Abstract:
A pressure-relief valve is provided. The pressure-relief valve having a valve body with a passage that can be closed by a moving closure element. An electromagnet is mounted detachably on the valve body and interacts with an armature in driving connection with the closure element. A spring interacts with the closure element the bias tension of which can be changed by an adjustment element that can be moved manually relative to the valve body. The adjustment element and the valve body are constructed such that a manual change of the bias tension of the spring by means of the adjustment element is then possible only when the electromagnet is detached from the valve body. Also provided is a round baler employing such a pressure-relief valve.

Description:
FIELD OF THE INVENTION 
     The invention herein relates to a pressure-relief valve having a valve body with a passage that can be closed by a moving closure element. An electromagnet interacts with a moving armature in driving connection with the closure element. A spring interacts with the closure element, a biasing tension of the spring being changeable by an adjustment element that can be moved manually relative to the valve body. The invention further relates to a round baler equipped with such a pressure-relief valve. 
     BACKGROUND OF THE INVENTION 
     In round balers of variable bale chamber design, the tension of the bale forming elements (e.g. belts) is typically defined by a hydraulic cylinder that interacts with a moving tension arm that carries a deflection roller abut which, in turn, the bale forming elements revolve (normally one or more belts or bar-chain conveyors). The pressure in the hydraulic cylinder is normally controlled by way of a pressure-relief valve whose opening pressure can be controlled electromagnetically. According to U.S. Pat. No. 4,611,535 a driving of the pressure-relief valve is provided in the sense of a reduction in the pressure in the hydraulic cylinder and thus the tension of the bale forming elements, when an excess torque is detected on the main drive shaft. In German Patent document DE 197 18 229 A1 it is proposed to drive the pressure-relief valve as a function of a desired bale density and bale size, while, in the non-energized state, the pressure-relief valve in European Patent Application EP 1 059 476 A2 is held in the closed state by a biased spring. 
     In the baler according to U.S. Pat. No. 4,611,635, the opening pressure of the pressure-relief valve can be adjusted by hand by means of plain washers that can be inserted between a spring and the armature of the electromagnet. In EP 1 059 476 A2, the opening pressure of the pressure-relief valve can be adjusted by hand by means of an adjustment screw changing the bias tension of a compression spring. This manual adjustability of the opening pressure of the pressure-relief valve is useful especially when the electronic controller of the pressure-relief valve is interfered with or has failed. 
     In both of the above-mentioned balers, a failure or interference in the electronic controller of the pressure-relief valve results in a relatively high opening pressure. Now, if the operator inadvertently sets the opening pressure of the pressure-relief valve even higher, then it is possible that a structurally specified maximum pressure in the hydraulic cylinder is exceeded, which could result in damage to the baler. This problem occurs to an elevated degree if the electromagnet (like in the embodiment according to FIG. 3 of U.S. Pat. No. 4,611,535) in the energized state increases the opening, and the error in the electronics leads to a permanent excitation of the electromagnet. 
     SUMMARY OF THE INVENTION 
     The problem forming the basis of the invention is thus seen in the need for a pressure-relief valve being provided with an opening pressure that can be changed electromagnetically and manually, and in which it is not possible that maximum permissible pressure cannot be exceeded. 
     Accordingly, a pressure-relief valve, as disclosed herein, comprises a valve body with a passage provided therein that can be closed by a moving closure element. The closure element interacts with a moving armature of an electromagnet and a spring. To be able to change the opening pressure of the pressure-relief valve by hand when an error occurs in the electronic controller of the electromagnet or in the electromagnet itself, the valve body and an adjustment element that can be moved manually are constructed such that a manual adjustment of the bias tension of the spring is possible by means of the adjustment element only when the electromagnet is detached from the valve body. 
     In this way, situations are prohibited in which the electromagnet and the spring set by hand, together increase the opening pressure of the pressure-relief valve to a value at which there is the risk of damage to the hydraulic system or to a round baler equipped with the pressure-relief valve. 
