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Ship Steering gear failures and safeguards
The vital importance of the steering gear is reflected in the regulations of the
government department with responsibility for shipping (usually Department
of Transport or Coastguard) the requirements of the classification societies
(Lloyd's Register, American Bureau of Shipping, Bureau Veritas and others)
and the recommendations of the International Maritime Organization (IMO).
Measures against steering gear failure
Some general requirements for steering gears, based on the various
regulations and SOLAS 1974, are given below:
(1) Ships must have a main and an auxiliary steering gear, arranged so that
the failure of one does not render the other inoperative. An auxiliary steering
gear need not be fitted, however, when the main steering gear has two or more
identical power units and is arranged such that after a single failure in its piping
system or one of its power units, steering capability can be maintained. To
meet this latter alternative the steering gear has to comply with the operating
conditions of paragraph 2 — in the case of passenger ships while any one of the
power units is out of operation. In the case of large tankers, chemical tankers
and gas carriers the provision of two or more identical power units for the main
steering gear is mandatory.
(2) The main steering gear must be able to steer the ship at maximum
ahead service speed and be capable at this speed, and at the ship's deepest
service draught, of putting the rudder from 35 deg on one side to 30 deg on the other
side in not more than 28 secs. (The apparent anomaly in the degree of
movement is to allow for difficulty in judging when the final position is reached
due to feedback from the hunting gear which shortens the variable delivery
pump stroke.) Where the rudder stock, excluding ice strengthening allowance,
is required to be 120 mm diameter at the tiller, the steering gear has to be
power operated.
(3) The auxiliary steering gear must be capable of being brought speedily
into operation and be able to put the rudder over from 15deg on one side to 15deg
on the other side in not more than 60 sees with the ship at its deepest service
draught and running ahead at the greater of one half of the maximum service
speed or 7 knots. Where the rudder stock (excluding ice strengthening
allowance) is over 230 mm diameter at the tiller, then the gear has to be power
operated.
(4) It must be possible to bring into operation main and auxiliary steering
gear power units from the navigating bridge. A power failure to any one of the
steering gear power units or to its control system must result in an audible and
visual alarm on the navigating bridge and the power units must be arranged to
restart automatically when power is restored.
(5) Steering gear control must be provided both on the bridge and in the
steering gear room for the main steering gear and, where the main steering
gear comprises two or more identical power units there must be two
independent control systems both operable from the bridge (this does not
mean that two steering-wheels are required).
When a hydraulic telemotor is
used for the control system, a second independent system need not be fitted
except in the case of a tanker, chemical carrier or gas carrier of 10000 gt and
over. Auxiliary steering gear control must be arranged in the steering gear
room and where the auxiliary gear is power operated, control must also be
arranged from the bridge and be independent of the main steering gear control
system. It must be possible, from within the steering gear room, to disconnect
any control system operable from the bridge from the steering gear it serves. It
must be possible to bring the system into operation from the bridge.
(6) Hydraulic power systems must be provided with arrangements to
maintain the cleanliness of the hydraulic fluid. A low level alarm must be fitted
on each hydraulic fluid reservoir to give an early audible and visual indication
on the bridge and in the engine room of any hydraulic fluid leakage. Power
operated steering gears require a storage tank arranged so that the hydraulic
systems can be readily re-charged from a position within the steering gear
compartment. The tank must be of sufficient capacity to recharge at least one
power actuating system.
(7) Where the rudder stock is required to be over 230 rnm diameter at the
tiller (excluding ice strengthening) an alternative power supply capable of
providing power to operate the rudder, as described in paragraph 3 above, is to
be provided automatically within 45 seconds. This must supply the power unit,
its control system and the rudder angle indicator and can be provided from the
ships emergency power supply or from an independent source of power
located within the steering compartment and dedicated for this purpose. Its
capacity shall be at least 30 minutes for ships of 1OOOOgt and over and 10
minutes for other ships.
Steering gear testing
Except in the case of ships regularly engaged on short voyages, the steering
gear should be thoroughly checked and tested within 12 hours before
departure. These tests should include testing of power unit and control system
failure alarms, the emergency power supply (when relevant) and automatic
isolating arrangements.
Every three months an emergency steering drill should be held and should
include direct control from within the steering compartment at which time the
use of the communications procedure with the navigating bridge should be
practised.
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.
......
- 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.
......
- 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.
......
- 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.
......
- 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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