By Ken Bannister, Contributing Editor
“Having implemented an in-house consolidation program, we hope to significantly reduce the number of lubricants that our site stocks and uses. We have a number of greases and gear lubricants of similar viscosity and want to know how to determine their compatibility when switching from one to another.”
The mixing of lubricants—greases or oils—is a major cause of equipment problems. For example, a lubricant containing acidic additives mixed with one containing base or alkaline additives can quickly neutralize the mixed product’s effectiveness and protective ability, and lead to catastrophic lubricant and bearing failure. Similarly, when changing to a new lubricant, depending on compatibility issues, you may or may not be required to flush the old product from the reservoir and bearings with neutral flushing oil prior to filling and using the new one.
Before proceeding with a lubricant “swap,” ALWAYS consult with the manufacturer of the replacement product to determine if it has already been tested for compatibility with the old product, and if not, would the manufacturer be willing to do so on your behalf. If no information is available or forthcoming, and you’re unable to establish compatibility, you can conduct your own tests, as follows:
- Taking samples of both oils, blend three mix samples in a 50:50, 90:10 and 10:90 ratio.
- Send samples to an oil-analysis laboratory and have them tested for filterability, sediment and color/clarity. In addition, have the lab perform a RPVOT (rotating pressure-vessel oxidization test) to determine an oil’s resistance to oxidation, and a storage-stability comparison.
- For accurate results, the tests should be performed three times and the results normalized.
- Ask the lab to assist you in determining any cross-contamination risk.
Note: RPVOT testing can be expensive. You could forego this procedure by having the manufacturer of the new lubricant recommend a neutral flushing oil (if you don’t have too many lube changeovers to complete).
When dealing with greases, a similar process is followed: Samples are mixed in 75:25 and 25:75 ratios then sent to the lab to test for consistency, dropping point and shear stability.
Understanding lubricant compatibility before a swap will protect against catastrophic failure due to incompatibility shortly after the changeover. MT
Lube questions? Ask Dr. Lube, aka Ken Bannister, author of the book Lubrication for Industry and the Lubrication section of the 28th edition Machinery’s Handbook. He’s also a Contributing Editor for Maintenance Technology and Lubrication Management & Technology. E-mail: email@example.com.