     In one preferred embodiment, the electromagnet or its housing forms a stop for the adjustment element. If the electromagnet is then mounted on the valve body, the adjustment element cannot be moved farther than specified by the stop and thus the bias tension of the spring does not increase. 
     The adjustment element can be equipped, in particular, with a thread that interacts with a thread of the valve body. Here, it is arbitrary whether the valve body has an internal or external thread; the adjustment element then has an external or internal thread. 
     The adjustment element can interact with the spring directly or via an intermediate element supported displaceably on the valve body. 
     The maximum bias tension that can be set with the adjustment element (which can be reached when the electromagnet is removed from the valve body) can be advantageously specified on the production side, in order to realize that a desired maximum opening pressure be reached but not exceeded. An adjustment screw, in particular, can be used for this purpose, wherein this adjustment screw is screwed into a body of the adjustment element. While the body can be moved relative to the valve body, the adjustment screw displaces one end of the spring relative to the body and consequently also relative to the valve body. In order to make later manipulations more difficult, the adjustment screw could be fixed permanently on the body of the adjustment element after its production-side adjustment, e.g., by adhesive or a coating, and/or be covered by a cover element. 
     The spring is preferably a compression spring that biases the closure element into a closure position blocking the passage. 
     When the electromagnet is energized it advantageously biases the closure element similarly into a closure position blocking the passage. In such a configuration, the construction according to the invention of the adjustment element and the valve body is especially useful, because, through the removal of the electromagnet, the increase of the opening pressure resulting from the electromagnet for a defective permanent excitation is avoided. The biasing of the spring corresponds to a minimum value when the electromagnet is fixed on the valve body. The minimum value advantageously corresponds to an opening pressure of 0 bar. The bias tension of the spring (and thus the opening pressure) can be increased by the adjustment element relative to the minimum value only when the electromagnet has been removed. In this way, the full displacement range of the pressure-relief valve is provided to the electromagnet. 
     The described pressure-relief valve can be used in an especially advantageous way on a round baler with a bale chamber bounded by one or more bale forming elements in which the tension of the one or more bale forming elements can be defined by means of a hydraulic cylinder whose pressure is determined by a pressure-relief valve according to the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the description that follows the preferred embodiment(s) of the invention are described in detail with reference to the accompanying drawings wherein: 
         FIG. 1  is a round baler in partially schematic side view; 
         FIG. 2  is a hydraulic diagram of the hydraulic cylinder and the associated hydraulic system that define the tension of the bale forming elements of the round baler; 
         FIG. 3  is a sectional view of a pressure-relief valve with mounted electromagnet; and, 
         FIG. 4  is a sectional view of the pressure-relief valve of  FIG. 3  with detached electromagnet. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a round baler  10  with a housing  12  that can be attached to a tractor (not shown) by means of a drawbar  14 , in order to be pulled across a field in a driving direction toward the right with respect to  FIG. 1 , and is supported on wheels  16 . The housing  12  is assembled from a front, rigid housing half  18  and a rear, pivoting housing half  20  that are pivotably connected to each other in a joint at the top. The housing  12  carries a plurality of rollers. Endless bale forming elements  22  that are arranged one next to the other run over the rollers. These bale forming elements mostly encompass, together with side walls of the housing  12 , a bale chamber  24 . In this embodiment, the bale forming elements  22  are constructed as belts. In the lower region of the bale chamber  24  there is an inlet  26  that is bounded at the back by a roller  28  and material taken up by a pick-up  30  can enter into the bale chamber  24 . 
     The round baler  10  comprises several bale forming elements  22  arranged laterally one next to the other. They are set in motion during the harvesting operation and enclose a bale  32  forming in the bale chamber at the front, top, and back. The setup and the function of such a round baler are known. Refer to the disclosure of German Patent document DE 101 53 540 A, the contents of which are hereby incorporated herein by reference. The reference symbols  32 ,  32 ′, and  32 ″ designate bales of increasing diameter. 
