Stop! Don’t throw that worn or bent ball screw out. It may be repairable, not just once but three or four times, saving you 30 to 50% of the cost of a new assembly
Many ball screws are repairable. Common problems, such as loss of repeatability due to wear can be fixed by regrinding the ball thread grooves and then using larger balls in the ball-screw assembly. If a minor crash bends the screw a bit, it can often be straightened to its original accuracy and returned to service. Surface problems such as spalling, brinelling, chipping, or feathering, Figure 1, can be reground and replated. All of these repairs may prove more economical than replacement with a whole new ball-screw assembly.
Evaluate ball screw condition
One of the best ways to determine when a ball screw needs repair, as well as determine approximately how much repair, is to measure its diametral backlash, or lash.
Diametral lash (not axial backlash) is a measurement that can be taken in the plant. The ball-screw assembly is placed in V blocks. An engineer lifts the ball nut vertically and with a gage, measures the play between the ball nut and screw. For a preloaded assembly, a diametral lash of 0.0005 in. indicates a wear factor of 50%. Minimal, or Level 1, repair is needed to bring the screw back into use. A lash of 0.0035 in. represents 80% wear and indicates either a Level IV repair or a dead ball screw. Similarly for nonpreloaded assemblies, a diametral lash of 0.009 in. is 50% wear and needs Level 1 and 0.015 in. is 80% wear and needs Level IV or replacement.
For ball-circle diameters ranging from 0.03934 to 0.04875 in., the acceptable lash is 0.002 to 0.004 in. For diameters ranging from 0.5625 to 0.6250 in., the acceptable lash is 0.004 to 0.008 in.
As noted, a ball screw with 80% or more wear is likely irreparable. There are a few other reasons a ball screw might be irreparable, including:
• Physical damage caused by a crash or mishandling.
• Case hardening of the ball screw is not deep enough to permit the necessary grinding and still provide a hardened track for the balls to ride.
• Replacement balls may be too large for the return system.
• A bent ball screw may crack if straightened because the root of the ball threads was hardened in the manufacturing process.
Some of these limiting cases may be the result of repeated repairs, such as reballing, where the assembly has simply reached the end of its usefulness. (Four repairs is about the maximum). Bury the ball screw with honors.
Levels of repair
When a ball screw arrives at a repair facility, it is inspected and evaluated for the type of needed repair. This process can take up to three or four days.
In general, there are four levels of cost-effective repair. While each successive level adds cost, this cost is still less than a new ball-screw assembly. After Level IV, a new ball screw is required because the old one can’t be repaired or the cost to repair would be excessive.
All repair levels involve the same four basic steps: inspect, clean, reball, and straighten.
• Level 1 (three days) takes care of the most common problem, loss of repeatability due to wear. Balls are the first to wear. Thus, during repair, new, larger balls are used at every level to restore preload and repeatability. The secret to proper ball replacement: for every 0.003 in. of wear, use a 0.001 in. larger ball.
The screw is also straightened because if it has a bow as little as a thousandth of an inch, it can put excess moment on the ball nut, which can later result in failure.
• Level 2 (seven days) adds regrinding of the ball nut to the steps taken in Level 1. Ball-nut thread grooves wear faster than the screw threads because they are subject to more ball travel.
• Level 3 (seven days) adds regrinding of the ball screw threads, and as required, rebuilding of the journal diameters with eutectic spraying and grinding them back to size.
• Level 4 (14 days) adds regrinding of the ball nut and ball screw. This level may cost 55% of a new assembly, but since a repaired ball screw will have a normal new-screw life, it pays to repair.
When the repair cost goes over 65%, buy a new assembly.
These four levels of repair are classified by the most common repairs and do not cover all contingencies.
Putting in the repaired ball screw
Before installing the repaired screw, be sure to make any necessary operation or procedure changes to ensure the same failure doesn’t occur again. For example, if brinelling was detected before grinding repair, it is probable that the ball screw was overloaded. This often happens when a machine is misapplied and the ball screw is not up to the new loads. Re-installing the same screw will only repeat the problem. Either install an assembly with greater capacity (perhaps by using larger balls), a multistart screw for added tracks, or a larger assembly.
If spalling (pitting) was found during inspection, this condition indicates that either contaminants entered the ball nut, lubrication was lacking, or through lack of lubrication seals were not working properly. Correct the cause before reinstalling the assembly.
In general, concentricity and squareness are important to proper assembly and installation. Figure 2 shows the correct measurement points and values. Care must be taken during installation to make sure the end-support bearings are preloaded and positioned properly and that any machining on the ball-screw assembly is compensated for during alignment.
A few points to keep in mind when you receive the repaired ball screw.
• Remember it may not necessarily “feel right.” The feel of a repaired ball screw is often deceptive. Human nature causes us to test a rebuilt unit, by hand, more critically than a new unit. Keep in mind that the balls don’t circulate smoothly until they are loaded and running at a 45 deg contact angle with the ball groove. Feel is subjective; it doesn’t count.
• Torque is often misused as a measure of preload since few companies have the necessary equipment to measure the proper specification — spring rate, which is load (lb) over deflection (in.). Spring rate measures axial movement at a given load. Beyond a certain point, increased torque does not increase the stiffness needed for a particular system’s design. Torque of 1 to 2.5 lb-in. is usually in the right ballpark.
• You can expect the same accuracy from a repaired ball screw as from a new one. Repeatability of preloaded ground and rolled ball screws is the same: about 0.000050 in. In general, the lead accuracy of a rolled ball screw will be 0.004 in./ft and that of a ground screw will be 0.0005 in./ft, although 0.0002 in./ft is achievable when needed.
When evaluating repair services, look for a facility with adequate replacement stock for the different ball screws you use. Also look for machines and operators dedicated to ball screw repair. Next, check for inspection equipment including a spring rate test machine, a laser measurement system for ball screws and a coordinate measurement system for ball nuts. Accessibility to a metals lab and ISO 9000 registration are pluses that will help ensure that your decision to repair is a profitable one. Your ball screw repair service should provide a one year warranty and if requested, certification of the ball screw spring rate, straightness, and squareness of related mounting surfaces.
Ernest J. Machelski is product manager of ball screw repair at Thomson Saginaw Ball Screw Co., Saginaw, Mich.