To reduce the frictional losses between metal to metal rubbing or sliding parts, lubrication is required. Below are the some essential qualities of a good engineering lubricant.
01) Viscosity: - When any lubricant offered resistance to the deforming forces, it is called viscosity of the lubricant. Good quality lubrication oil must maintain sufficient viscosity at higher temperatures and it should not be too viscous at lower temperatures.
For high speeds, low viscosity lubricants are suggested while for large clearance and high loads; high viscosity lubrication oil is recommended. The viscosity of oil increases, if the temperature of mating surfaces decreases.
02) Flash & Fire Point: - Flash point of the lubrication oil is the minimum temperature at which it gives off enough vapour to form a momentary flash when naked flame is brought near its surface, while the fire point is the lowest temperature of the lubrication oil, where it burns continuously. To avoid the possibility of fire, the flash point of oil must be higher than the temperatures likely to be developed in the rubbing surfaces.
The fire point is generally higher than the flash point and the difference between flash point and fire point must be 18 degree centigrade at higher side.
03) Oiliness: - It is the property of lubrication oil to spread and attach itself firmly to the lubricating surfaces. When mating surfaces are subjected to a high intensity of pressure, it is recommended to particularly high oiliness; so that the metal is protected by a layer of oil and the wear is reduced.
04) Volatility: - Lubrication oil losses a certain weight due to evaporation when it is subjected to high temperature for long hours. This loss is known as “loss by evaporation”. The consumption of oil is considerably increased with high volatility at normal working temperatures that is why low volatility is always suggested for lubrication oil.
05) Stability & Insoluble Residue: - Stability of oil is to resist oxidation that would yield acids and sludge. Good lubrication oil must have high stability.
Free carbon or hydrocarbons decomposing into carbon at high temperature is called as insoluble residue. Good quality lubrication oil should not have any insoluble residue.
I hope the information above will help you to select proper lubricants for your machines.
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