the upper face of the drive shaft plate is at a maximum. At any  given moment of operation, three of the pistons are moving  away from the top face of the cylinder block, producing a  partial vacuum in the bores in which these pistons operate.  This occurs over the inlet port, so fluid is drawn into these  bores at this time. On the opposite side of the cylinder block,  three different pistons are moving toward the top face of the  block. This occurs while the rotating group is passing over  the outlet port causing fluid to be expelled from the pump  A typical constant-displacement axial-type pump is shown  in  Figure 12-27.  The angular housing of the pump causes  a corresponding angle to exist between the cylinder block  and the drive shaft plate to which the pistons are attached.  It is this angular configuration of the pump that causes the  pistons to stroke as the pump shaft is turned. When the pump  operates, all parts within the pump (except the outer races of  the bearings that support the drive shaft, the cylinder bearing  pin on which the cylinder block turns, and the oil seal) turn  together as a rotating group. At one point of rotation of the  rotating group, a minimum distance exists between the top of  the cylinder block and the upper face of the drive shaft plate.  Because of the angled housing at a point of rotation 180°  away, the distance between the top of the cylinder block and  cylinder block, a piston for each bore, and a valve plate with  inlet and outlet slots. The purpose of the valve plate slots is  to let fluid into and out of the bores as the pump operates.  The cylinder bores lie parallel to and symmetrically around  the pump axis. All aircraft axial-piston pumps have an odd  number of pistons.  [Figure 12-26] Figure 12-26.  Hydraulic pump shear shaft. Figure 12-27.  Bent axis piston pump. 12-19 Bent Axis Piston Pump Drive shaft plate Seal Coupling shaft Universal link Piston A Cylinder bearing pin Cylinder block Valve plate Piston B Inlet port Outlet port Drain port Thrust bearing Radial bearing Radial bearing Splines Shearing point Pump side Engine side