Research On An Innovative Squeeze Film Bearing Utilizing Traveling Waves

Non-contact fluid lubricated bearings such as hydrostatic/hydrodynamic and aerostatic/aerodynamic bearings are widely utilized in ultra-precision machinery such as machine tools, measuring instruments, semiconductor manufacturing equipments et. al. The biggest advantage of non-contact bearings is that they can deliver low friction support, which is vital for achieving accurate positioning. However, usage of these types of bearings is limited due to their working principles. For example, hydrodynamic and aerodynamic bearings are designed based on self-lift feature at high rotation speed. As a result, they suffer from high friction and wear particularly during start and stop. On the other hand, hydrostatic and aerostatic bearings can overcome this, but they need external pressurized sources such as compressors, tanks, bulky regulators and tubing systems to supply constant pressurized bearing medium, which limits their use in compact ultra-precision machines and devices. In order to cope with the contradiction of perfor mance and size of conventional non-contact bearings, Salbu first developed squeeze air film bearings in 1964. Although squeeze film bearings do not require external pressurized sources and their performance is not influenced by slipping velocity of friction pair, their current performance could not been close to conventional non-contact bearings, furthermore squeeze liquid film bearings cannot realize non-contact lubrication. For improving squeeze film bearings’ performance and developing a non-contact squeeze liquid film bearing, based on peristaltic transport principle, author first proposes a novel non-contact liquid bearing, which utilizes traveling waves to generate liquid film. The feasibility and performance of the proposed bearing are analyzed numerically and experimentally in this paper.Based on fluid peristaltic transport, the traveling wave bearing which utilizes propagation of traveling waves on an elastic thin-film of bearing surface to squeeze lubricant fluid into the clearance between bearing and guide surface s for levitating the bearing and carrying load. Therefore non-contact lubrication can be realized. To verify the proposed bearing principle requires the analysis to the pressure distribution over the bearing clearance under the influence of traveling wave peristaltic transport. The flow inside the proposed bearing is too complicated to analytically study because of the complex traveling wave deformation, close-end flow, varied flow direction and fluid film height, as well as the large ratio of the fluid film length to height. Therefore, in this paper, with considering the principle and structure of the proposed bearing, a mathematic model for the traveling wave on an elastic thin-film is developed based on Euler-Bernoulli beam theory; with considering moving boundary and deforming solution field, a numerical simulation model for the unsteady flow due to the traveling wave motion of the bearing surface is developed based on transient 2-D Navier-Stokes equations over deforming control volumes. A solver for the developed simulation model is implemented using C++ language in an open-source computational fluid dynamics(CFD) software open FOAM. Utilizing this solver, a number of simulations are conducted to obtain pressure distributions in a variety of driving conditions, which provide a tool for the numerical analysis to bearings’ load-carry capacity.Based on simulation, the characteristics of lubricant fluid flow of traveling wave bearings are analyzed numerically. Simulation results depict that the traveling wave bearing successfully generates periodic floating force with a non-zero and steady-state value, by which non-contact lubrication can be realized. The periodic floating force causes small vibration, which restricts the bearing’s application in precision and ultra-precision field. For improving the accuracy of the bearing, author proposes a floating force stabilization technique, which utilizes compensating voltage to introduce additional squeeze into traveling waves for actively controlling average height of bearing clearance. The feasibility of the stabilization technique is proved by simulation. Utilizing the stabilization technique, a lot of numerical analysis in a variety of driving conditions are carried out to clarify the effect of amplitude and frequency of traveling waves, clearance height an d fluid viscosity on the steady-state value of floating force. The results of numerical analysis put forward the technique guidance to build a prototype based on the proposed bearing principle.In order to verify the proposed bearing experimentally, author design s a principle-verification prototype utilizing the thin-film piezoelectric actuators. The key structure dimensions are optimized in Ansys. A driving system utilizing a real-time DSP and a high speed voltage amplifier is developed. The traveling wave deformation on the bearing surface is examined by a laser confocal displacement sensor. The examination results reveal that the prototype successfully generates traveling-wave-like deformation which propagates inward as the driving voltage. Fluid peristaltic transport can be realized by utilizing this generated traveling wave.In order to clarify the practicability of the prototype, firstly the developed prototype is demonstrated to float. The experimental results show that the traveling wave bearing can levitate successfully, but the bearing is significantly inclined due to the flatness error of bearing surface and the asymmetry of traveling waves. The inclination of bearings lowers their accuracy and performance. For suppressing the inclination, an adjust apparatus is built based on differential screws structure. Utilizing this indication adjust apparatus, the parameter dependencies of amplitude and frequency of driving voltage and average bearing clearance height on the steady-state value of floating force is clarified experimentally to validate the correctness of simulation. The results of experiment and simulation basically consist. The experimental results state that the prototype can generate periodic floating force with a non-zero and steady-state value, by which non-contact levitation can be realized, but the prototype vibrates with amplitude of several micron s due to the periodic floating force. For proving the availability of the proposed stabilization technique, a steadier levitation with compensating voltages is demonstrated and the vibration of prototype is significantly suppressed.