Lubricant Additives

Organic Friction Modifi ers 205

in the BL regime. Recent investigations by the authors, carried out with a pin-on-ring tribometer,
showed that it is likely that organic FMs act predominantly in the BL regime as well.
Tests were carried out with CEC reference oil RL 179/2, which is applied in the CEC L-54-T-96
fuel economy test. RL 179/2 is a formulated 5W/30 engine oil that does not contain any FM and that
has a proven fuel economy benefi t CEC round-robin tests.
Frictional behavior was investigated by establishing stabilized Stribeck curves. By determin-
ing these, both boundary and mixed friction can be investigated. Stabilized Stribeck curves are
obtained by measuring the coeffi cient of friction over a speed range from ∼0.0025 to 2 m/s at
appropriate steps. A number of runs are carried out until two consecutive runs give a good match.
Usually, after four runs, the curve has stabilized, meaning that process roughness has stabilized to
a large extent.
Performance criteria in considering the results are frictional level in the BL and ML regimes in
combination with specimen wear. The reason for looking at wear is that this parameter corresponds
with contact pressure, which in turn infl uences the ML/EHL transition.
The relationship between wear and contact pressure is given by the expression

F

A

n =p

(7.3)

where
Fn = normal force (load)
A = wear scar
p = contact pressure

Consequently, a high-wear scar leads to a lower contact pressure, and a lower contact pressure does
shift the ML/EHL transition in the Stribeck curve to the left (see Figure 7.9).

7.8.1 STRIBECK CURVE DETERMINATIONS

Stabilized Stribeck curves have been determined with a pin-on-ring tribometer at which the ring
was a 100Cr6 stainless steel ring with a 730 mm diameter. These rings are high-quality materials
used in standard bearings and therefore easily available. The pin used was a cylinder from the same
material with an 8 mm diameter, also used in bearings. To get proper line contact, the cylinders
have been provided with fl exible ends to allow full alignment with the ring.

FIGURE 7.9 Infl uence of wear/contact pressure on ML/EHL transition.

Boundary lubrication

Mixed lubrication

Elasto-hydrodynamic

lubrication

Lubricant parameter

(viscosity × speed / load)

Contact pressure

(wear) High Low

Friction coefficient

f