Grease – the word is derived from the old French graisse which was based on the old Latin word crassus meaning fat - is man’s oldest and most widely used lubricant. It’s as old as the wheel -maybe older- and humungous amounts are used each year. World grease consumption was 1.2 million tons in 2011 (about 3 % of overall global lubricant usage).

Originally, lubricating greases were made of tallow rendered from animal fats. Today they are complex blends of lubricating oils, thickeners, and other additives. And they are used to lubricate everything from a simple journal bearing to complex components in space vehicles.

Definition & Functions Technically, a lubricating grease is: A solid to semi-fluid mixture of a fluid lubricant and a thickener. (Thickener is the agent used with an oil to create the gel or gel-like structure. Grease thickeners should not be confused with materials used to increase the viscosity of lube oils, e.g., viscous oils & VI Improvers)

Grease lubrication & oil lubrication serve the same purpose - to minimize friction and wear between moving surfaces. However, grease is generally used instead of oil where:

A lubricant must act as a seal to prevent entry of contaminants.
A lubricant must maintain its position in a mechanism (when the opportunity for relubrication is limited or impossible).

Because of their essentially solid nature, greases do not provide the same degree of cooling and cleaning as fluid lubricants. A characteristic feature of greases is that they possess a high initial viscosity, which upon the application of shear, drops to give the effect of an oil-lubricated bearing of approximately the same viscosity as the base oil used in the grease. This change in viscosity is called thixotropy. A satisfactory grease for a given application should:

REDUCE FRICTION & WEAR in the machine being lubricated under various operating conditions.
PROTECT against rust & corrosion.
PREVENT dirt,water, & other contaminants from entering the parts being lubricated.
RESIST leakage, dripping, & throw-off.
MAINTAIN structure & consistency during long periods of use.
PERMIT free motion of moving parts at low temperatures & itself pump freely at those temps.
HAVE SUITABLE PHYSICAL CHARACTERISTICS for method of application & retain those characteristics in storage.
BE COMPATIBLE with elastomer seals & other materials used in the components being lubricated.
TOLERATE some degree of moisture contamination without significant loss of performance.
TOLERATE some degree of moisture contamination without significant loss of performance.

These properties are imparted by the physical & chemical properties of the fluid lubricant, thickener & additives. Typically, a grease might contain 85% base fluid, 10% thickener and 5% additives. Depending on the shear conditions at the moving surfaces, the grease will deform and flow to provide lubrication and then regain its structured consistency as the shear decreases. GREASE VERSUS OIL LUBRICATION: ADVANTAGES & DISADVANTAGES

Grease vs. Oil Lubrication - Advantages and Disadvantages
  Grease Lubrication Oil Lubrication
Feeding device Grease sealed bearing does not require relubrication for an extended period. Requires a device that continuously feeds oil (drip-feed, splash feed, recirculation system).
Consumption Can be kept to the minimum necessary. Significant amount required.
Lubrication system Simple. Complex.
Leakage Unlikely because of its seal forming characteristics. Possible if sealing system is inadequate.
Use for high-speed application Limited. Yes.
Contaminant removal No. Continual removal by filtration or centrifuge.
Cooling efficiency No cooling capacity. High cooling capacity.
Friction loss Generally high, but torque reduction can be achieved by channeling in roller bearings. Generally low.