Double rotary piston positive displacement pump with variable offset transmission means
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to rotary pumps and more specifically to double rotary piston pumps. 
     2. Related Art 
     Conventional double rotary piston pumps operate by rotating a pair of cylindrical, i.e., generally disc-shaped, pistons within cylindrical chambers about offset parallel axes and in opposite directions. This rotation is typically accomplished by applying a driving rotation via a drive shaft or drive gear to the pistons at points offset from their centers to provide cooperating eccentric motion to each piston. To maintain pumping efficiency, the pistons should remain in mutual contact during the entirety of each cycle or period of rotation. However, in pumps of this type in the prior art, the pistons tend to move apart and lose contact with each other (&#34;gap&#34;) during a portion of each cycle of rotation. This gapping phenomenon is caused by relative translation movement of the eccentrically-mounted pistons and is well known in the art, as evidenced by the disclosure of U.S. Pat. No. 1,837,714 to Jaworowski dated Dec. 22, 1931 (see page 1, lines 29-67 and especially lines 62-67 and FIG. 8). The gap causes a loss of the pressure drop between the inlet and outlet conduits of the pump, thereby diminishing the pumping efficiency of the pump. 
     To address this gapping problem, some workers have attempted to modify the shape of the pistons. For example, U.S. Pat. No. 3,726,617 to Daido dated April 10, 1973 discloses a double rotary piston pump in which the rotors or pistons have a varying radius in order to accommodate the opposing gap-causing motions of the pistons and to insure that the pistons remain in contact at all positions in the pumping cycle. Likewise, U.S. Pat. No. 1,771,863 to Schmidt dated July 29, 1930 teaches the use of pistons having a cross-sectional profile which consists of arcs of circles of differing sizes (see page 1, lines 65-75). 
     Another approach has been to recognize that the gap may be accommodated by the use of slightly oversized pistons to allow for mutual contact even at the position where gapping is expected to be the greatest. However, when such oversized pistons rotate through each cycle or period of rotation, a high degree of compression is attained between the pistons in positions where gapping diminishes. To accommodate this compression, it is known to cover the contact peripheral surfaces of the pistons with a compressible material which, while maintaining contact without stress in the position where gapping would be the greatest, can compress to a sufficient degree to accommodate the relative closeness of the pistons in the aligned position. Such a solution is taught, for example, by U.S. Pat. No. 3,078,807 to Thompson dated Feb. 26, 1963. Still another solution taught in the art is to rotate the pistons at various speeds through the use of elliptical drive gears, as taught by Jaworowski, U.S. Pat. No. 1,837,714 dated Dec. 22, 1931. Other attempts at improving the performance of double piston rotary pumps include the modification of the piston surface to provide a ramp or step on the surface of the pistons as shown in U.S. Pat. No. 2,453,284, to Tornborg dated Nov. 19, 1948, col. 1, line 46 to col. 2, line 24, and U.S. Pat. No. 20,796 to Holly dated July 6, 1858, col. 2, lines 71-98). 
     SUMMARY OF THE INVENTION 
     Generally, a pump according to this invention comprises variable offset transmission means to rotate the pistons without gapping or compression by varying the rotational offset of the pistons during each cycle or period of rotation. 
     Specifically, in accordance with the present invention there is provided a dual piston rotary pump comprising a housing defining a pump cavity having first and second generally disc-shaped chambers whose peripheral walls intersect to form a passage therebetween. The housing further defines an inlet port and an outlet port each in flow communication with the pump cavity to establish a fluid flow path through the housing. First and second disc-shaped pistons are mounted within the first and second chambers, respectively, on respective first and second variable offset transmission means carried on the housing. The pump according to this invention further comprises first and second rotating drive means coupled to the first and second variable offset transmission means, respectively, each drive means defining a drive axis of rotation parallel to the axis of the associated cylinder, for rotating the pistons. The variable offset transmission means vary the offset of the pistons in relation to the drive axes of rotation during the course of a pumping cycle to allow the pistons to rotate in mutual contact and to reduce compression between them. 
     According to one aspect of the instant invention, the first and the second variable offset transmission means each comprise a slide member which carries the associated piston. The slide member is slidably engaged with the associated rotating drive means to allow the slide member to rotate the piston in response to the rotation of the rotating drive means and to vary the offset of the piston by sliding in relation to the rotating drive means. The variable offset transmission means may comprise a fixed cam having a semi-elliptical portion to provide a positive cam action for changing the position of the slide member in response to rotation by the rotating drive means. 
     According to another aspect of the instant invention the variable offset transmission means may comprise an extensible crank attached between a piston and a rotating drive means and may further comprise at least one of a fixed cam for determining the degree of extension of the crank and a biasing means for biasing the crank to bear against the cam. 
