Optimization of machining process for raceway surface of outer ring of double row self-aligning roller bearing

This paper focuses on the machining of the outer ring raceway surface of a double-row self-aligning roller bearing after heat treatment, specifically addressing the issue of tool vibration when the turning tool tip passes over the oil holes. The machining process of the outer ring raceway surface is optimized to reduce tool vibration, improve part accuracy, and enhance machining efficiency simultaneously.

 

PART 01 Introduction

Double-row self-aligning roller bearings are known for their high load capacity, self-aligning ability, and durability, which makes them crucial in the heavy industry, new energy, and high-end manufacturing sectors. As global industries undergo upgrades and technological innovations, the application scenarios for these bearings are expected to expand, especially in equipment that operates under complex conditions and requires high reliability.

Recently, with the rapid growth of the wind power industry, double-row self-aligning roller bearings have become the most commonly used bearings in the main shafts of doubly-fed wind turbine models. Their market share has exceeded 90%, making them essential for this equipment. As the power of gearboxes continues to rise, the demand for lubricating oil by the bearings also increases. To accommodate this need, the oil holes on the outer raceway surface must be enlarged. However, this adjustment can lead to increased tool vibration during machining, which negatively impacts part precision and machining efficiency.

 

PART 02 Double-Row Self-Aligning Roller Bearing Structure

Double-row self-aligning roller bearings possess a self-aligning feature that effectively compensates for installation errors and deformations that may occur during operation in gear shaft systems. This ensures accurate gear meshing while also reducing vibration and noise. These bearings are known for their high load capacity and long service life, making them essential components in industrial equipment.

The structure of a double-row self-aligning roller bearing includes an outer ring, an inner ring, two symmetrically arranged rows of rollers, and a cage. Additionally, the middle section of the outer ring contains an oil passage (referred to as the oil hole) that is used for lubricating the raceway.

PART 03 Original Process and Existing Problems

3.1 Post-Heat Treatment Machining Process

The conventional machining process for the outer ring raceway surface of double-row self-aligning roller bearings, after heat treatment, includes hard turning of the outer ring raceway surface, followed by finish turning, final grinding, and superfinishing of the outer ring raceway surface.

 

(1) Hard Turning of the Outer Ring Raceway Surface

The process consists of two steps. In the first step, a ‘low speed, high feed’ method is employed to remove the majority of the surface allowance from the outer ring raceway. The second step utilizes a ‘high speed, low feed’ method to refine the surface accuracy of the outer ring raceway. In both steps, the cutting tool follows the contour of the outer ring raceway surface.

(2) Finish Turning of the Outer Ring Raceway Surface

The processing method and machine tool parameters used in this step are similar to those in the second step of hard turning the outer ring raceway surface. However, the machine chosen for this process offers higher precision, and the calibration requirements for the outer ring raceway surface before machining are more stringent.

(3) Final Grinding of the Outer Ring Raceway Surface

Form grinding (see Figure 2) is used to further improve the dimensional and contour accuracy of the outer ring raceway surface, ensuring it meets the drawing requirements.

(4) Ultra-fine machining of the outer ring raceway surface

The surface of the outer ring raceway is meticulously machined on an ultra-precision machine using oil stones. The dimensions of the outer ring raceway remain largely unchanged; this process is designed solely to enhance the surface quality of the outer ring raceway.

 

3.2 Existing Problems
In conventional double-row self-aligning roller bearings, the oil hole size is relatively small, typically measuring less than 5 mm in diameter. This size is smaller than the tip of a standard 60° external round cutting tool. As a result, while some vibration occurs during machining in this area, the impact is minimal.

However, for the currently used double-feed machine types, the spindle bearing outer diameter is considerably larger, usually ranging from 1500 to 2000 mm. The oil holes in these machines are generally designed to be between 10.0 mm and 12.5 mm in diameter, which is significantly larger than the tip of a cutting tool. Consequently, during the hard turning and precision turning of the outer ring raceway surface, the cutting tool tip experiences noticeable vibrations when it passes over the oil holes. This can lead to significant production issues in three main areas:

1. There is an increased risk of tool edge chipping, which results in higher processing costs.
2. The machining process creates deep tool marks around the oil holes that cannot be entirely removed during subsequent grinding, raising the likelihood of defective products.
3. Significant fluctuations in the outer ring raceway diameter occur, necessitating additional machining allowances in the final grinding process to correct the outer ring raceway dimensions. This negatively impacts both processing costs and efficiency.

