Article
The Science of Breaking Bad... Failure Analysis of Machine Shafts

by Neville W. Sachs, P.E.

ABSTRACT

As the industrial arena grows more sophisticated, it seems as though operations are confronting fewer and fewer broken machine shafts. When shafts DO break, however, there are almost always as many theories regarding the suspected culprits as there are people involved.

*This copyrighted article was first published in the July 2012 issue of Maintenance Technology magazine (maintenancetechnology.com), which has granted permission to the Vibration Institute to post the material solely in the Member’s Only Selection of technical papers in the association’s library. This article may not be copied or distributed without the express permission of Maintenance Technology. All rights reserved.

PREVIEW

“There are only four basic failure mechanisms: corrosion, wear, overload and fatigue. The first two—corrosion and wear—almost never cause machine-shaft failures and, on the rare occasions they do, leave clear evidence. Of the other two mechanisms, fatigue is more common than overload failure. (NOTE: Keep in mind that many times corrosion will act in conjunction with fatigue loading to cause a shaft failure.) This article will focus on failures resulting from overload and fatigue factors.

Overload Failures
Overload failures are caused by forces that exceed the yield strength or the tensile strength of a material. As depicted in Fig. 1, the appearance of an overload failure depends on whether the shaft material is brittle or ductile. No shaft materials are absolutely brittle or absolutely ductile. The shafts used on almost all motors, reducers and fans are low- or medium-carbon steels and relatively ductile. As a result, when an extreme overload is placed on these materials, they twist and distort. The bent shaft shown in Photo 1 has been grossly overloaded by a torsional stress.

There are occasional cases when a ductile shaft will fail in a somewhat brittle manner. Photo 2 shows an example of this situation—i.e., what happened when a 200 hp, 3600 RPM motor suddenly stopped running. The result was a huge torsional stress and a cracked shaft. But because the material is ductile, the angle of the crack it is not at the 45° position shown in Fig. 1, and there is obvious distortion of the keyway. When ductile materials are grossly overloaded very rapidly, they tend to act in a brittle manner.

Fortunately, brittle fractures of machine shafts are extremely rare. Like all brittle fractures, they are characterized by a relatively uniform surface roughness—the crack travels at a constant rate, and surface features called “chevron marks” are evident. Photo 3 (page 18) shows the brittle fracture of the input shaft of a large reducer that was dropped. The “chevron marks” are the fine ripples on the surface that all point just to the left of the keyway.

Fatigue Failures
Fatigue is caused by cyclical stresses, and the forces that cause fatigue failures are substantially less than those that would cause plastic deformation. Confusing the situation even further is the fact that corrosion will reduce the fatigue strength of a material. The amount of reduction is dependent on both the severity of the corrosion and the number of stress cycles.”

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