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Principle of a Vane type gear which provides security of four ram steering gear
Vane type gear :
These 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. Figure 9.19 illustrates the principle.
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 movement obtainable from a gear with
two fixed and two moving vanes, may be 130°. Figure 9.19 (b) shows the detail
of a typical unit.
Figure 9.19 (b): Rotary vane unit
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.
Vertical clearance is arranged between the inside of the stator flanges and the
top and bottom of the anchor brackets to allow for vertical movement of the
rudderstock. This clearance varies with each size of the rotary vane unit, but is
approximately 38 mm in total and it is necessary that the rudder carrier should
be capable of restricting the vertical movements of the rudderstock to less than
this amount.
The method of control for these gears and also for the hydraulic supply
system is as described for electro-hydraulic gears.
Figure 9.19 (a): Rotary vane operation
Summarized below various ship steering gears general guideline:
- Ship Steering gear failures and safeguards
The hydraulic circuit incorporates an arrangement of stop and bypass valves in the chest VC, which enable the gear to be operated on all four or on any two adjacent cylinders but not with two diagonally disposed cylinders.
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- Four-ram electro-hydraulic steering gear mechanism
The hydraulic circuit incorporates an arrangement of stop and bypass valves in the chest VC, which enable the gear to be operated on all four or on any two adjacent cylinders but not with two diagonally disposed cylinders.
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- Enclosed hunting gear
The light construction of the combined control and hunting gears is possible
because the forces concerned are moderate. The self-contained unit is
self-lubricating, and contained in an oil-tight case.
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- Ship steering control mechanism- use of Hydraulic telemotor
The telemotor has become, on many vessels, the stand-by steering control
mechanism, used only when the electric or automatic steering fails. It comprises
a transmitter on the bridge and a receiver connected to the steering gear
variable delivery pump, through the hunting gear.
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- Two-ram electro-hydraulic steering gear with variable
delivery pumps
An arrangement of a two-ram steering gear with variable
delivery pumps may have a torque capacity of 120-650 kNm.
The cylinders for this gear are of cast steel but the rarns comprise a one-piece
steel forging with integral pins to transmit the movement through cod pieces
which slide in the jaws of a forked tiller end.
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- Rudder carrier bearing & Steering gear
The rudder carrier bearing takes the weight of the rudder on a
grease lubricated thrust face. The rudder stock is located by the journal
beneath, also grease lubricated
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- Small hand and power gears - Ship steering systems
A simpler variant of the electro-hydraulic gear, for small ships requiring rudder
torques below say, 150 kNm
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- Four ram gear with servo-controlled axial cylinder pumps
Variants of the servo-controlled swash plate axial cylinder pump
are capable of working at 210 bar. Each pump is complete with its own torque
motor, servo-valve, cut-off mechanism, shut-off valve and oil cooler.
......
- Vane type gear - provides security of four ram steering gear
These 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.
......
- Details of two ram hydraulic steering gear arrangement
When the main pumps are at no-stroke, the auxiliary pumps dischar.
to the reservoir via a pressure-limiting valve PC20, set at 20 bar, and to t
pump casings. When the main pumps are on-stroke, the auxiliary pump
discharge to the main pump suction.
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