a first embodiment of the present invention will now be described referring to fig1 through 7 . as shown in fig5 a fixed scroll 1 serves as the compressor &# 39 ; s center housing 1d and connects to a front housing 2 . a bearing 4 rotatably supports a rotary shaft 3 , in the front housing 2 . the rotary shaft 3 securely attaches to an eccentric pin 5 , here shaped in the form of a rectangular prism . a balance weight 13 and a bushing 6 are attached to the eccentric pin 5 . the bushing 6 has a nearly rectangular cylinder hole 6a fitted over the eccentric pin 5 . a movable scroll 7 which engages with the fixed scroll 1 is rotatably supported by the bushing 6 via a radial bearing 8 . the fixed scroll 1 has an end plate 1a and a spiral element 1b formed integral with the end plate 1a . likewise , the movable scroll 7 has an end plate 7a and a spiral element 7b integrally formed with the end plate 7a . the bushing 6 fits into a boss portion 7c integrally formed on the front face of the movable end plate 7a . a plurality of gas pockets p are formed between the end plates 1a and 7a and the associated spiral elements 1b and 7b . the volume of gas contained in each pocket p decreases as the pocket shifts toward the center from the periphery of the movable scroll 7 , as shown in fig7 . the front face of the movable end plate 7a forms a movable pressure receiving wall 7d . a fixed pressure receiving wall 2a is formed on the inner wall of the front housing 2 . an anti - rotation device k intervenes between both pressure receiving walls 2a and 7d . this device k prevents the movable scroll 7 from rotating about its own axis . device k , nonetheless , permits the orbital movement or revolution of the movable scroll 7 about the axis of the rotary shaft 3 . more specifically , this anti - rotation device k has a plurality of cylindrical collars 9 ( four in this embodiment ) which are fitted in the fixed pressure receiving wall 2a . device k also has a plurality of cylindrical collars 10 fitted in the front face of the movable end plate 7a , eccentrically displaced at predetermined distances from the associated collars 9 . a ring 11 is disposed between both pressure receiving walls 2a and 7d . formed in the ring 11 are a plurality of through holes 11a ( four in this embodiment ) in which pins 12 are respectively inserted . each pin 12 is engaged with the inner walls of a hole 9a of the associated collar 9 and a hole 10a of the associated collar 10 . as the rotary shaft 3 rotates , the eccentric pin 5 and the bushing 6 revolve . the engagement of each pin 12 with the associated holes 9a and 10a prevent the movable scroll 7 from rotating around its own axis , but allow it to revolve around the axis of the rotary shaft 3 . four elements 11b are formed integral with the front and rear faces of the ring 11 . these elements are spaced at equal angular distances to transmit the compressive reaction force of the refrigerant gas to the fixed pressure receiving wall 2a from the movable pressure receiving wall 7d . a suction port ( not shown ) is formed in the front housing 2 , and a suction chamber s is formed between the movable scroll 7 and the inner wall of the front housing 2 . a rear housing 14 in which a discharge chamber d is formed is securely joined to the rear face of the fixed scroll 1 . a discharge hole 1c is formed in the fixed end plate 1a , and a discharge valve 15 for opening and closing the discharge hole 1c is disposed in the discharge chamber d . the function of the scroll type compressor having the above - described structure will now be described . when the rotary shaft 3 rotates , rotation of the movable scroll 7 is inhibited by the anti - rotation device k . the movable scroll 7 does , however , revolve together with the eccentric pin 5 around the axis of the rotary shaft 3 . refrigerant gas is then supplied into the suction chamber s from the suction port and flows into the pockets p between both scrolls 1 and 7 . as the movable scroll 7 revolves , the pockets p converge toward the center of both spiral elements 1b and 7b . during this convergence , the volume of each pocket p decreases . as a result , the refrigerant gas is compressed in each pocket p and is discharged to the discharge chamber d from the discharge hole 1c . the operation of the anti - rotation device k will now be described with reference to fig6 . each pin 12 engages both the fixed and movable scrolls . a front end of each pin 12 engages the uppermost portion of the hole 9a of the associated collar 9 , while the rear end of each pin 12 is engaged with the lowermost portion of the hole 10a of the associated collar 10 . the movement of each pin 12 is therefore restricted by the inner walls of the associated pair of opposing collars 9 and 10 . as shown in fig6 at the beginning of a revolution , the bushing 6 , the movable scroll 7 and axis o r are located at an uppermost position in their revolution with respect to axis o s . when the eccentric pin 5 and bushing 6 rotate counterclockwise due to the rotation of rotary shaft 3 , the center axis o s of the bushing 6 moves to the lowest position of the movable scroll &# 39 ; s revolution . at this time , each pin 12 moves along the inner walls of the holes 9a and 10a of the associated collars 9 and 10 , maintaining their engagement with the holes 9a and 10a . though not illustrated , the front end of each pin 12 engages with the lowermost end of the hole 9a of the associated