Abstract:
Smaller and lighter hydraulic pump/motors are provided with pistons having body portions substantially as long as the axial length of the respective cylinders in which they reciprocate. A plurality of respective lubricating channels, formed circumferentially and radially transecting the walls of each cylinder, is each positioned to be closed at all times by the axial cylindrical body of each respective piston during its entire stroke. Each lubricating channel is interconnected, one to another, to form a single, continuous lubricating passageway entirely within the cylinder block and not connected by either fluid “input” or fluid “output” passageways, being replenished solely by blow-by entering from the valve end of each cylinder. A plurality of sealing members, each located near the open end of each cylinder, substantially eliminates blow-by from this lubricating passageway, thereby significantly increasing volumetric efficiency. The resulting improved lubrication, in combination with unique spring-biased hold-down assemblies, permits use of variable-angle swash-plate arrangements that require neither dog-bones at the outer ends of the pistons nor conventional nutating-only wobblers.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a Continuation-In-Part of U.S. copending parent application Ser. No. 10/229,407, filed Aug. 28, 2002, which application is hereby incorporated by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to hydraulic pump/motor machines that have elongated pistons reciprocating in cylinders and, more particularly, to a system for lubricating such pistons while maintaining contact between the heads of such pistons and the swash-plate of the pump/motor.  
         BACKGROUND  
         [0003]    Hydraulic pumps and motors are well known and widely used, having reciprocating pistons mounted in respective cylinders formed in a cylinder block and positioned circumferentially at a first radial distance about the rotational axis of a drive element. Many of these pump/motor machines have variable displacement capabilities; and they are generally of two basic designs: (a) either the pistons reciprocate in a rotating cylinder block against a variably inclined, but otherwise fixed, swash-plate or (b) the pistons reciprocate in a fixed cylinder block against a variably inclined and rotating swash-plate that is generally split to include a non-rotating (but nutating) wobbler that slides upon the surface of a rotating (and nutating) rotor. While the invention herein is applicable to both of these designs, it is particularly appropriate for, and is described herein as, an improvement in the latter type of machine.  
           [0004]    While hydraulic machines with fixed cylinder blocks can be built much lighter and smaller than the machines that must support and protect heavy rotating cylinder blocks, the mounting and support of the swash-plate wobblers has always been a major problem. For high-pressure/high-speed service, the wobbler must be supported in a manner that allows the relative motion between the non-rotating pistons and the wobbler to follow varying non-circular paths. Also, such fixed-cylinder-block machines have heretofore used a “dog-bone” extension rod (i.e., a rod with two spherical ends) to interconnect one end of each piston with the flat surface of the nutating-but-not-rotating wobbler. One spherical end of the dog bone is pivotally mounted into the head end of the piston, while the other spherical end is usually covered by a pivotally-mounted conventional “shoe” element that must be held at all times against the swash-plate wobbler. These just-mentioned elements greatly increase the complexity and cost of building the rotating swash-plates of these machines.  
           [0005]    Dog-bone rods are also sometimes used to interconnect one end of each piston with the inclined (but not rotating) swash-plates of hydraulic machines having rotating cylinder blocks. However, more often this latter type of machine omits such dog-bones, using instead elongated pistons, each having a spherical head at one end (again, usually covered by a pivotally-mounted conventional shoe element) that effectively contacts the non-rotating flat surface of the swash-plate. Such elongated pistons are designed so that a significant portion of the axial cylindrical body of each piston remains supported by the walls of its respective cylinder at all times during even the maximum stroke of the piston. This additional support for such elongated pistons is designed to assure minimal lateral displacement of each spherical piston head as it slides over the inclined-but-not-rotating swash-plate when the pistons rotate with their cylinder block.  
           [0006]    Generally, these elongated pistons are primarily lubricated by “blow-by”, i.e., that portion of the high-pressure fluid that is forced between the walls of each cylinder and the outer circumference of each piston body as the reciprocating piston drives or is driven by high-pressure fluid. Such blow-by provides good lubrication only if tolerances permit the flow of sufficient fluid between the walls of the cylinder and the long cylindrical body of the piston, and blow-by sufficient to assure good lubrication often negatively affects the volumetric efficiency of the pump or motor machine. For instance, a 10 cubic inch machine can use as much as 4 gallons of fluid per minute for blow-by. While smaller tolerances can often be used to reduce blow-by, the reduction of such tolerances is limited by the need for adequate lubrication.  
           [0007]    The invention disclosed below is directed to improving the volumetric efficiency of such elongated-piston machines while, at the same time, assuring (a) appropriate lubrication of the pistons and (b) simplification of the apparatus used to maintain contact between the pistons and the swash-plate.  
