Gear is the most commonly used mechanical transmission parts, gear failure is also a common failure of rotating equipment. According to relevant statistics, gear failure accounts for 10.3% of rotating machinery failure. Gear fault can be divided into two categories, one is bearing damage, imbalance, misalignment, gear eccentricity, shaft bending, etc., the other is gear itself (i.e. gear teeth) in the transmission process. Among the parts of the gearbox, the failure rate of the gear itself is the largest, which is more than 60% according to statistics. The common failure forms of gear itself are as follows.

Broken teeth

Tooth breakage is the most common fault of gear. The fracture of gear teeth usually occurs at the root of gear, because the bending stress at the root of gear is the largest and the source of stress concentration.

There are three cases of broken teeth: ① the bending stress at the root of fatigue broken teeth under load is fluctuating cyclic alternating stress, and fatigue cracks will be produced under the combined action of stress concentration sources such as root fillet, machining tool mark, material defect, etc. The crack propagates gradually, which leads to the fatigue fracture of gear teeth. ② For the gear made of brittle materials such as cast iron or high hardness alloy steel, the stress on the dangerous section of the tooth root will exceed the limit value due to serious overload or impact load. ③ When the machining accuracy of the tooth surface is low, or the quality of gear maintenance and installation is poor, the phenomenon of partial load will occur along the contact line of the tooth surface. The partial load makes the stress at the tooth root of the local contact increase obviously, and the partial tooth break occurs when the stress exceeds the limit value. Local tooth breakage always occurs at the end of the tooth.

Pitting

Pitting is a common damage form of closed gear transmission, which usually occurs on the tooth root surface near the pitch line. The reason is that the fluctuating cyclic contact stress on the tooth surface exceeds the ultimate stress of the material.

When the contact stress of fluctuating cyclic change at the tooth surface exceeds the limit stress of the material, the fatigue crack will appear on the tooth surface. When the crack is meshed, it will close and increase the oil pressure in the crack gap, thus accelerating the crack growth. In this way, the surface metal on the tooth surface will peel off and form pockmarks, i.e. pitting.

There are two kinds of pitting: ① the initial pitting (also known as convergent pitting) usually only occurs on the soft tooth surface (HB < 350), after pitting appears, it will not continue to develop, or even disappear. The reason is that the micro bulge becomes flat gradually, which enlarges the contact area and reduces the contact stress. ② Extended pitting occurs on the hard tooth surface (HB > 350). After pitting, the edge of the pit will not be flattened because of the high brittleness of the tooth surface, but will continue to crack until the tooth surface is completely damaged.

The fatigue crack on the tooth surface of the split gear is worn away before it forms or expands, so there is no pitting.

When the heat treatment of the hardened gear is not right, along the interface between the surface hardening layer and the core, the tooth surface sometimes flakes off, which is called flake corrosion.

Wear

The wear of tooth surface is caused by metal particles, dust and sand entering into the working surface of tooth. Uneven tooth surface and poor lubrication are also the causes of tooth surface wear. In addition, misalignment, coupling wear and torsional resonance will cause greater torque change at the gear engagement point, or increase the impact, which will accelerate the wear.

After the gear is worn, the thickness of the gear becomes thinner, the profile of the gear is deformed, and the side clearance becomes larger, which will increase the dynamic load of the gear, not only increase the vibration and noise, but also possibly lead to the broken teeth.

Bonding

The scuffing (scratch) of tooth surface is caused by the rupture of oil film when the meshing tooth surface is sliding relative to each other, and the tooth surface is in direct contact. Under the action of friction and pressure, the contact area will generate instantaneous high temperature, and the metal surface will be damaged by local fusion welding adhesion and peeling.

Scuffing often occurs under the conditions of too low viscosity of lubricating oil, too high operating temperature, too large load per unit area on the tooth surface, too high relative sliding speed, too small contact area, too low rotation speed (oil can not be brought up), etc. When the gear surface is glued, it will accelerate the wear of the gear surface and make the gear transmission tend to failure quickly.