How Clean Is Your Grease?
EP Editorial Staff | November 29, 2023
Because grease contaminants stay at the wear points, clean grease may be more important than clean oil.
By Mark Barnes, PhD, CMRP, Des-Case Corp.
It has been estimated that as many as 60% to 80% of lubrication-related mechanical failures can be attributed either directly or indirectly to oil contamination, followed closely by water content. Because of the significant impact on asset reliability, much has been written about what oil contaminants do to rotating and reciprocating assets. What is less well documented is how contamination can have an equally significant effect on greased assets.
In most industrial facilities, grease-lubricated assets are much more numerous than those that use oil. This stands to reason since 90% or more of rotating assets are driven by electric motors, the majority of which use grease-lubricated bearings. Add to that other grease-lubricated components and the percentage of all assets that use grease can exceed 60% in most plants.
While grease and oil function similarly at the component load zone by providing an oil-film thickness to separate moving parts, it could be argued that the impact of contamination on greased components is far greater than on oil-lubricated components for one simple reason. With oil lubrication the oil is constantly flowing through the contact zone. Grease is largely retained within the contact zone since this is one of its primary functions. As such, any contamination that enters a grease-lubricated component will tend to remain in the contact zone, causing repetitive cutting wear, fatigue stress, and other particle-induced failure modes.
Unlike oil, with a grease it’s next to impossible to filter out contaminants, i.e., contaminants are likely to stay within the housing. This changes only if excess amounts of grease are pumped into the housing, which can create other problems, particularly with high-speed applications. Because of this, the key to controlling contamination in greased assets is to keep particles and moisture from getting into the component.
This all starts with application. Most greased assets are manually lubricated using a hand- or battery-operated grease gun. Prior to applying grease, the grease zerk, or fitting, should be wiped with a clean, lint-free rag. A small amount of grease should be pumped from the end of the grease gun nozzle and again wiped away. Once regreasing is completed, the dust cap should be pressed back onto the grease fitting or, alternatively, a small dollop of grease should be left on the fitting to prevent fitting contamination. Another simple method to exclude contaminants is to use automatic lubrication systems.
Without special handling, it is well known that new oil is not clean oil. The same holds true for grease.
To minimize the amount of contamination in new oil, a simple step is to purchase lubricants in the smallest-sized container that’s practical for the needed volume. Grease supplied in single-use, 14-oz. tubes will likely be cleaner than grease dispensed from bulk grease kegs.
So how clean or dirty is new grease? Figure 1 shows the results from testing a number of new, unused greases from numerous lubricant manufacturers. Testing involved measuring particles in grease using an optical-imaging device capable of reporting particle sizes and shapes and gravimetrically recording the number of milligrams of particles greater than 25 microns/gram of grease. The data showed a wide disparity in new-grease cleanliness from a low of less than 0.1 mg/gram of grease to more than 10 mg/gram, equivalent to more than 10% of the total weight of grease.
A similar story holds true for moisture contamination. Figure 2 shows the amount of water present in a range of new greases.
Again, there is a wide range of contamination from as little 300 ppm of water to more than 15,000 ppm, or 1.5% by volume. With water, the type of thickener and grease matrix clearly plays a role. While some thickeners, such as polyurea, naturally resist water, others, particularly calcium-based greases, have an affinity for water.
In addition to highlighting the wide variation of contaminants found within new greases, what’s equally interesting is comparing greases of similar formulations from different suppliers. For example, comparing the particle and moisture content from two different lithium-complex greases—by far the most common multiple-purpose greases used in industry—shows that some manufactured greases contain 10 to 15 times more contamination than is found in grease from other suppliers.
The most obvious question is, “Why are new greases so dirty?” Just like oil, the answer lies in the manufacturing process. Greases are usually made in a grease kettle. While some kettles are sealed and operate as a continuous process, others are made in a batch process, in kettles that are left open to the ambient environment.
Even the manufacturing equipment can create problems. The author of the data presented in Figures 1 and 2 reported that one heavily contaminated new grease showed an unusually high preponderance of ferrous particles. Upon further inspection, it was determined that the gear pump used to dispense new grease into containers for distribution was wearing, introducing ferrous particles into the new grease!
While the case for contamination control in oils is well known, little has been done to look at grease cleanliness. Given that much more than 50% of assets in plants are lubricated with grease, maybe it’s time we paid more attention to how clean our grease is before re-greasing critical assets. EP
Mark Barnes, PhD, CMRP, is Senior Vice President, Global Business Development, at Des-Case Corp., Goodlettsville, TN (descase.com). He has more than 25 years of experience in lubrication management, oil analysis, and contamination control.