     The variable offset transmission means according to this invention may comprise biasing means to urge the slide member against the fixed cam. Alternately, the fixed cam may be an offset split cam which may have a first cam surface and a second cam surface to provide both increasing offset and decreasing offset. The first and second cam surfaces may each comprise a semicircular portion and a semi-elliptical portion in which the minor axis of a semi-elliptical portion equals the diameter of its associated semicircular portion. The slide member or crank comprises at least one cam follower to bear against the fixed cam. 
     According to another aspect of this invention, the interior walls of the chambers may each have a semicircular chamber portion and a semi-elliptical chamber portion, and the semi-elliptical chamber portions may have as their minor axes the diameter of their associated semicircular chamber portion. 
     In a pump according to the instant invention the direction of rotation of the pistons about their drive axes of rotation may be reversible. The pump may comprise check valve means interposed in the fluid flow path to prevent back flow through the pump. The check valve means may be switchable between first and second conditions to accommodate fluid flow through the fluid flow path in opposite directions. The pump may comprise automatic check valve switching means to reverse the check valve means in response to a reverse in the pumping direction. 
     In another aspect of the present invention, the pistons may comprise counterbalance means disposed within the pistons to reduce vibrations during use. 
     As used in this specification and in the claims, the term &#34;semi-elliptical&#34; does not necessarily refer to a contour which follows the strict mathematical definition of an ellipse, but rather is intended to refer to an ovoid contour which differs from a circle in the manner described herein below. 
     As used herein, the term &#34;oblique position&#34; refers to the position of pistons when their respective centers are at the point of greatest deviation from the centerline of the pump. 
     As used herein, the term &#34;aligned position&#34; refers to the position of the pistons when their respective centers are disposed along the centerline of the pump. The &#34;distal aligned position&#34; is the aligned position in which the piston is farthest from the passage between the chambers, and the &#34;proximal aligned position&#34; is the aligned position in which the piston protrudes into the passage between the chambers. 
     As used herein and in the claims, the term &#34;semi-elliptical chamber&#34; refers to a chamber having a semicircular portion and a semi-elliptical portion, as described below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic elevational view of a pump according to the prior art with the face plate removed; 
     FIG. 2A is a schematic elevational view of one embodiment of a pump according to the present invention with the face plate removed; 
     FIG. 2B is an exploded perspective view partly in cross section of a variable offset transmission means utilizable in the pump of FIG. 1 and comprising in the illustrated embodiment a fixed cam according to this invention; 
     FIG. 2C is a perspective view of the fixed cam of FIG. 2B; 
     FIG. 3 is a schematic elevational view of the first cam surface of the fixed cam of FIG. 2C showing the relationship thereto of the cam follower of FIG. 2B; 
     FIG. 4 is a schematic elevational view of the second cam surface of the fixed cam of FIG. 2C showing the relationship thereto of the cam follower of FIG. 2B; 
     FIG. 5A is a diagram illustrating a method for generating a semicircular, semi-elliptical path for a pump according to the present invention; 
     FIG. 5B is a diagram illustrating a method for generating a semicircular, semi-elliptical chamber wall for a pump according to this invention; 
     FIG. 6 is a partly exploded, partly cross-sectional perspective view of a counterbalanced piston utilizable in a pump according to one embodiment of the present invention. 
     FIG. 7 is a schematic elevational view of a reversible check valve mechanism utilizable in a pump according to one embodiment of the present invention; and 
    
    
     DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF 
     There is shown in FIG. 1 a double piston pump 210 according to the prior art with the face plate removed. The prior art pump includes a housing 212 which defines a pump cavity having two cylindrical chambers 214a and 214b which intersect to form a passage 215 therebetween, and which further defines an inlet port 213a and an outlet port 213b each in flow communication with the chambers to accomplish fluid flow through housing 212. Disc-shaped pistons 216a and 216b having center points 220a and 220b, respectively, (indicated by cross hairs) are mounted within cylindrical chambers 214a and 214b, respectively. Each piston is driven by a drive shaft (not shown) having a drive axis of rotation 228a, 228b, respectively. In FIG. 1, drive axes of rotation 228a and 228b are seen as points which are vertically aligned and together establish a vertical centerline C of the pump. When pistons 216a and 216b are positioned with their respective center points 220a and 220b  on centerline C, they are said to be in an &#34;aligned&#34; position. Pistons 216a and 216b are coupled to their respective drive means so that their center points 220a and 220b are offset by distance d from the drive axes of rotation 228a and 228b and the rotation of the drive means causes pistons 216a and 216b to rotate or orbit about the drive axes of rotation with a constant eccentricity or offset equal to distance d. 
