The variable-stroke axial-piston pump usually has either seven or nine single-acting pistons which are evenly spaced around a cylinder barrel. (Note that the term CYLINDER BARREL, as used here, actually refers to a cylinder block that holds all the cylinders.) An uneven number of pistons is always used so that pulsations in the discharge flow can be avoided. The piston rods make a ball-and- socket connection with a socket ring. The socket ring rides on a thrust bearing carried by a casting called the TILTING BOX or TILTING BLOCK.

When the tilting box is at a right angle to the shaft, and the pump is rotating, the pistons do not reciprocate; therefore, no pumping takes place. When the box is tilted away from a right angle, however, the pistons reciprocate and the liquid is pumped.

The variable-stroke axial-piston pump is often used as a part of a variable-speed gear, for for-ward and reverse rotation, such as electro-hydraulic anchor windlasses, cranes, winches, and the power transmitting unit in electrohydraulic steering engines. In those cases, the tilting box is arranged so that it may be tilted in either direc-tion. Thus it may be used to transmit power hydraulically to pistons or rams, or it may be used to drive a hydraulic motor. In the latter use, the pump is driven by a constant-speed electric motor and is called the A-end of the variable-speed gear. The hydraulic motor is called the B-end.

The B-end 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 perma-nently fixed angle. Thus, the B-end becomes a fixed-stroke axial-piston pump. Figure 13-13 il-lustrates an axial-piston hydraulic gear with the A-end and B-end as a single unit. It is used in gun turrets for horizontal and vertical drive and for 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 causes the piston to be pushed out. In being pushed out, the piston, through its connect-ing rod, will seek the point of greatest distance between the cylinder barrel 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 cylinder 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 to the suction inlet of the pump, and from there to suction pistons of the pump. The pump is constantly putting pressure on one side of the motor while it is con-stantly receiving hydraulic fluid from the other side. The fluid is merely circulated from pump to motor and back again.