Page 274 - Machinery Component Maintenance

256 Machinery Component Maintenance and Repair

In the first category, advantage is taken of the fact that a body free to
rotate always seeks that position in which its center-of-gravity is lowest.
Gravity balancing machines, also called nonrotating balancing machines,
include horizontal ways or knife-edges, roller stands, and vertical pendu-
lum types (Figure 6-14). All are capable of only detecting and/or indicat-
ing static unbalance.
In the second category, the amplitude and phase of motions or reaction
forces caused by once-per-revolution centrifugal forces resulting from
unbalance are sensed, measured, and indicated by appropriate means.
The rotor is supported by the machine and rotated around a horizontal or
vertical axis, usually by the drive motor of the machine. A centrifugal
balancing machine (also called a rotating balancing machine) is capable
of measuring static unbalance (singlc planc machinc) or static and couple
unbalance (two-plane machine). Only a two-plane rotating balancing ma-
chine can detect couple and/or dynamic unbalance.
Field balancing equipment, the third category, provides sensing and
measuring instrumentation only; the necessary measurements for balanc-
ing a rotor are taken while the rotor runs in its own bearings and under its
own power. A programmable calculator may be used to convert the vi-
bration readings (obtained in several runs with test masses) into magni-
tude and phase angle of the required correction masses.

(B) Roller Stand
(A) Horizontal Ways (C) Pendulum

Figure 6-14. Static balancing devices.