Answer EKG Question 19

Q19. Describe with the aid of sketches where necessary a vane type steering gear showing how to weight of the rudder and stock are carried and the arrangement that allow for wear down.

Answer: Rotary vane steering gear. 

Operation arrangements:-

pipe line is similar to 4-Ram hydraulic steering gear. The variable delivery pumps deliver the oil into one side of the rotary vane and receive from the other side, till the rudder is at required position. At this position the feed back lever brings the pump to a zero pumping position and the rudder gets hydraulically locked.

Advantages:- less space, lighter construction, less cost and low maintenance.

Rotary vane steering gear can be fitted without a rudder carrier. The gear applies a pure torque to the stock, eliminating any transverse loading on the rudder bearings. They are compact, hence are useful with twin rudder applications. It saves weight.  Initial cost is less.

Disadvantages:-  Efficient seal is difficult to achieve especially at higher pressures of 90 bar and above. It has a leakage path from high to low pressure side at higher pressures. It is limited to ships requiring low rudder torque.

Construction:-

Vane type gear may be regarded as equivalent to a two-ram gear, with torque capacities depending on size. An assembly of two rotary vane gears, one above the other, provides the security of a four ram gear.

The rotor C is fitted and keyed to a tapered rudder stock, the stator B is secured to the ship's structure. Fixed vanes, secured equidistantly in the stator bore and rotating vanes secured equidistantly in the rotor, form two sets of pressure chambers in the annular space between the rotor and stator. They are interconnected by a manifold. Fluid supplied at pressure to one set of these chambers will rotate C clockwise and the rudder will turn to port, or to starboard if the alternate set is put under pressure.

Three fixed and three moving vanes are usual and permit a total rudder angle of 70°, i.e. 35° in each direction. 

The fixed vanes also acts as rudder stopper and allow a maximum angle of 80°.

A mechanical operated relief valve is located in the rotary vanes. when the rudder is brought to hard over, the valve stem contacts the segment and opens the valve, thus allowing free passage of oil and preventing a rise of pressure. 

Rotation of the stator is prevented by means of two anchored brackets and two anchor bolts. the anchor brackets are securely bolted to the ship.

Builders stool and vertical clearance is arranged between the inside of the stator flanges and the top and bottom of the anchor brackets to allow to allow for vertical movement of the rudder stock. 

The clearance  varies with each size of rotary vane unit but is approximately 38mm total and it is essential that the rudder carrier should be capable of restricting the vertical movement of the rudder stock to less than this amount.

The anchor pin are fitted with special bushes in halves, shaped externally in order to preload the synthetic rudder stock absorbers, which is fitted between them and to the under brackets. The maximum deflection os the stock absorbers under full load is approximately 1mm.

The fixed and rotating vanes may be of spheroidal graphite cast iron. They are securely fixed to the cast steel rotor and stator by high tensile steel dowel pins and cap screws. Keys are also fitted along the length of the rotary vanes, for mechanical strength. Assembly of the gear would not be possible if the fixed vanes were keyed; they rely on the dowels to provide equivalent strength.

The vanes fixing is considered to be of sufficient strength to make them suitable to act as rudder stops. Steel sealing strips, backed by synthetic rubber, are fitted in grooves along the working faces of the fixed and rotary vanes, thus ensuring a high volumetric efficiency, of 96—98% even at the relief valve pressure of 100 bar or over. Rotation of B is prevented by means of two anchor brackets, and two anchor pins. The anchor brackets are securely bolted to the ship.

Thrust bearing made of bronze takes the weight of rudder and allow the rudder to move freely.