BACKGROUND OF THE INVENTION 
     This invention relates to a scroll compressor of a differential pressure oil-supply type, and, more particularly to a scroll compressor including means for maintaining a differential pressure oil-supply in a wide operation range. 
     A scroll compressor of the aforementioned type is proposed in, for example, U.S. Pat. No. 4,365,941 wherein an intermediate pressure chamber on the back of an orbiting scroll. 
     A pressure of the intermediate pressure chamber is governed by a pressure of an airtight chamber which is communicated with the intermediate pressure chamber through a communication hole and roughly maintained at an average pressure of the airtight chamber. The pressure of the airtight chamber is governed by a suction pressure according to characteristics of the scroll compressor and that value is determined by a position of the communication hole. In other words, the intermediate pressure is detemined by the suction pressure and the communication hole position independent of discharge pressure. 
     However, in the above-noted scroll compressor, as oil is supplied to the sliding part by a differential pressure between the discharge pressure and the intermediate pressure, if the differential pressure is small or null, the oil cannot be supplied to the sliding part so that the operation is impossible. Also, when the communication hole is provided at the position where the intermediate pressure is low in order to maintain the differential pressure, a force pressing the orbiting scroll against a stationary scroll produced by the intermediate pressure is insufficient when the pressure differential between the suction pressure and the discharge pressure is so great that sealing between tips of lapping parts and flat plate parts becomes impossible. 
     It is an object of the pressent invention to provide a scroll compressor which facilitates maintaining an oil-supply pressure within a wide operation range. 
     In a scroll compressor, when suction pressure is constant and discharge pressure is low, the force pressing the orbiting scroll against the stationary scroll can be small compared to when the discharge pressure is high. According to the present invention, a scroll compressor is provided with a valve which can switch connection of an intermediate pressure chamber to a lower intermediate pressure or to a higher intermediate pressure of a compressor part, and is constructed so that when the discharge pressure is below a certain value, the intermediate pressure chamber is connected to the lower intermediate pressure side of the compressor by the valve to maintain oil-supply pressure and, when the discharge pressure is above the certain value, the intermediate pressure chamber is connected to the higher intermediate pressure side of the compressor part by the valve which maintains the oil-supply pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of an embodiment of a scroll compressor of the present invention; 
     FIG. 2 is an enlarged sectional view of a main part of FIG. 1; 
     FIG. 3 is a sectional view for explaining an operation of the valve shown in FIGS. 1 and 2; 
     FIG. 4 is a sectional view of another embodiment of a scroll compressor of the present invention; 
     FIG. 5 is a sectional view of the main part of a scroll compressor according to further another embodiment the present invention; 
     FIG. 6 is a perspective view of a valve body of a valve shown in FIG. 5; and 
     FIG. 7 is a sectional view of FIG. 5 for explaining the operation of the valve. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings wherein like reference numerals are used throughout the various views to designate like parts and, more particularly, to FIG. 1 according to this figure, a scroll compressor includes a compressor part generally designated by the reference numeral 2 disposed at an upper part in an airtight container 1 and a motor part 3 disposed at a lower part in the airtight container 1, with the compressor part 2 including an orbiting scroll 6 and a stationary scroll 7 which mesh with each other. The orbiting scroll 6 includes a flat end plate 6a and a spiral lapping part 6b vertical to the flat end plate 6a. The stationary scroll 7 includes a flat end plate 7a and a spiral lapping part 7b vertical to the flat end plate 7a and fixed to a frame 5. A lower end of a crank shaft 4, joined with a rotating shaft of a motor 3, is immersed in an oil sink 9 defined at the bottom of the airtight container 1. An oil path 4a whose lower end is opened in a shaft axis and whose upper end is opened at an eccentric position with respect to the rotating shaft axis is provided in the crank shaft 4. A communication hole 10 is provided in the orbiting scroll 6 at a position where an intermediate or back pressure between a suction pressure and a discharge pressure is obtained and an intermediate pressure chamber 8 in which the pressure is at the intermediate value is also provided. An upper end of the crank-shaft 4 is coupled with a boss part 6c extending downwardly from the orbiting scroll 6. 
     As shown in FIG. 2, a valve 13 connects or disconnects a communication between a suction chamber 11a of the compressor part 2 and the intermediate pressure chamber 8, with the valve including a cylinder 14 formed in the stationary scroll 7, a valve body 15 incorporated in the cylinder 14, a compession spring 16 for pressure setting and a stopper ring 17 for preventing the valve body from coming out. One side of the cyliner 14 communicates with the airtight container 1 and the other side of the cylinder 14 communicates with the intermediate pressure chamber 8 through a communication hole 18 and also communicates with the suction chamber 11a through a communication hole 19. The valve body 15 includes a surface 15a for receiving the internal pressure of the airtight container 1 at one end and a surface 15b for receiving the internal pressure of the intermediate pressure chamber 8 on the back side of the pressure receiving surface 15a. The valve body 15 also has a needle part 15c which can open and close the communication hole 19 at its other end. The compression spring 16 is placed between the pressure receiving surface 15b of the valve body 15 and a side wall of the cylinder 14. The stopper ring 17 is fitted into an annular groove provided along an inside wall of the cylinder 14 and prevents the valve body 15 from coming out. 
