Abstract:
The subject matter described herein relates to a spin pump that includes a combination of a compressor and a vacuum pump on respective pistons extending from a common crankshaft in a rotating housing. Related methods, apparatuses, systems, techniques and articles are also described.

Description:
REFERENCE TO PRIORITY DOCUMENT 
       [0001]    The present application claims priority to co-pending U.S. Provisional Application Ser. No. 61/888,893 entitled “SPIN PUMP WITH SPUN-EPICYCLIC GEOMETRY” and filed Oct. 9, 2014. Priority of the aforementioned filing date is claimed and the provisional application is incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    There is a need for inexpensive, compact, high-efficiency oxygen concentrators comprised of compressors and vacuum pumps to drive the pressure-swing and/or vacuum/pressure-swing absorption cycles that separate oxygen from ambient air, such as for therapeutic use in patients with chronic obstructive pulmonary disease (COPD). Such oxygen concentrators typically come in stationary, transportable, and portable varieties. Patients generally prefer more and more the smaller, transportable and portable devices when the patients are still ambulatory. These smaller units have the most severe demands for compactness and weight, plus efficiency (as that drives the duration of the portable battery power source). Vibration can also be a problem when carrying or wearing a portable concentrator. 
         [0003]    Stationary concentrators are more cost-driven designs and use a pressure-swing adsorbent (PSA) cycle in which all air pumping in the absorbent beds is done at or above ambient pressure, enabling the use of inexpensive compressors to move the air. In portables however, it is preferred to use vacuum-pressure adsorbent swing (VPSA) cycles, in which the lower-pressure portions of the cycle are sub-atmospheric, because the known absorbents can deliver more oxygen per unit mass of absorber material when the pressures are at such ‘vacuum’ levels. Nonetheless, the need for these pumps (compressors or compressor-vacuum combinations) must also provide breathable quality gas, which can require that they be non-lubricated devices (i.e., do not use oils for lubrication). To date, all such concentrators have been low-stroke reciprocating devices driven with conventional motors. 
         [0004]    There is a long-held need for compact, low-vibration, efficient pressure-vacuum combination pumps and compressors that operate without oils and cost no more than conventional reciprocating types. 
         [0005]    Existing patents disclose basic kinematics that resemble some elements of the kinematics arrangements described herein. For example, U.S. Pat. No. 2,831,438 to Guinard describes a rotary piston pump having crossed-piston geometry with two sets of cross pistons riding on sliding “sole plates. (a scotch-yoke variant). Moreover, the Guinard system has a crankshaft that is directly connected to a rotor housing. U.S. Pat. No. 2,683,422 to Richards describes a rotary engine or pump having a similar kinematic geometry to the present disclosure, that is epicyclic motion, with a crankshaft rotating at twice the speed of the cylinders to give relative reciprocation between pistons and cylinders, but Richards drives the cylinders, requiring a gear to impart the required motion to the crank (itself a complex hollow construction over a stationary eccentric), and with separately attached cylinders at each piston face, which makes for a cumbersome construction that is difficult to align adequately (and hence requires gears for synchronization). Richards further leaves to the imagination the actual fluid connections required to function. DeLancey U.S. Pat. No. 2,121,120 is a crossed-piston flowmeter, but it is not epicyclic, and uses rollers and cams moved by its pistons, to produce uniform shaft rotation proportional to volumetric displacement in the chambers. There is no rotation of the cylinders. Smith U.S. Pat. No. 2,661,699 is a crossed-piston engine with a conventional crank, stationary cylinders and sliding (”Scotch&#39;) yokes connecting the pistons to the connecting rods, similar to Guinard&#39;s device. The Smith engine is not epicyclic. Johnson U.S. Pat. No. 2,684,038 is another crossed piston design with scotch yokes, but with yokes in the connecting rods&#39; centers, rather than at the pistons as in Smith. DeLancey, Smith, and Johnson are all cited by Richards. In addition, none of these patents describe a combination of pressure chambers and vacuum chambers in a single device. Moreover, the existing patents all describe oil-lubricated devices and do not describe a concentrator system in oil-free form. 
