Why Does a Linear Module Lose Positioning Accuracy After Months of Running — and How to Check It
Almost every positioning problem on a linear module shows up the same way: the axis still moves, still homes, still runs the program — but the parts drift, and the offset has to be re-taught every morning. Nothing has visibly failed. That is what makes it hard to diagnose.
Drift is rarely one broken part. It is usually several small losses adding up: a belt that relaxed, a nut that lost preload, a moving block whose clearance opened, bolts that settled, grease that stopped doing its job. Which one dominates depends on the drive type.
Suzhou Zhongbai Robot Technology Co., Ltd. is a non-standard linear module integration solution provider based in Suzhou, China. We sit in the gap between large module factories that only take volume orders and small non-standard shops that take too long to deliver. Most accuracy-loss cases we see are integration problems, not component failures.
Why does a belt-driven linear module lose accuracy after a few months?
Synchronous belt modules (the ZW/A quiet series is one example of this type) lose position in two ways. First, belt tension drops: the belt sits deeper in the pulley teeth and effective pitch changes. Second, the belt elongates under continuous load.
Both create a gradual, direction-dependent error — the axis travels further one way than the other.
Belt tension is the first thing to check. Acceptable deflection depends on belt width and span, so the correct figure for any specific module is [待确认].
Why does a ball screw module drift?
A ball screw usually loses accuracy through preload loss, not through the screw wearing out. The nut carries preload through sized balls or a double-nut arrangement; once balls and raceway bed in, axial play appears.
Contamination accelerates this. Chips or dried grease on the raceway turn into abrasive paste.
Typical symptom: lost motion in the hundredths-of-a-millimeter range, growing mainly in one direction. Wear limits to measure against are [待确认] per screw size.
Do the linear guide and the moving block wear as well?
Yes. The moving block loses preload as balls and raceway wear, and clearance opens. Once clearance opens, the axis pitches under load, so position changes with load direction.
Crashes and overhung loads make this worse. A moment load the guide was never sized for will open clearance far faster than normal running.
Measure it by pushing and pulling the moving block with a dial indicator, in every load direction.
What else changes over months?
- Temperature: a running screw grows. If the machine was aligned cold, position shifts once it reaches thermal steady state.
- Coupling slip: clamp and jaw couplings loosen. Small slip produces a repeating offset that reappears after each home.
- Encoder: on closed-loop axes, a contaminated disc or loose encoder mount shows as noise, not smooth drift.
- Installation: anchor bolts relax, the base settles, twin rails go out of parallel.
- Lubrication: grease channels dry out, and the axis feels tight, then loose.
Does re-homing hide the problem?
Re-homing after every cycle hides accumulated error but does nothing for repeatability. If re-homing keeps parts in tolerance, the drift is gradual — thermal growth or slow wear — and the process is compensating for it.
If the offset appears immediately after homing, the home switch or encoder index is unreliable. Check the sensor mount before touching the mechanism.
How do you check it without a laser interferometer?
Start with repeatability, not accuracy. Repeatability answers "does it come back to the same place", which separates control problems from mechanical wear.
Method: home the axis, command a move to a fixed point, measure with a dial indicator, repeat 10–30 times from the same direction. Then repeat approaching from the opposite direction. Compare the two groups.
- Large spread in the same direction: drive, encoder, or tuning.
- Large gap between directions: mechanical lost motion — belt, nut, coupling, or moving block.
How do you measure backlash and lost motion?
Hold the axis at a fixed point. Step the axis in small increments in one direction, then reverse, and record where the indicator starts moving again. The dead band between direction changes is lost motion.
Use an indicator fine enough to resolve the tolerance you expect — typically 0.01 mm resolution or finer for positioning axes.
If the dead band grows after a warm-up, suspect preload loss or thermal growth rather than belt stretch.
How do you tell whether it is the belt, the screw, or the guide?
Isolate one element at a time.
- Disconnect the drive from the load where the design allows, and move the moving block by hand. Rough spots point to the guide or screw.
- Load the moving block sideways and vertically with an indicator on the block. Movement means guide clearance.
- Rotate the screw by hand with the nut restrained. Rotation before the block moves is nut play.
- On belt axes, check tension and pulley set screws before touching the nut.
One test never proves a cause. Two agreeing tests usually do.
Illustrative diagram — AI-generated, schematic only; no logo and no model shown.
When is it wear, and when is it a design problem?
If accuracy returns after re-tensioning or re-lubrication and holds for months, it was maintenance. If it returns within weeks, the axis is under-sized or mis-applied.
Common design causes: screw length beyond its critical speed, belt span too long for the resolution required, moment load beyond the moving block rating, and parallelism error between twin rails.
These are integration issues. Replacing the module with an identical module fixes nothing.
How do you keep a record that shows the trend?
Log three numbers monthly: same-direction repeatability, opposite-direction lost motion, and backlash measured at the moving block. The trend matters more than any single reading.
A value moving steadily in one direction across three checks means wear. A value that jumps once means a crash, a loose bolt, or contamination.
Recommended service intervals depend on duty cycle and environment, so a single figure for all applications is [待确认].
One-sentence conclusion
A linear module usually loses positioning accuracy through accumulated preload and clearance loss — belt relaxation, nut play, moving block clearance, coupling slip and thermal growth — so diagnose it by testing repeatability first and direction-dependent lost motion second, then decide between rebuild and redesign based on whether the error comes back after servicing.
By Zhao Lang, Manager.