Marine Auxiliary Machinery

1. Marine Auxiliary Machinery Chapter 9 Lesson 2 Deck Machinery Hydraulic Drives By Professor Zhao Zai Li 05.2006

2. Hydraulic drives (1) • Hydraulic drives can be broadly sub-divided into constant pressure, constant volume and variable displacement systems. • Constant pressure systems for use with prime movers on deck machinery are rare, control and oil cooling problems being difficult to overcome with large equipment. • Constant volume systems are more common, one example being the high torque vane type system which operates between 25-45 bar.

3. Hydraulic drives (2) • As the pump (see under ‘Positive displacement pumps’ in Chapter 3) delivers a constant volume of oil speed control of the hydraulic motor is obtained by throttling the required amount of oil to the motor through a control valve, the remainder being by-passed to the pump suction. • The pump discharge pressure is determined by the load, speed and direction of rotation being controlled at the hydraulic motor by a reversing lever positioning a balanced spool valve.

4. Hydraulic drives (3) • A third form of hydraulic drive consists of a variable displacement axial piston pump (see Chapter 8)which is driven at constant speed, normally by an a.c. electric motor, and which supplies oil to a fixed displacement hydraulic motor coupled to the mechanical portion of the machinery (Figure 9.4). • The piston stroke and hence the pump delivery is controlled by servo motor, the operation of which is dependent on the movement of a pilot valve in either direction from the neutral position. • As the motor speed is directly related to the amount of oil available, should light line duties be required the full load speed of the motor is obtained when the pump is on some fraction of full stroke.

5. Figure 9.4 Figure 9.4 Norwinch hydraulic winch motor (open)

6. Hydraulic drives (4) • This type of system incorporates a constant horsepower device which limits the maximum amount of power absorbed from the pump drive motor under loaded conditions and provides an automatic speed/load discrimination feature similar in performance to the steam drive or Ward Leonard electrical system. • Full load system pressures normally fall within the range 140-240 Bar, though some work at lower pressures, using a vane-type motor.

7. Hydraulic drives (5) • Many of the hydraulic systems，fitted to deck machinery are of the ‘unit’ type, one pump driving one motor, but there are great advantages to be gained by the use of a ‘ring main’ system. • With this system one centrally located hydraulic pump is able to cater for the needs of a number of auxiliaries working simultaneously at varying loads, • As the equipment powered from this central pumping station need not be restricted to deck machinery, the system offers considerable savings on capital cost．

8. Hydraulic drives (6) • It is important with all hydraulic systems to ensure that interlocking arrangements provided for pump or motor control levers are in the neutral position before the pump driving motor can be started in order to avoid inadvertent running of unmanned machinery. • Overload protection on hydraulic systems is provided by the use of pressure relief valves set between 30-50％in excess of rated full load pressures.