Patent Publication Number: US-11022115-B2

Title: Controlled variable delivery external gear machine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present patent application is related to and claims the priority benefit of U.S. Provisional Patent Application Ser. No. 62/514,704, filed Jun. 2, 2017, the contents of which is hereby incorporated by reference in its entirety into the present disclosure. 
    
    
     STATEMENT REGARDING GOVERNMENT FUNDING 
     This invention was made with government support under 1543078 awarded by the National Science Foundation. The government has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     The present application relates to gear machines, and specifically to external gear machines used in fluid power management systems. 
     BACKGROUND 
     This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art. 
     External gear machines (EGMs) are used as primary flow supply units in many applications such as fuel injection systems, small mobile applications such as micro-excavators, turf and gardening machines. EGMs are also used in fixed applications such as hydraulic presses and forming machines. EGMs also find applications in auxiliary systems such as hydraulic power steering, fan drive systems and as charge pump in hydrostatic transmissions. 
     Referring to  FIG. 1 , an exploded perspective view of an EGM  100  of prior art is used as disclosed in WIPO publication WO2015131057. The EGM  100  includes a housing  112 , a drive gear  114 , which drives a driven gear  116 , both disposed inside the housing  112 . The drive gear  114  and the driven gear  116  are supported by bushings  118 A and  118 B inside the housing  112 . The drive gear  114  and the driven gear  116  are coupled together in a mesh zone (depicted in  FIG. 2 ) where a plurality of their respective teeth comes into contact with each other. End caps  126  and  128  enclose the housing  112  and are coupled to the housing by fasteners  119 , where the end cap  126  provides a journal support  127  for endshaft  115  of the drive gear  114 . The EGM  100  also includes an outlet  122  and an inlet  124 . The EGM also includes end caps  126  and  128 , 
     The EGM  100  also includes sliders  120 A and  120 B. These sliders  120 A and  120 B are coupled to the respective bushings  118 A and  118 B. A sealing member is fastened to the housing  120 . The positioning and coupling of the sliders  120 A and  120 B with respect to the bushings  118 A and  118 B is described below with reference to  FIG. 2 . 
     Referring to  FIG. 2 , a plane view of the drive gear  114  and the driven gear  116  in engagement with each other is provided. The drive gear  114  has a plurality of teeth, exemplified by  202 A and  202 B, while the driven gear  116  also has a plurality of teeth, exemplified by  204 A and  204 B. Tooth space volume  206  is identified as the space between any two consecutive teeth. Within this space, fluid is picked up and then trapped between any two consecutive teeth of the drive gear  114  and any two consecutive teeth of the driven gear  116  and the housing  112 . The engagement of the teeth creates a mesh zone  210  identified as the angular portion θ, and shown in  FIG. 3 . The tooth space volume  206  is a variable that is constant for most of its rotational path but begins to decrease and then increase within the mesh zone  210 . 
     The mesh zone shown in  FIG. 3  is divided into four portions. The first portion (identified as  1  in a circle) is the upper portion in  FIG. 2 , where the teeth just begin to engage each other. This portion is identified as the space between mesh-zone-start  214 A and upper-exterior-portion  216 A. As the teeth from both the drive gear  114  and the driven gear  116  come together in the first portion of the mesh zone ( 1 ), the space volumes  206  of the respective gears begin to interfere with each other and the overall tooth space volumes  206  decrease. As the tooth space volumes  206  decrease, fluid pressure increases, causing ejection of fluid through the outlet  122  at an output pressure. At this point fluid begins to be ejected from the EGM  100  via an outlet grove  222  (also referred to as the outlet fluid communication channel), identified in dashed lines for clarity, positioned below the mesh zone  210  as well as openings (not shown) to the outlet  122 . The bottom of the first portion is identified by the point “D” which signifies a point in the rotation where the teeth have trapped the fluid in the associated tooth space volumes  206  as a result of contact with each other. Beyond point “D” the only path for ejection of fluid is through the outlet groove  222  to the outlet  122 . In other words, point “D” corresponds to the switch point between i) fluid ejection via the outlet groove  222  and other openings (not shown) to ii) fluid ejection via the outlet groove  222  only by isolating tooth space volumes  206  with the outlet groove  222 . 
