| Hydro viscous drive clutch(HVD clutch)transmits torque by the viscous shear force of lubrication oil and adjusts the rotation speed by changing the gap distance between friction pairs.Now it is widely used for the soft start of large conveyors,fans and pumps in a cost-effective and energy-saving way.Previous studies show that the grooves can lower the temperature of friction pairs under the mixed friction condition and boundary friction condition,while they can generate hydrodynamic effect under fluid friction condition.The hydrodynamic effect will cause the cavitation bubbles and shrinkage of oil film and influence the torque transmission.In this thesis,the lubrication oil was taken as the research object and the effect of groove morphology,centrifugal force and surface micro texture were analyzed.The transmission mechanism of lubrication oil was studied with theoretical analysis,simulation and experimental tests.The main research subjects are as follows:Based on the N-S equations and the continuity equation,the characteristics of oil flow between non-groove friction pairs were analyzed and the mathematic model of flow field was established with the effect of inertia force.Analytical solutions of flow speed and pressure were deduced by step-by-step analytical method.Then the influences of inlet pressure,thickness of oil film,rotation speed and centrifugal force on flow field were analyzed.Based on the boundary condition,the load bearing capacity and torque transmission of oil film were obtained.Theoretical analysis indicates that without the effects of grooves,centrifugal fore is the main factor for the shrinkage of oil film.In order to reduce the torque loss,the inlet pressure needs to be adjusted in real time to ensure that the gap of friction pairs is fully filled with lubrication oil.When the speed of the friction disk reached 800 rpm,the oil film began to shrink.And when the speed ratio of the separator disk to the friction disk was larger than 0.9,the transmission efficiency of the oil film reduced sharply.Based on the characteristics of groove structure and oil flow,the N-S equations and Reynolds equation were simplified and the over-determined differential equations of flow field were obtained.With the hypothesis of flow continuity,the boundary conditions of groove region were gained.Combing the over-determined differential equations with theboundary conditions,the analytical solutions of flow speed and pressure were deduced.Compared with the simulation results,the accuracy of the analytical solutions were verified.The model of the load bearing capacity and torque transmission were modified taking the hydrodynamic effect into consideration,and the formula of torque transmission efficiency were obtained.Based on the Reynolds boundary conditions,the model of oil film shrinkage were established.The influences of hydrodynamic effect on flow field,load bearing capacity,cavitation and torque transmission were determined.Hydrodynamic effect causes the oil film to shrink sharply.The increase of the oil film thickness has an inhibitory effect on the oil film shrinkage.When the oil film thickness exceeded 0.3 mm,the equivalent radius of the oil film tended to be stable.The torque loss in groove region increases with the rotational speed.The torque loss in the non-groove region reached a maximum when the ratio of the oil film thickness to the oil groove depth was 0.5.The influences of groove parameters were investigated.When the effective area coefficient kept constant,the increase of groove width enhanced the load bearing capacity and transmission torque.When the groove depth was larger than the oil film thickness,the increase of oil groove depth reduced the bearing capacity and torque transmission.Under the speed regulation condition,the stress situation of oil film between multiple friction pairs was investigated.The thickness of oil film were inconsistent,varying the torque transmission.Therefore,micro texture of separator disks are verified to be feasible to enhance the load bearing capacity by.The simulation model with micro dimples were established.The influences of texture type,texture depth and texture rate were investigated on the load bearing capacity and torque transmission.The results show that the micro texture effectively improves the load bearing capacity of oil film,especially the cylindrical texture.When the texture rate was constant,the load bearing capacity increases linearly and evidently with the texture depth with a small loss of transmission torque.When the texture depth was constant,the increase of the texture rate enhanced the load bearing capacity especially when the texture rate was larger than 10%,the improvement was significant.While the increase of the texture rate reduced the effective area,causing the loss of transmission torque.The orthogonal test was designed to calculated the significant level of texture type,texture depth and texture rate.There was no colinearity among texture type,texture depth and texture rate.They all significantly influenced the load bearing capacity and torque.However,the order of influence factors were different.A visualized HVD clutch test bench was developed.The torque transmission under a parallel friction pair was tested.The effects of oil film thickness,inlet pressure and microtexture on the torque transmission were investigated.The experimental data were consistent with the simulation results.Cavitation phenomena and oil film shrinkage were captured using a high-speed camera.Experiment results show that with the rise of the rotational speed,cavitation bubbles increases and the oil film shrinks evidently.The cavitation bubbles and oil film shrinkage occurred periodically.The collapse of the cavitation bubbles iws the main reason for the shrinkage of the oil film.The surface roughness rise with surface texture and leaded to the generation of slender bubbles.The amount and volume of bubbles here are smaller than those in the groove region. |
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