186 Lubricant Additives: Chemistry and Applications
- Performance enhancement that extends the capability of the conventional liquid lubricant
- Performance enhancement that extends the service life of the conventional liquid lubricant
- Applications that undergo a “start/stop” routine
- Applications that require low sliding speed but heavy bearing load
- 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.