Page 311 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Power Transmission Gears 291

Vibration

All gear units operate at certain vibration levels. Generally speaking, Fig-
ures 6-32,6-33, and Table 6-2 depict vibration values expected of a gear unit
properly installed and in good condition. Vibration levels above these may
be perfectly acceptable but must be evaluated on a case-by-case basis.
Excessive vibration may be caused by:

1. Unbalance-This phenomenon is the most common cause of gear
unit vibration and can be produced by broken teeth, couplings, key
fitting practice, improper balancing during manufacture, poor as-
sembly of gear to shaft, and even oil inside the gear blanks. Al-
most any vibration specialist can isolate the cause of unbalance vi-
bration and either balance the parts or determine what must be
corrected.
2. Loose Foundation Bolts-This condition is usually detected by in-
spection. When retightening loose foundation bolts, be careful that
shims are not missing. Be sure that the housing foot is not “soft.”
3. Coupling Misalignment-Misalignment is a serious problem with
gear units, and many papers have been written on alignment con-
trol. A machinery train that is properly aligned today will change
over the years due to settling of the foundation. Misalignment se-
vere enough to cause high vibration levels will damage the gear set
and shorten the life.
4. Inadequate Foundation-This cause of vibration is self explana-
tory and is most generally due to improperly designed and manu-
factured steel bases under gear units.
5. Wear in Bearings and Gears-Wear in gear teeth most generally
shows up as an increase in the vibration or acceleration levels at
tooth mesh frequency. Bearing wear can be detected by excessive
clearance in journal-type bearings and pitting or spalling of rolling
element bearings.
6. Lateral and Torsional Critical Speed Response-On high speed
drives, lateral critical speeds of the shafts become very important,
and users should be very careful when changing couplings to be
sure that the weights and centers of gravity are the same as used
during design stages. Torsional critical response is very important
but is most common on reciprocating machines. In addition to
causing vibration, both of these responses reduce gear life and in
some extreme cases can cause immediate failure.
7. Coupling Lockup-Lockup is a form of coupling misalignment
that occurs when toothed couplings are unable to shift axially to