1. Introduction
1.1 Significance of Spiral Bevel Gears in Aeroengines
Spiral bevel gears play a crucial role in aeroengine central transmission systems. With the continuous increase in aeroengine thrust and thrust – to – weight ratio, the power and speed that gears need to transmit are also rising. Spiral bevel gears are favored for their advantages such as smooth transmission, high load – bearing capacity, and compact structure. They are the key components for power transfer in aeroengine central transmission, and any failure can lead to the loss of the entire transmission system’s function and the interruption of flight missions.
1.2 Common Failure Modes of Aeroengine Gears
The failure modes of aero – conical gears are diverse, mainly including tooth pitting, scuffing, wear, plastic deformation, and fracture. Tooth pitting occurs due to the repeated contact stress on the tooth surface, resulting in small pits on the surface. Scuffing is the adhesion and tearing of the tooth surface under high – load and high – speed conditions. Wear is caused by the relative movement between the teeth, gradually reducing the tooth profile. Plastic deformation happens when the load exceeds the yield strength of the material, deforming the tooth shape. Fracture is the most serious failure mode, which can lead to sudden gear failure.
1.3 Research Background and Motivation
Although many scholars have studied gear failures, there is a lack of research on the fatigue failure analysis of spiral bevel gears caused by edge contact, and it is difficult to distinguish from traditional resonance – induced failure modes. In a central transmission debugging test, the tooth fracture of an active spiral bevel gear made of the new low – carbon high – alloy steel \(15Cr_{14}Co_{12}Mo_{5}Ni_{2}\) occurred. This material has excellent properties such as high – temperature resistance, corrosion resistance, and good toughness. Similar failures have not been reported in this material before. Therefore, exploring the failure mechanism is of great significance for improving the design and manufacturing process.
2. Experimental Investigation of the Failed Spiral Bevel Gear
2.1 Macroscopic Inspection
The macroscopic morphology of the failed spiral bevel gear is shown in Figure 1. The active spiral bevel gear had 4 complete teeth broken at the web below the teeth, and after splicing, it was roughly intact. The driven spiral bevel gear was intact. The contact imprints on the working surfaces of the concave surface of the active gear and the convex surface of the driven gear were clearly visible, showing a slender eyebrow – like shape. The contact imprints of the entire circumference of the active spiral bevel gear were uneven, biased towards the small – end side and close to the tooth bottom. The contact imprints of the driven spiral bevel gear were distributed at the top of the small – end teeth, incomplete, and partially beyond the working surface. The detailed information of the contact imprints of the active gear is summarized in Table 1.
| Tooth Category | Imprint Width (mm) | Imprint Length (mm) | Distance from Tooth Top (mm) |
|---|---|---|---|
| 5# tooth (cracked tooth) | 1.0 | 23.0 | 5.0 |
| 6# – 9# teeth (lost teeth) | 2.0 | 20.0 | 5.3 |
| Other teeth (except 5# – 9#) | 1.7 | 18.0 | 5.3 |
2.2 Fracture Surface Analysis
The fracture surface of the lost teeth of the failed active spiral bevel gear can be divided into radial 1, circumferential, and radial 2 fracture surfaces. The radial 1 and circumferential fracture surfaces are relatively fine, with obvious fatigue arcs and radial lines, indicating that the fracture surface is of fatigue nature (Figure 2). The fatigue origin is judged to be at the corner of the transition between the small – end face and the convex root of the tooth, starting from a point source. The circumferential fracture surface is formed by the fatigue expansion of the radial 1 crack, and the radial 2 fracture surface is rough, with an area less than 20% of the entire lost – block fracture surface area, which should be the instantaneous fracture surface formed in the later stage of fatigue expansion. Microscopic observation of the radial 1 fracture surface by scanning electron microscopy further confirmed the fatigue origin and the characteristics of high – cycle fatigue, such as obvious fatigue arcs, radial lines, and fatigue striations.
2.3 Microstructure Examination
A complete double – tooth was cut from the area near the fracture of the failed active spiral bevel gear for microstructure observation. The results are shown in Figure 3. The carbide grade of the tooth profile carburized layer was 3, which met the standard requirements (1 – 4). The surface of the concave – surface meshing area showed deformation characteristics. The core structure was tempered martensite + a small amount of retained austenite, with no abnormalities. At the junction of the carburized layer and the matrix, there was white – block – shaped tissue. Micro – hardness testing showed that the hardness of this tissue was significantly lower than that of the core, indicating that it was retained austenite.
