A vertical axis on a grinding machine was running with a heavy-preload linear guide. The axis was smooth, rigid, and precise. The customer’s next machine used the same guide with light preload because “the axis moves faster.” The table wobbled under grinding load and the surface finish went from Ra 0.4 to Ra 0.9. The light preload was the cause. The guide manufacturer’s catalog lists preload grades for a reason — and the reason is that each one serves a different class of machine.
What preload does
A linear guide’s preload is the internal clearance (or interference) between the balls, the rail, and the carriage. With negative clearance (interference), the balls are squeezed between the raceways. The carriage is rigidly located — there’s no free play when the load direction reverses. The stiffness of the guide increases with preload.
The tradeoff: preload increases rolling resistance (friction) and reduces the guide’s life. The balls are under constant stress even when the carriage isn’t loaded. The manufacturer’s rated life is derated for preloaded operation.
| Preload grade | Clearance | Friction increase | Life factor | Stiffness |
|---|---|---|---|---|
| Light (Z0) | Zero to slight clearance | 1.0x | 1.0x | Baseline |
| Medium (Z1) | Negative (interference) | 1.3-1.5x | 0.7-0.8x | 1.5-2x |
| Heavy (Z2) | Strong interference | 1.8-2.2x | 0.5-0.6x | 2.5-3x |
The Z1/Z2 designations are the common Japanese standard grades (THK-style). The numbers vary by manufacturer but the concept is the same: more preload = stiffer, stronger friction, shorter life.
Why the grinding machine needs heavy preload
A grinding machine removes material by pressing the wheel into the workpiece. The grinding force is one-directional but the machine axis must hold position against that force without flexing. If the guide has play, the table lifts a few microns as the grinding force builds, the wheel cuts deeper, and the surface gets wavy. The heavy preload (Z2) gives the guide the stiffness to hold the table flat under a 500 N grinding force.
The math: a light-preload guide with zero clearance but no interference deflects about 5 microns under 500 N at the carriage center. A heavy-preload guide deflects about 2 microns — 2.5x stiffer. On a grinding machine, 3 microns of table deflection under load is the difference between Ra 0.4 and Ra 0.9.
Heavy preload also kills the “ball creep” problem. In a lightly loaded, light-preload guide, the balls can creep sideways in the raceway at low speed. The carriage wanders a few microns. For a grinding axis that must repeat to 2 microns, that wander is a failure. The Z2 preload locks the balls in place.
When light preload is correct
Light preload is for axes that move fast, carry light loads, and don’t need sub-micron positioning. Typical cases: pick-and-place robots (the load is a gripper, the requirement is speed), long-stroke transfer axes, and inspection stages where the axis moves between positions and measures in between (the measurement happens when the axis is still, so no dynamic stiffness needed).
The light-preload guide in the grinding machine’s sister machine wasn’t wrong — it was in the wrong machine. The same guide model, light preload, in a labeling machine runs 24/7 with zero issues. The axis carries a 5 kg labeling head and the tolerance is ±0.5 mm. Nobody can see 5 microns of play on a label placement.
The friction effect on servo tuning
Heavy preload changes the friction profile of the axis, which affects servo tuning. The rolling friction is higher and it’s more stick-slip at low speed. A servo tuned for a light-preload guide will oscillate on a heavy-preload axis (the friction becomes a nonlinear disturbance). The grinding machine’s servo was tuned with the heavy preload in place — the axis is stiff and the friction is consistent, so the loop gains stay low and the axis is rock stable.
The friction also matters for positioning at low speed. A heavy-preload guide has a breakaway friction that’s about 2x the running friction. The servo needs to deliver that breakaway torque cleanly or the axis stalls at start. This is why high-preload axes need properly sized servos — the catalog motor torque should include the breakaway friction of the guide.
The life derating in practice
A light-preload guide rated at 20,000 km of life (the standard L10 rating) drops to about 15,000 km at medium preload and 11,000 km at heavy preload. For a grinding machine that runs 2000 hours/year at 20 m/min average, that’s 2400 km/year. The heavy-preload guide lasts about 4.5 years. The customer replaces the guide at 5 years on the maintenance schedule. The medium-preload guide on the fast axis lasts 6 years. Neither is a problem — the machine is rebuilt at 10 years anyway.
The life derating is why you don’t spec heavy preload everywhere. On a long-stroke transfer axis at 60 m/min, 6000 km/year, the heavy preload would die in 2 years. The medium or light preload lasts 3-5 years. Match the preload to the duty, not to the brochure.
How to check preload on a used machine
When buying a used machine, the preload grade is stamped on the rail or carriage. If it’s not readable, a quick check: with the axis unpowered, push the carriage sideways with a spring scale. Measure the force to move it 0.1 mm. Compare to the manufacturer’s friction spec. A guide that moves too easily (light preload on a heavy-preload machine) has either the wrong grade or worn-out balls. A guide that’s too stiff to move at all has rust or damaged balls. The $20 spring scale check catches both before you buy the machine.
Preload is a stiffness-for-life trade. Heavy preload on a grinding axis holds microns; light preload on a transfer axis saves the balls. The grinding machine’s surface finish didn’t drop because the guide was bad — it dropped because light preload let the table flex 3 microns under the wheel. Pick the grade by the stiffness the axis needs, then check the life derating, then tune the servo to match.
