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PROPERTIES OF GREASE Greases are a type of ''shear-thinning'' or Pseudo-plastic Fluid , which means that the Viscosity of the fluid is reduced under Shear . After sufficient force to shear the grease has been applied, the viscosity drops and approaches that of the base mineral oil (or that of the EP additive for EP greases under heavy load). This sudden drop in shear force means that grease is considered a Plastic Fluid , and the reduction of shear force with time makes it Thixotropic . It is often applied using a Grease Gun . USES FOR GREASE Greases are employed where heavy Pressure s exist, where oil drip from the Bearing s is undesirable, and/or where the motions of the contacting surfaces are discontinuous so that it is difficult to maintain a separating lubricant film in the bearing. Grease-lubricated bearings have greater Friction al characteristics at the beginning of operation. Under shear, the viscosity drops to give the effect of an oil-lubricated bearing of approximately the same viscosity as the base oil used in the grease. Calcium- and sodium-based greases are the most commonly used; sodium-based greases have higher melting point than calcium-based greases but are not resistant to the action of Water . Lithium-based grease has a drip temperature at 350° to 400°F and it resists moisture, hence it is commonly used as lubricant in household products such as garage door openers. GREASE ADDITIVES Teflon is added to some greases to improve their lubricating properties. Gear greases consist of Rosin oil, thickened with Lime and mixed with mineral oil, with some percentage of water. Special-purpose greases contain Glycerol and Sorbitan Ester s. They are used, for example, in low-temperature conditions. Some greases are labeled "EP", which indicates "extreme pressure". Under high pressure or shock loading, normal grease can be compressed to the extent that the greased parts come into physical contact, causing friction and wear. EP grease contains solid lubricants, usually Graphite and/or Molybdenum , to provide protection under heavy loadings. The solid lubricants bond to the surface of the metal, and prevent metal-to-metal contact and the resulting friction and wear when the lubricant film gets too thin. OTHER GREASES Other types of lubricating material that are soft solids at room temperature are often called grease, though they may not exhibit the shear-thinning properties typical of the oil/soap grease. Silicone Grease is an amorphous fumed, silica-thickened, polysiloxane-based compound, which can be used to provide lubrication and corrosion resistance. Since it is not oil-based, it is often used where oil-based lubricants would attack Rubber seals. Silicone greases also maintain stability under high temperatures, and are often used, in pure form or mixed with Zinc Oxide , to join Heat Sink s to Computer CPU s. Petroleum Jellies , such as Vaseline , are also sometimes called greases, and are commonly used for lubricating food-handling equipment. EXTERNAL LINKS
SEE ALSO Dropping Point |
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