         SUMMARY OF THE INVENTION  
         [0008]    The invention is disclosed on two different hydraulic machines. Both have the preferred format of fixed cylinder blocks and rotating/nutating swash-plates. [However, persons skilled in the art will appreciate that the invention is equally applicable to hydraulic machines with rotating cylinder blocks and swash-plates that do not rotate with the drive elements of the machines.] Each disclosed machine can operate as either a pump or a motor. One has a swash-plate that, while rotating at all times with the drive element of the machine, is fixed at a predetermined inclined angle relative to the axis of the drive element so that the pistons move at a maximum predetermined stroke at all times. The swash-plate of the other disclosed machine has an inclination that can be varied throughout a range of angles in a manner well known in the art to control the stroke of the pistons throughout a range of movements up to a maximum in each direction.  
           [0009]    In each machine, each piston is elongated, having an axial cylindrical body portion that preferably is substantially as long as the axial length of the respective cylinder in which it reciprocates. Preferably, each piston also has a spherical head end that, by means of a conventionally pivoted shoe and relatively simple apparatus, is maintained in effective sliding contact with a flat face of the machine&#39;s swash-plate. The axial length of each cylindrical piston body is selected to assure minimal lateral displacement of the spherical first end of the piston at all times. Therefore, even when each piston is extended to its maximum stroke, that portion of the piston body which is still supported within its respective cylinder is sufficient to assure a minimal lateral displacement of the extended spherical end of the piston when it is in sliding contact with the rotating/nutating flat face of the swash-plate.  
           [0010]    According to the invention, each cylinder formed within the cylinder blocks of each machine is provided with a respective lubricating channel formed in the cylindrical wall of each cylinder. This lubricating channel is positioned so that at all times during reciprocation of the piston within its respective cylinder, each respective lubricating channel remains substantially closed by the axial cylindrical body of the piston during its entire stroke. Preferably, each respective lubricating channel is formed circumferentially and radially transects each cylinder.  
           [0011]    Also formed in the fixed cylinder block of each machine is a plurality of further passageways that interconnect each of the just-described lubricating channels. The interconnection of all of the lubricating channels, one to another, forms a single, continuous lubricating passageway in the cylinder block. This continuous lubricating passageway is formed entirely within the cylinder block, preferably transecting each cylinder and being centered circumferentially at substantially the same radial distance as the cylinders are centered about the rotational axis of the drive element.  
           [0012]    [NOTE: To facilitate explanation of the invention, each piston is described as having an axial cylindrical body portion and a spherical head end, while each respective cylinder has a valve end and an open head portion beyond which the spherical head end of each piston extends at all times.] In the preferred embodiments disclosed, the continuous lubricating passageway just described above is not connected by either fluid “input” or fluid “output” passageways but instead is substantially closed off by the cylindrical body portions of the pistons at all times during operation of the machine. During operation, this lubricating passageway almost instantly fills with initial blow-by of high-pressure fluid that enters at the valve end of each cylinder and then passes between the walls of each cylinder and the outer circumference of the body portion of each driven piston. This blow-by effectively maintains high pressure within the continuous lubricating passageway at all times. A plurality of sealing members, each located respectively near the open end of each cylinder, provides a relatively tight seal for substantially eliminating blow-by between the body portion of each piston and the open head portion of each respective cylinder, thereby allowing the escape of only minimal blow-by from this lubricating passageway past the open end of the cylinders.  
           [0013]    Nonetheless, the lubricating fluid in this closed continuous lubricating passageway moves constantly as the result of the ever-changing pressures in each of the respective cylinders as the pistons reciprocate. That is, as the pressure in each cylinder is reduced to low pressure on the return stroke of each piston, the high-pressure fluid in the otherwise closed lubricating passageway is again driven between the walls of each cylinder and the outer circumference of the body of each piston into the valve end of each cylinder experiencing such pressure reduction. However, this secondary blow-by is not “lost”, i.e., it does not return to the sump to be replenished into the closed loop hydraulic system by the charge pump. Instead, this secondary blow-by is immediately returned to the closed loop without requiring the use of a charge pump, and the closed continuous lubricating passageway is immediately replenished by the entrance of a similar flow of high-pressure blow-by from the valve end of each cylinder experiencing increased pressure.  
           [0014]    This just-described lubricating passageway provides appropriate lubrication to the high-speed reciprocation of the pistons while substantially reducing blow-by. During successful operation of commercial prototypes built according to the invention, blow-by was reduced by 90%. That is, the blow-by experienced by conventional commercial hydraulic machines of comparable specifications generally ranges between 4-5 gallons per minute, while the blow-by experienced by the invention&#39;s prototypes ranges between 0.5-0.7 gallons per minute, thereby remarkably increasing the volumetric efficiency of the invention&#39;s hydraulic machines.  
