Superior Ball Screw Repair

Ball Screw Repair Specialists > Why Ball Screws Fail

What part of a precision ball screw wears out and why? If you are experiencing excessive backlash in your ball screw, then likely the balls and the thread radius in the ball nut and the screw are starting to wear. This problem occurs for one or more of the following reasons:

· Improper lubrication or absence of lubrication.
· Contamination.
· Misalignment.
· Counterbalance not working (on vertical axis machines).
· Machine is being pushed beyond the capabilities of the ball screw.
· Ball screw has been abused, damaged, tampered with or assembled with excessive preload.
· Normal use over an extended time period.

Solution: This problem with wear can usually be corrected by abrasive lapping or in more extreme cases regrinding the thread radius. In addition, the worn balls are replaced with over-sized high grade chrome steel balls. If your machine is not positioning correctly and the ball screw assembly is found to have backlash, it is advised that the ball screw be removed and rebuilt before it is worn excessively or damaged beyond repair.

What causes a precision ball screw to lock up or skid? Ball nuts are engineered with a return system that re-circulates the balls internally or externally depending on the design. If the return system is badly damaged, it often causes the ball nut to seize up on the screw. In some instances, the return system is only slightly damaged preventing the balls from circulating. This usually results in damage to the screw because the balls skid around the ball grooves instead of rolling. Return systems get damaged for the following reasons:

· A damaging collision on the machine axis.
· The machine table or slide is bumped with a forklift or other heavy object.
· The ball screw has been abused, damaged, or tampered with.
· Normal use over an extended time period.

Solution: When this happens, the ball screw assembly must be reconditioned or repaired immediately. If rotation of the ball screw or ball nut is forced, permanent damage will likely occur to the return system and the ball screw could require more extensive repairs or replacement.

What steps should be taken when the ball nut has been run off the end of the screw by mistake? This is a common occurrence during machine tool servicing. Some balls usually fall out of the ball nut assembly and the service man is often in a panic trying to collect them up and insert them back into the ball grooves in an attempt to fix the problem.

· Do not attempt to rotate the ball nut back on the ball screw.
· Collect up as many balls as you can and any other parts that come out of the ball nut.

Solution: Remove the ball screw from the machine. Attempting to reassemble the ball nut without an understanding of exactly how the return system works usually causes permanent damage. If the screw has little or no wear and no damage, it can be re-balled by a reputable ball screw repair shop for much less than the cost of a complete rebuild or a new assembly.

Important: If your precision ball screw assembly does not seem to be functioning properly, it is more economical to have it looked at by a qualified repair shop right away, rather than wait until it is damaged beyond repair.