Abstract:
A valve assembly for use in regulating the flow of fluid between a first fluid side and a second fluid side of a hydraulic circuit. The assembly has an insert mounted in a hydraulic component having at least a portion of the hydraulic circuit formed therein, the insert also has a first end exposed to the first fluid side, a first end opening formed on the first end and a second end open to the second fluid side. The valve assembly additionally has a valve plug retained within the second end of the insert and a valve seat surface formed on the first end of the insert. The valve assembly also has a combination check valve and pressure relief valve located within the insert between the valve seat surface and the valve plug to selectively open and close the first opening.

Description:
CROSS-REFERENCE 
   This application is a continuation of U.S. patent application Ser. No. 10/704,893 filed on Nov. 10, 2003 now U.S. Pat. No. 6,761,182, which is a continuation of U.S. patent application Ser. Nos. 10/115,315 (now U.S. Pat. No. 6,719,005) and 10/115,149 (now U.S. Pat. No. 6,691,512), both filed on Apr. 3, 2002. These prior applications are incorporated herein in their entirety by reference. 

   BACKGROUND OF THE INVENTION 
   This invention relates generally to the design of valves and, more particularly, to the design of a combination check valve and pressure relief valve. 
   In the art, check valves and pressure relief valves are known. Generally, a check valve functions to restrict flow in one direction while a relief valve is used to regulate flow pressure. Furthermore, U.S. Pat. No. 4,948,092 discloses a combined check valve and pressure relief valve having a resilient duckbill valve body. Fluid passing through a cylindrical core around a valve actuator functions to open the lips of the duckbill valve body to permit the free flow of the fluid while back pressure functions to seal the lips of the duckbill valve. Manual depression of the valve actuator, however, causes the valve actuator to penetrate and open the valve lips to selectively permit backflow to provide the relief valve function. 
   A further combination check valve and pressure relief valve is shown in prior art  FIG. 13 . As illustrated, the combination check valve and pressure relief valve includes a check compression spring  78  and a relief compression spring  81 . The check compression spring  78  and relief compression spring  81  cooperate with a valve stem  82 , a machined valve seat  83 , and valve plug assembly  80  to provide the check valve and pressure relief functions. To establish the opening characteristics of the valve, a wet set procedure is utilized which involves setting the valve in a test stand and using fluid to activate the valve. The valve configuration is then adjusted, for example by turning a set screw, until the valve shows the desired amount of opening, based either on flow, or pressure, or both. While this wet set process works for its intended purpose, it does suffer the disadvantages of being time consuming and expensive. The use of a machined valve seat also increases the relative cost of the valve while further disadvantageously limiting the physical characteristics that can be provided to the valve seat. Still further, the large size of the compression spring  81 , disadvantageously requires the machining of holes in the valve seat to provide a means for relief fluid flow (since the size of spring  81  provides no fluid flow passages through spring  81 ). The size of spring  81  additionally increases the overall size of the valve assembly thereby preventing use of this valve in applications such as integrated hydrostatic transaxles. 
   SUMMARY OF THE INVENTION 
   To overcome these, and other disadvantages, a combination check valve and pressure relief valve is provided for use in regulating the flow of fluid between a first fluid side and a second fluid side. The combination check valve and pressure relief valve includes a valve plug, a valve guide moveably positioned in relation to the valve plug, a valve stem engaged to the valve guide, a check compression spring attached to the valve stem and positioned between the valve guide and the valve plug, a valve seat carried by the valve stem, and a relief compression spring positioned between the valve seat and the valve guide. The valve guide is adapted to move the valve stem relative to the valve seat against the force of the relief compression spring to allow fluid to pass from the first fluid side to the second fluid side through a fluid flow passage formed between the valve stem and the valve seat. Furthermore, the valve seat, valve stem, and valve guide are adapted to move relative to the valve plug against the force of the check compression spring to thereby remove the valve seat from an opening formed between the second fluid side and the first fluid side to permit fluid to pass through the opening. 
