13-05-2013, 04:27 PM
Fatigue failure of a helical gear in a gearbox
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Abstract
This paper presents a failure analysis of a helical gear used in gearbox of a bus, which is made from AISI 8620 steel.
The helical gear had been in service about three years when several teeth failed. The failed helical gear had a number of
adjacent teeth and random teeth breakage at one end of teeth. An evaluation of the failed helical gear was undertaken
to assess its integrity that included a visual examination, photo documentation, chemical analysis, micro-hardness measurement,
and metallographic examination. The failure zones were examined with the help of a scanning electron microscope
equipped with EDX facility. Results indicate that teeth of the helical gear failed by fatigue with a fatigue crack
initiation from destructive pitting and spalling region at one end of tooth in the vicinity of the pitch line because of
misalignment.
Introduction
Helical gears are widely used as power transmitting gears between parallel or crossed shafts, since not
only can they carry larger loads but also the dynamic load and the noise level experienced during the operation
are minimum.
Gears can fail in many different ways, and except for an increase in noise level and vibration, there is
often no indication of difficulty until total failure occurs. In general, each type of failure leaves characteristic
clues on gear teeth, and detailed examination often yields enough information to establish the cause of
failure. The general types of failure modes (in decreasing order of frequency) include fatigue, impact fracture,
wear and stress rupture [1]. Fatigue is the most common failure in gearing.
Experimental procedure
The failed helical gear was inspected visually and macroscopically; care was taken to avoid damage of
fractured surfaces. The failed helical gear was subjected to optical microscopy, photo documentation,
chemical analysis and micro-hardness measurement both at the failure zone and away from the failure
zone. The fractured surfaces were ultrasonically cleaned and examined with the help of a scanning electron
microscope (SEM) equipped with EDX facility.
Conclusion
This study was conducted on a failed helical gear used in gearbox of a bus. Spectrum analysis and microhardness
measurement revealed that the failed helical gear material was AISI 8620 steel as carburised. The
composition, microstructure, hardness and the case depth were found to be satisfactory and within the
specification. Fractographic features indicated that fatigue was the main cause of failure of the helical gear.
On the fracture surface of the teeth, the crack initiation region and beach marks could be seen clearly with
naked eye. It was observed that the fatigue crack originated from destructive pitting and spalling areas in
the vicinity of tooth pitch line and propagated toward tooth region of tooth. Failure analysis results indicate
that a part of tooth fatigue failure in this case was, therefore, as a result of an incorrect load distribution
on the gear teeth. It was concluded that the primary cause of failure of the helical gear was likely a
misalignment of the helical gear. Formation of the destructive pitting and spalling at one end of tooth
in the vicinity of the pitch line, and all the fatigue crack initiation sites that were close to one end of the
gear teeth supported this hypothesis.