Due to the tough operating conditions experienced by machinery used in agricultural applications, the prospects of attaining a long service life when using standard rolling bearings are extremely limited. Acutely aware of this fact, leading manufacturers of quality agricultural machines, such as the Kverneland Group, rely on application-specific bearings from NSK.
The Soester plain to the east of Dortmund, Germany, is one of the most fertile regions in Europe, making it the ideal location for the seeding technology competence centre and headquarters of Kverneland’s Soil Processing Division. In Soest, the company’s seeding and drilling machines are developed and manufactured in large quantities in order to meet high demand across Europe, as well as increasing uptake from customers in the USA and Asia.
During seedbed preparation, the machine’s components are subjected to extremely high stresses, particularly the disc harrow segment of the so-called U-drill. The disc harrow is used to loosen and break up the soil before the following sowing coulters dig the furrows ready to receive the seeds.
The disc-shaped tools of the harrow are required to keep turning in all types of ground and at different soil depths. As a result of this low friction characteristics are highly important. The tools drive forward with working speeds of up to 18 km/h over uneven ground and must withstand any impact from stones. As the tools sit at an angle to the direction of motion, the bearings are exposed to high axial forces, not to mention environmental hazards such as dust, moisture, dirt and corrosive fertiliser.
"The very diverse stress patterns that affect the bearings cannot be accurately predicted, which means that bearings for agricultural machinery are always designed to be much larger than needed,” says Volker Schanzenbach, R&D manager of Kverneland Group Soest GmbH. “Instead of sizing, the main challenge relates to the seal that protects the bearings from dust and moisture.”
This challenge is exaggerated by recent climate change that has led to much drier soil and, consequently, more dust. Furthermore, the efficiency demands of modern farming means that fuel consumption, working speed, maintenance costs and machine availability take ever-higher priority. Clearly, the performance of the bearings has never been more important.
The Kverneland machines feature eight tools for each metre of working width. This means that they can be equipped with up to 48 highly stressed rolling bearings. To ensure the bearings are up to the job, the company today uses a robust and durable NSK solution, known as the Agri-Disc Hub.
Developed especially for use in soil cultivation and seeding machines, the NSK hubs comprise of a double row of angular contact ball bearings (featuring a 40° contact angle) with a separate inner ring that is designed to withstand high axial and tilt loads. Supplying an added-value solution, NSK fits the bearings into housings ready to be installed, thus simplifying assembly for Kverneland.
To overcome the challenge of sealing the bearing, NSK added a static seal with three sealing lips to the disc side (opposite the bearing housing) and a dynamic seal with two sealing lips between the bearing unit and the disc carrier. A large pressure plate provides further protection for the bearing and also serves to generate the desired pre-tensioning.
According to Kverneland, this innovative sealing system has proved to be highly effective, even with heavy dust, moisture and abrasive or corrosive elements in the soil such as sand and chemical fertilisers. Water projection tests have shown that even when subjected to high pressure cleaning, there is no reduction in bearing durability or longevity.
The bearing is filled with special performance grease which has a high pressure absorption capacity designed to last throughout the service life of the bearing. Despite heavy use, this feature makes any subsequent lubrication unnecessary saving time and grease whilst preserving the environment.
After the positive experience provided by adopting the Agri Disc Hubs, engineers at Kverneland (which was taken over by Kubota in 2012) and NSK made a study of other machines with the goal of optimising bearing locations and increasing longevity. This work led to improved bearing points on Kverneland’s disc harrows, where double row angular contact ball bearings without a flange (but with a radial shaft seal on one side) are now used. At other bearing points, Kverneland seeding machines use single row ball bearings (four point bearings with a widened inner ring and a flinger seal on one side). Depending on the application, various seal shapes and materials are deployed.
NSK has also developed custom rolling bearings for the seed coulters of the Kverneland single seed drilling machines with underfoot fertilisation (an extremely abrasive application), as well as for other soil-working machines such as short disc harrows.