   Advantageously, a dry set procedure is used to set the valve configuration. Furthermore, a valve seat having complex geometries may be manufactured using metal injection molding. A better understanding of these and other advantages, objects, features, properties and relationships of the invention will be obtained from the following detailed description and accompanying drawings which set forth illustrative embodiments and which are indicative of the various ways in which the principles of the invention may be employed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, reference may be had to a preferred embodiment shown in the following drawings in which: 
       FIG. 1  illustrates a rear view of a tractor using an exemplary transmission constructed in accordance with the subject invention; 
       FIG. 2  illustrates a cross-sectional, front view of the transmission of  FIG. 1 ; 
       FIG. 3  illustrates a side view of the transmission of  FIG. 1  with the side housing removed to expose certain components; 
       FIG. 4  illustrates a cross-sectional back view of the transmission along line D—D of  FIG. 3  with the components missing from  FIG. 3  restored; 
       FIG. 5  illustrates a side view of a combination check valve and pressure relief valve and a valve plug useable in connection with the end cap of  FIG. 12 ; 
       FIG. 6  illustrates a cross-sectional view of the combination check valve and pressure relief valve along line B—B of  FIG. 5 ; 
       FIG. 7  illustrates a front view of the combination check valve and pressure relief valve of  FIG. 5  without a valve plug; 
       FIG. 8  illustrates a cross-sectional view of the combination check valve and pressure relief valve along line C—C of  FIG. 7 ; 
       FIG. 9  illustrates an exploded view of the combination check valve and pressure relief valve of  FIGS. 5–8  including a valve plug; 
       FIG. 10  is a flow chart diagram of exemplary steps used to configure the combination check valve and pressure relief valve of  FIGS. 5–8 ; 
       FIG. 11  illustrates a cross-sectional view of a center section for a single pump hydrostatic transmission in which the combination check valve and pressure relief valve of  FIGS. 5–8  is installed taken along line A—A of  FIG. 2 ; 
       FIG. 12  illustrates a cross-sectional view of a center section for a dual pump hydrostatic transmission in which the combination check valve and pressure relief valve of  FIGS. 5–8  is installed; and 
       FIG. 13  illustrates a prior art combination check valve and pressure relief valve. 
   

   DETAILED DESCRIPTION 
   Turning now to the figures, wherein like reference numerals refer to like elements, there is generally illustrated in  FIGS. 5–9  a combination check valve and pressure relief valve  10 . While the combination check valve and pressure relief valve  10  will be described in the context of a hydrostatic transmission, it is to be understood that this description is not intended to be limiting. Rather, from the description that follows, those of ordinary skill in the art will appreciated that the combination check valve and pressure relief valve  10  may be utilized in connection with a myriad of additional applications. 
   With reference to  FIGS. 1–4 , the combination check valve and pressure relief valve  10  is particularly suited for used in connection with a hydraulic circuit of a hydrostatic transmission  12 . In this regard, the hydrostatic transmission  12  generally operates on the principle of an input shaft  14  rotatably driving a hydraulic pump  16  which, through the action of its pump pistons  18 , pushes hydraulic fluid to a hydraulic motor  20  through a center section  22  to cause the rotation of the hydraulic motor  20 . The rotation of the hydraulic motor  20  causes the rotation of a motor shaft  24  which rotation is eventually transferred through a gearing system or the like  25  to drive one or a pair of axle shafts  26 . A motive force may be supplied directly to the input shaft  14  or indirectly by means of pulleys and belts which are connected to an internal combustion engine. For a more detailed description of the principles of operation of such a hydrostatic transmission, the reader is referred to U.S. Pat. Nos. 5,201,692, and 6,322,474 which are incorporated herein by reference in their entirety. 
