| High-frequency vibrations exist in many mechanical systems. Most often they are not desired. However, in ultrasonic motor (USM), high-frequency vibrations have been properly used. USM is a new type of driver which is developed in 1990s. Inverse piezoelectric effect and ultrasonic vibration are used to transmit power by friction in USM. Due to its unique operational principle, USM is considered to be a promising actuator in aerospace environment. However, there are a few studies on adaptability and friction drive mechanism of USM in vacuum and low temperature. These restrict the application of USM in aviation and aerospace field. So the above aspects will be studied systematically in this paper.Driving properties of USM were studied in vacuum and low temperature. Firstly, a simulating tester of driving properties of USM was designed. Then, variations of load property, stability of rotational speed and wear of friction material of with vacuum degree and temperature were experimentally studied using a 60 traveling wave type ultrasonic motor (TWUSM). Driving characteristics of ultrasonic motor in vacuum and low temperature were analyzed in according to the composition of USM and ultrasonic vibration antifriction. Research methods of friction drive mechanism of USM were proposed.The oval-shaped ultrasonic vibration between stator and rotor in USM was broken down into horizontal vibration and vertical vibration in order to reveal the friction drive mechanism of USM. Theoretical models of friction reduction in normal sliding both by horizontal and vertical ultrasonic vibrations were established individually. It was pointed out that the larger friction reduction by horizontal vibration and the smaller friction reduction by vertical vibration in normal sliding, the higher output torque would be obtained by USM. The effects of horizontal vibration and vertical vibration on wear of friction material (FM) were analyzed.A new friction rig with ultrasonic vibration used in vacuum and low temperature was developed for experimental study of friction reduction by ultrasonic vibration. It consisted of six parts: ultrasonic vibration part, sliding driving part, force applying part, friction measuring part, cooling part and mechanical supporting part. This offered reliable experimental platform for studying friction drive properties of USMAntifriction properties of ultrasonic vibration were studied experimentally in vacuum and low temperature. The experimental results were compared with theoretical predictions. Variations of friction reduction both by horizontal ultrasonic vibration and vertical ultrasonic vibration with influencing factors were obtained. The reasons of driving characteristics change of USM in vacuum and low temperature were found with respect to friction. In order to improve the output torque and stability of USM, some measures were proposed to weaken the friction reduction effect by ultrasonic vibration in USM.Effects of ultrasonic vibration on wear of FM of stator and rotor were studied in atmosphere and vacuum. Wear properties of FMs of stator and rotor were compared when they slid with different ultrasonic vibrations, friction pairs and in atmosphere and vacuum. Wear mechanism of FM under ultrasonic vibration were obtained.Contact state of two surfaces under vertical ultrasonic vibration was studied by an electric contact method. Variations of contact time with vibration amplitude, normal force and environmental pressure were obtained. Ultrasonic levitation testing system was built and levitation characteristics were studied under different environmental pressures. Experimental results were compared and analyzed with theoretical predictions. The experimental results had a guidance to analyze contact property of stator and rotor of USM in vacuum.
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