The Piston Pump Manufacturers describes that an axial piston pump has a plurality of pistons (usually odd numbers) that are arranged in a circular array within a housing, commonly referred to as a cylinder block, rotor or barrel. The cylinder block is driven about its axis of symmetry by an integral shaft that is more or less aligned (usually parallel but not necessarily) with the pumping piston.

Mating surfaces. One end of the cylinder block is convex and wears on a mating surface of the fixed valve plate. The inlet and outlet fluids of the pump pass through different portions of the sliding interface between the cylinder block and the valve plate. The valve plate has two semi-circular ports that allow the entry and exit fluids of the working fluid to escape.

Highlight the piston. The pumping piston projects from the opposite end of the cylinder block. There are many configurations for the exposed ends of the pistons, but in all cases they rest against the cam. In a variable displacement unit, the cam is movable and is commonly referred to as a swash plate, yoke or suspension. For conceptual purposes, the cam may be represented by a plane that combines with the rotation of the shaft to provide a cam action that causes the piston to reciprocate and thus pump. The angle between the vector perpendicular to the cam plane and the axis of rotation of the cylinder block (referred to as the cam angle) is a variable that determines the displacement of the pump or the amount of fluid that is rotated per revolution. The variable displacement unit is capable of changing the cam angle during operation, while the fixed displacement unit is not.

Reciprocating piston. When the cylinder block rotates, the exposed end of the piston is constrained to follow the surface of the cam plane. Since the cam plane is at an angle to the axis of rotation, the piston must reciprocate axially as the piston is advanced about the axis of the cylinder block. The axial movement of the piston is sinusoidal. During the rising portion of the piston reciprocating cycle, the piston moves toward the valve plate. In addition, during this time, the fluid sandwiched between the buried end of the piston and the valve plate is discharged to the discharge port of the pump through a semi-circular port-discharge port of the valve plate. As the piston moves toward the valve plate, fluid is pushed or displaced through the discharge port of the valve plate.

The impact of precession. When the piston is at the top of the reciprocating cycle (commonly referred to as top dead center or only TDC), the connection between the trapped fluid chamber and the discharge port of the pump is closed. Shortly thereafter, the same chamber was opened to the inlet of the pump. As the piston continues to move about the cylinder block axis, it moves away from the valve plate, thereby increasing the volume of the trapping chamber. When this occurs, fluid enters the chamber from the inlet of the pump to fill the gap. This process continues until the piston reaches the bottom of the reciprocating cylinder - commonly referred to as bottom dead center or BDC. At the BDC, the connection between the pump chamber and the inlet is closed. Shortly thereafter, the chamber was again opened to the discharge port and the pumping cycle was restarted.

Variable displacement. In a variable displacement pump, if the vector perpendicular to the cam plane (swash plate) is set parallel to the axis of rotation, the piston does not move in its cylinder. Therefore there is no output. The motion of the swashplate controls the pump output from zero to maximum. There are two types of variable axial piston pumps:

Direct displacement control pump, a direct displacement controlled axial piston pump. The direct displacement control uses a mechanical lever that is connected to the swashplate of the axial piston pump. Higher system pressures require more force to move the lever, making direct displacement control available only for light or medium pumps. Heavy duty pumps require servo control. The direct displacement control pump consists of a connecting rod and spring, and in some cases also a magnet instead of a shaft to the motor located outside the pump (thus reducing the number of moving parts), protecting the parts from protection and lubrication, and reducing the pair Resistance to liquid flow.

Servo control pump.

pressure. In a typical pressure compensated pump, the swashplate angle is adjusted by the action of a valve using pressure feedback such that the instantaneous pump output flow is just sufficient to maintain the specified pressure. If the load flow increases, the pressure will temporarily decrease, but the pressure compensating valve will detect a decrease and then increase the swashplate angle to increase the pump output flow to restore the required pressure. In fact, most systems use pressure as the control for this pump. With a working pressure of 200 bar (20 MPa or 2900 psi), the swashplate is driven to zero angle (the piston stroke is almost zero) and the inherent leaks in the system allow the pump to stabilize at the set delivery pressure. As demand increases, the swashplate moves to a greater angle, the piston stroke increases, and the fluid volume increases; if demand decreases, the pressure will rise and as the pressure rises, the pumping volume will decrease. At maximum system pressure, the output is again almost zero. If the fluid demand increases beyond the pump delivery capacity, the system pressure will drop to near zero. The swashplate angle will remain at the maximum allowed and the piston will run at full stroke. This situation continues until the system flow demand is reduced and the pump capacity is greater than the demand. As the pressure increases, the swashplate angle is adjusted to attempt to meet the flow demand while not exceeding the maximum pressure.

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