Radial-Piston  Motor

Radial-Piston  Motor continues as long as fluid under pressure enters the cylinders. The radial-piston motor operates in reverse of the radial-piston pump. In the radial-piston pump, as  the  cylinder  block  rotates,  the  pistons  press against the rotor and are forced in and out of the cylinders, thereby receiving fluid and pushing it out into the system. In the radial motor, fluid is forced  into  the  cylinders  and  drives  the  pistons outward.  The  pistons  pushing  against  the  rotor cause  the  cylinder  block  to  rotate. The   operation   of   a   radial-piston   motor   is shown in figure 10-15. This motor is shown with three pistons for simplicity. Normally it contains seven or nine pistons. When liquid is forced into the cylinder bore containing piston 1, the piston moves  outward  since  the  liquid  cannot  be compressed. This causes the cylinder to rotate in a   clockwise   direction.   As   the   force   acting   on piston  1  causes  the  cylinder  block  to  rotate,  piston 2  starts  to  rotate  and  approach  the  position  of piston  3.  (Note  that  the  distance  between  the cylinder block and the reaction ring of the rotor gets progressively shorter on the top and right half of  the  rotor.) As  piston  2  rotates,  it  is  forced  inward  and, in turn, forces the fluid out of the cylinder. Since there  is  little  or  no  pressure  on  this  side  of  the pintle valve, the piston is easily moved in by its contact  with  the  reaction  ring  of  the  rotor.  The fluid is easily forced out of the cylinder and back to the reservoir or to the inlet side of the pump. As  the  piston  moves  past  the  midpoint,  or  past the shortest distance between the cylinder block and the rotor, it enters the pressure side of the pintle valve and fluid is forced into the cylinder. Piston 3 then becomes the pushing piston and in turn  rotates  the  cylinder  block.  This  action Figure 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 fluid to it. Admitting fluid under pressure on the top side of the pintle valve forces piston 3 out of the cylinder block. This causes the cylinder to rotate in  the  counterclockwise  direction. Axial-Piston Motor The  variable-stroke  axial-piston  pump  is  often used  as  a  part  of  variable  speed  gear,  such  as electrohydraulic   anchor   windlasses,   cranes, winches,   and   the   power   transmitting   unit   in electrohydraulic steering engines. In those cases, the tilting box is arranged so that it maybe tilted in either direction. Thus it maybe used to transmit bidirectional  power  hydraulically  to  pistons  or rams,  or  it  may  be  used  to  drive  a  hydraulic motor. In the latter use, the pump is the A-end of the variable speed gear and the hydraulic motor is the B-end. The   B-end   of   the   hydraulic   unit   of   the hydraulic speed gear is exactly the same as the A-end  of  the  variable-stroke  pump  mentioned previously.  However,  it  generally  does  not  have a  variable-stroke  feature.  The  tilting  box  is installed at a permanently fixed angle. Thus, the B-end becomes a fixed-stroke axial-piston motor. Figure  10-16  illustrates  an  axial-piston  hydraulic speed gear with the A-end and B-end as a single unit. It is used in turrets for train and elevation driving  units.  For  electrohydraulic  winches  and cranes,  the  A-end  and  B-end  are  in  separate housings  connected  by  hydraulic  piping. Hydraulic fluid introduced under pressure to a cylinder (B-end) tries to push the piston out of the  cylinder.  In  being  pushed  out,  the  piston, through  its  piston  rod,  will  seek  the  point  of greatest distance between the top of the cylinder and  the  socket  ring.  The  resultant  pressure  of  the piston  against  the  socket  ring  will  cause  the cylinder  barrel  and  the  socket  ring  to  rotate.  This action  occurs  during  the  half  revolution  while  the piston  is  passing  the  intake  port  of  the  motor, which  is  connected  to  the  pressure  port  of  the pump.  After  the  piston  of  the  motor  has  taken all the hydraulic fluid it can from the pump, the piston passes the valve plate land and starts to discharge oil through the outlet ports of the motor 10-10


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