Abstract:
Smaller and lighter hydraulic pump/motors provide remarkably improved volumetric efficiency 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 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 a minimal flow of fluid to and from the valve end of each cylinder and passing between each respective cylindrical wall of each cylinder and the axial cylindrical body of each respective piston. Several embodiments are disclosed in combination with various spring-biased hold-down assemblies. The preferred embodiment included a fixed cylinder block, a roller bearing mounting between the wobbler and rotor of a split-swash plate, with piston shoes contacting the wobbler directly without any intermediary apparatus.

Description:
RELATED APPLICATIONS 
   This application is a Continuation-In-Part of U.S. parent application Ser. No. 10/229,407 filed 28 Aug. 2002 now abandoned and Continuation-In-Part Application Ser. No. 10/647,557 filed 25 Aug. 2003, now abandoned, which applications are hereby incorporated by reference. 

   TECHNICAL FIELD 
   This invention relates to liquid hydraulic pump/motor machines appropriate for relatively “heavy duty” automotive use, e.g., for hydraulic transmissions used for vehicle locomotion and/or for the storing and retrieval of fluids in energy-saving accumulator systems. [Note: the term “liquid” is used to distinguish from “gas” hydraulic pumps, e.g., pumps for compressing air and/or other gases.] 
   BACKGROUND 
   Hydraulic pumps and motor 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 often split to include-a non-rotating (i.e., nutating-only) “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 in which the pistons reciprocate in a fixed cylinder block. 
   As indicated above, this invention is directed to “liquid” (as distinguished from “gas”) type hydraulic machines and it should be understood that the terms “fluid(s)” and “pressurized fluid(s),” as used herein throughout the specification and claims, are intended to identify incompressible liquids rather than compressible gasses. Because of the incompressibility of liquids, the pressure and load duty cycles of the these two different types of hydraulic machines are so radically different that designs for the gas compression type machines are inappropriate for use in the liquid-type machines, and visa versa. Therefore, the following remarks should all be understood to be directed and applicable to liquid-type hydraulic machines and, primarily, to such heavy duty automotive applications as those identified in the Technical Field section above. 
   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. However, these lighter machines require rotating and nutating swash-plate assemblies that are difficult to mount and support. For high-pressure/high-speed service, the swash-plate assembly must be supported in a manner that allows for the relative motion between the heads of the non-rotating pistons and a mating flat surface of the rotating and nutating swash-plate. As just indicated above, such prior art swash-plates have often been split into a rotating/nutating rotor portion and a nutating-only wobbler portion, the latter including the flat surface that mates with the heads of the non-rotating pistons through connecting “dog bones”. 
   That is, 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 in full and flat contact against the flat surface of the swash-plate wobbler during all relative motions between the heads of the non-rotating pistons and a mating flat surface of the nutating swash-plate. As is well known in the art, these relative motions follow varying non-circular paths that occur at all inclinations of the swash-plate away from 0°. These dog-bones greatly increase the complexity and cost of building the rotating swash-plates of these lighter machines. 
   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 with 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. 
   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 effects 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 needs for adequate lubrication that increase with the size of the pressure and duty loads of the machine. Of course, such blow-by is accomplished by using fluid that would otherwise be used to drive or be driven by the pistons to accomplish work. Therefore, in the example just given above, the 4 gallons of fluid per minute used for blow-by lubrication, reduces the volumetric efficiency of the machine. 
   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 
   The invention is disclosed on various embodiments of hydraulic machines, all of which share a novel combination of simple structural features including elongated pistons reciprocating in a fixed cylinder block, cylinders provided with unique lubrication recesses, and shoes directly attached to each piston (without dog bones) that make sliding contact with a rotating and nutating swash-plate or, preferably, with the nutating-only wobbler portion of a split swash-plate. These simple structural features synergistically result in (a) a remarkable 90% increase in volumetric efficiency and (b) such increased mechanical efficiency that even the drive shafts of machines as large as 12-cubic inch capacity can be easily turned by hand when the machine is fully assembled. 
   Each disclosed machine can operate as either a pump or a motor. One embodiment 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 piston move at a maximum predetermined stroke at all times. The swash-plate of the other disclosed machine have inclinations 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. [However, persons skilled in the art will appreciate that the invention is equally applicable to hydraulic machines with rotating cylinder blocks and swash-plate that do not rotate with the drive element of the machines.] 
   In each machine according to the invention, each piston is elongated, having an axially 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, the preferable piston for this invention is “elongated”. That is, 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 at all times during machine operation. 