     The second portion (identified as  2  in a circle in  FIG. 3 ) is the upper-interior portion in  FIG. 2 . This portion is identified as the space between the upper-exterior-portion  216 A and the centerline  218 . As the tooth space volume decreases, fluid pressure increases. In this portion the teeth come in contact with each other and trap the fluid within the shrinking tooth space volume  206 . 
     Somewhere in this portion, the outlet groove ends, at which point fluid is no longer able to be ejected via the outlet groove  222 . At the center  212  of mesh zone  210  the tooth space volumes  206  are minimized. At any point beyond the center  212 , the tooth space volume  206  begins to increase. 
     The third portion (identified as  3  in a circle in  FIG. 3 ) is the lower-interior portion in  FIG. 2 . This portion is identified as the space between the centerline  218  and lower-exterior-portion  216 B. In this portion the teeth remain in contact with each other and continue to trap the fluid, however, now the tooth space volumes  206  begin to increase. Somewhere in this portion ( 3 ), an inlet groove  224  (also referred to as the inlet fluid communication channel), shown in dashed lines for clarity, ends; at which point fluid that is isolated to the inlet groove  224  can begin to be sucked in via the inlet groove  224  from the inlet  124 . The end of portion  3  is designated as “S” in  FIG. 3 , corresponding to a switch point between i) fluid suction via the inlet groove  224  only by isolating tooth space volumes  206  with the inlet groove  224  to ii) fluid suction via the inlet groove  224  and other openings (not shown) to the inlet  124 . 
     The fourth portion (identified as  4  in a circle in  FIG. 3 ) is the lower portion in  FIG. 2 , where the teeth just begin to separate from each other. This portion is identified as the space between lower-exterior-portion  216 B and mesh-zone-end  214 B. As the teeth from both the drive gear  114  and the driven gear  116  come apart from each other in the fourth portion of the mesh zone ( 4 ), the space volumes  206  of the respective gears continue to expand. As the tooth space volumes  206  increase, the fluid pressure decreases causing suction of fluid from the inlet  124  at an inlet pressure. At this point fluid continues to be sucked into the EGM  100  via the inlet grove  224  positioned below the mesh zone  210  as well as openings (not shown) to the inlet  124 . 
     The sliders  120 A and  120 B are positioned relative to each other so that placement of one determines the position of the other. The sliders  120 A and  120 B have a first end that sees pressure at the outlet  122 , and a second end that sees pressure at the inlet  124 . The cross-section of these two ends is about the same, namely A. 
     While, the sliders  120 A and  120 B provide the ability to selectively adjust displaced volume as seen in  FIG. 3 , the design of the sliders  120 A and  120 B and the fact that at least portions of the sliders  120 A and  120 B are under high pressure and therefore have large forces acting on them, it is impractical to dynamically adjust position of the sliders  120 A and  120 B of the prior art. In particular, the forces acting on one of these sliders is F=(P2−P1)·A, where P2 is the outlet pressure, P1 is the inlet pressure, and A is the cross-sectional area of the first end and the second end of the sliders  120 A and  120 B. Since both sliders  120 A and  120 B slide together, any sort of dynamic adjustment must be capable of overcoming 2*F. In certain applications these forces can reach hundreds of newtons. 
     There is, therefore, an unmet need for a novel approach to provide dynamic variable flow in gear machines. 
     SUMMARY 
