ML
www.machinerylubrication.com
|
September - October 2015
|
11
than is generally available, this test methodology is clearly a poor
choice for an in-service or condition monitoring test.
The cone penetration test can be performed in a worked or
unworked condition. The intent is to test the grease samples with a
similar level of pre-conditioning for a more consistent result. The
common pre-conditioning of a worked cone penetration sample
test is to be sheared for 60 strokes. However, this pre-conditioning
practice is only intended for new greases. In-service grease receives
no pre-conditioning prior to machine operation; the machine
simply starts.
The reporting method used for the cone penetration test is also
very crude, and the results can be misleading. The NLGI numbering
system appears to be linear (0, 1, 2, 3, etc.), but the grease consis-
tency changes are not linear. For example, it would be reasonable to
assume that an NLGI 4 grease would be twice as stiff as an NLGI 2
and four times as stiff as an NLGI 1. However, this is not the case.
Figure 1 was developed by determining the surface area in
contact with the grease for increasing depths of the cone. These
depths were then compared and plotted. It is clear that the change
in surface area in contact with the grease is non-linear.
Use of the NLGI scale permits tremendous product variation,
which becomes more pronounced with stiffer greases. This allows
the producer a huge manufacturing margin in the product's rheo-
· Series1
360
300
250
200
150
100
50
0
0 50 100 150 200 250 300 350 400 450 500
1/4 Scale Cone Area
Measured Penetration
Cone
Area
mm2
Figure 1. Cone area change with increasing depth
ML
