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A New Kind of Gear Pump to Solve Trapped Oil Problem
Views: 132 Update Date: Dec 11 , 2018

As the gear pump will inevitably encounter oil trap, the traditional solution is to unload the load groove, which is not only difficult to manufacture, but also can not completely eliminate the harm of oil trap, can only reduce its harm. The author once encountered a helical gear pump with zero gear tip clearance (no oil trap area), which not only solved the problem of oil trap of gear pump, but also improved the noise and low pressure of gear pump.

 

What is trapped oil? In order to make the gear pump supply oil continuously and smoothly, the overlap coefficient of gear meshing must be greater than 1 to ensure that at least one pair of gear teeth are engaged at any moment of work. Because of E > 1, there will be two pairs of teeth meshing at the same time, that is, the original pair of meshing teeth have not been separated, the latter pair of teeth have entered meshing. In this way, a closed volume between two pairs of meshing teeth, called oil trapping area, will be generated, so that the oil remaining between the two pairs of teeth will be trapped in this closed volume. As the gear continues to rotate, the volume of the trapped area changes. When the volume decreases, the oil in the trapped area is squeezed and the pressure rises sharply because of the inability to discharge oil. As the gear continues to rotate, the closed volume increases gradually (when the front pair of meshing gear teeth are in the position to be detached, the closed volume is the largest). Because of the inability to replenish oil, a local vacuum is formed in the trapped area. Oil is in the trapped area, there is no place to drain when it needs to be discharged, and when it needs to be filled, it can not be replenished. This phenomenon is called trapped oil.

 

Because of the small hydraulic shrinkage and the sealing volume of the trapped oil area, the trapped oil is forced to extrude from the cracks in the mating surface of the parts after being extruded, which makes the gears and bearings suffer great additional loads, reduces the life of the bearings, generates power loss, and also increases the oil temperature and affects the system. Normal working temperature. When the volume of trapped area becomes larger, local vacuum is formed in trapped area, gas in oil is precipitated, and gas bubbles are generated by oil vaporization, which enter the hydraulic system, causing vibration and noise (when the rotational speed is higher than 1 500 r/min, it increases rapidly). When the rear pair of gear teeth engage, the front pair of gear teeth have lost the ability of oil displacement, which reduces the flow rate of the pump and increases the fluctuation of the instantaneous flow rate, especially when the pressure rises sharply.

 

Oil trapping is an inevitable consequence of gear pump in order to ensure the sealing of oil suction chamber and oil discharge chamber. Generally, structural measures of unloading groove are used to reduce its harmful effects. For example, there are two unloading grooves milling on the front and back cover of the pump, one of which is connected with the oil drainage chamber of the pump and the other with the oil suction chamber of the pump. When the trapped oil volume becomes smaller, the trapped oil volume will be connected with the unloading groove leading to the drainage chamber, and the oil in the trapped oil area will be discharged; when the trapped oil volume becomes larger, another unloading groove will be used to connect the trapped oil volume with the oil absorption chamber to realize oil replenishment. The location of the unloading tank should be appropriate so that the oil in the trapped area can be both discharged and repaired. However, the distance between the two unloading grooves should not be too small to prevent the suction chamber and the discharge chamber from colluding through the trapped volume, which will affect the volume efficiency of the pump. The position accuracy of pressure unloading tank is high, which not only increases the manufacturing difficulty, but also can not completely eliminate the hazards caused by oil trapping, but can only reduce its hazards.

 

The new gear pump is a helical gear pump (or screw pump) with zero gear tip clearance (no oil trap area). Its greatest advantage is to avoid the oil trapping phenomenon of traditional gear pump, thus avoiding a series of problems caused by oil trapping. It also has the advantages of gentle change when pressure rises, low noise (speed up to 5000r/min), prolonging the service life of hydraulic system, and the gear will not bring the oil in the high-pressure area back to the low-pressure area, avoiding the internal leakage caused by the gear structure, improving efficiency and saving energy. Therefore, compared with the traditional gear pump, under the same conditions, higher rated pressure can be obtained. With the servo motor, the pressure and flow can be changed according to the different needs of the hydraulic system.

 

The working principle of the new gear pump is the same as that of the traditional gear pump, but the helical gear will produce axial force under the action of oil pressure. For this reason, the top pin of the gear shaft is designed. The top pin exerts force to the left under the action of high pressure oil, which counteracts the axial force of the helical gear. The technical difficulty of the pump lies in the manufacture of two helical gears (or screw). The biggest difficulty of the helical gear is the control of the dimension tolerance between the top circle DA and the root circle df, the control of the dimension tolerance between the top circle R1 and the root circle R2, and the arc connection between the top circle DA and the root circle df. In the high precision screw rotor grinder, the grinding wheel is trimmed to size and the forming grinding can effectively solve this technical problem.

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