Mechanism, Modelling And Experiment Investigation Of Trapped Oil In External Gear Pump

Phenomenon of trapped oil is an inherent characteristic of external gear pump; the accurate evaluating of value of trapped oil pressure is a precondition for controlling and relieving trapped oil pressure. With the targets of modeling and accurate simulation of trapped oil pressure, this paper performs a further research that contains the following main aspects:For a external gear pump with different teeth of drive gear and driven gear, by using a method of swept volume and taking engagement length of drive gear as a variable, the paper establishes formulas of average flow, uneven flow coefficient and trapped oil volume, the flow of trapped oil, all of them are a preparatory work for subsequent simulation of trapped oil pressure. The results show that the combination of more teeth of drive gear with fewer teeth of driven gear is favorable for improving the flow characteristic and unfavorable for reducing the phenomenon of trapped oil, while the effect of the combination of fewer teeth of drive gear with more teeth of driven gear is just on the contrary.The minimum area of trapped oil region and relief area are calculated by combining computation method with virtual measurement method. The results show that the curve of relief area is parabolic, different indices and coefficients of parabola can be adopted for simplificating calculation of actual relief area under different parameters in actual application.The simulation model of the trapped oil pressure is established in the paper, and the model is divided into static model and dynamic model according to with or without dynamic factor of gear pair. The influence of percentage of gas in oil, slope of side plate, effective bulk modulus, and centrifugal force and gear pair vibration on trapped oil pressure is studied by numerical simulation. The results show that gas in oil is the major cause of zero-approaching trapped oil pressure in the later stage of expansion, and that inlet gas in oil has little effect on the trapped oil pressure, while the effect of percentage of centrifugal gas and percentage of outlet gas in oil is amplified in the trapped oil areas; and smaller backlash and higher oil pressure will aggravate vibration of gear pair. Normally the peak of simulated pressure in dynamic model is lower than that in static model, making dynamic model more reliable. At peak of trapped oil pressure in driven gear side is greater than that in drive gear side under normal circumstances.Four kinds of leakages in the simulation model are investigated, among which, axial leakage takes slope of side plate and variation of trapped oil profile in a trapping period into account, in static state, meshing leakage from meshing tooth surface depends on the minimum film thickness therein, and leakage from backlash depends on original design values of backlash; in dynamic state, both of them depend on change of meshing position and vibration location. The results show that the relief leakage and backlash leakage and meshing leakage are basically the same as trapped oil flow in magnitude; these three kinds of leakages are main method for relieving the trapped oil pressure.Trapped oil pressure in driven gear side is measured on test device and compared with the results of theoretical analysis, thus the simulation model has been verified. There is a larger gap in simulation results of static model and dynamic model, and the results in dynamic model are closer to the test results, the volume of drainage has little influence on trapped oil pressure, therefore it is negligible.Finally, the influence of relevant parameters on trapped oil pressure is analyzed based on dynamic model. The results show that small modulus, number of teeth, pressure angle, gear width, addendum coefficient and big positive profile shift coefficient contribute to the relieving of trapped oil pressure. In particular, greater positive profile shift coefficient is very effective for relieving of the flow pulse and trapped oil. In addition, trapped oil pressure is forecasted in special working conditions such as high-pressure and low-speed or low-pressure and high-speed.In conclusion, the research offers an effective method for the accurate prediction of trapped oil pressure, and is useful in the design of gear pump, especially, in vibration and noise controlling, etc. this study is of significance both in theoretical and applications.