   For placing the hydraulic pump  16  in fluid communication with the hydraulic motor  20 , the center section  22  includes hydraulic porting. The hydraulic porting is in further fluid communication with a source of makeup fluid, such as a fluid sump or a charge gallery. Generally, the hydraulic porting comprises a high pressure side through which fluid moves from the hydraulic pump  16  to the hydraulic motor  20  and a low pressure side through which fluid returns from the hydraulic motor  20  to the hydraulic pump  16 . Since fluid tends to leak from the hydraulic porting, the hydraulic pump  16  generally requires more fluid than is returned from the hydraulic motor  20  via the low pressure side porting. This requirement for fluid may, however, be satisfied by using the combination check valve and pressure relief valve  10 . Generally, the combination check valve and pressure relief valve  10  functions to prevent the flow of fluid from the hydraulic porting to the source of makeup fluid while allowing fluid to flow from the source of makeup fluid into the hydraulic porting when the fluid pressure in the hydraulic porting is lower relative to the fluid pressure in the source of makeup fluid. In cases where the fluid pressure in the porting is excessive, determined on a application by application basis, the combination check valve and pressure relief valve  10  further functions to relieve this excess fluid pressure by allowing fluid to be discharged from the hydraulic porting to the source of makeup fluid. 
   To this end, the combination check valve and pressure relief valve  10  may be mounted in the center section  22  between a first fluid side A and second fluid side B. In the application described herein, the first fluid side A is associated with the source of makeup fluid while the second fluid side B is associated with the porting that provides a fluid path between the hydraulic pump  16  and hydraulic motor  20 . By way of example,  FIG. 11  illustrates the combination check valve and pressure relief valve  10  installed in an exemplary center section  22  for a single pump hydrostatic transmission carried as part of an integrated hydrostatic transaxle.  FIG. 12  illustrates the combination check valve and pressure relief valve  10  installed in an exemplary center section for a dual pump hydrostatic transmission which may used in connection with a stand-alone hydrostatic transmission. It will be understood, however, that these illustrated embodiments are not intended to be limiting. Rather, the combination check valve and pressure relief valve  10  may be used in connection with any closed hydraulic circuit where there is a need for makeup fluid. 
   To secure the combination check valve and pressure relief valve  10  in the center section  22 , a valve plug  30  may be threaded to mate with corresponding threads provided in the center section  22  as illustrated in  FIG. 12 . As further illustrated in  FIG. 12 , an O-ring  32  may be provided to prevent the leakage of fluid from the junction between the combination check valve and pressure relief valve  10  and the center section  22 . Other manners for securing the combination check valve and pressure relief valve  10  in the center section  22  are also contemplated. For example, as illustrated in  FIG. 11 , the valve plug  30 ′ may be carried within a threaded insert  22 A that is to be considered a part of the center section  22 . In this case, a retaining ring  21  may also be utilized to maintain the valve plug  30 ′ within the threaded insert  22 A. 
   To allow for the flow of fluid from fluid side A to fluid side B when fluid side B is under lower pressure relative to fluid side A, the combination check valve and pressure relief valve  10  includes a check compression spring  34  as illustrated in  FIGS. 5–9 . In this regard, the check compression spring  34  is positioned between the valve plug  30  and a valve guide  36  which is carried within and moveable with respect to the valve plug  30 . The valve guide  36  is, in turn, attached to a valve stem  38  which cooperatively engages a valve seat  40 . The check compression spring  34  may be attached to the valve stem  38  by providing the compression spring  34  with a portion  35  having a diameter sized to mate with grooves or threads formed on a first end  38   a  of the valve stem  38 , as illustrated in  FIG. 8 . 
   More specifically, when the force on the valve stem  38  caused by the fluid pressure differential is sufficient to overcome the restoring force of the check compression spring  34 , the fluid pressure differential will influence the valve stem  38  and the attached valve guide  36  to compress the check compression spring  34  into the valve plug  30 . During this movement of the valve stem  38 , an enlarged portion  38   b  of the valve stem  38  engages a second surface  40   b  of the valve seat  40  and causes the valve seat  40  to move away from a valve seat surface  42  formed in the center section  22 . The valve seat surface  42  can be integrally formed with the center section  22  as illustrated in  FIG. 12  or may be a part of the threaded insert  22 A as illustrated in  FIG. 11 . In this manner, the movement of the valve seat  40  away from the valve seat surface  42  breaks a sealing engagement between the valve seat  40  and the valve seat surface  42  to allow the fluid under pressure in fluid side A to flow into fluid side B through an opening defined within the valve seat surface  42 . 
