The Rail That Was Too Stiff (And Too Noisy)
We mounted a profile rail (article 122) on a slide. We chose the highest preload (Z5, the tightest). The slide moved, but it was stiff (hard to push by hand) and noisy (the balls rolled under high preload). The motor had to work harder (more torque). The problem: we over-preloaded the rail. The highest preload is for heavy, high-load applications. Our slide was light (5 kg). The high preload added friction (and noise) with no benefit. We switched to the standard preload (Z0 or Z1). The slide moved smoothly (less friction). The mistake was choosing the highest preload “for precision” without matching it to the load.
Linear guide preload classes balance stiffness against friction. This article covers the selection.
The Preload Classes
A profile rail (article 122) has a preload (how tightly the balls are pressed between the rail and the block). The classes (ISO) are:
- Z0 (light preload): Minimal preload. Low friction. For light loads, low precision (a conveyor, a drawer).
- Z1 (standard preload): Standard. Moderate stiffness. For general automation (a pick-and-place slide). The default.
- Z3 (medium preload): Higher stiffness. For moderate loads, some precision.
- Z5 (heavy preload): Maximum stiffness. For heavy loads, high precision (a machine tool). High friction (noisy, needs more torque).
Step 1: Match the Preload to the Load
The preload is chosen based on the load (not the desired precision alone). A light load (5 kg) on a Z5 rail is over-preloaded (the friction is high, the life is reduced). A heavy load (50 kg) on a Z0 rail is under-preloaded (the rail deflects, no stiffness).
For a general automation slide (10–30 kg), use Z1 (standard). For a heavy, precision axis (a machine tool), use Z3 or Z5. For a light drawer (5 kg), use Z0.
The preload rule: Don’t choose the highest preload “for precision.” The rail that was too stiff had Z5 on a 5 kg slide. Use Z1 (standard) for general automation. Use Z5 only for heavy, high-precision axes. The preload is matched to the load, not the ambition.
Step 2: Stiffness vs Friction
Higher preload = stiffer (less deflection under load) but more friction (the balls roll under pressure). More friction means:
- The motor needs more torque (to push the slide).
- The rail is noisier (the balls roll hard).
- The rail life is shorter (the balls wear faster under high preload).
For a light load, the stiffness gain is negligible (the load doesn’t deflect the rail anyway). The high preload only adds friction. Use the lightest preload that gives the needed stiffness.
Step 3: Mounting (Parallelism)
A high preload rail requires precise mounting (the rail must be parallel, article 122). If the rails aren’t parallel, the high preload binds (the block can’t move). A Z1 rail tolerates some misalignment. A Z5 rail needs perfect parallelism. If your frame isn’t flat (article 107, 146), don’t use Z5 (it binds).
Step 4: The Effective Load (Article 56)
The preload is also affected by the effective load (article 56). A high preload reduces the rated life (the preload adds to the load). For a long-life requirement (article 55), use a lower preload (longer life). For a short, precision move, a higher preload is OK.
| Preload | Friction | Best For |
|---|---|---|
| Z0 (light) | Low | Light loads, drawers, conveyors |
| Z1 (standard) | Medium | General automation (default) |
| Z3 (medium) | Higher | Moderate loads, some precision |
| Z5 (heavy) | High (noisy) | Heavy, precision (machine tool) |
A Preload Checklist
- What is the slide load? (kg?)
- Is it light (<10 kg)? Use Z0 or Z1?
- Is it general (10–30 kg)? Use Z1?
- Is it heavy/precision? Use Z3 or Z5?
- Is the mounting parallel? (Article 122?)
- Does the rail bind? (Misalignment?)
- Is the friction acceptable? (Motor torque?)
- Is the noise acceptable? (Z5 is noisy?)
- Is the life needed? (Lower preload = longer life?)
- Did you over-preload “for precision”? (Back off?)
The Bottom Line
Linear guide preload classes match the preload to the load. The rail that was too stiff had Z5 on a light slide. Use Z1 (standard) for general automation. Use Z5 only for heavy, precision axes (with perfect mounting). Higher preload adds friction, noise, and shortens life. The slide that moved smoothly wasn’t the highest preload — it was matched to the load.
