Ball Screw Repair
Choosing between rebuilding or replacing ball screws can be a difficult decision to make. New ball screws may be the right choice based on price and availability. It is usually wiser to have the screw reconditioned. Which can save business money and downtime. Often times ordering a new ball screw can take weeks to assemble. While a repair can take only days.
Assessing the Damage
Ball screw failure is often caused by gradual degradation of the screw surface. Or machine crashes that result in a bent screw. Visible damage or lack of functioning will alert you that the ball screw has a problem. Cracks, dents or physical breaks are an indication that the ball screw is not in optimal condition. If the screw is worn out, the backlash will increase and efficiency will decrease. Causing the screw to require extra torque or horsepower to turn. When the screw is worn out, the lead accuracy could be worse than required.
Measuring diametral backlash can determine if a ball screw needs repair. To do this, place the ball screw assembly in V blocks. Then lift the ball nut vertically and with a gauge measure the play between the ball nut and the screw. 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.
Types of Repair
- Level 1: Addresses the most common problems, such as loss of repeatability due to wear. Balls are the first to wear. During repairs, new, larger balls are used at every level to restore preload and repeatability.
- Level 2: Involves regrinding of the ball nut to the steps taken in Level 1.
- Level 3: 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: Adds regrinding of the ball nut and ball screw. This level is the most costly. But can restore a damaged ball screw to a normal new screw lifespan.
What to Consider
When considering a repair service it is important to make sure they hold adequate stock of supplies. Additionally, turn around time and quality are important factors. Some repair services used refurbished balls, which can impact the overall life of a ballscrew. Others may not use a ball a size larger when making repairs. Using a ball a size larger allows for the screw to restore the level of preload and repeatability it once had. Additionally, accessibility to a metals lav and ISO 9000 registration help ensure high quality.