Lubrication Theory by Thomas Yoon

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	Lubrication Theory
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	Submitted Friday, December 03, 2004
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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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» left by jayantha (2 years 137 days ago.)
variation of viscosity with temperature
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