How to select the right bearing lubricant

Ken Thayer | May 24, 2019

Bearing failure can be much more expensive than just the cost of buying a new bearing. Maintenance costs and production losses due to machine downtime can add up quickly. A leading cause of bearing failure is insufficient or improper lubrication, which can lead to contamination or excess heat buildup.

Source: JRK Bearings

During operation, bearing balls and rollers generate friction due to a sliding action between the rolling element and the bearing ring. The contact area of pure rolling motion is actually relatively small compared to the sliding contact area. Lubrication applied between these surfaces prolongs bearing life and minimizes the heat due to this sliding friction. Excess heat can cause surface degradation or cage breakage due to extreme stresses.

Prelubricated bearings

Many rolling element bearings are manufactured with integral double seals and are “lubricated for life.” These bearings are pre-lubricated with grease by the manufacturer and do not require routine maintenance.

Pre-lubricated bearings have some advantages and are desirable when the application presents some challenges or makes relubrication impractical.

Advantages of pre-lubricated bearings include:

Bearing already lubricated when purchased
No risk of contamination with external mechanisms
Lower assembly, purchasing and maintenance costs

Use pre-lubricated bearings if the application has the following limitations:

The housing cannot be kept free from dirt, water and other contaminants
Limited assembly space precludes the use of a grease-filled housing
Relubrication with external lubricators is not feasible

Although they do not require relubrication, routine monitoring of pre-lubricated bearings can avoid unexpected failures. Good preventive maintenance programs that combine replacement of a failing bearing with other maintenance items is a cost-effective practice. To detect a possible bearing problem as part of routine preventive maintenance, inspect bearing points and compare to baseline measurements for increases in vibration or excess heat generation.

Relubricatable bearings – Oil or grease?

Any bearing that does not contain integral lip seals will require an external lubrication source. Lubrication is critical to maintaining a bearing’s anti-friction characteristics and reducing the heat that is generated by excess sliding friction between the balls or rollers, bearing cage and bearing rings.

When selecting the best lubrication for an application, design engineers must decide between grease and oil. Different bearing types and applications factor into the lubrication selection.

(See industrial lubricants products and suppliers on IEEE GlobalSpec.)

Lubrication can serve purposes beyond reducing the heat due to friction, which is a common benefit of both oil and grease. A proper lubricant will also provide a load-bearing film between the rings and rolling element, which helps minimize wear. Oil and grease will also help keep moisture out and prevent corrosion. Most manufacturers will display speed limits for both oil and grease in their specification tables.

The table below outlines some of the additional benefits unique to oil and grease.

Source: IEEE GlobalSpec

Oil

Oil is the ideal method of lubrication for rolling element bearings. However, it is not practical for many applications. Several types of oil lubrication systems exist.

Oil bath lubrication is a simple method of supplying lubrication to the bearing by placing the bearing in a sealed housing filled to the middle of the lowest rolling element. It requires minimum maintenance other than checking oil levels as well as routine checks for contamination. Oil bath lubrication is suitable for low-speed applications. Constant level lubrication systems will maintain a specified level of lubricant in a housing. (See constant level lubrication system products on IEEE GlobalSpec)

Oil splash lubrication is used in gearboxes and other low-speed applications where a gear or rotating mechanical component splashes oil onto a bearing. Oil splash lubrication is not recommended for high-speed applications.

Circulating oil lubrication systems are closed-loop lubrication systems that circulate oil from a pump through the bearing via a piping system. The oil exits the bearing and returns to the storage tank, where it cools. Oil circulating systems are suitable for high-speed applications. (See circulating oil lubrication products on IEEE GlobalSpec)

Oil-air lubrication is a precision oil delivery method where small droplets of oil are metered at specific intervals into a constant flow of air that is directed at the bearing. This provides a steady supply of oil to the bearing in high-speed applications such as machine tool spindles. Oil-air systems are suitable for high-speed applications. Direct oil-air lubrication is a variant of oil-air for high speeds where the lubrication is directed through an orifice in the outer bearing ring directly to the surface of the bearing race. (See oil-air lubrication products on IEEE GlobalSpec)

Oil mist lubrication systems provide atomized oil suspended in a volume of pressurized air to fill a cavity with oil mist. As this method is less directed than an oil-air system, one supply line can lubricate multiple bearings. (see oil mist lubrication products on IEEE GlobalSpec)

Oil jet lubrication (also referred to as oil injection) is for extreme speed applications, such as jet engine bearings. One or more nozzles provide a steady stream of pressurized lubrication oil directly to the bearing. The velocity of the oil must be high enough to break through the turbulence around the rotating bearing.

Oil drop lubrication provides metered drops of oil directly to the bearing at specified intervals. Oil drop may be used in high-speed applications, but testing should be performed to ensure the oil is actually able to reach the bearing surface. Oil-air is preferred over oil drop for high-speed applications. (See oil drop lubrication products on IEEE GlobalSpec)

(See all lubricators and lubrication systems suppliers and products on IEEE GlobalSpec.)

