Selecting a Lubricant for Ball Screws

Lubricants on ball screws are as important to their performance and life as are their operating loads, speeds, accuracy, environments, and power requirements. Lubricant reduces the already low friction by minimizing contact between the balls and grooves, thus adding torque and increasing efficiency, while extending the screw’s life by a factor of 10. Proper selection and application of lubricants also affects contamination, a leading cause of premature ball-screw failure.

Oil or Grease

The right choice of oil or grease for ball screws reduces unscheduled downtime and repairs by ensuring the assemblies deliver their expected service lives. Pumps and metering devices deliver oil; grease gets applied through fittings on the ball nut or attachment flange. The main application differences are in flow control, risk of contamination, and cost.

It is easier to direct oil exactly to where it is needed. It will also clean out moisture and other contaminants as it runs through the ball nut and provide cooling. On the downside, oil requires an investment in pumps and meters, and excess oil can introduce contaminants, for example, by mixing with the cutting fluid in a machine.

Grease gets applied directly to the screw’s threads near the root of the ball track or pumped directly into the ball nut if there are lube holes. Grease fittings contain lubrication and repel contamination. They also cost less and are cleaner than oil pumps and meters. But grease is hard to keep inside the ball nut and tends to accumulate at the ends of ball-nut travel, where it attracts chips and abrasive particles.

Excess grease must be removed, leaving only a thin lubricating film on the screw shaft. Wipers may be needed to keep the grease inside the ball nut, but it is crucial to re-lubricate the assembly with grease at least every 600 to 800 hr for most applications. Incompatibility of old grease with re-lubrication grease could become another problem.

Both oil and grease fittings can be equipped with filters if precise motion and smooth, quiet operations are needed. Filtration also improves the homogeneity of the thickening agent and removes virtually all particulate matter.