When gears work, they bear two kinds of stress, so there are few two kinds of failure. One is that bending stress may cause tooth fracture, including fatigue fracture and brittle fracture; the other is that contact stress may cause fatigue pitting, spalling, plastic deformation, wear and abrasion.
According to a large number of investigation materials, it is confirmed that there are few broken teeth of large low-speed and heavy-duty gears, and a large number of failures are caused by contact stress. Due to the different service conditions of gears, there are different forms of tooth surface damage in different stress states. The load of reducer gears such as cement mill and mine hoist is relatively stable, mainly due to the early fatigue pitting on the tooth surface. Some theories believe that this early fatigue pitting may gradually disappear in the future use. The practice shows that there are two conditions for the disappearance of the so-called early fatigue pitting, that is, the tooth surface wear or the tooth surface plastic deformation, which are difficult to appear on the low-speed and heavy-duty gears. The gear wear of reducer in normal operation is very little. The gear wear of reducer in a factory is 0.09mm after 30 years of use. However, plastic deformation is impossible, because the plastic deformation of tooth surface is caused by the applied load exceeding the yield strength of tooth surface. If the tooth surface can cause plastic deformation, it is impossible to form early pitting after the plastic deformation is uniform. A large cement mill reducer with a center distance of 3.3m is produced by our factory. Due to the manufacturing and installation reasons, the tooth contact rate is very low. The gear is made of 40crni steel, and the hardness is hb270 after quenching and tempering treatment. After 2 years of use, the tooth stubble is still in use, without any signs of wear and plastic deformation, but pitting corrosion occurs on the contact line and continues to develop.
The increase and expansion of pitting corrosion in the early stage lead to the load can not be kept stable, and the tooth surface damage is more serious. Before reflecting the design bearing capacity and service life of the gear, it is in the state of running with defects or reducing the load until it must be replaced.
The causes of early fatigue pitting are poor contact of gears and insufficient contact rate of tooth length and tooth height. The original material has higher contact fatigue strength in the operation before the basic cycle number of fatigue limit. The uneven contact of the tooth surface causes local overload, which makes part of the tooth window work under the stress which is twice of the contact fatigue limit, resulting in overload fatigue damage, i.e. early pitting.
For example, the reducer of a four meter winch imported from abroad (center distance 1800mm) in a coal mine has been used for less than one year. Due to the poor contact of tooth length, the eccentric load in the direction of tooth length is caused, resulting in early pitting at one end, as shown in the figure.