Thursday, 21 June 2012

Steering gear

Steering gear
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A hydraulic steering gear consists of a bridge control which applies helm, an engine control which is operated jointly by the helm and hunting gear (when fitted) and a power pump and rudder actuator which constitutes the steering engine.

Telemotor systems

The telemotor system consists of a transmitter on the bridge and a receiver fitted on the steering gear forming a part of the hunting gear. The system may be electrical or hydraulic or a combination of the two.
Most modern vessels are fitted with electric or electro-hydraulic systems. Due to the increasing size of vessels pipe runs have lengthen causing lags in the operation of the receiver in hydraulic systems. In addition hydraulic only systems generally require more maintenance.
Hydraulic transmitter
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Hydraulic transmitter

Shown above is a typical hydraulic transmitter unit. The pinion driving the pistons is turned by the bridge wheel.
The casing is usually gun metal, with bronze rams, and copper pipes are led in by frilled leads on the casting.
To test the system, with the steering gear actuating pumps stopped, the wheel may be lashed at hard over and the pressure recorded. It should maintain this pressure for some time
To allow for expansion in the system and to allow topping up a 'by-pass valve' is fitted. It will also act as a safety valve.

Author note:
The main problem appears to be the effect of air entrained within it. Thus regular venting of the system is required.
By-pass valve
By-pass valve
The operating rod is pushed down making both line common whenever the wheel is at midships, generally by a cam fitted to the pinion. This ensures they system is always balanced.
Hydraulic receiver
The charging valves are opened only when filling or flushing.
The moving cylinder is attached to the hunting gear. When the bridge wheel is turned hydraulic pressure acts on the cylinder causing it to move. This in turn moves the hunting gear. The steering gear is then moved to compensate until the hunting gear is moved back to the neutral position. The total movement of the receiver is limited by stops.

Electro-hydraulic type telemotor system

simple elecro-hydraulic system Shown is a very simple system capable of operating a steering hunting gear. A pressure relief valve would normally be fitted after the valve and across the pump to prevent over pressurisation of the system.
The signal is derived from the action on the steering wheel, created by the autopilot or directly from the non-follow up control levers.

Telemotor fluid

    should be a good quality mineral oil with the following properties;
    1. low pour point
    2. non sludge forming
    3. non corrosive
    4. good lubricating properties
    5. high flash point
    6. low viscosity

Hunting Gear

hunting gear The steering gear system above consists of the telemotor which receives a signal from the bridge wheel. This acts on the hunting gear.
The hunting gear moves displacing a control rod, this rod acts on the pump displacement control gear to alter the delivery from the pump. The delivery from the pump causes the ram to move rotating the rudder stock and hence the rudder. The other end of the hunting gear is mounted on the rudder stock.
The rotation of the rudder stock moves the hunting gear returning the operating rod for the pump to the neutral position once the rudder has reached the correct angle. action of steering gear

Rudder Actuators

There are many different mechanisms by means of which hydraulic power can be converted into torque at the rudder stock some of which are as follows;

