New Designs in Ring Rolling
After several successful projects in the field of train-wheel manufacturing, Schuler Group is now offering complete systems for the production of seamless rings. The newly developed machinery offers innovations that are easily integrated into complete production lines.
Schuler decided to enter the market for ring-rolling applications about five years ago. Having received numerous customer requests, the first system we delivered was used for manufacturing train wheels. Since then, the company completed and delivered three additional projects – catapulting our technology and global market expertise in forming technology from a machine supplier to a system supplier and ultimately to a provider of turnkey systems. With a newly developed machine, Schuler has now added the capability of providing complete systems for the production of seamless rings.
Two Approaches to Ring Rolling
What are the characteristic features of a ring? For one thing, they are endless and represent a sense of stability and permanence. They also cannot be allowed to have any imperfections. And that is exactly the goal of the rolling process: to produce a part completely free of weld seams that could potentially tear.
This forming technique produces an excellent structure without any defects and with an uninterrupted grain flow and a small grain size. As a result, rings fabricated using this process are known for their high strength and toughness, and they are therefore ideally suited to withstand extremely high loads in everyday use (e.g., wind turbines). This material-saving and cost-efficient method produces rings with a small cross-section and low weight.
What does a typical production process look like? First, all material sections are cut to size, heated and, in some cases, descaled. Depending on the level of quality required, forming processes may then take place in a press (a step referred to as preforming). The preliminary final step is the rolling process, where the ring is given the desired final geometry, diameter and shape. Where necessary, a heat treatment can then also be performed before the ring enters final processing.
Schuler has sold many machines that use the preforming step. However, at this point it is important to distinguish between two basic approaches. In one approach, a multistage process is possible on a press into which three stages have been integrated: upsetting, pre-piercing and piercing.
For one customer in India, for example, we delivered the necessary dies for this configuration, along with the cooling and lubrication units. On this particular press, the cycle time is 20 seconds – and could be even shorter were it not limited by the constraints of upstream and downstream processes.
The other approach involves a host of different applications in which forming takes place over a single stage in the middle of the press. We have delivered these types of solutions to customers in Romania and Switzerland, for example. Multiple actions are required to preform the ring. Various different concepts are available in this case to swing the dies into place or to move the lower dies within the machine. For this to be possible, the parts must be centered and raised.
This means that the accompanying press concept is complex and the necessary sequences involve many different aspects. What’s more, a wide range of forming techniques is also available – piercing from above, piercing from below, die piercing – regardless of what the component will be used for in the end or what the desired grain flow will look like. The bigger the ring and the stricter the demands for the grain flow, the more likely it is that a single-stage press concept will be used.
Of course, a complete line is made up of more than just a ring roller and a press. Heating systems are also used, and we enjoy an extremely successful collaboration with Andritz Metals, a producer of industrial furnaces. We also provide the necessary automation equipment, such as robots and charging machines, because the parts are too hot and often too heavy to be transported manually.
Schuler has developed a series of ring rollers for rings or bushings with diameters of 1.25-4 meters but will now also offer machines for rings with diameters measuring up to 8 meters. As it is formed, the ring is rolled between the driven roll and the idler roll, with the gap between the two growing increasingly smaller. This reduces the ring’s cross-section, while axial rollers bring the ring’s height to the desired dimension. To achieve an adequate and stable rolling process – especially where larger rings are involved – it is critical that all machine parameters and all axes be linked to one another via algorithms.
Our ring roller aims to help customers achieve higher quality and more cost-efficient production operations. We have already implemented one such feature in our wheel roller line: an idler roll that rotates even when no rings are in the machine. The fact that it is only ever possible to perform cooling unevenly from just one side (in practice) leads to bending, or what is known as the bi-metal effect, on conventional machines. This means that the idler rolls are already stressed to the point of bending by the cooling alone. That’s why Schuler has integrated a motor in the upper bearing of the wheel roller, thereby allowing the idler roll and all other forming dies to rotate even during downtime. This makes it possible to cool the dies evenly from all sides.
Another advantage, which we have patented, can be found in the design of the machine itself. On conventional wheel rollers, the idler roll must be pulled out so that the part can be placed into or removed from the machine. The device used to lift the idler roll defines the height of the machine, but it also interferes during die changes.
Because these dies can weigh hundreds of kilograms, installing and removing them is a laborious process involving an overhead crane. Consequently, our designers intentionally left out a conventional idler-roll lifting device in the design of the new machine. Instead, the entire upper bearing was installed on a rail so that the whole assembly can be moved upward to provide the necessary clearance for changing the rings. This design also reduces the height of the machine, which helps improve accessibility.
In addition, the idler roll is designed as a cartridge that is clamped in place in the machine – hydraulically or mechanically, at the customer’s discretion – and is therefore easy to exchange with pre-equipped dies. The same thing applies to the driven roll, which can be completely removed from the machine and replaced with a pre-equipped driven roll. Operators of conventional ring rollers find that this operation typically takes an entire shift to complete. Thanks to the new wheel roller, this step is now possible in just a few minutes.
The movable upper bearing can also be used to shift the idler-roll bearing downward for smaller rings. This results in a shorter idler-roll length and significantly less bending stress caused by the process.
On existing machines, idler-roll breakage is the most frequently encountered form of wear. This occurs when the idler roll is subjected to excessively high loads due to large distances between supports as well as high bending stresses. By adjusting the idler-roll bearing to fit the height of the ring, we have been able to achieve a service life that is much longer when compared to conventional machines.
All of the machines integrated into this process are run by the same control system – as is the case for the wheel line that we delivered to Turkey. In addition, every system component can be monitored and operated from every control unit. Thanks to a holistic system concept that includes dies, forming machines and heating equipment, Schuler now also offers a single-source solution for ring manufacturing.
Co-author Georg Obermaier is the head of Product Management for Forging Systems at Schuler Pressen GmbH. He can be reached at Georg.Obermaier@schulergroup.com. Co-author Simon Scherrenbacher is in Integrated Communications for Schuler AG. He can be reached at firstname.lastname@example.org. For additional information, visit www.schulergroup.com.