Page 313 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Power Transmission Gears 293

and cure of the ailment. Before a gear is shut down and replaced because
of questionable or seemingly severe damage, periodic examination with
photographs or impressions is recommended to determine if the observed
condition is progressive.

Repair
The most common repair performed on a gear unit is the replacement
of bearings. This procedure is normally straightforward, and the only ba-
sic difference from replacing pump, turbine, or compressor bearings is
that the alignment between the gear teeth must be maintained. After in-
stalling the new bearings and before replacing the cover, the tooth contact
should be checked. The importance of maintaining good tooth contact
cannot be overemphasized.
When gear tooth failure occurs, different methods can be used to repair
the gear set depending on the original design, hardness, and manufactur-
ing method used.
When wear or pitting is the only problem, the gear and pinion can
sometimes be recut or reground and returned to like-new condition. If
wear or pitting is severe, the gear can be reduced on the outside diameter
and recut and a new oversized pinion manufactured. This repair method
can be used on almost all through-hardened gears and does not change
the ratio.
Case-hardened gears cannot be recut due to the high hardnesses, and
the outside diameters cannot be reduced since the hardened case is too
thin. Regrinding is possible but risky for the same reasons. When the
case is broken through by pitting, regrinding will probably only delay
ultimate failure. In some cases, these gear blanks can be normalized, re-
cut, reheat-treated, and reground.
When only a pinion tooth is broken, a new matching pinion can be
made or the gear can be recut for an oversized pinion as just described. In
many cases, if a gear tooth is broken, a new alloy steel band can be in-
stalled on the gear hub by shrinking or welding and a new pinion manu-
factured. This procedure is difficult to do on case-hardened gearing due
to heat treating requirements. Also, this method of repair cannot be used
safely at extremely high pitch line speeds.
In the event of a breakdown, these repair procedures can save time,
materials, and money. The most important saving is usually in repair
time when spares are not available for rapid replacement.

Bibliography

1. Dudley, Darle W., Gear Handbook. McGraw-Hill Book Com-
pany, Inc., New York, 1962, 1st ed.