     A controlled variable delivery external gear machine (VD-EGM) is disclosed. The VD-EGM includes a housing, an inlet, a drive gear, a driven gear, the drive gear configured to engage the driven gear in an angular mesh zone, an outlet, a first slider comprising a first longitudinal portion connected to a second longitudinal portion such that longitudinal forces applied to the first and second longitudinal portions substantially cancel each other thereby requiring between about 0 N to about 20 N to longitudinally moving the first slider, selective positioning of the first slider configured to vary net operational volumes of fluid communication between the inlet and the outlet, for a given rotational speed of the drive gear, and a first drive mechanism coupled to the first slider and configured to cause the first slider to slide in a longitudinal direction. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic of an external gear machine of the prior art depicting an exploded perspective view of various components including a drive gear and a driven gear each with a plurality of teeth. 
         FIG. 2  is a schematic view of the drive and driven gear of  FIG. 1  in coupling with each other depicting teeth in engagement with respect to each other. 
         FIG. 3  is a schematic graph of tooth space volume vs. angular position of the engaged teeth of  FIG. 2 . 
         FIG. 4  is a schematic of a controlled variable delivery external gear machine (VD-EGM) according to the present disclosure depicting an exploded perspective view of various components including a drive gear and a driven gear each with a plurality of teeth shown engaged therewith, a front cover, a back cover and casing having an inlet and an outlet, a first slider disposed in the front cover. 
         FIG. 5  is a schematic cross-sectional view of various components of the VD-EGM of the present disclosure depicting the first slider in a juxtaposed position with respect to the drive and driven gears of  FIG. 4 , according to the present disclosure. 
         FIG. 6  is a schematic perspective view of the first slider of  FIG. 4 , according to the present disclosure. 
         FIG. 7A  is a schematic collection of graphs of tooth space volume vs. angular position of the engaged teeth of  FIG. 4  showing a trapped volume of fluid as the drive and driven gears rotate, according to the present disclosure. 
         FIG. 7B  is a schematic collection of graphs of tooth space volume vs. angular position of the engaged teeth of  FIG. 4  showing changes in the tooth space volume as the position of the first slider changes, according to the present disclosure. 
         FIG. 7C  is a perspective schematic view of a front cover also shown in  FIG. 4 , according to the present disclosure, depicting insertion of the first slider into the front cover. 
         FIG. 7D  is a perspective schematic view of the front cover of  FIG. 7C , according to the present disclosure, depicting the first slider fully inserted into the front cover with a top plate, also shown in  FIG. 4  placed atop the front cover. 
         FIG. 8  is a perspective schematic view of the front cover, slider, and the top place of  FIG. 7D , according to the present disclosure, further depicting an actuator, also shown in  FIG. 4  placed atop the top plate. 
         FIG. 9  is a graph of flow (lpm) vs. pressure (bar) for various rotational speeds of the drive gear, with the slider kept at maximum displacement. 
         FIG. 10  is a graph of flow (lpm) vs. pressure (bar) for various rotational speeds of the drive gear, with the slider kept at minimum displacement. 
         FIG. 11  is a perspective view of a back cover and casing also shown in  FIG. 4 , depicting an inlet and outlet. 
         FIG. 12  is a schematic of another controlled variable delivery external gear machine (VD-EGM) according to the present disclosure depicting an exploded perspective view of various components including a front cover, a back cover, a casing having an inlet and an outlet, a first slider disposed in the front cover, and a second slider disposed in the back cover, the casing configured to receive a drive gear and a driven gear (not shown) each having a plurality of teeth (not shown), engaged therewith, the first slider and the second slider configured to balance pressure between lateral sides of the drive and driven gears. 
         FIGS. 13A and 13B  are front and perspective views, respectively, of a slider according to another embodiment, where the foot of the slider includes grooves. 
     