           [0015]    As indicated above, fixed-cylinder-block hydraulic machines can be built smaller and lighter than conventional rotating block hydraulic machines having similar specifications. As a result of the improved lubrication of the elongated pistons, the disclosed invention makes it possible to use these smaller and lighter designs to meet the high-speed/high-pressure specifications required for automotive use.  
           [0016]    Further, special attention is called to the invention&#39;s significantly simplified support assemblies for the variable rotating swash-plates of the invention&#39;s disclosed hydraulic machines. Each of the invention&#39;s support assemblies disclosed herein (a) omits dog-bones that normally are mounted between the outer end of each piston and the nutating-only wobbler portion of a conventional rotating/nutating swash-plate but (b) also omits the nutating-only wobbler portion of a conventional rotating/nutating swash-plate. Instead, a conventional shoe is mounted directly to the spherical head of each piston and is maintained in effective sliding contact with the flat face of the swash-plate&#39;s rotor portion by means of a minimal spring bias sufficient to maintain such effective sliding contact in the absence of hydraulic pressure at the valve ends of the pump&#39;s cylinders.  
           [0017]    Two simplified support mechanisms are disclosed: The first simplified support mechanism comprises a unique hold-down plate assembly biased by a coil spring positioned circumferentially about the rotational axis of the pump&#39;s drive element. The invention&#39;s second support mechanism is even simpler, comprising nothing more than a conventional shoe mounted directly to the spherical head of each piston, with the minimal bias being supplied by a plurality of springs, each spring being positioned respectively between the body portion of each respective piston and the valve end of each respective cylinder. While the second support mechanism is a little more difficult to assemble than the first, the latter is considerably simpler, lighter, and cheaper to manufacture.  
           [0018]    The important changes introduced by this invention not only provide hydraulic machines that are lighter and smaller than conventional machines having similar specifications but, further, provide machines with greater volumetric efficiency while reducing the weight and size of the machines as well as the cost of manufacture and simplifying assembly. 
       
    
    
     DRAWINGS  
       [0019]    [0019]FIG. 1 is a partially schematic and cross-sectional view of a hydraulic machine with a fixed cylinder block and a rotating/nutating swash-plate having a fixed angle of inclination, showing the invention incorporated in the cylinder block and at the piston/swash-plate interface.  
         [0020]    [0020]FIG. 2 is a partially schematic and cross-sectional view of the fixed cylinder block of the hydraulic machines of FIGS. 1 and 3 taken along the plane  2 - 2  with parts being omitted for clarity.  
         [0021]    [0021]FIG. 3 is a partially schematic and cross-sectional view of a hydraulic machine with a fixed cylinder block and a rotating/nutating swash-plate having a variable angle of inclination, again showing the invention incorporated in the cylinder block and at the piston/swash-plate interface.  
         [0022]    [0022]FIGS. 4A and 4B are partially schematic and cross-sectional views of the swash-plate and piston shoe hold-down assembly disclosed in FIGS. 1 and 3 when the swash plate is inclined at +25°, with parts removed for clarity, showing relative positions of the head ends of the pistons, shoes, and special washers, as well as the spring-biased hold-down element that biases each sliding shoe against the flat face of the swash-plate; the view in FIG. 4A is taken in the plane  4 A- 4 A of FIG. 3 in the direction of the arrows, while the view in FIG. 4B is taken in the plane  4 B- 4 B of FIG. 4A.  
         [0023]    [0023]FIGS. 5A and 5B,  6 A and  6 B, and  7 A and  7 B are, respectively, views of the same parts illustrated in FIGS. 4A and 4B when the swash-plate is operating at three other inclinations, namely, at +15°, 0°, and −25°.  
         [0024]    [0024]FIG. 8 is an enlarged, partial, schematic and cross-sectional view of only a single cylinder and piston for another hydraulic machine similar to those shown in FIGS. 1 and 3 but showing a more simplified second embodiment of a spring-biased hold-down assembly for the invention&#39;s piston shoes. 
     
    
     DETAILED DESCRIPTION  
       [0025]    The operation of hydraulic machines of the type to which the invention may be added is well known. Therefore, such operation will not be described in detail.  
