Study On Characteristics Of Ultrasonic Levitating Bearings With Ability Of Carrying Radial And Axial Loads

Bearings are the basic components of motive power machines and equipment.The primary function of bearings is to support the mechanical moving parts,reduce the friction in relative motion process between two moving components,and keep the kinematic accuracy in the meantime.Bearings have been widely used in the industrial field,and the bearing technologies have developed rapidly due to the continuous special requirements in different applications.Based on piezoelectric driving theory and Near Field Acoustic Levitation(NFAL)effect,the ultrasonic bearings suspend the loads depending on the acoustic radiation force.Compared with traditional noncontact bearings,ultrasonic bearings have simpler design process,easier control strategy,smoother start-stop process,and lower operation and maintenance cost.On account of the ability of self-aligning in the radial direction,this kind of bearings have higher limit speeds.The research on ultrasonic bearings will promote the development of the technologies of noncontact bearings,and exploit their application area.A novel ultrasonic bearing with a capability of carrying bidirectional loads is proposed,and the suspension theory and experimental study are conducted in this paper.According to actuating principle of the sandwiched type piezoelectric transducers,the acoustic wave's propagation path in the solid is changed by means of changing the vibration output end's sectional dimensions.Through the above approach,the vibration in one direction is transformed into vibrations in two directions.Based on near field acoustic levitation effect,a single-axis ultrasonic bearing with bidirectional load-carrying capacity driven by one piezoelectric transducer is presented.Using electric-acoustic equivalent method,the acoustic impedance network model is established to acquire the bearing's output impedance and predict the bearing's working frequency and minimum impedance accurately.Based on Finite Element Method(FEM),the modal analysis and harmonic response analysis are carried out to obtain the bearing's ideal mode shapes and output amplitude at the resonant frequency.Analysis results of finite element method can guide the determination of the bearing's key structural parameters and evaluate the performance of the bearing.The levitation experiments of the bearing in radial and axial directions are also conducted to validate the bearing's bidirectional load-carry capacity.The levitation force is one of the most important performance indexes of the ultrasonic bearings.Accurately prediction of the bearing's levitation force is one of vital steps in the bearing's development process.The radial and axial levitation forces of the single-axis ultrasonic bearing are motivated by the normal vibration of radiator's concave surface and flexural vibration of lateral surfaces,respectively.The vibration modes in the two direction are different,and sound fields generated by different vibrations are also different.Taking account of the ununiform of the bearing's air film under different levitation heights and surface roughness of the bearing and the rotor,the radial levitation theoretical model is built based on the nonlinear acoustics.According to the changing rule of gas film's volume and sound pressure,the bearing's axial levitation theoretical model is established based on thermodynamics.The radial and axial levitation experiments are carried out in order to verify the above two levitation theoretical models.For the sake of realizing the enveloping of the bearing's radial radiation surface to the rotor in the whole circumferential direction and guarantee the rotor's steady running,a three-axis levitating ultrasonic bearing actuated by three piezoelectric transducers is put forward.This kind of bearings have the ability of self-aligning which can improve the bearing's running stability at high speeds.Based on the modified Reynolds equation,the hydrodynamic equation of the three-axis ultrasonic bearing is built taking the gas film's inertial effect,surface topography,rarefaction effect,and boundary effect into consideration.The Crank-Nicolson weighted implicit format in finite difference method is adopted to calculate the hydrodynamic equations in the space and time.Then the sound pressure distribution in the bearing's air film is obtained and the radial static and dynamic levitation forces of the bearing are acquired by surface integral.A levitation testing system is built to prove the accuracy of the radial levitation force model of the three-axis ultrasonic bearing.Ultimately,the influence of the kinetic parameter of transducers,working medium,bearing clearance,and environmental factors on the bearing's radial levitation force is studied to guide the bearing's design and the selection of working medium and work environment.Friction characteristics exert a significant impact on the power consumption,temperature rising,and longevity of the ultrasonic bearings.The moment-equilibrium method is used to test the three-axis ultrasonic bearing's starting torque;Swinging rod method is presented to acquire the bearing's friction torque at low speeds;A friction torque measurement scheme at high speeds is also designed to reveal the relationship between the rotor's rotational speed and the bearing's friction torque.The stability-testing system of the bearing at high speeds is established which can be used to acquire the rotor center's trajectory in real time.Based on Short-Time Fourier Transform(STFT)method,the two-dimensional or three-dimensional time-frequency spectrum of the rotor center's trajectory data can be obtained to evaluate the bearing's running stability at high speeds.The experimental results demonstrate that the bearing shows a good stability at start-stop stage,and speed-up and speed-down process.The experiments also reveal that the bearing's limit speed can reach to 30000 r/min.At last,the effect of the rotary speed,carrying load,and bearing clearance exerting on the bearing's running stability is studied and the measures improving the bearing's running stability are also presented.