    
    
     DETAILED DESCRIPTION 
     For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of this disclosure is thereby intended. 
     In the present disclosure, the term “about” can allow for a degree of variability in a value or range, for example, within 10%, within 5%, or within 1% of a stated value or of a stated limit of a range. 
     In the present disclosure, the term “substantially” can allow for a degree of variability in a value or range, for example, within 90%, within 95%, or within 99% of a stated value or of a stated limit of a range. 
     Referring to  FIG. 4 , an exploded perspective view of a variable displacement external gear machine (VD-EGM)  400  according to one embodiment of the present disclosure is provided. The VD-EGM  10  includes a flange  401 , a back cover and casing  402  having an outlet  422  and an inlet  424 , a lateral plate  403 , a drive gear  404 A and a driven gear  404 B, a front cover  405 , a front cover top plate  406 , a slider  407 , an actuator  408 , and a plurality of fastening members  410 . It should be appreciated that the lateral plate  403  shown is optional for improved surface mating and is not required in all embodiments according to the present disclosure. While not intended to be a limiting factor of the VD-EGM  400  of the present disclosure, one difference between the VD-EGM  400  of the present disclosure and the EGM of the prior art for high pressure applications (e.g., with reference to  FIG. 1 ), is the VD-EGM  400  of the present disclosure does not require axial compensations; therefore, the lateral grooves controlled by the sliders  120 A and  120 B ( FIG. 1 ) can be realized directly on the front cover  405  of the VD-EGM  400 . The actuator  408  is mechanically coupled to the slider  407  and is configured to force the slider up and down in the respective cavities of front cover  405  and front cover top plate  406  as will be discussed in more detail below. The outlet  422  is configured to eject fluid at a selective variable flow rate and the inlet  424  is configured to receive fluid at a selective variable flow rate. The drive gear  404 A is coupled to and driven by a drive shaft  412  that passes through and is supported by collars  414  and  416  of the back cover and casing  402  and the flange  401 , respectively, and by a corresponding collar (not numbered) on the front cover  405 . The driven gear  404 B is similarly supported by collars (not shown) in the back cover and casing  402  and the flange  401 , respectively, and by a corresponding collar (not numbered) on the front cover  405 . Both the drive gear  404 A and the driven gear  404 B are received in a cavity in the back cover and casing  402  (see cavity  420  in  FIG. 11 , discussed below). The fastener members  410  pass through the front cover  405  and the back cover and casing  402  and thread into the flange  401  in order to bring these components in fluid operations together. 
     The design of the slider  407  represents an important aspect of the VD-EGM  400 . One of the goals realized by the design of the slider  407  is to minimize the longitudinal forces (i.e., vertical forces in  FIG. 4 ) acting on the slider resulting from the fluid pressure. This requirement is to permit a low force actuation of the VD-EGM  400 , so that the flow can be varied without significant energy consumption. This arrangement permits a low energy actuation by the actuator  408 . While a stepper motor is depicted in  FIG. 4  for the actuator  408 , it should be appreciated that other electromechanical and electrohydraulic approaches, including motors, cams, belts, and/or chains, known to a person having ordinary skill in the art, and electrohydraulic approaches known to a person having ordinary skill in the art can be used to effect the up and down motion of the slider  407 . 
     The slider  407  is now discussed in relationship with  FIGS. 5, 6, 7A, 7B, 7C, 7D, and 8 .  FIG. 5  is a schematic view of the slider  407  disposed in the front cover  405  and coupled to the drive and driven gears  404 A and  404 B. The slider  407  is an L-shaped member with three zones of interest. The bottom (right side of the foot of the slider  407 ) is a low-pressure zone  516  (marked in  FIG. 5  as “LP”). The central portion of the slider  407  (left side of the foot of the slider  407 ) is situated in high-pressure zones  514  and  512 , having the same high pressure side as the outlet  422  (see  FIG. 4 ). The top  510  of the slider  407  protruding out of the top plate  406  is mechanically coupled to the actuator  408 . A seal  508  (e.g., an O-ring) dynamically seals the slider  407  against the front cover  405  and the top plate  406 . The high-pressure zones  512  and  514  are designed to generate opposing forces (high-pressure zone  512  generates longitudinal force F1 which is pressure times the area of the high-pressure zone  512  while high-pressure zone  514  generates longitudinal force F2 which is pressure times the area of the high-pressure zone  514 , opposite F1). Depending on the application in which the VD-EGM  400  used, e.g., whether the low-pressure is at atmosphere or below or above atmospheric pressure, the low-pressure zone  516  generates longitudinal force F3 which is pressure times the area of the low-pressure zone  516 . The slider  407  is thus designed such that F1+F3−F2 is about zero. F3 can be ignored if the low-pressure is atmosphere. While no force is shown acting on the top  510 , a force can be used (either from atmospheric pressure, or an external force other than the actuator). If so, that force (e.g., F4) would be used in the algebraic relationship provided above between the other forces with the appropriate sign depending on the direction of the force. 
     Referring to  FIG. 6 , a perspective view of the slider  407  is provided. The slider  407  comprises two longitudinal portions  608  having a larger outer dimension and  610  having a smaller outer dimensions. While a cylindrical-shaped slider with a rectangular foot is discussed above and shown in the figures of the present disclosure, it should be appreciated that other shapes, e.g., elliptical and non-rectangular foot shapes, are also within the scope of the present disclosure. 
     The longitudinal portion  608  is sealingly coupled to the front cover  405  via the seal  508  (see  FIG. 5 ). The longitudinal portion  608  has an outer diameter  607  (d1 which is r1·2). The longitudinal portion  610  has an outer diameter  609  (d2 which is r2·2). Force F1 (see  FIG. 5 ) is defined by high-pressure acting on an area A1 defined in the embodiment shown by (d1 2 −d2 2 )·π/4. The longitudinal portion  610  terminates in a foot  606  defined by dimensions length  614  (L) and width  612  (W). Force F2 (see  FIG. 5 ) is defined by high-pressure acting on an area A2 defined in the embodiment shown by L·W. Force F3 (see  FIG. 5 ) is defined by low-pressure acting on an area A3 defined in the embodiment shown by L·W+d2 2 ·π/4. Therefore, from manufacturing considerations, the following approximation applies:
 
