Correct lubrication is critical to bearing performance.
For more information on our standard oils and greases, please see our LUBRICANT TABLES
Lubrication provides a thin film between the contact areas in a bearing to reduce friction, dissipate heat and inhibit corrosion on balls and raceways. The lubricant will affect maximum running speed and temperature, torque level, noise level and, ultimately, bearing life. There are a range of options depending on the application.
Mineral or synthetic based lubricants are the most commonly used. There are many different types, designed for general or high speed use, low noise applications, water-resistance or extreme temperatures.
Silicon lubricants have wide temperature ranges and change viscosity less with temperature. They also have good water-resistance and are safe to use with most plastics. They are not suitable for high loads and speeds.
Perfluorinated lubricants or PFPE lubricants are non-flammable, oxygen compatible and highly resistant to many chemicals. They do not react with plastics or elastomers. Many have low vapour pressure and are suitable for vacuum or clean-room applications while some can withstand temperatures of over 300°C.
Dry lubricants are used where standard lubricants may cause contamination such as vacuum environments. Popular materials such as molybdenum disulphide or tungsten disulphide may be burnished or sputtered on to the balls and raceways to give smooth operation and higher running speeds than unlubricated bearings.
Solid polymer lubricants consist of a synthetic polymer impregnated with lubricating oil which fills most of the internal space of the bearing. This type of lubricant is often used in sealed bearings in dusty environments or where lubricant leakage cannot be tolerated such as clean environments and vertical shaft applications. Solid lubricants have excellent water resistance and will withstand regular wash-downs. They will also tolerate high vibration and high centrifugal force.
Dampening greases are widely used in automotive parts to prevents rattles and squeaks. They are also used to give a “quality” feel to switches, slides, threads and gears. They can be used in slow rotating bearings in, for example, potentiometers for the same reason.
Food grade lubricants are required for the food and beverage industries to conform with strict hygiene regulations. HI approved lubricants are required for bearings were there may be incidental contact with food and H2 approved greases are used where there is no contact. These greases are also designed to be highly resistant to being washed out by cleaning processes.
Low viscosity oils and greases are used where low lubricant resistance is required such as sensitive instruments. Higher viscosity lubricants may be specified for high load, high speed or vertical shaft applications. Low viscosity oils (or greases with low viscosity base oils) are preferred for high speed applications as they generate less heat. Although greases often provide much greater resistance than oils, many modern low torque greases can produce torque figures that are similar to some oils, particularly where a low grease fill is used.
Most oils maintain their consistency well over a wide temperature range and are easy to apply. For very low torque applications, a light instrument oil should be specified. Higher running speeds are possible with oil but, as it tends not to stay in place, continuous lubrication must be applied by oil jet, oil bath or oil mist unless speeds are low or rotation is for short periods. An oil-impregnated phenolic retainer or a synthetic retainer made from a material with a very low coefficient of friction such as Torlon do not need continuous external lubrication. These types of retainer are often used in high speed, low torque dental bearings.
Greases are simply oils mixed with a thickener to so that they stay inside the bearing. Greases are generally more suitable for heavy loads and have the obvious advantage of giving constant lubrication over a long period without maintenance.
Surprisingly, too much grease can be bad for a bearing. A high grease fill will mean greater rolling resistance (higher torque) which may not be suitable for many applications but worse still is the risk of heat build-up. The free space inside a bearing is important in allowing the heat to radiate away from contact area between balls and raceway. As a result, too much grease can lead to premature failure unless speeds are low. The standard fill is 25% – 35% of the internal space but this may be varied if required. A smaller percentage may be specified for a high speed, low torque application while a much higher fill may be advisable for a low speed, high load application.
Grease Speed Rating
Greases have speed ratings sometimes called “DN” ratings. The calculation for the “DN” of an application is as follows:
Speed in rpm x (bearing ID + bearing OD) ÷ 2
Assume a bearing rotates at 20,000 rpm. The bearing ID is 8mm and the OD is 22mm. The above formula produces a DN of 300,000 so the grease should be rated above this figure. Many modern greases are suitable for high speeds with some rated at 1 million DN or more.