     Pistons 216a and 216b are positioned in mutual tangential contact at piston-piston contact point 240 when in the aligned position, as shown. 
     The respective drive means (not shown) rotate pistons 216a and 216b in opposite directions about drive axes of rotation 228a and 228b, respectively, so that center points 220a and 220b move in the direction of arrows 232a and 232b with equal angular velocity about their respective drive axes of rotation. As a result, during rotation the pistons 216a, 216b undergo relative translational movement about their respective axes of rotation. During such movement, the center points of pistons 216a and 216b will simultaneously move downwardly as sensed in FIG. 1 along centerline C while also moving away from centerline C in opposite directions, piston 216a moving to the left and piston 216b moving to the right. Since center points 220a and 220b are thus moving away from each other, the pistons can no longer remain in mutual tangential contact and a gap will occur between them. The gap G will be at its maximum when the center points of the pistons are at their maximum deviation from centerline C, i.e., when pistons 216a and 216b have rotated ninety degrees from the positions shown in solid line to the positions shown in dotted outline in FIG. 1, where center points 220a and 220b lie on horizontal reference abscissas 266 and 282, respectively. At this point, the pistons are described as being in oblique positions and the gap G is at its greatest width. As rotation continues, center points 220a and 220b of pistons 216a and 216b once again begin to approach centerline C, and gap G diminishes. 
     If, to avoid this gapping phenomenon, larger pistons are used so that they are in mutual tangential contact even in oblique positions, subsequent rotation as described above will produce a compression between the pistons as the center points approach centerline C and each other on further rotation. 
     To remedy the gapping and compression problems discussed above with respect to the prior art, a pump according to the present invention comprises variable offset transmission means to vary the rotational offset, i.e., the orbital eccentricity, of the pistons as they proceed through each cycle of rotation. Thus, the pistons may be in contact in the oblique position, and may rotate further without compression. The chambers enclosing the pistons are dimensioned and configured to accommodate the new orbital path of the pistons without gapping or compression between the piston and the chamber wall. 
     Accordingly, there is shown in FIG. 2A a pump 10 according to this invention comprising a housing 12 defining a pump cavity having an inlet port 13a and outlet port 13b to establish a fluid flow path through the pump, and two generally disc-shaped chambers 14a and 14b whose peripheral walls intersect to form a passage 15 therebetween. Ports 13a and 13b are equipped with check valves 17a and 17b in the fluid flow path to prevent backflow through housing 12. Within chamber 14a is mounted a disc-shaped piston 16a, and within chamber 14b, is mounted a like piston 16b. In this embodiment, the longitudinal axis of chamber 14a is parallel to the longitudinal axis of chamber 14b, and the disc-shaped pistons and chambers are generally cylindrical. It will be understood, however, that the chambers and pistons in a pump according to this invention need not be cylindrical. For example, if the longitudinal axes of the pistons (and chambers) lie in a common plane but are not mutually parallel, the pistons (and chambers) may have the general shape of a frustum. 
     Since the pump illustrated is symmetrical with respect to the construction of the cylinders, pistons and other elements to be described in each cylinder, this description will focus on the top cylinder and piston as shown in FIG. 2A, but it should be understood that this description applies equally to the other half of the pump, with corresponding structures in and around the two generally cylindrical chambers having like numbers and complementary letters &#34;a&#34; and &#34;b&#34;, e.g., 14a being the upper chamber and 14b being the lower chamber. 
     Piston 16a is mounted on a piston spindle 18a which is circular in cross section and concentric at center point 20a with the center of piston 16a. Between piston spindle 18a and piston 16a is a bearing 22a which allows piston 16a to be freewheeling, i.e., to rotate with little friction, clockwise or counterclockwise, about piston spindle 18a. A rotating drive means such as drive shaft 26a (shown in FIG. 2B) is coupled to piston spindle 18a by a transmission means described below, to rotate piston 16a within chamber 14a. Drive shaft 26a, which may be driven manually or by machine, establishes a drive axis of rotation 28a parallel to the longitudinal axis of chamber 14a about which piston 14a orbits. Piston spindle 18a is off-center from drive axis of rotation 28a by a distance referred to herein as the &#34;piston offset&#34; and represented in FIG. 2A by arrow 30a. Therefore, when drive shaft 26a rotates piston 16a about drive axis of rotation 28a, center point 20a (which represents the centers of both piston spindle 18a and piston 16a) rotates about drive axis of rotation 28a in an orbit having an offset equal in length to arrow 30a. The rotating drive means for piston 16b may be a drive shaft similar to drive shaft 26a or some other drive such as a drive gear driven with or from drive shaft 26a. Chamber 14a is designed so that radius 36a of piston 16a is sufficient to allow tangential contact of piston 16a with the wall of chamber 14a during the course of its rotation. Since piston 16a is free to rotate about piston spindle 18a, the rotation of drive shaft 26a in the direction of arrow 32a impels piston 16a to roll or &#34;walk&#34; around the inside wall of chamber 14a, thus imparting a rotation to piston 16a in the direction of arrow 34a about center point 20a. The pump is reversible in that by reversing the direction of rotation, the direction of fluid flow is reversed. This device may thus be used as a pump or as a compressor for liquids or gases. Fluid is contained in chambers 14a and 14b by securing face plates (not shown) to the front and rear of the pump, in a conventional manner. The front face plate may completely cover and seal the open chamber, whereas the rear face plate has an opening large enough to accommodate the drive means (i.e., drive shaft 26a) but small enough to seal the chambers between housing 12 and pistons 16a and 16b. 
