1. Introduction
Gear transmission is a crucial component in the power output system of aero – engines. It is widely used in various fields such as aerospace, ships, and automotive engineering due to its advantages like compact structure, high transmission efficiency, smooth operation, strong load – bearing capacity, and long service life. However, in practical applications, the harsh working environment, complex working conditions, and processing and installation errors of the gear system can affect the normal operation of the gear transmission system, leading to significant vibration and impact.
Aero – engine reducers play a vital role in aircraft power transmission. Any failure in the reducer gears can pose a serious threat to flight safety. Tooth surface spalling is one of the common fatigue failure forms in gear transmission. Studying tooth surface spalling failures is of great significance for preventing and early diagnosing such failures, which can avoid safety accidents caused by mechanical equipment failures and prevent significant losses of personnel and property. This article focuses on the tooth surface spalling failure of a certain aero – engine reducer gear, conducts in – depth analysis, proposes improvement measures, and verifies their effectiveness.
2. Gear Tooth Surface Spalling Failure
2.1 Failure Phenomenon
During the long – term test of an aero – engine reducer with the entire engine, a detector alarm was triggered. Inspection revealed the presence of exfoliated metal flakes. After disassembling the reducer, it was found that one tooth on a certain gear (the driving gear) had a tooth surface spalling problem. The spalling morphology is shown in Figure 1.
| Figure 1 | Gear Spalling Morphology |
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| [Insert the actual picture of gear spalling morphology here] | The figure shows the spalled area on the gear tooth surface, with obvious signs of material peeling off. |
2.2 Tooth Surface Spalling Failure Analysis Framework
Based on the structural characteristics of aero – engine reducer gears and combined with multiple tooth surface spalling failure cases, an analysis framework for aero – engine reducer gear tooth surface spalling failures has been established, as shown in Figure 2.
| Figure 2 | Tooth Surface Spalling Failure Analysis Framework |
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| [Insert the actual picture of the analysis framework here] | This framework includes aspects such as spalling area analysis, visual inspection, metallurgical analysis, design review, and manufacturing and assembly process review to comprehensively analyze the causes of tooth surface spalling. |
2.3 Failure Cause Analysis
2.3.1 Appearance Inspection
Among the faulty gears, one tooth (marked as Tooth 1) had obvious spalling on the working tooth surface. The tooth that was separated from Tooth 1 by four teeth (marked as Tooth 6) had a pit on its working surface, as shown in Figure 3.
| Figure 3 | Faulty Gear Tooth Damage Morphology |
|---|---|
| [Insert the actual picture of the damaged teeth here] | In the figure, the spalling position of Tooth 1 and the pit position of Tooth 6 are clearly visible, and both are basically located in the center of the tooth width. |
The spalling position of Tooth 1 and the pit position of Tooth 6 were basically at the center of the tooth width. Additionally, all teeth had a white bright band near the root on the working surface, while there was no such bright band on the non – working surface. From the perspective of tooth height, the lower edge heights of the spalling area of Tooth 1 and the pit of Tooth 6 were basically the same, approximately at about 1/3 of the tooth height from the root, and basically near the bright band at the root. There was a distinct bright spot damage on the visible bright band along the tooth length direction, and the bright spot damage positions of Tooth 1 and Tooth 6 were also basically the same.
2.3.2 Microscopic Inspection
Microscopic observation of the spalling damage of Tooth 1 showed that the spalling area had an inverted triangular shape and a certain directionality. It could be judged that the spalling originated from the lower edge of the inverted triangle and spalled towards the tooth height direction, as shown in Figure 4(a).
| Figure 4 | Microscopic Morphology of Tooth 1 Tooth Surface Spalling |
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| [Insert the actual picture of the microscopic morphology of Tooth 1 here] | This figure shows various details of the spalling area of Tooth 1, such as the spalling direction, the damaged area below, and the micro – cracks in the bright band area. |
There was a bright band damage area with a width of about 1.89mm from the meshing line at the root to the lower part of the spalling origin, as shown in Figure 4(b). Many fish – scale – shaped micro – cracks along the tooth length direction could be seen in the bright band area, which extended obliquely into the substrate, representing the early morphology of spalling, as shown in Figure 4(c). The tooth surface around the spalling origin area also showed more severe fish – scale – shaped damage, and some had even developed into small spalling pits, as shown in Figure 4(d). The un – meshed area at the root still had the remaining processing morphology, and there were individual original pit damages locally. Clear fatigue characteristics could be seen in the large spalling pit, as shown in Figure 4(e).
