During the meshing process of the gear pump, the number of gear pairs meshed at the same time should be more than one pair, that is, the overlap coefficient ε should be greater than 1 (ε=1.4) to work normally. The oil remaining between the teeth is trapped in an enclosed space formed by two pairs of teeth meshing at the same time, and the volume of this space will change with the rotation of the gears. This is the trapped oil phenomenon of gear pumps
If there is a certain period of time in the whole meshing process that the number of gear pairs meshed is less than 1 pair, that is, when ε< 1, the oil delivery rate of the oil pump is very uneven, and sometimes the pressure oil is fed, and the difference of the instantaneous flow can reach 30%, and the gear pump can not work normally. ε=1 does not guarantee that the gear pump will work properly.
Hazards of oil trapping: the bearing load increases, the power loss increases, the oil heats up, causes noise and vibration, and affects the working performance, stability and life of the oil pump.
Description: The volume of the enclosed space changes dynamically, from large to small and then from small to large. Small: The oil is incompressible, the oil is squeezed, the pressure rises, and it is forcibly extruded from the gap of the joint surface of the parts (this pressure is much higher than the working pressure of the oil pump, and even up to hundreds of atmospheres), so that the gear and bearing are subjected to great radial pressure and additional load. When it grows larger, a local vacuum is generated, and the air precipitates and vaporizes, causing cavitation.
Solution (the basis point of elimination and mitigation is pressure relief):
(1) Modify the tooth shape to minimize the volume change of the enclosed space, and this method is rarely used.
(2) Pressure relief hole method: drill radial through holes from the top of the tooth to the root of the driven gear, and mill out two grooves on the driven gear shaft (complex processing).
(3) Pressure relief groove (unloading groove) method On the inner side of the cover on both sides of the pump, along the tangent line direction of the gear tooth circle circle, four rectangular grooves are opened (one in the direction of oil inlet and discharge on each side cover). The distance between the grooves must be greater than the thickness between the teeth of one tooth, so as not to make the suction and discharge cavity communicate directly.
The pressure relief tank method is divided into:
Symmetrical pressure relief tank method: the pump can be reversed, which can greatly reduce the phenomenon of trapped oil, but it is not perfect;
Asymmetrical pressure relief tank method: that is, moving an appropriate distance to the suction side, this method can recover a part of the high-pressure liquid, and the noise is significantly reduced, but the pump is not allowed to reverse.
The most widely used method to reduce trapped oil is the pressure relief tank method
Causes of radial forces
(1) The pressure distribution acting on the outer circle of the gear is not the same, and the pressure distribution of the oil from the oil pressure chamber to the oil suction chamber is gradually graded and reduced, and the radial force generated by the existence of pressure difference;
(2) There is a radial gap between the tooth top and the inner surface of the pump; The resultant force of the uneven force of the oil acts on the pump shaft, causing the bearing to be subjected to a radial force caused by one-way pressure.
The higher the working pressure of the oil pump, the greater the radial force. The resultant force F1 of the radial force experienced on the driving gear: small. The resultant force F2 of the radial force on the driven gear is larger, F2 > F1. Because the meshing point of the gear is constantly changing, the magnitude and direction of the force change periodically.
Hazards of radial forces
Vibration and noise lead to early damage to the bearing and affect the service life.
Measures to reduce radial forces:
1. Reduce the size of the pressure port. The area of the oil pressure cavity acting on the gear is reduced to the range of 1-2 gears.
2. Open the hydraulic balance groove. Open two hydraulic balance grooves on the end cover or bearing in the transition area from the oil suction port to the oil pressure port, so that the oil pressure port and the oil suction port are communicated with the balance groove close to the oil suction port and the oil pressure port respectively, so that the radial force will be balanced to a certain extent.
3. Expand the high-pressure area. Enlarge the oil pressure chamber to the side close to the suction chamber, only keep the gap between the last two tooth tops and the housing small, and enlarge the gap between the other part of the tooth tops. So that the pressure in the large top clearance area is equal to the outlet pressure, and finally the radial force in the symmetrical area is balanced, and the purpose of reducing the radial force acting on the bearing.
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