         [0026]    Hydraulic Motor  
         [0027]    Referring to FIG. 1, hydraulic motor  10  includes a fixed cylinder block  12  having a plurality of cylinders  14  (only one shown) in which a respective plurality of mating pistons  16  reciprocates between the retracted position of piston  16  and the extended position of piston  16 ′. Each piston has a spherical head  18  that is mounted on a neck  20  at one end of an elongated axial cylindrical body portion  22  that, in the preferred embodiments shown, is substantially as long as the length of each respective cylinder  14 .  
         [0028]    Each spherical end  18  fits within a respective shoe  24  that slides over a flat face  26  formed on the surface of a rotor  28  that, in turn, is fixed to a drive element, namely, shaft  30  of the machine. Shaft  30  is supported on bearings within a bore  31  in the center of cylinder block  12 . Flat face  26  of fixed rotor  28  is inclined at a predetermined maximum angle (e.g., 25°) to the axis  32  of drive shaft  30 , being supported by an appropriate thrust bearing assembly  35 .  
         [0029]    A modular valve assembly  33 , which is bolted as a cap on the left end of cylinder block  12 , includes a plurality of spool valves  34  (only one shown) that regulates the delivery of fluid into and out the cylinders  14 . As indicated above, each of the machines disclosed can be operated as either a pump or as a motor. For this description of a preferred embodiment, the fixed-angle swash-plate machine shown in FIG. 1 is being operated as a motor. Therefore, during the first half of each revolution of drive shaft  30 , high-pressure fluid from inlet  36  enters the valve end of each respective cylinder  14  through a port  37  to drive each respective piston from its retracted position to its fully extended position; and during the second half of each revolution, lower pressure fluid is withdrawn from each respective cylinder through port  37  and fluid outlet  39  as each piston returns to its fully retracted position.  
         [0030]    In a manner well known in the art, fluid inlet  36  and outlet  39  are preferably connected through appropriate “closed loop” piping to a mating hydraulic pump (e.g., pump  110  shown in FIG. 3 and discussed below) so that, at all times, fluid pressure biases spherical ends  18  and respective shoes  24  against flat face  26 . The serial extension and retraction of each respective piston causes rotor  28  to rotate, thereby driving shaft  30 . Flat face  26  is fixed at the maximum angle of inclination so that, when the flow rate of hydraulic fluid being circulated in the closed loop through inlet  36  and outlet  39  is relatively small, pistons  16  reciprocate relatively slowly, resulting in a relatively slow rotation of drive shaft  30 . However, as the flow rates of fluid circulation in the closed loop increase, the reciprocation of the pistons increases accordingly and so does the speed of rotation of drive shaft  30 . When operated at automotive speeds or pressures (e.g., up to 4000 rpm or 4000 psi), lubrication of the pistons becomes critical, and blow-by losses can also greatly increase. Cylinder block  12  is modified by the invention to address such lubrication needs and to reduce such blow-by losses.  
         [0031]    Referring now to both FIGS. 1 and 2, the cylindrical wall of each cylinder  14  is transected radially by a respective lubricating channel  40  formed circumferentially therein. A plurality of passageways  42  interconnect all lubricating channels  40  to form a continuous lubricating passageway in cylinder block  12 . Each respective lubricating channel  40  is substantially closed by the axial cylindrical body  22  of each respective piston  16  during the entire stroke of each piston. That is, the outer circumference of each cylindrical body  22  acts as a wall that encloses each respective lubricating channel  40  at all times. Thus, even when pistons  16  are reciprocating through maximum strokes, the continuous lubricating passageway interconnecting all lubricating channels  40  remains substantially closed off. Continuous lubricating passageway  40 ,  42  is simply and economically formed within cylinder block  12  as can be best appreciated from the schematic illustration in FIG. 2 in which the relative size of the fluid channels and connecting passageways has been exaggerated for clarification.  
         [0032]    During operation of hydraulic motor  10 , all interconnected lubricating channels  40  are filled almost instantly by blow-by of high-pressure fluid from inlet  36  entering each cylinder  14  through port  37  and being forced between the walls of the cylinders and the outer circumference of each piston  16 . Loss of lubricating fluid from each lubricating channel  40  is restricted by a surrounding seal  44  located near the open end of each cylinder  14 . Nonetheless, the lubricating fluid in this closed continuous lubricating passageway of lubricating channels  40  flows moderately but continuously as the result of “secondary” blow-by in response to piston motion and to the changing pressures in each half-cycle of rotation of drive shaft  30  as the pistons reciprocate. As the pressure in each cylinder  14  is reduced to low pressure on the return stroke of each piston  16 , the higher pressure fluid in otherwise closed lubricating passageway  40 ,  42  is again driven between the walls of each cylinder  14  and the outer circumference of body portion  22  of each piston  16  into the valve end of each cylinder  14  experiencing such pressure reduction.  
