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
