Hydraulic gear pump is driven by an independent motor, which can effectively block upstream pressure pulsation and flow fluctuation. The pressure pulsation at the gear pump outlet can be controlled within 1%. Adopting a gear pump in the extrusion line can increase the flow output speed and reduce the shearing and residence time of the material in the extruder.

External gear pump is the most widely used gear pump, the general gear pump usually refers to the external gear pump.

Its structure as shown in Figure 5-14, mainly has driving gear, driven gear, pump body, pump cover and safety valve and so on. The seal space of pump body, pump cover and gear is the workshop of gear pump. The wheel shafts of the two gears are respectively arranged in the bearing holes on the two pump cover, and the driving gear shaft extends out of the pump body and is driven by the motor to rotate. The external gear pump is simple in structure, light in weight, low in cost, reliable in operation and wide in application.

When the gear pump works, the driving wheel rotates with the motor and drives the driven wheel to rotate. When the meshing teeth on one side of the suction chamber are gradually separated, the volume of the suction chamber is increased, and the pressure is lowered, and the liquid in the suction pipe is sucked into the pump; the suction liquid is divided into two parts to be pushed into the discharge chamber by the gear in the alveolus. When the liquid enters the exhaust chamber, the teeth of the two gears are continuously engaged, so that the liquid is squeezed and discharged from the exhaust chamber into the discharge pipe. The driving gear and the driven gear keep rotating, and the pump can continuously inhale and discharge the liquid.

The pump body is equipped with a safety valve, when the discharge pressure exceeds the required pressure, the delivery liquid can automatically open the safety valve, so that the high-pressure liquid returns to the suction pipe.

The internal hydraulic gear pump, which is composed of parts, a crescent of gears meshing with each other and their middle pump shell etc.. A crescent is the role of the suction chamber and a discharge chamber separated. When the driving gear rotates, forming a partial vacuum in out of gear meshing, the liquid is filled with the suction pump suction chamber between the teeth, and then along the inside and outside of a crescent divided into the discharge chamber. When the gear teeth come into engagement, the liquid between the teeth is squeezed and sent into the discharge pipe.

Hydraulic gear pump has the characteristics of self priming capability, flow and discharge pressure is independent of the pump casing, no suction valve and discharge valve, has the advantages of simple structure, uniform flow and reliable properties, but low efficiency, noise and vibration, easy to wear, mainly used for various oil conveying non corrosive and non solid particles with lubricating ability, the temperature is not more than 70℃, such as lubricating oil, edible vegetable oil. The general flow range is 0.045- 30ms/h, the pressure range is 0.7 - 20MPa, the working speed is 1200 - 4000r/min.