Study On Multi-field Coupling Lubrication Characteristics Of Water Lubricated Bearing Under Heavy Load And Start-up Conditions

Water lubricated bearings have been widely used in the propulsion shafting of ships and vessels with the advantages of saving resource and protecting environment.However,due to the low viscosity physical characteristics of water,water lubricated bearings are often in the mixed lubrication state.Especially under severe working conditions such as heavy load and start-up condition,the lubrication state of water lubricated bearings is worse,and there are problems such as increased wear and high temperature of the bearing,which can cause damage of the bearing system.In view of this,the transient start-up model of water lubricated bearing is established,to study the lubrication performance of water lubricated bearings under heavy load and start-up conditions by numerical method.And the influence of sea wave impact on transient performance of water lubricated bearings during start-up is explored.Aiming to provide theoretical reference for the structural design of water lubricated bearings and safe start-up of bearing system under severe working conditions.The main contents include:(1)Based on the mixed lubrication theory,the transient pressure field,temperature field control equation and shaft motion equation are established,and the transient start-up model of water lubricated bearings is constructed by coupling the physical field control equations.The start-up model is considered elastic deformation of bearings,partial asperity contact between shaft and bearing surface,and bearing system temperature rise.The numerical solution scheme for the start-up model is formulated,and the reliability of the model calculation results is verified by comparing the theoretical and experimental data in the literature.(2)The steady-state performance of water lubricated bearings under heavy load condition is studied,the lubrication state of water lubricated bearings under different rotating speeds is defined,the pressure and temperature distribution of the bearing system are obtained,the mechanism of temperature rise of water lubricated bearing under heavy load condition is analyzed,and the effects of length diameter ratio on performance parameters such as film thickness,pressure,capacity,friction coefficient and bearing temperature of water lubricated bearings are studied.The results show that there is intense asperity contact,and great risk of high temperature in water lubricated bearings under heavy load condition.Increasing length diameter ratio can increase capacity of water film and minimum film thickness,improve the bearing lubrication state,and reduce the bearing temperature.But excessively large length diameter ratio will hinder the heat dissipation of the bearing end leakage.And when the asperity contact between the shaft and the bearing surface is weak or no contact,increasing the length diameter ratio will increase the friction coefficient.(3)The transient performance of water lubricated bearings under start-up condition is simulated,and the transient variation regulation of lubrication parameters during start-up is analyzed.The shaft center orbit and the distribution of water film pressure are given,and the load-bearing mechanism of water lubricated bearings during start-up is revealed.The influence of surface roughness on the transient start-up performance of water lubricated bearings is discussed.It is found that a drastic instantaneous vibration occurs on the shaft at the initial stage of start-up,which cause the changing sharp in an instant of main parameters such as film thickness,pressure,capacity and coefficient of friction,and the vibration becomes more intense with the increase of surface roughness.The reduction of surface roughness can improve the capacity of water film,reduce the asperity contact strength and friction coefficient,conducive to reducing bearing temperature.(4)Considering the influence of sea wave impact on the start-up process of water lubricated bearing,the shaft center orbit under impact load is analyzed.The variation regulation of water film pressure and asperity contact pressure are studied,the response mechanism of water film capacity and asperity contact capacity to impact load is defined.When the direction and amplitude of the impact load are appropriate,the drastic vibration of shaft at the initial start-up stage can be suppressed or even eliminated.The impact load which earlier enters starting process,will cause serious asperity contact and damage the stability of the bearing system.In addition,with the increase of impact load amplitude and the advance of the enter time,the time taken for the shaft to achieve the take-off state is shorter.