The Hele-Shaw pump, commonly used in electrohydraulic steering gear systems, is a variable-displacement, reversible axial-piston pump. Its delivery and direction of flow are controlled by changing the position or angle of the circular floating ring/swash plate.

Working Principle

The pump consists of a rotating cylinder barrel containing a number of pistons, the outer ends of which are connected through slippers to a circular floating ring or swash-plate arrangement. The cylinder barrel rotates with the driving shaft, while the ports are arranged through a central valve arrangement.

1. Neutral Position – No Pumping

When the circular ring accommodating the slippers is concentric with the central valve arrangement, the pistons do not have any relative reciprocating motion inside their cylinders.

Therefore:

• No change in cylinder volume takes place.
• No oil is sucked into the cylinders.
• No oil is discharged.
• Although the pump and cylinder barrel continue to rotate, no fluid is delivered.

This is the neutral or zero-delivery position.

Similarly, in the swash-plate type arrangement, when the swash plate is in the vertical or neutral position, no pumping takes place.

2. Ring/Swash Plate Moved to One Side

When the circular floating ring is pulled to the right, or the swash plate is tilted in one direction, the pistons are forced to move to and fro within their cylinders as the cylinder barrel rotates.

This produces the pumping action.

For example:

• The lower piston moves inwards and discharges fluid through the lower port.
• As the cylinder barrel continues to rotate, the piston reaches the horizontal position and then starts moving outwards.
• During the outward movement, fluid is drawn into the cylinder through the upper port.

Thus, with the ring displaced to one side:

• Upper ports act as suction ports.
• Lower ports act as discharge ports.

The pump therefore delivers hydraulic oil in one direction.

3. Ring/Swash Plate Moved to the Opposite Side

If the circular ring is pushed to the left, or the swash plate is tilted in the opposite direction, the reciprocating movement of the pistons is reversed relative to the ports.

Consequently:

• The previous suction ports become discharge ports.
• The previous discharge ports become suction ports.

Thus, the direction of hydraulic oil flow is reversed.

This reversible flow enables the hydraulic rams of the steering gear to move in either direction, thereby turning the rudder to port or starboard.

Swash-Pump Operation – Summary

1. The driving shaft rotates the cylinder barrel and pistons.
2. An external trunnion shaft enables the swash plate to be moved or tilted about its axis.
3. When the swash plate is in the vertical/neutral position, no pumping takes place.
4. When the swash plate is tilted in one direction, the pistons reciprocate, causing one set of ports to act as suction ports and the ports on the opposite side of the centreline to act as discharge ports.
5. When the swash plate is tilted in the opposite direction, the direction of fluid flow is reversed.
6. The stroke length of the pistons, and hence the quantity of fluid delivered, depends on the angle of tilt of the swash plate. A greater angle of tilt produces a longer piston stroke and greater pump delivery.

In Summary

The Hele-Shaw pump provides:

• Zero delivery when the swash plate/floating ring is in the neutral position.
• Variable delivery depending on the angle of displacement or tilt.
• Reversible flow when the direction of displacement is reversed.

Hydraulic oil leakage along the ram is prevented by providing an effective ram sealing arrangement at the point where the ram passes through the cylinder cover or gland.

The arrangement generally consists of:

1. Gland packing or sealing rings: Special seals are fitted around the ram to prevent hydraulic oil from escaping along the reciprocating surface.
2. Multiple sealing elements: A combination of pressure seals, backup rings and scraper/wiper rings may be used to provide reliable sealing.
3. Wiper or scraper ring: This removes dirt, moisture and other contaminants from the ram surface before it enters the cylinder, thereby protecting the main sealing elements.
4. Drainage/leakage collection arrangement: The gland area may be provided with a leakage collection or drain arrangement so that any seal leakage is detected and prevented from spreading into the steering gear compartment.

The ram surface must also be kept smooth, clean and free from corrosion or scoring, since a damaged ram surface can rapidly destroy the seals and cause excessive oil leakage.

In heavy seas, a large external force acting on the rudder may cause sudden movement or vertical jumping of the rudder. Suitable arrangements are therefore provided to protect the steering gear, tiller and hydraulic rams from excessive shock loads.

1. Relief or safety valves

• When a heavy sea strikes the rudder, the external force can cause the hydraulic pressure in the steering system to rise sharply.
• Safety or relief valves are fitted to prevent excessive pressure from damaging the hydraulic system. If the pressure exceeds the preset value, the relief valve opens and allows hydraulic oil to bypass. This relieves the excessive pressure and permits controlled movement, thereby protecting the steering gear components.

2. Jumping clearance

• A specified vertical jumping clearance is maintained between the structural stops associated with the rudder and the ship's hull.
• This clearance is carefully designed to be less than the internal clearance between the tiller and the steering gear ram casing. Therefore, if the rudder moves vertically due to heavy seas, the external structural stop takes the load before the tiller or crosshead can strike and damage the steering gear components.

3. Jumping bars or stop pads

• Heavy-duty jumping bars or stop pads are fitted to the hull structure.
• If the rudder jumps upward, it contacts these solid structural stops first. The stops limit the vertical movement of the rudder and prevent the internal tiller or crosshead from striking the hydraulic rams or actuators, thereby avoiding serious mechanical damage.

4. Rudder carrier bearing

• A robust rudder carrier bearing supports the weight of the rudder assembly and limits excessive vertical or lateral movement.
• By reducing unwanted play, the carrier bearing helps reduce the severity of shock loading and impacts when the rudder is subjected to heavy sea forces.