Precision Alignment Techniques for Sun Gear Pitch Circles

This study examines the problem of inadequate grinding allowance in the sun gears of wind turbine gearboxes by analyzing the causes and suggesting process improvements. By utilizing a gear grinding machine to ensure proper pitch circle alignment, we can optimize grinding allowance, prevent component wastage from grinding steps, and address issues with insufficient tooth surface hardness. This approach enhances the overall quality of sun gear grinding. The method for aligning the pitch circle of sun gears is straightforward and practical, making it suitable for widespread application on sun gears that have been fine-turned on a vertical lathe following carburizing and quenching.

 

01 Introduction

As a core component of wind turbine gearboxes, the sun gear is the central gear within the planetary gear mechanism. This gear not only endures complex loads but also enables precise transmission and torque transfer. Consequently, all components of the sun gear require high dimensional accuracy, especially concerning the precision and strength of the teeth and splines.

The teeth of the sun gear are wide and long, featuring a large module. However, hobbing or milling with insert tools can lead to overlapping errors, and the deformation caused by carburizing and quenching is significant. Additionally, challenges arise when trying to align the pitch circle during fine turning, and the optimization of design and adjustments to the processing helix angle can result in inadequate grinding amounts. In extreme cases, these issues can lead to direct gear scrapping or the emergence of grinding steps, which can easily cause tooth breakage and lead to gear failure.

To address the problem of poor sun gear grinding, Gu Xiaoming and colleagues recommend using a center-of-gravity welded suspension and a hollow body design for carburized wind turbine sun gears to help reduce heat treatment deformation. Mou Xinghua proposed adding a crown to the tooth during gear hobbing, applying the principle of reverse deformation to counteract heat treatment deformation. Zhao Ping analyzed the causes of false grinding steps at the tooth root and identified measures to eliminate them. Similarly, Li Zejun examined the causes and negative effects of grinding steps, proposing methods for their prevention and repair. He Peng investigated the design of roughing tool cam angles before heat treatment, calculated tool overrun after heat treatment, and assessed how heat treatment deformation affects grinding steps. Zhang Lifeng and his team studied the causes and solutions for steps arising from poor transition curves at the tooth root.

While these findings offer promising solutions for conventional sun gear machining, they may be less effective in instances where multiple factors contribute to poor grinding. Issues such as significant heat treatment deformation, pitch circle alignment errors during finishing, and design changes in the helix angle can worsen grinding issues, increasing the risk of scrapping.

This article utilizes a specific model of sun gear as a case study to introduce a new method for aligning the pitch circle of sun gears during the finishing process. This method effectively optimizes the grinding allowance, addresses discrepancies in grinding allowance, and ultimately avoids grinding steps. This improvement enhances the quality of the sun gear and extends its lifespan.

 

02 Solar Gear Tooth Parameters and Material Properties

The sun gear described in Figure 1 has the following specifications: it features 33 teeth, a module of 18 mm, a pressure angle of 22.5°, and a helix angle of 6°8′. The gear is left-handed and has a tooth width of 455 mm. Its root diameter measures 551.912 mm. The normal modification coefficient is 0.14, and the normal length across five teeth ranges from 248.625 mm to 248.712 mm.

For heat treatment, the gear will undergo carburizing and quenching to achieve a surface hardness of 58 to 62 HRC and a core hardness of 33 to 45 HRC. The material used for this component is 18CrNiMo7-6, which is a carburized and hardened steel known for its high strength, excellent toughness, good hardenability, and corrosion resistance, making it suitable for heavy-duty load transmissions.

 

03 Plan Development

Based on the structural characteristics of the sun gear, the following process flow was developed: rough turning, drilling, gear hobbing, tooth profile chamfering, carburizing and quenching, finish turning, machining center processing, gear grinding, knurling, tooth profile chamfering, burn detection, magnetic particle inspection, and final inspection.

Drawing from years of production experience, we determined that the hollow body design of the sun gear, along with the inclusion of tooth crowning during gear hobbing, can minimize heat treatment deformation. Consequently, the process design specifies that the inner hole must be turned during the rough turning phase, and crowning should be added during the gear hobbing process. The normal hobbing length, W5, is set at (249.32 ± 0.03) mm, with a crowning allowance of 0.4 mm.

 

04 Existing Problems

The trial production of the sun gear, based on the specified process plan, encountered an issue with inadequate tooth surface grinding allowance during the gear grinding process. Here are the details: the sun gear has 33 teeth, and nine evenly distributed tooth grooves were selected for testing. The testing positions are illustrated in Figure 1, which includes the upper section (A), the middle section (B), and the lower section (C). The distribution of the tooth surface grinding allowance was evaluated using the Niles gear grinding machine ZP12. The results are presented in Table 1.