         [0006]    The more relevant patents citing Richards include Baker U.S. Pat. No. 3,977,303. Baker is epicyclic, but includes a free-rotating secondary eccentric between his crankshaft and pistons, all within a non-rotating cylinder block. Gail U.S. Pat. No. 5,375,564 teaches an oil-lubricated epicyclic engine with three or more piston axes (and cites Avermaete U.S. Pat. No. 3,665,811, another 3-cylinder epicyclic engine; Lamm U.S. Pat. No. 3,799,035 which teaches a spinning epicyclic engine or pump similar to the present invention; and Froumajou U.S. Pat. No. 3,921,602 which describes an engine of complex epicyclic form in which the pistons describe multiple strokes per revolution, where eccentricities of the rotating elements have non-unity integer ratio). Farrington U.S. Pat. No. 6,148,775 discloses an engine with the epicyclic kinematics of the present invention. 
         [0007]    What is needed is a compact, balanced, and low-cost oil-free pump that can serve either simple PSA, the more efficient and compact VPSA systems or both PSA and VPSA. Compactness and balance are of special value to portable concentrators, and low cost is of greater value in stationary units. 
       SUMMARY 
       [0008]    A rotary, positive displacement pump (also referred to as a spin pump) is described that in an embodiment includes a combination of a compressor and a vacuum pump on respective pistons extending from a common crankshaft in a rotating housing of the spin pump. The spin pump is advantageously compact, light in weight, inexpensive, portable, and produces no or minimal vibration due to a near perfectly balanced construction. 
         [0009]    The details of one or more variations of the subject matter described herein are set forth in the accompanying drawings and the description below. Other features and advantages of the subject matter described herein will be apparent from the description, the drawings, and the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  shows a perspective view of a spin pump assembly. 
           [0011]      FIG. 2  shows another perspective view of the spin pump assembly. 
           [0012]      FIG. 3  shows a perspective view of a rotor of the spin pump assembly. 
           [0013]      FIG. 4  shows another perspective view of a rotor of the spin pump assembly. 
           [0014]      FIG. 5  shows a crankshaft of the spin pump assembly. 
           [0015]      FIG. 6  shows a diagram illustrating kinematics of the spin pump assembly; 
           [0016]      FIG. 7  shows an alternate embodiment of the spin pump assembly in an exploded state. 
           [0017]      FIG. 8  shows an example of a two-piece rotor in an assembled state. 
           [0018]      FIG. 9  shows a first congruent piece of a two part rotor. 
           [0019]      FIGS. 10 and 11  show cross-sectional views of embodiments of the spin pump assembly in assembled states. 
       
    
    
       [0020]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0021]    Disclosed is a low-cost, easy to machine rotary or spin pump assembly for use in an oxygen concentrator. In an embodiment, the spin pump assembly operates as a compressor pump pursuant to a PSA cycle. In another embodiment, the spin pump assembly operates as a vacuum pump pursuant to a VPSA cycle. In an optional embodiment, the spin pump assembly combines both a compressor pump (PSA) and a vacuum pump (VPSA). The ease of machining is due to requiring only flat and circular surfaces on the components of the pump. In an embodiment, the components of the pump include operative part surfaces comprising portions of the pump components that define piston or fluid chambers or portions that abut adjacent portions either in a fixed or moving relationship. Some non-limiting examples of operative part surfaces include the internal walls of piston chambers, as well as the outer surface of the rotor that spins adjacent to a housing surface and surfaces of bearings. The operative part surfaces are those requiring precision for function, and here all such surfaces can all be substantially flat or cylindrical and/or machined at low cost. No special profiles such as those required in making other forms of pumps (e.g., a swing or scroll compressor) are required. 
         [0022]    As described in detail below, the spin pump assembly employs an epicyclic geometry, which uses a counter-rotating vectors approach to generating straight-line reciprocation for pistons in the cylinders of the pump. Moreover, a reference frame for the counter-rotating vectors is itself spinning. That is, both vectors can spin clockwise—but one vector can spins at 2× speed of the other vector. This contrasts with a normal epicyclic, where a bearing part is stationary (i.e., spin speed zero) and two-counter-rotating parts spin at opposite spin speeds (say, of −1 and 1). They combine to produce straight-line reciprocation, which can let a piston move relative to a stationary cylinder. In the system described herein, all parts receive additional forward spin relative to the surrounding ‘ground’, so a cylinder-bearing part (i.e., the rotor) changes from spin speed zero to 1, one previous rotator goes from −1 to zero and becomes the new ‘grounded’ part instead of the cylinders, and other rotator—the crankshaft—goes from speed 1 to 2. 
