Abstract:
A hydrostatic drive device comprising a hydrostatic transmission mounted in a housing and a single output shaft driven by the hydrostatic transmission and extending out of the housing. The hydrostatic transmission has a center section mounted entirely within the housing and a rotatable hydraulic pump and a rotatable hydraulic motor mounted on the center section and connected through hydraulic porting in the center section. A pump shaft extends into the housing to drive the pump block, and the longitudinal axis of the output shaft is perpendicular to the longitudinal axis of the pump shaft.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 10/178,873 filed Jun. 24, 2002, now abandoned which is a continuation of U.S. application Ser. No. 09/774,754, filed Jan. 30, 2001, now U.S. Pat. No. 6,427,443, which is a continuation of U.S. application Ser. No. 09/170,915, filed Oct. 13, 1998, now U.S. Pat. No. 6,192,682, which is a continuation of application Ser. No. 08/700,933, filed Aug. 23, 1996, now U.S. Pat. No. 5,819,535, which is a continuation of 08/451,162 filed May 26, 1995 now U.S. Pat. No. 5,557,931, which is a continuation-in-part of U.S. application Ser. No. 08/394,144, filed Feb. 24, 1995, now U.S. Pat. No. 5,555,727. All of these prior applications are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   This invention relates to charge pumps and other auxiliary pumps used on hydrostatic transmissions (“HST”) for use in light duty applications. Such HSTs can either have their own housing and be attached to an axle driving apparatus, or can be incorporated within a housing that includes the components of the axle driving apparatus. A unit that contains an HST within the housing of an axle driving apparatus is often referred to as a integrated hydrostatic transaxle (“IHT”). The operation of such units are described in U.S. Pat. Nos. 5,201,692 and 5,314,387, the terms of which are incorporated by reference. 
   SUMMARY OF THE INVENTION 
   This invention presents a unique and novel manner of providing and mounting charge pumps and auxiliary pumps for both IHTs or stand-alone HSTs that are mounted to a separate transmission. As described in the &#39;692 patent, an HST generally comprises a pump that receives and is rotated by an input shaft driven by a vehicle engine. The pump includes a plurality of pistons that contact a swashplate to cause axial movement thereof when the pump rotates. The pump is hydraulically connected to a motor, which is similar in construction to the pump. The motor receives hydraulic fluid from the pump, and movement of the motor pistons against a swashplate causes rotation of the motor, which is connected to and drives an output shaft. The hydraulic connection between the pump and motor is a closed circuit; however, in any such circuit there will be deliberate and incidental leakage due to lubrication requirements, the high pressure of the hydraulic fluid and manufacturing tolerances. Thus, the HST requires a mechanism to replace fluid leaked from the closed circuit. This replacement fluid is commonly called make-up fluid. 
   In present HST designs, the pump and motor are often mounted on a center section that includes the hydraulic circuit therein. The hydraulic circuit includes two sides: a high pressure side and a low pressure side. The low pressure side is sometimes referred to as the vacuum side. These two sides are reversed when the vehicle motion is changed from forward to reverse. 
   Typically, the center section is mounted in a housing, and the housing provides a hydraulic fluid sump. Make-up fluid is brought from the sump into the low pressure side of the hydraulic circuit to replace fluid which is lost therefrom due to leakage. Specifically, check valves mounted directly into the center section or mounted in a separate plate that is in communication with the center section, as shown in the &#39;692 patent, provide a fluid flow path between the sump and the hydraulic circuit. However, this arrangement often does not provide sufficient fluid flow into the low pressure side of the circuit to replace the lost fluid. Therefore, a charge pump may be used to assist in this process. In addition, the use of an auxiliary pump to supply pressurized hydraulic fluid for various purposes is generally known. This invention provides for an efficient manner of mounting a charge pump and, in some cases, an auxiliary pump, external to the housing but still in direct communication with the hydraulic circuit in the center section. 
   The external charge pump confers significant benefits with respect to the accessibility on the hydraulic system design and configuration. The external nature of the charge pump allows direct access via a simple hydraulic fitting to hydraulic fluid that can be used for auxiliary functions. Internal charge pumps, in comparison, generally require a complex series of chambers, connections and fittings in order for fluid to be accessible exterior to the housing. External pumps provide accessibility without unit disassembly, thereby allowing replacement, addition or upgrade of a charge or auxiliary pump. Previous configurations were not accessible without disassembly of the HST or IHT. 
