Knowledge

Gear pitting failure

 

I. Mechanism of Gear Pitting Failure

1.1 Definition and Phenomenon of Pitting

Pitting refers to small and shallow pits that appear in the contact area of gear teeth surfaces. These pits are usually formed due to local material spalling. At the beginning of pitting, they are generally tiny spots. With the passage of time and the continuous action of load, these spots gradually expand, deepen, and connect with each other, forming larger areas of spalling, which seriously affects the working performance of gears.

 

1.2 The mechanical principle of pitting

When a pair of gears mesh with each other, the tooth surfaces are subjected to alternating contact stresses. Under the repeated action of contact stresses, the tooth surface material will suffer fatigue damage. According to Hertz contact theory, very high local stresses will occur at the contact points of the tooth surfaces, especially in the area close to the pitch line, where the relative sliding speed is relatively small, the oil film is difficult to form, the lubrication conditions are poor, and the contact stress is more concentrated. When the contact stress exceeds the fatigue limit of the material, tiny cracks will occur on the surface of the tooth. As the gears continue to rotate, the cracks will gradually expand under the action of alternating stress, eventually leading to the shedding of the tooth surface material and the formation of pitting.

1.3 The Development Process of Pitting Corrosion

The development of pitting corrosion can generally be divided into three stages. The first stage is the initial pitting stage, during which small pits appear on the tooth surface. These are usually caused by microscopic defects or processing marks on the material surface that first develop cracks under the action of contact stress. These pits are generally small and relatively scattered, having a minor impact on the working performance of the gear. The second stage is the expansion stage. As the gear continues to operate, the initial pitting pits gradually expand and deepen, and adjacent pits connect with each other, forming larger spalling areas. At this stage, the vibration and noise of the gear increase significantly, and the transmission efficiency also decreases. The third stage is the severe pitting stage. At this point, the pitting areas on the tooth surface further expand, the spalling phenomenon becomes more severe, the surface roughness of the tooth significantly increases, the load-carrying capacity of the gear drops sharply, and it may even lead to complete gear failure.

 

II. Factors Affecting Gear Pitting Failure

2.1 Material Factors

Material composition: Steels with different compositions have different mechanical properties and resistance to pitting corrosion. For instance, alloy steels containing appropriate amounts of alloying elements such as chromium, nickel, and molybdenum have higher strength, hardness, and toughness, and thus possess relatively stronger resistance to pitting corrosion. In contrast, ordinary carbon steels have relatively weaker resistance to pitting corrosion.

 

Material microstructure: The microstructure of materials also has a significant impact on their resistance to pitting corrosion. For example, gears that have undergone proper heat treatment have uniform microstructures with fine grains, which can enhance the material's strength and toughness, thereby improving its resistance to pitting corrosion. Conversely, if the material's microstructure is not uniform, with large grains or other defects, stress concentrations are likely to form on the tooth surface, reducing the resistance to pitting corrosion.

 

Material hardness: Generally speaking, the higher the hardness of the tooth surface, the stronger the resistance to pitting corrosion. Increasing the hardness of the tooth surface through surface hardening, carburizing, nitriding, and other heat treatment processes can effectively reduce the probability of pitting corrosion. However, the hardness should not be too high, as excessive hardness can make the material brittle and prone to cracking.

 

2.2 Load Factors

Load magnitude: The greater the load that a gear bears, the higher the contact stress on the tooth surface will be, and the probability and development speed of pitting will increase accordingly. When the load exceeds the fatigue limit of the material, pitting will develop rapidly, seriously affecting the service life of the gear.

Load nature: If the gear is subjected to impact load or alternating load, the stress variation on the tooth surface becomes more complex, making it more likely to cause fatigue damage to the material and accelerate the occurrence of pitting. For example, in some construction machinery, gears often experience sudden impact loads, which places higher demands on the gear's resistance to pitting.