         [0023]    The spin pump assembly includes an offset between a crank axis and a rotor axis of the assembly. A crankpin represents or defines one vector and a center of the rotor location relative to the crank axis represents another vector. 
         [0024]    The rotor includes a first piston that is driven by the crank pin and trapped in the rotor&#39;s transverse cylinder. The first piston is driven to reciprocate in the rotor as the rotor rotates at half crank speed. In order to greatly reduce or remove side load from the pistons, an internal-external timing gear (such as a 2:1 timing gear) can be disposed on the outside ends of the crankshaft and can be fitted to move the rotor and crank together. The rotor also includes a second piston in the same rotor. The second piston is optionally axially offset relative to the first piston, with its reciprocation axis 90 degrees to the first (and the matching crankpin 180 degrees out). In another embodiment, fork-and blade rods are used, or rods offset from piston centerlines, so piston centerlines fit all in one plane even when bearings are offset along the crankshaft axis. 
         [0025]    In an embodiment, porting of the pistons is independent such that one piston serves as a vacuum pump and the other piston serves as pressure pump. 
         [0026]    There are now described some example embodiments of the spin pump assembly for use in an oxygen concentrator.  FIGS. 1 and 2  show perspective views of a spin pump assembly  105 , which includes an outer housing  110  that contains a rotor  205  (shown in  FIGS. 3 and 4 ) that is rotatably mounted inside the housing  110 . The rotor  205  is driven to rotate by a crankshaft  115  that defines a first axis A. The crankshaft  115  is rotatably coupled to the housing  110  such as, for example, via one or more bearing plates  120 . The rotor  205  contains a pair of cylindrical bores ( FIGS. 3 and 4 ), each of which contains at least one piston such that the piston(s) define at least one fluid chamber inside each of the bores. The bore(s) may be radial or diametral relative to a center axis of the rotor  205 . That is, the bore(s) may extend partially through the rotor or may extend entirely through the rotor such that the bore(s) intersect and form openings through two sides of the rotor. The kinematics of the spin pump assembly are described in detail below. The rotor is contained in a close fit alignment within the housing. For example, there may be a radial gap between the rotor and the housing of 0.001-0.002 inch. 
         [0027]    In the embodiment of  FIGS. 1 and 2 , the housing  110  has an outer shape that is rectangular with substantially flat surfaces, which provide ease of manufacturing. A full housing may not be required if the piston cylinders are fitted with heads that rotate with them. The housing  110  has a cylindrical bore in which the rotor  205  is rotatably positioned. As discussed in more detail below, the rotor  205  rotates about a second axis of rotation that is parallel to, but offset from, the first axis of rotation defined by the crankshaft  115 . In an embodiment, the second axis is offset from first axis by ¼ of the desired stroke and crankpin eccentrics are offset from crank rotation axis by ¼ of desired stroke. 
         [0028]      FIGS. 3 and 4  show perspective views of the rotor  205 , which surrounds the crankshaft  205 . The crankshaft carries pistons that ride in the rotor, but there is no direct attachment between the rotor and the crankshaft. Rotation of the rotor occurs because of the pistons pushing on their cylinder walls when the crankshaft rotates (unless a timing gear directly drives the rotor from the crank. The rotor  205  includes two cylindrical piston chambers  210  and  215 , each of which contains at least one piston. In an embodiment, the piston chambers are offset by 90 degrees relative to one another. In an embodiment, both of the piston chambers serve as a compression chamber (for example, for use in a PSA cycle). In another embodiment, both of the piston chambers serve as a vacuum pump chamber. In another embodiment, one piston chamber serves as a compression chamber and another piston chamber serves as a compression chamber (for example, for use in a VPSA cycle).  FIG. 5  shows the crankshaft  115  in a standalone state. 