   Additional benefits and features of this invention will be disclosed in the description of the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional side view of a charge pump and IHT in accordance with the present invention; 
       FIG. 2  is a sectional end view of the IHT shown in  FIG. 1 ; 
       FIG. 3  is a partial sectional side view of the porting plate and charge pump of the IHT shown in  FIG. 1 , with a typical hydraulic fluid flow path shown; 
       FIG. 4  is a partial sectional bottom plan view of the IHT shown in  FIG. 1 , with the lower housing and charge cover partially cut away; 
       FIG. 5  is a sectional side view of the IHT shown in  FIG. 1 ; 
       FIG. 6  is a sectional side view of another embodiment of an IHT in accordance with the present invention, with the center section enclosed in the housing and porting plate mounted to the exterior of the housing; 
       FIG. 7  is a sectional side view of another embodiment of an IHT in accordance with the present invention, with the porting plate incorporated as a part of the lower housing; 
       FIG. 8  is an elevational end view of an HST incorporating a charge pump in accordance with the present invention; 
       FIG. 9  is a further elevational end view of the HST shown in  FIG. 8 ; 
       FIG. 10  is a sectional side view of the HST shown in  FIG. 9  along the line  10 — 10 ; 
       FIG. 11  is a partial elevational side view of the HST lower housing, porting plate and charge cover shown in  FIG. 8 ; 
       FIG. 12  is a sectional side view of another embodiment of an HST with a charge pump in accordance with the present invention incorporating the porting plate into the lower housing; 
       FIG. 13  is a partial, sectional side view of the lower housing of the embodiment shown in  FIG. 12 ; 
       FIG. 14  is a sectional side view of an IHT incorporating an auxiliary pump in accordance with another embodiment of the present invention; 
       FIG. 15  is a sectional end view of the IHT shown in  FIG. 14 ; 
       FIG. 16  is a sectional side view of the auxiliary pump used in connection with the IHT shown in  FIG. 14  with a typical fluid path shown; 
       FIG. 17  is a partial sectional bottom plan view of an IHT as shown in  FIG. 14 , with the lower housing, porting plate and auxiliary pump cover partially cut away; 
       FIG. 18  is a hydraulic schematic of the embodiment as shown in  FIG. 14 ; 
       FIG. 19  is a bottom plan view of an HST incorporating an auxiliary and charge pump of the present invention, with a partial sectional view of the auxiliary pump cover; 
       FIG. 20  is a sectional side view of an HST incorporating an auxiliary and charge pump embodiment of the present invention; 
       FIG. 21  is a hydraulic schematic of an embodiment that incorporates separate charge and auxiliary pumps; 
       FIG. 22  is a sectional side view of a further embodiment of an IHT incorporating separate charge and auxiliary pumps; 
       FIG. 23  is a sectional side view of a further embodiment of the invention incorporating a gallery forming manifold; 
       FIG. 24  is a top view of the manifold shown in  FIG. 23 ; 
       FIG. 25  is a bottom view of the manifold shown in  FIG. 23 ; 
       FIG. 26  is a sectional side view of a further embodiment of the invention incorporating a gallery forming manifold; and 
       FIG. 27  is a sectional side view of a further embodiment of the invention incorporating a gallery forming manifold. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   A charge pump embodiment of the invention incorporated in an IHT will first be described in accordance with the accompanying drawings.  FIGS. 1 through 5  illustrate an IHT configured with a horizontally split housing with upper housing  21  and lower housing  22 . The invention does not require any specific housing configuration, and all known configurations can be accommodated. All specifics of an IHT are not shown in these figures as the general operating of an IHT is known in the art. 
   Pump  24  is disposed on center section  26  and receives input shaft  25 , which communicates with and is driven by a vehicle engine (not shown). Center section  26  includes internal porting  30  that hydraulically connects pump  24  and motor  40 . Pump pistons  23  engage adjustable swashplate  27  to create pressure within a center section internal porting  30 . Housings  21  and  22  form a sump or reservoir  32  external to center section  26 . Motor  40  is connected to and drives output shaft  41 , which in turn drives gear  42   a , which is connected to gear  42   b  mounted on counter shaft  48 . Gears  42   a  and  42   b  do not rotate with respect to one another; the teeth of gear  42   b  engage with gear  42   a . As shown most clearly in  FIG. 4 , counter shaft  48  is mounted to lower housing  21  by means of screws  49  and is located between motor shaft  41  and axles  45   a  and  45   b . Gear  42   b  then drives gear  43  and the remaining gears of differential  44 . Differential  44  is in turn operatively connected to the output drive axles  45   a  and  45   b  of the vehicle. The specifics of the output gearing and differential are not essential to this invention and are disclosed in the &#39;387 patent. 
