Page 278 - Machinery Component Maintenance

260 Machinery Component Maintenance and Repair

Hard-Bearing Balancing Machines
Hard-bearing balancing machines are essentially of the same construc-
tion as soft-bearing balancing machines, except that their bearing sup-
ports are significantly stiffer in the transverse horizontal direction. This
results in a horizontal resonance for the machine which occurs at a fre-
quency several orders of magnitude higher than that for a comparable
soft-bearing balancing machine. The hard-bearing balancing machine is
designed to operate at speeds well below this resonance (see Figure
6-16B) in an area where the phase angle lag is constant and practically
zero, and where the amplitude of vibration-though small-is directly
proportional to centrifugal forces produced by unbalance.
Since the force that a given amount of unbalance exerts at a given
speed is always the same, no matter whether the unbalance occurs in a
small or large, light or heavy rotor, the output from the sensing elements
attached to the balancing machine bearing supports remains proportional
to the centrifugal force resulting from unbalance in the rotor. The output
is not influenced by bearing mass, rotor mass, or inertia, so that a perma-
nent relation between unbalance and sensing element output can be estab-
lished.
Centrifugal force from a given unbalance rises with the square of the
balancing speed. Output from the pick-ups rises proportionately with the
third power of the speed due to a linear increase from the rotational fre-
quency superimposed on a squared increase from centrifugal force. Suit-
able integrator circuitry then reduces the pickup signal inversely propor-
tional to the cube of the balancing speed increase, resulting in a constant
unbalance readout. Unlike soft bearing balancing machines, the use of
calibration masses is not required to calibrate the machine for a given
rotor.
Angle of lag is shown as a function of rotational speed in Figure 16A
for soft-bearing balancing machines whose balancing speed ranges start
at approximately twice the resonance speed of the supports; and in Figure
6-16B for hard-bearing balancing machines. Here the resonance fre-
quency of the combined rotor-bearing support system is usually more
than three times greater than the maximum balancing speed.
For more information on hard-bearing and other types of balancing
machines, see articles on advantages of hard-bearing machines and on
balancing specific types of rotors. (Reprints are available through
Schenck Trebel Corporation.)
Both soft- and hard-bearing balancing machines use various types of
sensing elements at the rotor-bearing supports to convert mechanical vi-
bration into an electrical signal. These sensing elements are usually ve-
locity-type pickups, although certain hard-bearing balancing machines
use magnetostrictive or piezo-electric pickups.