Lubricant Additives

Organic Friction Modifi ers 209

Further studies on the mechanisms of FM action, for example, through molecular modeling
techniques, could also speed up the development of optimized additives and additive systems. Apart
from frictional properties, other important tribological parameters such as wear rate and surface-
metal geometry should be investigated as well. In most papers studied, this seems to be ignored,
although all three parameters should be considered in relation to one another.

7.10 BENCH TESTS TO INVESTIGATE FRICTION-REDUCING COMPOUNDS

Several bench tests can be thought of to investigate the frictional properties of base fl uids and for-
mulated products. In recent literature [8,23,24], the following test equipment has been used:

1. The high-frequency reciprocating rig (HFRR) to measure boundary friction. Although
   originally developed to measure diesel fuel lubricity, the equipment can be successfully
   applied to measure lubricant properties as well.

Frequency 10–200 Hz

Stroke length 20–2000 μm

Load 0–1000 g

Ambient temperature 200°C

A 6 mm diameter ball is the upper specimen and a 3 mm thick smooth disk with a

10 mm diameter is the lower specimen. HFRR specifi cations are the test conditions that

the authors applied to screen FMs; these include a 40 Hz frequency, a stroke of 1000 μm,

and a 400 g load.

2. A mini traction machine (MTM) to measure mixed and (E)HD friction, for example, by
   the determination of Stribeck curves. The MTM rig is capable of measuring at either con-
   stant or varying slide/roll ratios if required.

Speed range Up to 5 m/s

Slide/roll ratio 0–200% (Full rolling to full sliding)

Load 0–75 N

Ambient temperature 150°C

Standard specimens are a 19.05 mm diameter ball as upper specimen and a 50 mm

diameter disk as lower specimen. Both are manufactured from AISI 52100 bearing steel.

The standard disk is smooth, which allows measurement of mixed-fi lm and full-fi lm fric-

tion. Alternatively, rough disks are available for measurements in the BL regime. MTM

specifi cations are the test conditions that the authors applied to test FMs; these comprise a

speed range of 0.001–4 m/s, a 30 N load, and a 200% slide/roll ratio.

3. An optical rig provided with a disk coated with a spacer layer to measure EHD fi lm thick-
   ness. Such a rig enables fi lm-thickness measurements down to <5 nm with a precision
   between 1 and 2 nm.
   Some other literature refers to the low-velocity friction apparatus (LVFA). Alternative
   reciprocating rigs may be suitable as well.

REFERENCES

1. Wilk, M.A., W.D. Abraham, B.R. Dohner. An investigation into the effect of zinc dithiophoshpate on
   ASTM sequence VIA fuel economy. SAE Paper 961,914, 1996.


2. Houben, M. Friction analysis of modern gasoline engines and new test methods to determine lubricant
   effects, 10th International Colloquium, Esslingen, 1996.