 W×L ≈π( R   2   −r   2 )  (1)
 
     The longitudinal force required to move the slider  407  downward is thus defined by:
 
 F   net   =P   2 ·( A 1− A 2)+ P   1 ·( A 3), wherein
 
F net  is the net longitudinal force needed to move the first slider  407  downward,
 
P 2  is the pressure at the outlet  422 ,
 
P 1  is the pressure at the inlet  427 . In the embodiment shown, Eq (1) can be re-written as
 
 F   net   =P   2 ·(( d 1 2   −d 2 2 )·π/4− L·W )+ P   1 ·( L·W+d 2 2 ·π/4), wherein
 
d 1  is the diameter of the longitudinal portion  608 ,
 
d 2  is the diameter of the longitudinal portion  610 ,
 
L is the length of the foot  606 , and
 
W is the width of the foot  606 .
 
     It should be appreciated that fluid disposed atop the foot  606  is in fluid communication with the outlet  422  (see  FIG. 4 ) and fluid disposed below the foot  606  is in fluid communication with the inlet  424 . Similar to the tooth space volume shown in  FIG. 2 , the location of the foot  606  with respect to the drive gear  404 A and  404 B (see  FIG. 4 ) determines the volumetric selection of fluid transfer from the inlet  424  to the outlet  422 . Referring to  FIG. 7A  a schematic overview effect of slider position on fluid flow is provided. As shown in the top panel, with the slider position centrally within a mesh zone  700  of the drive gear and the drive gear  404 A and driven gear  404 B, the tooth space volume has a minimum trapped volume M. As shown in the middle panel, “D” representing the beginning of the trapped volume is equidistantly shown on the tooth space volume graph from “M” as is “M” from the end of the trapped volume (“S”). In the position of the slider  407  shown in  FIG. 7A , maximum fluid flow is established from the inlet  424  to the outlet  422 . 
     Referring to  FIG. 7B , a schematic overview effect of slider movement on fluid flow is shown. As shown in the left panel (similar to  FIG. 7A ), when the foot  606  (shown in dashed lines) of the slider  407  (also shown in dashed lines) is centrally positioned with respect to the mesh zone  700 , the point “M” is centrally positioned between maximum allowed fluid input from the inlet  424  and fluid output out of the outlet  422 . However, when the slider  407  is moved downward, the maximum allowed fluid input from the inlet  424  is decreased thereby decreasing the volumetric fluid flow through the VD-EGM  400 . It should be noted that if the slider  407  is allowed to travel downward beyond a threshold, the inlet  424  will be connected to the outlet  422 , thereby rendering the VD-EGM  400  inoperative (i.e., no fluid flow). While not shown, if the slider  407  was to move upward from the position shown in the left panel of  FIG. 7B , the maximum allowed fluid output out of the outlet  422  is decreased thereby decreasing the volumetric fluid flow through the VD-EGM  400 . Similarly, it should be noted that if the slider  407  is allowed to travel upward beyond a threshold, the inlet  424  will be connected to the outlet  422 , thereby rendering the VD-EGM  400  inoperative (i.e., no fluid flow). 
     Referring to  FIG. 7C , a schematic representation of insertion the slider  407  into the front cover  405  is shown. In  FIG. 7C , a sliding chamber  720  and two receiving collars  730  for the drive shaft and a shaft on which the driven gear is mounted are shown. 
     Referring to  FIG. 7D , a partially assembled VD-EGM  400  is shown with the slider  407  in place through the top plate  406  and the front cover  405 . 
     Referring to  FIG. 8 , the actuator  408  is provided on top of the top plate  406  and coupled to the slider  407 . The seal  508 , provides a dynamic seal between the slider  407  and the front cover  405  and the top plate  406 . The actuator  408  is activated by cables  810 . 
     The actuator  408  (stepper, or other actuators as discussed below) control precisely the position of the slider, so that the flow of the VD-EGM  400  can be electronically set. The actuator utilizes negligible power (between about 0 and 0.1 W) when it is not actuated. This means that the electronic controller will consume energy only when the slider has to be moved to realize a different flow through the VD-EGM  400 . 
     Referring to  FIGS. 9 and 10 , flow vs pressure curves are provided based on measurements for several rotational speeds with the slider kept at maximum displacement and minimum displacement, respectively. With reference to  FIG. 9 , at maximum displacement, the resulting derived displacement is about V d,max =8.87 cm 3 /rev—the displacement is the y-intercept divided by the speed as provided in the legend, where the y-intercept gives flow rate at zero pressure which when divided by angular speed provides displacement. With reference to  FIG. 10 , similar experiments were performed with the slider kept at minimum displacement position, see the right panel of  FIG. 7B . The resulting displacement is about V d,min =6.31 cm 3 /rev. 
