GREASE LUBRICATION
The field application of lubricating greases is related to their non newtonian
characteristics.
It is
usual to employ grease as a lubricant where temperatures and speed s 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 hig her than lubricating oil is general 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 confined 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 up to which losses and heating are tolerable, fall sharply.
Calcium
soap greases are mainly used where temperatures are moderate. Calcium based
greases possess preferable for bearing s which have to run in the presence of
excessive moisture. Other types of greases are required where working
temperatures and more satisfactory sealing properties than the conventional "c
up" 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. ln
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 rung sout of the bearing
and leaves behind dry soap which hardens and cakes. This interferes with
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.
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.
Compressing Cup or Pressure Gun 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 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 which has been tightly packed with
grease is liable to overheat
