Better bearings allow greater throughput

Building machines for such high loads requires the right partners, which is why the company has been working with the bearing specialist SKF for over 80 years. The first step in their cooperation at the time was to replace simple slide bearings with SKF self-aligning roller bearings. “This dramatically improved the load distribution within the bearing and allowed for misalignement  of the rollers,” adds Gereon Wollenhaupt, Designer and Project Manager at Köppern.

“We equip most of the machines we produce with bearings from SKF,” says Heuer. SKF is more than just the main supplier.  The bearing specialist is above all an active partner in new development. The trend toward ever greater throughput rates shows how important this is. “This demand comes from customers such as cement plants that we supply with grinding rolls,” he explains. “SKF develops solutions for and with us that are especially designed for these high requirements.”

To achieve greater power density in its machines, the roller specialist Köppern recently revised its 850-Series of high-pressure grinding rolls. They support a roll diameter of up to 2000 millimeters and in this way, for example, they can make possible throughputs of more than 1200 metric tons per hour for the cement industry. SKF developed a special self-aligning roller bearing in the Explorer performance class (see box) for this application.

During manufacturing, the bearings are subjected to a special heat treatment called the XBITE method, which yields greater surface hardness. This makes the bearings even more resistant to wear and contamination. The residual compressive stresses on the surface as well as the tough base material prevent cracks. A one-piece, double-pronged machined cage made of steel provides the required strength even on large and heavy rollers. The rollers are self-guiding in the bearing due to the interplay between the precisely co­ordinated surface topography of the raceways and the rolling elements as well as the loose guide ring and the symmetrical rollers.

An advantage when developing this special type of bearing was that the external dimensions did not have to follow ISO or DIN standards. Mr. Wollenhaupt explains: “This gave us the freedom to design our roller set specifically for this application by optimizing the internal bearing geometry in cooperation with SKF.”

Efficient and secure assembly

The grinding rolls of the 850-Series are subjected to extremely high loads. At speeds of just 15 to 20 revolutions per minute, the self-aligning roller bearings experience radial loads of more than 8000 kN per bearing. Using the right lubricant is therefore especially important to Köppern. SKF developed the LGEV 2 grease specifically for such extreme operating conditions. A special complex thickener, solid lubricant additives and a base oil viscosity of 1000 mm²/s ensure reliable formation of the lubricant film.

Using grease instead of oil as the lubricant has ­several advantages. “Grease lubrication generates a thicker, more stable lubricant film compared to oil lubrication,” explains Wollenhaupt. Using grease for bearing lubrication gives customers significantly better emergency running conditions for their machine. If a seal is damaged, the machine can continue to operate under supervision until it can be repaired. The system does not stop immediately, avoiding costly loss of production. Proper sealing is ensured by a multi-labyrinth seal and multiple V-rings. Together with lubricating grease discharged through the seal gap, this provides secure and reliable sealing against the ingress of contamination into the bearings, even under the extremely dusty conditions encountered in milling.

Maschinenfabrik Köppern values SKF as more than a reliable partner in development. “We’re also impressed by the high availability of the products,” explains Heuer. “SKF can always deliver reliably, even in economic conditions with extended delivery times.”