Rapson Slide Actuators - Ram type

rapson slide gear Steering gear incorporating the rapson slide principle are the most common in use on heavy duty applications.
The rapson slide acting on either a fork tiller or the more common round arm. The tiller drives the rudder stock by means of a key or keys. The crosshead is free to slide along the circular arm of the tiller so that the straight line effort of the rams is applied to the angular moving tiller. Each set of two cylinders in line are connected by a strong steel girder usually called a "Joist" which stiffens the system and forms a "guide bar" for the crosshead guide slippers to slide along. The joist is often designed to incorporate the steering engine stops.
An important consideration in all steering gears is the "wear down" of the rudder carrying bearing, this bearing takes all the weight of the rudder. Therefore there must be adequate clearance between the bottom of the tiller and the crosshead bearing, so as the rudder bearing wears down in service the tiller and crosshead bearing do not touch, clearance when new can be 22 mm at bottom and 12 mm at top; the top clearance is a precaution to stop the tiller bumping up the steering rams in the unlikely event of the rudder lifting in heavy weather. Should the bottom of the tiller and the crosshead bearing touch, then the weight of the rudder will be transferred from the rudder bearing to the steering rams with disastrous results such as leaking of working fluid from the cylinders and shearing of the rams.
In the case of forked tiller design, the thrust from the rams is transmitted to the tiller through swivel blocks. One advantage of this arrangement is that the overall length of pairs of rams is reduced compared to the round arm tiller design and this can be an important consideration in some cases. A disadvantage is that where as any slight misalignment in the case of the round arm tiller is not vitally important, it could lead to uneven loading of the swivel blocks in the forked tiller design and it is essential that the line of the rams be exactly at right angles to the rudder stock centre line if this is to be avoided.
With the Rapson Slide the torque reaction from the rudder is taken on the tiller by a force which is balanced by an equal and opposite force having two components one of which is produced by the ram and acts in the line of the ram, whilst the other is at right angles to the line of the ram and is produced by the guide reaction.
Force acting in a rapson slide system
Where guides are not fitted as is sometimes the case with smaller steering gears then the guide reaction force must be carried by bearings or the glands of the cylinders.
a = actuator area
p = Working fluid pressure
n = Number of effective rams ( 1 for 2 ram, 2 for 4 ram)
q = rudder angle
r = tiller radius at amidships
r' = tiller radius at qo of tiller helm
s = guide reaction force
f = force on ram with tiller amidships ( = p x a)
f' = effective force acting at 90o to tiller

r' = r / cos.q also f' = f / cos.q = p x a / cos.q
t = torque available = f' x r' x n
= ((p x a) / cosq). (r / cos.q) . n
t = (p x a x n x r) . (1 / cos.2q)
Showing that the rapson slide effect which gives increase of available torque with increases of rudder angle
The torque demanded from the steering gear increases and is at a maximum at maximum rudder angle when the mechanical advantage of the Rapson Slide gear is at a maximum. Ram type gears are also well adapted to take advantage of the high pressures which are currently available, since ram diameters and casing are relatively small and leakage paths are small or non-existent.

Oscillating Cylinder Actuators

The use of oscillating cylinders or pinned actuators is a recent development. They can be used as single cylinder units for hand only steering or two cylinder units for hand and power steering. While four double acting cylinders can cope with larger torque demands. These units are double acting because pistons work in the cylinders and pressure can be applied to either side as compared with ram gears which are single acting. In these cases, the torque T applied to the rudder stock varies with the rudder deflection angle and on the location of the actuator. In general the torque developed will be less at the maximum rudder angle than the maximum possible from the actuator.
Maximum torque from actuator = p.a.n.r.
Torque at 35o = p.a.n.r. cos (35 = o)
where o = angle traced out by the actuator
between o = 0o and o = 35o
Mechanical advantage at 35o = Cos 35o = 0.82
since the actuators are pivoting about their pin centre, they usually have their working fluid tank and pump mounted on the actuator cylinder, or they are connected to tank and pump by a flexible pipeline.

Rams Connected To Crossheads By Links

This type of gear is used if the athwartships space is limited, or the head room at the rudder head is restricted, as for example, in the case of a vehicle ferry having a slip way aft. The design enables the steering gear to be moved forward where there is reasonable head room for access. As in the case of the oscillating cylinder design the Mechanical Advantage of the Rapson Slide gear is lost in the links and the torque output of the gear is at a minimum at hard over when the torque demand created by the rudder hydrodynamic forces is at a maximum.