Microscopic observation of the “pit” damage of Tooth 6 also indicated fatigue spalling damage. The spalling size was 2 – 2.6mm, which was consistent with the spalling damage of Tooth 1. The spalling originated from the lower edge, that is, near the bright band position as shown in Figure 5(a).
| Figure 5 | Microscopic Morphology of Tooth 6 Spalling |
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| [Insert the actual picture of the microscopic morphology of Tooth 6 here] | This figure shows the spalling direction and fatigue characteristics of Tooth 6. |
Obvious fish – scale – arranged micro – cracks (early characteristics of fatigue spalling) could be seen around the spalling origin position and in the lower bright band area, and typical fatigue characteristics could be seen in the spalling pit, as shown in Figure 5(b). Similar to Tooth 1, fatigue spalling early – stage damage of different degrees could be seen in the bright band area along the entire tooth length direction.
2.3.3 Metallurgical Analysis
The metallurgical analysis of the spalled tooth surface included the inspection and analysis of chemical composition and metallographic structure. After dissecting the faulty gear, the composition of two positions, the spalling pit and the non – spalled working area of the tooth surface, was analyzed by energy – spectrum detection on the tooth. The results showed that they were mainly the matrix components, and a small amount of foreign elements could be detected, but there was no significant difference.
A random tooth mid – section was selected for metallographic corrosion. The results showed that there was a distinct carburized layer on the tooth surface, and the carburized layer depth of each tooth was relatively uniform, as shown in Figure 6(a). There was no significant difference in the carburized layer structure of each tooth. The carburized layer structure was hidden acicular martensite + carbide, as shown in Figure 6(b). The surface metallographic structure of the faulty part met the requirements.
| Figure 6 | Tooth Surface Metallurgical Analysis |
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| [Insert the actual picture of the carburized layer and its metallographic structure here] | This figure shows the carburized layer and its internal metallographic structure details. |
2.3.4 Design Review
The design of the faulty gear pair of the aero – engine reducer was reviewed. Component simulation analysis was carried out for the basic tooth parameters and structural dimensions. The calculation results showed that the strength of the faulty gear pair met the design requirements.
A system – level finite – element simulation analysis model was established based on the real structure of the aero – engine reducer. The contact imprints of the faulty gear pair under different design modification parameters were analyzed. The simulation analysis of the contact imprints of the faulty gear pair is shown in Figure 7, and the design modification schemes are listed in Table 1.
| Figure 7 | Contact Imprints of Faulty Gear Pair Tooth Surfaces (Design State) | |||
|---|---|---|---|---|
| [Insert the actual picture of the contact imprints in the design state here] | This figure shows the contact imprints of the gear pair tooth surfaces under different design modification parameters. | |||
| Table 1 | Faulty Gear Pair Modification Schemes | |||
| —- | —- | |||
| Design Requirements | Driving Gear | Driven Gear | ||
| —- | Tooth Profile Modification (μm) | Tooth Direction Modification (μm) | Tooth Profile Modification (μm) | Tooth Direction Modification (μm) |
| Design Maximum Value | 20 | 18 | 18 | 24 |
| Design Minimum Value | – | – | – | – |
According to the simulation analysis results, if the tooth modification parameters of the faulty gear deviated to the lower limit of the design requirements, the risk of edge contact near the root position and the meshing line at the root of the tooth surface contact heavy – load area of the faulty gear would be greatly increased.
2.3.5 Manufacturing and Assembly Process Review
The heat treatment, actual structural dimensions, modification parameters, tooth tip fillets, etc. during the processing of the aero – engine reducer gears were reviewed. It was found that the tooth tips of the mating parts (driven wheels) of the faulty gear were all chamfered and all too small. The tooth modification parameters of the faulty gear (driving gear) were within the design requirements, and the tooth profile modification amount of the mating parts (driven wheels) of the faulty gear was too small. Reviewing the assembly process records, it was found that after assembly, the reducer was smoothly transmitted, and all rotating parts rotated flexibly without abnormal noises or jams.
A reducer simulation model was established based on Romax software, and the contact imprints of the gear pair under the measured modification parameters of the faulty gear were analyzed. The contact imprints of the faulty gear are shown in Figure 8. According to the simulation analysis results, the contact heavy – load area under the measured modification parameters of the faulty gear deviated to the root position, which was in good agreement with the actual imprints.
| Figure 8 | Contact Imprints of Faulty Gear Pair Tooth Surfaces (Actual State) |
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| [Insert the actual picture of the contact imprints in the actual state here] | This figure shows the actual contact imprints of the gear pair tooth surfaces. |
2.3.6 Test Process Review
The faulty gear of the reducer completed over – torque and long – term tests with the engine respectively. After the over – torque test with the engine, disassembly and inspection revealed that there were contact wear marks on the working surface near the root of the faulty gear, as shown in Figure 9(a), and there were contact wear marks on the corresponding mating gear near the tooth tip, as shown in Figure 9(b).
| Figure 9 | Contact Imprints of Faulty Gear Pair Tooth Surfaces |
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| [Insert the actual picture of the contact imprints after the over – torque test here] | This figure shows the contact wear marks on the faulty gear and its mating gear after the over – torque test. |
During the long – term test, the test load, lubricating oil temperature, and pressure were within the required range, and there was no load mutation. After the long – term test, disassembly and inspection found that one tooth of the faulty gear had tooth surface spalling, and there was wear near the root position (see Figure 3). The mating gear had an elliptical pit near the tooth tip and an obvious bright band.
