# A Novel Noncontact Ultrasonic Levitating Bearing Excited by Piezoelectric Ceramics
## 1 Introduction  
Many sophisticated power machines and equipment such as high-speed precision machine tools, aero-engines, and micro gas turbines (MGT) demand greater bearing performance in terms of high-speed capacity, precision, reliability, longevity, low noise, etc. [1; 2; 3]. Traditional contact bearings hardly can meet these requirements due to problems such as wear, heat generation and so on [4]. Therefore, noncontact bearings have been extensively studied. The main types of noncontact bearings widely adopted today include hydrostatic bearings, hydrodynamic bearings, and electromagnetic-suspension bearings. However, these bearings also have limitations [5]. Gas pumps and pipelines are absolutely necessary for hydrostatic bearings, which results in high operation costs and bulky systems [6]. Serious wear will appear in the starting or stopping stage of hydrodynamic bearings, and an eccentricity phenomenon is inevitable [7]. With regard to electromagnetic-suspension bearings, demagnetization and magnetic leakage will affect their performance. Besides, a complicated control system is needed to guarantee their high precision and stability [8]. Hence, improvement of existing bearing technology and development of novel bearing concepts are of great significance. As a promising alternative solution, the ultrasonic levitating bearing based on near-field acoustic levitation (NFAL) was proposed [9; 10]. This kind of bearing carries a load using acoustic radiation force excited by piezoelectric ceramics. Compared with other noncontact bearings, air-supply devices and lubrication are not needed. In the starting or low-speed stage, ultrasonic bearing can generate identical carrying capacity, ensuring good stability. Bearing clearance and carrying capacity can be adjusted through changing the amplitude of the vibration surface. The control system of an ultrasonicbearing is simple, and does not cause electromagnetic interference to sensitive electronic components in the system. The levitation height of ultrasonic bearings can be controlled at the micron level, which can meet precision requirements. Moreover, the friction is very low as the bearing operates at a high speed, which makes it feasible to raise the speed limit [11; 12; 13]. Benefiting from the development of piezoelectric ceramics technology, a lot of ultrasonic bearing concepts and structures excited by piezoelectric ceramics have come into existence. [PERSON] et al. first proposed a novel ultrasonic linear bearing based on elastic hinges and piezoelectric ceramic stacks. Although elastic hinges can magnify the vibration amplitude under excitation of PZT (piezoelectric ceramics of PbZr03-PbTiO3 system) stacks, they also bring about structural complexity and high machining costs [14; 15]. Based on [PERSON]'s concepts, [PERSON] improved the design. The improved linear bearing can suspend an object weighing 800 g at a height of a few micrometers [16; 17]. [PERSON] and [PERSON] et al. presented a new kind of ultrasonic journal bearing, which integrates ultrasonic levitation with hydrodynamic lubrication aimed at solving the wear problem of aerodynamic bearings in the starting or low-speed stage. The force required from one piezo-actuator to produce deformation of lobe depth of 3.5 \(\upmu\)m was about 10 N [18; 19; 20]. [PERSON] proposed a tubular squeeze-film journal bearing. The bearing was designed to be driven at its 13 th mode at the frequency of 16.37 kHz at which the amplitude response was 3.22 \(\upmu\)m [21; 22; 23]. [PERSON] presented an active journal air bearing actuated by high-power piezoelectric transducers. A steel spindle with a diameter of 50 mm had been successfully levitated and driven at low rotational speed. Each of the three ultrasonic transducers can provide a load carrying force up to 51 N at an input power of 50 W [24; 25]. [PERSON] et al.