Abstract:
In order to reduce the shock that occurs in downshifting and up-shifting a hydraulic radial piston engine, a valve ( 1 ), which controls the displacement of the radial piston engine, is reversed with throttling. In order to prevent the throttled reversal from being affected by the level of pressure in the hydraulic fluid intake ( 3 ) to the hydraulic radial piston engine, a sealing point is provided in the valve ( 1 ), which controls the hydraulic fluid intake ( 3 ) to the torque-generating pistons. To prevent critical safety situations from arising, a valve ( 24 ) is positioned in front of the valve ( 1 ) that controls the displacement of the hydraulic radial piston engine; this valve ( 24 ) always switches the valve ( 1 ) that controls the displacement of the hydraulic radial piston engine, without pressure, when the hydraulic fluid intake ( 3 ) from a hydraulic fluid source to the hydraulic radial piston engine falls below a certain pressure level.

Description:
FIELD OF THE INVENTION 
     The invention relates to a hydraulic radial piston engine. 
     BACKGROUND OF THE INVENTION 
     Radial piston engines, preferably externally mounted, slow-running radial piston engines that generate high levels of torque can be designed as dual-range engines, in which all the torque-generating pistons can be acted upon by hydraulic fluid from a pump, hence the hydraulic engine is operated at maximum absorption volume, at which it will produce a maximum amount of torque, while the rpm is decreased; on the other hand, it is also possible to operate the hydraulic engine with only a portion of the torque-generating pistons, in which a portion of the torque-generating pistons remain connected to the flow of hydraulic fluid from the pump, while another portion of the torque-generating pistons are connected to the flow of hydraulic fluid back to the pump or to the hydraulic fluid reservoir. In this operating condition the hydraulic engine generates a lower level of torque at the same level of pressure, however it can generate a higher rpm while the capacity of the pump remains the same. 
     In German Patent Specification 19 05 455, a hydrostatic radial piston engine is disclosed, which is equipped with a valve that connects the flow of hydraulic fluid from the pump to all the torque-generating pistons; when this valve is reversed, a portion of the torque-generating pistons become connected to the return flow of hydraulic fluid, hence the hydraulic engine is operating with only a portion of the torque-producing pistons, causing it to generate a higher rpm and lower torque. The valve is switched over by hydraulic fluid acting on a piston of the valve, shifting it against the force of a spring, or if the piston is pre-stressed against the spring, the valve is switched over in that the hydraulic fluid that acts on the piston is connected to a hydraulic fluid reservoir, hence the spring forces the piston back to its initial position. With transmissions of this type, a portion of the torque-generating pistons is suddenly connected to either the hydraulic fluid intake or the hydraulic fluid return flow. Since the speed of the drive is not able to adjust to this new situation as quickly, a severe shifting shock is created 
     In DE 196 37 570 A1 a hydrostatic-mechanical wheel drive is disclosed, in which a hydraulic radial piston engine drives an inner, central gear of a planetary gear set. The hydraulic piston engine can be slowed via a brake, which is hydraulically lifted and is closed via spring resistance, and always becomes engaged when the hydraulic fluid in the fluid intake drops below a certain pressure level. In order to keep the brake from becoming engaged if the pressure level in the hydraulic fluid intake drops briefly below the predefined pressure level, the spring must force the hydraulic fluid against a throttle. If a hydraulic engine is used to drive a mobile construction vehicle, such as an excavator, as is disclosed in DE 196 37 570 A1, critical safety situations may result if an automatic valve control system is used to connect some or all of the torque-generating pistons to the hydraulic fluid intake. If the hydraulic drive system is equipped with an automatic shifting device, which, when a previously established level of pressure in the intake of hydraulic fluid from the hydraulic pump to the hydraulic engine is exceeded, shifts the displacement switchover valve such that all of the torque-generating pistons are connected to the hydraulic fluid intake, and, if the hydraulic fluid intake falls below this previously established level of pressure, the shifting device reverses the displacement switchover valve so that only some of the torque-generating pistons are connected to the hydraulic fluid intake, then when the vehicle is traveling uphill, when the level of pressure in the hydraulic fluid intake is above the established pressure level, the hydraulic engine will operate at its maximum absorption volume. The displacement switchover valve is thus reversed, such that all of the torque-generating pistons are connected to the hydraulic fluid intake. If the driver reduces the flow of hydraulic-fluid from the pump to the hydraulic engine, in order to come to a stop on an incline, then the level of pressure in the hydraulic fluid intake to the engine will drop, so that the displacement switchover valve is reversed, and only a portion of the torque-generating pistons are connected to the hydraulic fluid intake. Because the brake is opened by force of pressure and closed by force of a spring, when the brake is closed, which occurs automatically when the pressure in the intake to the