         [0029]      FIG. 6  is a schematic diagram  500  illustrating kinematics of the spin pump assembly  105 . The schematic diagram shows an example piston  505  movably mounted in the rotor  205 , which is rotatably positioned in the housing  110 . The crankshaft  115  drives the piston  505  to rotate and thereby to reciprocate within the rotor  205 , itself rotating in housing  110 , which includes a discharge port  517  and a suction port  518 . The piston may have any of a variety of structures. In an embodiment, the piston is formed of a pair of piston crowns on a connecting rod. 
         [0030]    Diagram  500  of  FIG. 5  schematically shows a sequence of steps in the operation and rotation of the components in the spin pump, proceeding from an arbitrary first position shown at upper left at position  502 , and sequentially from position  502  to position  516 . After a further equal increment subsequent to  516 , the sequence again goes to the first position shown at position  502 . 
         [0031]    As mentioned, the components of the spin pump assembly are arranged in a spun-epicyclic geometry, which allows a counter-rotating vectors approach for generating a straight-line reciprocating motion of the pistons  505  with respect to the rotor  205 . The center of rotation of the rotor  205  is concentric to the bore of stationary housing  105 . The center of rotation of the crankshaft  115  is parallel to but offset from the rotor center by a predetermined distance, such as a distance equal to one quarter of the desired piston stroke (as shown initially upward by diagram  500  at crank angle zero, at  502 ). The crankshaft has a crankpin offset from the center of rotation of the crankshaft  115  by one quarter of the desired piston stroke (also shown upward at  502 ). 
         [0032]    At position  502 : when a torque is applied to the crankshaft  115  by an external device (for example, a motor, which is not shown) at the position  502 , a lateral force is generated on the piston  505  at its mid-length where the crankpin fits. This force presses the piston  505  against the cylinder wall that contains it in rotor  205 . However, because of the combined offset of the crank rotation center and the crankpin (which combine to hold one piston end marked here with a dot  503  at a maximum proximity to the outer rim of the rotor), this force is applied to the rotor  205  away (for example, by a distance of two quarters or one half of the piston stroke) from its own center of rotation. This force causes a torque on the rotor  205  around its own rotation center. The torque compels the rotor  205  to spin on its bearings about the center of the rotor  205 . 
         [0033]    At  504 : the rotor  205  has turned 45 degrees clockwise, and the crank has rotated 90 degrees, maintaining the relative alignment of the crankpin, the piston, and rotor bore. Accordingly, the piston  505  (refer to the shown dot end  503 ) has retreated axially relative to the outer rim of the rotor  205 , thus beginning the suction stroke of the dot-end chamber in the spin pump assembly  105  (the chamber at opposite end of piston  505  simultaneously experiences compression). The space between the dot end  503  of the piston  505  and the rim of the rotor  205  is exposed to the suction port of the housing from times between position  502  and position  510 . 
         [0034]    With further rotation of the crankshaft  115 , parts continue to spin on their centers. As the crankshaft  115  spins around its axis, the piston  505  orbits around the center of the crankshaft  115 , as shown from  502  to  516 . The offsets between the center of the rotor  205  and the center of the crankshaft  115  move from an alignment position (where those offsets are additive, as shown in  502  and  510 ) to anti-alignment position (where those offsets are cancelling, as shown in  506  and  514 ). However, with respect to the rotor  205  (which is also rotating), the vector sum of the crank center eccentricity and the crankpin eccentricity remains aligned with the axis of the cylinder in rotor  205  and thereby the motion of the piston  505  in that cylinder. From the frame of reference of the rotor  205 , the first eccentricity (that is, a fixed-magnitude vector about the rotor center, and directed toward the crank center fixed in the housing) moves counter-clockwise, equal, and opposite to a vector associated with the second eccentricity (that is, a fixed-magnitude vector about the crank center, and directed toward the crank pin). During the addition of these vectors, the opposite component parts of the vectors cancel while the component parts of the complementary components of those vectors sum up, thereby resulting in a linear reciprocating vector of sinusoidal magnitude. This linear reciprocating vector with sinusoidal magnitude characterizes the stroke of the piston  505  relative to the rotor  205 . This movement of the piston is also referred to as an epicyclic movement. 