     The tension of the bale forming elements  22  is determined by rollers  28  that are fixed on a tension arm  34 . The tension arm  34  is hinged on the front housing half  18  about an axis  36  running horizontally and perpendicular to the forward direction and is coupled with a hydraulic cylinder  38  that is hinged on the front housing half  18  and whose piston rod is connected to the tension arm  34  by means of a flexible element  40 , such as a cable or a chain. The flexible element  40  wraps around a deflection roller arranged underneath the tension arm  34  by nearly 180°. The greater the pressure in the piston rod space of the double-acting hydraulic cylinder  38 , the more the tension arm  34  is biased downward and the greater the tension of the bale forming elements  22 . Shown differently in the figures, tension arms  34  and hydraulic cylinder  38  are provided on both sides of the round baler  10 . 
       FIG. 2  shows a hydraulic schematic of the hydraulic cylinder  38  and the hydraulic system used for its driving. The latter comprises a pump  44  for providing pressurized hydraulic fluid. The pump  44  could be located on board the round baler  10  or the tractor pulling the baler and is connected to a valve  46  with three positions and two paths and whose other inlet port is coupled with a storage container  48  for hydraulic fluid. With its one outlet, the valve  46  is connected through a line  50  to the piston chamber and, at its other outlet, through a different line  52  to the piston rod chamber of the hydraulic cylinder  38 . With its inlet  66 , a pressure-relief valve  54  is connected to the line  52  and the piston rod chamber of the hydraulic cylinder  38  and with its outlet  68  to the line  50  and the piston chamber. An electronic controller  56  is connected to an electromagnet  58  that can change the opening pressure of the pressure-relief valve  54 . The force transmitted by the hydraulic cylinder  38  onto the tension arm  34  is designated with the letter F and depends on the pressure in the piston rod chamber. 
     It would also be conceivable to connect the piston rod of the hydraulic cylinder  38  directly to the tension arm. Then, in  FIG. 2 , the ports of the piston chamber and the piston rod chamber would be interchanged. 
       FIG. 3  shows a sectional diagram of the pressure-relief valve  54  with the electromagnet  58  fixed on this valve. The pressure-relief valve  54  comprises a valve body  60  that contains a passage  62 . A closure element  64  that can move in the vertical direction in  FIG. 3  relative to the valve body.  60  can be moved between a position closing the passage and an illustrated, opened position. The inlet  66  is connected to the bottom side of the passage  60  lying opposite the closure element  64 , while the outlet  68  is connected to an annular space  70  surrounding the closure element  64 . The closure element  64  is attached on the lower end of a tappet  72  on which an armature  74  made from ferromagnetic material is fixed. The armature  74  is surrounded concentrically by the electromagnet  58 . In the energized state, the electromagnet  58  pulls the armature  74  downward and brings the closure element  64  into the closed position from which it can be opened again against the force of the electromagnet  58  by the pressure in the inlet  66 . The hollow cylindrical housing  60  there extends within the electromagnet  58  concentrically to the armature  74 . 
     Furthermore, the valve body  60  is equipped, on its end away from the passage  62 , with a thread  76  in which engages a thread of the body  78  of an adjustment element  80 . The body  78  has a hollow cylindrical construction and is equipped with a handle ring  82  projecting outward on its end facing away from the passage  62 . In the interior of the part of the valve body  60  that projects upward past the electromagnet  54  and is enclosed by the lower area of the body  78 , there is an intermediate element  84  that is supported so that it can be displaced relative to the valve body  60  in the direction of movement of the tappet  72 . Shoulders  86  of the intermediate element  84  and the part of the valve body  60  projecting upward past the electromagnet  54  form an upper limit for the movement range of the intermediate element  84 . A coil-shaped spring  88  contacts, with its upper end, the intermediate element  84  and, with its other end, an upper plate  90  of the tappet  72 . From above, an adjustment screw  92  that is equipped with a top hexagonal socket (or other engagement element, such as a slot or a Phillips) is screwed into a central thread of the body  78  of the adjustment element  80 . This adjustment screw is covered at the top by a cover  94 . 
