Abstract:
A pressure relief valve has a valve element that is biased by a spring into a closed position. A pressure being controlled is applied to the valve element to counter the force of the spring. When the pressure applied to the valve element exceeds the force of the spring the pressure relief opens, otherwise the valve closes. The pressure is applied through a damping orifice which controls the rate at which the applied pressure can increase and decrease thus delaying the response of the pressure relief valve to pressure changes in both directions. The relative size relationship between the inlet and outlet can be varied to define the pressure-flow characteristic of the pressure relief valve.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not Applicable 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to pressure relief valves which open when a pressure differential across valve exceeds predefined level. 
   2. Description of the Related Art 
   Pressure relief valves are provided at various locations in a hydraulic system to limit the maximum pressure to less then a predefined threshold level. For example, a pressure relief valve is normally provided at the outlet of a pump so that the supply line cannot be pressurized to an excessively high level. Should the pressure threshold level be exceeded, the relief valve opens creating a path usually to the hydraulic system reservoir, or tank. Another pressure relief valve is frequently provided at the workport of a control valve assembly to which a hydraulic actuator, such as a piston and cylinder combination, is connected. This arrangement protects against shock loading or inertia overloading due to the load which acts on the piston and cylinder. If that load creates an excessively high pressure within the hydraulic lines attached to the cylinder, the workport pressure relief valve opens to relieve the excessive pressure to the system tank. The workport relief valve remains open until the load condition no longer exists and pressure within the associated hydraulic line decreases below the threshold of the relief valve. 
   A common type of relief valve has a movable element biased against a valve seat by a spring to maintain the valve in the closed state. When pressure acting on the valve element exceeds the force of the spring, the valve element moves away from the seat, thereby opening the pressure relief valve. In this type of valve, the valve opens as soon as the spring force is exceeded and closes immediately upon the pressure dropping below the spring force. 
   In some relief valve applications, it is desired that the valve not open immediately when the pressure threshold is exceeded, but rather require that the excessive pressure should be required to exist for a given amount of time. Thus, the pressure relief valve will be immune from sporadic, short duration pressure excursions which exceed the threshold level. Nevertheless, when the pressure exceeds the threshold setting for longer than the given amount of time, the valve opens to relieve that pressure before damage to the hydraulic system can occur. Inversely, it is also desirable in certain situations to delay the valve closing to ensure that the pressure remains below the threshold. This prevents a momentary pressure decrease from closing the valve in a situation where pressure relief is still required. 
   Therefore, it is desirable to provide a pressure relief valve with bidirectional damping of the valve action, so as to provide hysteresis. 
   SUMMARY OF THE INVENTION 
   A pressure relief valve includes a body having a bore with an inlet and an outlet. A valve element is movably received within the bore and defines a chamber within the bore. In a first position of the valve element in the bore, a path exists between the inlet and the outlet, whereas in a second position the valve element closes that path. A damping orifice is formed in either of the body or the valve element to enable fluid to flow between the inlet and the chamber. A spring biases the valve element into the second position. 
   The pressure to be limited by the relief valve is applied to the inlet from which the pressure is transmitted through the damping orifice into the chamber in the bore of the housing. As the pressure at the input changes, the damping orifice delays application of that pressure change to the chamber and the size of the damping orifice determines the rate at which the relief valve responds to the pressure change. Thus a new pressure level must exist for at least a certain period of time before that pressure will occur in the chamber in the housing bore. 
   When the pressure in the chamber is greater than a threshold level set by the force of the spring, the valve element moves from the second position to the first position in which a path is opened between the inlet and outlet. 
   When the pressure decreases thereafter, the damping orifice delays application of the pressure decrease to the housing bore chamber so that the lower pressure level must occur for the certain period of time before that pressure occurs in the chamber. When the chamber pressure is less than the threshold level set by the spring, the spring force drives the valve element into the second position at which the path between the inlet and outlet is closed. 
