Figure 10-16.—Exploded view of a axial-piston hydraulic speed gear.to the suction pistons of the pump. The pump isconstantly putting pressure on one side of themotor and receiving hydraulic fluid from the otherside. The fluid is merely circulated from pumpto motor and back again.Both of the axial-piston motors described inthis section may be operated in either direction.The direction of rotation is controlled by thedirection of fluid flow to the valve plate. Thedirection of flow may be instantly reversedwithout damage to the motor.TURBINESTurbines are used in pneumatic systems toconvert kinetic energy of gases to mechanicalenergy. Turbines are used to drive electricgenerators, to convert mechanical energy intoelectrical energy, and to drive pumps to supplyfluid flow in hydraulic systems.The basic parts of a turbine are the rotor,which has blades projecting radially from itsperiphery; and nozzles, through which the gas isexpanded and directed. The conversion of kineticenergy to mechanical energy occurs on the blades.The basic distinction between types of turbinesis the manner in which the gas causes the turbinerotor to move. When the rotor is moved by adirect push or “impulse” from the gas impingingupon the blades, the turbine is said to be animpulse turbine. When the rotor is moved by forceof reaction, the turbine is said to be a reactionturbine.Although the distinction between impulseturbines and reaction turbines is a useful one,it should not be considered as an absolutedistinction in real turbines. An impulse turbineuses both the impulse of the gas jet and,to a lesser extent, the reactive force that resultswhen the curved blades cause the gas to changedirection. A reaction turbine is moved primarilyby reactive force, but some motion of the rotoris caused by the impact of the gas against theblades.IMPULSE TURBINEThe impulse turbine consists essentially of arotor mounted on a shaft that is free to rotate ina set of bearings. The outer rim of the rotor carriesa set of curved blades, and the whole assemblyis enclosed in an airtight case. Nozzles direct the10-11
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