   The charge pump includes an inlet porting plate  28  external to the lower housing  22  at the external surface thereof, i.e., attached to the lower housing  22 . Porting plate  28  is mounted so that its interior surface is in close proximity to center section  26 . Gerotor housing  29 , which is also known as a charge cover, is attached to the external surface of inlet porting plate  28 , and gerotor set  34  is held in position by the configuration of input shaft  25 . This embodiment also includes mounting the porting plate  28  on other housing elements, or parts that may be labeled by other nomenclatures, that would perform functions similar to the described upper and lower housings. Porting plate  28  includes an inlet  36  that is connected to an external reservoir  50  that contains hydraulic fluid and which is typically mounted on a supporting member of the vehicle in which the IHT is mounted. Inlet  36  could also be in communication with and receive fluid from sump  32  instead of reservoir  50 . 
   Input shaft  25  extends through pump  24  and center section  26  to drive gerotor set  34 , which is of a standard design known in the art. Rotation of gerotor set  34  by shaft  25  creates a low pressure or suction at inlet  36  that remains constant regardless of the forward or reverse status of the HST. The porting plate  28  also includes fluid passage  31  in communication with gerotor set  34 . The action of the gerotor will cause fluid forced from the gerotor set into passage  31  to become pressurized. Thus, fluid is drawn through inlet  36  by the suction created by rotation of gerotor  34 ; the pressurized fluid is then forced into passage  31  and then into gallery  37 . Gallery  37  is formed and defined by the interface between the inlet porting plate  28  and the center section  26 . In the embodiment shown in  FIG. 1 , lower housing  22  includes an opening for the bottom surface of center section  26  to be approximately flush with the external bottom surface of lower housing  22 . The attachment location of center section  26  in upper housing  21  establishes the position of the center section  26 . Other embodiments with different split lines to the housing elements would have a similar opening. The positioning of the interface between the center section and the porting plate is not critical. In the embodiment shown in  FIG. 1 , the interface is flush with the external housing, but may move from that position in other embodiments. 
   In an alternative embodiment shown in  FIG. 6 , center section  26  could be mounted entirely within the IHT housing and inlet porting plate  28  would then be mounted on the external surface of lower housing  22 . The charge pump is then connected to internal porting  30  of center section  26  with a lower housing passage  96  and a gallery formed inside the lower housing by a gallery housing  97 . 
     FIG. 7  shows another embodiment of this invention, similar to that shown in  FIG. 6 , wherein the porting plate has been integrally formed with the lower housing  22 . Center section  26  is seated on internal surface  99  of lower housing  22 . Gallery  37  for the hydraulic fluid is formed and defined by the lower surface of center section  26  and the internal surfaces of the integrally formed porting plate, i.e., lower housing  22 . An o-ring  73  is used to aid in positioning and seating these elements and prevents leakage of hydraulic fluid from gallery  37 . The principal benefit of this embodiment is the reduction in complexity by the elimination of a separate porting plate. In the embodiment shown in  FIG. 1  o-ring  73  seals gallery  37  from the interface between porting plate  28  and lower housing  22  as well as from sump  32 . With the integration of porting plate and lower housing  22  into a single piece, the requirement to seal between these two is eliminated, which thus increases the reliability of the IHT. 
   In the embodiment shown in  FIGS. 1–5 , check valves  39   a  and  39   b  are operatively connected to internal porting  30  of center section  26 . As shown in this embodiment, check valves  39   a  and  39   b  may be included in plugs that are threaded directly into the surface of center section  26 . Alternatively, they may be located in a separate plate that is then secured to center section  26 . 
