Abstract: Planetary gear, as one of the core components of RV reducers, have a significant impact on the transmission error, load sharing, wear, noise, and lifespan of the entire RV reducer. Strict control over the positional accuracy of both internal and external teeth of planetary gear is essential. Due to the numerous manufacturing steps, long process chains, and high positional accuracy requirements for both internal and external teeth, ensuring this accuracy has always been a challenging problem in the gear manufacturing industry. This paper focuses on the two typical abnormal conditions encountered during the trial production of planetary gear for an RV reducer: large angular phase fluctuations and significant eccentricity between internal and external teeth. Based on existing processing technologies, combined with theoretical analysis and process verification, effective measures have been implemented to reduce angular phase fluctuations and eccentricity, thereby ensuring the positional accuracy of both internal and external teeth of planetary gear and significantly improving the qualification rate. The optimized hobbing process has been gradually applied to batch trials of planetary gear, demonstrating stability and reliability.
1. Introduction
Currently, the mass production processes for planetary gear in RV reducers used domestically and internationally are predominantly gear grinding or gear hobbing. Gear grinding can modify the external teeth profile of planetary gear, achieving high precision but at a lower processing efficiency. Gear hobbing弥补了the deficiency of low efficiency in gear grinding, with the tooth shape of planetary gear controlled by the hob tooth shape. With the continuous development of the gear manufacturing industry, the precision of gear hobbing has continuously improved, even reaching the precision of gear grinding. Considering processing efficiency and the fact that the precision requirements for the external teeth of planetary gear is not excessively high, gear hobbing is adopted for the external teeth of planetary gear. High-precision hobbing machines equipped with the FANUC system and featuring gear tooth searching functionality are used in the gear hobbing of planetary gear external teeth. Taking the planetary gear of a certain RV reducer as an example, this paper elaborates on the causes and corresponding solutions to two typical abnormal conditions encountered during the trial production process.
2. Analysis of Typical Abnormal Conditions and Solutions
2.1 Large Angular Phase Fluctuations of Internal and External Teeth
2.1.1 Abnormal Condition
Unlike conventional gears, planetary gear in RV reducers simultaneously contain internal and external teeth with an angular relationship between them. the midpoint of the internal tooth groove of the planetary gear is denoted as a, the center of the planetary gear as o, and the midpoint of the external tooth as b. The angle formed by connecting these three points is defined as the angular phase between the internal and external teeth of the planetary gear, denoted as α. After hobbing, the angular phase range of the internal and external teeth of the planetary gear was ±0.1°, with large fluctuations, making it difficult to meet tolerance requirements and resulting in a low qualification rate. Therefore, this issue required attention and resolution.
2.1.2 Cause Analysis
After hobbing the external teeth of planetary gear, the problem of exceeding the angular phase tolerance between internal and external teeth often arises. This issue is typically addressed by compensating for the rotation angle of the C-axis of the machine tool to change the cutting amount of the hob on the left and right tooth surfaces of the planetary gear’s external teeth, thereby rotating the external teeth of the planetary gear relative to its internal teeth by a certain angle to ensure that the angular phase value between the internal and external teeth of the planetary gear after hobbing meets the tolerance requirements. However, during actual processing, it was found that compensating for the rotation angle of the C-axis of the machine tool often led to the phenomenon of no light reflection on one side of the external teeth of the planetary gear. Analysis determined that insufficient hobbing allowance for the external teeth of the semi-finished planetary gear was one of the main reasons for this phenomenon. During hobbing, the hob did not contact a certain tooth surface of the gear’s external teeth, resulting in a deviation between the position of the tooth surface without light reflection and the expected theoretical tooth surface position, leading to changes in the angular phase and large fluctuations. Before each hobbing process, the machine tool’s tooth searching program is run to determine the position of the external tooth groove of the planetary gear relative to the hob tooth, providing a reference for compensating for the rotation angle of the C-axis of the machine tool and then starting the hobbing process. Due to the quality of the semi-finished planetary gear, the relative position of the hob tooth and the external tooth groove of the planetary gear determined during the re-tooth search has an error compared to the previous piece. When compensating for the rotation angle of the C-axis of the machine tool based on the angular phase deviation value of the previous planetary gear and processing the next planetary gear, due to the error in the reference position obtained during the tooth search, the angular phase value of the internal and external teeth of the next planetary gear after hobbing deviates from the expected value or even exceeds the tolerance, which is another reason for the large angular phase fluctuations of the planetary gear after hobbing.
