Abstract
To meet the reliability requirements of aviation products, aerial spiral bevel gear must possess high precision, strength, wear resistance, and lightweight qualities. However, the complex structure and diverse forms of spiral bevel gear inevitably lead to increased processing difficulty. After heat treatment processes such as carburizing, quenching, and tempering, spiral bevel gear undergo significant deformation. This article delves into the deformation compensation techniques for typical spiral bevel gear after heat treatment, addressing the impact of heat treatment deformation on products, ensuring product conformity, and providing valuable experience for the development and processing of subsequent similar parts.
1. Introduction
Spiral bevel gear serve as the primary device for transmitting power in the main reduction system of aviation helicopters. To fulfill the service life and structural requirements of aviation products, aerial spiral bevel gear must meet the criteria of high precision, strength, and lightweight quality. The complex structure and diverse forms of spiral bevel gear also lead to increased processing difficulty.
The rotor transmission system of helicopters requires high-speed rotation during flight. The flexible blades, subjected to asymmetric airflow fields with rotation and sliding, endure severe oscillating, torsional, and high-frequency coupled vibration loads. Furthermore, helicopters are highly maneuverable, capable of forward, backward, sideward flight, hovering, turning, and vertical takeoff and landing, leading to complex and varied load conditions.
Input bevel gears have a complex structure and belong to thin-walled components, with the thinnest section of the auxiliary plate being only 4mm. Additionally, five locations on the entire component require carburizing, including the bevel teeth, spline, inner diameter, and outer diameter, with varying carburized layer depths requiring double carburizing, which results in greater heat treatment deformation. Double bevel gears are the most difficult components to control in terms of heat treatment deformation in current production. This component is a double-tooth thin-walled structure with an outer diameter of Φ490mm and a wall thickness of 8.13mm. The large and small teeth have different carburized layer depths and also require double carburizing, leading to significant heat treatment deformation. The metallurgical quality inspection of input bevel gears and double bevel gears after heat treatment is stringent, with specific regulations on the carburized layer and HRC60 depth, posing significant processing challenges. While ensuring metallurgical quality, the precision of the components must also be maintained, categorizing them as difficult-to-machine grade 4 gears.
This article addresses the deformation compensation technology for spiral bevel gear by focusing on input bevel gears and double bevel gears. Firstly, the heat treatment workshop improves tooling methods to control deformation and explores deformation patterns before and after heat treatment. Based on these deformation patterns, the machining workshop adjusts the machining routes to achieve the desired range through compensation, ensuring all processing requirements are met.
2. Effective Measurement Methods for Heat Treatment Deformation Compensation Control
Effective measurement methods for heat treatment deformation compensation control of spiral bevel gear primarily include tooth surface measurement and root cone measurement. Tooth surface measurement is conducted using a gear measuring machine, which evaluates the datum to produce a tooth surface topography map, tooth thickness, adjacent, and cumulative gear-related parameters. Root cone measurement is performed using a coordinate measuring machine (CMM), which measures the distance from the theoretical diameter position root cone to the mounting distance end face. By analyzing pre- and post-heat treatment data measured by the gear measuring machine and CMM, the deformation patterns during heat treatment can be assessed, and processing schemes for compensation and reverse compensation can be determined based on measurement data analysis.
3. Compensation Machining Methods for Input Bevel Gears
Installed in the initial reduction gearbox, the input bevel gear serves as the first stage of reduction, with a reduction ratio of 1:3.6 and a rotational speed of approximately 7000r/min. Bearing inner rings are assembled at the A and B shaft journals, while a bearing outer ring is assembled at the inner diameter F. The C location is the spiral bevel gear, all of which require carburizing. The D location is an internal spline, and the E location is an inner diameter, which mate with the spline and outer diameter of the clutch outer ring, also requiring carburizing. To enhance the loading capacity of armed helicopters, components must meet usage conditions while ensuring lightweightness, hence the design as hollow thin-walled components with the thinnest auxiliary plate being only 4mm. The spiral bevel teeth, bearing raceways on both ends, internal spline, and inner bore supporting bearing raceways of the input bevel gear all require carburizing, with varying depths of carburized layers that are difficult to control during heat treatment. The differing requirements for carburized layer depths necessitate a second carburizing process, which increases the difficulty of controlling carburizing deformation and directly affects the grinding of the bevel teeth, thereby failing to guarantee the effective depth of the carburized layer and the final metallurgical quality requirements of the component.
During the carburizing and quenching heat treatment process for the input bevel gear, the temperature field, phase transformation, and stress field changes during the carburizing quenching process are investigated in combination with the component’s structure, material composition, organization, and performance characteristics. The internal stress changes and stress relaxation process in the component during pressure quenching cooling conditions are studied, and the distortion characteristics of the final component are analyzed. Methods to control deformation, hardness, and stress gradient distribution during the actual carburizing quenching process of the component are explored.
Carburizing temperature is a significant factor affecting component deformation. A higher temperature allows for faster carbon atom diffusion but also leads to larger deformation at high temperatures. An important factor contributing to deformation is the influence of gravity. Sometimes, to provide a favorable deformation state for quenching, the loading method of the component can be considered to suppress or induce deformation. Additionally, when the component loading volume is large, the heating rate can be appropriately reduced, and a slower cooling rate can be adopted during the cooling process to minimize component deformation. To avoid carburizing deformation caused by the component’s own gravity, a quenching and tempering heat treatment process after high-temperature tempering of carburizing is adopted. During the carburizing process, the supporting wall thickness at the auxiliary plate can be appropriately increased to reduce deformation caused by the component’s own gravity. A machining process can be added between quenching and carburizing to remove the allowance affecting hardenability.
The quenching process determines the heat treatment state of the component and generates the maximum stress, involving thermal stress and structural stress, which affect the change in the component’s volume and shape. Simultaneously, the quenching process can also achieve pressure quenching, changing the deformation of the component, making the quenching process particularly important. The factors affecting quenching are extremely complex, mainly including quenching method, quenching temperature, quenching medium, and stirring.
In summary, the input bevel gear, as a critical component in the helicopter’s transmission system, faces significant challenges in heat treatment due to its complex structure and diverse material requirements. Effective control of deformation during the heat treatment process is crucial to ensuring the final quality and performance of the component.