The concept of gear oil pump is very simple, that is, its basic form is two gears of the same size meshing and rotating in a tightly fitted housing. The interior of this housing is similar to an "8" shape, with two gears installed inside, and the outer diameter and sides of the gears tightly fitting with the housing. The material from the extruder enters the middle of two gears at the suction port and fills this space. It moves along the shell with the rotation of the teeth and is finally discharged when the two teeth mesh.

In terms of terminology, a gear pump, also known as a positive displacement device, is like a piston in a cylinder. When one tooth enters the fluid space of another tooth, the liquid is mechanically squeezed out. Because liquid is incompressible, liquid and teeth cannot occupy the same space at the same time, thus the liquid is eliminated. Due to the continuous meshing of teeth, this phenomenon occurs continuously, providing a continuous discharge capacity at the pump outlet. The discharge capacity is the same for each revolution of the pump. As the drive shaft rotates continuously, the pump continuously discharges fluid. The flow rate of the pump is directly related to the pump speed.

In fact, there is a small amount of fluid loss inside the pump, which makes the operating efficiency of the pump unable to reach 100%. This fluid is used to lubricate both sides of the bearings and gears, and the pump body cannot fit without clearance, so it cannot be discharged 100% from the outlet. Therefore, a small amount of fluid loss is inevitable. However, the pump can still operate well and achieve an efficiency of 93% to 98% for most extruded materials.

For fluids with changes in viscosity or density in the process, this type of pump will not be greatly affected. If there is a damper, such as a filter or a limiter placed on the discharge side, the pump will push fluid through them. If the damper changes during operation, that is, if the filter becomes dirty, clogged, or the back pressure of the limiter increases, the pump will still maintain a constant flow rate until it reaches the mechanical limit of the weakest component in the device (usually equipped with a torque limiter).

The speed of a pump is actually limited, mainly depending on the process fluid. If oil is being transported, the pump can rotate at a high speed. However, when the fluid is a high viscosity polymer melt, this limitation will be greatly reduced.

It is very important to push the high viscosity fluid into the two tooth space on the suction side. If this space is not fully filled, the pump cannot discharge accurate flow, so the PV value (pressure x flow rate) is another limiting factor and a process variable. Due to these limitations, gear pump manufacturers will offer a range of products with different specifications and displacements (the amount discharged per revolution). These pumps will be combined with specific application processes to achieve optimal system capacity and price.