The hydraulic gear pump is a common type of positive-displacement pump found in hydraulic systems; its primary function is to convert mechanical energy into hydraulic energy. Its operating principle is based on the periodic variation of sealed volumes generated by the meshing of gears: as a pair of gears rotates within the pump housing, the gear teeth—together with the inner surface of the housing and the front and rear end covers—collectively form a series of enclosed working chambers. On the side where the gear teeth disengage, the volume of the working chambers gradually increases, creating a localized vacuum that draws in the hydraulic fluid. As the gears continue to rotate, the fluid is carried to the opposite side; when the gear teeth subsequently engage, the volume of the working chambers decreases, thereby displacing the fluid and delivering it into the system piping.
1.Main Classifications
（1）External gear pump：It consists of two meshing external gears, has a simple structure, and is the most widely used.
（2）Internal gear pump：It consists of an internal gear ring (outer rotor) and an external gear (inner rotor) that mesh eccentrically within the pump chamber, resulting in a compact structure and smooth operation.
2.Advantages and disadvantages
(1) Advantages:
① Simple structure, low manufacturing and maintenance costs
② Small size and light weight, suitable for medium and low pressure and high speed conditions
③ Good self-priming performance and low starting torque
④ High reliability and insensitive to oil contamination (compared to some precision pumps)
(2) Disadvantages:
① Volumetric efficiency is affected by leakage from the end face, radial, and tooth flank clearances, and efficiency decreases after long-term wear.
② Large flow and pressure pulsations lead to higher noise levels.
③ The fixed side clearance design limits the increase in working pressure, and the maximum efficiency is usually lower than that of a plunger pump.
④ Changes in the viscosity of the pumped medium have a significant impact on performance.
3.Different Comparison Dimensions
(1) Structure and cost：
Internal gear pumps: complex structure, high machining requirements, and high cost.
External gear pumps: simple structure, mature technology, and low cost.
(2)Volume and weight
Internal gear pumps: Compact structure, small size, and light weight.
External gear pumps: Relatively dispersed structure, larger size and heavier weight.
(3)Performance
Internal gear pumps: Less flow pulsation, lower noise, and generally higher volumetric efficiency.
External gear pumps: Relatively larger flow pulsation and noise.
(4)Tolerance and maintenance
Internal gear pumps: require high oil cleanliness and are complex to maintain.
External gear pumps: have better tolerance to contaminants and are easier to maintain.
4. Conclusion
Internal and external gear pumps each have their own technical characteristics and market positioning. A thorough understanding of their working principles and structural differences, combined with a balance between specific operating conditions and cost constraints, is key to making reasonable selections and optimizing system performance and reliability.