Device for pressing orbiting scroll member in scroll type fluid machine

Claim 1

A scroll type fluid machine comprising: a stationary scroll member having an end plate and a spiral wrap formed on one side of said stationary scroll member so as to protrude upright therefrom; an orbiting scroll member having an end plate and a spiral wrap formed on one side of said end plate so as to protrude upright therefrom, and adapted to be coupled with said stationary scroll member such that said wraps on both scroll members mate with each other to define a plurality of working chambers therebetween; a frame means fixed to said stationary scroll member; a driving means for causing an orbiting movement of said orbiting scroll member, said driving means including a main shaft rotatably carried by said frame means through bearings, an eccentric shaft adapted to transmit the movement of said main shaft to said orbiting scroll member at a position spaced by a distance substantially equal to a radius of relative orbiting movement between said scroll members; a high pressure port means; a low pressure port means; a rotation prevention means disposed between said orbiting scroll member and a stationary part of said machine so as to maintain a fixed angular relationship of said orbiting scroll member to said stationary scroll member; a first back pressure chamber defined by a cooperation of said frame means, said drive shaft and said orbiting scroll member, said first back pressure chamber being adapted to hold a fluid pressure which imparts an axial pressing force to said orbiting scroll member; a first back pressure passage means providing a communication between said first back pressure chamber and at least one of said working chambers which holds an intermediate pressure of the fluid and does not substantially communicate with said low pressure port means and never communicates with said high pressure port means; a second back pressure chamber defined by said eccentric shaft of said driving means and by a shaft supporting portion rotatably and axially movably engaging with said eccentric shaft, said second back pressure chamber being adapted to hold a fluid pressure for imparting an axial pressing force to said orbiting scroll member; a second back pressure passage means including a fluid passage formed in said driving means and adapted to introduce the fluid pressure into said second back pressure chamber so as to impart an axial pressing force to said orbiting scroll member; and wherein the first back pressure chamber and the second back pressure chamber have a volume such that a total axial pressing force applied therefrom to the orbiting scroll member is in accordance with the following relationship: Pb·Ab+Pd·Ad=Fpa+(Ms/Rb)+Fs where: Pb=a suitable intermediate pressure in the first back pressure chamber, Ab=an area of the back surface of the orbiting scroll member to which the pressure Pb is applied, Pd=a high pressure of gas or lubricating oil in the second back pressure chamber, Ad=an area to which the pressure Pd is applied, Fpa=an axial fluid force within the working chambers, Ms=moment imparted to the orbiting scroll member by the radial fluid force within the working chambers, Rb=radius of the end plate of the orbiting scroll member, and Fs=force applied to the sliding surfaces of the end plates caused by hydraulic pressure.