Lubricants Assignment Help




Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load (pressure generated) between the opposing surfaces. The interposed lubricant film can be a solid, (e.g. graphite, MoS2) a solid/liquid dispersion, a liquid, a liquid-liquid dispersion gasses exceptionally a gas. In the most common case the applied load is carried by pressure generated within the fluid due to the frictional viscous resistance to motion of the lubricating fluid between the surfaces.

Lubrication can also describe the phenomenon such reduction of wear occurs without human intervention (hydroplaning on a road). The science of friction, lubrication and wear is called tribology. Adequate lubrication allows smooth continuous operation of equipment, with only mild wear, and without excessive stresses or seizures at bearings. When lubrication breaks down, metal or other components can rub destructively over each other, causing destructive damage, heat, and failure.

The regimes of lubrication:-

As the load increases on the contacting surfaces three distinct situations can be observed with respect to the mode of lubrication, which are called regimes of lubrication:

1).Fluid film lubrication is the lubrication regime in which through viscous forces the load is fully supported by the lubricant within the space or gap between the parts in motion relative to one another (the lubricated conjunction) and solid-solid contact is avoided

  • Hydrostatic lubrication is when an external pressure is applied to the lubricant in the bearing, to maintain the fluid lubricant film where it would otherwise be squeezed out.
  • Hydrodynamic lubrication is where the motion of the contacting surfaces, and the exact design of the bearing is used to pump lubricant around the bearing to maintain the lubricating film. This design of bearing may wear when started or stopped, as the lubricant film breaks down.

2).Elastohydrodynamic lubrication: The opposing surfaces are separated but there occurs some interaction between the raised solid features called asperities, and there is an elastic deformation on the contacting surface enlarging the load bearing area whereby the viscous resistance of the lubricant becomes capable of supporting the load.

3).Boundary lubrication (also called boundary film lubrication): The bodies come into closer contact at their asperities; the heat developed by the local pressures causes a condition which is called stick-slip and some asperities break off. At the elevated temperature and pressure conditions chemically reactive constituents of the lubricant react with the contact surface forming a highly resistant tenacious layer, or film on the moving solid surfaces (boundary film) which is capable of supporting the load and major wear or breakdown is avoided. Boundary lubrication is also defined as that regime in which the load is carried by the surface asperities rather than by the lubricant.

Methods to Reduce Friction:

Pushing a plate on a dining table gives a rough feeling but if you were to 'accidentally' drop some water and then push the plate, it would easily glide through. This is because the layer of water between the plate and the table reduces the friction.

Below are some common ways to reduce the friction.

Polishing

Polishing the surface, reduces the irregularities and hence, reduces friction.

Lubrication

It provides a layer of the lubricant on which the bodies can easily slide.

Use of ball-bearing or roller bearing

ball bearings

Ball

/roller bearing help to convert sliding friction into a milder form - rolling friction. Rolling friction is lesser than sliding friction.

Streamlining

Fast cars, boats, planes etc., have a streamlined body. This is to allow air (or water in the case of boats) to easily flow by, without offering much resistance.

Use of correct combination of surfaces in contact

Use of alloys on moving and sliding parts reduces friction because alloys have a low coefficient of friction.

Why is it easier to pull a body than to push it ?

If you have ever mowed the lawn using a mower, you would have noticed that pushing the mower is more difficult than pulling it.

Have you ever wondered why?

Let us try to understand this by a simple block and plane example.

When the block is pushed, the vertical component of the push increases the normal force. Hence, there is an increase in friction.

 

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Now when you pull the body, the vertical component of the pull is in the opposite direction of the normal reaction and hence, reduces it, thereby reducing the friction.

 

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Hence, friction in (b) is less than friction in (a)

 Pulling a body is easier than pushing.


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