Abstract:
A proportional throttle valve includes a valve piston ( 2 ) displaceably guided in the longitudinal direction in a valve housing ( 3 ) having a fluid inlet ( 4 ) and a fluid outlet ( 5 ) and pretensioned by an energy accumulator, in particular in the form a pressure spring, against a stroke stop ( 6 ) defining the closure position. A pressure limiting valve ( 7 ) is integrated in the proportional throttle valve ( 1 ). The side ( 8 ) of the valve piston ( 2 ) to allow the piston to move away from the stroke stop ( 6 ) in opposition to the pretensioning force when a predefinable pressure is exceeded at the fluid inlet ( 4 ).

Description:
FIELD OF THE INVENTION 
     The invention relates to a proportional throttle valve comprising a valve piston movably guided in the longitudinal direction in a valve housing having a fluid inlet and a fluid outlet and pretensioned by an energy accumulator, in particular in the form of a compression spring, against a stroke stop defining the closed position. 
     BACKGROUND OF THE INVENTION 
     Proportional throttle valves are often used when the level of the fluid flow passing through these valves is to be continuously changed as a function of a prevailing magnetic flow of a coil actuating the valve piston. With a slide valve, the opening cross section is dependent on the piston position. If the coil is supplied with an electrical current, a magnetic force proportional to the magnetic flow is generated. As a result of the magnetic force, the valve piston can be moved into an open position. A spring located on the back side of the valve piston is generally tensioned, with its spring force opposing the magnetic force. If an equilibrium is established between the magnetic force and the spring force, the valve piston remains in its position. 
     The proportional throttle valve is largely independent of the pressure level to be controlled because the valve piston is pressure-equalized by the pressure prevailing on the two end surfaces as well as in the pilot chamber. 
     Proportional throttle valves are used, for example, in lifting-lowering applications, such as in industrial trucks. In these applications, a 2/2 directional valve is typically used in the lifting-lowering motion for lowering the load. At the same time, proportional throttle valves are used to control the volumetric flow. To protect the entire system of these two valves, a pressure limiting valve is used in addition to limit the maximum pressure. 
     The known systems thus have a plurality of valves requiring the corresponding installation space. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an improved proportional throttle valve that integrates in itself several functions with respect to the lifting-lowering motion. 
     This object is basically achieved by a proportional throttle valve where the proportional throttle valve integrates a pressure limiting valve to facilitate influencing the fluid pressure on the back side of the valve piston for opening on the pertinent valve seat when a definable pressure is exceeded at the fluid inlet. A combination of the function of the proportional throttle valve with the pressure limiting valve is created helping save components and installation space compared to the prior art. 
     The pressure limiting valve is preferably integrated into the valve piston for functional coupling of the control lines. The control lines cause at least the fluid pressure at the fluid inlet of the proportional throttle valve to act on a valve seat, especially made in the form of a cone seat of the pressure limiting valve. The fluid pressure from the fluid inlet is applied via a radially offset axial bore in the valve piston to the back side of the valve piston via an orifice bore and a bore extending axially in the valve piston as well as via a radial bore draining from the radial bore to a pilot chamber of the valve. Thus pressure equalization is effected. This fluid pressure also acts at the same time on the valve seat of the pressure limiting valve. If the pressure exceeds the value set permanently on a valve spring of the pressure limiting valve, a control channel to the fluid outlet is cleared. The pilot volumetric flow is established at this point from the fluid inlet via the lateral axial bore in the valve piston. The back side of the valve piston as well as the orifice with the connected axial bore to the valve seat of the pressure limiting valve effects a pressure drop at the orifice. This reduced fluid pressure is signaled to the pilot chamber, from which a resulting force against the main spring of the valve piston results and moves the valve piston in the direction of an “open position” between the fluid inlet and the fluid outlet. 
     This mechanism of action is possible essentially both in the deenergized and in the energized state of the proportional throttle valve so that a continuously available pressure limitation situation is integrated into the proportional throttle valve. 
     Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preffered embodiment of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the drawing which forms a part of this disclosure and which is schematic and not to scale: 
         FIG. 1  is a side elevational view in section of a proportional throttle valve according to an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a proportional throttle valve  1  directly controlled in terms of its fundamental design. In a magnet system housing, a magnet system  14  has an armature  16  actuatable by a DC coil  15 . The magnet system  14  is designed as a pushing, path-controlled proportional magnet system. The armature  16  and an actuating element  17  dynamically connected to actuating element are moved down in the direction of a fluid inlet  4  in the energized state of the DC coil  15 . 