Rotary Vane Gear


    These consist of two elements:
    1. a cylindrical static casing (stator) with usually three internal vanes which project radially inwards
    2. a rotor keyed to and concentric with the rudder stock, the rotor has rotor vanes which project radially outwards into the spaces formed by the stator vanes.
    The spaces formed between the stator and rotor vanes are used as high and low pressure chambers. The main advantage of the system is that it is compact, occupying about 1 / 10 the space of a ram system. The disadvantages are ;
    1. it has a long oil sealing path
    2. it is a constant torque machine at all angles of helm compared to the ram system where due to the Rapson slide effect, the torque available increases with increasing helm.
Where 100% redundancy is required two rotary vanes in piggy back are used.
All vanes are spheroidal graphite cast iron secured to the cast iron rotor and stator by high tensile steel dowel pins and cap screws. Rotor strength is maintained by keys fitted full length of the rotary vane. Steel sealing strips are fitted along the working faces, backed by synthetic rubber in grooves along the working faces which are elastically loaded, so as to ensure that contact with the mating surfaces is maintained in order to hold the hydraulic pressures.
rotary vane gear
The chambers are alternately connected to the suction and delivery from the hydraulic pump so that they can be used to produce the rudder actuating torque. Because the distribution of the pressure chambers is balanced around the rudder stock, only pure torque is transmitted to the stock and no side loading are imposed by the gear.
There are two main types of rotary vane steering gear in use today. One has its stator firmly fixed to the steering flat deck and the stator housing and cover are provided with suitable bearings to enable the unit to act as a combined rudder carrier and rudder stock bearing support. The other type of vane gear is supported where the stator is only anchored to the ships structure to resist torque but is free to move vertically within the constraints of the separate rudder head bearing and carrier which is similar to the bearing provided for ram type steering gears.
The rudder carrier ring bearing (Pallister Bearing) is taking the weight of the rotary vane steering gear and the rudder and stock.
Vertical view of rotary vane gear
rotary vane anchor arrangement
Rotation of the stator is prevented by means of two anchor brackets and two anchor bolts . The anchor brackets are securely bolted to the stool and 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 rudder stock. This clearance varies with each size of rotary unit but could be about 40 mm total . It is essential that the rudder carrier should be capable of restricting the vertical movements of the rudder stock to less than this amount.
The anchor bolts are fitted with special bushes in halves, shaped externally in order to pre-load the synthetic rubber shock absorbers , which are fitted between them and the anchor brackets. The maximum deflection of the shock absorbers under full load is approximately 1 mm.
The working angle of the gear is governed by the number of vanes and their thickness. Vanes act as rudder stops when a moving vane contacts a fixed vane. Valves at inlet to the chambers may be shut causing a hydraulic lock. In the rotary vane units the Mechanical Advantage is unity at all angles and hence torque is constant
Torque = p.a.n.r.
where n = number of rotating vanes

Tendfjord Rotary Piston Gear Actuator

This gear consists of a casing around the rudder stock which contains pistons of rectangular section sliding in angular compartments concentric with the rudder stock. The tiller projects into a gap between the cylinder, the piston ends abutting onto the tiller but not being attached to it so that axial movements of the rudder cannot be transmitted to the pistons. Steering gears of this type operate at hydraulic pressures up to 41 bar (600 lbf/in2) and are in general restricted to low power application. As with the rotary vane steering gears the Mechanical Advantage is unity at all angles and hence the torque is constant.
Torque = p.a.n.r.
where n in this case is unity.