     FIG. 2A shows pistons 16a and 16b in oblique positions and in mutual, tangential piston-piston contact at point 40. As piston 16a rotates about drive axis of rotation 28a in the direction of arrow 32a, the piston offset will remain constant until piston 16a has rotated 180°, including 90° to the proximal aligned position and a further 90° to the second oblique position which occurs within a single 360° pumping cycle. 
     As discussed above, if, while piston 16a moves in the semicircle from the oblique position shown to the opposing oblique position, piston 16b rotates about its drive axis of rotation 28b with a constant offset, center point 20a of piston 16a and the center point of piston 16b will approach one another along centerline 42 of pump 10, resulting in compression between the cylinders at piston-piston contact point 40. To alleviate this compression and to avoid the need for one or both of the pistons to be made from compressible material or to at least be covered with compressible material, the present invention provides that the offset of piston 16b increases while piston 16b rotates 180° from the oblique position through the distal aligned position to the opposing oblique position. To accommodate this varying offset, the contour of chamber 14b differs from the chambers of the prior art to provide a semi-elliptical surface 44b which allows piston 16b to travel beyond the constraints of a purely cylindrical chamber as taught in the prior art. Thus, while piston 16a travels clockwise in a semicircular orbit about drive axis of rotation 28a from the oblique position shown in FIG. 1 through the proximal aligned position to the opposing oblique position, piston 16b travels in a semi-elliptical orbit about its drive axis of rotation. Conversely, when piston 16b rotates in the counterclockwise direction indicated by arrow 32b from the oblique position which is 180° from the position shown in FIG. 1 through its proximal aligned position to the oblique position shown, this portion of its rotation will be semicircular and have a constant offset from its drive axis of rotation. The corresponding rotational path of piston 16a, however, will be altered by varying the offset to avoid compression as was the rotation of piston 16b described above. To synchronize the rotations of the first and second pistons, the respective drive means are synchronized, e.g., by timing gears. Thus, the piston offset of each piston is constant through part of the rotation about its drive axis of rotation, and varies in the other part. Like chamber 14b, chamber 14a is partially elliptical to accommodate the varying offset of the rotation of piston 16a. 
     To accomplish the varying offset described above, this invention comprises variable offset transmission means to rotate pistons 16a and 16b about drive axes of rotation 28a and 28b (not shown) and to change the offset of piston spindles 18a and 18b from drive axes of rotation 28a and 28b during part of the course of a full rotation as described above. 
     The variable offset transmission means comprises a slide member 48a, FIG. 2B, which carries piston spindle 18a on which piston 16a may be mounted. Slide member 48a is equipped with a tongue 50a, and drive shaft 26a is equipped with groove 52a to slidably receive tongue 50a. When tongue 50a is disposed within groove 52a, the rotation of drive shaft 26a about drive axis of rotation 28a will cause slide member 48a and piston spindle 18a to rotate. As discussed above, center point 20a of piston spindle 18a is offset from the drive axis of rotation 28a as indicated by arrow 30a. In addition to transmitting the driving rotational force, the tongue-in-groove configuration allows slide member 48a to slide within groove 52a during the course of a rotation, thereby allowing a change in the magnitude of the piston offset represented by arrow 30a. To allow for changes in the piston offset, tongue 50a is disposed longitudinally parallel to the piston offset. Therefore, when tongue 50a is disposed within groove 52a, piston spindle 18a can both rotate in response to the rotation of drive shaft 26a and can simultaneously provide radial variations in the piston offset. In an alternative embodiment, the drive shaft may carry the tongue and the slide member may be equipped with a corresponding groove. 