Oil types

The two types of oil used for bearing lubrication are petroleum based and synthetic. Synthetic fluids include synthetic hydrocarbons, diesters, polyesters, glycols, fluorinated compounds, silicones and phosphate esters. Synthetic oils are more expensive than petroleum oils and are typically only used in extremely high- or low-temperature applications.

(See synthetic oils, greases and lubricants products and suppliers on IEEE GlobalSpec.)

Viscosity

When selecting an oil type, the most important property to consider is viscosity. Viscosity is the resistance to flow at a specific temperature. Lower viscosity oils will flow more readily than thicker, high viscosity oils. Kinematic viscosity is typically represented in either Saybolt universal seconds (SUS) or centistokes (cSt) at either 100° F (38° C) or 210° F (99° C).

A Saybolt universal viscometer is used to measure kinematic viscosity in units of SUS. It measures the time it takes a fluid at a controlled temperature to fill a 60 cc container.

A kinematic viscometer measures the time it takes a fixed amount of fluid to flow through a calibrated capillary and is represented in centistokes (cSt). (Note: 1 cSt = 1 mm²/s)

SAE and equivalent ISO oil grades and viscosity; Source: IEEE GlobalSpec

Bearings with balls or rollers typically use an oil with a minimum viscosity of 70 SUS or 15 cSt. Note that viscosity decreases when heated and increases when cooled. Bearing temperature at operating speed and load should be taken into consideration when specifying oil viscosity.

Grease

Grease is a semi-fluid to a solid product of a thickening agent combined with a lubricant. The lubricant may be a mineral oil, ester, organic ester, glycol or silicone. The thickening agent may be a soap (lithium, sodium, barium, calcium or strontium), an inorganic non-soap (microgel, carbon black or silica-gel) or an organic soap (urea compound, terephthalate or organic dye).

Most bearing greases are comprised of a soap thickening agent combined with a petroleum oil. Lithium based greases are very popular as bearing grease due to their water resistance and performance characteristics in both high and low temperatures. Greases comprised of synthetic lubricating fluids perform well in extreme low and high temperature ranges.

NLGI grease grade and penetration number; Data source NLGIGrease consistencies vary from a semi-fluid similar to a viscous oil to a solid that is nearly as hard as a soft wood. Grease consistency is subject to change as the bearing turns and the grease shears. This causes the temperature to increase and the grease to soften.

Grease consistency is measured by a penetrometer. A penetrometer is an instrument that drops a weighted cone onto a grease sample and measures the depth of penetration. The National Lubricating Grease Institute (NLGI) provides standard grease consistency grade.

Advancements in grease technology have contributed to the increase in popularity of pre-lubricated bearings. One factor of greased bearings that must be considered is starting torque, especially at low temperatures. A cold grease may resist starting torque. This is more a function of the individual grease properties than it is the consistency of the grease.

A good guideline is that grease life is halved with every temperature increase of 25° F (14° C) and doubles with the same decrease in temperature.

For proper grease selection, refer to the bearing company catalog, or consult an application engineer.

(See industrial grease products and suppliers on IEEE GlobalSpec.)

Dry lubrication

Dry lubricants are used in environments where grease or oil are not viable options. They reduce the friction between the bearing surfaces and cannot be washed away. They do not require oil or any other lubrication source during operation. Bearings can be purchased with a dry lubricant or it can be applied to a standard ball or roller bearing post-purchase. Unlike oil and grease, most end users do not have the ability to apply dry lubricants so this is usually a process that is outsourced.

Solid film

Solid film, a dry lubricant, is a non-fluid coating that is applied to the bearing surfaces. It is intended for extreme cases where the use of oil or grease is not possible. the two most common types of dry film lubricants are graphite and molybdenum disulfide (MoS₂). Boron nitride and polytetrafluoroethylene (PTFE) are also forms of solid film dry lubricants.

Graphite should be used in an environment that has water vapor present as the adsorption of water helps the graphite bond to the bearing surfaces.
Molybdenum disulfide has a similar structure to graphite, but better lubrication performance. MoS₂ can also be used in vacuums.
Boron nitride is a ceramic powder with extremely high temperature resistance properties.
PTFE has an extremely low coefficient of friction (as low as 0.04) and can be operated in temperatures to 260° C.

Benefits of solid film lubrication include:

Reduced friction and wear
Low heat generation
Low startup torque
Increased bearing life and performance
Compatibility with corrosive chemicals and acids

Proper lubrication is critical to the performance and life of every rolling element bearing. When selecting the best lubrication for your bearing application many variables must be considered. Space constraints, maintenance requirements, bearing type and design, operating parameters, environment, cost and other considerations must be factored into the decision. If in doubt, contact an application engineer from a bearing manufacturer or distributor for assistance.

(See solid and dry film products and suppliers on IEEE GlobalSpec.)

Solid oil

Bearings with solid oil; Source SKFSolid oil lubricant, a relatively new form of dry lubrication, is a porous polymer material that is molded to the shape of the bearing and filled with oil to completely fill all free spaces in the bearing. Solid oil bearings are lubricated for life and do not require lubrication. Solid oil bearings are for extreme applications where the use of grease or oil is not feasible.