1, failure mode: tooth broken

Performance: The whole or part of the tooth is separated from the gear fracture or falls off, usually the root of the tooth is broken.

Impact: Tooth fracture is the most serious failure form. Broken teeth often cause instantaneous transmission stuck or damage to the gearbox under the action of centrifugal force, resulting in equipment shutdown, which must be avoided.

Diagnosis: The gear teeth bear the meshing force during the working process, of which the root part bears the largest bending stress, and the rounded corner of the root often has stress concentration. When the instantaneous impact load exceeds the rated load, it may cause overload fracture. When the alternating stress exceeds the bending fatigue limit of the material and is repeated continuously, fatigue cracks will occur on the strained side of the tooth root, and finally fatigue fracture will occur.

Countermeasure: When designing gear, the bending fatigue strength of tooth root should be checked. Usually, it can be improved by increasing the modulus, increasing the radius of the tooth root, and strengthening the tooth surface.

FIG. 1 Tooth fracture

2, failure mode: tooth pitting

Performance: The tooth surface will be subjected to alternating contact stress in the meshing process, if the fatigue crack generated by the extension of metal shedding and forming pitting pits is fatigue pitting.

Impact: After pitting occurs, the vibration of the transmission system increases and the noise is obvious, so that the gear can not work normally and the transmission fails.

Diagnosis: Pitting generally occurs in the closed soft-tooth gear box, because the higher the hardness of the tooth surface, the stronger the pitting resistance, in the open gear box, due to poor lubrication, the tooth surface wear faster, the crack has not been expanded on the ground, so less corrosion phenomenon.

Countermeasures: In order to avoid pitting of the tooth surface, the tooth surface contact fatigue check should be carried out, which can generally be improved by increasing the hardness of the tooth surface, increasing the diameter of the indexing circle, and reasonable selection of lubricating oil.

FIG. 2 Pitting of tooth surface

3, failure mode: tooth surface wear

Performance: Tooth surface wear, also known as particulate wear, is manifested as tooth surface wear and then tooth thinning.

Impact: tooth surface wear causes damage to the shape of the tooth profile, resulting in impact, vibration, noise and other problems, if not timely maintenance, will eventually lead to tooth fracture caused by tooth thickness reduction.

Diagnosis: According to the different working conditions, there will be different wear forms, such as metal chips, sand and other hard particles into the meshing position to destroy the tooth surface. The main failure form of open gear when the tooth surface is worn.

Countermeasures: Generally improve the tooth surface hardness, improve the lubrication cleanliness, or directly change to closed gear drive.

Figure 3 Tooth surface wear

4, failure mode: tooth surface gluing

Performance: The meshing two tooth surface materials are bonded together, so that part of the material on one tooth surface is glued to the other tooth surface, tooth surface bonding is the main failure form of high-speed heavy-duty gear.

Impact: Glued excess material is easy to cause scratches on other tooth surfaces, forming a vicious cycle, and eventually lead to transmission failure.

Diagnosis: Tooth surface gluing is generally divided into hot gluing and cold gluing. In high-speed and heavy-duty transmission, due to the high pressure and high speed, the instantaneous temperature at the mesh is too high, resulting in the metal adhesion and tear of the tooth surface, which is hot gluing. When the low speed and heavy load transmission, the oil film between the tooth surfaces is directly crushed by too much contact pressure and produces gluing, which is cold gluing.

Countermeasure: In order to avoid tooth surface gluing, the bearing capacity of anti-gluing should be checked. Generally, it can be improved by reducing the modulus, increasing the hardness of the tooth surface, reducing the contact roughness, and using the tooth profile modification.

Figure 4 Tooth surface gluing

5, failure mode: plastic deformation

Performance: permanent tooth deformation. When the sudden overload, the larger impact load may cause the gear tooth offset; When the load is too large and the oil film is destroyed, the friction force is intensified, the contact stress of the tooth surface is greater than the shear strength of the material, and the tooth surface material flows along the direction of the friction force and becomes deformed.

Impact: The plastic deformation of the gear teeth destroys the correct meshing position and meshing profile of the gear, and in serious cases, the deformed metal is full of the top gap, causing severe vibration, and even fracture.

Diagnosis: It generally occurs in the transmission of soft tooth surface, low speed and heavy load. The driving wheel forms a concave groove, and the driving wheel forms a convex edge.

Countermeasures: Usually, it can be improved by increasing the hardness of the tooth surface and the viscosity of the lubricating oil.

Figure 5 Plastic deformation

Source: Mechanical Engineering Literature