hydraulic engine drops below a predefined pressure level, then hydraulic fluid must be forced out of the brake actuating device. In order to prevent the brake from becoming engaged automatically with only a very brief drop in pressure, the hydraulic fluid in the brake-actuating device must be forced against a throttle position, causing the brake to engage at a delay. However, because switching the hydraulic fluid intake from all of the torque-generating pistons to only a portion of the torque-generating pistons creates a situation in the displacement switchover valve in which all of the torque-generating pistons are directly connected to the hydraulic fluid return flow, but the brake has not become engaged, the construction vehicle will roll down the hill until the brake is engaged, or until the connection is closed again. This can lead to critical safety situations. If the construction vehicle comprising an automatic shifting device, in the case of which below a certain pressure level only a portion of the torque-generating pistons are connected to the hydraulic fluid intake, moves downhill and if the driver of the construction vehicle reduces the flow of hydraulic fluid to the hydraulic engine in order to brake the vehicle, then the hydraulic engine is able to generate a maximum braking moment with only a portion of the torque-generating pistons with the pressure level of the brake valve or pressure limiting valve. This can result in a critical safety situation. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to create a hydraulic radial piston engine in which the shifting shock that occurs when the hydraulic fluid intake is switched from all of the torque-generating pistons to a portion of the torque-generating pistons, or when the hydraulic fluid intake is switched from a portion of the torque-generating pistons to all of the torque-generating pistons, is reduced, and in which no critical safety situations can occur in the operation of the hydraulic radial piston engine. 
     According to the invention, the valve that connects the torque-generating pistons to the hydraulic fluid intake is actuated via hydraulic fluid, the inward and outward flow of which is throttled, hence the throttled valve is forced against a spring tension, or the throttled valve is forced by the spring tension against the hydraulic fluid that is flowing off. Because the valve controls the inflow of hydraulic fluid to the pistons, and is itself reversed by throttled hydraulic fluid flowing in or out, a sealing point must be positioned between the hydraulic fluid intake to the hydraulic engine or the pistons and the actuating chamber for the hydraulic fluid of the valve, since otherwise hydraulic fluid could drain out of the hydraulic fluid intake to the hydraulic engine or to the pistons, via a gap in the valve actuating chamber, and could thus negatively affect the throttled actuation of the valve. The throttling point in the intake and the return for actuating the valve that connects the torque-generating pistons to the hydraulic fluid supply can have a defined value, or can be adjustable based upon operating parameters, such as the temperature of the hydraulic fluid or rpm. Preferably, the piston of the valve that connects the torque-generating pistons with the hydraulic fluid intake is designed such that when the valve is reversed from a first position to a second position, the cross-section, which connects or divides the two portions of torque-generating pistons, changes almost evenly over the shifting path of the piston, so that no sudden change in the cross-section of the flow of hydraulic fluid will occur; hence, undesirable shifting pressure is prevented. This is preferably realized in that the sides of the connecting grooves in the valve piston are designed as tapered surfaces, or are equipped with grooves, which are shaped such that the cross-section changes almost evenly with the shifting of the piston. The valve piston must be designed such that the cross-section changes nearly evenly with a shift from all of the torque-generating pistons to a portion of the torque-generating pistons, and with a shift from a portion of the torque-generating pistons to all of the torque-generating pistons. The valve that connects the torque-generating pistons to the hydraulic fluid intake is preferably positioned coaxially in a hydraulic fluid intake that supplies hydraulic fluid to a piston carrier. Thus the valve is housed in a structural chamber of the hydraulic radial piston engine that otherwise would be unused. It is also possible, however, for the valve that connects the torque-generating pistons with the hydraulic fluid intake to be positioned outside of the radial piston engine in order to reverse the displacement of the hydraulic radial piston engine. The valve that connects the torque-generating pistons with the hydraulic fluid intake is preferably reversed via spring tension such that when the flow of hydraulic fluid to the hydraulic radial piston engine drops below a certain pressure level, all of the torque-generating pistons become connected to the hydraulic fluid intake. This serves to ensure that, if there is a defect in the hydraulic fluid intake, the hydraulic radial piston engine can be operated with all of its torque-generating pistons, and thus at its maximum torque for the purpose of reversing the valve. Preferably, the hydraulic radial piston engine is connected via a planetary gear set to a wheel drive for a tracked vehicle, preferably an excavator, however it can also be directly connected to the driven shaft. This serves to ensure that with a compact wheel drive, sufficient torque will