The Tribological Problems Research Of Oil-free Scroll Compressor Faces Based On Fractal Theory

As a new high performance displacement compressor, scroll compressor has characteristics of compact structure, smooth running and low noise. With the development of industry, it has been more and more widely applied, from refrigeration and air conditioning to oil-free felids, such as medicine, food and fuel cell. There are many friction pairs due to the specific configuration and compression process of scroll compressor. In addition, the efficiency and performance of scroll compressor are largely influenced because of the friction loss. Consequently, in order to realize the oil-free technique, study of tribological properties of friction pair is one of the key problems in this research. The surface topography of friction pair has a big effect on performance of compressor as well as the friction, wear loss, mixed lubrication characteristics and contact rigidity of friction pair interaction surfaces. Friction force and friction coefficient are the two important parameters to reflect the friction pair tribological behavior, meanwhile, friction signal also contains much information which can reflect characteristics and status of tribological system. So the research on the correlation of friction signals and initial surface of friction pair will have great significance in process, quality control, service life improvement of machinery parts. Friction characteristics of oil-free scroll compressor friction pairs not only depend on the running conditions and the load, but also on the materials, surface initial characteristics and faces fractal in operation processing of friction pairs. So establish mathematical model of fractal force and predict friction behavior of oil-free scroll tribological system though analysis of real microcosmic contact form and force of friction pair faces will provide some reference to dynamic characteristics analysis and theory design of oil-free compressor.In this research, basing on the basic principle of tribology and fractal theory, self-lubricating materials PEEK,PI,PES and scroll compressor used material QT were selected to study the friction characteristics through serials experiments, and results show that friction characteristics of PEEK is better than PI and PES. In order to investigate the correlation of friction force and coefficient signals of QT/PEEK and initial surface of PEEK, different surface profiles of PEEK were measured with stylus profilometer, and their surfaces were scanned by JSM-5600LV electronic microscope , then friction tests of QT/PEEK were carried out on an UMT-3MT wear tester. Friction force and coefficient signals and data were collected during the tests. The fractal behaviors of initial surfaces and friction signals were studied with fractal theory and the fractal dimension of them were calculated using the method of structure function measurement. Moreover, according to the configuration, compression process and force analysis of oil-free scroll compressor, prediction friction force fractal model for fixed and orbiting scroll faces rubbing pair was derived based on M-B fractal model , fractal prediction model of sliding friction force and analysis of real microcosmic contact form and force of fixed and orbiting scroll faces. Verification and analysis of the model were made through an example.Results show that the initial surface of PEEK and friction signals both have obvious fractal characteristics, under the same velocity and same initial surface, the value of fractal dimension increased with the increase of contact load, and under the same velocity and each loads, friction signals of preliminary running-in stage and initial surfaces of PEEK are both positive correlations. And according to the prediction model, there is an optimum fractal dimension of scroll surface, on this optimum fractal dimension, friction force was reduced with the decreases of G*and A*r. In addition, material property of fixed and orbiting scroll faces make a great affect to friction force, so the choice of appropriate self-lubricating material and rubbing pair of oil-free scroll compressor is significant for its operational performance and working life.