Xona Rotors Bearings and Bearing Preload System

20 May 2015

Xona Rotor has implemented a new variable preload angular contact bearing system to the XR product line. The two most common methods to create preload are mechanical or by spring force. The mechanical method requires very tight axial tolerances to be held between opposing inner races and opposing outer races. Implementing a purely mechanical preload system in a turbocharger presents a challenging environment for such a system. The biggest challenge is the thermal operating conditions that are present in operating turbocharger. For a mechanical system to maintain preload it needs uniform operating temperatures across the entire bearing system and made from materials of equal coefficient of thermal expansion. Even with a watercooled bearing housing the temperature between the outer bearing races and the turbo shaft where the inner bearing races contact the turbocharger rotor shaft can have several 100 degree temperature gradient due to thermal transfer from the turbine wheel. This temperature differential will not allow preload to be maintain for all operating conditions. A spring preload is better solution than mechanical as it can maintain a relatively constant preload across the operating temperatures changes.

A variable system provides the best solution. It also solves the temperatures issues as well optimizing preload with bearing speed. Together as engine and turbocharger speed increase the preload is changed. When turbocharger speed and load increase ( boost ) axial thrust loads are carried by one of the angular contact bearings. In a mechanical preloaded or lightly preloaded (spring loaded) bearing system the bearing not carrying thrust load will allow the ball bearing element to slide and skid on the race due to lack preload creating excessive and uneven wear resulting in reduced bearing life. The variable preload system reduces bearing problems associated with ball sliding and scuffing due to high shaft speeds with insufficient bearing preload. Although hydraulically damped rotating assembly is not new ball bearing turbochargers, what is new is that the variable preload adds axial load damping to the bearing system resulting in extend bearing life.



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