Hydraulic mechanical transmission

A hydraulic mechanical transmission comprising:

• a first hydraulic unit having a first shaft;
• a second hydraulic unit connected in hydraulic fluid communication with the first hydraulic unit by high and low pressure lines, the second hydraulic unit including a second shaft and a rotating group that is coupled to the second shaft so as to rotate in unison with the second shaft about an axis defined by the second shaft, the rotating group including a cylinder block that defines a plurality of axial cylinders, the rotating group also including pistons mounted to slide within the axial cylinders, the second hydraulic unit further including a rotatable component that is rotatable relative to the second shaft about the axis of the second shaft, the rotatable component including a swashplate that opposes ends of the pistons, the rotatable component also including a commutation structure for providing fluid communication between the high and low pressure lines and the axial cylinders of the rotating group while maintaining separation between the high and low pressure lines;
• at least one of the first and second hydraulic units having variable displacement;
• a mechanical torque transfer arrangement for transferring torque between the first shaft and the rotatable component of the second hydraulic unit;
• and
• wherein in use one of the first and second hydraulic units operates as a hydraulic pump and the other of the first and second hydraulic units operates as a hydraulic motor.