collar 9 on the fixed side , and the rear end of each pin 12 engages with the uppermost end of the hole 10a of the associated collar 10 on the movable side . therefore , the engagement of each pin 12 with the associated collars 9 and 10 allows the movable scroll 7 to revolve with a radius of revolution corresponding to the distance , r , between the axes o s and o b . this is illustrated , for example , in fig3 . the balance weight 13 , shown in fig1 and 5 , has an elongated hole 13a where the eccentric pin 5 is inserted . with this pin 5 inserted in the hole 13a , therefore , the balance weight 13 is rotatable together with the pin 5 . the eccentric pin 5 has a pair of guide surfaces 5a on both sides , extending in parallel to the axis of the rotary shaft 3 . the elongated hole 13a and the elongated hole 6a of the bushing 6 are set longer than the cross sectional length of the eccentric pin 5 , i . e ., the short side of the guide surface 5a . therefore , the bushing 6 and the balance weight 13 can move slightly in the radial direction along the guide surfaces 5a of the eccentric pin 5 . a shallow recess 6b is formed in the front end face of the bushing 6 as shown in fig2 . a projection 13b is formed on the center portion of the balance weight 13 , and is fittable in the recess 6b to prevent the radial deviation of the projection 13b and the recess 6b . in this embodiment , the weights of the movable scroll 7 and the balance weight 13 are set in such a way that the centrifugal force f w produced by the revolution of the balance weight 13 is 80 to 97 % of the sum of the centrifugal forces f s and f b respectively produced by the revolution of the movable scroll 7 and the bushing 6 . the guide surfaces 5a of the eccentric pin 5 are inclined at an angle θ with respect to a straight line h passing through the center axis o s of the rotary shaft 3 and the center axis o b of the bushing 6 as shown in fig3 . at the time the eccentric pin 5 revolves , the balance weight 13 revolves together with the movable scroll 7 in the direction x , as shown in fig3 via the bushing 6 . since the sum of the centrifugal force f s of the movable scroll 7 and the centrifugal force f b of the bushing 6 is set greater than the centrifugal force f w of the balance weight 13 , the guide surface 5a of eccentric pin 5 guides the movable scroll 7 and bushing 6 to move with an increasing radius of revolution r , as shown in fig1 . consequently , the spiral element 7b of the movable scroll 7 is tightly pressed against the spiral element 1b of the fixed scroll 1 , thus improving the sealing of the pockets p . the above will be discussed more specifically . during the compressor &# 39 ; s operation , the centrifugal force f w acts on the balance weight 13 , the centrifugal force f b acts on the bushing 6 , and the centrifugal force f s acts on the movable scroll 7 , as shown in fig1 . those centrifugal forces f w , f b and f s can be expressed as a combined force f (= f w + f b + f s ) along the line h , as shown in fig4 . this combined force f consists of two component forces f 1 and f 2 . the first component force f 1 (= f × cos θ ) acts on the eccentric pin 5 itself in the direction perpendicular to the inclined surfaces 5a of the eccentric pin 5 . the second component force f 2 (= f × sin θ ) acts on the bushing 6 and the movable scroll 7 in the direction parallel to the inclined surfaces 5a , pressing the spiral element 7b of the movable scroll 7 against the spiral element 1b of the fixed scroll 1 . therefore , the second component force f 2 improves the sealing of the pockets p , and consequently , the efficiency with which the compressor can compress refrigerant gas . a description will now be given of the relationship between the centrifugal forces and the compressive reaction force of the refrigerant gas . the compressive reaction force f &# 39 ; of the refrigerant gas acts on the eccentric pin 5 in the direction opposing the direction of the first component force f 1 as shown in fig4 . practically , therefore , a bending load f &# 34 ; (= f &# 39 ;- f 1 ) acts on the eccentric pin 5 . this bending load f &# 34 ; is smaller than the compressive reaction force f &# 39 ; ( f &# 34 ;& lt ; f &# 39 ;). should the sum of the movable scroll &# 39 ; s centrifugal force and the bushing &# 39 ; s centrifugal force be unbalanced with the balance weight &# 39 ; s centrifugal force , the bending load f &# 34 ; will be reduced if the centrifugal force f w lies within 80 to 97 % of the sum of the movable scroll &# 39 ; s centrifugal force f 3 and the bushing &# 39 ; s centrifugal force f b . while the magnitudes of the compressive reaction force f &# 39 ; and the first component force f 1 may vary , depending on the number of rotations of the compressor , the compression ratio , etc ., the directions of these forces f &# 39 ; and f 1 will not . if the centrifugal force f w of the balance weight 13 is less than 80 % of the sum of the movable scroll &# 39 ; s centrifugal force f s and the bushing &# 39 ; s centrifugal force f s , the intended performance of the balance weight 13 will be less than desirable . on the other hand , should the centrifugal forces f w exceed 97 % of the sum of the movable scroll &# 39 ; s centrifugal force f s and the bushing &# 39 ; s centrifugal force f s , then the centrifugal force f w will be excessively large in comparison to the sum of the centrifugal forces f s and f b . this is due to