   Once the pressure differential and flow of fluid from fluid side A is no longer sufficient to overcome the restoring force of the compression check spring  34 , the compression check spring  34  return force urges the valve guide  36  and valve stem  38  back towards the valve seat surface  42 . This movement of the valve guide  36  and valve stem  38  functions to return the valve seat  40  into sealing engagement with the valve seat surface  42  to thereby prevent the flow of fluid through the opening defined by the valve seat surface  42 . In this regard, the valve seat  40  moves with the valve guide  36  and valve stem  38  owing to a relief compression spring  44  which is disposed around the valve stem  38  between the valve guide  36  and the valve seat  40  and which generally biases the valve seat  40  against the enlarged portion  38   b  of the valve stem  38  when the valve seat  40  is not engaged with the valve seat surface  42 . 
   To allow for the flow of fluid from fluid side B to fluid side A when the fluid in fluid side B is under excessively high pressure relative to the fluid in fluid side A, a flow passage  38   c  is formed in the valve stem  38 . While not intended to be limiting, the flow passage  38   c  is illustrated as being a reduced diameter portion formed in the valve stem  38  that cooperates with an opening in the valve seat  40 . In particular, when the force resulting from the fluid pressure differential is sufficient to overcome the restoring force of the relief compression spring  44 , the valve stem  38  is caused to move relative to the valve seat  40 , which is normally in sealing engagement with the valve seat surface  42 , such that the end of flow passage  38   c  extends beyond the valve seat  40  to thereby allow fluid to flow from fluid side B to fluid side A through the flow passage  38   c.    
   For allowing the valve stem  38  to move as a result of the excess pressure in fluid side B, a small gap (for example, 0.004 to 0.009 inches diametrically) is provided between the valve guide  36  and the valve plug  30 . This gap allows fluid to flow into the space B′ formed behind the valve guide  36 . In this manner, when the pressure within the space B′ behind the valve guide  36  builds to a certain point, which is established primarily by the hole diameter in the valve seat  40  and rate of the relief compression spring  44 , the fluid pressure differential causes the valve stem  38  and the attached valve plug  30  to move and compress the relief compression spring  44  in the manner described above. As further illustrated in  FIG. 11 , the valve plug  30 ′ may be provided with fins that provide fluid access around valve plug  30 ′ to the volume behind the valve guide  36  (as illustrated by the arrows in  FIG. 11 ) while also providing stability to the moving valve guide  36 . 
   As pressure continues to build in fluid side B relative to fluid side A, the valve guide  36  is compressed further, the valve stem  38  moves further relative to the valve seat  40 , and more of the fluid flow passage  38   c  is exposed to fluid side A. Thus, the opening of fluid side B to fluid side A via the fluid flow passage  38   c  is not abrupt, and pressure can continue to build in fluid side B. However, this pressure build up is at a steadily decreasing rate as compared to a hydraulic circuit in which no combination check valve and relief valve  10  is utilized. 
   Disadvantageously, the flow of fluid through the flow passage  38   c  can set up an oscillatory motion which, in some cases, can be detected as a vibration or pulse in certain applications such as hydrostatic transmissions. The small gap between the valve guide  36  and the valve plug  30 , however, functions to reduce or eliminate such oscillatory movement. In particular, this results from the time it takes for the fluid to move into and out of the space B′ behind the valve guide  36 . 
