Every month we get a call that starts with the same sentence: the pitch is still in spec, but it sounds wrong under load. Nine times out of ten the noise is not the lead. It is the recirculation.
What the numbers hide
A laser interferometer measures positional accuracy along the axis. It tells you nothing about the return path inside the nut, where the balls swap lanes. Once a return channel starts to score, the balls no longer roll cleanly. You get audible tick, a temperature rise after twenty minutes of continuous cutting, and a small but stubborn increase in following error at rapid.
What to check before you condemn the screw
- Take a listen at idle rapid, both directions. A one-sided rumble is a strong tell.
- Log axis load over a two-hour production run. A creeping baseline points at the nut, not the drive.
- Pull the wiper covers and look for grease that has gone chalky rather than glossy. Chalky means it has been running hot.
Ballbar readings looked fine. Vibration analysis flagged the Y axis inside a week. That was the nut, not the screw.
If it is caught early, a nut-only rebuild is a two-day turnaround and a fraction of the cost of a full screw and support-bearing pack. Left running, you are on borrowed time before it seizes mid-cycle.