Some of the hydraulic equipment that you maintain is found in electrohydraulic elevator installations. Modern carriers use elevators of this type. The elevators described in this chapter are now in service in some of the ships of the CV class. These ships are equipped with four, deck-edge airplane elevators having a maximum lift capacity of 79,000 to 105,000 pounds. The cable lift platform of each elevator projects over the side of the ship and is operated by an electrohydraulic plant.

Electrohydraulic Power Plant

The electrohydraulic power plant for the elevators consists of the following components:

1. A horizontal plunger-type hydraulic engine

2. Multiple variable-delivery parallel piston-type pumps

3. Two high-pressure tanks

4. One low-pressure tank

5. A sump tank system

6. Two constant-delivery vane-type pumps (sump pumps)

7. An oil storage tank

8. A piping system and valves 9. A nitrogen supply

The hydraulic engine is operated by pressure developed in a closed hydraulic system. Oil is supplied to the system in sufficient quantity to cover the baffle plates in the high-pressure tanks and allow for piston displacement. Nitrogen is used because air and oil in contact under high pressure form an explosive mixture. Air should not be used except in an emergency. Nitrogen, when used, should be kept at 97 percent purity.

The hydraulic engine has a balanced piston-type valve with control orifices and a differential control unit. This control assembly is actuated by an electric motor and can be operated by hand. To raise the elevator, move the valve off center to allow high-pressure oil to enter the cylinder. High-pressure oil entering the cylinder moves the ram. The ram works through a system of cables and sheaves to move the platform upward. The speed of the elevator is controlled by the amount of pressure in the high-pressure tank and the control valve.

When the elevator starts upward, the pressure in the high-pressure tank drops. The pressure drop automatically starts the main pumps. These pumps transfer oil from the low-pressure tank to the high-pressure system until the pressure is restored. An electrical stopping device automatically limits the stroke of the ram and stops the platform at the proper position at the flight deck level.

To lower the elevator, move the control valve in the opposite direction. This lets oil in the cylinder flow into the exhaust tank. As the platform descends, oil is discharged to the low-pressure tank (exhaust tank). The original oil levels and pressures, except for leaks, are reestablished. The lowering speed is controlled by the control valve and the cushioning effect of the pressure in the exhaust tank. Leak is drained to the sump tanks. It is then automatically transferred to the pressure system by the sump pumps. An electrically operated stopping device automatically slows down the ram and stops the platform at its lower level (hangar deck).

Safety Features

The following list contains some of the major safety features incorporated into modem deck-edge elevators:

1. If the electrical power fails while the platform is at the hangar deck, there will be enough pressure in the system to move the platform to the flight deck one time without the pumps running.

2. Some platforms have serrated safety shoes. If all the hoisting cable should break on one side, the shoes will wedge the platform between the guide rails. This will stop the platform with minimum damage.

3. A main pump may have a pressure-actuated switch to stop the pump motors when the discharge pressure is excessive. They may also have to relieve the pressure when the pressure switch fails to operate.

4. The sump pump system has enough capacity to return the unloaded platform from the hangar deck to the flight deck.

5. The oil filter system maybe used continuously while the engine is running. This allows part of the oil to be cleaned with each operation of the elevater.