Lubrication
Grease lubrication
In most applications with super-precision angular contact ball bearings, grease with a mineral base oil and lithium thickener is suitable. These greases adhere well to the bearing surfaces and can be used where temperatures range from –30 to +100 °C. For bearing arrangements that run at very high speeds and temperatures and where long service life is required, the use of grease based on synthetic oil, e.g. the diester oil based grease SKF LGLT 2, has been proven effective.Initial grease fill
In high-speed applications, less than 30% of the free space in the bearings should be filled with grease. The initial grease fill depends on the bearing size as well as the speed factor, which isG = K Gref
where
| A | = | speed factor [mm/min] |
| n | = | rotational speed [r/min] |
| dm | = | bearing mean diameter
= 0,5 (d + D) [mm] |
The initial grease fill can be estimated from
G = K Gref
where
G = K Gref
where
| G | = | initial grease fill [cm3] |
| K | = | a calculation factor dependent on the speed factor A (diagram 1) |
| Gref | = | reference grease quantity (table) |
Running-in of grease lubricated bearings
A grease lubricated super-precision bearing in the 718 (SEA) series will initially run with a relatively high frictional moment. If the bearing is run at high speed without a running-in period, the temperature rise can be considerable.
The relatively high frictional moment is due to the churning of the grease and it takes time for the excess grease to work its way out of the contact zone. This time period can be minimized by applying a small quantity of grease distributed evenly on both sides of the bearing during the assembly stage. Spacers between two adjacent bearings are also beneficial (see the section Bearing preload).
The time required to stabilize the operating temperature depends on a number of factors – the type of grease, the initial grease fill, how the grease is applied to the bearings and the running-in procedure (fig 1).
See also the section Grease lubrication – Running-in of grease lubricated high-precision bearings for more detailed information.
Oil lubrication
Oil lubrication is recommended for many applications, as the method of supply can be adapted to suit the operating conditions and design of the equipment.Oil-air lubrication method
For typical arrangements with bearings in the 718 (SEA) series, the high operational speeds and requisite low operating temperatures generally require an oil-air lubrication system (fig 2). With the oil-air method, also called the oil-spot method, accurately metered quantities of oil are directed at each individual bearing by compressed air. For bearings used in sets, each bearing is supplied by a separate oil injector. Most designs include spacers that incorporate the oil nozzles.
Guidelines for the quantity of oil to be supplied to each bearing for high-speed operation can be obtained from
Q = 1,3 dm
where
Q = 1,3 dm
where
| Q | = | oil flow rate [mm3/h] |
| dm | = | bearing mean diameter
= 0,5 (d + D) [mm] |
The calculated oil flow rate should be verified during operation and adjusted depending on the resulting temperatures.
Oil is supplied to the feed lines at given intervals by a metering unit. The oil coats the inside surface of the feed lines and "creeps" toward the nozzles, where it is delivered to the bearings. The position of the oil nozzles (table) is important to make sure that the oil can be introduced into the contact area between the balls and raceways and to avoid interference with the cage.
High quality lubricating oils without EP additives are generally recommended for super-precision angular contact ball bearings. Oils with a viscosity of 40 to 100 mm2/s at 40 °C are typically used. A filter that prevents particles > 5 μm from reaching the bearings should also be incorporated.