Components

Relief Isolating And Bypass Valves

Hydraulic actuators are provided with relief and bypass valves between complementary pairs of cylinders or chambers of vane gears. The relief valves are set to lift at pressures above the normal maximum. The bypass valves are normally closed but can be opened on a two cylinder gear to enable emergency steering to be used. On a four cylinder gear one pair of cylinders can be bypassed while the other pair provide emergency steering at a reduced torque, an instruction plate is fitted over the controls valve block giving a combination of failures and which valves have to be open or shut to cope with the emergency etc. It should be noted that if one ram or cylinder in a four ram system breaks down, then never isolate the cylinder diagonally opposite the damaged unit, since the steering gear will not operate due to the fact that the remaining two cylinders will be either on all pressure or on all suction at the same time.
Isolating valves are provided at each cylinder or rotary vane chamber which when closed will hold the rudder by trapping the oil in the chambers. Isolating valves are also fitted to pumps so that a pump can be completely shut off from the circuit and removed for servicing while steering is continued with the other pump.
In the case of gears with duplicated variable stroke pumps, in order to be able to bring a standby unit quickly into operation, the pump stroke mechanisms are permanently coupled together and both pumps are left open to the hydraulic circuit. Thus it is only necessary to start up a motor for the stand by pump to be operative. It is usual to run both pumps in restricted navigation waters. As a variable stroke pump can operate as a motor if pressure oil is applied to one side while it is on stroke, it is necessary to prevent wind milling or rotation of the pump which is on stand by duty.
Otherwise, the output of the operation pump, instead of moving the steering gear would be used up in rotating the stand by pump.
One method to prevent this,is using a fixed ratchet is provided concentric with the pump driving shaft. Pawls that can engage this ratchet are carried in the drive coupling. When the pump is on stand-by the pawls engage with the ratchet and prevent rotation when oil on the delivery side of the operating pump is on pressure. In this condition the tendency to motor the stand by pump will always be against its normal direction of rotation. As soon as the pump is started, rotation being in the opposite direction, the pawls disengage and by centrifugal action fling out against the inner flange of the coupling completely clear of the ratchet. When a pump is on stand-by and the rudder is being driven by water pressure in the direction in which it is being moved so as to generate pressure on what is normally the suction side of the operating pump, this will cause the stand by pump to rotate in its normal running direction. This means that the pawls will disengage and the pump will be motored round, allowing the rudder to move more quickly to a new steering position than the single operating pump will allow.
Another method of protection against rotation of the stand by pump is to fit Servo pressure operated automatic change over valves in the pipelines; these ensure that the pump can only be started in the unloaded condition (neutral) and in addition prevents the stand by pump from being motored by the pump in service.
On some ships it has been discovered that the ball bearing races on the stand-by pump have been failing due to brinelling of the ball bearings, caused by ship vibrations, and in these cases it is usual to fit devices which allows the stand by pump to be motored slowly.
When fixed delivery pumps are duplicated in supplying oil to a common hydraulically operated control valve, an automatic change over valve can be fitted which will isolate the stand by pump when it is at rest, but will connect it to the actuator when the pump is started up.

Stops And Limit Switches

External or stern posts stops are set at the absolute limit to hard over movement of the rudder , protects propeller and ship stern in the event of metal or other failure which allows rudder to swing in an uncontrolled manner. Mechanical stops on the rudder actuator operate before the external stop are reached .these take the form of travel limits. Stops on the bridge control operate before mechanical stops. local controls are set midway. auto pilot controls are set first. It should be noted that the vanes act as stops on rotary vane gears.

Drive Back Due To Heavy Sea's

Heavy seas acting on the rudder can force the actuator against the hydraulics sufficient to lift the relief v/v, in which case the rudder will move. Hunting gear will tend to return the gear to its correct position.

Hand And Power Hydraulic Steering Gears

For small ships during navigational course keeping hand steering can be used, whist during manoeuvring power steering can be used. These may take the form of chains or simple hydraulics operated by a fixed delivery pump attached to the steering gears.

"Follow Up" Steering

This is the normal method of steering and involves the feedback of steering angle to the helm. This is suited to both manual and automatic operation. The ships heading may be set into the autopilot which can then compare the actual to desired heading and adjust the rudder angle to suit

"Non-follow Up" Steering

Normally used for back up purposes only. Consists of a single lever per steering gear unit, by moving the lever in on direction the rudder will begin to turn, the rudder will continue to turn until the lever is released or it reaches the limit of its operation

Charging A System With Fluid

. In all cases high quality hydraulic oil should be used , containing inhibitors against oxidation , foaming, rust and wear and emulsification. In order to keep the transmission load as low as possible when hand steering , hand power systems must have oil of low viscosity.
The condition of the oil should be monitored and ensured at least clean and free of moisture.

Steering gear failure

A study of steering gear defects demonstrates that the most common are related to vibration and the working loose of components. The most common source of failure are the pump and the hydraulic system associated with it.

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