     With its top side, a housing  96  of the electromagnet  54  forms a stop on which the body  78  of the adjustment element  80  contacts when it is screwed as far as possible onto the thread  76 , as shown in  FIG. 3 . Then a slight gap still remains between the spring  88  and the intermediate element  84 , so that the spring  88  does not act on the tappet  72  and the closure element  64 . In the situation according to  FIG. 3 , the opening pressure of the pressure-relief valve  54  is defined solely by the electromagnet  58  that pulls the armature  74  downward by a greater or lesser extent and biases the closure element  64  downward. 
     The electromagnet  58  can be detached with its body  96  from the valve body  60  after the adjustment element  80  has been unscrewed from the thread  76 . This situation is shown in  FIG. 4 . Because the housing  96  of the electromagnet  58  no longer presents a lower stop for the adjustment element  80 , it can be rotated downward on the thread  76  until a shoulder  98  on the inside of the body  78  contacts the upper end of the valve body  60  (in  FIG. 3 , by means of the part of this valve body projecting upward past the electromagnet  54 ), as shown in  FIG. 4 . In this position, the spring  88  is biased to a maximum degree and a highest possible opening pressure of the pressure-relief valve  54  is set. This highest possible opening pressure is set to a desired value by the adjustment screw  92  on the manufacturer side and then the adjustment screw  92  is fixed in the body  78 , for example, by adhesive or a coating, and then covered from the outside by the cover  94 , in order to discourage unnecessary later adjustment. In  FIG. 4 , the pressure-relief valve  54  is shown in the opened position, i.e., an inlet  66  pressurized sufficiently for opening. 
     In the normal operation of the baler, the electronic controller  56  is acted upon with signals with reference to which it specifies the pressure in the piston rod space of the hydraulic cylinder  38  on the electromagnet  58 . Input parameters that could be used include, for example, the pick-up rate of the crop that can be detected, e.g., on the drive of the pick-up, or the bale size, or a contact pressure specified by the operator. For this purpose, refer to the disclosure of DE 197 18 229 A1. The electromagnet  58  pulls the armature  74  and the closure element  64  as a function of the size of the electrical current through the electromagnet  58  and/or its pulse width that depend, in turn, on the defaults by the controller  56 , more or less in the direction toward the passage  62 , while the spring  88  has no influence on the tappet  72  and the closure element  64 . As soon as the hydraulic pressure in the passage  62  is greater than the pressure exerted by the armature on the closure element  64 , the pressure-relief valve  54  opens. In this way, the pressure on the lines  50 ,  52  can be changed by the valve  46  that can be displaced, for pressure increase in the line  52 , toward the right from the home position shown in  FIG. 2  and, for pressure reduction in the line  52 , toward the left from the home position shown in  FIG. 2 . The tension of the pressing means  22  is consequently controlled by the controller  56 . 
     Now, if an error occurred in the electronic controller  56  or a sensor connected to this controller, it would be disadvantageous if the round baler  10  could no longer be used. In this case, the operator could detach the electromagnet  58  from the pressure-relief valve  54 , as described above with reference to  FIG. 4 , and set the opening pressure of the pressure-relief valve  54  by hand, as shown in  FIG. 4 . In this way, the full adjustment range of the opening pressure up to the maximum opening pressure specified on the manufacturer side by the position of the adjustment screw  92  is provided to the operator. The incorrectly driven electromagnet  58  can no longer affect the opening pressure. After a repair, the situation shown in  FIG. 3  can be reestablished. 
     Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.