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. ELECTROMECHANICAL   ELEVATORS Electromechanical elevators are used for freight, bombs, and stores. In this type of elevator, the platform is raised and lowered by one or more wire ropes that pass  over  pulleys  and  wind  or  unwind  on  hoisting drums. Hoisting drums are driven through a reduction gear unit by an electric motor. An electric brake stops and holds the platform. The motor has two speeds, full speed   and   low,   or   one-sixth,   speed.   Control arrangements allow the elevator to start and run on high speed. Low speed is used for automatic deceleration as the elevator approaches the selected level. The platform travels  on  two  or  four  guides.  Hand-operated  or power-operated  lock  bars,  equipped  with  electrical interlocks, hold the platform in position. LUBRICATING  SYSTEMS Most  equipment  is  provided  with  a  lubricating system that supplies oil under pressure to the bearings. The system consists of a sump or reservoir for storing the oil, an oil pump, a strainer, a cooler, temperature and pressure gauges, and the necessary piping to carry the oil to the bearings and back to the sump. The location and arrangement of these parts vary with each piece of equipment. This system allows the lube-oil system to perform  the  following  functions: l Supply lubrication to the bearings . Cool the bearings . Flush any wear products from the bearings The lube-oil pump is generally a gear-type pump. A definite pressure is maintained in the oil feed lines. A pressure  relief  valve  allows  excess  oil  to  recirculate  to the suction side of the pump. Quite often, dual strainers are connected in the line so that the system can operate on one strainer while the other one is being cleaned. The tube-in-shell type of cooler  is  generally  used  with  seawater  circulating through the tubes and the oil flowing around them. The temperature of the oil is controlled by adjusting the valve that regulates the amount of seawater flowing through the tubes. Oil  must  be  supplied  to  the  bearings  at  the prescribed pressure and within certain temperature limits. A pressure gauge installed in the feed line and a thermometer installed in the return line indicate oil system  functioning.  Thermometers  are  often  installed  in the bearings to serve as a warning against overheating. If there is a decided drop in oil pressure, shut down the equipment immediately. You should investigate even a moderate rise in the oil temperature. An oil-level float gauge indicates the amount of oil in the sump. Some bearings do not require a lot of cooling or flushing of 10-48