Oil vs. Grease Lubrication

Customers often ask us: what type of lubricant is best for my ball screw application? And which is better, oil or grease? The truth is that both oil and grease can perform well in a variety of situations.

If the duty cycle is such that an elastohydrodynamic (EHD) film can build over a significant part of the motion, then oil lubrication (with a properly selected grade and viscosity) will always outperform grease in terms of wear. Only oils with wear inhibiting additives should be used.* These have the ability to lubricate in conditions of boundary friction, when speeds for EHD-lubrication are insufficient
On the other hand, grease has an edge at slow speeds because it offers better wear protection under mixed friction or boundary friction conditions. Also, lubricant loss with grease is lower than with oil, so grease can be used for long-term or “for life” lubrication.  Attention must also be given to the wiper choice in all cases.
Steinmeyer has extensively studied the performance of a wide variety of oils and greases. Some of this work was conducted by the tribology lab of a major European research center. Details are available on our web site. This test set-up was designed to mimic the specific tribological conditions found in industrial ball screws, both in terms of lubrication film build-up and wear.
One key finding was that the coefficient of friction does not seem to be correlated to the wear rate. Some lubricants yielded low friction, but higher wear at the same time! With the exception of a high-pressure grease, which caused the lube film to collapse at high speeds, all greases performed well and yielded acceptable wear rates throughout the test scenario. At the same time it was proven that greases are unable to build a perfect fluid film like oils of proper viscosity, so the wear rate with grease lubrication is higher than the wear rate of oil when an EHD film is present.
So oil and grease both have different advantages. The final choice will depend greatly on your application and desired maintenance schedule. Please contact Steinmeyer for a detailed consultation with our engineers!

* We strongly recommend CLP grade gear oil per DIN 51517-3 or equivalent. Do not use way oils or hydraulic oils, even if they are labeled “high pressure”!  Look for the words “suitable for gear boxes”.

Learn more: http://www.steinmeyer.com/en/technology/drive-technology/lubrication-and-wipers/

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