   Thus the damping orifice provides energy dissipation to the bidirectional motion of the relief valve. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a pressure relief valve according to the present invention; 
       FIG. 2  is a view of the nose end of the pressure relief valve; 
       FIG. 3  is a longitudinal, cross-sectional view of the pressure relief, taken along line  3 — 3  of  FIG. 2  and mounted in a valve block; 
       FIG. 4  is an isometric view of a pressure sensing piston within the pressure relief valve; 
       FIG. 5  is a longitudinal, cross-sectional view of the pressure relief in an open state; and 
       FIG. 6  is a graph depicting how the pressure-flow characteristic of the pressure relief valve is a function of the relative sizes of the inlet and outlet openings. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With initial reference to  FIGS. 1–3 , a pressure relief valve  10  has a cylindrical housing  12  with a reduced diameter portion  14  for insertion into an aperture  15  of a valve block  17  or hydraulic manifold. A threaded region  16  of housing portion  14  engages threads in the aperture  15  to secure the valve in place. The valve block  17  has passages through which hydraulic fluid flows. A first of these passages  19  opens into the aperture  15  to one side of the reduced diameter portion  14  of the pressure relief valve  10  as seen in  FIG. 3 . As will be described, the pressure relief valve responds to the pressure level in this first passage  19 . A second passage  21  in the valve block  17  opens into the bottom of the block aperture  15  adjacent a nose  18  of the pressure relief valve on and typically leads to the system tank. 
   Referring to  FIG. 3 , the valve housing  12  has a generally tubular shape with a central bore  20  extending there through. The housing  12  has a first and second transverse apertures  22  and  23  extending through the wall of the reduced diameter portion  14  and communicating with first passage  19  in the valve block. A tubular sleeve  24  is secured within the portion of the housing bore  20  that is within the reduced diameter portion  14 . The housing  12  and the sleeve  24  form a body  25  of the valve  10 . The sleeve  24  has an inlet  26  and sensing aperture  28  extending transversely there through and opening into the first and second transverse apertures  22  and  23 , respectively, to provide a paths for fluid to flow from the first block passage  19  to different regions of a central bore  30  within the sleeve. The sleeve bore  30  has an open end facing a spring cavity  38  in the housing bore  20  and has a closed end at the nose  18  of the pressure relief valve. An outlet  32  extends through the sleeve  24  between the sleeve bore  30  and a longitudinal groove  34  that leads to the nose  18  along the outside surface of the sleeve. A smaller groove  36  connects the larger longitudinal groove  34  to the opposite end of the sleeve  24  and opens into the spring cavity  38 . As indicated by the section line  3 — 3  in  FIG. 2 , the two grooves  34  and  36  are offset 90° around the longitudinal axis of the valve sleeve  24  from the inlet  26  and sensing aperture  28 . 
   A sensing piston  40  is slidably received within the bore  30  of the sleeve  24  and abuts the closed end of that bore. With additional reference to  FIG. 4 , the sensing piston  40  has three lands  41 ,  42  and  43  formed there around with corresponding annular grooves  44  and  45  between adjacent lands. The middle land  42  has a smaller diameter than the outer lands  41  and  43  so that the annular grooves  44  and  45  are in fluid communication with one another when the sensing piston  40  is in the sleeve bore  30 . Thus the recess in the sensing piston formed by the middle land  42  and grooves  44  and  45  creates a sleeve cavity in bore  30 . A reduced diameter tip portion  46  is located between the first land  41  and a first end  48  of the sensing piston which end abuts the bottom of the sleeve bore  30  in the closed state of the relief valve  10  shown in  FIG. 3 . The tip portion  46  creates a sensing chamber  49  within the sleeve bore  30 . A damping orifice  50  is provided by a longitudinal notch in the first land  41  between the tip portion  46  and the first annular groove  44 . As seen specifically in  FIG. 3 , the sensing aperture  28  in the sleeve  24  opens into the first annular groove  44  of the sensing piston  40 . Thus pressurized fluid from the first passage  19  is applied to the first annular groove  44  and via the damping orifice  50  to the sensing chamber  49 . 
   The second end  52  of the sensing piston  40  abuts a valve piston  54  which also is slidably received in the bore  30  of the sleeve  24 . The sensing piston  40  and the valve piston  54  combined form a valve element  53 . Although the preferred valve element  53  is segmented into two pistons  40  and  54  a single piece valve element can be utilized. An intermediate chamber  56  is formed within the sleeve bore  30  at the interface between the two pistons  40  and  54 . In the illustrated closed position of the pressure relief valve  10 , the valve piston  54  closes communication of the inlet  26  and outlet  32  with the intermediate chamber  56 . 