         [0035]    By adding a spin to such a system in its entirety, the relative rotations of housing (crank eccentricity), rotor  205 , and crankshaft  115  (crankpin eccentricity) are changed from being negative, zero, and positive with respect to ground to being zero, positive, and twice positive, as shown in diagram  500 . The crankshaft  115  rotates at twice the rate of the rotor  205  and the housing is stationary, but their relative movements are the same as if the rotor  205  were stationary, the housing rotated opposite to the crankshaft, and the piston  505  reciprocated in the rotor  205 . 
         [0036]    As mentioned, an internal-external  2 : 1  timing gear may be connected to the crankshaft  115  and the rotor  505  to enforce their relative rotational speeds without delivering power through the piston-rotor contact surface (the rotor cylinder bore). The internal-external 2:1 timing gear moves the crankshaft  115  together with the rotor  205  such that the rotation of the crankshaft  115  is twice the rotation of the rotations of the rotor  205  and the piston. While such rotations occur, the housing stays static in a same position, as shown in  FIG. 6 . 
         [0037]    However, in some implementations (based on some empirical testing), the spin pump assembly  105  may not require such timing gears when both the crankshaft and rotor are independently supported on bearings with respect to the housing (or, equivalently, to ‘ground’). In these implementations, timing gears may be deleterious to the simplicity and efficiency of the spin pump assembly  105 . The inertia of the rotor  205  may be made sufficient to carry the motion smoothly through positions where the crankshaft torque exerts no net torque on the rotor to encourage its further rotation (for example, through positions  506  and  514 ). However, addition of a second piston that is oriented at ninety degrees to the piston  505  and that is driven by a second crankpin oriented 180 degrees from the crankpin  206  may be used to eliminate such zero-torque positions when both pistons share a common rotor  205  and crankshaft  115 . 
         [0038]    As a further explanation of the above-noted operation associated with the epicyclic movement, consider the effect of the fluid chamber  501  as the piston  505  rotates through one cycle from  502  to  516 . At  502 , the crankshaft  115  is at an angle of zero, the rotor  205  is at an angle of zero, and the chamber  501  is at the top dead center (TDC). The TDC characterizes a datum position where the face  503  of the piston is in a same angular position as the angular position of the crankshaft  115 . At the TDC, the volume of the chamber  501  is minimum. As the piston rotates clockwise to go towards  504 , the cylinder opens to the suction port and the volume of the chamber  501  expands. 
         [0039]    At  504 , the crankshaft  115  has already rotated ninety degrees while the rotor  205  and the piston have already rotated forty-five degrees. As noted above, the suction occurs here, and the volume of the chamber  501  keeps expanding until the suction ends. 
         [0040]    At  506 , the crankshaft  115  has already rotated one hundred and eighty degrees while the rotor  205  and the piston have already rotated ninety degrees. Suction continues, and the volume of the chamber  501  keeps expanding. 
         [0041]    At  508 , the crankshaft  115  has already rotated one two hundred and seventy degrees while the rotor  205  and the piston have already rotated one hundred and thirty five degrees. The volume of the chamber  501  keeps expanding until the suction ends. As the face  503  of the piston moves towards the position illustrated at  510 , the expansion of the volume of the chamber reaches a maximum and stops after suction ends and the chamber becomes sealed from the suction port  518 . 
         [0042]    At  510 , the crankshaft  115  has already rotated three hundred and sixty degrees while the rotor  205  and the piston have already rotated one hundred and eighty degrees. The chamber  501  is at the bottom dead center (BDC). At  510 , the suction has stopped (as the chamber  501  has become sealed from suction port), and the discharge has not yet begun. 
         [0043]    At  512 , the crankshaft  115  has already rotated four hundred and fifty degrees while the rotor  205  and the piston have already rotated two hundred and twenty five degrees. There is neither suction nor discharge from volume of the chamber  501 . Accordingly, the volume of the chamber  501  has decreased without substantial change in the mass of contained fluid, and pressure has risen therein. 
         [0044]    At  514 , the crankshaft  115  has already rotated five hundred and forty degrees while the rotor  205  and the piston have already rotated two hundred and seventy degrees. There is neither suction nor discharge from volume of the chamber  501 . Accordingly, the volume of the chamber  501  has further decreased and the pressure of the fluid contained in the chamber  501  has further risen until (just after this  514  moment) the chamber  501  reaches the discharge port and the discharge begins. The exact timing of such opening is preferably determined by positioning the discharge port such that the pressure rise achieved in chamber  501  matches the desired discharge pressure at the port. 
