Analysis of time-domain signals of piezoelectric strain sensors on slow spinning planetary gearboxes

In the following, time-domain signals of robust piezoelectric strain sensors applied on the surface of the ring gear of a gearbox are analyzed regarding their mechanical basics, measurement chains and inferable information. The gearbox specimen is a main gearbox of a wind energy converter (WEC), which is deployed on a WEC system test rig. It can be shown that the surface strain on fixed ring gears in tangential direction is mainly influenced by the transferred tooth forces between planets and ring gear but also by the stiffnesses and geometries of the ring gear itself and the supporting gearbox structure. A direct comparison of sensor connection in AC- and DC-coupling shows that with the utilized piezoelectric sensors in DC-coupling surface strain signals with very low frequencies down to 0.002 Hz can be obtained. The acquired signals show a very high signal-to-noise-ratio and high repeatability even at very low revolution speeds. Furthermore a direct correlation to the dynamic torque, which is transferred by the gearbox, and to the planetary load sharing is found.