Detail

Hydraulic steering gear integrated with the rudder system defines the complete ‘turning mechanism’ mandatory for each and every ship irrespective of size, type and operation.

As per standard requirements, the steering gear should be capable of steering the ship from 35 degrees port to 35 degrees starboard and vice-versa with the vessel plying forwards at a steady head-on speed for maximum continuous rated shaft rpm. and summer load waterline within a time frame of maximum 28 seconds. With one of the power units inoperative, the rudder shall be capable of turning 15 degrees port to 15 degrees starboard (and vice-versa) within a time frame of 1 minute with the vessel moving at half its rated maximum speed or 7 knots (whichever is greater) at summer load line. The major power units and the control systems are to be duplicated so that if one of them fails, the other can easily substitute for them as standby

The steering gear provides a movement of the rudder in response to a signal from the bridge. The total system may be considered made up of three parts, control equipment, a power unit and a transmission to the rudder stock. The control equipment conveys a signal of desired rudder angle from the bridge and activates the power unit and transmission system until the desired angle is reached. The power unit provides the force, when required and with immediate effect, to move the rudder to the desired angle. The transmission system, the steering gear, is the means by which the movement of the rudder is accomplished.

Certain requirements must currently be met by a ship's steering system. There must be two independent means of steering, although where two identical power units are provided an auxiliary unit is not required. The power and torque capability must be such that the rudder can be swung from 35° one side to 35° the other side with the ship at maximum speed, and also the time to swing from 35° one side to 30° the other side must not exceed 28 seconds. The system must be protected from shock loading and have pipework which is exclusive to it as well as be constructed from approved materials. Control of the steering gear must be provided in the steering gear compartment. Tankers of 10000-ton gross tonnage and upwards must have two independent steering gear control systems which are operated from the bridge. Where one fails, changeover to the other must be immediate and achieved from the bridge position. The steering gear itself must comprise two independent systems where a failure of one result in an automatic changeover to the other within 45 seconds. Any of these failures should result in audible and visual alarms on the bridge.

Steering gears can be arranged with hydraulic control equipment known as a 'telemeter', or with electrical control equipment. The power unit may in turn be hydraulic or electrically operated. Each of these units will be considered in turn, with the hydraulic unit pump being considered first. A pump is required in the hydraulic system which can immediately pump fluid in order to provide a hydraulic force that will move the rudder. Instant response does not allow time for the pump to be switched on and therefore a constantly running pump is required which pumps fluid only when required. A variable delivery pump provides this facility. The steering gear system is to be provided with additional power unit (hydraulic pump etc.) connected to the emergency power supply from Emergency Generator, which shall be capable of turning the rudder from 15 degrees from one side to other side within 60 seconds with the vessel moving at a maximum service speed or 7 knots, whichever is greater

There are two types of commonly used steering gear systems present that are hydraulic and electro-hydraulic type. Though the system has undergone some major evolution, the basic physics of operation remains the same. In hydraulic and electro-hydraulic systems, hydraulic pressure is developed by hydraulic pumps which are mainly driven by electric motors (electro-hydraulic systems) or sometimes through purely mechanical means (hydraulic systems). However, mainly advanced electro-hydraulic systems are predominant in ships nowadays. These hydraulic pumps play a crucial role in generating the required pressure to create motions in the steering gear which can trigger the necessary rotary moments in the rudder system.

Ram Type Steering Gear System
Ram type steering gear is one of the commonly used steering gear construction and is quite expensive in construction. The basic principle is same as that of a hydraulically-driven motor engine or lift. There are four hydraulic cylinders attached to the two arms of the actuator disc, on both sides. These cylinders are directly coupled to electrically driven hydraulic pumps which generate hydraulic pressure through pipes. This hydraulic pressure field present in the pumps imparts motion to the hydraulic cylinders, which in turn corresponds with the actuator to act upon the rudder stock. As we know, rudder stock is an indispensable part of the entire steering gear arrangement of ships and dictates the exact behavior of the rudder response. The sense of turning the rudder is guided by the action of the hydraulic pump. The ram type steering gear arrangement produces a considerably high value of torque for a given applied power. The hydraulic oil pressure varies from 100 bars to 175 bars depending on the size of the rudder and torque required.

Rotary Vane Steering Gear
In rotary vane steering gear, there is a fixed housing in which two vanes rotate. The housing along with the vanes form four chambers. The physics behind its operation is similar to the ram type with a small difference. Rotary vane type arrangement is used when the pressure requirement is 60 to 100 bar for producing required torque. This is the main advantage of rotary vane type steering gear, requiring lesser hydraulic pressure and thus power for producing the same amount of torque as ram type. There are 3 fixed and 3 moving vanes, which can make rudder angles up to 70 degrees, i.e 35 degrees on each side. This arrangement has several other advantages like lower installation cost, less weight and smaller space required. The fixed and rotating vanes are of spheroidal graphite cast iron. Keys are often provided in the rotary vanes for proper strength and orientation.