         [0045]    At  516 , the crankshaft  115  has already rotated six hundred and thirty degrees while the rotor  205  and the piston have already rotated three hundred and fifteen degrees. There is discharge from volume of the chamber  501 . Accordingly, the volume of the chamber  501  continues to decrease as the rotor  205  moves toward its initial TDC position again, even as the chamber  501  remains open to discharge port and fluid is pressed out of chamber  501 , as seen at  516 . 
         [0046]    Finally, for this one complete cycle, at  502  again the crankshaft has rotated seven hundred and twenty degrees while the rotor  205  and piston have rotated three hundred sixty degrees to return to the original condition, at TDC, with substantially all of the inducted fluid (from the suction port) having been compressed and delivered out of the discharge port, from which chamber  501  has already been sealed by passing beyond it, and approaching again the suction port to begin a new cycle. 
         [0047]    In some implementations, a one-way valve can be included at either suction or discharge ports to reduce or substantially eliminate back flow or cross flow between ports. Such a one-way valve can be provided on the piston in place of the suction or discharge port. The crankshaft area of the housing communicating with chamber  501  through the valve can be used as a source or sink of the pumped fluid, respectively. The bore of rotor  205  can be capped by valves or ducts adjacent to the bore within the rotor  205 . Conduction and direct flow in and out of the chamber  501  may not use ports in the housing addressing the periphery of the rotor  205 , but rather may occur through crankshaft area or axial end faces of rotor to external ports there. 
         [0048]    Based on such kinematics and the selection of compatible dry-lubricating materials for piston and rotor, the need for oil in the spin pump assembly  105  as a lubricant is advantageously obviated. Materials for the assembly may include, for example: polymers selected from PTFE, polyethylene, acetal, or other known low-friction materials for one part (for example, the piston or a coating thereon); anodized aluminum, nickel plating, vapor-deposited diamond graphite or other known hard, smooth surfaces (e.g. for the rotor bore). 
         [0049]    As there is no oil lubrication, the spin pump assembly  105  can provide breathable quantity of compressed gas, such as oxygen. Additionally, the rotational movements associated with the above-noted kinematics advantageously prevent vibration that is caused in conventional pumps due to linear or oscillatory movements of their moving parts with respect to ground, because each component part in the present invention is either spinning about its own center or orbiting around another spin center. So, rotating balancing masses can be applied for substantially perfect elimination of forces and vibration from unbalanced mass in motion. 
         [0050]    Further, the components  202  used in the spin pump assembly  105  are light in weight (for example, between 0.2 kilograms and 0.5 kilograms for a two-piston unit with swept volume of 20 cc/ rotor revolution, as shown with respect to the spin pump assembly  105 .). In other implementations, the weight of the components  202  can be based on the scale of the device. For example, the components  202  can weigh a few micrograms or a few kilograms. Light weight and pure rotational motion combine to enable high operating speeds, further reducing the required size and mass for a desired output flow. 
         [0051]    The spin pump assembly  105  is inexpensive to manufacture because all key part shapes or features are simple cylinders or planes and all relative orientations of shapes or features are parallel or orthogonal. Additionally, the spin pump assembly  105  is inexpensive as compared to many conventional pumps. Further, the spin pump assembly  105  is small, portable, and affordable. Further, the spin pump assembly  105  can operate in concentrators based on the principle of vacuum pressure swing adsorption (VPSA), where lower pressure portions of the kinematics cycle are sub-atmospheric, because the adsorbent substances can deliver more oxygen per unit mass of the adsorbent substance when pressures are at the vacuum levels. This is most advantageously achieved by dedicating one piston (two faces) to pressure, and another piston (two other faces) to vacuum, with those pistons operating axially separated and with axes ninety degrees apart on a crankshaft  115  with crankpins  206  one hundred eighty degrees apart, and with each piston addressing separate suction and discharge ports connected to their respective cycle control valves. Alternatively, two rotors with one piston each can be driven by a single crankshaft, but with an intervening partition to isolate the vacuum pump rotor from the pressure rotor. 
