The field of application of lubricating grease is related to their non-Newtonian characteristics. It is usual to employ grease as a lubricant where temperatures and speeds are not excessive and the sealing arrangements do not offer satisfactory lubrication by oils. In addition to being a lubricant, grease possesses excellent self-sealing properties and protects the bearing against the entry of contaminants, where the neck sealing is inadequate or elementary. The viscosity of grease at low or moderate rates of shear remains much higher than lubricating oil in general and so the bearing reaches hydrodynamic or full-film stage more rapidly than with oil.
However at high speeds frictional losses are relatively high and more frictional heat is also generated. Since the effectiveness of grease as a coolant is limited, grease lubrication of plain bearings is therefore confirmed to units of low or moderate speed often under 200 rpm and rarely over 300 rpm; with journal surface speeds generally 1 to 2 m/s. This speed limit is however for large bearings of 50-cm. dia. or more. With reduction in size of the bearing the maximum journal surface speed upto which losses and heating are tolerable, fall sharply.
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Calcium soap greases are mainly used where temperatures are moderate. Calcium-based greases are preferable for bearings that have to run in the presence of excessive moisture. Other types of greases are required where higher working temperatures and more satisfactory sealing properties are required than the conventional 'cup' greases. Such grades include lithium soap and inorganic-base greases and some soda soap greases, but it should be noted that Sodium soap greases are not recommended where the bearing is subjected to excessive washing by water.
Where loading is exceptionally severe for example, in roller-bearing assemblies fitted to the roll necks of a metal rolling mill, it may be necessary to use a grease with extreme pressure properties. Such greases are designed to give effective lubrication under conditions of shock loads and constant high loading. In certain applications they also require resistance to moisture; on a metal-rolling mill, for example, water is frequently used to cool the roll faces.
Whatever type of grease is used, it should have no tendency to separate under the operating conditions. When separation occurs, the oil runs out of the bearing and leaves behind dry soap that hardens and cakes. This interferes with the movement of the rolling elements; overheating and mechanical failure may result. Excessive softening is also undesirable, because the grease might leak out of the bearing and leave the working surfaces unprotected.
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The methods used to apply grease are governed by the design of the bearings and by conditions of operation. They are:
- Grease Packing - Applicable where periodic addition of fresh grease is not required.
- Compression Cup or Pressure Gum Application - Employed where the addition of fresh grease is periodically necessary.
- Centralized Pressure Systems - For severe service where the rate of consumption makes it necessary to add fresh grease at frequent and regular intervals (e.g. roller-bearing assemblies of roll necks of metal-rolling mills).
In selecting lubricants for ball and roller bearings, the needs of each application must be studied. Due regard must be paid to the type of bearing, method of lubrication and operating conditions. Thereafter, to ensure satisfactory operation in service, the main points to bear in mind are the exclusion of moisture, dust and other contaminants and regular replenishment or renewal of the lubricant. Care should be taken to avoid over-filling because this may cause operating troubles; for example, a bearing that has been tightly packed with grease is liable to overheat.
Open Gears & Other application
Greases are used in large open gears in Industries. Greases are manually applied on the open gear tooth's or in special application like Cement Mill girth gears, it is sprayed on the gear tooth by a pneumatic centralised spray system. Greases also used for lubrication of rail flanges.
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