Page 282 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Power Transmission Gears 263

may be exceeded on many high performance designs. If in doubt,
refer to the instruction manual furnished with the gear. Also, mea-
sured bearing temperature rise will depend on where and how the
measurement is made.
3. Run gear under full load and speed and check for unusual noise,
vibration, oil temperature, and bearing temperature. After temper-
ature stabilization, the oil temperature downstream from the oil
cooler (if used) should generally be no greater than 140°F. How-
ever, some special units are designed to operate with oil inlet tem-
peratures up to 180°F or more.
4. After the unit has run several hours (six or eight) under load, shut it
down, check coupling alignment, tighten any bolts that may be
loose, and recheck tooth contact.

Vibration Levels
A well manufactured and properly installed gear unit should run with
very little vibration; actual vibration levels may vary depending on the
type of foundation used. Expected vibration in terms of peak-to-peak dis-
placement levels for gears on permanent foundations and in good condi-
tion are shown in Figures 6-32 and 6-33 for the shafts and housings respec-
tively. Housing vibration velocity levels can be measured on the bearing
caps, and areas that may be in resonance should be avoided. In addition,
vibration velocity levels can be measured on the housing; acceptable levels
are shown in Table 6-2. Very low speed units with high shock loading can
have vibration levels several times those shown in Figures 6-32 and 6-33
and still be acceptable. Refer also to chapters on vibration monitoring in Vol-
ume 3, and later in this volume.

Acceleration Levels
To monitor gear tooth condition using accelerometers, a reasonably
stable area of the housing should be selected, such as a bearing cap. Re-
cord base-line acceleration levels when the gear is in good condition if
possible. Levels can be monitored continuously on a regular schedule.
Slight changes wi€l indicate that something in the system is different.
Large increases at tooth mesh frequency will usually indicate gear wear
or deterioration.
Very often large increases in acceleration will indicate that changes
have taken place in the alignment, bearings, or couplings, causing un-
even loading on the gear teeth. If external forces are causing the change,
early detection and correction can prevent gear failure.
There are no good rules as to what are acceptable acceleration levels
for gear drives. These levels will vary with design, installation, coupling