Page 299 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Power Tmmission Gears 279

Measurement of bearing clearances may be done by lifting the shaft
and measuring the distance traveled by the shaft with a dial indicator.
Also, feeler gauges or plastic gauge material can be used. Some wear
should be expected, especially on a gear unit that is stopped and started
or slow-rolled frequently. The bearing may be considered operational as
long as the measured clearance does not exceed the design clearance by
more than .004 to .005 in. for bearings in the 3 to 8 in. diameter range.
This value for clearance increase is acceptable for most applications and
can be used when the manufacturer’s recommendation is not available.
Note: If high shaft vibration develops, this clearance increase may not be
acceptable.
The thrust bearing clearance provides room for the formation of an oil
film between the bearing face and the gear hub and for thermal expansion
of the shafting. Normally, wear of the thrust bearing is not very critical
unless it is enough to cause loss of oil pressure in the lubrication system.
Measurement of thrust bearing clearance wear may be done by pushing
the slow speed shaft axially to one side of the unit, setting a dial indica-
tor, and then pulling the shaft axially to the other side of the unit against
the dial indicator.
On gear units with double helical gearing where there is one thrust
bearing located on the low speed shaft, the end play of the high speed
shaft may be determined by holding the slow speed gear stationary and
measuring the axial movement of the pinion against the gear. This value
should be added to the thrust bearing clearance to obtain the total end
play of the high speed shaft. When both the high speed and low speed
shafts have thrust bearings, the high speed thrust bearing clearance may
be measured in the same manner as the low speed.

Bearlng High Spots

Evidence and location of high spots in the bearing are indicated by
bright spots or areas. These spots are caused by wear resulting from a
ruptured oil film around the high point. Bright spots should be lightly
scraped and polished with fine steel wool or crocus cloth until they blend
in with the rest of the bearing. Caution: Do not use sand paper.
To check the bearing contact, install the lower half of the bearing in the
housing with the journal and thrust face clean and dry. Check outside
diameter of bearing with a .0015 in. feeler gauge to be sure the lower half
is seated in the housing. Apply a light coat of soft blue to the journal and
to each thrust face. The journal should show blue transfer for a minimum
of 80 percent of bearing length. Thrust faces should show a minimum
contact of 60 percent of load area. Contact may be spread out to the de-
sired amount by removing high spots using a bearing scraper and fine