Page 305 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Power Transmission Gears 285

Preparation for Assembly

Before assembling a gear unit, several steps should be taken to prepare
it. These steps will help to ensure trouble-free, satisfactory operation of
the gear unit after it is put back into service.
First of all, remove all of the old split line sealant from the machined
surfaces of the housing and cover. Also remove the excess sealant that
ran into the bores and oil passages. Many bearings have been failed by
the presence of excessive sealant or old sealant. As a final step in prepar-
ing the split line, flat-file the machined surfaces to remove all sealant
residue and nicks.
While a gear unit is disassembled, try to keep any dirt or trash out of
the housing and off the parts. In addition, before assembling the gear
unit, ensure that the parts and housing are as free as possible of dirt and
trash. If feasible, the gear housing should be washed down with solvent
during assembly, then the system should be flushed with oil after assem-
bly is completed. During the flushing procedure, the oil temperature
should be raised to 110 to 140°F and the shafts rotated in both directions
by hand to dislodge any trash.
Cleanliness cannot be over-emphasized. With higher load and higher
precision gearing, cleanliness is more important since very high preci-
sion gears operate with a lubricant film thickness at the gear mesh of a
few microns. Any minute piece of foreign material present can pass
through the mesh and damage the gear teeth. Furthermore, journal type
bearings operate with film thickness of less than .001 in., and any trash
present will become embedded in the bearing, causing damage to the
bearing and the shaft. Rolling element bearings are a little more forgiving
of foreign material than journal bearings are since any foreign particles
tend to pass through the bearing rather than embed in it.
Before installing the bearings-in the gear unit, it is important to be sure
that they fit down in the bores and that the bores do not crush the bearings
at the split line. For journal type bearings, it is often necessary to fit the
bearing outside diameter to the housing bore and also to fit the bearing
inside diameter to the shaft journal. While fitting the bearing to the jour-
nal and bore, gear tooth contact should be checked using the soft blue
procedure. In the event of poor contact, the bearings should be refitted
until the contact pattern is acceptable.
One of the aspects most influential on the satisfactory operation of a
gear unit is the handling of the rotating elements. Many gear sets have
been destroyed or have required extensive rework due to improper han-
dling. Before assembling gear elements in a housing, they should be in-
spected visually and by feel for nicks and bruises. The importance of the
sense of touch during this inspection should not be underestimated since