Radial-Piston Motorcontinues as long as fluid under pressure entersthe cylinders.The radial-piston motor operates in reverse ofthe radial-piston pump. In the radial-piston pump,as the cylinder block rotates, the pistons pressagainst the rotor and are forced in and out of thecylinders, thereby receiving fluid and pushing itout into the system. In the radial motor, fluid isforced into the cylinders and drives the pistonsoutward. The pistons pushing against the rotorcause the cylinder block to rotate.The operation of a radial-piston motor isshown in figure 10-15. This motor is shown withthree pistons for simplicity. Normally it containsseven or nine pistons. When liquid is forced intothe cylinder bore containing piston 1, the pistonmoves outward since the liquid cannot becompressed. This causes the cylinder to rotate ina clockwise direction. As the force acting onpiston 1 causes the cylinder block to rotate, piston2 starts to rotate and approach the position ofpiston 3. (Note that the distance between thecylinder block and the reaction ring of the rotorgets progressively shorter on the top and right halfof the rotor.)As piston 2 rotates, it is forced inward and,in turn, forces the fluid out of the cylinder. Sincethere is little or no pressure on this side of thepintle valve, the piston is easily moved in by itscontact with the reaction ring of the rotor. Thefluid is easily forced out of the cylinder and backto the reservoir or to the inlet side of the pump.As the piston moves past the midpoint, or pastthe shortest distance between the cylinder blockand the rotor, it enters the pressure side of thepintle valve and fluid is forced into the cylinder.Piston 3 then becomes the pushing piston and inturn rotates the cylinder block. This actionFigure 10-15.—Operation of a radial-piston motor.The direction of rotation of the motor (fig.10-15) is changed by reversing the flow of fluidto it. Admitting fluid under pressure on the topside of the pintle valve forces piston 3 out of thecylinder block. This causes the cylinder to rotatein the counterclockwise direction.Axial-Piston MotorThe variable-stroke axial-piston pump is oftenused as a part of variable speed gear, such aselectrohydraulic anchor windlasses, cranes,winches, and the power transmitting unit inelectrohydraulic steering engines. In those cases,the tilting box is arranged so that it maybe tiltedin either direction. Thus it maybe used to transmitbidirectional power hydraulically to pistons orrams, or it may be used to drive a hydraulicmotor. In the latter use, the pump is the A-endof the variable speed gear and the hydraulic motoris the B-end.The B-end of the hydraulic unit of thehydraulic speed gear is exactly the same as theA-end of the variable-stroke pump mentionedpreviously. However, it generally does not havea variable-stroke feature. The tilting box isinstalled at a permanently fixed angle. Thus, theB-end becomes a fixed-stroke axial-piston motor.Figure 10-16 illustrates an axial-piston hydraulicspeed gear with the A-end and B-end as a singleunit. It is used in turrets for train and elevationdriving units. For electrohydraulic winches andcranes, the A-end and B-end are in separatehousings connected by hydraulic piping.Hydraulic fluid introduced under pressure toa cylinder (B-end) tries to push the piston out ofthe cylinder. In being pushed out, the piston,through its piston rod, will seek the point ofgreatest distance between the top of the cylinderand the socket ring. The resultant pressure of thepiston against the socket ring will cause thecylinder barrel and the socket ring to rotate. Thisaction occurs during the half revolution while thepiston is passing the intake port of the motor,which is connected to the pressure port of thepump. After the piston of the motor has takenall the hydraulic fluid it can from the pump, thepiston passes the valve plate land and starts todischarge oil through the outlet ports of the motor10-10
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