Enhanced Design Solutions for Critical Components in Imported CNC Gear Hobbers

Large cast steel gear rings often face issues during the gear hobbing process. A failure analysis has been conducted to address the problems of wear on the guide rails of the gear hobbing machine worktable and cracking in the support component bearings.

To improve these issues, the following plan has been proposed: The load capacity of the bearings will be enhanced by using heavy-duty support roller bearings, and the guide rails will be hardened to above 58HRC through ultrasonic quenching. This solution effectively resolves the problems of bearing cracking and guide rail wear.

This improvement plan is significant for the stability of the equipment and the effective processing of large, high-precision parts.

 

01 Preface

In the heavy machinery industry, as the specifications for mills, cement kilns, and power generation equipment continue to increase in size, the key components—specifically cast steel large gear rings—are evolving towards larger modulus and greater diameters. For super-large gear rings with diameters exceeding 7 meters and weights over 40 tons, the processing involves the use of a large high-speed CNC gear hobbing machine. However, since the positioning and clamping of the workpiece exceeds the maximum range of the main worktable, an auxiliary rotary worktable (referred to as the worktable) is added to the main worktable. The guide rails of the worktable are supported by an 8-point auxiliary support assembly of the machine tool. The workpiece clamping method is illustrated in Figure 1.

The company has extensive experience in processing large gear rings. For instance, consider the 10-meter-high high-speed CNC gear hobbing machine currently in use. During the processing, the significant diameter and weight of the workpiece, combined with the high cutting speeds and depths typical of gear hobbing, subject the worktable and auxiliary support system to substantial impact forces every time the tool enters or exits the workpiece. These repeated stresses, along with other contributing factors, significantly reduce the lifespan of the support components. The main serious consequences of this situation are as follows.

 

1) During the gear hobbing process, the outer ring of the auxiliary support bearing near the hob frame frequently cracks (see Figure 2), necessitating shutdowns for replacement and causing prolonged machine tool downtime.

 

2) Handling large workpieces requires frequent removal, placement, realignment, and clamping, which is time-consuming and labor-intensive, significantly affecting production efficiency.

 

3) The damaged bearing is making contact with the worktable’s guide rail, leading to significant wear on the guide rail (see Figure 3) and a loss of axial circular runout accuracy for the worktable. To ensure the machine tool maintains its accuracy, the worktable’s guide rail requires frequent repairs, which ultimately reduces the overall service life of the worktable.

 

4) When a bearing cracks, it causes the worktable to become instantly unbalanced. This unbalance can lead to the workpiece’s tooth surface being damaged during gear hobbing, which significantly impacts product quality.

For high-speed, heavy-loaded, and high-precision workpieces, the stability of the worktable is essential. This article will analyze the causes of failure related to the worktable’s guide rails and bearings. It will also propose optimizations for the design of the worktable and bearing support components. The goal is to address the ongoing failures of these support components and ensure both the accuracy and stability of the worktable.

 

02 Failure Analysis

The company’s 10-meter high-speed CNC gear hobbing machine originally used eight auxiliary support components equipped with FAG21319-E1-TVPB spherical roller bearings (see Figure 4) to support the worktable with guide rails. The surface hardness of the guide rails was between 350 and 400 HBW.

 

FAG spherical roller bearings are designed with two rows of rollers, primarily to support radial loads while also being capable of accommodating axial loads in any direction. These bearings have a high capacity for bearing radial loads and are particularly well-suited for heavy or vibrational loads. However, they are not designed to handle pure axial loads.

In the actual working conditions, we have a workpiece mass of 40 tons, a worktable mass of 42 tons, and a tooling mass of 20 tons, leading to a total weight of 102 tons (excluding cutting forces). The rated speed of the worktable is 8 revolutions per minute (r/min), and the average single support must bear more than 124.95 kN. Based on the current issues with broken bearings on-site, the main reasons for bearing failure can be summarized as follows:

1. In addition to supporting the weight of the tooling and workpiece, the auxiliary support near the hob frame also bears the cutting load. Overloading in this area can lead to bearing cracks.

2. When hobbing or milling gears, the CNC metal cutting process generates significant and frequent impact forces, which can cause the bearings to crack.

3. During the rotation of the worktable, the TVPB spherical roller bearings are unable to withstand the torsional and axial forces generated by the rotation around the center of the hobbing machine table.

4. The clearance between the bearing and the guide rail cannot be adjusted, which may contribute to operational issues.

 

The main reasons for the failure of the workbench guide rail are as follows.
- After the bearing cracks, it continues to engage with the guide rail, and the damaged section of the bearing keeps scraping against the guide rail, leading to further damage on its surface.
- The guide rail surface is not hard enough.
- The guide rail surface and the bearing lack lubrication.

 

03 Improvement plan

3.1 Improve the bearing capacity

The heavy-duty support roller bearing NNTR90×220×100.2ZL has been redesigned for improved performance. This type of bearing features an outer diameter that is both cylindrical and arc-shaped. It has a full-fill cylindrical roller bearing with a relatively thick outer ring, allowing the rollers to roll directly on the raceway and effectively withstand heavy and impact loads. NNTR heavy-duty roller bearings are primarily used on heavy machinery tracks.

