• The variable delivery pump can run continuously in one direction, but have the cabpability of infinitely changeable discharge i.e., from zero to maximum in either direction.
  • This is attained by changing the stroke of the pump piston. For small ruder movements, piston stroke is small and for full rudder movement the stroke becomes full.
  • Normally variable delivery pumps are classified as
  1. Axial piston ( swash plate pump)
  2. Radial piston pump (hele-shawpump)


  • Construction :
  1. This has a circular cylinder block with axial cylinders on it’s pitch circle around the input shaft with which it revolves.
  2. The individual port holes terminate in a drilled port holes at the end face of the block with valve plate.
  3. The port holes are connected by external pipes to the steering cylinders.


  1. The driving shaft rotates the cylinder barrel and piston.
  2. The external trunnion shaft enables the swash plate to be moved about it’s axis.
  3. When the swash plate is in vertical position, no pumping takes place.
  4. When swash plate is tilted to one direction one side port becomes suction and the other port on other side of the centreline, become discharge because of the pumping action.
  5. For opposite direction of tilt, the direction of flow is reversed.
  6. Stroke length of the piston and hence the quantity of flow of fluid depend on the angle of tilt of the swash plate.


  1. The circular cylinder body driven by constant speed electric motor rotates around a central valve (tube) arrangement.
  2. The cylider body is supported by ball bearings which are installed on inner side of the body casing.
  3. The cylinder body is connected to the central valve arrangement by ports, which are further connected to the outer casing and finally to the ram cylinders for supply & delivery of oil
  4. A number of piston fit in to the radial cylinders & these pistons are fastened to slipper rings by gudgeon pins.
  5. The slipper rings fit in to the circular track (grove) in floating ring.
  6. The floating ring is supported by ball bearings on either side, which are further installed on a guide block.
  7. The guide block attached with the control rod can be moved horizontally at right angle to the pump shaft axis.
  8. Two spindles which pass out of the pump casing is attached to the guide block, control the movement of the floating ring.
  9. Pump is usually provided with odd number of cylinders, usually seven or nine to produce even hydraulic flow and better pump balance.