A bearing preload apparatus may include a gearbox assembly, a coupling assembly, and a sensor assembly. The gearbox assembly may include a gearbox housing, a first shaft and a second shaft. The first and second shafts may extend out of the housing and may be rotatable simultaneously with each other at different speeds. The coupling assembly may include a first driver coupling connected to the first shaft and a second driver coupling connected to the second shaft. The sensor assembly may be mounted to a stationary structure and the gearbox housing. The sensor assembly may measure a reaction torque of the gearbox assembly relative to the stationary structure.