| Water lubrication rubber stern bearing is the key device to the propulsion shafting of underwater vehicle.Under special working conditions such as low speed,overload,the bearing is easy to produce boundary or mixed lubrication status,the contact area between both bearing and shaft is increased,and the tribologial performance deteriorated,which is easy to induce friction self-excited vibration,produce chatter and howling,and seriously affect the concealment,reliability,and occupant comfort of the underwater vehicle.So far,there is no consensus about the textured surface effect on friction,wear,lubrication and friction vibration(promote or inhibit),especially the influence law of textured surface of high polymer material(such as rubber,etc.)on frictional vibration.Therefore,it is of important theoretical significance and engineering application for improving the performance,life and safety of the water lubricated rubber stern bearing of underwater vehicle that both tribological performance and friction vibration mechanism of water lubricated stern bearing are systematically carried out,the textured friction surface of the bearing liner optimized,the lubrication performance and carrying capacity improved,friction reduced and friction vibration suppressed.In this paper,the nitrile rubber samples of water lubricated stern bearing(including ring-block and slat ones)are researched,and the textured surface with different structure and size is build on the friction surface of the samples.Under low speed,overload and boundary lubrication and mixed lubrication conditions,the tribological performances,friction vibration inhibition mechanism and their influencing factors of the textured surface are explored,which provides the theoretical support for the reshaping design of friction surface of the bearing.The main research contents and results are as follows:1)The two-dimensional finite volume element models both untextured and texture(cylindrical pit,helical cylindrical pit and spherical cap pit)are set up with Fluent software,respectively,the simulation calculation of parameters such as the flow velocity distribution of the flow field,water film pressure distribution,and fluid viscous friction resistance are carried out to analysis the influence of the geometrical shape,diameter,depth and arrangement of textured surface on the tribological property of both untextured and texture surfaces.The research results show that the fluid viscous resistance of three kinds of texture surfaces is relatively close,and all lower than one of untextured surface.2)According to the morphology of friction surface,the friction surface is divided into the contact region,concave valley region and textured surface region,the ratio of normal load to total normal load in the various regions are analyzed,the total friction force and friction forces of the contact region,concave valley region,textured surface and their corresponding characterization methods of load ratio are given,and the cylindrical pit surface is chose for both tribological properties and vibration tests under different lubrication states(hydrodynamic pressure lubrication and mixed lubrication,etc.),and the friction coefficient,the vibration frequency and amplitude are measured for evaluating both tribological characteristics and the inhibitory effect of friction vibration of textured surface.The results show that under boundary or mixed lubrication conditions,friction force mainly comes from the contact region,in which load ratio is related to whether there is textured surface.Both friction force and friction coefficient of a textured sample are less than untextured one.3)With the friction system of both water lubrication bearing and shaft as an example,the friction coefficient-relative speed curve is divided into both rise and fall feature regions by the critical speed.In fall feature region,the lubricating status of the friction system is mainly probed into and both dynamics model and characteristic equation of the system are established,and the characteristic roots of the characteristic equation are obtained with Root()function of the Matlab.The relationship among the characteristic roots and load,friction coefficient difference,load coefficient,and the effect of textured surface and the positive and negative damping on the system instability are discussed.The research results indicate that the load,friction coefficient difference,load coefficient and negative damping etc.parameters of the friction system have to be controlled in a certain range to make the real components of characteristic roots negative,and the equivalent damping ratio of the friction system positive,so friction vibration can be inhibited and the stability of the friction system maintained.4)Based on the test results of the textured rubber ring-block sample,under the working conditions of different pressure and rotational speed,the influence of the nitrile rubber hardness and the shape,size and depth to diameter ratio parameters of the texture on friction torque and vibration situation are discussed,the contribution level of the various parameters and multi-parameter combined action result analyzed.Under certain pit depth,the friction torque of inclined cylindrical pit is obviously smaller than that of cylindrical pit.The pit depth is obviously affected by the rubber hardness so that both water storage volume and film thickness of the pit are changed,which affects the 'secondary lubrication' of the textured surface and friction vibration inhibition.5)Based on the friction vibration inhibition mechanism tests of the rubber slat of water lubrication bearing,the influence law of the shape,size,layout of different textured surfaces,rubber material properties and testing conditions(speed,load,temperature,etc.)on the tribological characteristic of the slat was studied,the effect of textured rubber slat on both friction coefficient,lubrication status evaluated in contrast to one of untextured rubber slat,and the rationality of the theoretical calculation model for the stability of friction system verified.In the research,the friction vibration suppression mechanism of the surface texture was revealed that can improve the surface lubrication effect,enhance the hydrodynamic effect of the surface texture,decrease friction coefficient,and reduce or eliminate the exciting source of friction vibration. |
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