Five common faults of gears and preventive measures

Due to different factors such as structural form, material and heat treatment, operating environment and conditions, the failure forms of gears are also different. Common gear failures include the following forms.

1 Gear failure form and preventive measures
1) Tooth surface fatigue (pitting, peeling)
Cause and phenomenon:
It is mainly due to the micro-fatigue crack caused by the alternating stress of the working surface. After the lubricating oil enters the crack, the inlet may be closed first and then squeezed during the meshing process. The lubricating oil in the micro-fatigue crack causes the crack to expand under high pressure, resulting in the small metal falling off the tooth surface, leaving a small pit and forming pitting corrosion. If the fatigue crack on the surface grows deeper and farther, or a series of small pits are connected due to the failure of materials between pits, causing large areas or large pieces of metal to fall off, this phenomenon is called spalling.

Occurring position: near the tooth root surface of the pitch line.
Occurrence: closed transmission.
Preventive measures: improve the hardness of the tooth surface, reduce the surface roughness value, reasonably select the viscosity of the lubricating oil and adopt the correct degree of gear transmission.

2) Worn or scratched tooth surface
Cause:
(1) Adhesive wear.
Under the conditions of low speed, heavy load, high temperature, poor tooth surface roughness, insufficient oil supply or low oil viscosity, the oil film is easy to be damaged and adhesive wear occurs. The viscosity of lubricating oil is high, which is beneficial to prevent adhesion wear.

(2) Abrasive wear and scratches.
The abrasive wear and scratch can be caused by the impurity particles in the lubricating oil, the foreign sand particles in the open gear transmission or the metal debris generated in the friction process. Generally, the friction of tooth top and tooth root is more serious than that of the pitch circle, because the pitch circle is rolling contact during gear meshing, while the tooth top and tooth root are sliding contact.

(3) Corrosion and wear.
The tooth surface is damaged due to the chemical reaction between some chemicals in the lubricating oil, such as acid, alkali or water, and the tooth surface.

(4) Burns.
Burn is the local area overheating caused by excessive friction caused by overload, overspeed or insufficient lubrication. This temperature rise is enough to cause discoloration and over-aging, which will re-quench the surface layer of several microns thick steel and cause white layer. The damaged surface is easy to produce fatigue cracks.
Occurrence: open drive.
Preventive measures: adopt closed transmission, improve the hardness of the tooth surface, reduce the contact stress, reduce the surface roughness value, and keep the lubricating oil clean.

3) Tooth surface gluing
Cause:
For high-power soft tooth surface or high-speed and heavy-load gear transmission, when the lubrication condition is poor, it is easy to produce tooth surface gluing (bite welding) damage, that is, part of the material on one tooth surface is glued to another tooth surface, leaving pits on this tooth surface. In the subsequent meshing transmission, the excess material on this part of the gluing is easy to cause scratches and grooves on other tooth surfaces, forming a vicious cycle.

Occurring position: the top surface of the tooth near the pitch line.
Occurrence: high speed, low speed and heavy load gear.
Preventive measures: appropriate lubricating oil, improve hardness, reduce surface roughness value, and use gear materials with strong anti-seize ability.

4) Plastic deformation of tooth surface (flash)
Cause:
When the gear with soft tooth surface is frequently started and heavily overloaded, the contact stress of the tooth surface will exceed the shear strength of the material under the action of excessive friction between the tooth surfaces, and the material on the tooth surface will enter the plastic state, resulting in the plastic flow of the metal on the tooth surface, forming a concave groove on the tooth surface near the driving gear pitch circle, and forming a convex ridge on the tooth surface near the driving gear pitch circle, thus damaging the correct tooth shape. Sometimes "flash" may appear on the driven tooth surface of some types of gears. In serious cases, the extruded metal will fill the top gap, causing severe vibration and even fracture.

Occurring position: the driving wheel forms a concave groove, and the driven wheel tooth surface forms a convex ridge.
Preventive measures: improve the hardness of the tooth surface, select the lubricating oil with higher viscosity, and avoid frequent startup and serious overload.

5) Broken gear teeth
reason:
Variable load (fatigue, overload), if it is caused by sudden overload or impact overload, it is easy to cause overload fracture at the tooth root. Even if there is no stress condition of impact overload, when the gear tooth is repeatedly loaded, it is easy to generate fatigue cracks due to stress concentration, and gradually expand, resulting in fatigue fracture of the gear tooth at the root. In addition, quenching cracks, grinding cracks and excessive thinning of tooth thickness after severe wear may cause fracture at any part of the gear teeth. The fracture of gear teeth is the most serious fault of gears, which often causes equipment shutdown.

Occurrence: in open gear transmission and closed gear transmission with hard tooth surface.
Preventive measures: select appropriate magic and tooth width, use appropriate materials and heat treatment process to reduce the stress concentration at the tooth root.

2 Main causes of gear failure
1) Manufacturing error
Gear manufacturing error mainly includes eccentricity, tooth pitch error and tooth profile error. Eccentricity means that the geometric center and the rotation center of a gear (generally a rotating body) do not coincide. Pitch deviation means that there is a large error between the actual tooth pitch and the nominal tooth pitch of the gear, while tooth profile error means that there is an error in the involute tooth profile.

2) Poor assembly
Improper assembly of gears will cause deterioration of working conditions. When a pair of meshing gear shafts are not parallel, only one end of the gear will contact in the direction of the tooth width, or the straightness deviation of the gear will occur, which will make the load borne by the gear uneven in the direction of the tooth width and can not transmit the dynamic torque smoothly. This situation is called "one end contact", which will make the part of the gear bear excessive load and may cause tooth breakage.

3) Poor lubrication
For high-speed and heavy-load gears, poor lubrication will cause local overheating of the tooth surface, resulting in color change, gluing and other faults. There are many reasons for poor lubrication. In addition to the clogging of the oil circuit and the clogging of the oil injection hole, water ingress in the lubricating oil, deterioration of the lubricating oil, and high oil temperature will cause poor lubrication of the tooth surface.

4) Overload
For gear drive devices with unstable working load (such as ore crusher, mining machine, etc.), overload often occurs. Without proper protective measures, it will cause gear tooth overload and fracture, or long-term overload will cause a large number of gear tooth root fatigue cracks and fractures.

5) Operation error
Operation errors usually include oil shortage, overload, long-term overspeed, etc., which will cause gear damage and damage.

Source: Network