Comparing Grease and Oil for Lubrication

Lubrication of your industrial machinery is a vast topic; most engineers and plant owners are strongly opinionated an many of its aspects. One of the most important debates that have been around for decades in the global machinery lubrication industry is the Debate of Oil vs. Grease. There are compelling arguments supporting each of them and equally compelling counterpoints by the opposing party. In this article, we will look at some of the classic arguments and a few new ones.

Why choose Grease

1. Grease helps maintain a better stop-start performance. When a machine shuts down, oil drain back to stump, whereas grease remains in the component that needs it. This lowers the risk of a dry start.  

Counterpoint – Most oil-systems like splash, oil or circ-oil systems can relubricate almost immediately on restart. In case a large volume of oil is involved, the additives present in the oil are also large in quantity, this gives them the ability to wash contaminants out of the frictional zones which extends the service life of the oil.    
2. When the seals and connectors get worn out, they retain grease better than oil. This lowers the risk of leakage and lubricant starvation. It also reduces the risk of lubricant damaging the work product.

Counterpoint – Grease may be less prone to leakage, but leakage is the sign of a machine exposed to the environment. If a seal is worn, it’s in need of repairs and it shouldn’t go unnoticed. This puts you at a risk of a more dangerous failure in the future.

3. Seals and connectors get tightly sealed by grease as the surplus grease packs around it; this prevents particle and water from entering the system. Regreasing periodically purges the contaminants out of the working surfaces of the machine.

Counterpoint – The regressing of most industrial machines can’t reliably remove the contaminants from cavities seals and side of the bearings, rather it may even drive the contaminants directly inside the bearing and cause a catastrophic failure. Most industrial machines are not even regreased so often to rely on this method. 

4. The use of solid additives such as zinc oxide, molybdenum disulfide or graphite is possible with grease, while they would get filtered or settle in the bottom of the tank if used in most oils.

Counterpoint – Additives that are used today in oil formulation are soluble. They provide comparable performance against both abrasion and adhesive wear in similar applications.

Why choose oil

1. Oil effectively regulates heat inside the machinery, as it can flow freely, unlike grease. This retains the base oil viscosity and minimizes additive depletion and the risk of heat-induced oxidation.

Counterpoint - When grease is properly selected for the desired application, its temperature and risk of oxidation remains reasonably low. Many machines have easily exceeded ten years of service using grease.

2. Churning of grease in bearings and gears consumes higher energy and it generates more heat. Energy consumption by oil in this aspect would be only a fraction of that of grease.

Counterpoint – When the grease is properly applied with the correct consistency, energy consumption for churning is negligible. Excess amount of grease is pushed away from the moving parts. 

3. Oil doesn’t use thickeners, thus it avoids the risk of inconsistency due to incompatible thickeners, and other such problems. 

Counterpoint – Oil may not have thickeners but the risk of additives and base oil incompatibility still remains. Mixing two grease products is arguably much safer than mixing two oil products. 

4. Oil can briefly suspend certain contaminants such as dirt or water and transport them to filters, settling zones and separators. Grease suspends them permanently. 

Counterpoint – Oil circulating in the system carries harmful contaminants to as far as the system goes. This puts the entire system at risk of wear and corrosion. Grease localizes most of the contaminants, preventing them from moving into critical surfaces.
5. The volume of oil in a machine can be easily and precisely monitored using sight glasses and level gauges. Measuring the volume of grease in a machine is a near impossible task. Bearing failures happen commonly due to over and under greasing. 

Counterpoint – Lubrication technicians that are well-trained with proper tools and procedures can easily apply safe quantities of grease in any component of a machine.

6. Changing the oil of a machine doesn’t require it to be dismantled. Regreasing requires considerable costs and labour associated with periodic repackaging. 

Counterpoint – Grease-lubed machines usually run for years without the need of repackaging. On the contrary, oil compartments sometimes require frequent drains and refills.

7. The sampling of oil lubricated machines is much easier than grease. A laboratory analysis of contaminants, wear metals and fluid properties requires a sample that represents the entire lubricant in the system. Such a sample is nearly impossible to obtain when using Grease.

Counterpoint – Almost 90% of the components and bearings lubricated with grease serve noncritical functions and hence they do not require sampling and analysis. Yet, new methods are being researched that can enable proper grease sampling.

8. Contrary to grease, the handling and disposing of oil does minimal harm to the environment. Machines lubricated by grease are a mostly total loss system, which means that there is no eco-friendly way to dispose of degraded or contaminated greases.

Counterpoint – As discussed above, oil can actually be more damaging to the environment as it leaks out of machines contaminating soil, water and plant life.
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The information contained herein has been compiled from sources deemed reliable and it is accurate to the best of our knowledge and belief. However, Castrol cannot guarantee its accuracy, completeness, and validity and cannot be held liable for any errors or omissions, as the results change depending on the working condition/environment.  Changes are periodically made to this information and may be made at any time.
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