Preventing lightning damage in bearings by using mechanical preloading

High positioned systems such as wind turbines or radar system onboard a ship can be easily struck by lightning. The lightning current has to be conducted via defined paths to prevent damage. A key element to protect is the bearing system between the rotating and stationary part. Providing a bypass current path via additional measures such as a slip ring is the conventional way of protecting bearing. The arcing due to high voltage difference between rollers and raceway is however the main cause of damage to the bearing system. It is commonly assumed that, if the wind turbine blades, or the radar, is rotating, the lubrication between the rolling elements and the raceway is a non-electrical conducting hydrodynamic lubrication layer, and thus high voltages can be developed. But if the bearing is sufficiently preloaded it is still providing a conductive path via boundary lubrication. No arcing occurs, and no damage. The concept of pre-loading the bearing system has been evaluated using many experiments on stationary and rotating bearings, and after performing endurance testing.