SKF Bearing Lubrication
SKF Bearing Lubrication

This SKF training film is about the lubrication of rolling bearings Overcoming friction has challenged man ever since he invented the wheel Friction occurs between contacting surfaces that are in relative motion If the loads and relative movement increases sufficiently without lubrication the surfaces will fuse together and seize up The main purpose of lubrication is to provide a separating film to prevent this happening For almost a century SKF have been involved in the reduction of friction and wear by the development and manufacture of a range of rolling bearings and lubricants Rolling bearings can carry very heavy loads at extremely high speeds and it is essential to have a film of lubricant between the surfaces Mineral oil is generally used to create the very thin oil film which is between 1/10 of a micron and up to 1 micron Very smooth surfaces and clean conditions are required to prevent the film being penetrated and wear started To achieve an adequate oil film thickness within the bearing an number of variables must be considered the most important is the viscosity of the oil which influences the film thickness This is specified as centistoke or millimeters square per second As viscosity changes with temperature this is usually quoted at a reference temperature of 40 degrees centigrade Viscosity is used for bearing lubrication ranging from two centistoke which is very fluid like water up to 500 centistoke which is comparable to syrup The operating speed has an important effect on the lubrication film thickness As the speed increases a condition known as hydrodynamic lubrication develops which significantly increases the oil film thickness The effect is similar to aquaplaning At slower speeds high viscosity oils are required to maintain an adequate oil film in very slow and heavily loaded applications additives to improve lubrication are required Temperature within the bearing must be considered the oil viscosity will reduce with increase in temperature with the corresponding reduction in oil film Oils with a high viscosity index limit the effect of temperature changes within the bearing The operating load has to be considered as this has a direct effect on the contact of raceways and all rolling elements Grease is a combination of approximately 85 percent mineral oil and 15 percent soap or thickener The three main soaps or thickeners are lithium; a good all-round grease with a wide temperature range calcium; which is recommended for installations exposed to water but has a 60 degree centigrade maximum temperature and sodium; which has good sealing properties but should not be used in wet applications All the soaps function in the same way they retain the oil in a similar manner to a sponge retaining water The oil is allowed to bleed out as the bearing requires it Greases are measured by their stiffness or consistency This, like the oil viscosity, can change with temperature For bearings use the normal range of the grease consistency between one and three with three being the stiffest Grease is by far the most popular method for bearing lubrication It easy to apply and retain within the bearing housing and can also function as a seal to protect the bearing The bearing should be filled with grease and the free space in the housing up to 50 percent This is to allow room for the excess grease to be ejected from the bearing during startup In slow speed conditions where there is no overheating a complete fill is desirable Oil lubrication is a very effective way to lubricate bearings but can present difficulties with sealing There are several methods which are used the static oil bath being the most common The oil level in this instance should be half way up the lowest rolling element In most gearbox applications the bearings are lubricated by oil splash On critical machinery such as very large gearboxes or the drying sections of paper-making machines heat and contamination can be a problem In such applications a recirculatory system is used Oil is fed by pump to the bearings and then allowed to drain back to a sump where it can be cooled filtered before re-use With replacement bearings it is essential to make sure that the bearing is completely full and the housing partially full of the correct grade of clean grease Make sure the grease like the bearing is kept protected at all times In applications where the operating conditions are hot and where the environment is harsh it may be necessary to re-grease the bearings In these cases provision must be made for the excess grease to escape from the housing Re-greasing should be done whilst the equipment is still rotating In some applications continuous lubrication is preferred This can be achieved by a centralised system or by the use of an automatic lubricator Ensure that the lubrication fitting is clean that the right type of lubricant is used and that the correct quantity of lubricant supply is set Sealed in shielded bearings are greased for life and should not be interfered with

3 thoughts on “SKF Bearing Lubrication”

  1. Haider Naqvi says:

    Very nice

  2. Kuppuswamy Mathu says:

    very good information

  3. Rajesh G says:

    i have kiln roller bearing now i'm using high temp grease, i'm grease put on bearing surface it's mechanically correct or wrong

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