     When the pressure differential between the pressures applied to both pressure receiving surfaces of the valve body 15 of the valve 13 is higher than the spring force of the compression spring 16, the valve body 15 is shifted to the position where the needle part 15c closes the communication hole 19, in other words, it disconnects the connection between the intermediate pressure chamber 8 and the suction chamber 11a. When the pressure differential is lower than the spring force of the compression spring 16, the valve body 15 is shifted by the spring force to the position where its needle part opens the communication hole 19, in other words, it communicates the intermediate pressure chamber 8 to the suction chamber 11a. 
     Referring to FIG. 1, the orbiting scroll 6 orbits as the boss part 6c is rotated in the intermediate pressure chamber 8 of the frame 5 by the rotation of the crankshaft 4. By the movement of the contact points between the orbiting lapping part 6b and the stationary lapping part 7b, the gas drawn through a suction tube 11 is compressed while flowing through the spiral chamber from the outside to the inside, is discharged into an airtight container 1 through a discharge outlet 7c provided at the center of the stationary scroll 7 and flows through a path 7d provided on an outer periphery of the stationary scroll 7 and a path 5a of an outside periphery of the frame 5. A part of the gas having passed through the paths 4d and 5a flows through outside periphery paths 3a and 3b of the motor 3 and the other part of the gas flows between the frame 5 and the motor 3, and all gas is discharged out of the machine through the discharge tube 12. The volume of the airtight chamber, defined by the lapping parts 6b and 7b and the flat end plates 6a and 7a of the orbiting scroll 6 and stationary scroll 7, is reduced according to the movement from the outside to the center so that the pressure of the gas contained therein is increased according to the movement. The pressure in the intermediate pressure chamber 8, defined by the orbiting scroll 6 and the frame 5, is, as mentioned above, maintained at the intermediate pressure between the suction pressure and the discharge pressure by the communication hole 10. With the pressure differential between the intermediate pressure and the internal pressure of the compression part, the orbiting scroll 6 is pressed against the stationary scroll 7 to maintain the tight contact of the sealing part of a gap between the tips of the lapping parts 6b and 7b and the flat plate parts 7a and 6a. Moreover, as the pressure in the airtight container 1 is the discharge pressure and higher than that in the intermediate pressure chamber 8, freezer oil in the oil sink is pushed up through the oil path 4a in the crank shaft by the differential pressure between those two pressures and supplied to a sliding part. 
     Referring to FIGS. 2 and 3, an operation of the valve 13 is hereinunder described. Ps denotes the suction pressure; Pb, the intermediate pressure; Pd, the discharge pressure; K, the spring force of the compression spring; SA, the cross sectional area of the cylinder 14; and SB, the cross sectional area of the communication hole 19. Because the relation Ps=Pb=Pd is maintained before start, the valve body 15 of the valve 13 is pressed against the stopper ring 17 by the spring force K as shown in FIG. 3. In other words, the valve body 15 is shifted to the position where it fluidly connects the intermediate pressure chamber 8 to the suction chamber 11a. When the compressor is started at this state, if the discharge pressure Pd is increased to the value which conforms to following formula (1): 
     
         Pd×SA&gt;Pb×SA+K                                  (1) 
    
     the valve body 15 is shifted to the position shown in FIG. 2 and the communication hole 19 is closed by its needle part 15c. In this state, the inside of the airtight container 1, the suction chamber 11a and the back pressure chamber 8 are separated from each other and the compressor is driven essentially under the same condition as if the valve did not exist. 
     When the discharge pressure Pd cannot reach a value high enough to conform to the formula (1), in other words the differential pressure between pressures applied to both pressure receiving surfaces of the valve body 15 is lower than the spring force K, the valve body 15 is maintained at the position shown in FIG. 3 and the intermediate pressure chamber 8 is communicated with the suction chamber 11a through communication holes 18 and 19 so that the intermediate pressure Pb is low. The fact that the intermediate pressure Pb is maintained at a value lower than the normal intermediate pressure means that the differential pressure between the discharge pressure and the intermediate pressure is large and the oil-supply pressure (the differential pressure between the discharge pressure and the intermediate pressure) is maintained; therefore, a wide range of operation can be obtained. When the intermediate pressure is maintained relatively low, a pressing force of the orbiting scroll 6 is small. However, as the discharge pressure is also low, the pressing force may be small and this is also a convenient point. 
     Then the case when the discharge pressure is reduced during operation, for instance by reduction of the load, is hereinunder described. While the needle part 15c of the valve body 15 closes the communication hole 19, if the discharge pressure is decreased to the value which conforms to the following formula (2): 
     
         Pd×SA&lt;Pb×(SA-SB)+Ps×SB+K                 (2) 
    
     the valve body 15 is shifted to the direction of opening the communication hole 19 and the intermediate pressure is maintained at a pressure lower than a normal pressure. 
