Page 258 - Machinery Component Maintenance

240 Machinery Component Maintenance and Repair

I

Centri fuga1
Force (F)

ala\nce Mass (m)

Flgure 6-1. Unbalance causes centrifugal force.

Often, balancing problems can be minimized by symmetrical design
and careful setting of tolerances and fits. Large amounts of unbalance
require large corrections. If such corrections are made by removal of ma-
terial, additional cost is involved and part strength may be affected. If
corrections are made by addition of material, cost is again a factor and
space requirements for the added material may be a problem.
Manufacturing processes are the major source of unbalance. Unma-
chined portions of castings or forgings which cannot be made concentric
and symmetrical with respect to the shaft axis introduce substantial un-
balance. Manufacturing tolerances and processes which permit any ec-
centricity or lack of squareness with respect to the shaft axis are sources
of unbalance. The tolerances, necessary for economical assembly of sev-
eral elements of a rotor, permit radial displacement of assembly parts and
thereby introduce unbalance.
Limitations imposed by design often introduce unbalance effects which
cannot be corrected adequately by refinement in design. For example,
electrical design considerations impose a requirement that one coil be at a
greater radius than the others in a certain type of universal motor arma-
ture. It is impractical to design a compensating unbalance into the arma-
ture.
Fabricated parts, such as fans, often distort nonsymmetrically under
service conditions. Design and economic considerations prevent the ad-
aptation of methods which might eliminate this distortion and thereby re-
duce the resulting unbalance.