Abstract:
An adjustable pressure relief valve for use in hydraulic circuits, particularly for controlling pressure transients—acceleration and deceleration—of hydraulic motors subject to inertia forces (progressive anti-shock pressure relief valves) is characterized by reduced leakage, involving a reduced influence of flow forces during valve control, as well as reduced pressure losses; these characteristics prevent the hydraulic motor from rotating when it is subject to a drive load and is not powered, eliminate vibrations and instability of valve-regulated pressure, especially at low flow rates, and ensure an adjustment value varying according to the flow rate through the valve. A further characteristic of the valve is the reduced number of its components, resulting in greater cost effectiveness.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of adjustable pressure relief valves for use in hydraulic circuits, particularly for controlling pressure transients—acceleration and deceleration—of hydraulic motors subject to inertia forces (progressive anti-shock pressure relief valves). 
     2. Description of the Related Art 
     Adjustable pressure relief valves are widely used in the art. 
     Such pressure relief valves have the purpose of damping regulating pressure increases by compressing a main spring via a spring pressing member actuated by the pressure of hydraulic fluid carried from the feed line into a chamber above it. An adjusting spring is also mounted in such chamber, for downwardly pressing the movable member within the pressure relief valve; the force with which such adjusting spring operates on the movable member may be adjusted by operating an adjustment means situated outside the valve. 
     Thus, the valve may be easily adjusted. 
     The main drawback of the prior art is that, due to considerable leakages, these valves mounted to hydraulic motors with transient control and anti-shock purposes cannot prevent the hydraulic motor from rotating when it is subject to a driving load and is not powered; therefore, additional motor braking systems shall be provided to prevent load displacement while the machine in which the motor is mounted is still. 
     A further drawback of prior art pressure relief valves is their complex construction architecture and the large number of their components. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a leak-proof valve that can solve the above mentioned leakage problems, without providing additional motor braking systems. 
     A further object of the present invention is to provide a valve whose construction architecture can reduce the influence of flow forces generated during adjustment and reduce pressure losses, thereby providing adjustment values varying to a reduced extent with the flow rate through the valve, and eliminate vibrations and instability of regulated pressure. 
     Also, such construction architecture has the advantage of reducing the number of the valve components, for easier assembly and lower costs thereof. 
     These objects and advantages are achieved by the progressive pressure relief valve of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       These and other characteristics will be more apparent from the following description of a few embodiments, which are shown by way of example and without limitation in the accompanying drawings, in which: 
         FIG. 1  is a sectional view of a first embodiment of the pressure relief valve of the present invention; 
         FIG. 2  is a sectional view of a second embodiment of the pressure relief valve of the present invention; 
         FIG. 3  is a sectional view of a third embodiment of the pressure relief valve of the present invention; 
         FIG. 4  is a detailed view of the closure member within the pressure relief valve of the present invention; 
         FIG. 5  is an exemplary view of the hydraulic circuit in which the pressure relief valve of the present invention is usually used; 
         FIG. 6  shows a typical curve of regulated pressure as a function of time, during operation of the pressure relief valve of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , the characteristics and operation of a first preferred embodiment of the valve V of the present invention will be now described. 
     The valve V consists of an internally hollow cartridge  6 , with a spring-holding plug  3  screwed to its upper end; a ring of radial through holes  6   a  and one or more holes  6   b  are formed in the cartridge  6 . 
     The cartridge  6  contains a closure member  10  with an axial through hole  10   a , a main spring  8 , a piston  5  with an axial through hole  5   a , a setting spring  12  and a plate  11  with an axial through hole  11   a  interposed between the spring  12  and the closure member  10 . 
     In a preferred construction embodiment, two calibrated throttles  7  and  9  are mounted in the closure member  10 , which throttles are disposed in series and have respective calibrated holes  7   a  and  9   a ; such throttles may be equipped with a steel music wire  7   b  to prevent clogging of the calibrated holes. 
     In a further construction embodiment, there may also be a single throttle with a calibrated hole of smaller diameter. 
     In yet another construction embodiment, the calibrated holes ( 7   a ,  9   a ) may be replaced by a compensated flow-rate regulator, appropriately sized to ensure a constant flow rate regardless of the pressure within the feed line P through the hole  10   a.    
     As show in greater detail in  FIG. 4 , the closure member  10  is composed of portions having different shapes: a head  10   b , whose frustoconical shape and geometric precision can ensure tightness of the inner edge S 1  of the cartridge  6 ; a cylindrical surface  10   c  that can axially slide with a few millimeters clearance along the inner surface of the cartridge  6 ; a central part and an end part  10   d  having a cylindrical shape but a smaller diameter for a main spring chamber C 1  to be defined by the inner surface of the cartridge  6 , the outer surface of the closure member  10  and the bottom surface of the piston  5 . 
     When the valve is at rest, the main spring chamber C 1  is maintained at the pressure of the discharge line T by one or more holes  6   b  formed in the cartridge  6 . 
     The head  10   b  of the closure member  10  is characterized by the presence of a baffle  15  which appropriately directs the hydraulic fluid to reduce the influence of the flow forces generated as fluid flows from the feed line P to the discharge line T; the baffle  15  is an annular discharge formed by mechanical machining at the base of the frustoconical head  10   b  of the closure member  10 . 
     The surface  10   c  of the closure member mates with the inner surface of the cartridge  6  and, due to the high degree of geometric precision with which the surface  10   c  of the closure member and the inner surface of the cartridge mate, no fluid leakage actually occurs between such contact surfaces. 
     As a result, the flow rate of the fluid that leaks between the chamber C 1  and the chamber C 3  (the latter being defined by the baffle  15  and the edge S 1  of the cartridge  6 ) is much lower than the flow rate of fluid that can leak between the chamber C 1  and the discharge line T through the holes  6   b , and pressures in the chambers C 1  and C 3  are independent from each other. 
     Also, the surface  10   c  of the closure member has such a size as to leave a portion of the ring of holes  6   a  exposed as the valve closes; this arrangement reduces pressure losses and prevents vibrations and instability of the regulated pressure, especially at low flow rates. 
     The flow through the valve from the feed line P to the discharge line T increases pressure in the chamber C 3  to a value above the pressure in the chamber C 1  due to the holes  6   a  that are partly obstructed by the surface  10   c  of the closure member  10 , which generates an additional thrust upon the closure member  10 , that helps to overcome the force of the springs  8  and  12 . The end part  10   d  of the closure member  10  is held within the hole  5   a  of the piston  5 ; the section along which the hole  5   a  and the end part  10   d  of the closure member mate is designed with such a length and geometric precision, as to obtain actually no leakage or a leakage lower than 5 ml/min, at the operating pressure of the valve, between such contact surfaces. 
     The piston  5  can axially slide within the cartridge  6 ; due to the high degree of geometric precision with which the outer surface of the piston  5  and the inner surface of the cartridge  6  mate and to the addition of the seals  13  and  14 , no fluid leakage actually occurs between such contact surfaces. 
     In the second embodiment of the valve V of the present invention, as shown in  FIG. 2 , an optimized sealing effect may be obtained between the mutually mating hole  5   a  and the end part  10   d  of the closure member  10  by forming an annular groove  17  in this portion, with a low friction seal  16  therein. 
     In the third embodiment of the valve V of the present invention, as shown in  FIG. 3 , the cartridge  6  is omitted and its components are directly mounted to a specially machined body  1   b ; the holes  6   b  formed in the cartridge  6  are replaced by holes  1   d  directly formed in the body  1   b  and the ring of holes  6   a  is replaced by an annular recess  1   c  formed in the discharge line T. 
     The head  10   b  of the closure member  10  seals against the edge S 2  which is also obtained by direct machining in the body  1   b.    
     As a result of the characteristics of these three embodiments:
         the chamber C 1  and the chamber C 2 , defined by the spring holder  3  and the piston  5  are isolated from each other;   the chamber C 1  and the feed line P are isolated from each other when the valve is closed.       

     The spring holder  3  has an adjustment screw  2  mounted at one end thereof and an inner cavity  3   a  in which a spring plate  4  and a setting spring  12  may be received. The setting spring  12  presses the end part of the closure member  10 , through the plate  11 , towards abutment of the head  10   b  against the edge S 1  and presses the spring plate  4  against the adjustment screw  2 . As a result, the adjustment screw  2  may be used to adjust the force with which the closure member  10  is pressed against the edge S 1 . 
     The valve V is closed until the force generated by the pressure of the hydraulic fluid in the feed line P is lower than the force with which the closure member  10  is pressed against the edge S 1  or the edge S 2 , which force is given by the preload of the main spring  8  and the preload of the setting spring  12 ; in this condition the chamber C 3  is separated by the feed line P and the surface  10   c  of the closure member leaves a portion of the ring of holes  6   a  of the cartridge  6  or the annular recess  1   c  in the body  1   b  exposed. 
     When the pressure generated by the hydraulic fluid in the feed line P and exerted on the head  10   b  of the closure member  10  exceeds a predetermined value, the assembly composed of the closure member  10  and the plate  11  moves upwards; a passage is formed between the head  10   b  of the closure member and the edge S 1  of the cartridge  6  or edge S 2  of the body  1   b , to allow part of the fluid in the feed line P to flow into the chamber C 3  and then, through the ring of holes  6   a  or the annular recess  1   c , into the discharge line T. 
     Another part of the hydraulic fluid in the feed line P flows through the holes  10   a  and  11   a  and through the calibrated holes  9   a  and  7   a  of the throttles  7  and  9 , into the chamber C 2 ; the action of this fluid causes the piston  5  to move downwards and press the main spring  8 , thereby increasing the regulating pressure. 