     To control the sliding motion of slide member 48a in groove 52a the variable offset transmission means according to this invention includes a fixed cam 54a (shown for clarity in partial cross section) which surrounds slide member 48a to define its position in relation to drive axis of rotation 28a during the course of a full 360° rotation. Fixed cam 54a is disposed in fixed relation to drive axis of rotation 28a and for this purpose may be mounted wherever appropriate, for example, it may be attached to housing 12. Fixed cam 54a functions as a split cam having two offset cam surfaces, first cam surface 60a and second cam surface 62a. To facilitate the function of fixed cam 54a, slide member 48a is equipped with a first follower member 56a which bears against first cam surface 60a, and a second follower member 58a which bears against second cam surface 62a. Second follower member 58a is radially opposed and axially set off from first follower member 56a. Follower members 56a and 58a may be polished protrusions from the surface of slide member 48a or may be rollers or bearings mounted in slide member 48a. 
     As shown in FIG. 2B, piston spindle 18a and piston 16a (not shown) are in the distal aligned position. As discussed above, this means that piston 16a, the position of which is identified with respect to center point 20a, is at the apex of the semi-elliptical portion of its orbit where the piston offset is at its greatest magnitude. As drive shaft 26a rotates piston spindle 18a ninety degrees in the direction of arrow 32a, piston 16a (not shown) will approach the oblique position, with the piston offset diminishing until, in the oblique position, the piston offset is equal once again to the original semicircular radius, and thence remains constant for the subsequent 180° of rotation. 
     Fixed cam 54a is shown in its entirety in FIG. 2C, where its annular configuration and the offset relationship of surfaces 60a and 62a are illustrated. The contours of surfaces 60a and 62a are described with respect to FIGS. 3 and 4, respectively. 
     To achieve the varying offset described above, first cam surface 60a, as shown in FIG. 3, has two distinct portions, which are described in reference to reference abscissa 66a. The two distinct portions are a semi-elliptical portion 64a spanning quadrants I and IV above reference abscissa 66a, and semicircular portion 68a in quadrants II and III beneath reference abscissa 66a. Together, semi-elliptical portion 64a and semicircular portion 68a are configured to alternately determine and accommodate the necessary travel of first follower member 56a, also shown in FIG. 2B, to produce the varying offset effect discussed above. 
     Due to the construction of slide member 48a and first follower member 56a thereon, and because first follower member 56a bears against first cam surface 60a, it is necessary to define the first follower offset as the distance from drive axis of rotation 28a to the point of contact of first follower member 56a with fixed cam 54a, indicated by arrow 70a, FIG. 2B. The first follower offset is the sum of the piston offset indicated by arrow 30a and the follower margin, which is the distance from the center point 20a to the point of contact of first follower member 56a with fixed cam 54a, indicated by arrow 72a. Since the follower margin is fixed and does not vary, the first follower offset varies with the piston offset. Likewise, the second follower offset, the distance from drive axis of rotation 28a to the point of contact of second follower member 58a with second cam surface 62a, indicated by arrow 71a, varies inversely with the piston offset. 
     In the distal aligned position shown in FIG. 3, first follower member 56a bears upon the semi-elliptical portion 64a of first cam surface 60a with a first follower offset 70a from drive axis of rotation 28a. The first follower offset, represented in FIG. 3 as arrow 70a, is the distance from axis of rotation 28a to the point on first cam surface 60a where first follower member 56a makes tangential contact with first cam surface 60a, and for descriptive purposes can be considered the radius of first cam surface 60a at the contact point, measured from drive axis of rotation 28a. In the position shown, the first follower offset is at a maximum magnitude. As discussed previously, as piston 16a rotates about drive axis of rotation 28a ninety degrees in the direction of arrow 32a, the first piston offset decreases. This occurs because, in quadrant I, the radius of semi-elliptical portion 64a decreases by degrees until it is equal to radius 74a of semicircular portion 68a, causing slide member 48a to slide within groove 52a, thereby bringing center point 20a (FIG. 2) closer to drive axis of rotation 28a. 
     Due to the geometrical relationship between semi-elliptical portion 64a and semicircular portion 68a, radius 74a of semicircular portion 68a represents one half of the minor axis of semi-elliptical portion 64a. The difference between the cam radius at the apex of semi-elliptical portion 64a (i.e., the length of arrow 70a) and the radius at any point in semicircular portion 68a (mirrored for clarity in quadrants IV and I as dotted curve 78a) is designated as d in FIG. 3, which will be understood to decrease as first follower member 56a rotates through quadrant I. 
