screws
- Unknown forces like vibrations
As a general rule, start considering reference edges for rails when using about 5% of the dynamic load rating (C) from the catalog.
For carriages, the threshold is around 10%—which often means the application doesn’t need reference edges.
Note: This applies to lateral forces.
Another factor to consider is tensile force—that is, if the carriage is being pulled off the rail. If the force is too high, the screws may break. As a rule of thumb, don’t exceed 50% of C under tensile loads. This varies greatly depending on the size and type of carriage. We recommend checking the values for the specific guide carriage in the manufacturer’s catalog.
Here’s a video on the topic featuring Carry Lindholm from Bosch Rexroth:
