activation of trailing edge flaps, leading edge slats, and  stabilizer trim. Some equipment uses a variable-displacement  piston motor where very wide speed ranges are desired.  Although some piston-type motors are controlled by  directional control valves, they are often used in combination  with variable-displacement pumps. This pump-motor  combination is used to provide a transfer of power between  a driving element and a driven element. Some applications  for which hydraulic transmissions may be used are speed  reducers, variable speed drives, constant speed or constant  torque drives, and torque converters.  Some advantages of hydraulic transmission of power over  mechanical transmission of power are as follows: The RAT is installed in the aircraft to provide electrical and  hydraulic power if the primary sources of aircraft power are  lost. Ram air is used to turn the blades of a turbine that, in turn,  operates a hydraulic pump and generator. The turbine and  Besides having the ability to move a load into position, a  double-acting cylinder also has the ability to hold a load in  position. This capability exists because when the selector  valve used to control operation of the actuating cylinder is  placed in the off position, fluid is trapped in the chambers on  both sides of the actuating cylinder piston. Internal locking  actuators also are used in some applications. Rotary actuators can mount right at the part without taking up  the long stroke lengths required of cylinders. Rotary actuators  are not limited to the 90° pivot arc typical of cylinders; they  can achieve arc lengths of 180°, 360°, or even 720° or more,  depending on the configuration. An often-used type of rotary  actuator is the rack and pinion actuator used for many nose  wheel steering mechanisms. In a rack-and-pinion actuator,  a long piston with one side machined into a rack engages a  pinion to turn the output shaft.  [Figure 12-54]  One side of the  piston receive fluid pressure while the other side is connected  to the return. When the piston moves, it rotates the pinion. Piston-type motors are the most commonly used in hydraulic  systems.  [Figure 12-55]  They are basically the same as  hydraulic pumps except they are used to convert hydraulic  energy into mechanical (rotary) energy. Hydraulic motors  are either of the axial inline or bent-axis type. The most  commonly used hydraulic motor is the fixed-displacement  bent-axis type. These types of motors are used for the  12-34 Figure 12-54.  Rack and pinion gear. Figure 12-53.  Linear actuator operation. Ram Air Turbine (RAT)  Rotary Actuators Hydraulic Motor  •  Quick, easy speed adjustment over a wide range while  the power source is operating at a constant (most  efficient) speed  •  Rapid, smooth acceleration or deceleration •  Control over maximum torque and power •  Cushioning effect to reduce shock loads •  Smoother reversal of motion R P R P Hoses Actuating cylinder Selector valve On/Extend Retract A B Piston Fluid pressure and return Piston Cylinder Actuator Rack Pinion gear Rack and pinion