To protect the tooth surface from damage during long – term operation, reduce friction losses, and decrease wear and heat generation, the gear transmission system needs to operate under lubricated conditions. After the test, disassembly and inspection and a flow test on the nozzles were carried out. It was found that there was a nozzle blockage phenomenon, which reduced the lubricating oil flow of the faulty gear pair by 26.7%.
2.4 Failure Location
Through macroscopic, microscopic, metallographic analysis of the faulty gear, inspection of design parameters, and review of the gear test process, the following judgments can be made:
- The nature of the gear tooth surface damage is fatigue spalling, which originates near the meshing line at the root. The fatigue spalling at the meshing line at the bottom of the gear may be related to the local stress concentration at the meshing line at the bottom when the gear engages at the bottom and the top.
- The actual chamfers of the tooth tips of the mating parts (driven wheels) of the faulty gear are all too small and the fillets are irregular, which is likely to produce sharp edges, resulting in edge contact at the roots of the faulty gear (driving gear) and the tooth tips of the meshing gears, causing initial damage to the tooth surface.
- The tooth profile modification amount of the mating parts (driven wheels) of the faulty gear is too small, causing the lower boundary of the contact imprint heavy – load area of the faulty gear (driving gear) to extend towards the root position, resulting in meshing – in and meshing – out interference.
- The tooth surface spalling of the gear is related to the previous over – torque test. Under high – load conditions, the lower boundary of the contact imprint heavy – load area of the faulty gear tooth surface extends towards the root position, and it is sensitive to the tooth tip fillets of the mating parts.
- The faulty gear pair has insufficient lubrication. The combined effect of heavy load and poor lubrication further expands the tooth surface damage, ultimately leading to tooth surface spalling.
3. Improvement Measures and Verification
3.1 Tooth Surface Contact Stress Analysis
An overly large tooth tip fillet can reduce the gear tooth contact area and decrease the contact ratio. Generally, when the normal module is between 2.5 – 5.0, the tooth tip fillet or chamfer is 0.1 – 0.3. The module of the faulty gear is 3.75.
Based on the design theoretical values of the faulty gear pair, edge contact analysis was carried out on gear pairs with different tooth tip fillets. The calculation results of the mating gears are shown in Figure 10. The simulation results show that when the tooth tip fillet is R0.1, the maximum stress appears at the tooth tip position, resulting in edge contact, and the stress increases by 29.5% compared to the tooth surface. As the tooth tip fillet increases (R0.5), the maximum stress appears at the tooth surface position, and the tooth tip stress gradually decreases, and the contact imprint tends to be normal. According to the simulation results, if the tooth tip fillet of the gear is too small, the risk of edge contact will be greatly increased, which is likely to lead to edge contact at the roots of the faulty gear and the tooth tips of the mating gears, and further contact expansion will lead to tooth surface spalling.
| Figure 10 | Tooth Surface Stress Distribution Cloud Diagram of Mating Parts of Faulty Gears |
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| [Insert the actual picture of the stress distribution cloud diagram here] | This figure shows the stress distribution on the tooth surface of the mating parts of the faulty gear under different tooth tip fillet conditions. |
It can be seen that appropriately increasing the tooth tip fillet of the gear can improve the contact imprint of the gear pair and avoid edge contact.
A reducer system – level simulation model was established based on Romax software. The simulation analysis results of the contact imprints of the faulty gear under the over – torque test state are shown in Figure 11, indicating that the lower boundary of the contact imprint heavy – load area extends towards the root position in this state. At the same time, edge contact analysis was carried out under the over – torque test state, and the results are shown in Figure 12. There is obvious edge contact at the tooth tip of the mating gear, and the stress has reached 2113MPa, with obvious local stress concentration.
| Figure 11 | Contact Imprints of Faulty Gear Pair under Test Load |
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| [Insert the actual picture of the contact imprints under the test load here] | This figure shows the contact imprints of the gear pair under the test load. |
| Figure 12 | Tooth Surface Stress Distribution Cloud Diagram of Mating Parts of Faulty Gears in Over – Torque State |
| —- | —- |
| [Insert the actual picture of the stress distribution cloud diagram in the over – torque state here] | This figure shows the stress distribution on the tooth surface of the mating parts of the faulty gear in the over – torque state. |
3.2 Mechanism Analysis
The meshing process of gears is shown in Figure 13. The driving gear near the root and the tooth tip of the driven gear enter meshing at point A and exit meshing at point D. Due to factors such as gear manufacturing, installation errors, and elastic deformation, the pitch of the driving and driven gear teeth changes, causing meshing interference at the meshing – in point A. Generally, it is necessary to modify the part of the gear tooth near the tip to compensate for manufacturing, installation errors, and elastic deformation, thereby improving the dynamic performance of the gear transmission system.
