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“The motor shaft: not as simple as it looks" With
trilingual summary
Although only a few standard grades of steel are
used for laminations in electric motor stators and rotors, many
varieties are available for machine shafts. Difference in alloys and
heat treatments allow a wide range of stress limits. No typical or
average material is used everywhere.
Stresses in steel shafts are complex, normally
involving combinations of bending, torsion, and shear. Some stresses
alternate or reverse in direction with each shaft revolution; others
may be cyclically varying. Good design dictates holding maximum
stress below the endurance or fatigue limit – the fairly low stress
which can be sustained indefinitely through repeated cycles of
application.
Shock loading is encountered in motors driving
crushers and similar machines. That greatly increases shaft stress.
It may be sudden load (in which two drive components are in contact
before force is applied between them) or impact (in which one
component moves through a distance before striking the other.)
Unlike strength, shaft stiffness (resistance to
bending) is independent of steel composition. Shaft critical speed or
resonance is a function of shaft deflection under the influence of
gravity. Calculated shaft deflection depends upon how the designer
considers rotor/shaft weight to be applied to the shaft, and upon the
method chosen to allow for the non-uniform diameter of most shafts.
In large machines, shaft stiffness is often
enhanced by welding radial arms to the shaft, on which the rotor is
mounted. The welding process must be carefully controlled, from
preheat to stress relief, to minimize the likelihood of fatigue
cracking in service.
Left to itself, any shaft-rotor assembly will
assume a deflection curve shape by bending under its own weight. In
some large drive trains, containing several coupled machines, the
various support bearings may have to be located and shaped so that
the full length of shafting is free to follow its natural curvature.
Otherwise, depending upon the couplings used, some shaft areas may be
subjected to unacceptably high bending stress.
When an existing motor is redesigned for higher
horsepower, or is applied to a different type of load (particularly
with a belt drive), a design analysis is necessary to ensure that
increased stress does not risk shaft failure.
From “The
motor shaft: not as simple as it looks," Electrical Apparatus
September 2006
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