   When the fluid pressure differential is no longer sufficient to overcome the restoring force of the relief compression spring  44 , the relief compression spring  44  forces the valve guide  36  and attached valve stem  38  back towards the valve plug  30 . This movement of the valve stem  38  causes the fluid flow passage  38   c  to move back into the valve seat  40 . This movement of the fluid flow passage  38   c  back towards the valve seat  40  causes less of the fluid flow passage  38   c  to be exposed to fluid side A until such time as the valve stem  38  sealingly engages the valve seat  40  to close the fluid flow passage  38   c . It will be appreciated that the movement of the valve stem  38  under the influence of the relief compression spring  44  is dampened as the movement of the valve guide  36  towards the valve plug  30  causes fluid to be forced from the volume behind the valve guide  36  through the gap between the valve guide  36  and the valve plug  30 . 
   To reduce cost, the valve seat  40  of the combination check valve and pressure relief valve  10  is preferably manufactured using a metal injection molding (“MIM”) process. The metal injection molding process also allows the valve seat  40  to be provided with a bleed orifice  46 , a rib structure  48  (which provides fluid access passageways to the center opening in the valve seat  40  as well as engagement surfaces for the spring  44 ), and the opening configuration that cooperates with the fluid flow path  38   c . Importantly, the MIM process allows the small bleed orifice  46  to be provided with orifice diameters depending upon the application in which the combination check valve and pressure relief valve  10  is to be utilized by easily changing inserts used in the MIM process. It would be extremely difficult and costly to machine the valve seat  40  to achieve the features above described. 
   Cost of assembly of a hydrostatic transmission utilizing the described combination check valve and pressure relief valve  10  is also reduced. In this regard, since the check compression spring  34  is attached to the back end  38   a  of the valve stem  38 , the combination check valve and pressure relief valve  10  can be installed using a simplified process. To this end, the valve plug  30  as shown in  FIG. 12  need only be installed after the configured components of the check valve and pressure relief valve  10  are dropped into the center section  22 . Note that the O-ring would be positioned on the valve plug  30  at the time of assembly. In contrast, prior techniques for installing valves required springs, such as spring  78  in prior art  FIG. 13 , to be installed loosely into a device. Thus, the presently described manner of assembly also has the advantage of generally eliminating the risk that the parts are misassembled or that the spring  34  becomes loose within the hydraulic circuit. 
   To configure the combination check valve and pressure relief valve  10  for use in connection with a given application, a dry set procedure, as illustrated in  FIG. 10 , may be utilized. The dry set procedure is utilized to set the relief compression spring  44  to a certain spring compression within the assembled valve  10  to allow the valve  10  to operate with a particular pressure and to provide a specific amount of fluid flow through the fluid flow path  38   c  at a particular pressure. The dry set procedure is performed by selecting a compression spring  44  that has a spring rate that is believed will provide the desired fluid flow at a desired opening pressure considering the valve stem diameter. The spring compression force required to achieve the desired opening pressure using the actual spring rate chosen is then calculated in a manner well known to those of skill in the art. The valve guide  36  is then moved to a point where the compression spring  44  is compressed between the valve guide  36  and the valve seat  40  at the calculated compression force upon which the valve guide  36  is crimped to attach the valve guide  36  to the valve stem  38 . 
   The valve  10  may then be tested to ensure that the proper spring rate and the proper valve guide  36  set position were selected to achieve the desired flow rate. This testing may be performed by installing the valve  10  in a test stand to measure the pressure required to achieve the required flow rate. If the testing proves successful, production parts can be manufactured using the selected spring rate and valve guide set position. While this dry set procedure is not as accurate as using fluid to set the opening point of the valve, the dry set procedure does give adequate tolerances for hydrostatic transmission application (e.g., approximately +/−9% psi variation from valve to valve). It is contemplated that the dry-set procedure may be modified to improve these tolerances if needed for a given application. By way of example, the compression spring  44  can be set in a manner where the influence of frictional forces are minimized. 
   While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. For example, while the valve  10  is illustrated as being positioned in the forward side of an hydraulic circuit, it will be appreciate that such a valve  10  can also be positioned in the reverse side of an hydraulic circuit. Accordingly, the particular arrangement disclosed is meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any equivalents thereof.