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The major reasons for shaft failure in a turbine can be broken down into the following groups:
Before the cause of failure can accurately be determined, it is necessary to understand loadings and stresses acting on the shaft. The ability to properly characterize the microstructure and the surface topology of a failed shaft are also important steps.
The most common tools available can be categorized as visual inspection, optical microscope, electron microscope and metallurgical analysis. Based on experience, a significant number of failures can be diagnosed with a fundamental knowledge of shaft failure causes and visual inspections. This may then lead to seeking confirmation through a metallurgical laboratory.
The following table shows the main reasons for turbomachinery shaft failure (as a rule of thumb):
There are other studies which suggest that fatigue-caused failures could be more important (perhaps around 50%). The failure mode is also dependent upon the type of turbomachinery and the application. In certain turbines, fatigue accounts for more than 55% of all failed shafts.
Causes of Shaft Failure[/caption]
The source of cracks caused by fatigue is usually the presence of surface discontinuities commonly referred to as stress raisers. Examples on shafts include keyways, steps, shoulders, collars, threads, holes, snap ring grooves, shaft damages or other flaws which produce a stress raiser (wherever there is a surface discontinuity a stress raiser exists). Corrosion can also create stress raisers. For typical turbines, two problematic places are the shoulder on the bearing and the coupling keyway region.
In the case of fatigue caused by axial loads, the thrust bearing carrying the axial load would most often suffer from fatigue before the shaft. However, there are numerous examples where the shaft is damaged before the turbomachinery is stopped.
Keyways are commonly used to secure rotating components, rotor cores and couplings to the shaft. The keyway on the take-off end or drive or driven end of the shaft is one of the biggest concerns because it is located in the area where the highest amount of shaft loading occurs. Fatigue cracks usually start in the fillets or roots of the keyway.
A keyway that ends with sharp steps has a higher level of stress concentration than a keyway uses a “sled-runner” design. In the case of heavy shaft loading, cracks frequently emanate from this sharp step. A connection using any form of key should be avoided to the maximum extent possible. In special cases, when non-key connections cannot be used, it is important to obtain an adequate radius on the edges of the keyway.
Fatigue-type failures usually follow the weak-link theory. The shaft fatigue failure process usually goes like this:
Residual stresses or initial deflections are usually independent of external loading on the shaft. There is a wide variety of manufacturing or repair processes which can affect the amount of residual stress or the initial deflection. These include drawing, bending, straightening, machining, grinding, surface rolling, shot blasting and polishing. All of these operations can produce residual stresses and initial deflection via plastic deformation. In addition, thermal processes which can introduce residual stress and deflection include hot rolling, welding, torch cutting and heat treating.
Shaft fretting can cause serious damage to the shaft and its mating part. Typical locations are points on the shaft where a “press” or “slip fit” exists. The presence of rust between mating surfaces is a strong confirmation that fretting has occurred. The cause of this condition is some degree of movement between the two mating parts and oxygen. Once fretting occurs, the shaft could become very sensitive to fatigue cracking. Shaft vibration can worsen this situation.