Question

What are the conditions for use of solid lubricants? Discuss the structure and uses of Graphite.

Answer 1

Dry lubricants or solid lubricants are material which can reduce the friction without a liquid medium . they are used where,
1. Operating conditions are such that a lubricating film cannot be formed or maintained.
2. Contaminations of liquid or semi-solid lubricant, with dust or dirt is not desirable ;e.g., open gears.
3. Combustible lubricants must be avoided due to the high operating temperature and pressure.
4. Heavy machinery working on a crude job at very high loads and slow speed.
5. Where the parts to be lubricated are not easily accessible.

GRAPHITE:
Graphite is the most widely used of all solid lubricants. It consists of a layered lattice structure. A layer of hexagonally arranged sp² hybridized carbon atom in the planar graphite molecule bonded covalently. The two neighbouring parallel layers are 3.7A^o apart and are held together by weak vander waals forces. Therefore the forces required to shear layers is very low and the molecules can slip over each other easily by mechanical forces. Hence, graphite powder is very soapy to touch or vey soft and can act as lubricant. It is not inflammable and not oxidised in air below 375⁰c. In the absence of air, It can be used up to very high temperature. Usually some organic substances are mixed with solid lubricants so that they may stick firmly to the metal ( called "Aqua dag'') or oil{ called "Oil dag''). Oil dag is used in IC engines. Aqua dag is useful in air compressors, general machine shop work, lathes and for machinery used in food industry. Graphite is also mixed with greases to form graphite greases, which are used at higher temperature.