2.4 Hardness Testing
The hardness of the concave – surface of the failed active spiral bevel gear was tested. The results are shown in Table 2. The surface hardness \(H_{cc}\) of the tooth surface was 63.1, 63.4, 63.2 (drawing requirement: \(H_{0}=60 – 67\)), and the core hardness \(H_{1}\) was 50.1, 51.5, 51.4 (drawing requirement: \(H_{bc}=48 – 52\)). In addition, a soft – point zone was found in the transition area between the carburized layer and the core structure of the tooth – root fillet of the failed active spiral bevel gear, corresponding to the white – block – shaped retained austenite tissue.
| Testing Location | Hardness Values | Drawing Requirements |
|---|---|---|
| Tooth Surface (\(H_{cc}\)) | 63.1, 63.4, 63.2 | \(H_{0}=60 – 67\) |
| Core (\(H_{1}\)) | 50.1, 51.5, 51.4 | \(H_{bc}=48 – 52\) |
2.5 Composition Analysis
The energy – dispersive spectrometry (EDS) analysis results of the matrix of the failed conical gear are shown in Table 3. The mass fractions of the main alloying elements showed no significant abnormalities, indicating that the material composition was within the normal range.
| Element | Mass Fraction (wt%) |
|---|---|
| V | 0.6 |
| Cr | 13.1 |
| Co | 13.0 |
| Fe | 66.7 |
| Ni | 2.0 |
| Mo | 4.5 |
2.6 Meshing Imprint Simulation Analysis and Verification
In the technical requirements of spiral bevel gears, the contact imprints of the gear pair are divided into static and dynamic contact imprints. The static contact imprint is the rolling contact trace obtained on the working surface of the gear after the installed gear pair is rotated under the slight braking force of the gear – rolling inspection machine. The dynamic contact imprint is expanded on the basis of the static contact imprint within the elastic range of the gear material after loading. The design requirements for the colored meshing imprints of the failed spiral bevel gear are that they should be continuous ellipses, always within the tooth surface range, and without edge contact. The analysis and verification results of the contact imprints using commercial software and finite – element software are shown in Figures 4 – 6. The results show that the static contact imprints analyzed by the commercial software are basically consistent with the actual rolling – inspection results of the processed parts, but the imprints of some inspection positions are close to the tooth edge, which is prone to edge contact under working load. The simulated loaded contact imprints of the two software are basically the same. The tooth – surface imprints of the active spiral bevel gear at the theoretical position and position 1 are far from the tooth edge, with a contact stress of about 1100 MPa, while at position 8, the imprints are close to the tooth edge, resulting in edge contact and a significant increase in contact stress, reaching 1600 – 2000 MPa.
3. Analysis and Discussion
3.1 Failure Mechanism of High – Cycle Fatigue
Based on the fracture – surface inspection of the failed active spiral bevel gear, the crack originated from the corner of the transition between the small – end face and the convex root of the tooth, with a point – source characteristic. The crack first expanded from the small – end face to the large – end face, then simultaneously along the axial and radial directions, and after reaching the web – 转接部位，it expanded circumferentially along the rotation direction. After passing through 4 teeth, it cracked and fell off due to radial overload. The expansion area accounted for about 80% of the entire fracture – surface area, with sufficient fatigue expansion. Microscopically, the expansion area had the characteristics of large – area tearing – edge expansion texture and fine fatigue striations. According to the judgment basis of high – cycle fatigue fractures, the failure of the failed active spiral bevel gear was of high – cycle fatigue nature.
3.2 Influence of Edge Contact on Gear Failure
The uneven distribution of contact imprints on the failed spiral bevel gear pair indicates that edge contact occurred during the working process. Edge contact can cause abnormal stress concentration on the tooth surface, especially at the tooth root. The contact area of the tooth surface affects the smooth operation, load distribution, service life, and noise of the gear. In this case, the edge contact of the active spiral bevel gear led to an increase in stress at the tooth root. Under the cyclic abnormal meshing action, fatigue cracks were prone to initiate at the corner of the transition between the small – end face and the convex root of the tooth, which was an important factor leading to gear failure.
3.3 Relationship between Gear Processing Parameters and Failure
The contact imprints of spiral bevel gears are closely related to the processing parameters. The formation of processing parameters of spiral bevel gears is based on the local conjugate principle, and the tooth – contact analysis (TCA) technology is used to simulate the meshing process to obtain the contact imprints and transmission error curves. In actual production, multiple parameter adjustments are often required to obtain suitable contact imprints and good meshing performance. The simulation and verification of the meshing imprints of the failed active spiral bevel gear show that the static imprints of some typical inspection positions of the gears processed according to the existing detailed design parameters of the tooth surface are at the tooth – profile edge. Under the action of the load during the working process, the dynamic imprints move towards the small – end and tooth – root directions, resulting in edge contact and an increase in tooth – root stress. Therefore, insufficient consideration of gear – tooth – surface processing parameters is the main cause of the failure.
