It’s no secret that oil contamination has a detrimental effect on bearings in the form of gear abrasion, three-body cutting wear and surface fatigue. In fact, research has indicated 80 percent of bearing failures are caused by particle-initiated failure modes across several industries. Additionally, high water levels in oils contribute to corrosion, reduced lubricating film strength and impeded function of vital additives.
Oil contamination is often combatted through filtration, breathers, and oil analysis, but what about grease contamination? Hard particles in grease often meet bearing and gear surfaces, resulting in similar failure modes. Water in grease can also impact lubricating films along with grease and additive functionalities, but it seems grease cleanliness programs often get put on the back burner.
Why does grease go neglected?
It can be speculated that grease cleanliness gets little attention because the problem is hard to see. When oil encounters moisture, the oil becomes foggy or separates into a free water phase. Even high dirt concentrations will become apparent as a deposition layer if left to settle, although the individual particles may not be visible.
On the other hand, grease contaminants stay suspended within the thickener, concealing even severe cases of contamination. Unfortunately, grease analysis is mostly an afterthought and a tool only used in the most specialized cases. Prioritizing grease cleanliness will not only save costs in the form of grease used, but also in the form of reduced equipment failure and healthier bearings.
Setting Grease Cleanliness Standards
The purpose of keeping grease clean is to safeguard the longevity of the lubricated parts or systems. This is because the lubricant film separating the contacting surfaces—whether rolling or sliding—is usually very thin (less than 1m up to 10m). Particles larger than the film thickness can be detrimental to the smooth running and service life of the bearing.
There are a few military standards and test methods for aircraft grease contaminant limits. MIL-G-81322 is used to rate aircraft grease and MIL-G-81937 is used to rate (ultra-pure requirement) instrumentation grease. The MilSpec for aircraft grease is less than 1,000 particles per cubic centimeter with particle sizes between 25 and 74 microns, and no particles larger than 75 microns. The specification for the ultraclean instrument grease is less than 1,000 particles per cubic centimeter with particle sizes between 10 and 34 microns, and no particles larger than 35 microns. This would roughly equal an ISO code of 19/16.
The European DIN standard 51-825 is also a recognized standard for rating solid contaminants in lubricating greases. The result is a gravimetric rating that measures the amount of particles greater than 25 microns per kilogram of grease.
The grease test used to quantify solid particles in grease, as defined in test procedure DIN 51-813, is performed by pressing 500 grams of grease through a 25-micron mesh screen, capturing the remains from the screen, dissolving the grease portion and then draining this portion through a filter. The filter is weighed and compared to the original filter weight to determine the quantity of particles sized 25 micron and larger that were captured. A successful test result would be less than 20 mg of particles greater than 25 microns per kilogram of grease. This test result limit would be equivalent to an ISO Cleanliness code of 23/19.
Preventing Contamination in Grease
Unlike oil, grease cannot be filtered effectively after formulation for two main reasons. Mixing and flow of a fluid is necessary in filtration to ensure uniform opportunity to remove contaminants from the fluid, which is not typically achievable with grease.
Secondly, normal filtration methods generally disrupt and break down the grease’s thickener, which is designed to give it consistency and keep it in place within the machine. Due to the inability to filter, you have to control grease contamination through careful manufacturing, packaging, handling and clean introduction to the machine.
Figure 1 Particle levels in new greases (green), from grease guns (orange) and from the machine (blue).
The first step to minimize contamination is to ensure grease is clean when it enters the system. Often when a bearing is greased manually, you run the risk of capturing dust and dirt in the grease as it’s being dispensed. A simple solution for this is using a single-point lubricator—by doing so, grease will remain in an enclosed space until it enters the bearing. Singe-point lubricators are also useful in situations where a bearing is dangerous to lubricate, such as heavy-duty air handling units.
While in the machine, it’s important to use effective seals to keep out contaminants or utilize a strategy of grease purging to keep the contaminants at bay. In addition to particles and the damage they can do, you must consider moisture in greases as well. When high moisture is present in oils, phase separation or cloudiness allows you to see these very high levels. An appropriate step would be understanding the moisture levels your new and stored greases are starting with, then working with your suppliers and improving your storage methods to reduce the initial moisture levels as low as possible. By doing so, you stand to gain substantial increases in bearing life and grease performance.