 

Table 1 presents the results for each tooth groove tested in lower section C. It shows that the minimum grinding amount on the left tooth surface is 0 mm at groove 6, while the minimum grinding amount on the right tooth surface is 0.06 mm at groove 9. To achieve a uniform minimum grinding amount, conventional calculations suggest determining this amount during gear grinding using the formula: (0 + 0.06)/2 = 0.03 mm. This calculation averages the minimum grinding amounts from both surfaces, resulting in a consistent minimum grinding amount of 0.03 mm on each surface.

 

Additionally, Table 1 reveals that the maximum grinding amount on the left tooth surface is 0.88 mm at groove 1, whereas the maximum grinding amount on the right tooth surface is 0.73 mm at groove 5. After compensating for the minimum grinding amount, the maximum grinding amount on the left tooth surface for groove 1 increases to 0.88 mm + 0.03 mm = 0.91 mm. Conversely, the maximum grinding amount on the right tooth surface for groove 5 decreases to 0.73 mm – 0.03 mm = 0.70 mm.

 

At this point, the maximum grinding allowances are as follows: 0.91 mm for the left tooth surface, 0.70 mm for the right tooth surface, and a minimum grinding allowance of 0.03 mm.Direct grinding would present the following problems:

1) The maximum grinding allowance on the left tooth surface is 0.91 mm, while the hob cam angle is 0.75 mm. This maximum grinding allowance is significantly larger than the transition radius of the sun gear tooth root, making additional grinding steps unavoidable after the initial grinding process.

2) The minimum grinding allowance is set at 0.03 mm, but this does not ensure a smooth finish on the tooth surface. Additional grinding is necessary, which could lead to a failure in meeting tolerance standards. Moreover, this may further increase the maximum grinding allowance to 0.91 mm on the left tooth surface, resulting in a deeper grinding step.

 

Excessive grinding allowance can reduce the hardness of tooth surfaces and lead to severe grinding steps. These steps can affect the bending fatigue strength of gear tooth roots, ultimately reducing product quality and lifespan.

 

05 Cause Analysis

(1) The influence of gear hobbing

To enhance the efficiency of gear manufacturing, insert tools are commonly utilized for gear hobbing or milling. The accuracy of tool manufacturing and any errors in blade alignment can significantly affect the amount of grinding required. After gear hobbing, the sun gear was tested for accuracy using the Niles gear grinding machine ZP25. The results indicated that the tooth shape exhibited an inward concave phenomenon, which led to an impact of approximately 0.07 mm on the single-side grinding amount.

 

(2) The influence of heat treatment

According to the literature, several factors influence the deformation of carburized and quenched gears. These factors include the gear’s structural design, the quality of the raw materials, the forging process, the initial heat treatment of the blank, machining (both hot and cold), and the carburizing and quenching process. Given the multitude of factors that affect gear deformation, along with the interactions between these factors, controlling them becomes quite challenging. As a result, deformation has emerged as a significant technical difficulty in gear manufacturing.

 

(3) The influence of pitch circle alignment in fine turning

According to the structural characteristics of the sun gear, a high-precision vertical lathe is utilized for precision turning. However, achieving pitch circle alignment poses challenges, resulting in suboptimal alignment outcomes. An analysis of the alignment data revealed that the alignment is random, potentially causing a maximum impact of approximately 0.2 mm on the single-side grinding amount.

Regarding the impact of helix angle changes, the design process optimizes the machining helix angle based on results from prototype trials. The helix angle is adjusted from 6°6′52″ to 6°8′. Specifically, the processing helix angle is maintained at 6°6′52″ during hobbing and adjusted to 6°8′ during grinding. Regardless of whether the processing helix angle increases or decreases, the grinding amount also increases. Moreover, the greater the absolute value of the change in the processing helix angle, the more grinding material must be reserved.

The width of the sun gear teeth is 455 mm. Due to the difference in the helix angle between hobbing and grinding, this change will theoretically impact the grinding amount by 0.15 mm.

 

06 Improvement measures

Based on the reasons mentioned above, several improvement measures can be proposed. These include enhancing the manufacturing accuracy of gear hobbing tools and blades, optimizing the hobbing drum shape, refining the heat treatment process, improving the quality of fine-tuning pitch circle alignment, and minimizing variations in the processing helix angle. While these methods are relatively standard, their effectiveness may not be ideal. This paper presents a new technical solution that can effectively address the issue of variability in grinding amounts. The specific implementation process is outlined as follows.