         [0033]    However, special attention of persons skilled in the art is called to the fact that this just-mentioned secondary blow-by back into cylinder  14  is not “lost”. Instead, it is immediately returned to the well-known closed hydraulic fluid loop that interconnects the pump and motor. Further, this secondary blow-by does not return to a sump and, therefore, does not have to be replenished into the closed loop hydraulic system by a charge pump. Finally, closed continuous lubricating passageway  40 ,  42  is immediately replenished by the entrance of a similar flow of high-pressure blow-by from the valve end of each cylinder experiencing increased pressure.  
         [0034]    As mentioned above, there is minimal blow-by loss from closed continuous lubricating passageway  42  that interconnects all lubricating channels  40 , That is, there may be a minimal fluid flow that leaks from this closed continuous lubricating passageway past the seals  44  at the end of each cylinder  14 . However, any such minimal blow-by is instantly replenished by a similar flow of blow-by entering around the opposite end of each piston  16 .  
         [0035]    The just-described lubrication arrangement is not only remarkably simple, but it also permits a similar simplification of the pinion/swash-plate interface apparatus of the hydraulic machine to further reduce the cost of manufacture and operation.  
         [0036]    To complete the description of hydraulic motor  10 , the pinion/swash-plate interface apparatus shown in FIG. 1 comprises only (a) rotor  28  mounted on drive shaft  30  using conventional needle and thrust bearings and (b) a simple spring-biased hold-down assembly for maintaining piston shoes  24  in constant contact with the rotating and nutating flat surface  26  of rotor  28 . [Note: Two embodiments of the invention&#39;s simplified pinion/swash-plate interface assemblies are described in greater detail in a separate section below.] 
         [0037]    The first embodiment of the invention&#39;s hold-down assembly, as shown in FIG. 1, includes a coil spring  50  that is positioned about shaft  30  and received in an appropriate crevice  52  formed in cylinder block  12  circumferentially about axis  32 . Spring  50  biases a hold-down element  54  that is also positioned circumferentially about shaft  30  and axis  32 . Hold-down element  54  is provided with a plurality of openings, each of which surrounds the neck  20  of a respective piston  16 . A respective special washer  56  is positioned between hold-down element  54  and each piston shoe  24 . Each washer  56  has an extension  58  that contacts the outer circumference of a respective shoe  24  to maintain the shoe in contact with flat face  26  of rotor  28  at all times.  
         [0038]    The just-described hydraulic motor, with its remarkable simplification of both lubrication and the piston/swash-plate interface, is efficient, easy to manufacture, and economical to operate.  
         [0039]    Variable Hydraulic Pump  
         [0040]    A second preferred embodiment of a hydraulic machine in accordance with the invention is illustrated in FIG. 3. A variable hydraulic pump  110  includes a modular fixed cylinder block  112  which is identical to cylinder block  12  of hydraulic motor  10  shown in FIG. 1 and described above. Cylinder block  112  has a plurality of cylinders  114  (only one shown) in which a respective plurality of mating pistons  116  reciprocates between the retracted position of piston  116  and variable extended positions (the maximum extension being shown in the position of piston  116 ′). Each piston has a spherical head  118  that is mounted on a neck  120  at one end of an elongated axial cylindrical body portion  122  that, in the preferred embodiment shown, is substantially as long as the length of each respective cylinder  114 . Each spherical piston head  118  fits within a respective shoe  124  that slides over a flat face  126  formed on the surface of a rotor  128  that, as will be discussed in greater detail below, is pivotally attached to a drive element, namely, shaft  130  that is supported on bearings within a bore  131  in the center of cylinder block  112 .  
         [0041]    In a manner similar to that explained above in regard to hydraulic motor  10 , variable pump  110  is also provided with a modular valve assembly  133  that is bolted as a cap on the left end of modular cylinder block  112  and, similarly, includes a plurality of spool valves  134  (only one shown) that regulates the delivery of fluid into and out of cylinders  114 .  
         [0042]    As indicated above, each of the machines disclosed can be operated as either a pump or as a motor. For the description of this preferred embodiment, the variable-angle swash-plate machine  110  shown in FIG. 3 is being operated as a pump, and drive shaft  130  is driven by a prime mover (not shown), e.g., the engine of a vehicle. Therefore, during the one-half of each revolution of drive shaft  130 , lower pressure fluid is drawn into each respective cylinder  114  entering a port  137  from a “closed loop” of circulating hydraulic fluid through inlet  136  as each piston  116  is moved to an extended position. During the next half of each revolution, the driving of each respective piston  116  back to its fully retracted position directs high-pressure fluid from port  137  into the closed hydraulic loop through outlet  139 . The high-pressure fluid is then delivered through appropriate closed loop piping (not shown) to a mating hydraulic motor, e.g., motor  10  discussed above, causing the pistons of the mating motor to move at a speed that varies with the volume (gallons per minute) of high-pressure fluid being delivered in a manner well known in the art.  
