Chemistry Reference
In-Depth Information
derivatives differ in the chain length of the carboxylic acid substituents on
the PHMWO. The longer carboxylate groups in the polyvalerates relative to
those in the polyacetates leads to a dramatic difference in the density and
viscosity of these two materials (Tables 10.2-10.4). As shown in Tables 10.2-
10.4, both the viscosity and the density of HMWO decreased due to ester-
ification of the hydroxyl groups. In addition, both the viscosity and the
density of the esterified MWOs were functions of the chain length of the
substituent groups. Both properties also decreased with increasing chain
length, but the decrease in viscosity was the more dramatic. Viscosity, along
with entrainment speed, are the two major parameters affecting lubricant oil
film thickness, and can be used to predict film thickness using the
Hamrock-Dowson (H-D) relationship.
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Predicted film thicknesses of
AMWO and VMWO oils using the H-D equation is illustrated by the lines in
Figure 10.13. Examination of the measured vs. predicted film thickness data
in Figure 10.13 reveals a number of interesting features. For AMWO at 70 1C,
the measured and predicted values showed excellent agreement at all en-
trainment speeds. For VMWO at 70 1C, an excellent agreement between ex-
perimental and predicted film thickness was observed mainly in the high
entrainment speed region, which also corresponds to high film thicknesses.
In the low entrainment speed region, however, VMWO displayed a negative
deviation, i.e., the measured film thickness was smaller than that predicted
by H-D theory. Such deviations between measured and H-D predicted EHD
film thicknesses, in thin and ultrathin film regions, have been reported for a
number of oils.
15,25-28
Various mechanisms have been proposed to explain
these deviations but more work is needed to develop a comprehensive theory.
10.3.5 Friction and Wear
The friction properties of these fluids were investigated using a 4-ball trib-
ometer configured for AW evaluation using ASTM D4172.
12
In this method, a
steel ball is loaded (392 N) against three balls and rotated (1200 rpm) while
completely immersed in the test lubricant maintained at 75 1C. During the
test, which lasts 60 min, the frictional torque, temperature, load, and speed
are continuously monitored and recorded. At the end of the test, the wear
scar diameters (WSDs) on the three balls are measured parallel and trans-
verse to the wear direction, and the six measurements averaged. The re-
corded frictional torque and load were used to calculate the COF using the
procedure outlined in ASTM D5183-95.
13
Two tests were conducted on each
test lubricant and the resulting COF and WSD values from each measure-
ment averaged and used in further analysis. An example of COF vs. time data
from a repeat measurement on a test lubricant is illustrated Figure 10.14.
The data in Figure 10.14 is for neat VMWO. As shown in both measurements,
the COF displayed an initial sharp increase and decrease. This was followed
by a more or less steady-state value from about 500 s until the end of the test.
The average and standard deviation of the COF values in the steady-state
region of the repeat measurements for this lubricant were 0.051
0.008 and
