Abstract:
Methods of controlling fluid pressure, including using a valve mechanism, are disclosed. In one embodiment, a valve mechanism for use in a hydrostatic transmission including a closed porting system for hydraulic fluid and a sump is provided. The valve mechanism includes a valve body mounted to the hydrostatic transmission whereby the valve body is open to the closed porting system at one end thereof and open to the sump at the other end thereof. The valve body has a first open position whereby hydraulic fluid is pulled into the closed system from the sump when the pressure of the fluid in the closed system is below a first pressure, a second open position whereby hydraulic fluid exists the closed system to the sump when the pressure of the fluid is at a second pressure higher than the first pressure, and a closed position when the pressure of the hydraulic fluid in the closed system is at a third pressure higher than the second pressure.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. App. Ser. No. 09/225,368, filed Jan. 4, 1999 issued as U.S. Pat. No. 6,073,444; which is a continuation of U.S. App. Ser. No. 08/658,364, filed Jun. 5, 1996, issued as U.S. Pat. No. 5,855,116, which is a continuation of U.S. App. Ser. No. 08/392,484, filed Feb. 23, 1995, issued as U.S. Pat. No. 5,546,752, and claims benefit of an earlier filing date under 35 U.S.C. 120. 
    
    
     BACKGROUND OF THE INVENTION 
     The disclosure of U.S. Pat. App. Ser. No. 09/225,368 is incorporated herein by reference. 
     This invention relates generally to hydrostatic transmissions (“HST”) commonly used with riding lawn mowers and similar small tractors. Such tractors generally use an engine having a vertical output shaft which is connected to a transaxle via a conventional belt and pulley system. Other designs use horizontal output shafts or direct shaft drive to the transaxle. The HST may be connected to an axle driving apparatus or it may be integrally formed therewith in an integrated hydrostatic transaxle (“IHT”). The general structure and benefits of HSTs and IHTs are discussed in U.S. Pat. No. 5,201,692, to Johnson and Hauser issued Apr. 13, 1993, the text of which is herein incorporated by reference. 
     A standard HST for a transaxle includes a hydraulic pump which is driven by the engine output shaft, and a hydraulic motor, both of which are preferably mounted on a center section containing porting to hydraulically connect the pump and motor. Rotation of the pump by an input shaft creates an axial motion of the pump pistons through use of the swash plate. The oil pressure created by this axial motion is channelled via porting to the hydraulic motor, where it is received by the motor pistons, and the axial motion of these pistons against a thrust bearing causes the motor to rotate. The hydraulic motor in turn has an output shaft which drives the vehicle axles through differential gearing. 
     As described, the hydraulic system has two pressure zones, the high pressure side which includes that portion of the circuit handling the movement of the fluid from the pump to the motor, and the low pressure side which includes the remainder of the circuit wherein fluid from the motor is returned to the pump. When the tractor is in reverse, the high and low pressure sides of the system are switched. It is generally understood in such designs that the pump requires more oil than is returned from the motor due to leakage from the hydraulic system into the sump. This requirement of oil is satisfied by using check valves on each side of the hydraulic system. The check valve consists of a means for preventing flow out of the system when under high pressure and a means for allowing flow into the system when under low pressure. Such check valves can be inserted directly into the center section or can be mounted in a separate check valve plate which is secured to the center section. 
     Furthermore, in the prior art, it is known to separately provide a mechanism for the relief of excess oil pressure (such as when neutral is desired) from the pressure side of the system. A first method of accomplishing this is by providing bleed orifices in the system from which oil will leak. However, these bleed orifices do not have the ability to close and it is seen that efficiency is lost as a result. A second method of accomplishing this is to provide a spring biased neutral valve that allows oil to pass, at a substantially constant rate, from the pressure side until a set pressure is reached, which overcomes the bias of the spring, whereby the valve will thereafter close. 
     While these valves work well for their intended purpose, it is seen that, among other things, these valve suffer the disadvantages of not providing smooth transition between closed and open positions and of having a rapid rate of closure whereby the neutral band is narrowed. Therefore, a need exists for an improved neutral valve. 
     As a result of this existing need, it is an object of the present invention to provide a combination neutral and check valve assembly. It is a further object to provide a neutral valve which has an increased neutral band. It is yet another object of the present invention to provide a neutral valve which incorporates a smooth transition between open and closed positions. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a valve mechanism for use in a hydrostatic transmission including a closed porting system for hydraulic fluid and a sump is provided. The valve mechanism includes a valve body mounted to the hydrostatic transmission whereby the valve body is open to the closed porting system at one end thereof and open to the sump at the other end thereof. The valve body has a first open position whereby hydraulic fluid is pulled into the closed system from the sump when the pressure of the fluid in the closed system is below a first pressure, a second open position whereby hydraulic fluid exists the closed system to the sump when the pressure of the fluid is at a second pressure higher than the first pressure, and a closed position when the pressure of the hydraulic fluid in the closed system is at a third pressure higher than the second pressure. 
     A better understanding of the objects, advantages, features, properties and relationships of the invention will be obtained from the following detailed description and accompanying drawings which set forth an illustrative embodiment and is indicative of the various ways in which the principles of the invention may be employed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the attached drawings, described briefly below, generally only enough of the invention is illustrated to enable one of skill in the art to practice the invention without undue experimentation. 
     FIG. 1 is a cross-sectional view of a valve cartridge manufactured in accordance with this invention; 
     FIG. 2 is a partial cross-sectional view of an HST center section using a valve pursuant to a second embodiment of this invention, with the valve in the fully closed position; 
     FIG. 3 is a partial cross-sectional side view of the hydrostatic transmission and valve shown in FIG. 2, with the check valve in the fully closed position and the neutral valve in the fully closed position; 
     FIG. 4 is a partial cross-sectional view of the HST and valve shown in FIG. 2, with the check valve portion fully closed and the neutral valve portion partially open; 
     FIG. 5 is a partial cross-sectional view of the HST and valve shown in FIG. 2, with the check valve portion closed and the neutral valve portion in the fully open position; 
     FIG. 6 is a partial cross-sectional view of the HST and valve shown in FIG. 2, with the check valve in the fully open position and the neutral valve in the fully open position; 
     FIG. 7 is a prior art check valve using a popper and spring with the check valve in the closed position; 
     FIG. 8 is a prior art check valve as in FIG. 7 with the check valve in the open position; 
     FIG. 9 is a partial cross-sectional view of a hydrostatic transmission incorporating a combination valve in accordance with the present invention; 
     FIG. 10 is a partial cross-sectional view of a HST incorporating a separate check valve and neutral valve in accordance with the present invention; 
     FIG. 11 is a partial cross-sectional view of a HST similar to that shown in FIG. 10 in which a different form of check valve is utilized; and 
     FIG. 12 is a partial cross-sectional view of the neutral valve portion of the combination valve which is illustrative of the neutral valve used in conjunction with the embodiments shown in FIGS. 10 and 11. 
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     FIG. 1 shows a cross-sectional view of the valve  10  in accordance with a first embodiment of the present invention. Valve  10  comprises head  12  at one end thereof, which may be formed in a hexagonal shape as a nut for securing valve  10  to the HST center section, and valve body  14 , which is generally cylindrical in shape. Valve body  14  is partially hollow and is open at a second end thereof 
     Retainer  16  is generally cylindrical and is shaped to fit into the opening at the second end of valve body  14 . In the preferred embodiment retainer  16  may be composed of plastic. Retainer  16  has a closed end with opening  18  formed therein and an open end. Retainer  16  may be held in place by the internal portion of valve body  14  by means of friction. It is to be understood that the retainer  16  may also be held in place by the center section  50 . Flange  21  is formed on retainer  16  to rest against the second generally open end of valve body  14  to secure retainer  16  in place. 
     Opening  23  is formed at the first generally closed end of valve body  14  to allow oil flow to and from the internal position thereof A seat  24  is formed on the internal portion of valve body  14 . 
     Check spool  32  and neutral spool  38  are formed to fit within retainer  16  and valve body  14 . Check spool  32  is generally cylindrical and has an internal chamber  33  shown with at least two areas of different diameters, namely chambers  33   a ,  33   b . It is understood that this design could use any number of sub-chambers of different internal diameters in internal chamber  33 . The body of check spool  32  is generally closed at a first end  36 , and includes opening  34  formed on the end  36  and communicating with internal channel  35  to allow oil flow between internal chamber  33  of check spool  32  and opening  23  of valve body  14 . Closed end  36  of check spool  32  is shaped to fit against seat  24 , although check spool  32  is movable within the internal chamber of retainer  16 . The check spool  32  also includes a needle valve projection  37  having a generally arcuate surface  37   a  disposed into the passage  33  in the vicinity of channel  35 . Generally, the needle valve projection  37  has a conical like shape having a smaller diameter near the top thereof than at the bottom thereof. 
     Neutral spool  38  has a generally cylindrical head  38   a  which has an external diameter sized such that head  38   a  slidably fits within internal chamber  33   b . Neutral spool  38  also has a cylindrical arm  38   b  integrally formed with and extending from head  38   a . Passage  39  is bored or otherwise formed in neutral spool  38  to allow the passage of oil therethrough. In a preferred embodiment, neutral spool  38  is composed of screw machined steel while check spool  33  may be manufactured using injection molding. Arm  38   b  has an outer diameter sized to slidably engage with the internal chamber  33   a  of check spool  32 . 
     As shown in FIG. 1, neutral spool  38  and check spool  32  are in slidable engagement with one another. Spring  40  is mounted around arm  38   b  and contacts head  38   a  and spring seat  41  formed on check spool  32  to control the movement of neutral spool  38  into and out of check spool  32 . Needle valve projection  37  is formed on the internal portion of check spool  32  to communicate with passage  39 . 
     Valve body  14  as shown in the embodiment of FIG. 1 has threads  22  formed thereon. However, it is not required to use threads  22  to secure valve  10  in the HST center section in such an embodiment. Another possible method would be to press-fit the entire valve into the center section. Thus, the drain passages in valve  10  would be sealed from the hydraulic circuit by the interference fit between valve  10  and center section. In this embodiment the valve  10  could be formed out of powdered metal. 
     Valve  10  has several positions, including fully open wherein oil flow between the HST&#39;s center section and sump is substantially unobstructed, and fully closed, wherein there is no oil flow absent normal leakage through the structure. These various positions are shown in FIGS. 2-6, which show a second embodiment of this invention. The general relationship and operation of neutral spool  38  and check spool  32  are the same in either embodiment and identical elements have been given identical reference numerals in the figures. 
     FIG. 2 shows a second embodiment of valve  10  mounted in a center section. Rather than having a separate valve body as in FIG. 1, the embodiment in FIGS. 2-6 has a check valve plate  52  which is secured to a surface of center section  50  by a cap screw  54  or similar means. Ring  56 , which may be a crush ring or sealing ring, functions to create a seal between this element and, if desired, a filter  58  maybe secured to center section  50  and/or check plate  52  to filter the hydraulic fluid before it enters center section  50 . Retainer  16  is shaped differently and incorporates head  16   a  to secure it to the center section  50 . This second embodiment is preferred due to lower manufacturing costs involved. 
     The following description of hydraulic fluid flow is generally given with respect to the second embodiment of this invention. It is understood that it applies as well to other embodiments shown and disclosed herein. When the hydraulic transmission is near the true neutral position, the small oil flow resulting therefrom flows from the center section to the sump through the neutral valve. As the transmission is moved out of neutral, this flow out of the center section is slowly reduced to zero as the neutral valve is closed. A key benefit of the present invention is that it allows for this flow reduction to be smooth and controlled, regardless of the speed at which the transmission is shifted out of neutral. This controlled cutoff is preferrably accomplished through the use of spring  40  and/or dampener spaces built into the design. 
     FIGS. 1 and 5 shows valve  10  in the fully open neutral position where hydraulic fluid can flow out of center section  50  to the sump as shown by the arrows in FIG.  5 . This fluid flows first through opening  18  in retainer  16  and it is ultimately discharged to the sump through opening  23  in valve body  14  or opening  63  in check plate  52 . One of the paths the oil can take is through passage  39 , through channel  35  and out opening  34  in check spool  32 . As the oil pressure in the hydraulic circuit adjacent the valve increases, the oil pressure increases on neutral spool  38 , and it is forced further into the internal section of check spool  32 , compressing spring  40  as shown in FIG.  4  and acting against the fluid trapped in chamber  33   b . Ultimately, when the oil pressure reaches a set level, as shown in FIG. 3, the distal end of arm  38   b  will obstruct passage  35  thus cutting off flow through passage  39 . The pressure at which these changes occur can be varied by changing the tolerances of the various parts as well as the constant of spring  40  and/or the dimensions of chamber  33   b.    
     As can be seen in FIG. 4, as the neutral spool moves towards the passage  35 , oil flow through passage  39  will slowly diminish due to the interaction of the arcuate surface  37   a  of the needle valve projection  37  and the side walls of the internal passage  39  whereby the opening leading to the passage  35  will be caused to slowly decrease in size. In addition, it is seen that oil is permitted to flow between head  38   a  and internal chamber  33   b  of check spool  32  in which spring  40  is mounted. Oil accumulates in this chamber  33   b  and is forced out between arm  38   b  and internal chamber  33   a  of check spool  32  when the neutral spool moves to the closed position. Specifically, the rate of oil flow from the internal chamber will move from 0, before the neutral spool  38  moves, to a generally constant rate of dispersement which rate of dispersement is known to depend upon the viscosity of the fluid and the size of the opening between the check spool  32  and the neutral spool  38 . Therefore, owing to the fluid trapped within the chamber  33   b , a pressure is built therein which pressure acts against the pressure of the fluid acting upon the head  38   a  such that the rate of closure is slowed with the result being a smooth rate of closure. While the spring  40  may or may not be used to further control the rate of closure of the neutral spool  38 , the spring  40  does function to bias the spool  38  towards the open position when pressure is removed from the head  38   a . It will also be appreciated that, since the rate of flow into the chamber  33   b  is also substantially constant, the movement of the spool  38  to the open position will be controlled by the rate of flow of fluid into the chamber  33   b . Specifically, the reverse pressure caused by the suction of fluid into the chamber  33   b  will act against the bias of the spring  40  whereby movement of the neutral spool  38  is controlled and the opening of the valve smoothed. As illustrated, the fluid in the chamber  33   b  acts to dampen the movement of the neutral spool  38  such that the valve will move at a rate slower than the rate of the pressure acting thereupon. 