         [0043]    Once again referring to modular cylinder block  112 , it is constructed identical to cylinder block  12  which has already been described. That is, the cylindrical wall of each cylinder  114  is transected radially by a respective lubricating channel  40 ′ formed circumferentially therein. A plurality of passageways  42 ′ interconnects all lubricating channels  40 ′ to form a continuous lubricating passageway in cylinder block  112 . A cross section of cylinder block  112  taken in the plane  2 - 2  looks exactly as the cross-sectional view of cylinder block  12  in FIG. 2.  
         [0044]    In effect, almost all of the discussion above relating to the invention&#39;s continuous lubricating passageway  40 ,  42  with reference to the apparatus of hydraulic motor  10  shown in FIGS. 1 and 2 applies equally to the operation of continuous lubricating passageway  40 ′,  42 ′ in cylinder block  112  of hydraulic pump  110  shown in FIG. 3, including the minimization of loss of lubricating fluid from each lubricating channel  40 ′ by a surrounding seal  144  located near the open end of each cylinder  114 . Similarly, the flow of lubricating fluid in closed continuous lubricating passageway  40 ′,  42 ′ is moderate but continuous as the result of “secondary” blow-by in response to piston motion and to the changing pressures in each half-cycle of rotation of drive shaft  130  as the pistons reciprocate. Of course, as is different in pump  110 , lower fluid pressure is present in each cylinder  114  when each piston  116  is moving to an extended position, while the source of the high-pressure fluid that is forced between the walls of the cylinders and the outer circumference of each piston  116  occurs as each piston  116  is being driven from its extended position to its fully retracted position by the rotation of drive shaft  130  by the prime mover (not shown).  
         [0045]    However, once again special attention of persons skilled in the art is called to the fact that this just-mentioned secondary blow-by back into each cylinder  114  is not “lost”. Instead, it is immediately returned to the well-known closed hydraulic fluid loop that interconnects the pump and motor. That is, this secondary blow-by does not return to a sump and, therefore, does not have to be replenished into the closed loop hydraulic system by a charge pump. Also, while there may be a minimal fluid flow that leaks from closed continuous lubricating passageway  40 ′,  42 ′ past the seals  144  at the end of each cylinder  114 , any such minimal blow-by is instantly replenished by a similar flow of blow-by entering around the opposite end of each piston  116  experiencing increased pressure.  
         [0046]    As discussed in the preamble above, the invention permits the machine&#39;s swash-plate apparatus to be simplified by (a) the omission of the dog-bones that normally are mounted between the outer end of each piston and a nutating-only wobbler portion of a conventional rotating/nutating swash-plate and (b) the omission of the wobbler portion itself as well as the apparatus conventionally required for mounting the non-rotating wobbler to the rotating/nutating rotor portion of the swash-plate.  
         [0047]    Rotor  128  is pivotally mounted to drive shaft  130  about an axis  129  that is perpendicular to axis  132 . Therefore, while rotor  128  rotates with drive shaft  130 , its angle of inclination relative to axis  130  can be varied from 0° (i.e., perpendicular) to α25°. In FIG. 3, rotor  128  is inclined at +25°. This variable inclination is controlled as follows: The pivoting of rotor  128  about axis  129  is determined by the position of a sliding collar  180  that surrounds drive shaft  130  and is movable axially relative thereto. A control-link  182  connects collar  180  with rotor  128  so that movement of collar  180  axially over the surface of drive shaft  130  causes rotor  128  to pivot about axis  129 . For instance, as collar  128  is moved to the right in FIG. 3, the inclination of rotor  128  varies throughout a continuum from the +25° inclination shown, back to 0° (i.e., perpendicular), and then to −25°.  
         [0048]    The axial movement of collar  180  is controlled by the fingers  184  of a yoke  186  as yoke  186  is rotated about the axis of a yoke shaft  190  by articulation of a yoke control arm  188 . Yoke  186  is actuated by a conventional linear servo-mechanism (not shown) connected to the bottom of yoke arm  188 . In this preferred embodiment, while the remainder of the elements of yoke  186  are all enclosed within a modular swash-plate housing  192  and yoke shaft  190  is supported in bearings fixed to housing  192 , yoke control arm  188  is positioned external of housing  192 .  
