Page 304 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

284 Major Process Equipment Maintenance and Repair

Passage of Electric Current Through the Bearing

In certain applications where electrical machinery or electrical equip-
ment is in use, there is the possibility that electric current will pass
through a bearing. Current that seeks ground through the bearing can be
generated from magnetic fields in the machinery or can be caused by
welding on some part of the machine with the ground attached so that the
circuit is required to be completed through the bearing. There are many
other causes of this phenomenon varying from electrostatic discharge and
belts to manufacturing processes involving leather, paper, or rubber.
When the current is broken at the contact surfaces between rolling ele-
ments and raceways, marking results; this marking produces localized
high temperature, and consequently, the surfaces are damaged. This
damage is usually manifested as very small pits in the raceways and the
rollers. Very moderate amounts of electrical pitting do not necessarily
result in failure that may cause bearing life to be reduced. Severe electri-
cal pitting will cause almost immediate bearing failure when the machine
is started.

Gear Unit Disassembly and Assembly

Due to the large variety of gear housing or enclosure designs, it is not
practical to describe disassembly and assembly procedures for all differ-
ent types. Alternatively, the basic steps involved and some important pre-
cautions to observe will be discussed.

Dlsassem bly
The first step in disassembly is to remove the housing cover. Be careful
not to damage internal and external piping and instrumentation that may
be routed from the housing to the cover. The upper halves of journal-type
bearings also tend to stick in the cover half bores and may fall out as the
cover is being lifted, thereby damaging the bearing halves.
After the cover is removed, disconnect the internal instrumentation
and remove the upper bearing halves if present. On antifrktion bearings
with bearing carriers or capsules, it may be necessary to either remove or
support the carriers before removing the gear elements.
Extreme care must be used when removing, lifting, and handling gear
elements. Use soft slings and protect all gear surfaces when lifting and
handling. Do not set gear or pinion on a hard surface if it is to be reused.