The Schaeffler Group (Germany; parent of INA and FAG bearings; 5251R9N7) is introducing a new paired bearing solution for wind turbine rotor shafts. With the new approach, Schaeffler said it is addressing the key limitations of the current solution, which is a pair of spherical roller bearings.

The main problem is that, due to sheer size and weight, turbine shafts put tremendous stresses on the bearings which cause them to vibrate and deflect. Also, turbine shafts can turn very slowly, which can cause the support bearings' rolling elements to slide or skid if there is a large axial deflection. Over time, the bearings develop radial and axial play which sets up vibrations, which causes further bearing deterioration and the eventual need for replacement. Vibrations are also hard on every other part of the turbine.

The traditional installation on turbine shafts has been spherical roller bearings, standing apart in separate housings. While these work well as long as they are kept within a narrow operating envelope, misalignment, changing wind forces and wear can set up vibrations and shorten the bearing in-service life. Also, spherical roller bearings generally operate with high radial internal clearances; as the needed clearance on the locating bearing side increases, the other bearing, controlling axial motion, must be set up with 6x to 7x that clearance. So even in the most tightly controlled designs, the allowable axial motion is far higher than ideal for the application. And that axial motion creates yet more stress on the locating bearing, particularly during gusty wind conditions. When that happens at slow speed, the rollers aren't moving fast enough to track the motion and so begin to slide across the bearing race, shortening bearing life and introducing additional play.

Schaeffler's solution is to deploy two different bearings to support the rotor shaft. Rather than sphericals for both, a different design is deployed for each function -- shaft location and axial motion control.

The locating bearing unit employs a pair of tapered roller bearings, which can be set up with very tight clearance to allow only minimal axial motion. Tight clearances reduce displacement, cut vibration, and even out the load distribution across the bearings, for a longer service life. With that design, even high axial forces and low speeds do not produce lateral sliding against the races.

In the second position on the shaft, the TRBs are teamed with a double cylindrical roller bearing. The CRBs can also run minimal internal clearance; the load distribution is more uniform because of the dual TRBs. And the bearing housing itself no longer has to handle axial length compensation, which is now limited to within the bearing's rolling elements.

The final installation issue is compensating for angular misalignment of the two bearing blocks relative to the shaft itself. In this TRB / CRB installation, angular misalignments are handled by building the TRBs and CRBs with spherical outers, coated to reduce sticktion and improve corrosion resistance.

Schaeffler said its new turbine shaft pairing will be displayed at the Husum Wind Energy Fair and production will begin in mid-2011.