Jul. 23, 2022
There are two types of lubricants on the market: oil and grease.
Selecting the right lubricant for your application depends on a handful of factors.
These include the type of machine, bearing type, size, temperature, load conditions, speed range, operating conditions (such as vibration and horizontal/vertical orientation of the shaft) and external environment.
In most cases, the best place to start selecting the right lubricant is to follow the bearing manufacturer's recommendations.
However, there are other guidelines to follow when selecting the correct one for the job.
As a general rule, bearings run the coolest and with the least amount of friction when a minimum amount of the lightest-bodied lubricant that will keep the bearing surfaces apart is used.
A good example of this is the "splash method," where oil is distributed by wicking or submerging.
Often a heavier lubricant is used in three unique scenarios:
ㆍIt's required by operating conditions
ㆍIt's called for specifically in the application
ㆍThe load is too heavy for the current lubricant
Oil lubricant is generally used for high-speed or high-temperature applications that require heat transfer away from working bearing surfaces. The oils consist of either natural mineral oil (with additives that prevent oxidation and rust) or synthetic oil.
That said, the four types of oils commonly used include petroleum oils, diesters, silicones, and fluorocarbons.
Oil systems for the above types of oils include:
ㆍAir/ oil mist
The base in synthetic oils is usually polyalphaolefins (PAO), polyalkylene glycols (PAG), esters, and silicones for cold and low torque conditions.
Although they are similar, these two types of oils offer unique properties and cannot be interchangeable. Mineral oils are more common than synthetic oils.
Viscosity is one of the key characteristics when specifying the right oil for a bearing. A good rule of thumb: low-viscosity fluids are thinner like water, and high-viscosity fluids are thicker like molasses.
The oil's viscosity correlates to the film thickness it can create, which is crucial to the separation of the rolling and sliding parts of a bearing.
While some bearing applications use oil as a lubricant, grease is the lubricant of choice for 80 to 90 percent of bearings.
Why is grease often a better lubrication option for more options?
Grease adheres to bearing surfaces better than oil, has a longer lifespan, and is less likely to run off or be ejected from rotating parts.
It also can be pre-lubricated, which eliminates the need for an external lubrication system and means less maintenance in the future.
Grease lubricants are made up of three components: additives (usually rust inhibitors), a base oil, and a thickener. When you are selecting a grease lubricant, it's important to note that the viscosity of the base oil (referred to as "base oil viscosity") determines how the lubricating film develops.
According to the National Lubricating Grease Institute (NLGI), the grease's consistency class indicates how the grease will flow and disperse within the bearing.
It's important to remember that regardless of which type of lubricant you chose, it will naturally lose its lubricating properties over time and if not maintained properly with the help of an experienced lubrication service, will eventually lead to bearing failure.
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