     FIG. 4 shows another embodiment of the present invention and this embodiment is different from the one shown in FIG. 2 at the point that the position where a communication hole 19a is opened is provided in an airthight chamber 11b of a compressor part 2 in which a pressure is lower than the intermediate pressure. 
     Also in this embodiment, operation principle is basically the same as the one shown in FIG. 2 and, if Ps is substituted by Ps&#39; (a pressure in the airtight chamber llb) in the formulae (1) and (2), the same performance and effect as described by FIG. 2 can be realized. 
     Still another embodiment of a scroll compressor of the present invention is hereinunder described with reference to FIG. 5 through FIG. 7. This embodiment is the same as FIG. 1 except for a valve 113 and a part of a compressor part 200. 
     In FIG. 5, the scroll compressor is provided with a valve generally designated by the reference numeral 113 at the stationary scroll 107. The valve 113 is constituted by a cylinder 114 formed in the stationary scroll 107, a valve body 115 incorporated in the cylinder 114, a spring 116 and a stopper ring 117. A pipe 118 communicating with the intermediate pressure chamber 8, a first port 119 communicating with a lower intermediate pressure side of the compressor part 200 and a second port 120 communicating with a higher intermediate pressure side of the compressor part 200 are connected to the cylinder 114. One end surface 115a of the valve body 115 is formed so as to receive a fluid pressure which is normally the discharge pressure and a seat part 115c which opens and closes the first port 119 protrudes from the other end surface 115b of the valve body 115. An annular groove 115d is provided at a central part of the valve body 115 and a valve hole 115e which is opened at the annular groove 115d and the end surface 115b is provided. The spring 116 is placed between a side wall of the cylinder 114 and the end surface 115b of the valve body 115 and actuates the valve body 115 toward the direction of opening the first port 119. The stopper ring 117 is attached to an inner wall of the cylinder 114 and prevents the valve body 115 from projecting out. 
     With the valve 113 of the above constitution, when the valve body 115 is shifted to the position where the first port 119 is closed by the seat part 115c, the pipe 118 is fluidly connected to the second port 120 through the annular groove 115d and the valve hole 115e and, when the valve body 115 is shifted to the position where the valve body 115 touches the stopper ring 117 as shown in FIG. 7, the pipe 118 is connected to the first port 119. 
     Then the operation of the embodiment of FIG. 5 is as follows: 
     Because the pressures in the compressor are normally balanced when the compressor starts, the valve body 115 of the valve 113 is shifted to the position where it touches the stopper ring 117 by the spring 116 as shown in FIG. 7. At this state, the pipe 118 is connected to the first port 119. In other words, the intermediate pressure chamber 8 is connected to the lower intermediate pressure side of the compressor part 200 when the compressor starts. 
     After the compressor starts, when the discharge pressure exceeds a certain value, the discharge pressure applied to one end surface 115a of the valve body 115 overcomes the spring force of the spring 116 and the valve body 115 is shifted to the position shown in FIG. 5. Then the first port 119 is closed by the seat part 115c and at the same time the pipe 118 is connected to the second port 120 through the annular groove 115d and the valve hole 115e. In other words, when the discharge pressure exceeds a certain value, the intermediate pressure chamber 8 is connected to the higher intermediate pessure side of the compressor part 200. 
     As described above, when the discharge pressure is below a certain value, the intermediate pressure is maintained at a relatively low value and, when the discharge pressure exceeds a certain value, the intermediate pressure is maintained at a relativey high value. The fact that the intermediate pressure is maintained at a relatively low value when the discharge pressure is low means that the oil-supply pressure, the pressure differential between the discharge and the intermediate pressure, can be maintained, and the range of operation can be widened in terms of the oil-supply. When the intermediate pressure is maintained at a relatively low value, the pushing-up force for the orbiting scroll 6 is small. However, the discharge pressure is also low in this case and the pushing-up force may be small and this is also a convenient point. 
     As described above, the intermediate pressure is controlled in accordance with the fluctuation of the discharge pressure by a valve of simpler structure to facilitate maintaining sufficient differential pressure between two pressures required for oil-supply and hence a wider range of operation is possible.
A scroll compressor including a stationary scroll and an orbiting scroll, with an intermediate pressure chamber being defined on a back side of the orbiting scroll member and being in communication with a pressure chamber through a communication hole to produce a force for pressing the orbiting scroll against the stationary scroll. Oil in an oil sink is supplied to sliding portions by a differential pressure between a pressure in the air tight container and a pressure in the intermediate pressure chamber. A valve is provided for switching the connection of the intermediate pressure chamber to a higher intermediate pressure side or to a lower intermediate pressure side of the compressor. The valve is constructed such that the intermediate pressure chamber is connected to the lower intermediate pressure side of the compressor when a discharge pressure of the compressor is lower than a certain value and connected to the higher intermediate pressure side of the compressor when the discharge pressure is higher than a certain value so that a proper oil supply pressure is maintained over a wide operational range.