A positive displacement hydraulic pump (hereinafter referred to as a positive displacement pump) is a power component of a hydraulic system that relies on changes in the volume of a sealed working chamber to achieve oil suction and pressure. It is widely used in the field of fluid dynamics and motion control. Its rotating and stationary parts have extremely small clearances, and it can deliver a constant volume of oil to the system per revolution of the pump shaft.
Polyposition pumps can be further divided into two main categories: fixed displacement pumps and variable displacement pumps. Fixed displacement pumps have a fixed displacement per revolution; variable displacement pumps have a single-cycle displacement that can be steplessly adjusted from zero to the maximum theoretical displacement. Common positive displacement pumps include gear pumps, piston pumps, and vane pumps.
1.Gear pump
（1）External gear pump
External gear pumps, as one of the most commonly used pump types in modern hydraulic systems, rely on the change in volume between the teeth of two meshing gears to transport oil. They feature a simple structure, low procurement and maintenance costs, can adapt to heavy-duty conditions and achieve high-density energy transfer, and possess excellent hydraulic, mechanical, and volumetric efficiency. They are also characterized by a compact structure and high power density. This balance between efficiency and economy makes them a popular choice for auxiliary hydraulic systems in various mechanical equipment.
External gear pumps can use spur gears (most common), helical gears, or herringbone gears. During operation, the driving and driven gears rotate synchronously, creating a local vacuum in the pump’s suction zone (the area where the gears disengage, the low-pressure zone) to draw in oil. In the discharge zone (high-pressure zone), the gear teeth re-mesh, stably delivering oil to the system.
（2）Internal gear pump
Internal gear pumps employ an internal gear driving an external gear, and their working principle is similar to that of external gear pumps. In traditional designs, the internal gear has one less tooth than the external gear, creating a dynamic sealing point on the tooth surface during meshing. Due to its longer high-low pressure transition zone (crescent plate area), its noise level is lower than some other pump types.
The gears in gear pumps are mostly made of special alloy steel, carburized, quenched, and precision ground. A reasonable tooth profile design and geometric ratio can effectively reduce flow pulsation and noise during pump operation.
2.Plunger pump
（1）Axial piston pump
Axial piston pumps convert the rotational motion of the main shaft into axial reciprocating motion through a piston assembly: the swashplate rotates, driving the pistons to reciprocate within the cylinder, and oil is drawn in and discharged through the suction and discharge windows of the distributor plate. The swashplate-type structure eliminates the swashplate; the main shaft directly drives the piston assembly to rotate, and displacement is adjusted by changing the cylinder’s tilt angle.
Due to the longer sealing path along the piston wall, it has higher volumetric efficiency. Furthermore, variable displacement pumps can precisely adjust the output flow rate according to system requirements, achieving cost reduction and energy saving.
（2）Radial Piston Pump
Radial piston pumps (fixed displacement type) are particularly suitable for high-pressure, low-flow applications, with rated pressures up to 35 MPa. Although continuous variable displacement is not possible, through special design, each piston can be individually controlled, thus achieving a function similar to a variable displacement pump.
Radial piston pumps employ a cylinder structure with pistons arranged radially, achieving oil delivery through the reciprocating motion of the pistons. Structurally, they can be divided into piston type and ball piston type; according to the flow distribution method, they can be divided into valve-type flow distribution (cam-driven pistons) and shaft-type flow distribution (cylinder rotation, piston ends contacting eccentric stator rings).
3.Vane pump
Vane pumps (fixed displacement – typically double-acting and simple variable displacement – typically single-acting) generally offer higher efficiency and lower noise than gear pumps. They are suitable for medium-pressure systems up to 21 MPa, and newer vane pumps can exceed 31.5 MPa under continuous operation.
Some vane pumps can achieve variable displacement by adjusting the stator eccentricity. These variable displacement pumps are mostly for constant pressure or constant power control: when the output pressure or power reaches the set value, the displacement automatically decreases until the system is balanced.
The core design principle of vane pumps lies in the reliable sealing between the vanes and the stator’s internal curves. Common vane tips feature multi-chamfered structures to ensure sealing while reducing wear. Vane clamping methods include spring clamping and hydraulic self-tightening designs.
