Fault Analysis and Improvement of Bevel Gears in the Drive Axle of Vibratory Rollers

The movement of the YZ10 vibratory roller is achieved through the drive axle. This type of model adopts single-axle drive. The drive axle, especially the circular arc hyperbolic bevel gear pair, is a key component of the entire machine. During operation, it bears a relatively large load and operates under harsh conditions. In the actual usage process, it is found that faults such as severe early wear of bevel gears occur from time to time, seriously affecting the overall quality of the machine. Therefore, it is extremely necessary to analyze the design and process of the bevel gears in the drive axle, identify the weak links of the gear pair, and provide a theoretical basis for future design. Now, the causes of faults and technical improvements of gear pairs are analyzed and introduced.

The structure of the central transmission of the drive axle

The drive axle is mainly composed of the axle housing and the central transmission, with the core part being the central transmission device. Central drive features a compact structure, small volume and large transmission ratio. The main parameters of the gear pair are Z1 = 5, Z2 = 39, m = 6.16mm, and βm/c = 47°29 '59 ".

The failure forms of 2 bevel gears

This gear pair is a circular arc tooth double-curved bevel gear. The bevel gear is nitrided. Before fine processing, the hardness of the quenched and tempered surface is HB190 to 217, the depth of the nitrided layer is 0.7 to 1.1mm, the hardness of the tooth surface is HRC58 to 63, and the hardness of the core is HRC35 to 42. Analysis and research on the gears fed back from failure show that the main forms of early failure are severe adhesive wear on the tooth surface and surface spalling. The teeth of the driving gear are severely worn. Some of the tooth tops are ground into knife-edge shapes, while others are all ground smooth. The shaft teeth have become smooth shafts. The driven gear undergoes severe and uniform adhesive wear, with the degree of wear on the concave side being greater than that on the convex side.

Analysis of the Failure Causes of 3 Bevel gears

Based on the basic relationship of gear pair transmission and the key points of geometric parameter design, after extensive calculation and research, the geometric dimensions of the drawing design of this bevel gear pair were checked, indicating that the geometric parameters are correct and error-free. The main reasons for the failure of bevel gears are the low precision of the gear contact zone and the failure of heat treatment to meet the mechanical property requirements of the gears.

3.1 Reasons for the low accuracy of the tooth surface contact zone

During the processing of the driving gear blank, the axial positioning process dimension 2000.05 exceeded the tolerance. According to the on-site random sampling inspection, the dimensional variation range was 0.24, with the maximum dimension being 20+0.14 and the minimum dimension being 20-0.10. The variation range was relatively large, which inevitably affected the position of the tooth surface contact zone.

(2) The gear processing technology is unreasonable, resulting in poor contact zones on the tooth surfaces. Especially after heat treatment, the processing technology of the inner hole of the driven gear is improper. Due to the large diameter of the driven gear, although a quenching press is used during quenching, certain deformation still occurs. Actual detection shows that the deformation of the inner hole is generally about 0.15mm. As our factory does not have a vertical internal cylindrical grinding machine and does not perform heat treatment on the inner hole before grinding, but instead uses the deformed inner hole and end face to position and align the processed gear teeth, it will inevitably lead to radial runout errors of the gear ring and deteriorate the quality of the contact area of the gear surface.

(3) Poor processing quality of related parts leads to poor contact areas on the tooth surfaces. The central support is a supporting component of gears. Its machining accuracy plays a crucial role in the contact area of the tooth surface, transmission quality, and reducing the workload of assembly and adjustment. Particularly, several parameter requirements include the verticality of the installation holes for the driving and driven gears, the 30mm offset distance of the driving gear's axis, and the 214±0.2mm distance from the end face of the driving gear installation hole to the axis of the driven gear. Actual tests show that the accuracy of these form and position tolerances as well as dimensional tolerances all have varying degrees of out-of-tolerance. In addition, the dimensional accuracy of 21 between the positioning end face of the bearing housing and the positioning end face of the lower bearing also affects the installation position of the driving gear, and thereby influences the quality of the tooth surface contact zone. Since the tolerance of dimension 21 on the drawing was not marked, this dimension was not well controlled.

(4) Poor assembly quality affects the contact area of the tooth surface. Assembly is the final process to ensure product quality, and the quality of assembly directly affects the quality of the contact area of the tooth surface. When assembling the central transmission components, there is a lack of detection methods to check the contact area of the tooth surface. The correctness of the adjustment position is mainly judged based on the tooth side clearance. After the drive axle is fully assembled, conduct an idle run test on the test bench. Indirectly judge the quality of the tooth surface contact area based on the running sound. On the other hand, if the meshing quality is poor, the central transmission component must be removed and readjust, which is labor-intensive and time-consuming.

3.2 The Impact of Improper Heat Treatment processes

Through the material inspection, analysis of the failed bevel gears and the evaluation of the current heat treatment process technology, it was determined that the poor structure of the infiltration layer and the core as well as the unreasonable hardness distribution of the infiltration layer were the main reasons for the early failure of the gears.

We conducted hardness and metallographic structure analyses on the failed gears that were fed back. Hardness tests show that the surface hardness of some gears in co-permeation is HRC57, and the core hardness is HRC28, both of which are lower than the requirements of the design drawings. Metallographic examination revealed that the carbo-nitriding concentration of this gear was insufficient, resulting in no primary nitrogen-carbon compounds on the carbo-nitriding surface and no network precipitation of secondary nitrogen-carbon compounds in the secondary layer. When reheating and quenching, the heating temperature was too low, resulting in a large amount of undissolved ferrite in the core, which greatly reduced the hardness of the core. When reheating and quenching, if the holding time is too long, it will cause severe decarburization of the hardened layer of the gear, resulting in a decrease in the surface hardness.

Quality improvement measures for 4 bevel gears

In response to the causes of bevel gear failure, we have taken the following measures:

During the gear processing, the quality inspection of the 200-0.05 size was strengthened, and inspection gauges for this size were made to facilitate the control of this size by the operators.

(2) Improve the processing technology flow of the driven gear and add a grinding process after heat treatment of the inner hole. Before purchasing the vertical internal cylindrical grinding machine, we adopted the method of outsourcing the processing of this procedure to achieve it.

(3) In response to the issue of low processing accuracy of related parts such as the central support, the precision control of the processing machine tool was strengthened, the process was strictly followed, and the boring processing fixture for the central support was redesigned and manufactured to improve the processing accuracy of the parts. And improve the design drawing, and add tolerance requirements for the bearing support size 21.

(4) Develop a simple, practical and easy-to-operate dedicated test device for central transmission components to facilitate the inspection and adjustment of the tooth surface contact area.

(5) To improve the heat treatment conditions, based on the existing conditions, we have taken appropriate measures such as increasing the co-diffusion temperature and quenching temperature, shortening the time from furnace removal to quenching cooling, improving the furnace gas circulation, and purchasing controllable gas-nitrogen heating furnaces. At the same time, improve the design and add heat treatment technical requirements, that is, requirements for metallographic structure, in the design drawings to enhance the guiding role for the heat treatment process.

After implementing the above solutions, the feedback of early severe wear and failure of the bevel gears in the drive axle has been significantly reduced in recent years, and the quality of the drive axle has been significantly improved. As a result, the overall quality of the machine has been further enhanced, and the economic and social benefits are very obvious.