   [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. Further, for all preferred embodiments, it is assumed that each disclosed hydraulic machine (e.g., whether motor or pump) is paired with a similar hydraulic machine (e.g., a mating pump or motor) in a well known “closed loop” arrangement (see  FIG. 10 ) wherein the high-pressure fluid exiting from the outlet  139  of each pump  110  is directly delivered to the input  36  of the related motor  10 , while the low-pressure fluid exiting from the outlet  37  of each motor  10  is directly delivered to the input  136  of the related pump  110 . As understood in the art, a portion of the fluid in this closed loop system is continually lost to “blow-by” and is collected in a sump; and fluid is automatically delivered from the sump back into the closed loop, by a charge pump, to maintain a predetermined volume of fluid in the closed loop system at all times.] 
   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 almost completely closed by the axial cylindrical body of the piston during its entire stroke. [The movement of fluid in these lubricating channels is discussed in greater detail beginning two paragraphs below.] Preferably, each respective lubricating channel is formed circumferentially and radially transects each cylinder. 
   Also formed in the fixed cylinder block of each machine are 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. 
   Special attention is called to the fact that, 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 almost completely closed off by the cylindrical body portions of the pistons at all times during operation of the machine. Therefore, the only source of lubricating fluid supplying this continuous lubricating passageway is a secondary minimal flow of fluid between each of the respective cylindrical walls of each cylinder and the axial cylindrical body of each respective piston. During operation, this lubricating passageway almost instantly fills with an initial minimal flow 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 secondary minimal flow effectively maintains high pressure within the continuous lubricating passageway at all times. If necessary, a plurality of sealing members, each located respectively near the open end of each cylinder, can optionally provide 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. However, in actual practice it has been found that only a relatively minimal blow-by from the open end of the cylinders moves past the elongated pistons of the invention and, since a small amount of blow-by mist is required for adequate lubrication of the drive shaft bearings, etc., such optional sealing members may not be necessary. 
   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, the lubricating fluid that is driven toward low pressure is not “lost”, i.e., it is not “blow-by” and is in returned to the sump to be replenished into the closed loop hydraulic system by the charge pump. Instead, this low pressure lubricating fluid 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 fluid from the valve end of each cylinder experiencing increased pressure. 
   The just-described lubricating passageway provides appropriate lubrication for 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. 
   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. 
   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. All of the invention&#39;s support assemblies disclosed herein omit 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. Further, one embodiment also omits the nutating-only wobbler portion of a conventional rotating/nutating swash-plate. In all embodiments, a conventional shoe is mounted directly to the spherical head of each piston and is maintained in effective sliding contact with the flat face portion of the swash-plate 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. 
   Three simplified support mechanisms are disclosed: The first simplified support mechanism comprises a unique hold-down plate assembly biased by a single 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 within the body portion of each respective piston 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. 
   The third of the disclosed simplified support mechanisms is the preferred arrangement. Namely, it includes a traditional split swash-plate, but modified by adding needle bearings to support the nutating-only wobbler portion on the nutating/rotating rotor member. While this third embodiment also includes a unique hold-down plate assembly similar to the first embodiment, this latter hold-down plate is biased by a plurality of springs, each spring being positioned, respectively, circumferentially about the sliding shoe associated with the head of each piston. This third embodiment provides a dramatic change in the dynamics of operation of the sliding shoes, significantly reducing the surface speed of the relative motion between the shoes and the swash-plate and, thereby, resulting in a reduction in wear and costs, and in a significant increase in machine efficiency. 
   The important changes introduced by this invention provide hydraulic machines that are lighter and smaller than conventional machines having similar specifications. Further, as indicated above, actual testing of working prototypes have proven that this invention provides machines with significantly increased volumetric and mechanical efficiency. In short, the invention disclosed herein provides machines having remarkably greater efficiency while significantly reducing the weight and size of the machines as well as the cost of manufacture and simplifying assembly. 

   
     DRAWINGS 
       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 features of the invention incorporated in the cylinder block and at the piston/swash-plate interface. 
       FIG. 2  is a partially schematic and cross-sectional view of the fixed cylinder block of the hydraulic machine of  FIG. 1  taken along the plane  2 — 2  with parts being omitted for clarity. 
       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 features of the invention incorporated in the cylinder block and at the piston/swash-plate interface. 
       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 , 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 when the swash plate is inclined at +25°, the view in  FIG. 4A  being 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 . 
