Knowledge · Linear Motion
Accuracy classes on linear guides: N, H, P, XP, SP and UP compared
The accuracy class sets how tight a linear guide's tolerances are, from the height of the runner block to parallelism across the whole rail. Five criteria define the tolerance, and the six classes from N through H, P, XP and SP to UP grade it by the verified values in the Bosch Rexroth catalog. With that you choose the class your application really needs, without paying for unnecessary precision.
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What an accuracy class defines
The accuracy class describes how tightly the dimensional tolerances of a runner-block-and-rail system are manufactured. It sits as an identifier in the type code (see Rexroth type code). Rexroth manufactures ball rail systems so precisely that any runner block is interchangeable on any rail of the same size and class. There are six classes for the runner block and five for the rail: the runner-block class XP runs, by manufacturer design, on a rail of class SP.
Higher accuracy means tighter tolerances, but also a higher price and longer lead time. The right choice is therefore the lowest class that still holds your tolerance chain, not the highest available.
The five tolerance criteria
An accuracy class combines several individual tolerances. Five criteria are decisive:
- Height tolerance (ΔH abs): the permissible deviation of the runner block height H above the rail, measured over the entire rail length and for any combination of runner block and rail.
- Height difference (ΔH rel): the maximum height difference between several runner blocks at the same rail position. Decisive when two or more blocks carry load in parallel.
- Width tolerance (ΔA3 abs): the permissible deviation of the lateral dimension A3 from the reference edge, likewise over the entire rail length.
- Width difference (ΔA3 rel): the maximum difference in lateral dimension A3 between several runner blocks at the same position.
- Running parallelism (P1): the parallelism of the runner block running surface to the reference edge of the rail over the travel. P1 depends on rail length and is stated by Rexroth as a curve over length.
Tolerance values by accuracy class
The following table states the verified tolerance values for steel ball rail systems (BSHP). The values apply when runner block and rail are ordered in the same class. For the steel guide, height difference and width difference share a single value.
| Class | Height tolerance ΔH abs | Width tolerance ΔA3 abs | Height/width difference ΔH rel, ΔA3 rel |
|---|---|---|---|
| N | ±100 | ±40 | 30 |
| H | ±40 | ±20 | 15 |
| P | ±20 | ±10 | 7 |
| XP | ±11 | ±8 | 7 |
| SP | ±10 | ±7 | 5 |
| UP | ±5 | ±5 | 3 |
Reading example: a class-N runner block may deviate in height by ±100 µm, a class-UP block by only ±5 µm, that is twenty times tighter. From class to class, every criterion gets tighter. Class N is therefore not imprecise but the standard class for general machine building; the high classes are reserved for metrology and machining tasks where a few micrometers count.
From N to UP: which class for what
- N (normal): general machine building, handling, packaging, standard axes. The most-chosen class.
- H (high): raised requirements, standard machine tools, axes with several runner blocks.
- P (precision): precise machining, good positional repeatability, demanding machine building.
- XP: special runner-block class on an SP rail for very high travel accuracy at lowest pulsation, for example precision scanners and eroding technology.
- SP (super precision): high-precision machining, grinding machines, measuring tasks.
- UP (ultra precision): measuring machines and applications with the highest accuracy requirements.
Combining accuracy class and preload
Accuracy class and preload are linked. Two rules are fixed: for rails of class N, Rexroth recommends preload C0 or C1, because at high preload the larger tolerances would otherwise create constraint stresses. And per the manufacturer rule, the high preload C3 is reserved for the high accuracy classes SP, XP and UP; in the PTS online range, C3 is currently available with SP and XP. How to choose the matching preload is covered in the article Choosing preload.
As a rule: runner block and rail belong in the same accuracy class. Only then is the resulting system tolerance clearly defined, and the values from the table above hold unchanged. The only exception provided by the manufacturer is the runner-block class XP, which is designed for the SP rail.
Choose the accuracy class in the configurator
In the linear configurator the selection offers only valid class combinations. Runner block and rail are kept in the same class, and impermissible pairings with the preload cannot be selected at all.
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