     After first follower member 56a proceeds 90° in the direction of arrow 32a (i.e., to the oblique position), it comes to bear against semicircular portion 68a of first cam surface 60a. Semicircular portion 68a will not change the first follower offset during the course of the next 180° of rotation through quadrants II and III because the radius of semicircular portion 68a of cam surface 60a is constant. 
     As first follower member 56a, as shown in dotted outline (FIG. 3), leaves the semicircular portion of its rotation path from quadrant III, it is free to move beyond radius 74a of semicircular portion 68a and thus travel in a semi-elliptical path extending beyond the complementary semicircular path 78a (shown in dotted outline) it would otherwise travel. If first follower member 56a were to follow first complementary semicircular path 78a, the first follower offset would be constant throughout the rotation, as would the piston offset, and the advantage of this invention would be lost. Therefore, some mechanism is required to insure that slide member 48a, FIG. 2B, to which first follower member 56a is attached, slides within groove 52a to increase, by degrees, the offset of first follower member 56a, FIG. 3, as it follows semi-elliptical portion 64a through quadrant IV toward the distal aligned position shown in FIG. 3. To provide the necessary outward sliding motion, second follower member 58a, which is radially opposed to and axially offset on slide member 48a from first follower member 56a, bears upon second cam surface 62a as shown in FIGS. 2B, 2C and FIG. 4. 
     Second cam surface 62a has a semi-elliptical portion 80a spanning quadrants II and II beneath reference abscissa 82a and semicircular portion 84a spanning quadrants IV and I above reference abscissa 82a. Semi-elliptical portion 80a is smaller than semicircular portion 84a, with the radius of semicircular portion 84a being the major axis of semi-elliptical portion 80a rather than the minor axis as was the case for first cam surface 60a. 
     While first follower member 56a is entering quadrant IV of FIG. 3 and is in need of an outward force to bear against semi-elliptical portion 64a, second follower member 58a (also seen in FIG. 2B) is entering quadrant II of FIG. 4, where it bears upon semi-elliptical portion 80a of second cam surface 62a. In quadrant II, semi-elliptical portion 80a decreases by degrees the second follower offset (represented by arrow 71a, FIG. 2B) of second follower member 58a thus pushing slide member 48a (FIG. 2B) within groove 52a and forcing first follower member 56a to follow semi-elliptical path 64a in quadrant IV of FIG. 3. When second follower member 58a reaches the smallest radius of its travel (at the point on semi-elliptical portion 80a between quadrants II and III where second follower member 58a is shown in dotted outline), first follower member 56a is pushed to its greatest offset, in the position shown in solid line in FIG. 3. At this point, the deviation of the radius of semi-elliptical portion 80a from the radius of semicircular portion 84a (reproduced for clarity in quadrants III and II as dotted line 85a) is at its greatest and is the same deviation d shown in FIG. 3. As drive member 26a continues to rotate, semi-elliptical portion 80a of second cam surface 62a can no longer push second follower 58a because the radius of the semi-elliptical portion 80a increases in quadrant III. However, while second follower member 58a is in quadrant III, first follower member 56a is in quadrant I, FIG. 3, where, as discussed above, its offset decreases, pushing slide member 48a toward a lesser offset, and keeping second follower member 58a against semi-elliptical portion 80a despite the increasing radius. The offset surfaces of fixed cam 54a thus provide a positive cam action to alternately increase and decrease the offset represented by arrow 30a by sliding slide member 48a within groove 52a. 
     Fixed cam 54a functions as a split cam because only part of each cam surface 60a and 62a provides a positive action to vary the piston offset as described above; no change in piston offset is effected by semicircular portions 68a and 84a. Specifically, the portion of semi-elliptical surface 80a spanning quadrant II of FIG. 4 increases the piston offset of piston 16a, and the portion of semi-elliptical portion 64a spanning quadrant I of FIG. 3 decreases the piston offset of piston 16a. In this embodiment, then, the semi-elliptical portion of fixed cam 54a which provides the positive cam action is split between two axially offset surfaces 60a and 62a. 
