Precision Enhancement Techniques for Planetary Gear Hobbing in Hydraulic Bulldozers

Due to the issue of misalignment in the planetary gears of the fully hydraulic bulldozer produced by our company during the hobbing process, which led to excessive accuracy, we conducted an analysis of the hobbing process. Our findings revealed that the clearance between the inner hole of the workpiece and the spindle significantly affects the radial circular runout of the planetary gear’s tooth surface and the accuracy of the tooth direction. This misalignment was identified as the primary cause of the excessive hobbing accuracy. We improved the processing technology to address this issue and achieve the desired results.

 

01 Introduction

The fully hydraulic bulldozer is a key product of our company, primarily utilized for pushing, excavating, and backfilling earth, stone, and other bulk materials. It is commonly employed in operations on roads, railways, mines, and airports.

A critical component of the bulldozer’s final drive is the planetary gear, which works in conjunction with the inner gear ring and the sun gear shaft to enable efficient tooth surface transmission. To ensure proper transmission for the bulldozer’s movement, three planetary gears are required within the final drive.

However, during the hobbing process, the planetary gear is susceptible to machining offsets, which can lead to excessive hobbing inaccuracies. This directly impacts the accuracy of gear transmission and can result in severe issues such as tooth breakage.

 

02 Planetary gear structure and processing

(1) Product structure
The planetary gear features an external tooth structure, and its transmission mode involves tooth surface transmission. The internal structure includes two equally sized bearing inner holes, as illustrated in Figure 1.

 

(2) Processing technology

The manufacturing process for the planetary gear consists of seven steps, from initial blank processing to the finished product. The steps are as follows:

- Blank forging
- Rough turning
- Gear hobbing
- Heat treatment
- Fine turning of the inner end face
- Fine turning of the inner hole
- Gear grinding

Each step plays a crucial role in ensuring the final product meets the required specifications.

 

(3) Gear hobbing processing process

The current gear hobbing process employs nut clamping and pressure plate clamping for alignment. The steps involved in this milling process are as follows:

- Place the workpiece on the gear hobbing fixture and conduct preliminary positioning (refer to Figure 2).
- Use a dial indicator to align the outer circle of the workpiece, ensuring that the positioning accuracy is ≤0.03 mm.
- Tighten the nut with a wrench to secure the workpiece under pressure.
- Proceed with gear hobbing according to the specified dimensions.

During the entire gear hobbing process, it is crucial to maintain the positioning accuracy stated in step 2 at ≤0.03 mm, as this is key to achieving the desired accuracy in gear hobbing.

03 Analysis of Planetary Gear Hobbing Precision Exceeding Tolerance

The analysis of planetary gear hobbing precision exceeding tolerance focuses on six key aspects: people, machines, materials, methods, environment, and measurements. The investigation systematically eliminates various factors contributing to the deformation of the inner hole. This deformation can occur due to the incomplete release of internal stress during heat treatment and excessive coaxiality tolerance between the inner hole and the outer circle of the workpiece.

Additionally, we examine the significant gap between the inner hole of the workpiece and the tooling mandrel, which poses a hidden risk.

As illustrated in Figure 3, the workpiece is positioned axially by precision CNC turning the end face, while the inner hole is adjusted radially to address the gap. Nuts are used for securing the workpiece. During alignment, the dial indicator shows a runout value of 0.03 mm, but after full tightening, this value increases to 0.08 mm. This detected runout exceeds the required positioning accuracy limit of 0.03 mm. The gap between the inner hole of the workpiece and the tooling mandrel (highlighted in the red circle in Figure 3) is too large, which may lead to uneven forces acting on the workpiece after alignment, tightening, and hobbing. This uneven force can cause deviations, resulting in the workpiece dimensions exceeding the specified tolerance.

 

In order to verify the influence of the alignment between the inner hole and the spindle gap on the workpiece, the workpieces with different spindle gaps were recorded, and the gear measuring instrument was used to verify the workpiece tooth surface runout, tooth direction accuracy, etc. (see Table 1).

 

Table 1 demonstrates that the clearance between the inner hole of the workpiece and the mandrel significantly impacts the radial circular runout of the planetary gear’s tooth surface as well as the accuracy of the tooth direction. It can be concluded that the unavoidable error resulting from the mandrel clearance and the inner hole installation is the primary cause of the reduced accuracy in hobbing.

 

04 Countermeasures for excessive hobbing accuracy of planetary gears

To ensure accurate hobbing, the most straightforward approach is to minimize clearance. However, theoretically, enhancing the accuracy of the inner hole processing and tooling cannot completely eliminate the presence of clearance during positioning. Currently, the workpiece is clamped and aligned using a dial indicator (see Figure 4). During the clamping process, the workpiece must be continually tapped for proper alignment. Achieving precise alignment is challenging, especially given the large number of planetary gears being processed. Therefore, improving alignment accuracy could significantly reduce processing efficiency.

 

 

In summary, the current gear hobbing method, which uses a pressure plate to clamp the gear, faces challenges of low processing efficiency and high difficulty. To enhance this process, an expansion fixture (see Figure 5) is being utilized in place of a gear hobbing wheel. This innovation aims to reduce clamping clearance and improve both product quality and processing efficiency.

The advantages of the expansion fixture are significant: it eliminates alignment errors without any need for manual adjustments, and it prevents axial workpiece offset, which enhances hobbing accuracy. The fixture is designed with a stopper that ensures proper positioning of the end face. When axial force is applied by the nut and pressure plate, the taper sleeve moves axially. This movement transforms the axial force into radial force, causing the elastic taper sleeve to deform and securely tighten the workpiece.

 

05 Verification of Improved Hobbing Accuracy

After using the tensioning fixture, the average radial runout of the planetary gear tooth surface decreased from 113 µm to 65 µm. The maximum radial runout on the sampled workpiece was 78.3 µm (see Figure 6), which meets the Grade 9 precision requirement of ≤85 µm. Additionally, the average tooth profile tolerance improved from 60 µm to 31 µm, thus achieving Grade 9 precision for the planetary gear tooth profile.

 

6 Conclusion

After analyzing the machining issues and implementing the appropriate countermeasures, we verified their effectiveness by inspecting the tooth profiles of all 300 workpieces that underwent gear hobbing using a coordinate measuring machine. The inspection results showed a 100% pass rate, successfully resolving the problem of out-of-tolerance gear hobbing accuracy and achieving our desired goals. Additionally, the need for alignment and positioning backlash was eliminated, which effectively improved machining efficiency.

 

 

If you wanr to know more or inquiry, please feel free to contact info@anebon.com

At Anebon, we firmly believe in “Customer First, High-Quality Always”. With over 12 years of experience in the industry, we have been working closely with our clients to provide them with efficient and specialized services for CNC milling small parts, CNC machined aluminum parts, and die-casting parts. We take pride in our effective supplier support system that ensures excellent quality and cost-effectiveness.