Methods of oil lubrication

Oil lubrication - Methods of oil lubrication

Oil bath

The simplest method of oil lubrication is the oil bath . The oil, which is picked up by the rotating components of the bearing, is distributed within the bearing and then flows back to the oil bath. The oil level should be such that it almost reaches the centre of the lowest rolling element when the bearing is stationary. The use of constant oil levellers is recommended to provide 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.

Oil pick-up ring

For bearing applications where speeds and operating temperature are such that oil lubrication is necessary and high reliability is required, the oil pick-up ring lubrication method is recommended . The pick-up ring serves to bring about oil circulation. The ring hangs loosely on a sleeve on the shaft on one side of the bearing and dips into the oil in the lower half of the housing. As the shaft rotates, the ring follows and transports oil from the bottom to a collecting trough. The oil then flows through the bearing back into the reservoir at the bottom. SKF plummer block housings in the SONL series are designed for the oil pick-up ring lubrication method.

Circulating oil

Operation at high speeds will cause the operating temperature to increase and will accelerate ageing of the oil. To avoid frequent oil changes and to achieve a fully flooded condition, the circulating oil lubrication method is generally preferred . Circulation is usually produced with the aid of a pump. After the oil has passed through the bearing, it generally settles in a tank where it is filtered and, if required, cooled before being returned to the bearing. Proper filtering leads to high values for the factor η _c and thus to long bearing service life.

Cooling the oil enables the operating temperature of the bearing to be kept at a low level.

Oil jet

For very high-speed operation a sufficient but not excessive amount of oil must be supplied to the bearing to provide adequate lubrication, without increasing the operating temperature more than necessary. One particularly efficient method of achieving this is the oil jet method where a jet of oil under high pressure is directed at the side of the bearing. The velocity of the oil jet must be high enough (at least 15 m/s) to penetrate the turbulence surrounding the rotating bearing.

Oil-spot
With the oil-spot method - also called the oil-air method - very small, accurately metered quantities of oil are directed at each individual bearing by compressed air. This minimum quantity enables bearings to operate at lower temperatures or at higher speeds than any other method of lubrication. The oil is supplied to the leads by a metering unit, such as the VOGEL OLA oil + air systems, at given intervals. The oil is transported by compressed air; it coats the inside of the leads and "creeps" along them. It is projected to the bearing via a nozzle. The compressed air serves to cool the bearing and also produces an excess pressure in the bearing arrangement that prevents contaminants from entering.

Oil mist

Oil mist lubrication has not been recommended for some time due to possible negative environmental effects.

A new generation of oil mist generators permits to produce oil mist with 5 ppm oil. New designs of special seals also limit the amount of stray mist to a minimum. In case synthetic non-toxic oil is used, the environmental effects are even further reduced. Oli mist lubrication today is used in very specific applications, like the petroleum industry.

Oil lubrica tion - Oil change

The frequency with which it is necessary to change the oil depends mainly on the operating conditions and the quantity of oil.

With oil bath lubrication it is generally sufficient to change the oil once a year, provided the operating temperature does not exceed 50 °C and there is little risk of contamination. Higher temperatures call for more frequent oil changes, e.g. for operating temperatures around 100 °C, the oil should be changed every three months. Frequent oil changes are also needed if other operating conditions are arduous.

With circulating oil lubrication, the period between two oil changes is also determined by how frequently the total oil quantity is circulated and whether or not the oil is cooled. It is generally only possible to determine a suitable interval by test runs and by regular inspection of the condition of the oil to see that it is not contaminated and is not excessively oxidised. The same applies for oil jet lubrication. With oil spot lubrication the oil only passes through the bearing once and is not recirculated.