The on-service hydraulic pump is running at all times and is a constant-speed pump. Unless steering is actually taking place, the tilt box of the main hydraulic pump is at zero stroke, and no oil is being moved within the main system. The auxiliary pump provides control oil and super-charge flows for the system. Assume that a steering order signal comes into the differential control box. It may come from either the remote steering system in the ship’s wheelhouse or the trick wheel. The control box mechanically positions the tilt box of the main hydraulic pump to the required angle and position. Remember that direction of fluid and flow may be in either direction in a hydraulic speed gear. It depends on which way the tilt box is angled. For this reason, a constant speed, unidirectional motor can be used to drive the main hydraulic pump. The pump will still have the capability to drive the ram in either direction.
With the main hydraulic pump now pumping fluid into one of the cylinders, the ram will move, moving the rudders. A rack and pinion gear assembly is attached to the rudder yoke between the rudder links. As the ram and the rudder move, the rack gear moves, driving the follow-up pinion gear. Each pinion drives a follow-up shaft which is mechanically coupled to the differential control box. This feedback (servo) system tells the directional control box when the steering operation has been completed and that the rudder angle is at the position ordered by the helm. As the ordered rudder angle is approached, the differential control box will begin to reduce the angle of the tilt box of the main hydraulic pump. By the time the desired rudder angle is reached, the tilt box should be at zero stroke. When this happens, the ordered signal (from the pilot house or trick wheel) and the actual signal (from the follow-up shafts) are the same. (NOTE: The ram is held in positon between the two cylinders by “hydraulic lock” due to the incompressible oil, which is prevented by the transfer valve from flowing into or out of the supply and return piping.) If either of these changes, the differential control box will react accordingly to cause the main hydraulic unit to pump oil to one end or the other of the ram. The trick wheels provide local-hydraulic control of the steering system in case of failure of the remote steering system. A hand pump and associated service lines are also provided for local-manual operation of the ram in case of failure of both hydraulic pump units.