Page 279 - Machinery Component Maintenance

Balancing of Machinery Components 261

Measurement of Amount and Angle of Unbalance

Three basic methods are used to obtain a reference signal by which the
phase angle of the amount-of-unbalance indication signal may be corre-
lated with the rotor. On end-drive machines (where the rotor is driven via
a universal-joint driver or similarly flexible coupling shaft) a phase refer-
ence generator, directly coupled to the balancing machine drive spindle,
is used. On belt-drive machines (where the rotor is driven by a belt over
the rotor periphery) or on air-drive or self-drive machines, a strobosco-
pic lamp flashing once per rotor revolution, or a scanning head (photo-
electric cell with light source) is employed to obtain the phase reference.
Whereas the scanning head only requires a single reference mark on
the rotor to obtain the angular position of unbalance, the stroboscopic
light necessitates attachment of an angle reference disc to the rotor, or
placing an adhesive numbered band around it. Under the once-per-revo-
lution flash of the strobe light the rotor appears to stand still so that an
angle reading can be taken opposite a stationary mark.
With the scanning head, an additional angle indicating circuit and in-
strument must be employed. The output from the phase reference sensor
(scanning head) and the pickups at the rotor-bearing supports are pro-
cessed and result in an indication representing the amount-of-unbalance
and its angular position.
In Figure 6-17 block diagrams are shown for typical balancing instru-
mentations.
Figure 6-17A illustrates an indicating system which uses switching be-
tween correction planes (i.e. a single-channel instrumentation). This is
generally employed on balancing machines with stroboscopic angle indi-
cation and belt drive. In Figure 6-17B an indicating system is shown with
two-channel instrumentation. Combined indication of amount of unbal-
ance and its angular position is provided simultaneously for both correc-
tion planes on two vectormeters having illuminated targets projected on
the back of translucent overlay scales. Displacement of a target from the
central zero point provides a direct visual representation of the displace-
ment of the principal inertia axis from the shaft axis. Concentric circles
on the overlay scale indicate the amount of unbalance, and radial lines
indicate its angular position.

Plane Separation

Consider the rotor in Figure 6-15 with only an unbalance mass on the
left end of the rotor. This mass causes not only the left bearing to vibrate
but, to a lesser degree, the right also. This influence is called correction
plane interference or, for short, “cross effect.” If a second mass is at-