In hydraulic systems, common hydraulic oil pumps mainly include vane pumps, gear pumps, and plunger pumps. Today we will mainly talk about internal gear pumps.

There are two types of internal gear pumps, involute tooth shape, and cycloid tooth shape, as shown in the figure below. Its working principle and main features are the same as those of the external gear pump, except that the sizes of the two gears are different, and the moving internal gears rotate in the same direction with their respective centers.

Internal gear pump structure

In the involute internal gear pump, a crescent plate is installed between the pinion and the internal gear to separate the oil suction chamber from the pressure oil chamber. When the pinion drives the internal gear to rotate, the left half of the gear teeth withdraw from meshing to form a vacuum to complete the oil absorption process. The oil that enters the tooth groove is brought to the oil pressure chamber, and the right half of the gear teeth enter into the mesh to squeeze out the hydraulic oil and discharge the oil from the oil pressure port. Compared with the external gear pump, it has the advantages of small flow pulsation, a more compact structure, lightweight, low noise and high efficiency, and no trapped oil phenomenon. Its disadvantage is that the tooth shape is complicated, and special high-precision processing equipment is required for processing.

In the cycloid internal meshing gear pump (also known as the cycloid rotor pump), the pinion and the internal gear differ by one tooth. When the pinion drives the internal gear to rotate, the teeth of all the pinion gears are engaged to form several independent The sealed cavity does not need to be equipped with a crescent plate. With the meshing and rotation of the internal and external gears, the volume of each sealing chamber changes, thereby performing oil absorption and oil pressure. It is characterized by a compact structure, few parts, large working volume, high speed, stable movement, and low noise. However, due to the small number of teeth, the flow pulsation is large, and the gap leakage at the meshing point is large, so the working pressure of this pump is low (generally 2.5-7MPa), and it is usually used as an auxiliary pump for lubrication and oil replenishment.

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