What is Steering Gear ?
Steering gear, is the equipment provided on ships to turn the ship from port to star board or starboard to Port while the ship is in motion during sailing .
The capacity of steering gear system should be such that the ruder can be turned from 35degree port to 35 degree starboard and vice versa and it should be able to turn the rudder from 30° one side to 35 degree other side within 28 seconds.
the steering gear is effective only when the ship is in motion.
In the steering system, the steering gear provides a movement of the rudder In response to a signal from the bridge.
The total system consists of three parts:-
c)transmission unit to the rudder stock.
a)control equipment convey a signal of desired rudder angle from the steering flat where it is received to activate the power unit and transmission system until the desired angle is reached.
b)power unit provides the force when required & with immediate effort to move the rudder to the desired angle.
c)transmission system (steering gear) is the means by which the movement of the rudder is accomplished.
control equipment can be hydraulic control equipment ( known as telemotor ) or an electrical electronic control equipment.
Types of hydraulic powered transmission unit or steering gears:-
It is of two types :-
a)Ram type ( may be 2 ram or 4 ram )
b)Rotary vane type
Here we will Discuss About Ram Type Steering Gear System
If the oil is pumped into the LHS (port) hydraulic cylinder and drawn from RHS ( starboard) hydraulic cylinder, there will be a left to right sliding movement of the rams.Through the RAM cross head, the tiller arm will move towards starboard and the rudder towards port .
The instant pumping is stooped , movement of tiller arm will stop and rudder will be hydraulically locked in this new position as oil now has no way to leak and is fully blocked within the ram cylinders and vice – versa.
Two ram Steering Gear
Sketch shows the arrangement of a 2 Ram the steering gear.
When the steering wheel is turned signal goes to telemotor receiver through the telemotor transmitter which operates the hunting gear.
Hunting gear inturn Moves the variable delivery pump (hele shaw or swash plate type ) operating spindle from neutral position.
Thus pump start pumping to one hydraulic cylinder and from the other hydraulic cylinder, oil is sucked to pump.
Thus the RAM starts moving and this movement is transmitted through the tiller to the rudder stock and rudder.
The movement of rudder cause the hunting gear to move in such a way that once the desired angle of rudder is reached the pumps operating spindle comes back to neutral positions, and no further pumping & rudder remains in desired angular position.
4 ram Steering Gear
Above Diagram shows the arrangement of a four ram steering gear.
As the signal from the telemotor system operates the hunting gear it actuates the variable delivery pumps to pumping position,thus oil flows to two numbers diagonally opposite cylinder ( say 1 and 4 ) from other two cylinder (two and three ) oil is sucked into the pump.
This causes the rudder to turn in clockwise direction. If due to the action of telemotor system hunting gear is moved to other direction then oil will be supplied to cylinder 2&3 and from 1& 4 oil will flow into the pump and thus the rudder turns in anticlockwise directions.
Turning of the rudder will move the hunting gear in such a way that so that once the desired position is reached , pump operating Lever will bring the pump into neutral position and the desired rudder position will be maintained.
Each cylinder is fitted with cylinder isolating valve. There are two bypass valve and relief valve connecting cylinder 1 and 2 & cylinder 3 and 4.
In case of excessive pressure in the cylinder ( which may occur due to shock on the rudder )
The spring loaded bypass valve will open automatically causes the connected cylinder to bypass.
Replenishing tank is provided into the system to make up any loss of Hydraulic oil .
Explain the following in a two ram steering gear system.
A) isolating valve :- isolating valves are provided at each cylinder & Rotary vane number which when closed it will hold the rudder by trapping the oil in the Chambers.Pumps are also equipped with an isolating valves so that the pump can be totally shut off from the circuit and removed for servicing while the steering is continued with the other pumps.
B) bypass valve:-. Bypass valves are normally closed but can be opened on two cylinder gear to allow for the use of emergency steering.One pair of cylinder can be bypassed on a four cylinders – gears while the other pair of cylinders provide emergency steering at reduced torque.
C)relief valves :- relief valves are provided between complementary pair of cylinder or Chambers of vane gears.They are set to lift at pressure above the normal maximum.
D) Shock valves are fitted in pipework of the system between cylinders to prevent excessive shock load from the rudder due to heavy seas being transmitted to the hydraulic system