The design includes loose-fitting ribs on both sides of the inner ring, which provide axial guidance for the outer ring through the roller group. This feature allows the cylindrical roller bearing to handle significant torsional torque and axial force even when tilted or deflected, enhancing its load capacity.

Notably, the outer ring of the NNTR heavy-duty roller bearing is thicker than that of conventional bearings, enabling it to support greater radial loads. Additionally, the contact surface width between the bearing and the guide rail has been increased from the original 45 mm to 100 mm, allowing for greater load-bearing capacity. For reference, the original FAG 21319-E1-TVPB has a rated static load of 430 kN, while the upgraded NNTR90×220×100.2ZL boasts a rated static load of 750 kN, representing a 74.4% increase in load capacity.

 

3.2 Redesign of support assembly

The size of the newly selected bearing differs significantly from that of the original bearing. As a result, the original support assembly is not compatible with the new bearing. Consequently, the support assembly has been redesigned and optimized, as illustrated in Figure 5. The structural design optimization process is detailed as follows.

 

- A 45° oil filling hole is designed to facilitate the manual addition of lubricating grease.
- A chip scraper baffle is included to prevent hobbing chips on the guide rail from becoming lodged in the bearing contact surface.
- The bearing bracket features a nearly fully enclosed structure to keep chips from entering the ball bearing.
- An adjustment ring is located at the bottom of the bearing bracket, allowing for precise control of the gap between the bearing and the track by fine-CNC turning process the thickness of the adjustment ring.

 

3.3 Install the support assembly

To ensure consistency in height among the eight bearings, adjustments must be made to control the gap between the guide rail and the support assembly. When the system is unloaded, it is essential that this gap remains uniform, and the runout error of the worktable must be maintained within 0.03 mm. Additionally, minimizing the impact between the guide rail and support assembly will help reduce the risk of bearing cracking and decrease the wear rate of the track.

To achieve uniform bearing height, a special tooling designed for measuring bearing height has been created, as shown in Figure 6. This tooling is mounted on the main workbench of the machine tool, with a dial indicator placed at the front end. As the machine tool rotates, the dial indicator detects the highest point of each bearing. By adjusting the adjusting ring on each support component, the height of the bearings can be standardized, ensuring that the clearance between the eight bearings and the track remains within 0.02 mm.

 

3.4 Improve the wear resistance and replaceability of the guide rails

The original workbench is made of tempered and processed steel casting, with the hardness of the guide rail surface measuring between 350 and 400 HBW. If the guide rail becomes damaged, material can only be removed through turning. However, if the processing is done multiple times, the entire workbench may need to be scrapped.

In this design improvement, we aim to enhance the hardness and wear resistance of the guide rail. Additionally, the guide rail and workbench will be designed as a combined structure. This way, if the guide rail becomes severely damaged, only the guide rail will need to be replaced, significantly reducing costs.

1) The design features a replaceable guide rail, where the guide rail and the rotary workbench are designed separately. The guide rail is secured to the workbench using bolt connections. To minimize the impact force when the guide rail joints make contact with the bearings simultaneously, the guide rail is constructed by splicing multiple steel plates using large-angle bevels (as illustrated in Figure 7).

2) The guide rail surface is hardened, but due to the constraints of the actual workbench setup, the guide rail can only be designed as a flat plate, with a length exceeding 1 meter and a thickness of only 20 mm. A challenge we face is how to enhance the surface hardness of the guide rail while minimizing deformation during heat treatment.

To manage manufacturing costs, the replaceable guide rail is made from 45 steel. The commonly used heat treatment methods for this material include tempering and surface quenching. Given the potential for deformation during heat treatment, as well as the required surface hardness and hardened layer depth, we find that the deformation resulting from surface quenching is relatively controllable.

After extensive testing, we implemented ultrasonic quenching, which provides a deeper hardened layer and higher hardness. The actual surface hardness achieved after quenching is above 60HRC. Following heat treatment, any deformation is corrected using a press, allowing us to maintain the final deformation within 0.3 mm. Post-processing, the surface hardness of the finished product reaches over 58HRC, closely approaching the hardness typical of bearings. The hardness distribution remains uniform after processing.

In summary, we have successfully controlled heat treatment deformation while improving the surface hardness of the guide rail.

 

04 Actual application effect

After three years of actual use with this improved solution, the bearings have not cracked. Long-term use reveals that the turning marks still appear on the surface of the guide rails, yet there is no significant wear on them (see Figure 8). The plane runout error of the rotary table has been effectively maintained within 0.03 mm. As a result, production efficiency has significantly improved, and product quality is now controllable.

 

05 Conclusion

The redesign and improvement of the guide rails and support components have effectively addressed issues such as bearing cracking, guide rail wear, and instability of the rotary table when processing super-large gear rings on large rotary tables.

These enhancements are particularly beneficial for large gear hobbing machines that handle heavy-loaded, high-precision parts. The modifications have increased the bearing load capacity, reduced the impact forces acting on the bearings and guide rails, and enhanced the wear resistance of the guide rails. As a result, the stability and accuracy of the machine tool have improved, which helps to extend its service life and maintain product accuracy.

Since the hob is now capable of enduring continuous high-speed cutting under heavy loads, overall processing efficiency has improved. This advancement also minimizes the likelihood of product quality issues stemming from bearing cracks and prevents economic losses due to machine shutdowns and repairs.

 

 

 

 

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