(1) To calculate the total tooth surface grinding amount for each tooth groove under inspection, you need to sum the left and right tooth surface grinding amounts. For example, the tooth surface grinding amount for the upper section A of the first groove is calculated as follows: the left tooth surface grinding amount (0.45 mm) plus the right tooth surface grinding amount (0.43 mm), which equals a total of 0.88 mm. The results of the calculated tooth surface grinding amounts and their distribution are presented in Table 2.

 

(2) Confirm the adjustable position.

To confirm the adjustable position based on the distribution of the tooth surface grinding amounts, the following process should be followed:

1. Calculate the difference in grinding amounts for each test section. The grinding amount difference is determined by subtracting the minimum tooth surface grinding amount from the maximum tooth surface grinding amount.

- For the upper section A:
Maximum tooth surface grinding amount = 0.90 mm (9th groove)
Minimum tooth surface grinding amount = 0.74 mm (5th groove)
Grinding amount difference = 0.90 mm – 0.74 mm = 0.16 mm

 

- For the middle section B:
Maximum tooth surface grinding amount = 0.91 mm (5th groove)
Minimum tooth surface grinding amount = 0.70 mm (2nd groove)
Grinding amount difference = 0.91 mm – 0.70 mm = 0.21 mm

 

- For the lower section C:
Maximum tooth surface grinding amount = 1.12 mm (3rd groove)
Minimum tooth surface grinding amount = 0.49 mm (7th groove)
Grinding amount difference = 1.12 mm – 0.49 mm = 0.63 mm.

 

By following these calculations, you can assess the grinding amount differences in each section.

2) Confirm the adjustment section.

The adjustment section exhibits the largest difference in grinding amounts among all the test sections. Lower section C shows a grinding difference of 0.63 mm, which is significantly greater than the differences observed in upper section A (0.16 mm) and middle section B (0.21 mm). Therefore, lower section C is identified as the adjustment section due to its substantial grinding difference.

 

3) Confirm the adjustable position.

The adjustable position is located on the side opposite to the groove with the minimum grinding sum. The groove with the minimum grinding sum is identified within the adjustment section. For clarity, the grinding sums of all tooth surfaces in lower section C are illustrated on a top view of the tooth profile, as displayed in Figure 2. The nine measured grooves listed in Table 1 are numbered from 1 to 9, with the corresponding grinding sums of the tooth surfaces in lower section C noted alongside each groove. According to Figure 2, groove 7 has the minimum grinding sum, indicating that the adjustable position is on the side opposite groove 7.

 

(3) Calculate the adjustable distance.

The adjustable distance is calculated based on the distribution of the adjustable position and the tooth surface grinding amount. The calculation process is as follows.

 

1) Confirm the tooth groove with the maximum grinding amount.

The tooth grooves that experience the highest amount of grinding are the two closest to the adjustable position. In Figure 2, these grooves are identified as the second and third tooth grooves. Therefore, the second and third grooves can be considered the ones with the maximum grinding amount.

 

2) Confirm the average grinding amount.

The average grinding amount is calculated as half of the sum of the minimum grinding amounts of the left and right tooth surfaces across all cross sections of the tooth grooves with the maximum grinding amounts. For clarity, the data for the tooth grooves with the maximum grinding amounts (the second and third grooves) and the tooth groove with the minimum grinding amount (the seventh groove) are listed separately in Table 3.

Among all the sections of the tooth grooves with the maximum grinding amounts (the second and third grooves), the two smallest grinding amounts are 0.32 mm for the left tooth surface of section A in the third groove, and 0.29 mm for the right tooth surface of section B in the second groove. To find the average grinding amount, we take half of the sum of these two values: (0.32 mm + 0.29 mm) / 2 = 0.305 mm. Therefore, the average grinding amount can be approximated as 0.30 mm.

 

This case analysis focuses on the operator’s inspection of the grinding depth of nine tooth grooves. The groove that has the minimum grinding depth (the seventh groove) is situated between the second and third grooves (see Figure 2). Since the gear grinding machine did not detect this groove, the grinding depths of the other grooves were summed to calculate an average grinding depth. This approach is not fixed and should be analyzed based on the specific locations of the grooves being inspected.

 

If eight grooves are inspected instead, the groove with the minimum grinding depth will be directly detected. In this scenario, the minimum grinding depth for this groove (considering the left and right tooth surfaces in the upper, middle, and lower sections) will represent the average grinding depth.

