Gear failure index

1.0 Tooth breakage

Click on photograph to enlarge
	Figure 1a A spiral bevel gear with broken teeth due to a high shock load
	Figure 1b A gear with a fractured tooth showing adjacent teeth with incipient cracks. There are also surface cracks due to heavy grinding which line up with the fatigue cracks and might have acted as stress raisers.
	Figure 1c Typical tooth breakage from fatigue cracks starting at the root of a tooth and arising from bending loads from the driving torque
	Figure 1d Bending fatigue failure of the teeth of a hardened double helical gear. The driving torque on the gear has been unidirectional, as is indicated by the beach marks on the loose teeth, which show fatigue cracks spreading from one side only.
	Figure 1e A tooth which has broken off under reversing bending stresses. The fatigue cracks have spread inwards equally from each side. This suggests that the tooth was probably from an idler gear.
	Figure 1f Tooth breakage at one end of a gear associated with misalignment. The light scuff marks towards the end of the teeth also indicate misalignment.
	Figure 1g Although fractures usually start at the root of teeth, surface pitting or scuffing can cause stress raisers on the surface can initiate fatigue cracks at other positions.
Main Characteristics
Cause	There are two basic types of tooth breakage in gears, one arising from shock overloading and one from fatigue. Shock overloading Shock overloading can arise from some major torsional shock in the drive system, often associated with the failure of a machine driven by the gear box. In these cases the fractured surfaces show a single brittle break with no sign of fatigue marking, and usually one or two teeth are affected. Apart from the broken teeth, the others are usually in good condition with no signs of cracks. Similar effects can also occur if a foreign hard body enters the gear mesh. This is generally obvious from the nature of the impression in the teeth and the gear blank, and the body which has caused the problem can generally be found in the failure debris in the bottom of the gearbox. Fatigue Tooth breakage can also arise from fatigue failures under the cyclic bending loads applied to teeth in normal gear operation, and this is one of the main failure criteria used in gear design. The failure starts as a crack which is usually at the root of the tooth and proceeds across the base of the tooth until the complete tooth breaks away from the gear. When failure arises from this cause there are often other adjacent teeth showing cracks at an earlier stage of development.
Note
Possible Confusion with Other Types of Damage
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