Abstract: A helicopter rotor transmission assembly for translating motion of a drive shaft driven by a helicopter engine into rotary motion of a helicopter rotor mast that carries rotor blades. The transmission assembly includes a fixed outer housing, an inner housing carried within the outer housing and adapted to rotate relative to the outer housing by the drive shaft, and a ball received in the inner housing so as to be rotatable relative to the inner housing about one axis, and constrained from rotation about the two orthogonal axes. The ball is adapted to receive the rotor mast so that the mast can be driven to both spin and tilt.