Page 289 - Machinery Component Maintenance

Bahncing of Machinery Components 271

When maintenance requires antifriction bearings to be changed occa-
sionally on a rotor, it is best to balance the rotor on the journals on which
the inner races of the antifriction bearings fit. The unbalance introduced
by displacement of the shaft axis due to eccentricity of the inner races can
be minimized by use of high-quality bearings.

Driving the Rotor

If the rotor has its own journals, it may be driven in a horizontal bal-
ancing machine through:

1. A universal-joint or flexible-coupling drive from one end of the ro-
tor
2. A belt over the periphery of the rotor, or over a pulley attached to
the rotor
3. Air jets
4. Other power means by which the rotor is normally driven in the
final machine assembly

The choice of end-drive can affect the residual unbalance substantially,
even if the design considerations listed later in this text are carefully ob-
served. (See also “Balance Errors Due to Drive” on page 296.) Belt-
drive has the advantage here, but it is somewhat limited in the amount of
torque it can transmit to the rotor. Driving belts must be extremely flexi-
ble and of uniform thickness. Driving pulleys attached to the rotor should
be used only when it is impossible to transmit sufficient driving torque by
running the belt over the rotor. Pulleys must be as light as possible, must
be dynamically balanced, and should be mounted on surfaces of the rotor
which arc square and concentric with the journal axis. The belt drivc
should not cause disturbances in the unbalance indication exceeding one-
quarter of the permissible residual unbalance. Rotors driven by belt
should not drive components of the balancing machine by means of any
mechanical connection.
The use of electrical means or air for driving rotors may influence the
unbalance readout. To avoid or minimize such influence, great care
should be taken to bring in the power supply through very flexible leads,
or have the airstream strike the rotor at right angles to the direction in
which the balancing machine takes its readings.
If the electronic measuring system incorporates filters tuned to a spe-
cific frequency only, it is essential that means be available to control pre-
cisely the rotor speed to suit the filter setting.