Page 303 - Machinery Component Maintenance

Balancing of Machinery Components 285

interface with the drive flange of the universal-joint shaft that transmits
the driving torque from the balancing machine headstock.
If a horizontal machine with belt-drive is to be used, and if the rotor
has no surface over which the drive belt may be placed, the arbor must be
provided with a belt pulley, unless the belt can run over the arbor itself.
In either case, balancing speed and drive power requirements must be
taken into consideration. On machines with fixed drive motor speeds, the
ratio between drive pulley diameter and driven rotor (or arbor pulley)
diameter determines the desired balancing speed.
If arbors are to be used often, for instance for production balancing,
they should be hardened and ground. Special care must be taken during
storage to prevent corrosion and damage to locating and running sur-
faces.

Biasing an Arbor

This method is helpful whenever the runout (primarily radial runout)
of the arbor surface which locates the rotor represents a significant factor
in the error analysis. Biasing means the addition of artificial unbalance(s)
to the (otherwise balanced) arbor. The bias masses are intended to com-
pensate for the unbalance error caused by rotor displacement from the
arbor's axis of rotation; rotor displacement being caused, for instance, by
radial runout of the arbor surface which locates the rotor and/or, on verti-
cal machines, runout of the machine spindle pilot.
Since the attachment of masses to a (horizontal machine) arbor may
prevent it from being inserted in the rotor bore, biasing is often accom-
plished by grinding or drilling two light spots into the arbor, equidistant
to the left and right of the rotor. The light spots must have the same angu-
lar location as the high spot of the arbor surface which locates the rotor
radially.
The combined approximate unbalance value (g - in.) of the two high
spots may be calculated by multiplying the rotor weight (g) by 112 of the
TIR (in.). On vertical machines the addition of bias masses to the arbor is
often the simpler method. Whether the proper bias has been reached can
be tested by balancing a rotor to the machine's minimum achievable re-
sidual unbalance, and then indexing it 180" on the arbor. One half of the
unbalance which shows up after indexing is corrected in the rotor, the
other half in the arbor. This indexing procedure is repeated until no fur-
ther residual unbalance is detectable. The total correction made in the
arbor is now considered its bias correction compensating for its runout,
but only for the particular type of rotor used. If the rotor weight changes,
the bias will have to be corrected again. Bias correction requires a good
rotor fit. It will not overcome locating errors caused by loose fits.