       FIGS. 5A and 5B ,  6 A and  6 B, and  7 A and  7 B are views of the same parts illustrated in  FIGS. 4A and 4B  when the swash-plate is inclined, respectively, at +15°, 0°, and −25°, the respective views in  FIGS. 5B ,  6 B, and  7 B being taken in the respective planes  5 B— 5 B,  6 B— 6 B, and  7 B— 7 B of  FIGS. 5A ,  6 A and  7 A. 
       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. 
       FIG. 9  is a partially schematic and cross-sectional view of another embodiment of the invention, showing a portion of another hydraulic machine with a fixed cylinder block substantially identical to that disclosed in  FIG. 3  but including an improved version of a conventional split swash-plate with a variable angle of inclination and having a nutating-only wobbler mounted on a rotating/nutating rotor, this view omitting the valve end of the cylinder block and portions of the housing as well as other parts for clarity. 
       FIG. 10  is a view of a prior art “closed loop” arrangement of two hydraulic machines. 
   

   DETAILED DESCRIPTION 
   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. As indicated above, it can be assumed that each disclosed machine is connected in a well known “closed loop” hydraulic system with an appropriately mated pump or motor. 
   Hydraulic Motor 
   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  reciprocate 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 . 
   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 rotor  28  is inclined at a predetermined maximum angle (e.g., 25°) to the axis  32  of drive shaft  30 . 
   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. 
   In a manner well known in the art: fluid inlet  36  and outlet  39  are preferably connected through appropriate “closed loop” piping to a matching hydraulic pump so that, at all times, fluid pressure biases spherical ends  18  and respective shoes  24  against flat surface  26 . The serial extension and retraction of each respective piston causes rotor  28  to rotate, thereby driving shaft  30 . 
   Also, as well known in the art, motor  10  is connected in a closed loop of circulating hydraulic fluid with a mating hydraulic pump (e.g., pump  110  shown in  FIG. 3  and discussed below); and 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. 
   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 and has been exaggerated for clarification. 
   During operation of hydraulic motor  10 , all interconnected lubricating channels  40  are filled almost instantly by a minimal flow 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 a continuous minimal flow of fluid between each of the respective cylindrical walls of each cylinder and the axial cylindrical body of each respective piston 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. 
   However, special attention of persons skilled in the art is called to the fact that this just-mentioned minimal flow of fluid 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 minimal flow of fluid 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 minimal flow of high-pressure fluid from the valve end of each cylinder experiencing increased pressure. 
   As mentioned above, there is minimal blow-by loss from closed continuous lubricating passageway  42  that interconnects all lubricating channels  40 . That is, there is still some 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 minimal flow of high pressure fluids entering around the opposite end of each piston  16 . 
   The just described lubrication arrangement is not only remarkably simple, and 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. 
   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: Three embodiments of the invention&#39;s simplified pinion/swash-plate interface assemblies are disclosed. While only the first of these hold-down assemblies is shown in combination with the motor and pump illustrated in  FIGS. 1 and 3 , each is described in greater detail in a separate section below.] 
   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. 
   Just described hydraulic motor  10 , with its remarkable simplification of both lubrication and the piston/swash-plate interface, is efficient, easy to manufacture, and economical to operate. 
   Variable Hydraulic Pump 
   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  reciprocate 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 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 in the center of cylinder block  112 . 
   In a manner similar to that explained above in regard to hydraulic motor  10 , variable pump  110  also is 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 regulate the delivery of fluid into and out cylinders  114 . 
   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; and 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 pump, e.g., pump  12  discussed above, causing the pistons of the mating pump 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. 
   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  140  formed circumferentially therein. A plurality of passageways  142  interconnect all lubricating channels  140  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 . 
   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  140 ,  142  in cylinder block  112  of hydraulic pump  110  shown in  FIG. 3 , including the fairly extreme minimization of loss of lubricating fluid from each lubricating channel  140  by optionally including a surrounding seal  144  located near the open end of each cylinder  114 . Similarly, the flow of lubricating fluid in closed continuous lubricating passageway  140 ,  142  is moderate but continuous as the result of a secondary minimal fluid flow 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). 
   However, once again special attention of persons skilled in the art is called to the fact that this just-mentioned secondary minimal fluid flow 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 fluid flow 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 blow-by that leaks from closed continuous lubricating passageway  140 ,  142  past the seals  144  at the end of each cylinder  114 , any such minimal blow-by is instantly replenished by a similar minimal fluid flow entering around the opposite end of each piston  116  experiencing increased pressure. 
   As discussed in the preamble above, the invention permits the machine&#39;s swash-plate apparatus to be simplified (a) by 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) in the embodiments illustrated in  FIGS. 1 and 3 , by 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. 