   The two check valves  39   a  and  39   b  each communicate with a different side of the internal hydraulic porting  30 . As previously stated, the hydraulic circuit, including internal hydraulic porting  30 , consists of a high pressure and a low pressure side, which are reversed when the vehicle direction, of motion is changed. Thus, while the HST is in the forward position, one side of the hydraulic circuit is under high pressure and the corresponding check valve is closed. At the same time the other side of the hydraulic circuit is under low pressure and that corresponding check valve is open and receives hydraulic fluid from gallery  37 . When the HST is moved to reverse, the two sides switch and the formerly low pressure side is under high pressure and the corresponding check valve is now closed and the high pressure side is now under low pressure, closing that corresponding check valve. 
   When the HST is in the neutral position, there is no demand for make-up fluid. However, since the rotational speed of input shaft  25  does not vary, the fluid pressure supplied to gallery  37  from gerotor set  34  will remain constant, requiring fluid to be diverted back to sump  32  through relief passage  38  formed into center section  26 . Relief valve  35 , which may be located at any point in passage  38 , controls the flow through passage  38 . While the relief passage was selected to be through the center section in this embodiment of the invention, alternative passages can be formed through the porting plate  28  or through the housing  22 ; the locations of the relief passage in other than center section  26  are considered obvious variations of the invention. Also, relief valve  35  can be a variety of devices in configurations that are known in the art, and would remain in communication with passage  38  regardless of the location of that passage. 
   A charge pump embodiment of the invention incorporated in an HST will now be described in accordance with the accompanying drawings. As discussed above, another embodiment of this invention uses the charge pump in a stand-alone HST that is mounted within a housing separate from that of the components that transfer power from the HST to one or more axle shafts. In such an embodiment, the specifics of the charge pump and related structures shown in  FIG. 3  can remain the same.  FIGS. 8–11  show such a stand alone HST unit incorporating a charge pump embodiment of the invention. The general operation of the HST is identical to that described above and elements that are structurally identical to those described above are given identical numerals. 
   As shown in  FIGS. 9 and 10 , HST  60  includes upper housing  61  and lower housing  62 . In these figures, the rotation of motor  40  caused by action of the motor pistons  45  against fixed swashplate  46  drives output shaft  41 . Output shaft  41  is supported by bearings  63 , and extends through the transmission mounting plate  68  that is integrally formed with upper housing  61  so as to engage a separate axle driving apparatus (not shown). 
   As shown is  FIG. 10 , inlet  36  receives fluid from the housing for the axle driving apparatus to which the HST  60  is mounted. Fluid may also be received from a reservoir  50  that would be mounted to the support structure of the vehicle in which the HST and axle driving apparatus were attached. As in the IHT configuration, inlet  36  could also be in communication with sump  32  formed by the upper housing  61  and lower housing  62 . 
   Another embodiment of the charge pump and lower housing is shown in  FIGS. 12 and 13 , wherein the inlet porting plate  28  is integrally formed with the lower housing  62 , similar to the embodiment shown in  FIG. 7 . The explanation provided above for the IHT version of this embodiment is applicable for the HST. The same benefits that accrue to the IHT from this embodiment also accrue to the HST. 
   An auxiliary pump embodiment of the invention incorporated in an IHT or HST will be described in accordance with the accompanying drawings. A further embodiment encompasses the use of a pump that provides hydraulic fluid to power attachments and implements, known in the art as an auxiliary pump, in addition to the charge pump with an IHT or with a stand-alone HST that is mounted with a separate axle driving apparatus. Auxiliary pumps for use with HSTs are generally known in the art and are used to create hydraulic fluid flow to power attachments and implements such as a hydraulic mower deck lift.  FIGS. 14 through 18  show an IHT  120  with a charge pump and auxiliary pump mounted thereon in accordance with this invention. The general operation of IHT  120  in this embodiment is substantially identical to that of the IHT depicted in  FIGS. 1–5 , and similar parts are indicated with the same numeral preceded by the numeral “1”. For the sake of brevity, such parts will not be discussed herein except as such specifically relates to this 20 embodiment of the invention. 
   As discussed above, the auxiliary pump and charge pump combination disclosed herein can be used with an IHT, as shown in  FIGS. 12–17 , and can also be used with an HST as shown in  FIGS. 19 and 20  that has its housing attached to a separate axle driving apparatus (not shown). The general operation of the auxiliary pump and charge pump in  FIGS. 19 and 20  is identical to that shown in  FIGS. 12–17  and the following discussion is applicable to both applications of this embodiment. The mounting of the HST housing through transmission mounting plate  168  is the same as was described for  FIG. 10  above. 
