A while ago, a customer called me. Their CNC milling machine's Z‑axis positioning was getting worse and worse, and backlash measured nearly 0.05mm. He asked if he should buy a new ball screw. I asked him how long the screw had been in use – three years. Then I asked if he had ever checked the bolts on the nut seat. He paused: "Bolts? Do those come loose?"
I told him to try tightening those bolts first. A little later he called back: "I tightened them, and the backlash dropped by half. So it wasn't the screw after all."
This is not an isolated case. When a machine loses positioning accuracy after some use, most people immediately think "the screw is worn." But in about half of the cases, the problem is somewhere else. Today, let's talk about a few common "false failures" of ball screws, to help you save some unnecessary expense.
1. Loose nut seat bolts – backlash comes from there
The ball screw nut is fixed to the slide table through the nut seat. Over time, machine vibration can loosen the bolts. Once the bolts are loose, the nut seat can move slightly, causing exactly the same symptoms as a worn screw – increased backlash and poor positioning.
How to check: Try tightening the bolts on the nut seat with a wrench. If any feel loose (sometimes you can turn them by hand), that's the culprit. After tightening, measure the backlash again – it will often return to normal.
The most striking case I remember: a customer said his screw had failed after six months and was ready to buy a new one. We went over and found two of the four bolts on the nut seat clearly loose. After tightening, backlash dropped from 0.08mm to below 0.01mm. The customer saved a lot of money, and we felt pretty good about it.
2. Loose lock nut – axial play in the screw
The lock nut at the end of the screw secures the bearings and eliminates axial clearance. Machine vibration can gradually loosen it. When the lock nut is loose, the screw can move axially, and backlash increases suddenly.
How to check: Mount a dial indicator against the end face of the screw, then push and pull the slide table. If the needle moves, the lock nut may be loose. Tighten it with a hook spanner, and the problem may be solved.
I've fallen into this trap too. Once, a machine's positioning accuracy was erratic. We checked the screw, the guide rails, the motor – two days wasted. Finally, we found the lock nut loose. Tightened it, everything worked fine. Since then, every time I inspect a screw, I first touch the lock nut.
3. Insufficient lubrication – dry running creates backlash
Another case, not bolts or nuts, but lubrication. When a screw lacks grease, the balls and raceway contact directly, wear accelerates quickly, and backlash develops.
How to check: Remove the dust cover and look at the screw surface. If it's dry, or the grease has turned black and lumpy, lubrication is the problem. Clean it out, relubricate, and run for a while. If backlash doesn't increase further, it was just a lubrication issue – no need to replace the screw.
4. When should you really replace the screw?
Of course, not all backlash problems can be fixed. In these cases, replace without hesitation:
- Obvious spalling pits on the raceway (you can feel the cavities with your finger)
- Screw bent, straightness exceeding 0.05mm/m
- Balls inside the nut are already fractured
- Repair cost exceeds half the price of a new screw
The rule is simple: if you fix it once and it fails again soon after, just buy a new one.
5. A note on the SFU series
One of the most common ball screw families on the market is the SFU series – for example, SFU1605, SFU2010, SFU3210, and so on. This series is popular because it has a mature design, easily available accessories, and good cost‑effectiveness. Many automation machines and ordinary CNC machine tools use this series. When selecting a screw, if the load is moderate and accuracy requirements are not extremely high, the SFU series is a reliable choice. Of course, the exact model still depends on your load, stroke, and speed.
A few honest words
When backlash increases, first check three things: nut seat bolts, lock nut, and lubrication. At least three out of ten times, the problem is not the screw itself. Don't rush to buy a new screw – spend half an hour on inspection first, and you might save a lot of money.
Zhejiang Baili Guide Rail offers ball screw repair and selection services. If you're not sure what's wrong with your machine, send us photos or a video, and we'll help you diagnose it.