 

2.3 Lubrication Factors 

The viscosity of lubricating oil: The viscosity of lubricating oil has a significant impact on the lubrication effect on the tooth surface. Lubricating oil with a higher viscosity can form a thicker oil film on the tooth surface, providing better lubrication and buffering effects, reducing the contact stress on the tooth surface, and decreasing the occurrence of pitting. However, excessively high viscosity will increase the rotational resistance of the gears and lower the transmission efficiency. Therefore, it is necessary to select lubricating oil with an appropriate viscosity based on the working conditions of the gears.

 

The quality of lubricating oil: High-quality lubricating oil has good anti-wear, anti-oxidation, and anti-corrosion properties, effectively protecting the tooth surface and extending the service life of the gears. On the contrary, low-quality lubricating oil is prone to oxidation and deterioration, generating acidic substances and impurities, which can corrode the tooth surface and accelerate the development of pitting.

 

Lubrication methods: Different lubrication methods also have different effects on the lubrication of gears. Common lubrication methods include oil immersion lubrication and oil spray lubrication. Oil immersion lubrication is suitable for low-speed and light-load gear transmissions; oil spray lubrication is suitable for high-speed and heavy-load gear transmissions, as it can promptly remove the heat and wear particles generated on the tooth surface, maintaining good lubrication conditions.

 

2.4 Processing and Installation Factors

Processing Accuracy: The processing accuracy of gears has a significant impact on the contact quality of the tooth surfaces. If the processing accuracy indicators such as tooth profile error and tooth alignment error do not meet the requirements, it will lead to uneven contact on the tooth surfaces, local stress concentration, and thereby increase the probability of pitting. For instance, excessive tooth profile error will cause interference between the tooth surfaces during meshing, resulting in excessively high local contact stress.

Installation Error: During the installation of gears, if there are installation errors such as non-parallel shaft lines and center distance deviation, it will also lead to poor contact on the tooth surfaces, uneven force distribution on the tooth surfaces, and accelerate the development of pitting. For example, non-parallel shaft lines will cause eccentric loading during gear meshing, resulting in excessive load on local tooth surfaces.

 

III. Detection Methods for Gear Pitting Failure

3.1 Visual Inspection

Visual inspection is a simple and intuitive detection method. By directly observing the tooth surface of the gear, it is possible to determine whether there is pitting and the distribution of the pitting. Generally, tools such as magnifying glasses can be used to assist in observing tiny pitting spots. Visual inspection can initially determine the degree of pitting on the gear, but it may not accurately detect early cracks and tiny pitting hidden inside the tooth surface.

 

3.2 Hardness Testing

Hardness testing can reflect the hardness changes of the tooth surface material. If pitting occurs on the tooth surface, its hardness may change. By conducting hardness tests on different parts of the tooth surface and comparing them with the original hardness values, it can be determined whether pitting has affected the performance of the tooth surface material. Common hardness testing methods include Rockwell hardness testing and Brinell hardness testing.

 

3.3 Non-destructive Testing

Ultrasonic Testing: Ultrasonic testing can detect whether there are cracks and other defects inside the tooth surface. When ultrasonic waves propagate in materials, if they encounter cracks and other defects, reflected waves will be generated. By analyzing the characteristics of the reflected waves, the location, size and shape of the defects can be determined. Ultrasonic testing has the advantages of large detection depth and high sensitivity, but it may not be effective in detecting some tiny surface cracks.

 

Magnetic Particle Testing: Magnetic particle testing is suitable for detecting surface and near-surface cracks in ferromagnetic materials. A magnetic field is applied to the tooth surface and magnetic particles are scattered on it. If there are cracks on the tooth surface, the magnetic particles will gather at the crack to form obvious magnetic traces, which can visually display the location and shape of the cracks. Magnetic particle testing is simple to operate and has a fast detection speed, but it can only be used for ferromagnetic materials and has limited detection capability for non-surface open cracks.

 

3.4 Vibration and Noise Monitoring

During the process of pitting failure of gears, their vibration and noise will undergo significant changes. By installing vibration sensors and noise sensors to monitor the vibration and noise signals of gears in real time and analyzing and processing the signals, it is possible to determine whether the gears have pitting and the extent of the pitting development. For instance, when pitting occurs on the tooth surface, specific frequency components will appear in the vibration signal spectrum, and the noise level will also increase significantly.