Study On The Tribological And Lubrication Properties Of Modified UHMWPE Water Lubricated Stern Bearings

As an important part of the ship propulsion system,the stern bearing's tribological and lubrication characteristics have an important influence on the service performance and life reliability of the ship.The low-speed and heavy-load working conditions of water-lubricated stern bearings combined with the low viscosity of the water usually make these bearings work in mixed lubrication or even boundary lubrication situation,which will inevitably lead to direct contact between the shaft and the bearing and the abrasion of the bearing material.Therefore,water-lubricated stern bearings have higher requirements on wear resistance of bearing material compared with other types of bearings.Studies show that wear failure has become one of the most critical factors leading to the degradation of water-lubricated stern bearing performance.In addition,the elastic modulus of the water-lubricated stern bearing material is only a few percent of that of the metal material,which makes the elastic deformation of the bearing and the thickness of the water film in the same order of magnitude,thus the influence of the elastic deformation on the lubrication performance and structural design of bearing can not be ignored.The design of water-lubricated composite bearings is very difficult,because the performance of bearings is sensitive to materials and structural parameters.The traditional numerical methods of bearing lubrication performance do not take into account the influences of elastic deformation and wear deterioration,so it is difficult to obtain an ideal calculation accuracy.According to statistics,the failure of propulsion system of ship caused by excessive wear of the stern bearing and design defects has become a critical problem that limits the promotion of the ship's reliability.Therefore,it is urgent to carry out researches on the water-lubricated stern bearing materials and design to improve the bearing's performance,operational reliability,and service life.In this article,the effects of filler type,filler contents and counter surface morphology on the friction and wear performance of the water-lubricated bearing materials are studied.Based on this,a kind of modified ultra-high molecular weight polyethylene(UHMWPE)is developed with low friction coefficient and high wear resistance.The influence of bearing structure,working conditions,counter surface morphology and wear deterioration on the bearing performance is studied with CFD-FSI method.Finally,the proposed numerical calculation method,material properties and tribological and lubrication mechanism are verified by bearing tests.The main work of this article is as follows:(1)A simulation analysis approach and an optimum design method are proposed for the water-lubricated polymer composite bearings with the consideration of bearing deformation and wear deterioration.Based on the fluid-structure interaction analysis,the influences of bearing deformation and wear deterioration are taken into account in the simulation of bearing lubrication characteristics.The effects of material parameters,structural parameters and working conditions on the performance of the water-lubricated bearings are studied.The influence of elastic deformation and wear deterioration on the lubrication characteristics of the polymer composite bearings is analyzed.The determination method of the allowable values of elastic deformation and wear depth is also proposed for the water-lubricated stern bearings.In addition,an optimum design method of water-lubricated bearing considering elastic deformation and wear deterioration is elaborated,and a bearing structure is optimized using this optimum method.(2)The friction and wear properties of UHMWPE water-lubricated polymer composites reinforced by different proportions of single and hybrid fiber fillers are studied.The results show that the friction coefficient and wear rate of UHMWPE can be effectively reduced by using carbon fiber,glass fiber or the hybrid of them,and additionally,the wear rate of UHMWPE filled with hybrid filler is smaller than that of single filler,just about 50%.The addition of fiber filler increases the mechanical strength and load-carrying capacity of the matrix,and inhibits the generation and propagation of the fatigue cracks.In addition,the fiber filler polishes the counter surface,and fiber filler can also promote the formation of complete and compact transfer film on the counter surface.The transfer film transforms the friction between metal and polymer into the friction between the transfer film and polymer,thus,the friction coefficient and wear rate of the polymer can be significantly reduced.(3)The effects of the counter surface morphology,the working condition parameters,like the rotational speed and load,on the friction and wear properties of polymer composites are studied,which shows that the counter surface morphology and working conditions have important effects on the friction and wear behavior of the composites.Rough counter surface contributes to the formation of transfer film,which decreases the friction coefficient but increases the wear rate of the polymer composites rapidly.In boundary lubrication,the friction coefficient of modified UHMWPE composites decreases with the increase of rotational speed and load.In dry friction condition,the friction coefficient increases with the increase of rotational speed.This result can be widely used in the practical application of the polymer composites.(4)Water-lubricated UHMWPE stern bearings are prepared based on the modification of polymer bearing materials and the optimum design of water-lubricated bearings,and their tribological and friction properties are also tested.The lubrication characteristics of Thordon SXL,NBR and modified UHMWPE bearings are studied using the prototype test rig of water-lubricated bearing principle.It is found that the starting torque and friction coefficient of the modified UHMWPE bearings are the lowest.At the same time,it is found that the addition of fiber fillers is an effective method to develop the water-lubricated bearing materials with low friction,low wear rate and high load carrying capacity.In addition,the bearing service life test rig is established.Using this equipment,the durability of the modified UHMWPE bearing is evaluated by frequent start-stop tests,and the water film thickness and film pressure distribution of the bearings are also tested.The results show that UHMWPE bearings can form good hydrodynamic lubrication and have good wear resistance,which can meet the service life requirements of the marine stern bearings.