The efficiency of the scroll compressor which employs the axial and the radial sealing techniques is competitive with that of the other rotary and the reciprocating compressors. The bearing load and the torque are smooth so that quiet operation is realized. 'l'herefore the machine is suitable particularly for air conditioning application. The authors have studied the scroll compressor which utilizes an involute of a circle. The displacement volume is calculated. The axial, tangential and radial pressure components are then obtained by assuming the isentropic or polytropic processes. The equations of motion are established for the orbiting scroll and the Oldham coupling. These equations are solved analytically and the motion of the orbiting scroll and the Oldham coupling is calculated. The motion of the orbiting scroll is influenced by the inertia force of the Oldham coupling, and the radial sealing force changes with a frequency twice that of the rotation. The overturning moment \lhich acts on the orbiting scroll is estimated, and the stability condition for the orbiting scroll is clarified. INTRODUCTION The scroll machine was invented by Creuxll in 1905 and is currently attracting the designers' attention because of its high efficiency and smooth operation. Application for car compressors and air conditioning is commercialized in Japan2l ,3), 4) and further development is expected. Axial and radial sealing are the most critical techniques and have been recognized from the old days. Creuxll referred to the tip seal (steam-tight in Reference (1)) which was pressed axially by the spring. The radial sealing techniques are 487 also known;S) ,6) they employ the centrifugal force of the rotating parts or produce the radial component from the pressure load by a mechanism like the swing link. The idea of the scroll fluid machinery was introduced; it was eguipped with axial and tangential sealing7),8). The practical development of this scroll compressor was also conducted9 l ,10). The scroll compressor appeared on the market in 1981 utilizing these two sealing techniques2l . The authors realized that the scroll compressor has inherent advantages (good sealing, smooth operation, stillness, no valve, simple structure, low rubbing speed, etc.) and analyzed the machine theoretically. The displacement volume is obtained from the geometric character of the involute of a circle. The tangential, radial and axial pressure loads are calculated by assuming the isentropic or polytropic compression processes. The motion of the orbiting scroll and the Oldham coupling has not been studied so far. To clarify the design conditions, the equations of motion are established for these two parts of the scroll compressor, including the friction. The equations are solved analytically and the forces acting on the orbiting scroll and the Oldham coupling are calculated. The inertia force of the Oldham coupling istransmitted to the orbiting scroll. The radial sealing force is necessarily influenced and changes with a frequency twice that of the rotation. The overturning moment which acts on the orbiting scroll is estimated, and the stability condition is given for the orbiting scroll. GEOMETRIC THEORY OF SCROLL COMPRESSOR Oisplacement Volume and Build-in Volume Ratio The major geometric parameters of the scroll are shown in Fig. 1. y a fbllus cl a Ba!e Cirde SOOII Atdl Scroll Thickness Scroll~ I 1\UTber d Sat>lls4 1:1 lrM:lklle IniTial twje )