Steering gear

The machinery, rudder actuators, steering gear power units and the means of applying torque to the rudder stock necessary for eff ecting movements of the rudder. Two types of electrohydraulically-powered steering gears are in common use: the ram and the rotary vane.

Auxiliary steering gear  – The equipment other than any part of the main steering gear necessary to steer the ship in the event of failure of the main steering gear but not including the tiller, quadrant or components serving the same purpose, (acc. to SOLAS).

Main steering gear  – The machinery, rudder actuators, steering gear power units, if any, and ancillary equipment and the means of applying torque to the rudder stock (e.g. tiller or quadrant) necessary for effecting movement of the rudder for the purpose of steering the ship under normal service conditions, (acc. to SOLAS).

Ram-type electrohydraulic steering gear  – A ram-type electrohydraulic steering gear consists of two or four hydraulic rams, connected by a link mechanism or a Rapson slide mechanism to the tiller which turns the rudder. A link mechanism transfers the ram movement to the tiller and imparts maximum torque at 35° of rudder movement. The Rapson slide mechanism consists of a block or a sleeve, pivoted to the ram and guided by a crosshead, and arranged to slide on the tiller arm so that the moment arm increases as the rudder angle increases. The rams are moved by hydraulic fluid supplied under pressure by one or two pumps. Usually, two independent pumping units are provided. They are connected so that either may be used to operate the gear, thus eliminating the classification society rule requirement for auxiliary steering gear.

Note: In a passenger ship, under normal service conditions, one unit works. In a cargo ship both units work.

Rotary vane steering gear – The rotary vane system works by introducing pressure into compartments formed between a stator fixed to the ship's structure and a rotor attached to the  rudder stock. There are two or three vanes on the rotor and an equal number on the stator to form the compartments. When steering effort is required, the pressure is increased in the appropriate compartments. The pressure reacts against the fixed vanes and pushes the rotor (and the rudder stock) in the required direction.

To increase the available torque, the diameter oft the unit is enlarged, although it is generally smaller than an equivalent ram type arrangement. Hydraulic pressures are also lower as the working area is larger than the total of the rams on the ram-type gear. Another advantage is the degree of rudder movement: that is up to 65° for Porsgrunn system, and up to 45° for Ulstein's Frydenbo. With ram operated gear the maximum degree of rudder movement is limited by the stroke of the cylinders and the scope of the slider mechanism. One potential disadvantage of the rotary vane system is that if is a fault inside the unit, all steering can be lost and specialist repair is needed. With ram type gear for larger vessels there are four single-acting cylinders so if one ram fails then the steering is not totally disabled. The working parts are also accessible in the event of a necessary repair and the rams are relatively simple to replace if a spare is carried.

According to the Motor Ship July 1996.

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