2.1.3 Solution
The compensation angle of the C-axis of the machine tool is generally converted into a chord compensation value. The chord compensation value refers to the chord length value obtained by converting the angle value of the planetary gear’s external tooth pitch circle rotating around the center of the planetary gear into a chord length value according to trigonometric function relationships, which corresponds to the removal amount of the unilateral tooth thickness of the planetary gear’s external teeth. The calculation formula for the pitch circle radius of the planetary gear’s external teeth is as follows:
r′=r⋅aa′=rhob⋅m⋅z/2r
Where r′ is the pitch circle radius of the planetary gear’s external teeth, r is the reference circle radius of the planetary gear’s external teeth, a′ is the actual center distance during hobbing of the planetary gear’s external teeth (obtained from the machine tool coordinates), a is the theoretical center distance during hobbing of the planetary gear’s external teeth, rhob is the hob radius, m is the module of the planetary gear’s external teeth, and z is the number of teeth of the planetary gear’s external teeth.
Through measurement, it was determined that the current unilateral tooth thickness allowance of the semi-finished planetary gear’s external teeth is approximately 0.05 mm. Based on the angular phase tolerance of the planetary gear, it was calculated that the unilateral hobbing allowance of the planetary gear’s external teeth should be controlled at about 0.3 mm. After verification, increasing the hobbing allowance of the planetary gear’s external teeth eliminated the phenomenon of no light reflection on its tooth surfaces. Additionally, to avoid the adverse effects of the quality of the semi-finished planetary gear’s external teeth on the tooth search before hobbing, the chord value of the C-axis of the machine tool was compensated based on the angular phase value between the internal and external teeth of the first precision-hobbed planetary gear, and the planetary gear underwent a second hobbing to correct its angular phase within tolerance. During subsequent hobbing processes, the tooth search function was turned off to reduce tooth search errors. Through the above verification, during hobbing, the tooth search was only performed on the first piece, and the tooth search function was turned off during subsequent batch hobbing, resulting in significantly reduced angular phase fluctuations of the internal and external teeth of the obtained planetary gear.
2.2 Significant Eccentricity of Internal and External Teeth
2.2.1 Abnormal Condition
The center offset between the internal and external teeth of the planetary gear was excessively large and out of tolerance, with the center offset evaluation item being radial runout (Fr). A high-precision gear tester was used to measure the radial runout (Fr) between the internal and external teeth of the planetary gear. The measurement results showed that the radial runout (Fr) value of the internal and external teeth of the planetary gear was as high as 0.05 to 0.08 mm, which could not meet the precision grade requirements of the gear, resulting in a low qualification rate and requiring attention and resolution.
2.2.2 Cause Analysis
The precision of the hobbing machine was verified for the above-mentioned abnormal condition of excessive radial runout (Fr) between the internal and external teeth of the planetary gear. Through inspection, it was confirmed that the circular runout and end face runout of the machine tool’s base and pressing head were within 0.005 mm, indicating that the equipment’s working condition met the usage requirements and had minimal impact on the radial runout (Fr) of the internal and external teeth of the planetary gear. Furthermore, the planetary gear hobbing tooling was verified. The hobbing tooling was equipped with a cylindrical spline that mates with the internal teeth of the planetary gear. According to the mating principle of the internal teeth of the planetary gear and the cylindrical spline, the tightness of their fit could be determined by measuring the tooth thickness of the internal teeth of the planetary gear and the cylindrical spline.