         [0049]    It will also be noted that swash-plate rotor  128  is balanced by a shadow-link  194  that is substantially identical to control-link  182  and is similarly connected to collar  180  but at a location on exactly the opposite side of collar  180 .  
         [0050]    Piston Shoe Hold-Down Assemblies  
         [0051]    Fluid pressure constantly biases pistons  116  in the direction of rotor  128 , and a thrust plate  198  is provided to carry that load. However, at the speeds of operation required for automotive use (e.g., 4000 rpm), additional bias loading is necessary to assure constant contact between piston shoes  124  and flat surface  126  of rotor  128 . In view of the invention&#39;s omission of conventional dog-bones and omission of the conventional wobbler as well as its required mounting assembly, the variable hydraulic machines of this invention are able to provide such additional bias by using either of two simple spring-biased hold-down assemblies, the first being similar to that already described above in regard to hydraulic motor  10  in FIG. 1.  
         [0052]    (a) Hold-Down Assembly with Single-Spring Bias  
         [0053]    The following description of the invention&#39;s first embodiment for a hold-down assembly continues to refer to FIG. 3, but reference is now also made (a) to FIG. 4A, which shows an enlarged view taken in the plane  4 A- 4 A of FIG. 3 when viewed in the direction of the arrows, and (b) to FIG. 4B, which shows an enlargement of the same view of shown in FIG. 1 with parts removed for clarity.  
         [0054]    The hold-down assembly for pump  110  includes a coil spring  150  that is positioned about shaft  130  and received in an appropriate crevice  152  formed in cylinder block  112  circumferentially about axis  132 . Coil spring  150  biases a hold-down element  154  that is also positioned circumferentially about shaft  130  and axis  132 . Hold-down element  154  is provided with a plurality of circular openings  160 , each of which surrounds the neck  120  of a respective piston  116 . A plurality of special washers  156  is positioned, respectively, between hold-down element  154  and each piston shoe  124 . Each washer  156  has an extension  158  that contacts the outer circumference of a respective shoe  124  to maintain the shoe in contact with flat face  126  of rotor  128  at all times.  
         [0055]    The positions of the just-described parts of the swash-plate and piston shoe hold-down assembly change relative to each other as the inclination of rotor  128  is altered during machine operation. These changes in relative position are illustrated at various inclinations of rotor  128 , namely, at +25°, in FIGS. 4A and 4B; at +15° in FIGS.  5 A and  5 B; at 0° in FIGS. 6A and 6B; and at −25°, in FIGS. 7A and 7B. [NOTE: Persons skilled in the art will appreciate that each piston shoe  124  has a conventional pressure-balancing cavity centered on the flat surface of shoe  124  that contacts flat face  126  of rotor  128 , and that each respective shoe cavity is connected through an appropriate shoe channel  162  and piston channel  164  to assure that fluid pressure present at the shoe/rotor interface is equivalent at all times with fluid pressure at the head of each piston  116 . Since piston channel  164  passes through the center of spherical head  118  of each piston  116 , the position of channel  164  can be used to facilitate appreciation of the relative movements of the various parts of the hold-down assembly.] 
         [0056]    Referring to the relative position of these parts at the 0° inclination shown in FIGS. 6A and 6B, each piston channel  164  (at the center of each spherical head  118  of each piston  116 ) has the same radial position relative to each respective circular opening  160  in hold-down element  154 . As can be seen from the views in the other illustrated inclinations of swash-plate rotor  128 , at all inclinations other than 0°, the relative radial position of each piston channel  164  is different for each opening  160 , and the relative positions of each special washer  156  is also different.  
         [0057]    It must be appreciated that, at each of these illustrated swash-plate inclinations, the different relative positions at each of the nine openings  160  are themselves constantly changing as rotor  128  rotates and nutates through one complete revolution at each of these inclinations. For instance, at the 25° inclination shown in FIG. 4A, if during each revolution of rotor  128 , one were to watch the movement occurring through only the opening  160  at the top (i.e., at 12:00 o&#39;clock) of hold-down element  154 , the relative position of the parts viewed in the top opening  160  would serially change to match the relative positions shown in each of the other eight openings  160 .  
         [0058]    That is, at inclinations other than 0° (e.g., at −25° shown in FIG. 7A), during each revolution of rotor  128 , each special washer  156  slips over the surface of hold-down element  154  as, simultaneously, each shoe  124  slips over the flat face  126  of rotor  128 ; and each of these parts changes relative to its own opening  160  through each of the various positions that can be seen in each of the other eight openings  160 . These relative motions are largest at ±25°; and each follows a cyclical path (that appears to trace a lemniscate, i.e., a “figure-eight”) that varies in size with the angular inclinations of swash-plate rotor  128  and the horizontal position of each piston  116  in fixed cylinder block  112 .  
