Schaeffler claims to have built the world’s largest and most powerful test rig for large bearings. The €7m rig, located at Schaeffler’s Schweinfurt plant in Germany, will allow bearings weighing up to 15 tonnes and with outside diameters up to 3.5m, to be tested in realistic conditions that simulate a wide range of operating environments.

The rig – called Astraios after a mythical Greek titan who fathered the four wind gods – will be used mainly to test rotor bearings for multi-megawatt wind turbines, thus helping to cut wind turbine development times, as well as improving the reliability, safety and cost-effectiveness of future wind turbine designs.

The rig, which took almost two years to develop, will also help to improve the understanding of wind turbine systems and inter-relationships between drive train components. In addition, it will provide insights into wind turbine operation and maintenance, and information on optimising the design of adjacent assemblies.

At the opening ceremony recently, Schaeffler group partner Maria-Elisabeth Schaeffler, described the Astraios rig as “a logical step towards further developing renewable energy as a strategic growth sector” for the group.

The rig will perform realistic simulations of static and dynamic loads that act on rotor bearings and slewing rings in wind turbines with outputs up to 6MW. The functional tests will provide insights into rolling bearing kinematics, temperature and friction behaviour, loads and deformation. More than 300 sensors on the test rig and in the bearings will be used to gather data.

Four radial and four axial hydraulic cylinders fixed to the loading frame will recreate the loads and moment forces that can occur in a wind turbine. The radial cylinders simulate the weight of a rotor hub with rotor blades, while the axial cylinders generate the wind loads.

The rotors and hub on a large wind turbine can weigh more than 100 tonnes. This weight acts on the bearing and generates a static radial load and a static “nodding” moment. The test rig cylinders need to be extremely powerful to simulate these loads.

The radial cylinders can generate forces of up to 1MN – equivalent to 100 tonnes of weight – while the axial cylinders deliver up to 1.5MN to simulate static axial loads and dynamic nodding and yawing moments. These movements are comparable to the lifting, lowering and turning of the wind turbine nacelle.

Different wind speeds are simulated using a drive train with a planetary gearbox. Typical speeds are 4–20 rpm, although it is also possible to simulate much higher speeds.

A tensioning frame acts as the connection side of a wind turbine nacelle. Wind seldom blows at the same speed or from the same direction, but acts with varying intensity and at different points on a turbine. It generates varying moments on the rotor hub, depending on the position of the rotor blades. For example, if the wind is acting on the top or bottom of the blades, it generates a dynamic nodding moment. And when it turns and blows more strongly from the side, it creates a dynamic yawing moment.

The continuously changing wind conditions mean that wind turbines are subjected to extremely complex stresses. A system called Sara (Schaeffler’s Automation System for Research & Development Applications) controls the test and measurement program, as well as all open- and closed-loop control processes on the test rig.

In addition to generating target values according to the wind load, Sara also controls all of the test rig`s components including dynamic servo cylinders. It measures and records all of the data; is responsible for the telemetry systems; displays all target, actual and limit values; and evaluates all measurement data.