Knowledge · Linear Motion
Preload on linear guides: choosing C0, C1, C2 and C3 correctly
Preload decides how stiff and how clearance-free your linear axis runs, and it drives friction and service life. More of it buys stiffness but costs running performance, and every class beyond the actual need shows up as friction and wear rather than precision. The recommendations are checked against Bosch Rexroth catalog data and real PTS order data.
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What preload means on a linear guide
Preload means there is no longer any clearance between runner block and rail; instead, a defined internal force is at work. It is achieved with oversized balls: Rexroth fits balls into the runner block whose diameter is slightly larger than the free space between the raceways. The balls are therefore permanently pressed against both raceways. The result is negative bearing clearance, a state with no play at all.
The practical benefit: a preloaded guide responds immediately to a load reversal, without the dead travel a guide with clearance would first have to cross. At the same time stiffness rises, because the preloaded balls deform less under load. For machining, metrology and any reversing motion, that is exactly the precondition for precision.
The classes C0, C1, C2 and C3 at a glance
Rexroth grades preload into four classes from C0 to C3. The class sits as an identifier in the runner block type code (see the article Rexroth type code). Basic rule: the higher the class, the stiffer the guide, but also the higher the displacement force and friction.
| Class | Preload | Stiffness | Typical application |
|---|---|---|---|
| C0 | No preload, slight clearance (1 to 10 µm) | Baseline | Handling, safety guards, non-critical adjustment axes |
| C1 | Moderate preload, clearance-free | Increased | General machine building, standard axes, widest use |
| C2 | Average preload | High | Machining, alternating loads, vibration-prone and single-rail axes |
| C3 | High preload | Very high | High-stiffness machining, high moment load |
C0 runs without preload and with a slight clearance of 1 to 10 µm between block and rail; with two rails and several blocks this clearance is usually equalized by the parallelism tolerances. C0 is the right choice when smooth running matters more than stiffness, or when the mounting surface is not perfectly flat. The guide becomes clearance-free from C1 upward, the pragmatic standard: noticeably stiffer than C0, yet still tolerant of small mounting inaccuracies. C2 and C3 belong in stiff, precise machines whose mounting surfaces can actually absorb the higher preload.
Stiffness, displacement force and service life
Higher preload is not a free win. As stiffness rises, so do displacement force and friction, which burdens the drive and generates heat. And preload feeds into service life: Rexroth states as a limit that preload force should not exceed one third of the bearing load F, otherwise service life drops. Reaching for C3 without need therefore throws away running performance. How preload works out in the calculation is shown in the article Calculating service life.
Conversely: if vibrations occur, Rexroth explicitly recommends a higher preload of at least C2. The preload then damps the axis and keeps it from oscillating. Choose preload as high as the stiffness demands, and not one class higher.
Choosing preload and accuracy class together
Preload and accuracy class cannot be combined freely. The low preloads follow the tolerance of the rail: on class-N rails it stays at C0 or C1, since their larger tolerances would create constraint stresses under higher preload. Conversely, the high preload C3 requires one of the tight classes, by manufacturer rule SP, XP or UP; in the PTS online range it can currently be ordered with SP and XP. What each accuracy class means in tolerance is spelled out there.
| Preload | Combinable accuracy classes |
|---|---|
| C0 | N, H |
| C1 | N, H, P, SP, XP, UP |
| C2 | N, H, P, SP, XP |
| C3 | SP, XP |
Which preload for which application
C1 is the most-chosen class and covers general machine building. As soon as stiffness and freedom from vibration take priority, for example in machine tools, C2 is the usual standard. C0 and C3 round out the scale at either end.
- C0 (no preload): handling, safety doors, adjustment axes, applications with an uneven mounting surface or where smooth running matters.
- C1 (light): general machine building, manual axes, packaging, most standard axes. Robust, most-chosen default.
- C2 (medium): machine tools, machining, alternating and shock loads, vibration-prone structures; also recommended for single-rail systems.
- C3 (high): high-stiffness machining centers, large moment loads, high accuracy classes only.
For a self-built CNC mill, C1 is the right choice in most cases. It runs clearance-free, is stiff enough for clean cutting results, and forgives small imperfections in home-made mounting surfaces. C2 only pays off when you machine steel, fight chatter marks, and your machine bed is genuinely stiff and flat. C0 is enough for light gantries that only cut wood or plastic. Too much preload on a not perfectly flat home build creates constraint stresses, raises friction and costs service life without improving precision.
Set the preload in the configurator
In the linear configurator you choose the preload per runner block and see price and availability instantly per system line. Every combination of preload and accuracy class is checked live as you go.
Prices and stock are visible in the catalog without signing in.