   As shown in  FIG. 14 , center section  126  is attached to upper housing  121  such that the lower surface is generally at the same level as the external surface of lower housing  122 . The center section  126  may be but need not be attached to the surface of lower housing  122  in this embodiment, but is positioned by the hole in the lower housing  122  through which the center section  126  extends. Inlet porting plate  128  is attached directly to lower housing  122 . Center section  126  includes two check valves  138  that, as per the preceding discussion, may be directly pressed into the bottom of the center section or which may be incorporated in a separate plate. Gerotor housing  129  is mounted to porting plate  128  and houses gerotor set  134 . Auxiliary pump cover  90  is then mounted to gerotor housing  129 . O-rings  91   a  and  91   b  are used to prevent leakage of hydraulic fluid from the gerotor housing  129 . 
   Rotation of gerotor set  134  created by rotation of input shaft  125  creates a suction at inlet  136 . The hydraulic fluid drawn into gerotor set  134  is then forced into auxiliary outlet passage  92  under pressure to an implement circuit. The implement circuit returns hydraulic fluid via the auxiliary inlet passage  93 , and then into gerotor housing  129 . The gerotor housing provides a path for the hydraulic fluid that is connected to fluid passage  131  in the porting plate, and thus to gallery  137 . The path that returns fluid from the implement circuit is not critical, and may be implemented in a variety of configurations that are well known in the art. The direction of fluid flow is generally shown by the arrows in  FIG. 16 . 
   As shown in  FIG. 17 , auxiliary pump cover  90  also includes a charge relief valve  94  and a check valve  95 . These valves can be implemented in a variety of configurations that are well known in the art, and are not unique to this invention. As the  FIG. 17  hydraulic schematic indicates, charge relief  94  acts to maintain hydraulic pressure in gallery  137 . Excess fluid not demanded by check valves  139   a  and  139   b  is diverted through charge relief valve  94  and returned to gerotor set  134 , where it is again routed through auxiliary outlet passage to the implement circuit  89 . If either the motor or pump requires more hydraulic fluid than is available through implement circuit  89 , then the suction caused in gallery  137  when either check valve  139   a  and  139   b  opens will cause check valve  95  to open to provide the additional hydraulic fluid required. 
   Another embodiment shown in  FIGS. 21–22  uses two completely separate gerotor sets  98  and  134  to serve as auxiliary and charge pumps, respectively. In this embodiment, the charge and auxiliary circuits are independent of each other, as shown in the hydraulic schematic in  FIG. 21 . An implementation of this embodiment is shown in  FIG. 22 , where input shaft  25  has been lengthened to extend through a modified charge cover  129 , into the auxiliary pump  98 . The operation of this embodiment follows the description above until return to the auxiliary cover  90 , where flow is directed back into the line that leads to reservoir  50  or sump  132 . The operation of the charge pump circuit follows the description provided in the charge pump section above. 
   Turning now to  FIGS. 23–26 , there is illustrated two further embodiments of the present invention. In particular, both of these embodiments include a charge pump, comprising a gerotor  34  and gerotor housing  29 , which is externally mounted to a lower housing cap  200 . The lower housing cap  200  is, in turn, mounted over an opening in a housing section  202 . As described previously, the gerotor  34  is drivingly engaged to the pump shaft  25  for creating a quantity of high pressure hydraulic fluid. The high pressure hydraulic fluid is stored in the gallery  37 , which is disposed in an area between the center section  26  and the charge pump, for use in the hydraulic pump and motor circuit as needed. 
   More specifically, the gallery  37  is defined by a manifold  204  which is cooperably positioned between the center section  26  and the housing cap  200  such that the upper surface of the manifold  204  matingly engages the center section  26 , opposite its pump running surface, while the lower surface of the manifold  204  engages the interior surface of the housing cap  200 . Preferably, the manifold  204  is constructed from a generally resilient and high pressure and heat resistant material, such as molded plastic or the like, to allow the manifold  204  to be compressionally captured between the center section  26  and the housing cap  200 . To provide further structural integrity to the manifold  204 , the manifold  204  may be provided with a series of strengthening ribs. The compressional capturing of the manifold  204  between the center section  26  and the housing cap  200  is desirable as it functions to minimize leakage from the gallery  37 . To further assist in preventing leakage of the high pressure fluid as it flows from the charge pump to the gallery, an optional O-ring  205  may be positioned between the manifold  204  and the housing cap  200  which O-ring  205  seals the fluid flow path therebetween. 