         [0052]      FIG. 7  shows another embodiment of the rotor assembly wherein the rotor is formed of a first piece  705  and a second piece  710  (i.e., a pair of congruent halves) that mate with one another to collectively form the rotor. The rotor may be cylindrical as shown or it may be rectangular or any other shape. The two pieces also collectively form the two piston chambers when mated to one another wherein each piece forms the entirety of a single piston&#39;s bore(s) such that each piece can contain a piston without having to be mated to the other piece. Each of the two pieces  705  and  710  individually form a cylindrical portion of two coaxial piston chambers aligned perpendicular to the rotation axis of the respective piece. When the two pieces are engaged or mated to one another, the rotational axis of one piece co-axially aligns with the rotational axis of the other piece to form the rotor. 
         [0053]    By splitting the rotor into two congruent halves that interlockingly engage one another, full cylinders are formed in each half (or each piece) for each double-ended piston. This enables single-piece, double-ended pistons  715  to be inserted into the cylinders (i.e., piston bores) before the two pieces of the rotor are assembled over the two ends of the crankshaft  115 . With the single-piece rotor, only one piston can be inserted into the cylinders before they are assembled over the two ends of the crankshaft, hence at least one multi-piece piston is required in one-piece rotor approach. With a hub-and-caps rotor (see for example, Richards U.S. Pat. No. 2,683,422), alignment of cylinder axes across the hub and rotation axis of the rotor is difficult and effectively precludes oil-free operation that requires greater precision to minimize incident lateral loads on pistons and cylinders. 
         [0054]      FIG. 8  shows an example of a two-piece rotor in an assembled state. The two congruent or substantially congruent pieces  705  and  710  are mated to one another so as to collectively form the rotor. The piece  710  is shown in phantom to illustrate internal components of the rotor. Note that each piece  705  and  710  includes an entire cylindrical piston bore that fits a single piston. The single piece pistons  715  are each positioned in a respective piston bore in each piece, with the rotor comprising all bores being collectively formed by the first and second pieces when assembled together. 
         [0055]    In a method of assembly, the pair of single piece, double ended pistons are positioned or otherwise inserted into the respective piston bores of the first and second, generally congruent pieces of the rotor. In this manner, there are two rotor pieces with each rotor piece containing a piston in its respective piston bore. The first piston-filled piece is then assembled over one end of the crankshaft, by aligning and fitting the piston (at its central cross-bore, where a bearing may be located) onto an eccentric  730  ( FIG. 7 ) of the crankshaft. The second piston-filled piece is then assembled over the other, opposite end of the crankshaft, by aligning and fitting the piston (at its central cross-bore, where a bearing may be located) onto another eccentric  730  of the crankshaft. The second piece of the rotor thereby becomes mated or engaged with the first piece of the rotor and can be joined by bolts or other known fastener means, so that the pistons are seated within the piston bores and such that the first and second pieces collectively form the piston bores and the rotor. 
         [0056]      FIG. 10  shows a cross-sectional view of the spin pump assembly  105  in an assembled state. The rotor  205  is mounted over the crankshaft  115  with a piston  505  movably positioned in a piston bore and coupled to the crankshaft  115  and enclosed by housing  110  and bearing plates  120 . In another embodiment shown in  FIG. 11 , the piston bore ends in rotor  205  are coupled to heads  1105 . Valve plates  1110  may include valves  1120  and  1125  that regulate fluid inflow and fluid outflow routed to respective side ports in bearing plates  120 . Thus, at least one valve is coupled to one of the piston bores. In an embodiment, one of the valves is a outlet valve on a piston head and another valve is an inlet valve on a piston, whereby inflow may be drawn through the central crankcase portion of the pump and outflow discharged through the head. 
         [0057]    The embodiments shown in the figures are examples and it should be appreciated that changes are possible and within the scope of this disclosure. For example, in an embodiment the pistons are rectangular or non-cylindrical and are mounted in complementary-shaped bores. In another embodiment, the rotor is rectangular or non-cylindrical. Other variations are within the scope of this disclosure. 
         [0058]    Although a few variations have been described in detail above, other modifications can be possible. For example, the logic flows depicted in the accompanying figures and described herein do not require the particular order shown, or sequential order, to achieve desirable results. Other embodiments may be within the scope of the following claims.