Tolerance classes on linear guide rails
2024-05-28
There are different tolerance classes when it comes to linear guide rails. These classes are defined by a common standard that specifies how large the manufacturing tolerances are allowed to be for the installation dimensions.
In practice, there are four types of tolerances that are evaluated.
The first one (H) is the difference in total height from the top of the carriage to the bottom of the rail, compared to the nominal dimension. So, for example, if a carriage and rail combo has a catalog height of 32 mm, the actual measurement can vary between 31.9 and 32.1 mm in the lowest tolerance class. In the highest tolerance class, however, it must be between 31.995 and 32.005 mm. That’s a tight window!
The second tolerance (A3) controls how much the measurement can vary sideways between the edge of the rail and the edge of the carriage.
The third tolerance deals with how much the dimensions are allowed to change if you only replace the carriage on an existing rail.
The fourth one governs how much the height and side-to-side measurements can vary during motion along the rail.
When it comes to the dimensions between the rail edge and the carriage edge, reference surfaces come into play. This means you need to know which side is the reference when performing a precise installation. On the carriage, the reference side is usually marked with a shiny, machined edge.
Rails come in a few variations:
- Some are ground so precisely that both sides can be used as reference surfaces.
- Some have a line marking either on the reference surface or on the underside of the reference side.
- Others may have the material number engraved on the non-reference side.
If both sides aren’t reference surfaces, you need to be extra careful when using an asymmetrical hole pattern on the rail.
With Bosch Rexroth linear guides, you can replace the carriage on an existing rail without having to replace the rail—100% interchangeability. That’s not the case for all brands. With some, you’ll need to swap both the rail and the carriage, which can be much more time-consuming in the event of a failure.
Here's a video on the topic featuring Carry Lindholm from Bosch Rexroth:
