Patent Publication Number: US-6341621-B1

Title: Valve structure for a fluid operated device

Description:
This is a continuation-in-part of U.S. patent application Ser. No. 09/246,847, filed Feb. 9, 1999 (now U.S. Pat. No. 6,035,634) and claims the benefit thereof. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to manually actuated, hydraulically operated tools of the type having working elements such as jaws or cutters which close over a workpiece and valving thereof. More particularly, the invention relates to a hand tool having a hydraulic circuit contained entirely within a housing containing two pistons. One piston converts manual input force to fluid pressure. The other piston converts fluid pressure to output force for imposing on the work. The tool enables three speeds of closure of jaw or corresponding tool movement at one input speed. 
     The field of endeavor most likely to benefit from this invention is the construction industry in that the device is specifically intended for use in creating effective hand tools which are often used in the building trades. However, the general fields of mechanical assembly and automotive repair could also benefit from the apparatus herein disclosed. For example, any process requiring crimping, bending, punching, cutting, pressing, etc. could significantly benefit from the performance characteristics of the instant hydraulic tool. 
     It can be appreciated that the potential field of use for this invention are myriad and the particular preferred embodiment described herein is in no way meant to limit the use of the invention to the particular field chosen for exposition of the details of the invention. 
     2. Description of Related Art 
     Gripping, clamping, pressing, and punching tools frequently employ hydraulic circuits for actuating solid moving parts of the tool. Hydraulics are quite practical to magnify manual force which can be applied to a work piece. Magnification of force is readily accomplished by varying respective areas of driving and driven components, such as a pump plunger and a driven piston, subjected to fluid pressure. Overpressure relief valves and manual release valves are also easily incorporated into hydraulic circuitry. However, the incorporation of such valving features has previously added considerable expense and complexity to the mechanism. This expense has been a major reason that small hydraulic hand tools have not achieved widespread success in the marketplace. 
     Thus, there is a need to provide hydraulic tool having valve structure of reduced complexity and cost. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to fulfill the needs referred to above. In accordance with the principles of the present invention, this objective is obtained by providing a bi-stable valve arrangement which includes a body having first and second opposing ends and constructed and arranged to define a passage between the body and another element so that the passage extends from the first end to the second end. A seal member is associated with the body. A first spring structure biases the seal member in a first direction and a second spring structure biases the seal member in a direction opposite the first direction. The first and second spring structures are constructed and arranged so that the seal member may seal the passage. The first and second spring structures have spring loads such that under certain fluid pressure conditions on the first and second ends of the body, the seal member moves against the bias thereon to permit fluid flow through the passage in one direction, and under different pressure conditions on the first and second ends of the body, the seal member moves against the bias thereon to permit fluid to flow through the passage in a direction opposite the one direction. 
     In accordance with another aspect of the invention a check valve includes a body having a passage therein in open communication with a source of fluid. A spring support structure is coupled to and extends from the body. A seal structure includes an elastomer seal member disposed generally adjacent to the passage. A spring is supported by the spring support structure and biases the seal structure so that the seal member is in a sealing position preventing fluid from the source from exiting the passage. A load of the spring is such that when fluid pressure in the passage exceeds the spring load, the seal structure moves against the bias of the spring, permitting the seal member to move to an unsealing position to permit fluid to exit the passage. 
     In accordance with yet another aspect of the invention, a pressure releasing valve arrangement includes a valve structure having a valve member constructed and arranged to be disposed in a housing chamber of an element to seal an opening in the element. The opening communicates a fluid pressure chamber of the element with the housing chamber. The valve member separates the fluid pressure chamber from the housing chamber. The valve structure is constructed and arranged to be operatively associated with a movable member mounted for movement within the element. A spring biases the valve member towards a sealing position to seal the opening. When fluid pressure in the fluid pressure chamber reaches a pre-determined pressure, the valve member moves from the sealing position against the bias of the spring to unseal the opening permitting fluid pressure in the fluid pressure chamber to be reduced below the pre-determined pressure, due to fluid entering the housing chamber. Further, when the movable member moves to an over-travel condition, the valve structure is engaged by the movable member and moved therewith which causes the valve member to move from the sealing position to unseal the opening. 
     Other objects, features and characteristic of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification. 
     Various other objects, features, and advantages of the present invention will become more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, wherein like parts are given like numerals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic, side cross-sectional view of a hydraulic device provided in accordance with the principles of a first embodiment of the present invention; 
     FIG. 2 is a diagrammatic, side cross-sectional view of a hydraulic tool provided in accordance with the principles of a second embodiment of the present invention; 
     FIG. 3 is an enlarged view of a floating seal valve assembly associated with the barrier of the hydraulic tool of FIG. 2; 
     FIG. 4 is an enlarged view of a spring retainer member of the floating seal valve assembly of FIG. 3; 
     FIG. 5 is an enlarged view of the pump piston and bulkhead of the hydraulic tool of FIG. 2; 
     FIG. 6 is a floating seal valve provided in accordance with another embodiment of the invention; 
     FIG. 7 is a diagrammatic, side cross-sectional view of another embodiment of a hydraulic tool provided in accordance with the principles of the present invention; 
     FIG. 8 is an enlarged cross-sectional view of the check valve of the tool of FIG. 7 shown in a closed position; 
     FIG. 9 is an enlarged cross-sectional view of the check valve of the tool of FIG. 7 shown in an open position; 
     FIG. 10 is an enlarged view of a bi-stable valve structure of the tool of FIG. 7 shown in sealing position; 
     FIG. 11 is an enlarged view of a bi-stable valve structure of the tool of FIG. 7 shown in an open position allowing fluid flow in one direction; and 
     FIG. 12 is an enlarged view of a bi-stable valve structure of the tool of FIG. 7 shown in an open position allowing fluid flow in another direction; 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a three-speed hydraulic device preferably in the form of a tool is shown, generally indicated at  10 , provided in accordance with the principles of the present invention. The hydraulic tool  10  includes a cylindrical bulkhead  12  disposed within an interior bore  14  of a unitary cylindrical housing structure  16 . Interior bore  14  encloses a ram piston  18  driven by pressurized fluid and a pump piston  20  for developing this pressure. At a first end  15  and a second end  17  of the housing  16 , a removable housing end cap  22  and  24 , respectively, is provided. The end caps are shown as being threaded into the housing  16  but other forms of attachment, such as bolts or the like, could be used. In the broadest aspect of the invention, the end caps  22  and  24  may be considered to be part of the housing  16 . The cylindrical housing, piston, and ram could be of square, hexagonal or other cross-section if desired. Furthermore, the housing structure  16  may be composed of separate housings, such as, a pump housing and a ram housing. 
     In the illustrated embodiment, interior bore  14  is subdivided into a pumping chamber D, a driving chamber C, a pump reservoir chamber E, a ram reservoir chamber B and an accumulator chamber A. The chambers A, B and E receive and dispense fluid displaced during operation of the tool  10 . The pumping chamber is defined by a first end surface  25  of the pump piston  20  and surfaces of the bulkhead  12  and of the housing  16 . Pump reservoir chamber E is defined by the surfaces of the first end  15  of the housing  16  and a second end surface  27  of the pump piston  20 . The drive chamber C is defined by surfaces of the bulkhead  12  and of the housing  16  and a first or rear surface  72  of the ram piston  18 . Ram reservoir chamber B is defined by surfaces of the housing  16 , of surface  73  of the barrier  22 , and of a second or front surface  74  of the ram piston  18 . Finally, accumulator chamber A is defined by surfaces of the housing  16 , of surface  75  of the barrier  22 , and of surface  77  of an accumulator piston  30  which is located at the second end of the housing  16 . 
     The total volume of all the chambers is slightly variable due to fluid displaced by the pump piston rod  26  and the ram piston rod  28  during movement of the pump piston  20  and ram piston  18 . This rod displacement volume variation is accommodated by a spring loaded accumulator piston  30 , which forms a movable end wall sealing chamber A at the left side thereof, as depicted in FIG.  1 . Accumulator piston  30  has an opening closely cooperating with ram piston rod  28 . A spring  32  urges the accumulator piston  30  to the right as show in FIG.  1 . Spring  32  is suitably entrapped within housing  16  so that it acts continuously against piston  30 . In the broadest aspect of the invention, the accumulator piston  30  may be considered part of the second end of the housing  16 . The area within housing  16  enclosing spring  32  is open to the atmosphere via ports  34  to avoid fluid pressures below atmospheric pressure, which would tend to interfere with operation of the tool  10 . 
     The bulkhead  12  includes a ram piston return and overpressure valve structure, generally indicated at  36  in FIG.  1 . The valve structure  36  is preferably a spring loaded valve having a spring  38  which acts on valve member  40  to seal opening  42  in the bulkhead  12 . Opening  42  communicates with drive chamber C and with chamber  43  which houses the valve structure  36 . A conduit  44  is operatively coupled with the valve member  40  at one end thereof. The other end of the conduit  44  is operatively associated with the pump piston  20  and communicates with pump reservoir chamber E through check valve  46 . Conduit  44  communicates with bulkhead chamber  43  via passage  45 . O-rings  48  and  50  are provided about the conduit  44  to permit the normal pump stroke without moving the conduit  44  or the valve structure  36 . A conduit  52  is in communication with chamber  43  and communicates with an external conduit  54 . Conduit  54  is in communication with accumulator chamber A and together with conduit  52 , chamber  43 , conduit  44  define communication structure fluidly communicating the accumulator chamber A with the pump reservoir chamber E. Check valve  46  may be considered to be part of the communication structure. 
     Although the conduit  54  is shown to be external to the housing  16 , it can be appreciated that the conduit  54  may be a channel defined in the wall of housing  16 . In addition, it can be appreciated that configuration of the communication structure is not limited to that described above, but includes any structure which permits fluid communication from the accumulator chamber A to pump reservoir chamber E. 
     A first mode of operation of the tool  10  is a high-speed, low force mode in which jaws (not shown) or other working elements associated with the hydraulic tool  10  are moved into engagement with a workpiece. There is little need for force beyond moving the working elements to the point of contact with the work piece. Hence, force is exchanged for increase speed of closure of the jaws during positioning of the tool on the workpiece. 
     With reference to FIG. 1, the high-speed mode for closing of a the working elements will now be described. Force is applied via input shaft  26  of pump piston  20  in the direction of arrow P. This may be accomplished, for example, by actuating a hand operated trigger (not shown in FIG. 1 ). Fluid contained in pumping chamber D is pressurized and flows through connecting structure to enter drive chamber C thereby urging ram piston  18  toward the left in FIG.  1 . In the illustrated embodiment, the connecting structure comprises conduits  58  and  60 , and an annular channel  62  so as to fluidly communicate chambers C and D. A unidirectional valve in the form of a check valve  64  in conduit  58  of the bulkhead  12  opposes back flow from chamber C to chamber D. A filter  66  is provided in channel  62  to filter out any foreign material in the fluid so as to not disrupt operation of any of the valves in the tool  10 . 
     When no resistance is imposed upon ram rod  28 , fluid is ejected from ram reservoir chamber B through conduit  68  past a unidirectional high-speed control valve structure, preferably a check valve  70  and into drive chamber C. This is possible since the net effective area of rear surface  72  of piston ram piston  18  exceeds that of front surface  74  due to the presence of ram rod  28  reducing effective area of front surface  74 . Thus, pressure in chamber B is incrementally greater than that in chamber C which expresses fluid from chamber B to chamber C until the pressures are equal in chambers B and C causing the ram rod  28  to move rapidly in the direction of arrow W. Equilibrium is accomplished when the opposing force of friction or resistance from engaging the work equals the pressure in chamber C divided by the cross-sectional area of the ram rod  28 . This action increases speed of pump piston  20  relative to that which would result if pumping chamber D were the only source of fluid entering drive chamber C. In addition, the accumulator chamber A communicates with pump reservoir chamber E as explained above which further causes the pump piston  20  to move in the direction of arrow P. The increased speed of pump piston  20  gives rise to the aforementioned high speed mode. 
     When ram rod  28  encounters a predetermined degree of resistance which would correspond to engagement of the workpiece, the pressure in chamber B builds and overcomes spring loaded check valve  78  thereby opening conduit  76 . At this time, an intermediate speed mode prevails as fluid is continuously pumped from pumping chamber D to drive chamber C through conduit  58  past check valve  64 . The fluid from ram reservoir chamber B is now diverted to the accumulator chamber A, rather than back to pumping chamber D through conduit  68  and valve  70 , since the back-pressure on valve  70  from chamber C now keeps valve  70  closed. Fluid from the accumulator chamber A moves through conduit  54 ,  36 , chamber  43 , conduit  44  past check valve  46  to back-fill the pump reservoir chamber E. 
     When still greater resistance is encountered requiring added force over that available in the intermediate mode, a low speed, high force mode prevails. When increased pressure developed in pumping chamber D opens control valve structure in the form of a spring loaded check valve  80  in conduit  82 , some fluid ejected from pumping chamber D flows into pump reservoir chamber E. This action bypasses the surface area of pump piston  20  thus bringing the cross-sectional area of the pump rod  26  into play. The pressure produced from the mechanical input force, which remains constant, is therefore increased by the ratio of the pump piston surface and the cross-sectional area of the pump rod  26 . As an example, assuming that the diameter of the pump rod  26  is one-third of the diameter of he pump piston, then the pressure in chamber B would be 9 times greater than that before the shift to this high force mode. In this mode, pumping chamber D communicates with drive chamber C through conduit  58 ,  60  and channel  62  via valve structure  64  and ram reservoir chamber B communicates with the accumulator chamber A through conduit  76  via valve structure  78 . It can be appreciated that for a given force applied to piston rod  26  in the low speed, high force mode, the pressure generated in pumping chamber D increases in proportion to the decrease in the net effective area of piston  20 . This increased pressure is translated to ram piston  18  which in turn delivers an increased force to the ram rod  28 . 
     Anytime the pump piston  20  is retracted to the right (in the direction opposite that of arrow P in FIG.  1 ), by pulling on shaft  26 , a pump piston return stroke is initiated. Just prior to this action, chamber E has been back-filled by action of the accumulator chamber A expressing fluid through conduits  54  and  36 , chamber  43 , conduit  44 , past check valve  46 . Now as the pump piston  20  is moved to the right, the pressure in pump reservoir chamber E begins to increase which closes valve  46  and cracks open check valve  86  and allowing fluid to pass into to pumping chamber D. 
     The valve structure  36  functions as a combined over-pressure relief and pressure release mechanism. During the normal course of operations, fluid pressure in the tool  10  continues to increase by action of the pump piston  20  which in turn imparts increased force on ram piston  28 . When pressure in the drive chamber C reaches a pre-determined pressure as regulated by spring  38 , valve  40  disengages from its seat, thus permitting fluid flow through opening  42 . Fluid moves into bulkhead chamber  43  until the pressure in the drive chamber C returns to the pre-determined maximum pressure. Fluid entering chamber  43  is distributed to piston reservoir chamber E through conduit  44  and secondarily through conduits  52 ,  54  and into chamber A. This overpressure relief mechanism prevents the tool  10  from becoming too aggressive for its work and provides the user a cautionary measure of safety. Now once the tool  10  has performed its work, valve structure  36  becomes the mechanism for releasing and resetting the tool  10 . Over-travel of the pump piston  20  away from the bulkhead  12  beyond its normal pumping range will cause shoulder  61  to be engaged causing it to travel to the right in FIG.  1 . This action unseats valve  40  permitting fluid in drive chamber C to communicate with accumulator camber A, and through conduit  59  and valve  57 , to communicate with ram reservoir chamber B, and through chamber  43  and conduit  44 , to communicate with the piston reservoir chamber E, and through conduit  84  and valve  86 , to communicate with pumping chamber D. While in this mode, ram  28  may be retracted into the tool  10  by hand or some other external force. Once the tool  10  has been reset, the pump piston is released form its over-traveled position and spring  38  will reseat valve  40 . 
     When the ram piston  18  is to be retracted into the tool  10  by some external force (not shown), the pump piston  20  is pulled to its over-traveled position, thereby unseating valve member  40  and opening passage  42 . Retracting the ram piston  18  forces fluid from chamber C through bulkhead chamber  43 , conduits  52  and  54  into the accumulator chamber A. Fluid from the accumulator chamber A passes through conduit  59  and valve  57  in the barrier  22  to back fill chamber B. The net addition of the fluid to the accumulator chamber A is essentially the volume of the ram rod  28  now pushed back into the tool  10 . At the point that the pump piston  20  is in its over-traveled position and valve member  40  is opened, all chambers are communicating with one another and pressures are equalizing. When valve member  40  is opened, fluid in the drive chamber C communicates with the pump reservoir chamber E via conduit  44  and fluid in the pump reservoir chamber E communicates with the pumping chamber D via passage passages  86 . Fluid demands for chambers D and E have essentially already been supplied, accumulator chamber A now expands to take up the fluid displaced by the ram rod  28  as it is retracted into the tool  10 . 
     In summary, the ram piston  18  moves at increased speed and reduced force relative to the pump piston  20  when fluid is routed from one side of the ram piston  18  to the other side thereof. Similarly, ram piston  18  moves at a reduced speed and with increased force relative to the pump piston  20  when fluid is routed from one side of the pump piston  20  to the other side thereof. When neither of these flow routs occur, an intermediate speed, intermediate force mode prevails. 
     The check valves described in FIGS. 1-6 are conventional and preferably of the spring-actuated, ball or needle valve type. 
     A second embodiment of the invention is shown in FIGS. 2 and 3. The second embodiment of the tool  100  functions the same as the first embodiment, (e.g., provides three speeds of operation). However, in the second embodiment, certain of the valve structures are in the form of floating seal valves, not check valves. 
     Since it is difficult to provide the proper volumetric flows in the small tool package using check valves, FIGS. 2 and 3 show a second embodiment of the invention. Thus, instead of providing conduits and check valves in the barrier  122 , valve structure in the form of a floating seal valve assembly is associated with the barrier  122 . As shown, the floating seal valve assembly includes a first floating seal valve, generally indicated at  113 , comprising an O-ring  115  sealing a passage  131  between an outer periphery of the generally cylindrical barrier  122  and the annular wall defining inner bore  114  of the housing  116 , and a spring retainer member  117  coupled to face  119  of the barrier  122  and operatively associated with the O-ring  115 . In the illustrated embodiment, the floating seal valve  113  also includes a glide member  111  provided between the O-ring  115  and retainer member  117 . The spring retainer member  117  slides the glide member  111  on the bore  114  and holds it against a stepped shoulder  134  defined in the barrier  122 . The stepped shoulder dimensions as related to the cylinder bore  114  are typical of those required to provide a seal when the glide member  111  is in place. The axial length of the stepped shoulder and/or it&#39;s slope are such that a small hydraulic pressure can move the glide member  111  off of the shoulder  134 . The glide member has a passage  136  therethrough such that when the hydraulic force deflects the spring retainer member  117 , a very large fluid flow path is provided. Thus, since the glide member  111  is bearing against the shoulder  134 , the glide member can support a high pressure in one direction yet permit easy flow of fluid in the opposite direction. In certain applications, the spring force on the glide member  111  may be high enough to require a predetermined pressure before the glide member  111  is moved off the stepped shoulder  134 . The retainer member  117  is preferably composed of spring material such as metal and gently biases the O-ring  115  in the direction of arrow J of FIG. 2 to seal the passages  131  and  136 . In the broadest aspect of the invention, the glide member  111  may be omitted. 
     A second, similar floating seal valve, generally indicated at  121 , comprises O-ring  123 , spring retainer member  125 , and glide member  124  between the retainer member  125  and the O-ring  123 . The O-ring bears against shoulder  138 . The retainer member  125  is fixed to a surface of the barrier  122 . The second floating seal valve is provided so as to selectively seal a passage  141  through the glide member  124  and passage  133  between the outer surface of the ram rod  128  and an inner wall defining bore  139  of the barrier  122 . The spring load of retainer member  125  is selected such that when conditions are such that fluid may flow from ram reservoir chamber B to accumulator chamber A, the retainer  125  will flex to permit fluid to flow past the O-ring  123  and through passages  131  and  141  in the direction of arrow J. Similarly, the spring load of the retainer member  117  is such that in a ram piston retracting mode, fluid may flow past O-ring  115  through passages  141  and  133  in the direction opposite to arrow J such that fluid in the accumulator chamber A may move into ram reservoir chamber B. In the broadest aspect of the invention, the glide member  124  may be omitted. 
     Floating seal valve structure  127 , including O-ring  129 , glide member  126  and spring retainer member  135 , is provided at the ram piston  112 . As with floating seal valve structure  113  associated with the barrier  122 , the retainer member  135  biases the O-ring  129  against a shoulder to seal a passage  137  between the periphery of the ram piston  112  and the housing inner bore  14 . Thus, retainer member  135  is constructed and arranged to prevent fluid communication between the drive chamber C and ram reservoir chamber B and when required, permit large volumetric flow from ram reservoir chamber B to drive chamber C. The spring load of floating seal valve  121  is greater than that of floating seal valve  127  so as to effect the shift between the high-speed/low force and the mid-speed/mid force modes of operation. In the broadest aspect of the invention, the glide member  126  may be omitted. 
     The spring retainer member  117  preferably has a plurality of fingers  180  extending from a central portion  182  thereof as shown in FIG.  4 . Spring retainer member  135  is configured similarly. 
     The pump piston  120  of the second embodiment has a different valve structure associated therewith than in the first embodiment of the invention. With reference to FIG. 5, an enlarged view of the generally cylindrical pump piston  120  of FIG. 2 is shown. Instead of providing conduits and check valves  80  and  86  in the pump piston as in the first embodiment of the invention, valve structure in form of a bi-stable floating seal valve arrangement, generally indicated at  132 , is provided. The floating seal valve arrangement  132  comprises an O-ring  160  positioned to seat on a raised ridge  161  of the pump piston  120 . Two opposing spring loaded guide rings,  162  and  164 , keep the O-ring  160  on the ridge  161  and in a sealed position. Stop surfaces  163  limit the movement of the guide rings toward the O-ring  160 . During operation, when the pressure in pumping chamber D reaches that planned for the transition to the high force/low speed mode, loaded spring  170  is overcome by the force of the fluid on the O-ring  160 , thus moving the O-ring  160  off its seat and permitting the fluid to flow through passage  166  from the pumping chamber D to the pump reservoir chamber E. Spring  168  is normally loaded, and accommodates the passage of fluid from chamber E to chamber D during the pump refilling operation pursuant to another stroke. 
     The embodiment of FIG. 2 includes a handle structure, generally indicated at  150 , which is operatively associated with pump rod  26  of the pump piston to actuate the same. The handle structure  150  includes a hand-operated trigger member  152  which, when actuated or squeezed, causes actuation of the tool  100  and which, when released, causes the return stroke of the ram piston  112 , thus resetting the tool  100 . It can be appreciated that the handle structure  150  can be provided on the tool  10  of the embodiment of FIG. 1 as well. 
     A mechanical linkage, generally indicated at  154 , is operatively associated with the over-pressure release valve structure  36  and is used to move the valve member  40  of the valve structure  36  to an open position so that fluid may flow from the drive chamber C to the accumulator chamber A and to the pump reservoir chamber E, as noted above. The mechanical linkage is connected to the pump piston  120  with a limited slip connection so that over travel of the pump piston  120  beyond a the normal stoke moves the valve member  40  to the opened position. 
     FIG. 6 shows yet another embodiment of a bi-stable floating seal valve associated with the barrier  222 . A first O-ring  215  disposed in groove  216  between bore  114  of the housing  16  and the periphery of the barrier  222  so seal a flow path between chamber A and B. The seal valve includes a second O-ring  223  positioned to seat on a raised ridge  224  of the barrier  222 . Two opposing spring loaded guide rings,  225  and  227 , keep the O-ring  223  on the ridge  224  and in a sealed position. The guide rings  225  have fluid flow passages therein to permit fluid flow between chambers A and B when desired. Finger springs  228  and  229  load the guide rings  225  and  227 . The spring load of spring  229  is greater than that of spring  228 . The spring load of spring  229  is selected such that when conditions are such that fluid may flow from ram reservoir chamber B to accumulator chamber A, the spring  229  will flex to permit fluid to flow past the O-ring  223  in the direction of arrow J and through passages in the guide rings. Similarly, the spring load of the spring  228  is such that in a ram piston retracting mode, fluid may flow past O-ring  223  through passages in the guide rings in the direction opposite to arrow J such that fluid in the accumulator chamber A may move into ram reservoir chamber B to effect the shift between the high-speed/low force and the mid-speed/mid force modes of operation. 
     FIG. 7 shows another embodiment of a hydraulic tool  300  provided in accordance with the principles of the of the invention. The tool  300  operates the same as the tool  100  of FIG. 2, but certain of the valves employed in the tool  300  are different from those of the tool  100 . More particularly, a novel check valve, generally indicated at  310 , is provided instead of the conventional check valve  86  of FIG.  2 . The check valve  310  may be employed in any system to permit fluid flow under certain conditions yet prevent fluid flow under different conditions. Thus, the tool  300  is one example of use of the check valve  310 . As best shown in FIGS. 8 and 9, the check valve  310  comprises a body  312  having a passage  314  therein which is in open communication with a source of fluid (not shown) which communicates with port  316  of a housing  318 . A seal structure  320 , including an elastomer seal member  322 , is disposed generally adjacent to the passage  314 . In the illustrated embodiment, the seal structure  320  comprises an O-ring  322  sandwiched between rigid support members in the form of a first washer  324  and a second washer  325 . The washers are made of rigid material such as metal or hard plastic. A spring support structure  326  is coupled to and extends from the body  312 . A spring  328  is supported by the spring support structure  326  and biases the seal structure  320  so that the seal member  322  is in a sealing position preventing fluid from the source from exiting the passage  314 . A load of the spring  328  is such that when fluid pressure in the passage  314  exceeds the spring load, the seal structure  320  moves against the bias of the spring  328  moving the seal member  322  to an unsealing position to permit fluid to exit the passage  314  in the direction of arrow P in FIG.  9 . Thus, when the check valve  310  is open, fluid can fill chamber E behind pump piston  329  (FIG.  7 ). In the illustrated embodiment, the support structure  326  is a shaft threadedly engaged with the body  312  at one end thereof. The other end of the shaft has a spring seat  330  upon which one end of the spring  328  rests. The other end of the spring  328  acts on the first support member  324  to bias the seal structure  320  in the direction opposite arrow P. 
     The check valve  310  further includes a cylindrical spacer  332  disposed within a bore  334  in the body  312  and about the shaft  326 . An outer periphery of the spacer  332  and surfaces defining the bore  334  cooperate to define at least a portion of the passage  314 . One end of the spacer  332  is engaged with a surface  336  of the body  312  so as to prevent movement of the spacer  332  in a direction opposite arrow P. The other end of the spacer  332  defines a stop surface  338  (FIG. 9) which the second rigid support member  325  contacts when the sealing structure  320  is in the sealing position. The body  312  also includes a stop surface  340  which is coplanar with the stop surface  338  of the spacer  332 . The second rigid support member  325  also contacts the stop surface  340  when the sealing structure  320  is in the sealing position. 
     The body  312  is cylindrical and includes external threads  337  which engage internal threads  339  in bore  341  of housing  318 . The bore  341  includes a tapering surface  342  (FIG. 8) and the body  312  includes a surface  344  disposed in sealing engagement with the tapering surface  342  to prevent flow between the body  312  and the housing  318 . Also, when surface  344  of the body  312  engages the tapering surface  342 , the passage  314  is aligned with the port  316  when the body  312  is installed fully into the bore  341 . As shown, port  316  is disposed transversely with respect to the bore  341 . An O-ring  343  seals the body  312  in housing  318  downstream of port  316 . 
     Although the check valve  310  has been disclosed for use in tool  300 , it can be appreciated the check valve  310  may be pre-assembled and simply inserted into a bore for use in any device requiring a check valve. Furthermore, the fluid is not limited to hydraulic fluid, thus, the check valve  310  may be used with air. 
     FIGS. 10-12 are enlarged views of the pump piston  329  of FIG. 7, including a bi-sable valve structure, generally indicated at  350 , of the invention. The bi-stable valve structure  350  comprises a seal member  352  positioned to seat on a raised seat  354  which is part of the pump piston  329 . In the illustrated embodiment, the seal member  352  is an O-ring of circular cross-section. Two opposing first and second spring structures, generally indicated at  356  and  358 , respectively, keep the seal member  352  on the seat  354  in a sealed position sealing a passage  359  defined between piston bore  360  and a periphery of the pump piston  329 . The first and second spring structures have spring loads such that under certain fluid pressure conditions on a first end  361  and a second end  363  of the pump piston  329 , the seal member  352  moves against the bias thereon to permit fluid flow through the passage  359  and opening  353  in pump piston  329  (in the direction of arrow S in FIG.  11 ). Under different pressure conditions on the first and second ends of the pump piston  329 , the seal member  352  moves against the bias thereon to permit fluid to flow through opening  353  and through the passage  359  (in a direction of arrow T in FIG.  12 ). 
     The first spring structure  356  comprises a first retainer member  362  and a first spring  364  coupled to pump piston  329 . The spring  364  biases the first retainer member  362  and the first retainer member  362  biases the seal member  352  in a direction of arrow R (FIG.  10 ). The second spring structure comprises a second retainer member  366  and a second spring  368  coupled to the pump-piston  329 . The spring  368  biases the second retainer member  366  and the second retainer member  366  biases the seal member  352  in a direction opposite of arrow R (FIG.  10 ). The second spring  368  has a spring load greater than the spring load of the first spring  364  so that less pressure is required to move the first spring  364  than is required to move the second spring  368 . As best shown in FIG. 12, the second spring  368  is a latch spring and the second retainer member  366  moves from a seal member biasing position to a latched position to maintain the passage  359  in an open condition permitting fluid flow through the passage  359 . The latch spring  368  causes the second retainer member  366  to remain in the latched position until being mechanically reset to the retainer biasing position. More particularly, and as best shown in FIG. 12, the latch spring  368  has an arm member  370  having a latching portion  372  at an end thereof. The second retainer member  366  has first and second latch engaging surfaces  374 ,  376 , respectively, disposed in spaced relation. The latching portion  372  engages the first latch engaging surface  374  when the second retainer member  366  is in the biasing position (FIG. 10) and engages the second latch engaging surface  376  when the second retainer member  366  is in the latched position (FIG.  12 ). The second retainer member  366  includes a cam surface  378  between the first and second latch engaging surfaces  374 ,  376 . The latching portion  372  of the latch spring rides on the cam surface  378  as the second retainer member  366  moves from the biasing position to the latched position. 
     During operation of tool  300 , when the pressure in pumping chamber D reaches that planned for the transition to the high force/low speed mode, loaded second spring  368  is overcome by the force of the fluid on the seal member  352 , thus moving the seal member  352  off its seat (FIG. 10) and permitting the fluid to flow through passage  359  from the pumping chamber D to the pump reservoir chamber E (FIG.  12 ). First spring  364  is normally loaded and accommodates the passage of fluid from chamber E to chamber D during the pump refilling operation pursuant to another stroke. The second retainer member  366  is preferably mechanically reset to the biasing position by actuating the handle  382  of the tool  300  to pull-back the pump piston  329 . 
     The O-rings described herein may be conventional, elastomeric, circular cross-section O-rings. However, other cross-sectional shapes may be used, such as, for example, rectangular, square, and U-shaped cross-sections. It can also be appreciated that other seal members, such as gaskets, may be used instead of the O-rings in the tools described herein. 
     The check valve  310  and bi-stable valve structure  350  of the tool  300  of FIG. 7 provide the following advantages when compared to conventional check valves and spool valves: 
     1) reduce or eliminate plumbing requirements, 
     2) are less expensive to build, and 
     3) provide larger volumetric through-puts and thus provide very fast response. 
     The foregoing preferred embodiment has been shown and described for the purposes of illustrating the structural and functional principles of the present invention, as well as illustrating the methods of employing the preferred embodiments and are subject to change without departing from such principles. Therefore, this invention includes all modifications encompassed within the spirit of the following claims.