 

3) Determine the superposition allowance. Superposition allowance = (average grinding depth – minimum grinding depth on the tooth surface across all sections of the minimum grinding depth groove) / 2. The minimum tooth surface grinding amount for the minimum tooth groove (slot 7) across all sections is 0.15 mm on the right tooth surface of lower section C. Therefore, the superposition allowance = (0.30 – 0.15) / 2 = 0.075 mm. This means that the tooth surface grinding amount of slots 2 and 3 needs to be reduced by 0.075 mm, while the tooth surface grinding amount of slots 7 needs to be increased by 0.075 mm. This will compensate for the larger tooth surface grinding amount of slots 2 and 3, which have been offset by the smaller tooth surface grinding amount of slot 7, thus ensuring a more uniform grinding amount.

 

4) Calculate the adjustable distance. To calculate the adjustable distance, use the formula:

Adjustable distance = superposition allowance / sin(α)

where α represents the gear pressure angle. In this case, the tooth surface grinding amount is 0.075 mm, which must be converted to the radial adjustable distance. Given that the sun gear pressure angle is 22.5°, we can determine the adjustable distance as follows:

Adjustable distance = 0.075 mm / sin(22.5°) ≈ 0.2 mm.

 

Next, we need to align the pitch circle. Adjust the position of the sun gear according to the determined adjustable distance. To do this, set a dial indicator on the opposite side of the tooth groove where the adjustable position is located. Then, use a copper rod to gently tap the adjustable position. Based on the reading from the dial indicator, knock the sun gear by approximately 0.2 mm. This completes the alignment process before grinding.

After alignment, use the Niles gear grinding machine ZP12 to re-check the grinding amount of each tooth groove. The grinding amounts after alignment are displayed in Table 4.

Table 4 reveals a change in the distribution of grinding amounts. The minimum grinding amount on the left tooth surface is 0.34 mm in the 1st and 2nd grooves, while the minimum on the right tooth surface is -0.01 mm in the 5th groove. The maximum grinding amount on the left tooth surface is 0.87 mm in the 6th groove, and on the right tooth surface, it is 0.43 mm in both the 8th and 9th grooves.

 

Using the conventional method, the minimum grinding amount during gear grinding is calculated as follows: (-0.01 + 0.34) / 2 = 0.165 mm. After optimizing and compensating for the minimum grinding amount, the maximum on the left tooth surface in the 6th groove is adjusted to 0.87 – 0.165 = 0.705 mm. For the right tooth surface in the 8th and 9th grooves, the maximum grinding amount is adjusted to 0.43 + 0.165 = 0.595 mm.

 

Once the pitch circle is aligned using this method, the maximum grinding amount on the left tooth surface is 0.705 mm, while the maximum on the right tooth surface is 0.595 mm, with a minimum grinding amount of 0.165 mm. This approach ensures that the tooth surface is polished effectively without generating grinding steps, resulting in a significant improvement in the grinding amount distribution.

 

(5) Use a gear grinding machine to complete rough grinding.

Clamp and lock the aligned sun gear, leaving a 0.2mm normal allowance for gear grinding.

 

(6) Turning.

Transfer the rough-ground sun gear to a CNC lathe turning. Using the fine turning alignment method, position the sun gear with the pitch circle as the reference. Since the teeth of the sun gear have already been rough-ground, fine turning alignment is straightforward. Re-turn the outer circle alignment reference, ensuring that it corresponds with the gear’s pitch circle reference.

 

(7) Fine grinding.

Transfer the finely tuned sun gear to a gear grinding machine for precision grinding. When performing rough grinding on the sun gear, leave a normal allowance of 0.2 mm. Ensure that the alignment of the outer circle reference matches the gear pitch circle reference. At this stage, align the outer circle reference and proceed with fine grinding to complete the sun gear grinding process.

 

07 Conclusion

This paper addresses the variation in grinding loss of sun gears by integrating theoretical analysis with process validation to develop a new method for aligning the pitch circle of sun gears. This method scientifically identifies specific adjustable positions and distances for the sun gear based on the distribution of tooth grinding loss. By adjusting the sun gear’s position to shift areas with higher grinding loss toward those with lower loss, this approach optimizes overall tooth grinding loss. It also helps prevent issues such as sun gear scrapping or tooth breakage resulting from grinding processes or insufficient tooth hardness. This innovative and practical sun gear pitch circle alignment method can be widely applied to sun gears that undergo fine-turning on vertical lathes after processes like carburizing and quenching. It provides valuable insights for similar gear machining applications.

 

 

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

With Anebon’s leading technology and our spirit of innovation, mutual cooperation, and development, we aim to build a prosperous future together with your esteemed enterprise as an OEM manufacturer of custom high-precision precision metal stamping parts, turned metal parts, and CNC precision milling steel components. Additionally, we have many overseas friends who visit us for sightseeing or trust us to purchase other products on their behalf.