   Also preferably supported between the center section  26  and  10  the housing cap  200  is an oil filter  206  which surrounds the manifold  204 . While not required in the preferred embodiment the upper portion of the oil filter housing includes an extension  207  which is trapable between the manifold  204  and the center section  26  when the manifold  204  is secured hereagainst. This cooperation between the housing extension  207 , center section  26 , and manifold  204  functions to further secure the oil filter  206  in its desired position. A pair of optional O-rings  208 ,  210  are also preferably positioned between the oil filter housing and the center section  26  and housing cap  200 , respectively, to prevent the flow of hydraulic fluid therebetween. 
   In a preferred embodiment of the invention, the manifold  204  is constructed to have a first generally circular upper portion  204   a  and a second generally circular lower portion  204   b  of smaller diameter. It is to be understood that this configuration, of the manifold  204  is not meant to be limiting and that the manifold  204  may be provided with other geometric arrangements while maintaining its ability to form the gallery  37 . Nevertheless, the circular configuration provided to the upper portion  204   a  is desirable as it is more economical to machine the portion of the center section which preferably mates with the manifold  204  to prevent the side to side motion thereof as a circle. Additionally, the differing diameters of the upper portion  204   a  and the lower portion  204   b  is preferred as it creates a storage area for unpressurized, filtered fluid which area is in fluid flow communication with the charge pump. 
   During operation, filtered hydraulic fluid will be drawn into the charge pump through an inlet hydraulic passage  212  formed in the lower housing  200 . The resulting pressurized hydraulic fluid, created by the action of the charge pump, will be forced into the gallery  37  through an outlet hydraulic passage  214  formed in the lower housing  200  and an opening  216  formed in the manifold  204 . Check valves  39   a ,  39   b  mounted within the center section  26 , operatively connect the gallery  37  and the high pressure hydraulic fluid stored therein with the hydraulic porting formed within the center section  26 . 
   A charge relief valve  35  may optionally be provided to allow pressurized hydraulic fluid to be dumped from the gallery  37 . In particular, the charge relief valve  35  may be maintained in the manifold  204  ( FIGS. 23–25 ), in the center section  26  ( FIG. 26 ), in the housing cap  200 , or in the charge cover  29  (not shown). When the charge relief valve  35  is maintained in the manifold  204 , it is preferred that the valve body  218  be formed integrally with manifold  204  itself, thus obtaining a valve body  218  at no additional cost within the manifold  204 . This embodiment is particularly desirable since the elimination of the valve body also reduces the number or parts and simplifies the assembly process to the point where such assembly can occur at an original equipment manufacturer. Specifically, the component parts constituting the valve function are inserted into this body and retained therein by a retaining ring or like type of securing device. Similarly, when the charge relief valve  35  is maintained in the center section  26 , it is preferred that the center section  26  itself be used as the valve body. This embodiment retains the advantages above-described but at a slightly higher cost owing to the need to machine the center section  26  to achieve the features required to maintain the charge relief valve  35  therein. 
   Should the charge relief valve  35  not be utilized, it is preferred that a fixed diameter bleed orifice be formed through the manifold  204 . Specifically, the manifold  204  may be provided with an aperture of predetermined size which will allow the pressurized hydraulic fluid to be dumped from the gallery  37  at a rate dependent upon the viscosity thereof. Alternatively, while not preferred, the manifold  204  could be captured between the center section and the charge pump with an imperfect seal whereby the pressurized hydraulic fluid may escape from the gallery  37  therethrough. 
   While the embodiments of the invention illustrated in  FIGS. 23 and 26  have been shown with an externally mounted charge pump, it is also contemplated that the charge pump could be internally mounted within the housing without departing from the spirit of the invention. Accordingly, in a further embodiment, it is contemplated that the charge pump could be mounted to the internal surface of the housing cap with the manifold being cooperably disposed between the center section and charge pump cover. 
   It is to be understood that the above description should not be read as limiting the scope of this invention, as further features and benefits will be obvious to one skilled in the art. This invention should be read as limited by the claims only.