Lubricant Additives

186 Lubricant Additives: Chemistry and Applications

3. Performance enhancement that extends the capability of the conventional liquid lubricant


4. Performance enhancement that extends the service life of the conventional liquid lubricant


5. Applications that undergo a “start/stop” routine


6. Applications that require low sliding speed but heavy bearing load


7. Applications that require “fool-proofi ng” for potential catastrophic lubrication failures that
   result from lubricant starvation

For successful incorporation of a solid lubricant as a secondary additive into liquid lubricants, a
well-formulated colloidal dispersion is required. As an example, consider a case study where gear
oil performance is enhanced above that of a conventional liquid lubricant by use of colloidal solids.
The addition of 1% colloidal molybdenum disulfi de to AGMA No. 7 and AGMA No. 8 gear oils
reduced the break-in times and steady-state operating temperatures of low-viscosity synthetic oils as
compared to nonfortifi ed gear oils [11]. Table 6.7 summarizes a comparison of the performance of
various blended gear oils to the measured output criteria as tested on a worm gear dynamometer.
Another example concerns the potential lubrication improvement from solid lubricants for
friction-modifi ed engine oils. Because of the burnishing property that solid lubricants such as
colloidal graphite or colloidal MoS 2 would have on metal surfaces, friction reduction in engine and

5 Kg

1 Kg

FIGURE 6.10 Lubrication of sliding surfaces—friction reduction.

TABLE 6.7

Worm Gear Dynamometer Tests

Description

Performance Parameters Output Torque = 113 N m

Mean Input

Torque (N m)

Percent

Effi ciency

Mean Oil Sump

Temperature (°C)

AGMA #8 gear oil 6.02 62.6 92.1
AGMA #8 gear oil + 1% 5.92 63.6 95.5
colloidal MoS 2 dispersion
AGMA #7 gear oil 6.05 62.3 93.6
AGMA #7 gear oil + 1% 5.89 64.0 93.4
colloidal MoS 2 dispersion
Synthetic PAG #2 oil 6.09 61.8 108.8
Synthetic PAG #2 oil + 1% 5.79 65.1 88.4
MoS 2 dispersion

Source: Pacholke, P.J., Marshek, K.M., Improved worm gear performance with colloidal molybdenum disulfi de containing
lubricants, ASLE paper presented at the 41st Annual Meeting in Toronto, Ontario, Canada, May 12–15, 1986.