         [0059]    Therefore, to assure proper contact between each respective shoe  124  and flat face  126  of rotor  128 , in preferred embodiments, a size is selected for the boundaries of each opening  160  so that the borders of opening  160  remain in contact with more than one-half of the surface of each special washer  156  at all times during each revolution of rotor  128  and for all inclinations of rotor  128 , as can be seen from the relative positions of special washers  156  and the borders of each of the openings  160  in each of the drawings from FIG. 4A through FIG. 7A. As can be seen from the drawings, a circular border is preferred for each opening  160 .  
         [0060]    Finally, attention is called to the suggested manufacture of each shoe  124  and its respective mating special washer  156  using reinforced thermoplastic resin materials. These mating parts can also be combined to form a single thermoplastic shoe/washer combination, with the shoe portion being manufactured so that it is formed about the spherical head  118  of each piston  16 ′,  22 . Similarly, the cost and complexity of thrust bearing assembly  35  can be significantly reduced by the use of reinforced thermoplastic resins.  
         [0061]    (b) Hold-Down Assembly with Multiple-Spring Bias  
         [0062]    The second embodiment of the invention&#39;s hold-down assembly, while slightly more difficult to assemble, is considerably simpler and less expensive. This second embodiment is shown schematically in FIG. 8 in an enlarged, partial, and cross-sectional view of a single piston of a further hydraulic machine  210  according to the invention. Piston  216  is positioned in modular fixed cylinder block  212  within cylinder  214 , the latter being transected radially by a respective lubricating channel  40 ″ formed circumferentially therein. In the same manner as described in relation to the other hydraulic machines already detailed above, lubricating channel  40 ″ is interconnected with similar channels in the machine&#39;s other cylinders by a plurality of passageways that forms a continuous lubricating passageway in cylinder block  212 ; and, similarly, a surrounding seal  244  is located near the open end of each cylinder  214  to minimize the loss of lubricating fluid from each lubricating channel  40 ″.  
         [0063]    The only difference between fixed cylinder block  212  and the modular cylinder blocks disclosed in FIGS. 1 and 3 is that fixed cylinder block  212  includes neither a large axially circumferential coil spring nor an axially circumferential crevice for holding the same.  
         [0064]    While not shown, the modular fixed cylinder block  212  of hydraulic machine  210  can be connected to either a modular fixed-angle swash-plate assembly (as shown in FIG. 1) or a modular variable-angle swash-plate assembly (as shown in FIG. 3); but in either case, hydraulic machine  210  provides a much simpler hold-down assembly. Specifically, the hold-down assembly of this embodiment comprises only a respective conventional piston shoe  224  for each piston  216  in combination with only a respective coil spring  250 , the latter also being associated with each respective piston  216 .  
         [0065]    Each piston shoe  224  is similar to the conventional shoes shown in the first hold-down assembly just discussed above and, similarly, is mounted on the spherical head  218  of piston  216  to slide over the flat face  226  formed on the surface of the machine&#39;s swash-plate rotor  228  in a manner similar to that explained above. Each coil spring  250  is, respectively, seated circumferentially about hydraulic valve port  237  at the valve end of each respective cylinder  214  and positioned within the body portion of each respective piston  216 .  
         [0066]    Again, in the manner just explained above, each shoe  224  slips over flat face  226  of rotor  228  with a lemniscate motion that varies in size with the horizontal position of each piston  216  and the inclination of rotor  228  relative to axis  230 . During normal operation of hydraulic machine  210 , shoes  224  are maintained in contact with flat face  226  of the swash-plate by hydraulic pressure. Therefore, the spring bias provided by coil springs  250  is only minimal but still sufficient to maintain effective sliding contact between each shoe  224  and flat face  226  in the absence of hydraulic pressure at the valve end of each respective cylinder  214 .  
         [0067]    It has been found that the just-described minimal bias of springs  250  not only facilitates assembly but is also sufficient to prevent entrapment of tiny dirt and metal detritus encountered during assembly and occasioned by wear. Further, special attention is again called to the fact that this second embodiment provides this necessary function with only a few very inexpensive parts.  
         [0068]    The just-described pump/motor as well as the invention&#39;s other hydraulic machines described earlier, all provide both lubrication and a piston/swash-plate interface that are remarkably simple and relatively inexpensive to manufacture and provide further economies by reducing the number of parts required for efficient operation and increasing volumetric efficiency.