Page 316 - The Tribology Handbook

c4 Greases

CONSISTENCY
The consistency of grease depends on, amongst other called Penetration.
things, the percentage of soap, or thickener in the grease. Penetration has been classified by the National Lubri-
It is obtained by measuring in tenths of a millimetre, the cating Grease Institute (NLGI) into a series of single
depth to which a standard cone sinks into the grease in five numbers which cover a very wide range of consistencies.
seconds at a temperature of 25°C (77°F) (ASTM D 217-IP This classification does not take into account the nature
50). These are called ‘units’, a non dimensional value which of the grease, nor does it give any indication of its quality
strictly should not be regarded us tenths of a millimetre. It is or use.

Table 4.4 NLGI consistency range for greases

Penetration range
Description NLGI no. (ASTM D 217-IP 50) Types generally available Some common uses
Semi-fluid 000 44-75 Not dye Centralised systems
Semi-fluid 00 400-430 Not dye
Very soft 0 355-385 Not dye Total loss systems

Soft I 3 10-340 All Rolling bearings
Medium soft 2 265-295 All
Medium 3 220-250 All General purpose
Stiff 4 175-205 Na or Ca only Plain bearings so
Very stiff 5 130-160 Na or Ca only called ‘Block’ or
Very stiff 6 85-1 15 Na or Ca only ’Brick‘ greases

The commonest consistencies used in rolling bearings use a grease stiffer than NLGI 2 and often a grease as soft
are in the NLGI 2 or 3 ranges but, since modern grease as an NLGI 0 may be found best. The extremes (000, 00,
manufacturing technology has greatly improved stability 0 and 4, 5, 6) are rarely, if ever, used in normal rolling
of rolling bearing greases, the tendency is to use softer bearings (other than 0 in centralised systems), but these
greases. In centralised lubrication systems, it is unusual to softer greases are often used for gear lubrication applica-
tions.

GREASE SELECTION

When choosing a grease consideration must be given to Likewise, in very low-temperature conditions, the
circumstances and nature of use. The first decision is always ambient temperature often has little effect after start-up
the consistency range. This is a function of the method of due to internal heat generation of the bearing. It is always
application (e.g. centralised, single shot, etc.). This will in advisable, if possible, to measure the temperature by a
general dictate within one or two NLGI ranges, the grade thermocouple or similar device. A measured temperature,
required. Normally, however, an NLGI 2 will be found to even ifit is not the true bearing temperature, will be a much
be most universally acceptable and suitable for all but afew better guide than a guess. By using Tables 4.1, 4.2 and 4.3
applications. above, the soap and fluid can be readily decided.
The question ofoperating temperature range comes next. Normally, more than one type of grease will be found
Care should be taken that the operating range is known with suitable. Unless it is for use in a rolling bearing or a heavily-
a reasonable degree of accuracy. It is quite common to loaded plain bearing the choice will then depend more or
overestimate the upper limit: for example, if a piece of less on price, but logistically it may be advisable to use a
equipment is near or alongside an oven, it will not neces- more expensive grease if this is already in use for a different
sarily be at that oven temperature-it may be higher due purpose. For a rolling bearing application, speed and size
to actual temperature-rise of bearing itself, or lower due to are the main considerations; the following Table 4.5 is
cooling effects by convection, radiation, etc. intended as a guide only for normal ambient temperature.
c4.3