Lubricants
Lubrication is very necessary in the gears, piston and valves of air compressors so that they can operate smoothly and reduce the effects of harsh friction. Regular lubrication improves an air compressor's life and functioning. All oiled compressors require lubrication and oil-free ones need them only for some gears and parts outside the cylinders. The lubricant oil gets mixed up with the passing air flow and compressed air filters are employed to separate them out.
Generally synthetic oils, petrochemical refinery produced mineral oils and a few natural oils are used as lubricants. Three types of synthetic oil are very common as air-line lubricants; these are Fluorosilicones, Di-ester based and Phosphate ester based oils. Mineral oil based lubricants are categorized according to viscosity index (VI) as defined by Society of Automotive Engineers (SAE). There are five groups (Group I- group IV) having separate VI, different quantity of saturates and amount of sulphur. Manufacturers incorporate various additives to enhance lubricating quality reduce oxidation of end product lubricant when applied in compressors. Selection of these additives is very important and varies in different VI levels. Use of naturally obtained oils as lubricant is less than the other two groups and these are mainly triglyceride esters obtained from plant and animal sources.
These were liquid lubricants for oiled air compressor. Oil-free compressors do not need oil-lubrication as they contain a PFTE or Teflon non-stick coating which acts as permanent solid lubricant and withstand high temperatures as 350°C. However at very high pressure ceramic or alloy metal coating is used as solid lubricant. Oiled compressors come with sufficient lubrication from the factory but throughout lifetime they need lubricants at regular intervals.
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