Frictional behavior under oil lubricated condition
One of the earliest experimental studies in this category is the
work of Denny [15]. His main conclusion was that in addition to
144 T.J. Goda / Tribology International 93 (2016) 142–150
the adhesion component of rubber friction, an additional contribution
of comparable magnitude may arise due to the roughness
of the track surface. Among others the friction of NBR with 60, 75
and 90 Shore A hardness was measured at constant sliding velocity
of 0.1 mm/s and against various lubricated apparently smooth
track surfaces. The velocity dependence of friction was negligible
below this sliding velocity. At the same time the low sliding
velocity implies that the effect of frictional heat generation on
friction force can also be neglected. The size of the rectangular
shape rubber specimens varied from 5 mm thick and 10.000 mm2
area to 1 mm thick and 8 mm2 area. In relation to the load effect
he found that the coefficient of friction decreases with increasing
pressure and depends only on the nominal contact pressure and
not on shape or size of the specimen (see Fig. 2). When NBR 75
slides on olive oil lubricated polished steel track under nominal
contact pressure of 0.25 and 0.75 MPa the coefficient of friction
was 0.31 and 0.22, respectively. The back-calculated frictional
shear stress vs. nominal contact pressure curve (see Fig. 3) shows
clearly that, in case of NBR 60, the frictional shear stress (friction
force per unit nominal contact area) reaches a practically constant
value with increasing contact pressure. The constant friction force
suggests that the magnitude of real contact area reached the
magnitude of nominal contact area (complete contact). The pressure
inducing complete contact, as seen, becomes higher and
higher with increasing rubber hardness.
Measurement results showing the effect of track surface
roughness are particularly interesting. Friction tests on NBR 90
against light mineral oil lubricated polymethyl methacrylate
(PMMA) track surfaces with Ra¼0.01 (0.01), 0.13 (0.25), 0.2 (0.38),
0.38 (1.37) and 0.38 (1.62) mm (values without and with brackets
are valid along and across finishing marks) showed that at any
contact pressure between 0.01 and 10 MPa the coefficient of friction
increases as the track surface becomes rougher (see Fig. 4).
Measured Ra values indicate anisotropic surface roughness which
is in accordance with the fact that they were prepared by unidirectional
abrasion with emery cloth of various grades. Fig. 5
shows the variation of coefficient of friction as a function of
nominal contact pressure for NBR 90 sliding on smooth PMMA
track. In respect of track material Denny mentioned that when
replacing steel track with PMMA the coefficients of friction
became about 60% higher. This latter allows us to estimate the
coefficient of friction for NBR 90/smooth steel sliding pair under
oil lubrication (see Fig. 5). As the track is smooth Denny hypothesized
that friction is due to friction mechanism other than
micro-hysteresis. In order to represent the effect of track roughness
on the coefficient of friction Denny subtracted the coefficients
of friction measured on smooth track (Ra¼0.01 mm) from the ones
measured on rougher tracks. In [15], the difference is termed
average excess coefficient above smooth track value (contribution
of surface roughness to the coefficient of friction). Fig. 6 shows the
contribution of surface roughness back-calculated from [15] (data
Fig. 2. Coefficient of friction vs. nominal contact pressure (back-calculated from
[15]). The shaded area represents the range of nominal contact pressure values
appeared at room temperature in [6].
Fig. 3. Mean frictional shear stress vs. nominal contact pressure (back-calculated
from [15]). The shaded area represents the range of nominal contact pressure
values appeared at room temperature in [6].
Fig. 4. Variation of coefficient of friction with nominal contact pressure in log–log
scale when sliding across finishing marks (based on [15]). The shaded area represents
the range of nominal contact pressure values appeared at room temperature
in [6].
Fig. 5. Variation of coefficient of friction with nominal contact pressure for smooth
PMMA and steel track (based on [15]). The shaded area represents the range of
nominal contact pressure values appeared at room temperature in [6].
T.J. Goda / Tribology International 93 (2016) 142–150 145
points) and as reported in [15] (solid line) as a function of track
roughness along direction of sliding. In Denny’s opinion, the solid
line can be considered as a largely pressure independent upper
limit for the average excess coefficient above smooth track value.
This statement is based on the fact that friction test results complicated
with neither frictional heat generation nor elastohydrodynamic
effects showed only slightly increasing track roughness
effect on rubber friction as the nominal or mean contact
pressure increased (see Fig. 6). Denny po