       FIG. 11  shows a schematic perspective view of the back cover and casing  402  showing the inlet  424  and the outlet  422  in relationship to each other and to the back cover and casing  402 . The cavity  420  is shown in the back cover and casing  402  that is configured to receive drive gear  404 A and the driven gear  404 B. 
       FIG. 12  depicts another embodiment of a variable displacement external gear machine (VD-EGM)  500  where two sliders are used, one identified as  407 A in the front cover  405 A, as shown in  FIG. 4 , and one identified as  407 B in a back cover  405 B. A casing  402 A is shown, having an outlet  422 A and an inlet (not shown). Also, while not shown, a drive gear and driven gear are configured to be received within a cavity  420 A disposed within the casing  402 A. Also, while not shown, either a separate electrical actuation, or as discussed earlier with respect to the actuator  408  other electromechanical or electrohydraulic actuators known to a person having ordinary skill in the art, can be utilized to actuate the second slider  407 B or the same electrical actuation used for the first slider  407 A. The purpose for use of two sliders  407 A and  407 B is to provide a pressure balancing between the inlet  424  and the outlet  422 . In other words, in high pressure applications, use of only one slider can generate lateral forces on the drive gear  404 A and the driven gear  404 B, resulting in pre-mature failure of internal components of the VD-EGM  400 . In particular, the two-slider implementation shown in VD-EGM  500  causes the pressure distribution on the two lateral surfaces of the gears to be uniform. This ensures there is no lateral moment resulting from lateral forces and the gears are laterally balanced, thereby maintaining a lateral lubricating gap (not shown) which is sufficient and thus allows the internal components to bear the resulting load. At high pressures this lateral gap needs to be controlled to minimize leakages and to prevent contact between the gears lateral surface and the front and back covers  405 A and  405 B, thus resulting in low wear and longer life. 
     As discussed above, while an electrical actuation in the form of a stepper motor is described, herein, it should be appreciated that other types of actuation are within the scope of the present disclosure. For example, alternate actuation technologies include electrical (e.g., solenoid), manual, mechanical, e.g. using a lever or a cam, pneumatic, hydraulic, as well as other actuation techniques known to a person having ordinary skill in the art. 
     Referring to  FIGS. 13A and 13B , front and perspective views, respectively, of a slider  507  according to another embodiment, of the present disclosure are presented. The slider  507  is similar to the slider  407  shown in  FIG. 6 , with one difference that the foot of the slider  507  includes grooves. In other aspects, not shown, the foot of the slider can have an elliptical cross section instead of a rectangular (as shown in  FIG. 6 ) or a pseudo-rectangular as shown in  FIGS. 13A and 13B . In yet other aspects, not shown, the foot of the slider can have a hybrid cross-section. The important aspect of the foot design is that when the foot of the slider  407  or  507  is coupled to a lateral side of the drive gear  404 A and the driven gear  404 B—or when the foot of the first slider  407 A is coupled to a first lateral side of the drive gear  404 A and a first lateral side of the driven gear  404 B and the foot of second slider  407 B is coupled to a second lateral side of the drive gear  404 A and a second lateral side of the driven gear  404 B—that a high-pressure zone coupled to the outlet  422  and a low-pressure zone coupled to the inlet  424  be generated about the first and second longitudinal portions of the respective slider(s), thereby generating the counterbalancing forces about these longitudinal portions requiring only a longitudinal force of between about 0 N and about 20 N to longitudinally move the respective slider. 
     In the present disclosure a combination of the front cover, the rear cover, and the back cover and casing are used synonymously as a housing. 
     While the variable delivery external gear machine (VD-EGM) of the present disclosure is described generally as a pump, it should be appreciated the VD-EGM of the present disclosure can be selectively operated as a pump or a motor. 
     Those having ordinary skill in the art will recognize that numerous modifications can be made to the specific implementations described above. The implementations should not be limited to the particular limitations described. Other implementations may be possible.