3.4 Influence of Residual Austenite on Gear Performance
The existence of a “soft – point” zone in the transition area between the carburized layer and the core structure of the failed active spiral bevel gear is due to the presence of retained austenite. The high – alloy content of the \(15Cr_{14}Co_{12}Mo_{5}Ni_{2}\) material makes carburizing difficult. If the carbon potential is too high and the diffusion time is insufficient during the long – time carburizing process, carbon and alloy elements will accumulate in the carburizing transition area, resulting in a retained – austenite zone and a decrease in hardness. However, in this failure case, since the crack originated from the surface of the corner of the transition between the small – end face and the convex root of the tooth, and there were no obvious retained – austenite and other tissue defects in the source area, the influence of retained austenite on the initiation of gear fatigue cracks was not significant.
4. Optimization Strategies to Avoid Gear Failures
4.1 Optimization of Tooth – Surface Processing Parameters
To avoid similar failures, it is necessary to optimize the tooth – surface processing parameters. This can be achieved by using advanced processing technology and simulation software. First, based on the gear design requirements, the geometric parameters and processing parameters of the gear are accurately calculated. Then, through TCA technology, the meshing process is simulated to predict the contact imprints and transmission errors. According to the simulation results, the processing parameters are adjusted to ensure that the contact imprints of the gear pair meet the design requirements, evenly distribute the load on the tooth surface, and reduce stress concentration.
4.2 Improvement of Meshing Quality through Simulation and Experiment
Meshing – imprint simulation and experimental verification are important means to improve meshing quality. Before manufacturing the gear, simulate the meshing process of the gear pair under different working conditions through software to obtain the distribution of contact imprints and contact stresses. Then, conduct experiments on the actual gear pair using a gear – rolling inspection machine and other equipment to verify the simulation results. By comparing the simulation and experimental results, the processing parameters are further optimized to ensure that the gear has good meshing performance during the working process, reducing the occurrence of abnormal meshing.
4.3 Control of Carburizing Process to Reduce Residual Austenite
For the \(15Cr_{14}Co_{12}Mo_{5}Ni_{2}\) material, it is necessary to control the carburizing process to reduce the retained austenite in the carburized layer. This can be achieved by adjusting the carburizing temperature, time, and carbon potential. For example, appropriately reducing the carbon potential and increasing the diffusion time can prevent the accumulation of carbon and alloy elements in the carburizing transition area, reducing the formation of retained austenite. In addition, heat – treatment processes such as quenching and tempering can also be optimized to reduce the stability of austenite during the cooling process, thereby reducing the content of retained austenite.
5. Conclusion
5.1 Summary of Failure Analysis
The failure of the active spiral bevel gear in the central – transmission debugging test was a high – cycle fatigue fracture. The fatigue originated from the corner of the transition between the small – end face and the convex root of the tooth, which had nothing to do with the material and metallurgical defects. The main cause of the failure was the abnormal meshing during the working process, which was caused by insufficient consideration of the tooth – surface processing parameters, resulting in edge contact and an increase in tooth – root stress. Although there was a retained – austenite zone in the transition area between the carburized layer and the core structure of the gear, it had no obvious relationship with the initiation of fatigue cracks.
5.2 Significance of Optimization Strategies
The proposed optimization strategies, including the optimization of tooth – surface processing parameters, the improvement of meshing quality through simulation and experiment, and the control of the carburizing process to reduce retained austenite, are of great significance for avoiding similar gear failures. These strategies can improve the design and manufacturing level of spiral bevel gears, enhance the reliability and service life of aeroengine central – transmission systems, and ensure the safe operation of aircraft.
5.3 Future Research Directions
Although this study has analyzed the failure mechanism of spiral bevel gears and proposed corresponding optimization strategies, there are still some areas that need further research. For example, the influence of complex working conditions (such as high – temperature, high – humidity, and variable load) on the failure of spiral bevel gears needs to be further studied. In addition, the development of new materials and manufacturing processes for spiral bevel gears can also be an important research direction to improve the performance and reliability of gears.
In conclusion, the failure analysis and optimization of spiral bevel gears in aeroengine central – transmission systems are of great significance for the development of aeroengines. Through continuous research and improvement, the reliability and performance of aeroengine transmission systems can be effectively enhanced, promoting the progress of the aviation industry.
