Functions of Lubrication
When used in a gearbox, the lubricant provides two primary two benefits: to lubricate the teeth and to remove heat generated from the gear operation.
The lubricant is also often used for lubricating the various bearings found in the gearbox.
If the correct lubricant is selected for use in a gear system, it will provide slip-free power transmission at high mechanical efficiency, with good reliability, low maintenance, and long life.
Methods of Lubrication
There are three primary methods of lubricating gears:
1. Grease lubrication
2. Splash lubrication
3. Spray lubrication
1. Grease Lubrication (0 to 6 m/s tangential gear speed )
Grease lubrication is suitable for any gear system that is open or enclosed, so long as it runs at low speed. The grease should have a suitable viscosity with good fluidity especially in a enclosed gear unit. Grease is not suitable for high loads and continuous operation and there is virtually not cooling effect. There must be sufficient grease to ensure the gear teeth are lubricated but an excess can result in viscous drag and power losses.
2. Splash Lubrication (4 to 15 m/s tangential gear speed )
Splash lubrication is the normal method for lubricating spur, helical, bevel and worm gears. The gears simply dip into a bath of oil as the rotate. Splash lubrication needs at least 3 m/s tangential speed gear speed to be effective. It is important that provisions are made to ensure the teeth are not immersed in the bath such that excessive losses result from the oil being churned up. The oil level should be monitored under static and dynamic conditions to ensure it is correct for the application.
3. Spray Lubrication (above 12 m/s tangential gear speed )
For the higher speed units (10 to 20 m/s peripheral speed) engineered spray lubrication is generally provided using shaped nozzles with oil at a circulated pressure of about 0.7 barg. At higher speeds the system for directing the oil at the teeth needs to be carefully engineering to ensure the oil actually reaches the contacting surfaces as centrifugal forces and escaping air flow will tend to deflect the oil jet.