the influence of the weight of the movable scroll 7 , the balance weight 13 and variations in manufacturing tolerances of the various component sizes . consequently , this reduces the effectiveness with which the gas pockets can be sealed , and prevents reductions from being made to the bending load f &# 34 ; on the eccentric pin 5 . a second embodiment of the present invention will be described below with reference to fig8 . as mentioned earlier , the combined force f of the centrifugal force f w of the balance weight 13 , the centrifugal force f b of the bushing 6 and the centrifugal force f s of the movable scroll 7 acts on the eccentric pin 5 . this combined force f is transmitted via the eccentric pin 5 to the rotary shaft 3 . in this embodiment , a recess 3c is provided at the outer surface of the large diameter portion 3a , of the rotary shaft 3 . a second balance weight 3d helps to prevent rotary shaft 3 from being dynamically unbalanced by the balance weight 13 and the movable scroll 7 . to form the second balance weight 3d , a recess 3c needs to be formed on the large diameter portion 3a . the rotary shaft 3 can be formed by forging or molding , and the inner wall of the recess 3c may be left as a forged surface . in this case , the recess 3c can be formed without carrying out unnecessary post working . the reduced number of steps needed to manufacture the compressor , as well as improving the yield of manufacturing materials , contributes to reduce the overall cost of the compressor . according to the second embodiment , any deficiency in the centrifugal force f w produced by the balance weight 13 can be compensated by centrifugal force f 3 produced by the balance weight portion 3d of the rotary shaft 3 . this allows the rotary shaft 3 to rotate smoothly , reducing the load on the radial bearing 4 , thereby increasing its durability . a modification of the second embodiment will be briefly described below with reference to fig9 . in this modification , a second balance weight 16 is disposed between the radial bearing 4 and the balance weight 13 in place of the recess 3c and balance weight portion 3d of the rotary shaft 3 . it is therefore possible to counteract the combined force f acting on the rotary shaft 3 with the second balance weight 16 , allowing smooth rotation of the rotary shaft 3 . a third embodiment of the present invention will be described below with reference to fig1 . in this embodiment , a recess 103c in the rotary shaft 3 is formed deeper than the recess 3c in the second embodiment . accordingly , centrifugal force f 3a greater than the centrifugal force f 3 described in the second embodiment is generated on a balance weight portion 103d . in order to generate a centrifugal force f 17 opposite to the direction of the centrifugal force f 3a , a third balance weight 17 is secured to the small diameter portion 3b of the rotary shaft 3 by welding , adhesion or other similar procedure . next , the combined force f is set equal to the centrifugal force f 17 , while the centrifugal force f 3a , produced by the balance weight portion 3d , is set twice as large as the combined force f . further , the distance between the application of the combined force f and the centrifugal force f 3a is set equal to the distance between the application of both centrifugal forces f 3a and f 17 . according to the third embodiment , therefore , the combined force f and the centrifugal forces f 3a and f 17 are completely counteracted and the rotary shaft 3 rotates smoothly , thus preventing excessive loads from affecting the radial bearing 4 . the present invention is not limited to the above - described embodiments , and may be embodied in the following forms . ( 1 ) a circular eccentric pin 5a as shown in fig1 and 12 may be used in place of the eccentric pin 5 having the shape of a nearly rectangular prism . in this embodiment the bushing 6 and the balance weight 13 are pivotable about the pin 5a . in this case , the angle between a line h 1 connecting the center o 5a of the eccentric pin 5a to the center o 2 of the bushing 6 and the aforementioned line h is expressed by γ . the combined force f on the line h consists of a first component force f 1 and the second component force f 2 both of which are determined according to the angle γ . the compressive reaction force f &# 39 ; is similar to those in the above - described embodiments , and acts on the line h 1 in the direction opposite to that of the first component force f 1 , thereby reducing the bending load f &# 34 ; acting on the eccentric pin 5a . the second component force f 2 improves the sealing of the pockets p . ( 2 ) instead of forming the recess 3c in the rotary shaft 3 , a separate balance weight of a material having a greater specific weight than that of the material for the rotary shaft 3 is inserted in the large diameter portion 3a . ( 3 ) a plurality of screw holes ( not shown ) are formed in the outer surface of the balance weight 13 , and the centrifugal force f w is adjusted by changing the number of screws to be engaged with the screw holes or the material of the screws . ( 4 ) in the embodiment shown in fig1 , the weights of the balance weight 13 , the balance weight portion 103d , the balance weight 17 and the like and the distances between points of action of the individual forces are altered so as to counteract the combined force f , the centrifugal force f 3a and the centrifugal force f 17 as a whole .