   The end of the valve piston  54  remote from the sensing piston  40  engages a spring assembly  60 , which exerts a force on the valve piston to urge the valve piston into the illustrated closed state. The spring assembly  60  has a ball bearing  62  which fits within recesses in the valve piston  54  and a guide disk  64  to provide a pivot coupling between those components. The guide disk  64  abuts one end a compression spring  66  that extends through the spring cavity  38  in housing  12  with another end that abuts a guide  68  slidably located within the housing bore  20 . A retainer  70  is threaded into an end of the bore  40  in housing  12 . By varying the depth to which the retainer  70  is threaded into the housing bore  20 , the force that the spring  66  applies to the valve element  53  can be adjusted, thereby defining the pressure threshold of the relief valve  10 . 
   The pressure being controlled is applied from the first block passage  19  through the second transverse aperture  23  in the valve housing  12  and the sensing aperture  28  in the sleeve  24 , thereby reaching the annular grooves  44  and  45  around the sensing piston  40  ( FIGS. 3 and 4 ). The pressure then bleeds through the damping orifice  50  into the sensing chamber  49  at the bottom of the sleeve bore  30 . As the pressure at the second transverse aperture  23  changes, the damping orifice  50  delays application of that pressure to the sensing chamber  49 . 
   Assuming that the pressure increases above the threshold setting of the pressure relief valve  10 , that pressure level eventually occurs within the sensing chamber  49  and is applied to the end surfaces of the sensing piston  40 . This creates a force that tends to move the sensing piston  40  to the right in the orientation of the pressure relief valve  10  in  FIG. 3 . Because this force exerted on the sensing piston  40  is greater than the force from the spring  66 , the sensing piston and the valve piston  54  move to the right within bore  30  as depicted in  FIG. 5 . 
   As a shoulder  55  on the valve piston  54  moves past the inlet  26  in the sleeve  24 , the inlet  26  is opened into the intermediate chamber  56 . The relatively high pressure is applied through this path to the surface of shoulder  55  which adds to the force that counteracts the spring force. Movement of the valve piston  54  also opens the intermediate chamber  56  to the outlet  32  leading into the longitudinal groove  34 . This provides a fluid path through the pressure relief valve  10  from the first passage  19  in the valve block  17  to the second passage  21  leading to the hydraulic system tank, thereby relieving the pressure in the first passage. 
   As the pistons  40  and  54  move into the spring cavity  38  of the housing  12 , any fluid displaced by that motion is exhausted to the system tank through the longitudinal grooves  34  and  36  along the sleeve  24 . Thus, pressure does not build-up in the spring cavity  38  which would otherwise impede the movement of the pistons and the operation of the pressure relief valve. 
   Assume that with the pressure relief valve  10  is in the open state; and pressure at the first passage  19  then decreases below the predefined threshold level set by spring  60 . Now the high pressure in the sensing chamber  49 , which caused the valve to open, is gradually relieved through the dampening orifice  50  and then via the sensing aperture  28  and the second transverse housing aperture  23 . As the pressure within sensing chamber  49  decreases, the pistons  40  and  54  move toward the closed end of the sleeve  24  (to the left in the orientation shown in  FIG. 5 ). Eventually, the first end  48  of the sensing piston  40  abuts that closed end of the sleeve  24  and the pressure relief valve closes, as shown in  FIG. 3 . Specifically in the closed state, the location of the valve piston  54  within bore  30  blocks the openings of the inlet  26  and outlet  32  into the intermediate chamber  56 , thereby shutting the path through the pressure relief valve  10 . The expanding volume of the spring cavity  38 , produced as the valve piston  54  moves farther into the sleeve bore  30 , is filled by fluid drawn through the smaller longitudinal sleeve groove  36 . 
   The damping orifice  50 , provided in the external surface of the sensing piston  40 , limits the rate at which pressure within the sensing chamber  49  is able to change, thereby preventing rapid transitions of the pressure relief valve  10  between open and closed states. The size of the damping orifice  50  can be varied during manufacture to tailor the response rate for a particular application of the valve. 
   The pressure-flow characteristic of the pressure relief valve  10  also can be tailored by varying the relative sizes of the inlet  26  through which the fluid enters the intermediate chamber  56  and the outlet  32  through which the fluid exits the intermediate chamber. Specifically when the inlet  26  is smaller than the outlet  32  the pressure continues to increase as the flow increases after the relief valve  10  opens at point  75 , as indicated by the upper curve in  FIG. 6 . In contrast, when the inlet  26  is larger than the outlet  32 , the pressure generally decreases as the flow increases as depicted by the lower curve of the graph. 
   The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.