Page 309 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Power Transmission Gears 289

causes of pump noise are cavitation in the pump suction area and
piping resonance.

Gear noise can be controlled to some extent by three measures: very
careful design and super quality manufacturing, extra heavy cast iron or
double wall steel housings, and acoustical enclosures. Any of these mea-
sures can be used singly or in combination with one or both of the other
measures to effectively reduce gear noise.
Very careful design and super quality manufacturing is the most expen-
sive way to control gear noise in addition to being the most difficult to
apply. Contrary to some opinions, the perfect gear is useless for power
transmission because of tooth deflections under load. The trick is to ob-
tain a gear which has a perfect involute (or conjugate) form under load.
The harder the gear, the more deflection there is due to higher allowable
loading; as a result, good mesh conditions are more difficult to obtain
since the involute produced is distorted more.
Secondly, extra heavy cast iron or double wall housings used with rea-
sonably accurate gearing can be very effective in controlling gear noise.
This method is less expensive than the first and is easier to apply consis-
tently in manufacturing. Also, detuning techniques can be used on hous-
ings and gear blanks to reduce noise based on calculated and experimen-
tal data.
Using an acoustical enclosure is the least expensive way to reduce gear
noise. Almost any noise level can be attained if space is not a problem.
However, sound enclosures have a very definite disadvantage when
maintenance is required. In addition, the inability of operators or mainte-
nance personnel to actually place their hands on the equipment or hear
the noises emitted if problems begin to appear may allow total destruc-
tion of the gear unit to occur instead of just minor damage. No matter
how sophisticated the monitoring equipment, the senses of touch and
hearing are still the best indications of a machine’s condition.

Overheating
Before it can be determined whether or not a gear unit is overheating,
the expected operating temperature must be determined. Very low speed
gears will run near the ambient temperature, and some high speed drives
may operate above 250°F.
Overheating in gear units may be caused by:
1. Low Oil Level-This condition will lead to both overheating and
gear failure should the level fall below a point where the gear teeth
or the oil pump can pick up the oil.