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Oil lubrication methods

Several methods are available and are described below. The best method to use will depend on operating conditions.

● Oil bath lubrication
This is a common method used where bearings are operating below the listed oil limiting speed. The housing is designed to provide a sump through which the rolling elements of the bearing will pass. Generally, the oil level should be no higher than the center point of the lowest rolling element. If speed is high, lower oil levels should be used to reduce churning. Gages or controlled elevation drains are used to achieve and maintain the proper oil level.

●Drip feed lubrication
This lubrication method is often used for small bearings operated at relatively high speeds. In the illustration, a visible oiler is used. The oil drip rate is controlled by a screw valve located at the top of the oil cup. In case of drop lubrication, the number of drops should be appropriately adjusted according to the specific conditions; however, several drops per minute should be sufficient under normal conditions.

● Splash lubrication
In this lubricating method, oil is splashed onto the bearings by gears or by a simple rotating disc. This method is commonly used in automobile transmissions, differentials and gear boxes. The illustration shows splash lubrication used on a reduction gear.

● Circulating system
This method is commonly used for high speed operation and for bearings used at high temperatures. A typical circulating oil system consists of an oil reservoir, pump, piping and filter. As shown in the illustration, oil from the supply pipe circulates through the bearings and exits to an external reservoir. After cooling in the reservoir, it returns to the bearing through a pump and filter. In this system, the oil outlet should be larger in diameter than the supply pipe so that an excessive amount of oil will not remain in the housing.

This system has the advantages of:
An adequate supply of oil for both cooling and lubrication. Metered control of the quantity of oil delivered to each bearing. Removal of contaminants and moisture from the bearing by flushing action. Suitability for multiple bearing installations. Large reservoir, which reduces deterioration.

● Jet Lubrication
Jet lubrication is often used for ultra-high speed bearings, such as the bearings in machine tool spindles. In this method, lubricating oil is sprayed under pressure from one or more nozzles directly into the rolling elements of the bearing. The lubricant quantity will be adjusted by the oil pressure and the nozzle bore diameter. It is important to ensure that the supplied oil will not accumulate at the bearing section. The illustration shows an example of typical jet lubrication.

● Oil-mist lubrication.
Oil-mist lubrication is called also oil fog lubrication, uses air to atomize the oil and carry it into the bearing. This system permits close control of the amount of lubricant reaching the bearings. The air is filtered and supplied under sufficient pressure to assure adequate lubrication of the bearings. This method is used in high-speed, continuous operation applications. To ensure “wetting” of the bearings and to prevent possible damage to the rolling elements and races, it is imperative that the oil mist system be turned on for several minutes before the equipment is started. The importance of “wetting” the bearing before starting cannot be overstated and has particular significance for equipment that has been idled for extended periods of time.

● Oil replacement
The need for oil replacement depends on operating conditions. If a bearing is used at a temperature of 50?C or lower in a favorable environment with little dust or dirt, an oil change intervals up to one year should be sufficient. If the bearing is used at an operating temperature exceeding 100?C with an operating temperature exceeding 100?C with an external heat source, the oil should be replaced every two or three months or more frequently, even if the used oil is thermally stable.

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