Electrical Apparatus magazine, September 2006 Motor shaft: not as simple as it looks

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.

Industry news...

Featured Technical Article
September 2006

On this page is a summary of the Electrical Apparatus September 2006 featured technical article, by Richard L. Nailen, P.E. , "Motor shaft: not as simple as it looks" Die Motorwelle: nicht so einfach, wie sie aussieht ... Les arbres des moteurs: ils ont l'air simples mais ne le sont pas ... El eje del motor: no tan sencillo como parece

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