     The structure of the hydrostatic transmission, and in particular the flow path of the hydraulic fluid is shown in more detail in FIG.  9 . The general operation of hydrostatic transmissions is known and is described in the above-referenced U.S. Pat. No. 5,201,692, and will not be described in detail here. In general, it is known that the input shaft causes the rotation of the pump, and the movement of a plurality of pump pistons against a swash plate causes hydraulic fluid to flow through hydraulic passages to the motor and a plurality of motor pistons which abut another swash plate. As shown in FIG. 9, the hydraulic fluid may also be diverted to flow through valve  10 , to exit from the system to the case as described herein. 
     A hydraulic circuit is located within center section  50  and incorporates elements in addition to those shown in FIG. 9, including the internal portions of pump  57  and a motor (not shown) and pump pistons  59  and motor pistons (not shown) and the porting in center section  50  between the pump and motor. 
     A benefit of this invention is in the combination of check valve functions and neutral valve functions in a single valve. As shown in FIG. 6, when the hydraulic circuit is under “vacuum,” or a very low pressure with respect to that of the pressure side of the circuit, the check spool  32  is lifted off of seat  24 , and the fluid flows essentially in reverse of what has been previously described with respect to the neutral spool assembly and hydraulic fluid is pulled from the transmission housing or a sump into the hydraulic circuit through valve  10 . Fluid may be drawn through filter element  58  before being drawn through check plate opening  63 , opening  34  in check spool  32  to the internal passages  35 ,  33   a  and  39 , and also between said retainer  16  and the center section  50 . Then when the pressure in that portion of the hydraulic circuit adjacent to the valve reaches a certain pressure, check spool  32  is reseated on seat  24 , and the valve is prepared to function as a neutral valve as described above. Specifically, in the embodiment illustrated, when the pressure is equal on both sides of the check spool  32 , owing to gravity acting upon the check spool, the check spool with reseat. In an alternate embodiment, not shown, the valve could be positioned such that gravity will maintain the valve unseated when the pressure upon both sides of the check spool  32  are equal. 
     From the previous descriptions it is seen that an initial pressure, or pressure equalization, will first cause the check spool  32  to seat against seat  24 . 
     Thereafter, an increase in pressure will start to close the neutral spool against the pressure of the fluid trapped within chamber  33   b  and/or the bias of spring  40 . A decrease in pressure will either unseat the check spool  32  at which time the bias of the spring  40  will open the neutral spool  38  or only be sufficient enough to allow the spring  40  to open the neutral spool  38  while the check spool  32  remains seated. 
     A prior art version of a check valve is shown in FIGS. 7 and 8 with FIG. 7 showing the check valve in the fully closed position and FIG. 8 showing the check valve in the fully open position. The reference numerals for similar elements are the same as those used in other figures. As can be seen, valve  82  can be mounted in center section  50  and secured therein by check plate  52 . Popper  81  is mounted inside valve  82  and the flow of oil through the body of valve  82  is controlled by spring  83 . The oil flow in such a design will be essentially fully open or fully closed, and this design does not provide for any neutral valve function. 
     It will also be appreciated by those skilled in the art that improved neutral valve described herein may be used in conjunction with the prior art check valves described above. Illustrated in FIGS. 10-12 is a neutral valve assembly  10 ′ which does not include the freely movable check valve spool  32  but instead uses a plug  32 ′ which is fixedly mounted to the center section  50  whereby only the neutral spool  38  is free to move therein. The operation and movement of the neutral spool  38  is as described hereinbefore with respect to the combination valve  10 . The operations and configuration of this type of hydraulic system utilizing a separate check and neutral valves will be appreciated by those skilled in the art and need not be described herein. 
     It should be apparent from the preceding description that this invention has among other advantages, the advantage of providing a single valve which is capable of allowing make-up flow into the hydraulic system, stopping neutral flow from the hydraulic system, and cushioning the acceleration and deceleration of the vehicle. 
     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. Accordingly, the particular arrangements disclosed are 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 ′equivalent thereof