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Oil lubrication methods

Oil bath
The simplest method of oil lubrication is the oil bath (fig. 1). The oil, which is picked up by the rotating components of the bearing, is distributed within the bearing and then flows back to a sump in the housing. Typically, the oil level should almost reach the centre of the lowest rolling element when the bearing is stationary. SKF recommends the use of oil levellers such as the SKF LAHD 500 to maintain the correct oil level.

When operating at high speed, the oil level can drop significantly and the housing can become overfilled by the oil leveller. If this occurs, contact the SKF application engineering service.
Oil pick-up ring
For bearing applications where speeds and operating temperatures require oil to be delivered reliably, SKF recommends using an oil pick-up ring (fig. 2). The pick-up ring hangs loosely on a sleeve on the shaft on one side of the bearing and dips into the oil sump in the lower half of the housing. As the shaft rotates, the ring follows and transports oil from the sump to a collecting trough. The oil then flows through the bearing back into the sump. SKF SONL plummer block housings are designed for the oil pick-up ring lubrication method.

For additional information, contact the SKF application engineering service.
Circulating oil
In general, high-speed operation increases frictional heat, elevates operating temperatures and accelerates ageing of the oil. To reduce operating temperatures and avoid frequent oil changes, the circulating oil lubrication method is generally preferred (fig. 3). Circulation is usually controlled by a pump. After the oil has passed through the bearing, it generally settles in a tank where it is filtered and cooled before being returned to the bearing. Proper filtering decreases the contamination level and extends bearing service life (→ SKF rating life). Cooling the oil can also significantly reduce the operating temperature of the bearing.

For information about the SKF CircOil system and SKF flow monitoring devices, refer to Lubrication solutions.

Oil jet
The oil jet lubrication method (fig. 4) is an extension of circulating oil systems. A jet of oil under high pressure is directed at the side of the bearing. The velocity of the oil jet should be sufficiently high (≥ 15 m/s) to penetrate the turbulence surrounding the rotating bearing. Oil jet lubrication is used for very high speed operation, where a sufficient, but not excessive, amount of oil should be supplied to the bearing without increasing the operating temperature unnecessarily.
Oil-air
The oil-air method (fig. 5), also called the oil-spot method, uses compressed air to transport small, accurately metered quantities of oil as small droplets along the inside of feed lines to an injector nozzle, where it is delivered to the bearing. This minimum quantity lubrication method enables bearings to operate very high speeds with relatively low operating temperature. The compressed air serves to cool the bearing and also produces an excess pressure in the bearing housing to prevent contaminants from entering.

For additional information about the design of oil-air lubrication systems, refer to Lubrication solutions.

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