| Lubricants and process liquids make fluid film bearings to reduce friction and wear,to provide load capacity,and to add damping to dissipate undesirable mechanical vibrations.Journal bearings represent the vast majority of applications where the mechanical surfaces shear the fluid causing it to flow and to produce the physical-wedge where a hydrodynamic pressure generates to carry,without contact,an applied load.Costs associated with stern tube bearing problems are high because it forces a ship out of service.The risk of uneven contact and seizure of stern tube bearing increases if misalignment of a propeller shaft exists.Stern tube bearings are exposed to different operating conditions in terms of load,speed and temperature.The conditions range from pure hydrodynamic,when the lubricating film is sufficiently thick,to severe asperity contact when the lubricating film is too thin between the adjacent surfaces.Therefore how to choose the stern bearing structure,lubrication style,apply the real-time monitoring during operation and diagnose bearing wear are significance for bearing reliability.In factual operating conditions,the load acted on journal causes the deformation of journal,and this deformation will affect the lubrication pressure distribution and asperity contact forces in mixed lubrication analysis;meanwhile,the lubrication pressure and asperity contact also act on the journal to influence the deformation of journal.Moreover,when the width of bearing is too large,such as the marine stern tube bearing,the deformation of shaft in the bearing hole cannot be described by only two angles(β and γ).In this condition,the shaft in the bearing hole is under the concentrated load of propeller screw together with the lubrication pressure and asperity forces,and the bending deformation of shaft.The stern shaft is subjected to complicated loading conditions.If the bending deformation of propeller shaft under complicated loading conditions is small,the shaft material is considered as elastic,the displacement superposition method may be used to calculate the bending deformation.The general statement for the principle of superposition asserts that the resultant stress or strain in a system subjected to several loading forces is the algebraic sum of their effects when applied separately.The algebraic sum of these solutions may be used to present the deformation of a stern shaft under these combined loading conditions from propeller loading force and asperities contact force.Based on the displacement superposition method,the bending deformation of journal under the concentrated load of propeller screw together with the lubrication pressures and asperity pressures can be determined accurately.Then the influences of the bending deformation of shaft on the Stribeck Curves are discussed.Because of the cavitation collapse can cause severe surface material damage,the cavitation phenomenon should be tried as a very important study direction.Moreover,its onset and extent can change the load capacity of a fluid film bearing.Furthermore,the appearance of cavitation largely effects on the rotor dynamic stability of a rotor-bearing system and its maximum whirl amplitude of vibration in the hydrodynamic bearings.In common,the effects of hydrodynamic pressures are looked as contribution of two different actions: wedge and squeezing.And the Jakobsson-Floberg-Olsson(JFO)model applies to dynamic load situations in which the surfaces are also under squeeze film motions.The temporal change of the ruptured film volume is included in the liquid flow continuity equation and without consideration of the bubble dynamics.The shape of the cavitation zone changes instantaneously as the surface squeeze speed changes.In the JFO model,however,since the cavitation zone is unknown during operating condition,it is hard to solve Reynolds equation in the numerical.This study establishes a dynamic flow balance in the cavitation zone,and the implementation of the universal cavitation algorithm in a computational program leads to mixed success and little improvement over more crude techniques.Moreover the value of pressure in cavitation zone of bearing is specific in this study using the numerical calculation method.Prior to investigating the cavitation effects on the hydrodynamic performance of journal bearings,the results of simulation with Reynolds condition and JFO condition respectively are compared with prior experimental measurements under steady-state situation,and the film pressure predicted by the JFO boundary has a good agreement with experimental results.Then based on the lubrication model develop in this study,the effect of boundary conditions on ML-HL transition point during operation is studied by using the Stribeck Curve.As an external load is applied to a journal bearing,the bending deformation of journal is happened,the oil film thickness is reduced at the sections closed to the bearing end;as a result,the oil pressure will increase rapidly at those sections.In this situation,in order to analyze of consequences for the pressure distribution and wear phenomena better,it is necessary to include the elastic deformation of the bearing in the EHD model.This study includes the elastic deformation effect in the analysis of dynamic bearing lubrication characteristics across a wide range of external loads together with the thermal effect coused by friction force.By calculating the deformation of bearing in different condition and adding it into the thickness value in the numerical procedure,the effects of elastic deformation on lubrication characteristics can be analyzed.Moreover,the thermal system of bearing-lubricant-jounal has been bulid to analyze the heat transfer process.According to the loading characteristics of stern bearing,and based on the theoretical model analysis,a test rig considering end load is designed and built.The details of friction testing system and oil pressure tesing system are given in this study.And using the experimental viscosity of oil and roughness of bearing and journal surface,the prediction results are compared with the experimental results.The comparison result shows that the simulation model build in this study is accurate and reliable,and this model can be used to guide the bearing design.Moreover,the influences of load,speed,oil supply pressure on oil pressure distribution are analyses based on the experimental results;meanwhile,the friction curves under strarved lubrication and hydrodynamic lubrication are compared.The bearing stiffness and damping properties affect the critical speeds and the stability of the journal.To simplify the stability analysis,the bearing forces are linearised and expressed as a linear function of the displacements and velocities of the rotor.This linearization yields 4 stiffness coefficients and 4 damping coefficients.These eight coefficients are normally called “dynamic coefficients”.Based on the numerical model developed above,a calculation procedure is developed for the analysis of marine stern bearing,and in the dynamic part,a combination of FDM and FP methods is applied.And as an important parameter in stability analysis,the property of equivalent stiffness in hydrodynamic lubrication regime is given.Moreover,due to the interaction among bending deformation of propeller,bearing deformation,asperities contact force and film force,the natural frequency is various in different running speeds. |
PDF Full Text Request