Lubrication Strategies: Beware Grease Incompatibilities

The table shows the results of compatibility tests on 10 different types of grease. The“worked penetration test” was used because it is rapid and reliable. Each tested grease proved to be incompatible with at least one other grease. Courtesy NSK Americas.

When two incompatible greases are mixed, either deliberately or accidentally, the results can be disappointing, if not devastating.

By Jane Alexander, Managing Editor

The problems associated with grease incompatibility manifest themselves in various ways. According to bearing manufacturer NSK Americas, Ann Arbor, MI, scenarios include:

• A lab technician tests grease from a problem bearing and finds that, while it meets all specifications, it doesn’t perform as it should.
• A mill switches grease based on glowing reports from other facilities, then finds the new product doesn’t deliver the promised results.
• A critical plant motor fails during rush production, despite having been lubricated as specified in the maintenance manual.

In each case, NSK representatives said, these sites had changed from one type of grease that met specifications to another type, which also met specs. They all fell victim to grease incompatibility.

Although maintenance organizations should be well aware of this information, not everyone remembers that some greases can’t be mixed with other greases—even when both types meet specifications. Unless incompatibility is understood and accounted for, switching grease can be catastrophic.

The table shows the results of compatibility tests on 10 different types of grease. The“worked penetration test” was used because it is rapid and reliable. Each tested grease proved to be incompatible with at least one other grease. Courtesy NSK Americas.2

Defining ‘incompatible’

Incompatibility occurs when a mixture of two greases shows properties or performance significantly inferior to those of either grease before mixing. Some grease bases are intrinsically incompatible. Different fatty acids and/or additive packages also affect compatibility. To make things even more confusing, sometimes two types of greases that are manufactured as a mixed-base product are incompatible when mixed in operation.

Unfortunately, issues associated with grease incompatibility usually aren’t apparent until the bearing is in use. At that point, major problems can develop. Thus, it’s best to know in advance which types of greases can be used together and which can’t (see table).

Make grease changes safely

What if switching grease is necessary? Several steps ensure safe changeovers. The good news is that incompatible greases don’t have to be eliminated completely. Preparing carefully and paying close attention to details can prevent problems:

• Ask your lubricant supplier(s) about product compatibility.
• Use up as much of the old grease as possible before introducing the new grease into the system. The ideal course of action is to completely drain and clean the system before changing over.
• Once the new product is added, grease flow should be increased temporarily. This will move the interface (the area of grease mixing) through and out of the system as quickly as possible. The increased flow also assures good lubrication and proper sealing while overly soft grease may be in the bearings.
• When in doubt, expect incompatibility and watch for problems. MT

Learn More

NSK Americas

National Lubricating Grease Institute