Electrohydraulic  equipment  for  the  crane  consists of one or more electric motors running at constant speed. E a c h    m o t o r    d r i v e s    o n e    o r    m o r e    A - e nd variable-displacement  hydraulic  pumps.  The  pump strokes  are  controlled  through  operating  handwheels. START,  STOP,  and  EMERGENCY  RUN  pushbuttons at the operator’s station control the electric motors. Interlocks prevent starting the electric motors when the hydraulic pumps are on stroke. B-end hydraulic motors are connected to the A-end pumps by piping. They drive the  drums  of  the  hoisting  and  topping  units  or  the rotating  machinery. Reduction gears are located between the electric motor and the A-end pump and between the B-end hydraulic motor and the rotating pinion. Each hoisting, topping, and rotating drive has an electric brake on the hydraulic motor output shaft. This brake is interlocked with the hydraulic pump control. It will set when the hydraulic control is on neutral or when electric power is lost. A centering device is used to find and retain the neutral  position  of  the  hydraulic  pump. Relief valves protect the hydraulic system. These valves are set according to the requirements of chapter 556 of the NSTM. Cranes usually have a rapid slack take-up device consisting of an electric torque motor. This motor is connected to the hoist drum through reduction gearing. This device works in conjunction with the pressure stroke  control  on  the  hydraulic  pump.  It  provides  fast acceleration of the hook in the hoisting direction under light  hook  conditions.  Thus,  slack  in  the  cable  is prevented  when  hoisting  is  started. Some cranes have a light-hook paying-out device mounted on the end of the boom. It pays out the heisting cables when the weight of the hook and cable beyond the boom-head sheave is insufficient to overhaul the cable as fast as it is unreeled from the hoisting drum. When the mechanical hoist control is in neutral, the torque motor is not energized and the cable is gripped lightly by the action of a spring. Moving the hoist control to LOWER energizes the torque motor. The sheaves clamp and pay out the cable as it is unreeled from the hoist drum. When the hoist control is moved to HOIST, the torque motor is reversed and unclamps the sheaves. Alimit  switch  opens  and  automatically  de-energizes  the paying-out  device. Maintain cranes according to the PMS requirements or the manufactured instructions. Keep the oil in the replenishing tanks at the prescribed levels. Keep the system clean and free of air. Check the limit stop and other mechanical safety devices regularly for proper operation. When cranes are not in use, secure them in their stowed positions. Secure all electric power to the controllers. ELECTROHYDRAULIC   ELEVATORS 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. 2. 3. 4. 5. 6. 7. 8. 9. A horizontal plunger-type hydraulic engine Multiple  variable-delivery  parallel  piston-type pumps Two  high-pressure  tanks One low-pressure tank A  sump  tank  system Two  constant-delivery  vane-type  pumps  (sump pumps) An oil storage tank A piping system and valves 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 10-47