Page 98 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

OVERHUNG ROTORS

Overhung rotors are often found in fans or blowers that have
a W/D ratio of less than .5, and most often operate at speeds of
less than 1,000 rpm. The single plane method of balancing can
often be employed to correct unbalance conditions for this type of
rotor.
a b

•
2 1

▲ ▲
•

Figure 5-23. An Overhung Rotor

Referring to Figure 5-23, the vibration readings are taken at
bearing 1 and the corrections are made in plane a. Once the cor-
rections have been made, the vibration is measured at bearing 2.
If the vibration levels are unacceptable, corrections must be made
in plane b using the vibration data from bearing 2.
Adding a trial weight in the b plane will upset the balancing
effort done in the a plane. To compensate for this, an equal but
opposite weight is added in the a plane, to establish a couple
force. This second trial weight is used to preserve the static bal-
ance in plane a.
Placement of the second trial weight in the a plane, must be
exactly 180 degrees opposite the trial weight placed in the b plane
and produce the same unbalance. That is, it is of the same weight
and placed at the same radius, or of different weight but placed at
a radius that creates the same inch-ounces of unbalance.
Now the single plane method is performed for the b plane
using the data from bearing 2. Once the correction is made, be
sure to remove the trial weight from the a plane, and then recheck