Hatch covers

Hatch covers are used to close off the hatch opening and make it watertight. Steel hatch covers, comprising a number of linked steel covers, are now fitted universally. Various designs exist for particular applications, but most offer simple and quick opening and closing, which speed up the cargo handling operation. The hatch covers are arranged to move on rollers along a track on top of the hatch coaming. The individual covers are linked together by chains and ride up and tip onto a stowage rack at the hatch end. A hydraulic power unit, operated from a control box at the hatch end, is used to open and close the hatch cover. It is possible to open and close the covers with a single wire pull from a crane or winch. Watertigtitness of the closed covers is achieved by pulling them down on to a compressible jointing strip. This is done by the use of cleats which may be hand-operated or automatically engaged as the hatch closes. Hatch covers below the weather decks are arranged flush with the deck. In the arrangement shown a self-contained hydraulic power pack with reservoir pump and motor is mounted into a pair of hatch covers. This power pack serves the operating cylinder for the pair of covers. Control is from a nearby point and hydraulic piping is reduced to a minimum. Maintenance requirements for this equipment are usually minimal but regular inspection and servicing should be undertaken. Most hatch covers can, if necessary, be removed manually. (1,200)

 

Exercise 9. Translate into Russian.

 

Part A. Hydraulic systems.The open-loop circuit takes oil from the tank and pumps it into the hydraulic motor. A control valve is positioned in parallel with the motor. When it is open the motor is stationary, while when it is closed the motor will operate. The exhaust oil returns to the tank. This method can provide stepless control, i.e. smooth changes in motor speed. The live-line circuit, on the contrary, maintains a high pressure from which the control valve draws pressurized oil to the hydraulic motor (in series with it), as and when required. In the closed-loop circuit the exhaust oil is returned direct to the pump suction. Since the oil does not enter an open tank, the system is considered closed. Low-pressure systems use the open-loop circuit and are simple in design as well as reliable. The equipment is large, inefficient in operation and overheats after prolonged use. Medium-pressure systems are favoured for marine applications, using either the open or closed circuit. Smaller installations are of the open-loop type. Where considerable amounts of hydraulic machinery are fitted the live-circuit, supplied by a centralised hydraulic power system, would be most economical.

Part B.Electrical operation.Early installations used d.c. supply with resistances in series to provide speed control. This inefficient power-wasting method was one possibility with d.c., but a better method was the use of Ward Leonard control. The high cost of all the equipment involved in Ward Leonard control and its maintenance is a considerable disadvantage. Machines operated on an a.c. supply require a means of speed control with either pole-changing or slip-ring motors being used. Slip-ring motors require low starting currents but waste power at less than full speed and require regular maintenance. Pole-changing motors are of squirrel cage construction, providing for perhaps three different speeds. They require large starting currents, although maintenance is negligible.

Apart from the advantages and disadvantages for each of the drive and control methods, all electric drives have difficulty with heavy continuous overloads. Each system has its advocates and careful design and choice of associated equipment can provide a satisfactory installation.

 

Exercise 10. Translate into English.

 

1. Электрический привод - это электромеханическая система для приведения в движение механизмов рабочих машин и управления этим движением.

2. Гидропривод представляет собой своего рода «гидравлическую вставку» между приводным двигателем и нагрузкой (машиной или механизмом) и выполняет те же функции, что и механическая передача (редуктор, ремённая передача, кривошипно-шатунный механизм и т. д.).

3. В гидроприводе с замкнутой схемой циркуляции рабочая жидкость от него возвращается во всасывающую гидролинию насоса, кнедостаткам следует отнести плохие условия для охлаждения рабочей жидкости и необходимость спускать из гидросистемы рабочую жидкость при замене или ремонте гидроаппаратуры.

Рабочая жидкость постоянно сообщается с гидробаком или атмосферой в гидроприводе с разомкнутой системой циркуляции, достоинства такой схемы являются хорошие условия для охлаждения и очистки рабочей жидкости, а недостатком можно считать громоздкость и большую массу.