Page 312 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

2% Major Process Equipment Maintenance and Repair

account for thermal growth. This condition can be caused by full
load starts, poor lubrication, wear, and centrifuging of the cou-
pling lubricant.
8. Coupling Weur-On toothed couplings, wear can cause both cou-
pling lockup and shifting of the loose pieces on the coupling. As
an example, the teeth on the outer element of a gear tooth coupling
can wear and allow the sleeves and spacer to shift off center. This
shifting then produces an unbalance equal to the weight of the
shifted coupling parts times the distance shifted.
9. Lack of Coupling Lubricant-Inadequate coupling lubricant will
prevent the coupling from performing as required by the design
and is equivalent to having a rigid shaft connection.
10. CoupZing Not us Designed-The use of couplings with weights
and stiffnesses different than the original design can cause en-
croachment on lateral and torsional natural frequencies of the sys-
tem. On very low speed drives, coupling weight has much less im-
portance than on pinions operating at high rates of speed.
11. Improper Installation-This subject covers a broad range of prob-
lems from foundation to lube oil and cooling water piping connec-
tions. When planning a gear unit installation, all environmental
conditions must be carefully considered since operating conditions
will vary with the cold wind, hot sun, and all other external influ-
ences.

Tooth Fallure and lnspectlon

The most up-to-date work on gear tooth distress is ANSUAGMA stan-
dard 110.04, “Nomenclature of Gear Tooth Wear and Failure.” The term
“gear failure” is in itself subjective and a source of considerable dis-
agreement. One observer’s “failure” can be another observer’s “wear-
ing-in.” For a summary of this AGMA standard, refer to pages 131-147 of
Volume 2 of this series, Machinery Failure Analysis and Troubleshooting.
Suffice it to say that during the initial period of operation of a set of
gears, minor imperfections will be smoothed out, and the working sur-
face will polish up, provided that proper conditions of design, applica-
tion, material manufacture, installation, and lubrication have been met.
Under continued normal conditions of operation, the rate of wear will be
negligible.
Failure in a gear train can in many instances be prevented. When it
does occur, the proper remedial action or redesign will ensure a trouble-
free unit. Regardless of when the trouble is rectified, the most important
faculty of those concerned with the problem is the ability to recognize the
exact type of incipient failure, how far it has progressed, and the cause