     The precise configurations of semi-elliptical portions 64a and 80a of first cam surface 60a and second cam surface 62a, respectively, are designed to provide the variable offset described above while the pistons rotate and remain in mutual tangential contact without gapping or compression. The shape of the orbit in which the center of the pistons must travel can be determined graphically as shown in FIG. 5A, in which semicircular piston paths 88a and 88b (also shown in FIG. 2A) represent the proximal portions of the paths of piston center points 20a and 20b, respectively. As discussed above, pistons which are in tangential contact have a determinable distance between their centers, and to avoid gapping or compression during rotation, this intercenter distance must remain constant. To determine the elliptical portion of the travel of center point 20a as it moves about drive axis of rotation 28a, one should assume the pistons to be in their mutually oblique positions with centers at points 90a and 90b, respectively, at which point pistons 16a and 16b are in tangential contact and their respective centers are separated by a fixed inter-center distance 92. Then, project the orbit of piston 16b about drive axis of rotation 28b in the direction of arrow 32b in a fixed angular increment θ 1  to position 94b, with a corresponding angular rotation of θ  1  of center 20a in the direction of arrow 32a. Prior art mechanisms would attempt to place center 20a at point 93a, a position at which the inter-center distance is reduced with corresponding compression between the pistons. According to this invention, however, the inter-center distance 92, now represented by connecting chord 92&#39;, is preserved by the variable offset transmission means described above, which increases the offset of piston 16a. An extension of radial chord 96a to a point 94a which is equally distant from point 94b, i.e., at the end of connecting chord 92&#39;, establishes the new position of center point 20a of piston 16a for that given increment of rotation. 
     The position of piston 16a at each progressive increment determines the configuration which chamber 14a must have to provide for tangential, noncompressing contact with piston 16a. As shown in FIG. 5B, piston 16a is in the oblique position signified by center point 20a&#39;, which corresponds to point 90a of FIG. 5A. In this position, chamber 14a must meet piston 16a in tangential contact at a point 134a which is the most remote point on piston 16a from drive axis of rotation 28a. During the angular incremental rotation θ 1 , piston center point (20a&#39;) moves along semi-elliptical path 98a (also shown in FIG. 2A) to point 20a&#34;. For clarity, only the most distal portion of piston 16a, indicated as 16a&#39;, is shown in this and in succeeding positions. Chamber 14a must be configured to touch piston 16a at remote point 134a&#39;. By projecting a number of such increments, a generatrix can be produced by tracing the most distant portions of piston 16a as it travels through the semi-elliptical portion of its rotation, to define semi-elliptical chamber portion 44a, FIG. 2A, which differs from the circular contour of cylindrical chambers of the prior art to accommodate the increased offset of piston 16a according to this invention. A complementary process is performed for piston 16b and chamber 14b when piston 16a travels on semicircular path 88a, establishing semi-elliptical path 98b (also shown in FIG. 2A) of center 20b (not shown), which similarly extends beyond the prior art semicircular path (shown in dotted outline.) 
     The required distance at each angular increment from drive axis of rotation 28a to the semi-elliptical chamber wall according to this invention determines the configuration of the cam surfaces 60a and 62a of fixed cam 54a by dictating the degree of departure of semi-elliptical cam portions 64a (FIG. 3) and 80a (FIG. 4) from a circular configuration (i.e., the variance in offset at that angle, previously designated deviation d). The radius of the semicircular cam portions 68a and 84a are chosen to accommodate slide member 48a and follower members 56a and 58a, and to provide a smooth transition between the semicircular and semi-elliptical portions. To foster a smooth transition, the radius of the semicircular portion should be significantly larger than the maximum deviation d in the semi-elliptical portion. 
     While the foregoing embodiment is preferred, it is not necessary that the fixed cam be a split cam. For example, it is possible to provide a single cam surface and a single follower member on the slide member, provided that the variable offset transmission means includes some means for assuring the follower will bear against the cam surface during all portions of the rotation, regardless of whether the offset increases or decreases. In such case, the semi-elliptical portion of the fixed cam which provides a positive cam action is disposed on a single cam surface. This may be accomplished, for example, where the groove in the variable offset transmission means is a closed-end groove in which there is a spring to bear against the side of the tongue when it is disposed in the groove. The spring may force the slide member radially outward so that the single follower member, radially opposed from the spring, bears against the single cam surface at all times. In an alternative embodiment, the variable offset transmission means includes an extensible crank attached to the drive member at one end and the cam piston at the other end, thus providing an offset. The crank may include biasing means such as a spring to bias the crank toward extension and may further include a follower member. In this embodiment, the variable offset transmission means includes a cam against which the follower member bears to vary the offset as required. 
     According to another aspect of this invention, the fixed cam may provide a circuitous slot for engaging a follower to determine the offset of the piston without the need for a split cam or biasing means. 
     Since each piston faces the same offset constraint, the rotating drive means of each are synchronized by timing gears to assure a one-to-one angular rotation at all times. 
     Preferably, the pistons include a resilient outer surface which is resistant to wear and which facilitates sliding contact between the pistons as they rotate. The thickness of the resilient coating is included in the diameter of the piston for purposes of determining the shape of the elliptical portion of the rotation path. 