     As the piston  5  moves down, the fluid in the chamber C 1  is pressed into the discharge line T through the holes  6   b ; in this condition, pressure in the chamber C 1  increases and opposes the downward motion of the piston  5 , which is thus slowed down. The size of the holes  6   b  or  1   b  is one of the valve operation time control parameters. 
     Referring to  FIG. 5 , an exemplary hydraulic circuit, in which the pressure relief valve of the present invention is usually used, will be now described. Two pressure relief valves V and V 1  are usually mounted to the body  1  or  1   b  of a hydraulic block; the valve V is in communication with a feed line P and a discharge line T and the valve V′ is in communication with a feed line P′ and a discharge line T′, formed in the body  1  or  1   b . The body  1  or  1   b  is also internally machined for the feed line P to be in communication with the discharge line T′ and for the feed line P′ to be in communication with the discharge line T. The hydraulic block  1  or  1   b  is incorporated in a hydraulic circuit comprising a directional control valve D and a motor M connected together by the hydraulic lines L 1  and L 2 . 
     Due to the presence of the pressure relief valves V and V′ of the present invention, when the control valve D is driven to impart a rotary motion to the motor M, a damped, gradual acceleration is obtained; conversely, when the control valve D is driven to abruptly stop the flow to the motor M, the latter tends to maintain its own rotational speed due to the inertial load connected thereto. The fluid that comes out of the motor M cannot flow through the directional control valve D and increases pressure in the line L 1  or L 2 , according to the direction of rotation, until the set pressure for the valve V or V′ is reached; this causes the valve to open and the fluid to recirculate on the opposite hydraulic line L 2  or L 1 . As fluid passes through the valve V or V′, it undergoes a pressure loss by energy dissipation, and allows gradual deceleration of the motor M until the latter stops, without requiring any additional braking system in the circuit. 
     The valves V and V′ are designed to operate at a low pressure value, and to gradually increase such set pressure to a final higher value in a predetermined time, which provides damped, gradual acceleration and deceleration transients of the motor M. When the directional control valve is driven to stop the flow to the motor M, the valves V and V′ ensure negligible leakages between the lines L 1  and L 2  and thus prevent any rotation of the motor M, even when the latter is subject to and tends to be driven by external loads. 
     The above described operation may be effectively represented by the typical curve of the regulated pressure as a function of time during valve operation, as shown in  FIG. 6 . From the time at which pressure starts to operate on the feed line P, pressure increases to the value P 1  at time t 1 , whereupon the closure member  10  starts to partially open the fluid passage between the feed line P and the chamber C 3  and from the latter through the ring of holes  6   a  or the recess  1   c  to the discharge line T; in this first step fluid passes through the holes  7   a ,  9   a  and  10   a  and reaches the chamber C 2 . From the time at which a pressure value is reached in the chamber C 2 , which generates a force that can displace the piston  5  (time t 1 ), pressure in the chamber C 2  remains substantially unchanged until time t 1 ′; from time t 1  to t 1 ′, the piston  5  translates and increases the preload of the main spring, thereby increasing the preload of the main spring  8  and hence the force required to lift the closure member  10 , i.e. the pressure in the feed line P increases the value P 1  to P 1 ′. At time t 1 ′, the piston  5  reaches an abutment and the pressure value P 1 ′ is reached, from now on pressure increases in the chamber C 2  until, at time t 2  a final pressure value P 2  is reached in the feed line P; this pressure value P 2  is then maintained unchanged with time as long as there is flow from the feed line P to the discharge line T. The calibrated holes  7   a  and  9   a  can control the time required for the piston  5  to run its stroke to the abutment; this allows adjustment of the operation time (t 2 -t 0 ) of the present valve, by only making changes to the calibrated throttles  7  and  9 . During this adjustment step, the size of the head  10   b  of the closure member and the presence of the baffle  15  reduce the influence of flow forces and pressure losses, eliminate vibrations and stabilize the regulated pressure at low flow rates. 
     The pressure relief valve of the present invention provides a number of advantages over prior art pressure relief valves:
         The chamber C 1 , the chamber C 2  and the chamber C 3  are wholly isolated from each other: the presence of the chamber C 3 , as well as the geometry of the baffle  15  on the closure member  10  can provide a sufficient damping effect to the closure member  10 -plate  11  system; this can avoid the provision of additional components to form another damping chamber;   With the valve at rest, fluid leakage between the feed line P and the discharge line T is very little and allows the hydraulic motor with which the valve is connected to be held in position even under the action of a driving load;   The influence of flow forces and pressure losses is reduced;   Vibrations and instability of regulated pressure at low flow rates are eliminated.   In a preferred construction embodiment, the valve operation time may be adjusted by two or more calibrated throttles in series: the series arrangement of the two or more throttles allows them to be formed with calibrated holes of increasing diameters and be less exposed to clogging; an additional guarantee against clogging of the calibrated holes is given by the possibility of providing a steel music wire  7   b  therein. The use of two throttles also avoids any complication to valve component processing.