Lubrication is absolutely essential to the proper operation of ball and roller 

Lubrication is absolutely essential to the proper operation of ball and roller bearings. A proper lubricant will reduce friction between the internal sliding surfaces of the bearings components and reduce or prevent metal-to-metal contact of the rolling elements with their raceways. Proper lubrication reduces wear and prevents corrosion, ensuring long service lives for bearings.

Lubrication, especially circulating oil will also remove heat from the bearing.

There are two basic types of bearing lubricants readily available: Oil and Grease. The former is fairly simple to understand, being a free-flowing liquid, while the latter is a little more complex.  In order to be a lubricant, all greases have oil that is entrained in a thickened base. It is this base that gives the impression that grease is a more viscous type of oil; however, it is the oil in the grease that does the actual lubricating. Each type of lubricant has its own advantages and disadvantages and is selected by the nature of the application. The major advantages of the two basic types of lubricants are:

Lubricant

Advantage

Disadvantage

Oil

Easy to distribute, lubes other components, less drag, easier to drain out and change.  Better for high temperature.

May leak (environmental concern), then no more lubrication

Grease

Remains in place, doesn't leak out easily, improves sealing, and does not require monitoring.

Requires more labor to clean out and replenish.  High temperature grease is very expensive.

Each manufacturer of a lubricant can supply a specification sheet for each of their products, and each sheet will have a list of about 20 properties and their values related to this lubricant. The most important property of any lubricant for rolling element bearings is its oil viscosity. If the specification sheet is for an oil, the viscosity values will be for the oil. If it’s a grease, it should refer to “Base Oil Viscosity” or another similar term, depending on the manufacturer. Usually, four viscosity values are shown as follows:

cSt @ 40° C(104°F) SI units

cSt @ 100°C (212°F) SI units

SUS @ 100°F(38°C) Imperial units

SUS @ 210°F(99°C) Imperial units

It is very important to select a lubricant that will provide a minimum acceptable viscosity at the bearing’s operating temperature, which will usually be between the lowest and highest reference temperatures shown above. Typically, oil viscosity numbers decrease very rapidly with increasing temperature. Determining a bearing’s operating temperature is a fairly complicated calculation that is beyond the scope of this catalog. Calculating the viscosity of a lubricant at this temperature from the lubricant manufacturer’s specifications is yet another matter. Often, previous experience with an existing similar machine will indicate an acceptable lubricant. In house testing of a prototype or the first machine can indicate operating temperatures. Most machines use a lubricant that is selected to match the most severe demand of one component in the machine such as a bearing, gear, etc. 

Additives are a very important feature of modern oils and greases and can often mean the difference for successful, long term operation of bearings and other machine components. The selection of any lubricant over another should always consider additives.