The Enduring Grease vs. Oil Debate

Jim Fitch, Noria Corporation

Many who claim to be purists tout that they prefer oil over grease. But then again, there are those die-hard grease advocates who have their own orthodoxy. Indeed, both schools of thought are supported by strong, compelling arguments. It seems for every point in support there is an equally strong counterpoint. As it has been for decades, the grease versus oil debate does not die or even fade away. Let’s hear what they are saying today, including several classic arguments along with a couple of new ones.

In Favor of Grease

1. Grease has superior stop-start performance. When a machine is shut down, oil will drain back to sump but grease remains in the component where it is needed, lowering the risk of a dry start.

Oil’s Counterpoint. Most bath, splash and circ-oil systems can lubricate almost immediately on restart. Larger volumes of oil (compared to grease in the same application) means a larger supply of additives (extended service life) and the ability to wash contaminants away from the working frictional zones.

2. Worn seals and connectors can retain grease better than oil, which lowers the risk of lubricant starvation and leakage. This also reduces the risk of the lubricant staining or damage to the work product (food, newsprint, textile, etc.).

Oil’s Counterpoint. True, grease is less prone to leakage, but then again leakage is a sign of a machine that is exposed (internally) to the environment and needs repair. Worn seals that go unnoticed don’t get better over time. This presents risk of a more serious or even catastrophic failure in the future.

3. Surplus grease packed tightly around seals and connectors serves as a sealant, preventing the ingress of particles and water. Periodic regreasing can purge contaminants out and away from the working surfaces of the component.

Oil’s Counterpoint. Most industrial machines are not regreased frequently enough to rely on this for displacing contaminants from seals and cavities adjacent to bearings. In fact, the practice of regreasing may actually drive these contaminants directly into the core of the bearing and cause imminent failure.

4. Grease enables the use of solid additives such as graphite, zinc oxide or molybdenum disulfide. These additives would settle or become filtered if used in many oils.

Oil’s Counterpoint. Soluble additives that are used in oil formulations today, will provide comparable performance to control both adhesive wear and abrasion in similar applications.

In Favor of Oil

5. Unlike grease, oil flows freely, enabling it to conduct and carry away unwanted heat (thermal convection). This keeps base oil viscosity stable and reduces the risk of heat-induced oxidation and additive depletion.

Grease’s Counterpoint. When properly selected for a given application, the temperature of grease will remain reasonably low and out of risk of premature oxidation. Many lubed-for-life grease applications have exceeded ten years of service.

6. Unlike oil, the churning of grease in gears and bearings results in high energy-consumption losses and heat generation. Energy consumed by oil in a similar application may be only a fraction of that of grease.

Grease’s Counterpoint. When bearings are properly lubricated with the right amount of grease having the correct consistency, churning losses are negligible. Unneeded amounts of grease will push to the side and away from moving parts (such as rollers and cage). This forms a convenient channel, leaving only the needed amount of grease and oil for lubrication. Oil, on the other hand, will continue to flow back into the pathway of moving parts, causing churning and heat.

7. Because oil doesn’t use thickeners there is no risk of incompatible thickeners clashing, causing changes in grease consistency and other problems.

Grease’s Counterpoint. True, oils don’t employ thickeners in their formulation but there still remains the risk of base oil and additive incompatibility upon accidental mixing. In fact, one could argue that there is no greater security when two oils are mixed compared to mixing two grease products. And unlike some oils, grease additives will not settle out during storage or when machines are at rest.

8. Oil lubricants enable certain contaminants such as water and dirt to be briefly suspended and transported to filters, separators and settling zones. Grease suspends these types of contaminants permanently. In fact, some grease products will suspend 100 percent of their weight in water.

Grease’s Counterpoint. Unlike grease, circulating oil carries harmful contaminants to the far reaches of a system, risking wear and corrosion to many surfaces at the same time. Grease keeps most contaminants localized and immobilized and even displaced away from critical surfaces.

9. Oil volume in machine components can be precisely controlled using level gauges and sight glasses. Grease volume is nearly impossible to monitor and control. Over- and undergreasing are common causes of bearing failures.

Grease’s Counterpoint. Well-trained lube techs using proper tools and procedures have no problem introducing safe quantities of grease into bearings and similar components.

10.Oil can be changed without dismantling machine hardware. Grease must be repacked periodically, which involves a considerable cost associated with labor, material and downtime.

Grease’s Counterpoint. Many grease-lubed machines can run for years without the need to repack the bearings. Oil compartments, however, sometimes require constant drains and refills.

11. Oil-lubricated machines are easier to sample for laboratory analysis of wear metals, contaminants and fluid properties. A representative sample of an in-service grease is nearly impossible to obtain.

Grease’s Counterpoint. Perhaps 90 percent of bearings and components lubricated with grease are noncritical and don’t require routine sampling and analysis. However, new methods are being developed to enable grease sampling on the run.

12. Used oils can be safely handled and disposed of with minimal impact to the environment. Most grease-lubricated machines are total-loss systems, meaning there is no environmentally simple way to recover and dispose of degraded or contaminated products.

Grease’s Counterpoint. Oil is actually more environmentally problematic than grease. As stated above, it more readily leaks out of machines and contaminates water, soil, plant life, etc.

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About the Author

Jim Fitch, a founder and CEO of Noria Corporation, has a wealth of experience in lubrication, oil analysis, and machinery failure investigations. He has advise...