     The actuating element  17  is dynamically connected to a valve piston  2  representing an orifice function. In the deenergized state of the DC coil  15 , the armature  16  (not detailed), the actuating element  17  and the valve piston  2  are moved by a spring  18  in the direction of a stroke stop  6 . This piston corresponds to the closed state of the proportional throttle valve and the closed state between fluid inlet  4  and fluid outlet  5 . The armature  16  is mounted inside in the pole tube. 
     The magnet system housing with a cylindrical guide  19  is designed to project into a valve housing  3  and is sealed for this purpose relative to the exterior. The cylindrical actuating element  17  likewise projects into the cylindrical guide  19  and has a detachable positive connection to a prolongation  20  of the valve piston  2 . Prolongation  20  guides the spring  18  on the mandrel side. The prolongation  20  undergoes transition through a cylindrical space  21  with a bottom  22  defining the stroke stop  6  into the valve piston  2  configured as an “orifice piston.” The stroke stop  6  separates the back side  8  of the valve piston  2  from its main control section  23 . 
     The valve piston  2  has the main control section  23  for triggering the flow rate from the fluid inlet  4  to the fluid outlet  5 . For this purpose, the main control section  23  has an inlet opening  24  and radial outlet openings  25 . As a function of the positioning motion of the armature  16  of the magnet system  14 , the outlet openings  25  can be caused to partially or completely line up with the fluid outlet  5  on the valve housing  3  when the valve piston  2  is moved in the direction of the fluid inlet  4 . 
     In order to be able to superimpose a pressure limitation function on the volumetric flow control function of the proportional throttle valve  1 , the main control section  23  of the valve piston  2  has an inner bore concentric to its longitudinal axis  26  and receiving a pressure limiting valve  7 . The pressure limiting valve  7  has a cylindrical housing  27  in which a valve cone  29  is placed on a valve seat  9  and is pretensioned by a compression spring  28 . The housing  27  of the pressure limiting valve  7  is permanently or fixedly connected to the sleeve-shaped main control section  23  of the valve piston  2  and is guided to be able to move in the longitudinal direction with valve piston  2 . Within the framework of the fluid to be routed, the main control section  23  thus communicates with different control bores for triggering, especially relating to the fluid pressure on the back side  8  of the valve piston  2 . 
     As a rule, a boundary pressure, which can be set via a compression spring  28  at the fluid inlet  4 , causes an actuation of the valve piston  2  for displacement motion of the valve piston  2  in the direction to the fluid inlet  4  and enables a volumetric flow to pass through the proportional throttle valve  1 . 
     The pressure at the fluid inlet  4  is transferred via an axial bore  10 , which axial bore is radially offset in the main control section  23 , to the back side  8  into the spring space  21 . The axial bore  10  in the region of the stroke stop  6  discharges from the valve piston  2 . Out of the spring space, the pressure propagates via an orifice bore  11  in prolongation  20  via a bore  12 . This pressure is routed in this prolongation  20  through bore  12  closed by the valve cone  29  of the pressure limiting valve  7 . At a small axial distance from the valve cone, a radial bore  13  branches off from the bore  12  and leads into a pilot chamber (not detailed) resulting in the main piston or valve piston  2  being pressure-equalized. 
     If at this point the pressure at the fluid inlet  4  exceeds a definable value, the valve cone  29  opens the valve seat  9  of the pressure limiting valve  7 . Fluid can then flow via an obliquely radially routed branch duct  30  through the housing  27  of the pressure limiting valve  7  and the wall of the main control section  23  to a secondary connection  31  connected to a pressure medium tank. On the orifice  11  of the bore  12  on the back side of the piston in the spring space  21 , a pressure drop arises and propagates to the pilot chamber. This pressure drop results in the valve piston  2  moving in the direction of the fluid inlet  4  and in enabling a flow connection to the fluid outlet  5 . 
     The described combination valve can replace the independent pressure limiting valve of the prior art to protect the system pressure by the integration of the pressure limiting function into the proportional throttle valve. As a result, an expensive valve can be completely omitted. In addition, a cost reduction can be achieved by a simplified “drilling” in the control block (eliminating valve installation space and various connecting bores), and installation space on site can be saved. In particular, for applications in which a pressure limitation, which is variable depending on the operating state of the lifting means, is desired and is implemented electronically, the valve according to the invention is suitable for implementation of an additional mechanical protection for safety reasons in case of a fault in the electrical control circuit. In the electronic pressure limitation function, the pressure is detected via pressure sensors and evaluated by a control electronics system that, when a pressure is exceeded, electrically actuates the proportional valve, and thus, the opening cross section is set such that the pressure at the fluid inlet  4  is kept constant according to the setpoint stipulation. This setting is critical, based on safety-relevant aspects and, in addition, should be mechanically protected for protection of the system components, for which a conventional pressure limiting valve (not shown) would also be necessary, but can be omitted at this point within the scope of the inventive solution. 
     While one embodiment has been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims.