Study On Water Lubrication Mechanism Of Rubber Bearings

Water-lubricated rubber bearings (WLRB) are widely used in ships, water pumps, etc., in virtue of their well shockproof characteristic, bedload and wear resistance. But the mechanism of lubrication and friction are still unclear. In this dissertation, the investigation is focused on supplying the gap by computer simulation and experimentation.Due to the low viscosity of water and the superelasticity of rubber, when the elastohydrodynamic lubrication (EHL) theory is applied to WLRBs, we encountered two difficulties, one is the choice of water lubrication equation and the other is the computation of rubber's elastic deformation. Taking into account of the inertia effect, a Reynolds equation based on cylindrical coordinate system, is derived from continuity equation and Navier-Stokes equation. From the stress-strain curves of compression test results, it is founded that the rubber used for WLRB can be considered as linear elastic within the specified range in this investigation. Assuming that rubber is linear elastic, an EHL lubrication model for WLRB is established, after comparing with the numerical results in different liquid states to choose the lubrication equation. According to this model, the EHL analysis of the concave-type and flat-type bearing are carried out in aid of a finite element software MARC.EHL analysis reveals that at low load, for concave-type bearing, it is impossible to form a hydrodynamic water film to separate the shaft and the rubber completely. There exists continuous lubricant film and macro-scale direct contact. Therefore, the concave-type bearing is in mixed-lubrication state. In addition, numerical results show that liquid pressure can produce 10-5m radial displacements on WLRB, which is coincided with the liquid pocket phenomenon discovered by test study. EHL results also reveal that for flat-type bearing, it is unable to develop a liquid film, so it is in boundary-lubrication state.Geometry is the main reason to cause the great difference of liquid pressure between concave-type and flat-type bearings. Concave-type bearing's convergent clearance is thin and long, whereas the flat-type bearing's is thick and short. After journal and bearing contacting each other, contact configuration is different due to the difference in geometry. Contact analysis shows that the contact area of concave-type bearing is a continuous region except the groove, whereas for flat-type bearing, only the center part of theload-carrying stave on WLRB is contacted with the shaft. Within the range of load in this computation, the contact area of concave-type bearing is almost twice of flat-type bearing. The thin and long convergent clearance, the big continuous contact area and the small contact stress are propitious to produce higher liquid pressure.The friction performance testes are carried out for concave-type and flat-type bearings. Results show that, at low load, the experimental data is consistent with the EHL analysis result, and whether a part of liquid film can be generated largely affect the friction performance of WLRB. Accordingly, structure design is one of the important ways to improve friction performance. On the other side, it should be realized that when the lubrication state is changed, the condition availing to generate liquid pressure may become the factor for reducing friction performance.Statistic method is used to analyze the correlation between static contact area and dynamic friction force. The results clarify that there is a linear correlation with significant level. Therefore, it is proposed that boundary-lubrication controls the friction performance, and material selection is an important approach to improve the performance of WLRB.