Some Key Points About Plunger Pumps

　　 Plunger pumps are critical components in hydraulic systems. They operate by the reciprocating motion of plungers within pump cylinders, which periodically changes the volume between the plunger and the cylinder wall, thereby repeatedly drawing in and discharging fluid to build up system pressure. Based on the orientation of the plungers, plunger pumps are classified as axial-plunger pumps and radial-plunger pumps. These pumps offer advantages such as high flow output, low pressure pulsation, stable operation, and easy maintenance, and can be configured as variable-displacement units. Owing to their excellent performance, plunger pumps are widely used in applications requiring high pressure, large flow rates, and precise flow regulation, including construction machinery, aerospace, and defense industries. However, they have a complex structure, high manufacturing costs, stringent requirements for oil cleanliness, poor self-priming capability, and demanding operational and maintenance standards.

　　An axial piston pump is a type of hydraulic pump in which the pistons are arranged circumferentially around the cylinder block, with the axis of the pump and the cylinder block either parallel to the central axis or inclined at a certain angle. Depending on the method of fluid distribution, it can be classified into valve-plate-type and valve-type distribution valves. Furthermore, based on differences in the drive mechanism and structural features, the flow-control valves in piston pumps can be categorized as swashplate-type and tilted-axis-type.

　　Structure and Operating Principle of the Swash-Plate Axial Piston Pump When the swash plate and valve plate are fixed, the motor drives the drive shaft, cylinder block, and pistons to rotate. The spring tails, piston ball heads, and the working surface of the swash plate maintain close contact at all times with the valve plate and the bore walls of the piston cylinders, thereby forming a sealed chamber. As the drive shaft rotates, the piston top—shown in the figure—constrains the right side of the swash plate to move toward the closed position, where the volume gradually decreases; oil is then pressurized by the valve plate and enters the hydraulic system, completing this process. Meanwhile, the lower end of the piston slides along the inclined surface of the swash plate, gradually disengaging from the pressure chamber and allowing oil to enter the cylinder bore. Under atmospheric pressure, the oil is drawn in.

　　The primary performance characteristic of an axial piston pump is that its flow rate gradually declines until it falls below the minimum operational threshold, or it may fail abruptly. This typically results from one of the following factors: interference between the drive shaft and the bearings; increased clearance in the piston bores; piston wear and jamming; failure of the check valve seal; breakage of the internal relief spring; or complete immobilization of the pistons.

　　Although the overall efficiency of a swashplate piston pump is slightly higher than that of a bent-axis piston pump, the swashplate pump has a larger footprint and achieves flow regulation by using a swing cylinder to vary the angle between the cylinder axis and the drive shaft axis. Due to its large inertial mass and slow dynamic response, the swashplate pump is widely used in fuel-handling systems.

　　Causes of Excessive Operating Noise in Plunger Pumps

　　(1) Air is present in the oil pump. This fault typically occurs when a new pump is installed. Upon startup of the new pump, oil should first be added to the pump to lubricate its bearings, plungers, and cylinders.

　　(2) The oil level in the tank is too low, and blockage of the suction pipe increases the pump’s suction resistance, leading to cavitation or air leaks in the suction line, causing the pump to ingest air.

　　(3) Improper installation of the oil pump and motor—specifically, misalignment between the pump shaft and the motor shaft—results in radial forces on the oil pump bearings, causing noise.

　　(4) Excessive hydraulic oil viscosity reduces the pump’s self-priming capability and volumetric efficiency.