Created on:2021-10-26 08:54

Basic structure of external gear pump

Basic structure of external gear pump

The structure of CB-B gear pump is shown in Figure D. Its main structure adopts a three piece structure of pump body 7 and front and rear end covers. The three pieces are positioned by two cylindrical pins 17 and fixed by six screws 9. The driving gear 6 of the two gears is fixed on the transmission shaft 12 with the key 5 and driven by the motor for continuous rotation, so as to drive the driven gear 14 to rotate. The rear end cover is provided with an oil suction port and an oil pressure port. The large opening is an oil suction port, which is connected with the oil inlet pipe to ensure that the oil suction chamber is always connected with the oil fluid; The other with a small opening is the pressure oil port, which is connected with the system through the pressure oil pipe. In order to make the gear rotate flexibly and ensure that the internal leakage is as small as possible, a very small axial clearance is left between the gear end face and the two end caps; In order to reduce the oil pressure between the pump body and the end face, reduce the screw tightening force, and prevent oil leakage outside the pump, unloading grooves 16 are opened on both ends of the pump body to lead the pressure oil at the ends of the two gears back to the oil suction chamber for pressure relief.


The external gear pump adopts the gear tooth meshing pump oil structure of ordinary gear, which forms the following problems of this gear pump.

(1) Internal leakage is serious. One of the main disadvantages of external gear pump is large leakage, which is only applicable to low pressure, and the volumetric efficiency is too low under high pressure. Inside the gear pump, the hydraulic oil in the pressure oil chamber can leak into the oil suction chamber through three ways: one is the clearance at the gear meshing, which is called meshing leakage; The second is radial clearance, which is called tooth tip leakage; The third is the end clearance, which is called end leakage. Among them, the end leakage through the end clearance is the largest, accounting for about 75% ~ 80% of the total leakage. Therefore, in order to improve the pressure and volumetric efficiency of gear pump, the end clearance must be automatically compensated to reduce the leakage of end clearance.

(2) Oil trapping in gear meshing area. For the gear pump to work smoothly, the overlap coefficient of gear meshing must be greater than 1, that is, before the previous pair of gears are about to disengage from meshing, the latter pair of teeth shall enter meshing. In this way, in this part of the area where the two pairs of teeth mesh at the same time, some oil will stay between the overlapping areas of the two teeth, as shown in Figure e (a) and (b). With the continuous rotation of the gear, the latter pair of teeth will continuously enter the meshing, which means that the teeth just entering the meshing will be squeezed against the opposite tooth slot. At this time, the meshed tooth slot is basically closed due to the structural condition that the tooth thickness of the two teeth is equal, as shown in Figure e (b), This part of the oil trapped in the tooth groove will be strongly squeezed due to the continuous meshing movement of the teeth and the continuous reduction of the sealing space of the tooth groove, as shown in Figure e (c). Due to the extremely small compressibility of the oil, the pressure of the trapped oil will rise sharply. This part of the oil will look for any gap to squeeze out to the outside, and even hinder the continuous rotation of the gear. The squeezed oil has brought great radial force to the gear. After turning the meshing node P, as shown in Figure e (d), the teeth will gradually disengage from the meshing, and the closed alveolar space will continue to expand, which will cause the vacuum negative pressure of the closed space. If there is no oil added in time, the air in the oil will be separated and precipitated, resulting in cavitation of the oil, causing vibration and noise.


The above phenomenon occurs in the meshing area of each pair of teeth. This phenomenon that the oil enclosed between the teeth is squeezed first and then vacuum negative pressure due to the equal tooth thickness is called the oil trapping phenomenon of gear pump.

The trapped oil of the gear pump causes cavitation of the oil, which will cause transmission vibration and noise, destroy the stability of the hydraulic transmission, and bring great additional radial dynamic load to the rotation of the pump, which will do great harm to the normal operation of the pump. Therefore, the trapped oil of the pump needs to be eliminated.

At present, the method to eliminate trapped oil is usually to mill two unloading grooves on the end covers on both sides of the gear pump, as shown in Figure e (E). When the trapped oil is strongly squeezed, the extrusion space is connected with the oil pressure chamber through the unloading groove; When a vacuum negative pressure is formed in the oil trap area, it is connected with the oil suction chamber, which can partially solve the problem of oil trap. However, it should be noted that the existence of two oil unloading grooves will increase the end face leakage. At the same time, the distance between the two oil unloading grooves should not be too close to avoid the collusion between the two chambers of the pressure suction oil chamber. In general, the two unloading slots of gear pump are opened asymmetrically, and the position is often offset to the oil suction cavity, but in any case, the distance between the two slots. It must be ensured that the oil suction chamber and oil pressure chamber cannot collude with each other at any time α= 20 ° standard involute gear with modulus m, α= 2.78m。 When the unloading groove is asymmetric, B = 0.8m must be ensured on one side of the oil pressure chamber. On the other hand, in order to ensure that the unloading groove is unblocked, the requirements of groove width C > 2.5m and groove depth h ≥ 0.8m shall be met.

(3) The radial force of the gear is unbalanced. When the two gears in the gear pump are working, the radial pressure acting on the gears is unbalanced. As shown in Figure F, the teeth of the gear in the oil pressure chamber are subjected to great radial pressure due to the high pressure of the liquid, while the teeth in the oil absorption area are subjected to small radial pressure. It can be considered that the pressure gradually decreases from the high pressure of the oil pressure chamber to the pressure of the oil absorption chamber, which is equivalent to a large radial unbalanced force exerted by the oil on the gear, Make the gear and bearing bear great eccentric load. The greater the working pressure of the oil, the greater the radial unbalanced force. The radial unbalanced force will bend the shaft, resulting in contact friction between the tooth top and the radial unbalanced force of the gear pump in Figure F of the shell. At the same time, it will accelerate the wear of the bearing and reduce the service life of the bearing. Therefore, the unbalanced radial pressure of the gear pump is the main reason that hinders the further improvement of the working pressure of the pump.

In order to reduce the unbalanced radial pressure of the gear pump, the method of reducing the pressure oil port is adopted on some gear pumps, so that the radial pressure of the pressure oil acts only in a small range of 1 ~ 2 teeth, as shown in the A-A section shown in Figure D. at the same time, the radial clearance can be appropriately increased, so that the tooth top will not contact and friction with the shell under the action of unbalanced pressure.

The utility model has the advantages of simple structure, small size, light weight, convenient manufacture, low price, reliable operation, strong self-priming capacity (large allowable oil suction vacuum), insensitive to oil pollution and easy maintenance. Its disadvantage is that gears, bearings and shafts will bear great unbalanced radial force, serious wear and leakage, which limits the increase of pump working pressure. In addition, its flow pulsation is large, so the pressure pulsation and noise are relatively large. Gear pump is generally used in low-pressure and small flow occasions.

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