Computer-vision-based Research On Friction-induced Vibration Mechanism Of Water-lubricated Rubber Stern Bearing

Water-lubricated rubber stern bearing is an important part of the ship's propulsion system.Under conditions of low speed,heavy load,startup and shutdown,the water-lubricated rubber stern bearing is frequently in a state of mixed or boundary lubrication,because it is difficult to establish a complete water film.The larger frictional excitation makes the propulsion shafting system produce abnormal vibration and noise.Frictional vibrations induced by the water-lubricated rubber stern bearing will not only affect the reliability of the ship,but also be transmitted to the hull through the stern shaft support,causing vibration and noise of the hull,which will further affect the stealth performance of the ship.In this thesis,frictional vibrations induced by the water-lubricated rubber stern bearing are taken as the research object from perspectives of numerical simulations and visual experiments.The mechanism of friction-induced vibrations of the water-lubricated rubber stern bearing,the coupling relations between vibrations of the propulsion shafting system,and the influence laws of material and structural parameters of the water-lubricated rubber bearing on friction-induced vibration characteristic are thoroughly analyzed.The main contents and achievements are summarized as follow:(1)Embarking from the actual problems,the main vibration components and vibration types induced by frictional excitation of the propulsion system are considered adequately.The velocity-dependent “Stribeck” friction model is used to describe the dynamic friction characteristics between the rubber bearing and the journal,and a six-degree-of-freedom nonlinear dynamic model is established.The indirect method of Lyapunov is used to analyze the stability of the propulsion shafting under frictional excitation and the influence of system parameters on the stability of the nonlinear vibration system.The Runge-Kutta numerical integration method is used to solve the vibration response of the stern bearing support,stern bearing and shaft in the propulsion shaft system.The research results show that control parameters of the friction model,rubber layer parameters and stern shaft parameters have variable influence on the stability of the system.Among them,control parameters of the friction model and stern shaft parameters mainly affect the system stability by changing the friction excitation.The parameters of the rubber layer mainly affect the system stability through changing the vibration characteristics of the system itself.In addition,by analyzing the dynamic response of the rubber layer and the stern shaft support,the occurrence mechanisms of the low-frequency "chatter" and the high-frequency "squeal" are revealed.The coupling process of the friction vibration of the water-lubricated rubber bearing and the torsional vibration of the stern shaft is analyzed in detail from the point of the numerical simulation by introducing the friction force,the speed of the rubber layer,the rotation speed of the stern shaft and the relative speed of the friction pair.(2)According to the established six-degree-of-freedom dynamic model,a water-lubricated rubber stern bearing friction-induced vibration in-situ measurement system is built in strict accordance with US MIL-DTL-17901C(SH)standard.In the system,a transparent glass shaft is used(rather than a metal shaft)to form a friction pair with a rubber test block.In order to develop the related research of the water-lubricated rubber stern bearing friction-induced vibration comprehensively and systematically,the dynamic responses of rubber layer,shaft and test block fixture(simulating stern shaft support)is recorded by a high-speed camera.Simultaneously,vibration,noise and friction signals are monitored by conventional test methods.(3)Based on the computer image processing technology,the dynamic response of rubber friction surface,glass shaft and test block fixture(simulating the stern bearing support)is tracked and calculated by the test rig with visual friction pair.The digital image signals,vibration,noise and friction force signals are analyzed jointly.The impacts of operating conditions and the surface roughness of the rubber layers on the friction-induced vibration are investigated,and the occurrence mechanism of water-lubricated rubber stern bearing friction-induced vibration under different frictional excitations and the vibration coupling process of multiple components of the shaft system are visually explored.The results show that the "chatter" and "squeal" phenomena are two completely different dynamic responses caused by different frictional excitations.Among them,the cause of "chatter" is that when the friction excitation is relatively large,the vibration of the rubber bearing and the stern shaft support are in the same phase.When the friction excitation is relatively low,the friction vibration of the rubber bearing does not significantly affect the stern shaft support.The main vibration component is the rubber layer,and the vibration form is "squeal" with relatively high frequency and loudness.In addition,while the friction vibration of the water-lubricated rubber stern bearing occurs,the torsional vibration of the stern shaft will occur under the action of the friction excitation.Through the analysis of image signals,the coupling process of frictional vibration and torsional vibration is shown visually,and this can verify the mechanism of frictional vibration of the water-lubricated rubber stern bearing and its coupling mechanism with the torsional vibration of the shaft in the numerical simulation results.(4)With the assistant of the in-situ measurement system of friction-induced vibrations,the vibration,noise,friction force and dynamic responses of rubber layer with different parameters are tested.Based on the mechanism of frictional vibration of water-lubricated rubber stern bearing,the vibration,noise,friction and digital image information of each component are analyzed to explore the influence of the hardness,thickness and the shape of the rubber layer on the friction-induced vibration.The research results show that when the influence mechanism and parameter optimization of rubber layer parameters on friction-induced vibration are investigated,comprehensive evaluation is required by using multiple indicators;the hardness and thickness of rubber layers mainly change the friction excitation and the tangential vibration absorption performance of the rubber layer by changing the dynamics of rubber layers in the tangential and normal directions.The shape of the rubber layer mainly changes the friction excitation by changing the contact area of the friction pair,which subsequently affects the frequency and amplitude of friction-induced vibration.And the experiment results of the effect of the structural parameters and material parameters of the rubber layer on the friction-induced vibration of the water-lubricated rubber stern bearing also verify the effect of the rubber layer parameters on the system stability in the numerical simulation results.