The function of a lubrication medium is:

1. To form a film between moving bearing components so that metal to metal contact is prevented.

2. To reduce friction and eliminate wear

3. To protect against corrosion

4. To seal against impurities like dust, dirt, water.

In order for the oil film to be formed between the moving bearing components, the film must be sufficiently thick even under heavy load, high temperatures or vibrations.

Some sleeve bearings that has very heavy loads, like in the crosshead bearings of diesel engines, usually have provisions for injecting pressurized oil to float the shafts. This method is called hydrostatic lubrication.

However, the most common method of lubrication for sleeve bearings is by the hydrodynamic method. When the two surfaces of a bearing and shaft move rapidly relative to one another, the oil is carried along the shaft to fill the gap between shaft and bearing. When the moving components become completely separated by a cohesive film of lubricant, hydrodynamic lubrication occurs. Hydrodynamic lubrication prevents wear in moving parts, as there is no metallic contact between them. The bearing metals can last for a long time.

During starting time, the rotating shaft does not have sufficient speed to pick up the lubricant. The film separating the moving surfaces is very thin - with only the thickness of a molecule. This is a condition called boundary-layer lubrication. With this condition, friction losses increases, producing heat, which raises the temperature of the lubricant, thereby reducing its viscosity so that the load-carrying capacity of the film is even lower. In worst case conditions, the surfaces can even seize together.

For rolling contact bearings like ball bearings, a condition called elasto-hydrodynamic lubrication occurs. At the point of contact, the ball deflects and flattens out slightly for a moment under the high pressure. When the ball rolls on, the contact surfaces return to their original shape. However, the lubricant is not forced away from the point of contact due to the dramatic increase in viscosity. When the ball has passed, the viscosity falls back again.

When grease is used for lubricating ball bearings, they also act as protection against impurities like dust, dirt and water that will cause wear down and corrosion.

More about oils and grease for lubrication could be the subject of the next topic.

Until next time...

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