by these pistons. The continuous and rapid action of the  pistons is overlapping in nature and results in a practically  nonpulsating pump output. The simplest type of axial piston pump is the swash plate  design in which a cylinder block is turned by the drive shaft.  Pistons fitted to bores in the cylinder block are connected  through piston shoes and a retracting ring so that the shoes  bear against an angled swash plate. As the block turns, the  piston shoes follow the swash plate, causing the pistons to  reciprocate. The ports are arranged in the valve plate so  that the pistons pass the inlet as they are pulled out, and  pass the outlet as they are forced back in. In these pumps,  displacement is determined by the size and number of pistons,  as well as their stroke length, which varies with the swash  plate angle. This constant-displacement pump is illustrated  in  Figure 12-26 . The vane-type power pump is also a constant-displacement  pump. It consists of a housing containing four vanes (blades),  a hollow steel rotor with slots for the vanes, and a coupling  to turn the rotor.  [Figure 12-28]  The rotor is positioned off  center within the sleeve. The vanes, which are mounted in  the slots in the rotor, together with the rotor, divide the bore  of the sleeve into four sections. As the rotor turns, each  section passes one point where its volume is at a minimum  and another point where its volume is at a maximum. The  volume gradually increases from minimum to maximum  during the first half of a revolution and gradually decreases  from maximum to minimum during the second half of the  revolution. As the volume of a given section increases, that  section is connected to the pump inlet port through a slot in the sleeve. Since a partial vacuum is produced by the increase  in volume of the section, fluid is drawn into the section  through the pump inlet port and the slot in the sleeve. As the  rotor turns through the second half of the revolution and the  volume of the given section is decreasing, fluid is displaced  out of the section, through the slot in the sleeve aligned with  the outlet port, and out of the pump. A variable-displacement pump has a fluid output that is varied  to meet the pressure demands of the system. The pump output  is changed automatically by a pump compensator within  the pump. The following paragraph discusses a two-stage  Vickers variable displacement pump. The first stage of the  pump consists of a centrifugal pump that boosts the pressure  before the fluid enters the piston pump.  [Figure 12-29] With each revolution of the drive shaft and cylinder block,  each piston goes through the pumping cycle described  above, completing one intake and one discharge stroke.  High-pressure fluid is ported out through the valve plate, past  the blocking valve, to the pump outlet. The blocking valve  is designed to remain open during normal pump operation.  Internal leakage keeps the pump housing filled with fluid  for lubrication of rotating parts and cooling. The leakage is  returned to the system through a case drain port. The case  valve relief valve protects the pump against excessive case  pressure, relieving it to the pump inlet. 12-20 Variable-Displacement Pump  The fluid contained in the bore is precompressed, then  expelled through the outlet port. Discharge pressure holds  the piston shoe against the yoke bearing surface during  the discharge stroke and also provides the shoe pressure  balance and fluid film through an orifice in the piston and  shoe subassembly. As the piston begins to withdraw from the cylinder block,  system inlet pressure forces fluid through a porting  arrangement in the valve plate into the cylinder bore. The  piston shoes are restrained in the yoke by a piston shoe  retaining plate and a shoe plate during the intake stroke.  As the drive shaft continues to turn the cylinder block, the  piston shoe continues following the yoke bearing surface.  This begins to return the piston into its bore (i.e., toward the  valve block). The aircraft’s engine rotates the pump drive shaft, cylinder  block, and pistons via a gearbox. Pumping action is generated  by piston shoes that are restrained and slide on the shoe  bearing plate in the yoke assembly. Because the yoke is at  an angle to the drive shaft, the rotary motion of the shaft is  converted to piston reciprocating motion. Vane Pump Basic Pumping Operation Figure 12-28.  Vane-type power pump. Inline Piston Pump Sleeve Vanes Rotor Inlet port Outlet port