Accurate measurement of torque -- a key parameter in understanding gearbox, braking and generator interactions in wind turbines -- is hard to achieve, but Binsfeld Engineering in the US state of Michigan says its TorqueTrak Revolution system enables measurement of torque, power and rotational speed continuously, so facilitating appraisals of turbine design and efficiency. To collect the data, a strain gauge sensor is attached to the shaft and signals are transmitted via a telemetry system to a stationary receiver, where they are collected for analysis or for real-time process control. The system was initially tested on a small vertical axis wind turbine and further trials are being undertaken using two gearboxes in the 600-750 kW power range as part of a program run by the National Renewable Energy Laboratory. Measurements are also being made on the output shafts of a test-bed gearbox of the type used in the Clipper 2.5 MW turbine. Here, the design splits the torque from the rotor shaft to drive four generators, with the aim of reducing component failures and extending the operating life. The manufacturer envisages that torque measurements will become standard on many machines in the future as they will provide useful information to condition monitoring systems.