   Still referring to  FIG. 3 , rotor  128  of pump  110  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 ±25°. 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°. 
   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 . 
   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 . 
   Piston Shoe Hold-Down Assemblies 
   Fluid pressure constantly biases pistons  116  in the direction of rotor  128 , and the illustrated conventional thrust plate assembly 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, the variable hydraulic machines of this invention provide such additional bias by using one of three simple spring-biased hold-down assemblies, the first being similar to that already briefly described above in regard to hydraulic motor  10  in  FIG. 1 . 
   (a) Hold-Down Assembly with Single-Spring Bias 
   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. 
   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  are 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. 
   The positions of the just-described parts of the swash-plate and piston shoe hold-down assembly change relative to each other as the inclinations 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. 5A and 5B ; 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.] 
   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. 
   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 . 
   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 . 
   Therefore, to assure proper contact between each respective shoe  124  and flat surface  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 . 
   (b) Hold-Down Assembly with Multiple-Spring Piston Bias 
   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, each lubricating channel  40 ″ is interconnected with similar channels in the machine&#39;s other cylinders to form a continuous lubricating passageway in cylinder block  212 ; and, similarly, an optional surrounding seal  44 ″ may be located near the open end of each cylinder  214  to further minimize the loss of lubricating fluid from each lubricating channel  40 ″. 
   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 same. 
   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. Namely, 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 . 
   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 . 
   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 . 
   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. 
   (c) Hold-Down Assembly with Multiple-Spring Shoe Bias 
   Referring to  FIG. 9 , a preferred hold-down assembly is disclosed in a preferred hydraulic machine, namely, pump  310  that, while being substantially similar to pump  110  illustrated in  FIG. 3  and described in detail above, includes an improved conventional split swash-plate arrangement. 
   As with the other hydraulic machines described above, a plurality of pistons  316 , each including a respective sliding shoe  324 , reciprocate in respective cylinders  314  formed in cylinder block  312  that is identical to cylinder blocks  12  and  112  as described above. Each shoe  324  slides on the flat face  326  formed on a wobbler  327  that is mounted on a mating rotor  328  by appropriate needle bearings  372 ,  374  that permit wobbler  327  to nutate without rotation while rotor  328  both nutates and rotates in the manner well known in the art. 
   It will be apparent to those skilled in the art, that the inclination of wobbler  327  and rotor  328  about axis  329  is controlled by the position of a sliding collar  380 , a control link  382  and a balancing shadow link  394  in exactly the same manner as described above in regard to pump  110  illustrated in  FIG. 3 . 
   Shoes  324  are held down by a hold-down assembly substantially identical to the first hold-down assembly described in detail in sub-section (a) above. However, in this preferred embodiment, the large single coil spring  150  is replaced by a plurality of smaller individual coil springs as follows: 
   A hold-down plate  354  is fixed to wobbler  327  and is otherwise identical to hold-down element  154  described in detail above with reference to  FIGS. 4–7 . Similarly, each shoe  324  receives the circumferential extension of a respective special washer  356  that is identical to each special washer  156  as described in detail above, and the neck of each piston  316  is positioned within one of a corresponding plurality of respective openings  360  formed through hold-down plate  354 , all exactly similar to the apparatus of the first hold-down assembly described in detail in sub-section (a) above. 
   While wobbler  327  does not rotate with rotor  328 , the nutational movement of wobbler  327  is identical to the nutational movement of rotor  328  and, therefore, the relative motions between shoes  324  and the flat surface  326  of wobbler  327  are also identical to that described in detail in sub-section (a) above. 
   In this embodiment, a plurality of individual coil springs  350  provides the minimal spring bias that is necessary, in the absence of hydraulic pressure at the valve end of each cylinder  314 , to maintain effective sliding contact between each shoe  324  and flat face  326  of wobbler  327 . Each coil spring  350  is positioned circumferentially about each shoe  324 , being captured between each special washer  356  and a collar formed just above the bottom of each shoe  324 . 
   The preferred embodiment that has just been described provides the same remarkable improvement in volumetric efficiency with full lubrication as the other embodiments disclosed. Further, it also provides a dramatic change in the dynamics of the operation of the sliding shoes, greatly improving efficiency and significantly reducing wear and the concomitant costs related to such wear. 
   The invention&#39;s hydraulic machines all provide remarkably improved volumetric efficiencies with effective lubrication as well as piston/swash-plate interface assemblies that provide further economies by being relatively simple and inexpensive to manufacture and by reducing the number of parts required for efficient operation.