| Figure 13 | Gear Meshing Process |
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| [Insert the actual picture of the gear meshing process here] | This figure shows the meshing process of the driving and driven gears, including the entry and exit points of meshing. |
Ding Y et al. found through experimental research on the formation of AISI4340 gear spalling that the formation of spalling is caused by the development of cracks below the gear tooth contact surface and the connection of cracks in the plastic – collapsed metal ligaments between the crack tips and the adjacent gear tooth contact surfaces. In this aero – engine reducer under high – load conditions, there is obvious heavy contact near the root of the faulty gear (driving gear). At the same time, the tooth tip fillets of the mating parts (driven wheels) of the faulty gear are all too small, the chamfers are irregular, and there are sharp edges, resulting in interference and edge contact. Due to the repeated contact of the tooth surface, the contact compressive stress on the tooth surface generates cyclic tensile stress, causing the cracks in the spalling area to expand towards the tooth tip.
3.3 Improvement Measures
The main reason for the tooth surface spalling of the driving gear is that the tooth tip fillet and tooth profile modification amount of the driven gear are too small, resulting in heavy contact between the root of the driving gear and the tooth tip of the driven gear during gear meshing. Simulation analysis results show that appropriately increasing the tooth tip fillet and tooth profile modification amount of the driven gear can improve the contact pattern of the faulty gear pair and avoid edge contact. The following improvement measures are taken:
- Change the tooth tip fillet or chamfer of the driven gear from R0.1 – R0.2 to R0.4 – R0.5, and clearly define the starting circle diameter of the tooth tip fillet.
- Increase the tooth profile modification amount of the driven gear from 8.5 – 18μm to 17 – 23μm.
- Before assembling the reducer, conduct a flow test on the nozzles and nozzle components. Only those that pass the test can be installed for use to ensure that the nozzle flow meets the design requirements and reduce the risk of tooth surface damage caused by insufficient lubrication.
After implementing these improvement measures, the contact pattern of the gear pair was analyzed using Romax software. The contact pattern of the faulty gear is shown in Figure 14, and the edge contact analysis result of the mating gear is shown in Figure 15. According to the simulation results, after the improvement, the lower boundary of the contact heavy – load area of the faulty gear moves upward, and the risk of edge contact near the root meshing line is low. When the tooth tip fillet is R0.4, the maximum stress of the gear tooth is located on the tooth surface, and no edge contact occurs.
| Figure 14 | Contact Pattern of Faulty Gear Pair |
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| [Insert the actual picture of the improved contact pattern here] | This figure shows the new contact pattern of the gear pair after improvement. |
| Figure 15 | Tooth Surface Contact Pattern of Mating Parts of Faulty Gears |
| —- | —- |
| [Insert the actual picture of the mating gear’s contact pattern here] | This figure shows the contact pattern of the mating gear after improvement. |
3.4 Verification Results
After implementing the improvement measures on the faulty gear of the aero – engine reducer, it ran with the entire engine for nearly 200 hours. After disassembly and inspection, it was found that the contact pattern on the working surface of the faulty gear was normal. There were no abnormal situations such as obvious heavy – contact lines and local inverted – triangle micro – pitting corrosion pits near the root position, as shown in Figure 16.
| Figure 16 | Tooth Surface Stress Distribution Cloud Diagram of Improved Faulty Gear |
|---|---|
| [Insert the actual picture of the improved gear’s stress distribution here] | This figure shows the stress distribution on the tooth surface of the improved faulty gear. |
The test results indicate that the measures of increasing the tooth tip fillet and tooth profile modification amount of the driven gear to improve the contact pattern of the driving gear are feasible and effective.
4. Conclusion
By conducting a comprehensive analysis and investigation of the design, manufacturing, assembly, and testing of the faulty gear pair of the aero – engine reducer, as well as a failure analysis of the faulty parts, the causes of the gear tooth surface spalling have been determined. Corresponding improvement measures have been taken for the gears according to the failure causes. After implementing the improvement measures, the gears were tested with the engine, and after disassembly and inspection, it was found that the tooth surface pattern of the gears was normal, and there were no abnormal situations near the root position. This indicates that the failure analysis and positioning are accurate, the mechanism is clear, and the improvement measures are effective and reliable, providing valuable reference for the design of aero – engine reducer gears. Future research can focus on further optimizing gear design parameters, improving manufacturing and assembly processes, and enhancing the monitoring and early – warning capabilities of gear transmission systems to prevent similar failures more effectively.
