Static Electricity 2000 Edition

9.4.5 Pulleys and Shafts.
9.4.5.1
Metal pulleys can accumulate a charge equal and
opposite to that carried by the belt that runs over them, and
this charge typically will pass to the supporting shaft and then
through bearings to the equipment and to ground. Where
nonconductive components isolate metal parts, separately
bonding or grounding those parts could be necessary.
9.4.5.2
Lubricated bearings are still sufficiently conductive to
allow dissipation of a static electric charge from the shaft.
However, the conductivity across bearings that operate at very
high speed is not necessarily sufficient to prevent accumula-
tion of charge when the rate of generation is also high. For this
reason, shafts that rotate at high speed should be checked for
accumulation of static electric charge and should be bonded
or grounded by means of a sliding metal contact to the hous-
ing, if necessary.
9.4.5.3
The effective resistance across a bearing while in oper-
ation can be measured with a common ohmmeter. One of the
probes should be placed on the grounded machine frame and
the other probe should be allowed to rest against the rolling
member. A value of approximately 10
4
ohms is to be expected.
If a value greater than 10
5
ohms is found, an auxiliary ground-
ing brush or shoe might be needed to prevent changes with
time that could exceed 10
6
ohms. The grounding brush needs
periodic checking and maintenance. Critical bearings without
a brush should be measured periodically.
9.4.6 Maintenance of Belts and Conveyors.
Belts and convey-
ors should be inspected frequently for slipping or jamming to
lessen the chance of generation of static electricity. Drive sys-
tems should be designed to stall without slipping when oper-
ating in hazardous environments. Lubricant does not prevent
the removal of static electric charges. Therefore, all bearings
should be properly lubricated. However, the flow of static elec-
tricity across the film of lubricant sometimes results in pitting
of bearing surfaces. A conductive grounded brush running on
the shaft or pulley will prevent pitting of the bearings.