Page 257 - Machinery Component Maintenance

Balancing of Machinery Components 239

Unbalance in just one rotating component of an assembly may cause
the entire assembly to vibrate. This induced vibration in turn may cause
excessive wear in bearings, bushings, shafts, spindles, gears, etc., sub-
stantially reducing their service life. Vibration sets up highly undesirable
alternating stresses in structural supports and frames that may eventually
lead to their complete failure. Performance is decreased because of the
absorption of energy by the supporting structure. Vibrations may be
transmitted through the floor to adjacent machinery and seriously impair
its accuracy or proper functioning.

The Balancing Machine as a Measurlng Tool

A balancer or balancing machine is necessary to detect, locate, and
measure unbalance. The data furnished by the balancer permit changing
the mass distribution of a rotor, which, when done accurately, will bal-
ance the rotor. Balance is a zero quantity, and therefore is detected by
observing an absence of unbalance. The balancer measures only unbal-
ance, never balance.
Centrifugal force acts upon the entire mass of a rotating element, im-
pelling each particle outward and away from the axis of rotation in a ra-
dial direction. If the mass of a rotating element is evenly distributed about
its shaft axis, the part is “balanced” and rotates without vibration. How-
ever, if an excess of mass exists on one side of a rotor, the centrifugal
force acting upon this heavy side exceeds the centrifugal force exerted by
the light side and pulls the entire rotor in the direction of the heavy side.
Figure 6-1 shows the side view of a rotor having an excess mass m on one
side. Due to centrifugal force exerted by m during rotation, the entire
rotor is being pulled in the direction of the arrow F.

Causes of Unbalance

The excess of mass on one side of a rotor shown in Figure 6-1 is called
unbalance. It may be caused by a variety of reasons, including:

1. Tolerances in fabrication, including casting, machining, and assem-
bly.
2. Variation within materials, such as voids, porosity, inclusions.
grain, density, and finishes.
3. Nonsymmetry of design, including motor windings, part shapes,
location, and density of finishes.
4. Nonsymmetry in use, including distortion, dimensional changes,
and shifting of parts due to rotational stresses, aerodynamic forces,
and temperature changes.