 

PART 04 Process Optimization

To address the machining difficulties caused by tool vibration, the process for 4 axis CNC machining the outer ring raceway surface after heat treatment has been optimized as follows: hard turning of the outer ring raceway surface, followed by fine grinding, finish grinding, and finally, ultra-precision machining of the outer ring raceway surface.

 

(1) Hard turning of the outer ring raceway surface
The process consists of two steps.

In the first step, the outer ring raceway surface is turned to leave a remaining thickness of 0.08 mm, which aligns with the original specifications.

The second step, illustrated in Figure 3, involves turning the remaining 0.08 mm of stock from the outer ring raceway surface. It is important to avoid the stock areas above and below the oil hole (designated as areas A1–A4 in Figure 3) during this turning operation. The intention behind this approach is to alter the original tool path to bypass the oil hole area. This ensures that any turning marks created while machining the oil holes remain on the stock of the outer ring raceway surface, facilitating easier removal in subsequent processes. Additionally, by removing more stock, this method reduces the machining cycle time for the following steps.

 

(2) Fine grinding of the outer ring raceway surface

After dressing the grinding wheel to match the shape of the outer raceway, ensuring that the center of the wheel aligns with the center of the outer raceway, proceed with profile grinding to finely grind the outer raceway surface (see Figure 4). No machining is performed at the chamfer positions at both ends in the Y-axis direction, as these areas are reserved for the final grinding process. Fine grinding serves as a replacement for finish turning. The increased contact area of the grinding wheel effectively removes tool marks from the previous process, which helps to reduce the number of defective products and minimizes the risk of chipping on the cutting edge of the lathe tool.

Since the optimized rough-turning process removes more material than the original method, this grinding operation takes a similar amount of time as the previous finish turning step, while achieving higher machining accuracy. This improvement facilitates the subsequent correction process, ensures the precision of the machined parts, and enhances overall processing efficiency.

 

(3) Grinding the outer ring raceway of the final grind
Use form grinding (see Figure 5) to eliminate the remaining stock and make fine adjustments to the previously shaped intermediate areas, ensuring they meet the design specifications. Form grinding involves positioning the rotation center of the workpiece and the rotation center of the grinding wheel in the same plane and perpendicular to each other. During this process, the workpiece rotates, while the grinding wheel not only spins but also moves laterally along the axis of its rotation. This technique creates a cross-hatched grinding pattern on the outer ring raceway (see Figure 6). Compared to the original process, form grinding is more efficient.

 

(4) Ultra-fine machining of the outer ring raceway surface
Same as the original process.

 

PART 05 Actual Machining Results

The optimized hard turning process avoids the oil hole area, and the subsequent fine grinding further eliminates tool marks. As a result, the occurrence of vibrations during cutting is reduced, which in turn decreases the likelihood of edge chipping. Actual machining tests have been conducted, and a comparison of production shifts before and after process optimization is presented in Table 1.

Two batches of 24 parts each were randomly selected for processing using methods before and after process optimization. Both batches were produced continuously, and the diameter of the outer ring raceway surface was measured. The diameter range for the parts processed using the original method was from φ612.980 mm to φ613.645 mm, showing relatively large dimensional fluctuations. In contrast, the parts processed with the optimized method had a diameter range of φ613.165 mm to φ613.435 mm, demonstrating smaller dimensional fluctuations.

In summary, the effects of the process optimization are as follows: 1) Altering the tool cutting path reduced the likelihood of tool edge chipping. 2) After optimization, tool vibration was minimized, tool marks from machining oil holes were eliminated, the risk of defective products was reduced, and machining efficiency improved. 3) Comparing the output and diameter data from both groups, it is evident that the diameter fluctuation range decreased significantly after process optimization, resulting in a notable improvement in dimensional accuracy.

 

PART 06 Conclusion

This paper examines the causes of tool vibration during the machining of the outer ring raceway of double-row spherical roller bearings, particularly at the oil hole, and proposes an optimized machining process. The process for machining the heat-treated outer ring raceway was divided into four distinct steps: hard turning, fine grinding, finish grinding, and ultra-precision machining.

In the optimized hard turning phase, the area around the oil hole was avoided to decrease the likelihood of tool edge chipping due to vibration. Following this, form grinding and oil stone fine grinding were employed to eliminate tool marks, minimize fluctuations in the outer ring raceway diameters, enhance dimensional accuracy, reduce machining costs, and improve machining efficiency.

 

 

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