     Since the cylindrical pistons rotate about a point which is not coincident with their respective centers of gravity, vibrations may occur. To ameliorate these vibrations, pistons according to this invention may comprise a counterbalance to more evenly distribute the total piston mass about the drive axis of rotation. This may be accomplished by constructing each piston from two circular plates which define an annular void between them as shown in FIG. 6 where piston 16b&#39; comprises inner piston half 114 (shown in partial cross section) and outer piston half 116 (shown in partial broken cross section, for clarity). Piston 16b&#39; is shown in an oblique position similar to that of piston 16b of FIG. 2A, evidenced by the horizontal orientation of the piston offset indicated by arrow 30b&#34;. Piston spindle 118 is equipped with a key 119b and collar portion 120 of counterbalance 122 is equipped with key slot 121b to receive key 119b. The portion of counterbalance 122 which appears in cross section encircles and is integral with collar portion 120. Counterbalance 122 further includes radial sector portion 124 which lies in a plane substantially perpendicular to, and which intersects, drive axis of rotation 28b&#34;. About the periphery of radial sector portion 124 is throw portion 126. Inner piston half 114 is mounted on collar portion 120 by means of first collar bearing 128 (shown in cross section), which allows inner piston half 114 to rotate freely, clockwise or counterclockwise, about collar portion 120. Inner piston half 114 defines a first annular recess 130 to accommodate at least part of throw portion 126. After inner piston half 114 is mounted to collar portion 120, collar portion 120 may then be fitted snugly upon piston spindle 118. Next, outer piston half 116 (shown in partial break-away view for clarity) is mounted with outer collar bearing 132 onto collar portion 120. Outer piston half 116 is configured to cooperate with inner piston half 114 to form a smooth piston surface 134 and defines a second annular recess 131 which cooperates with first annular recess 130 to form piston recess 133, within which throw portion 126 is fully disposed. Like inner piston half 114, outer piston half 116 is free to rotate freely about collar portion 120. The two piston halves may be fixed together so that they rotate together, e.g., by welding, adhesion, or through a mechanical bond. 
     Inner piston half 114 and outer piston half 116, being radially symmetrical about centerpoint 20b&#34;, would cause vibrations when rotated about drive axis of rotation 28b&#34; because their centers of mass are offset from the drive axis of rotation. Counterbalance 122, which is noncircular, occupies a portion of piston recess 133, in partial opposition to the centers of mass of piston halves 114 and 116 about drive axis of rotation 28b&#34;, about which piston 16b&#39; rotates. This counterbalance relationship is fixed by the disposition of key 119b in slot 121b and is effective to move the center of mass for the piston assembly closer to drive axis of rotation 28b&#34;, thereby reducing vibrations. 
     The direction of rotation of the rotating drive means may be reversed without defeating the advantages of this invention. When used with check valves, the check valves may be reversible. This may be accomplished by, for example, mounting the check valves in cylindrical plugs mounted in the inlet and outlet apertures. To reverse the check valves, all that need be done is to rotate the cylindrical plugs 180°, reversing the direction of the valve. This may be done manually or mechanically in response to a change in direction of the rotating drive means, the timing gears or some other related structure. For example, the cylindrical plugs bearing the check valves may be equipped with sheaves 100a and 100b, as shown in FIG. 7, which are connected by line 102. Catch member 108 is slidably mounted on a track or slide bar (not shown) so that it may slide in a path which defines a chord near the perimeter of timing gear 106 shown in dotted outline. Spring 110a biases catch member 108 toward the center point of chord 109 due to compression against stops 111a or 111b. Timing gear 106 is equipped with a plurality of bosses 112, only some of which are shown about the periphery of timing gear 106. As timing gear 106 rotates in the direction of arrow 32, bosses 112 sequentially bear against catch member 108, causing it to remain in the position shown in FIG. 7. However, upon reversal of the direction of rotation, catch member 108 will move toward the center of chord 109, when a boss will push catch member 108 from behind to the other end of chord 109 and compress spring 110b against stop 111b. Catch member 108 will they remain near stop 111b due to the repeated contacts of bosses 112 until the direction of rotation is again reversed. When catch member 108 moves in this manner, line 102 rotates sheaves 100a and 100b and the associated check valves. The radial displacement of bosses 112 and the diameter of sheaves 100a and 100b is adjusted to assure that the movement of catch member 108 from one end of chord 109 to the other corresponds to a 180° rotation of the associated check valves. To prevent accidental over-rotation, sheaves 100a and 100b are equipped with bosses 113a and 113b, respectively, which prevent rotation beyond stops 111a and 111 c, and beyond 111b and 111d, respectively. 
     While some features are shown with respect to some embodiments and not to others, this is not intended as a limitation to the invention. These and other embodiments and improvements will be evident to one skilled in the art, and are within the spirit and scope of the invention and the following claims.