Page 265 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Power Transmission Gears 247

should be paid to several checks that need to be made before and after
start-up to help ensure trouble-free operation. During the first day or two
of operation, special emphasis should be focused on bearing and oil tem-
peratures and on housing and shaft vibration to catch any potential prob-
lems.

Alignment

Securing proper shaft alignment is one of the most important phases of
setting up a gear unit. Even though flexible couplings are used on the
shaft extensions, any appreciable amount of misalignment can cause a
multitude of gear problems ranging from nonuniform bearing and gear
tooth wear to vibration and coupling problems. Therefore, it is essential
that good alignment be established and maintained and that thermal
growth and bearing clearances be anticipated in shaft alignment.
Gear units experience both thermal and mechanical movements when
they are operating under load. The thermal movements are due to tem-
perature changes, both environmental and operational. Mechanical
movements are due to internal gear loads causing the shafts to move
within the bearing clearances; in severe cases, operating torques and
thrust loads may also distort the housing causing apparent shaft move-
ment. All of these factors should be accounted for when aligning a gear
to other equipment.
As is evident, machinery alignment is one of the most important fac-
tors contributing to satisfactory gear operation. However, it is too com-
plex a subject to be covered in full detail in this chapter. Although the
following discussion of alignment procedures is very brief, it does cover
the high points and give a good general procedure that may be followed.
The reader should refer to Volume 3, Chapter 5, for a more complete
description of current alignment practices.

Checking indicator Arm Sag
Prior to taking indicator readings for coupling alignment check, the
spanner arm that clamps to the shaft and supports the indicator arm and
indicators must be checked for possible gravity sag. This spanner device
must be rigid enough to minimize deflection due to the weight of the arm
and indicators. This check can be made by using the following method, as
illustrated in Figure 6-23.
1. Rotate clamp while attached to shaft; observe change in outside di-
ameter indicator reading at 90" positions.