be generated. Because the piston carrier of the hydraulic engine is connected to a brake, the braking torque is intensified via the planet stage. To ensure that no critical safety situations can occur, a valve is positioned in front of the hydraulic fluid intake to the valve that is used to reverse the displacement of the radial piston engine; this additional valve is held in an initial position by force of spring tension, until the pressure in the hydraulic fluid intake to the hydraulic radial piston engine exceeds a certain level. When this valve is in its initial position, the hydraulic fluid intake for reversing the valve designed to connect the torque-generating pistons to the supply of hydraulic fluid is interrupted. This serves to ensure that below a certain pressure level in the hydraulic fluid intake to the hydraulic engine, all of the torque-generating pistons are constantly connected to the hydraulic fluid intake. This ensures that in the case of an intentional braking process when the vehicle is traveling downhill the hydraulic engine will brake with all of the torque-generating pistons against the brake valve or the pressure control valve. If the vehicle is to be stopped while driving uphill, and if in driving up the hill all of the torque-generating pistons are connected to the supply of hydraulic fluid in order to generate maximum torque, then even when the vehicle is held in a stationary position on a hill, all of the torque-generating pistons will remain connected to the hydraulic fluid intake, without causing an undesired shift, thus keeping the vehicle stationary on the hill until the brake has been engaged. In this, the level of pressure required to actuate the valve to reverse the displacement switchover valve is above the level of pressure required to lift the brake. Because the valve for reversing the valve that connects the torque-generating pistons with the hydraulic fluid supply is positioned in the hydraulic fluid intake, any shifting while the vehicle is stopped on the hill is prevented, independent of a manual or automatic shift command by the driver. Because this valve is positioned in front of the valve that connects the torque-generating pistons to the hydraulic fluid intake, this valve is preferably equipped with the throttle points through which the hydraulic fluid flows, in order to reverse the valve that connects the torque-generating pistons with the hydraulic fluid intake. If the hydraulic radial piston engine has a brake, then the pressure that lifts the hydraulic brake is also preferably used to reverse the valve that is positioned in front of the valve that connects the torque-generating pistons to the hydraulic fluid intake. This offers the advantage that no additional valve that constantly stems the flow of hydraulic fluid is necessary. 
     Because the flow of hydraulic fluid that controls the valve that connects the torque-generating pistons to the hydraulic fluid intake is throttled in its inward and its outward flow, and because it is guaranteed via a sealing point that this throttled inward and outward flow will not be negatively affected by any external hydraulic fluid, any undesirable shifting shock is prevented with an upward or downward shift. Because the valve that connects the torque-generating pistons to the hydraulic fluid intake is preceded by a valve, which, when the flow of hydraulic fluid to the hydraulic engine drops below a certain pressure level, interrupts the flow of hydraulic fluid that controls the valve that connects the torque-generating pistons to the hydraulic fluid supply, it can be ensured that, when the flow of hydraulic fluid to the hydraulic engine drops below a certain pressure level, all of the torque-generating pistons will be connected to the flow of hydraulic fluid to the hydraulic engine. In this manner, critical safety conditions are prevented. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example, with reference to the accompanying drawings in which: 
     FIG. 1 a cross-section of a hydraulic engine; 
     FIG. 2 a hydraulic diagram of the hydraulic engine; 
     FIG. 3 a diagram illustrating a shift in the connection of the flow of hydraulic fluid from all the torque-generating pistons to a portion of the torque-generating pistons, and 
     FIG. 4 a diagram illustrating a shift in the connection of the flow of hydraulic fluid from a portion of the torque-generating pistons to all the torque-generating pistons. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG.  1 : 
     A valve  1  that connects the torque-generating pistons  2  to the hydraulic fluid intake  3  is held in its initial position by force of a spring  4 , below a certain pressure level. In this initial position, a connection in the hydraulic fluid divider  6  is produced via a recess  5 , in which a first portion  7  of the torque-generating pistons and a second portion  8  of the torque-generating pistons are supplied with hydraulic fluid, so that all of the torque-generating pistons are connected to the hydraulic fluid intake  3 . In this position, the hydraulic engine generates its maximum torque. The piston carrier  9  can be held in a stationary position, without rotation, by a brake  10 . When hydraulic fluid flows into the chamber  11 , the piston  12  of the valve  1  is shifted against the force of the spring  4 , and the second portion  8  of the torque-generating pistons are connected to the hydraulic fluid return flow  13 , so that only the first portion  7  of the torque-generating pistons are acted upon by hydraulic fluid, and will generate torque. If a sealing point  14  is positioned between the hydraulic fluid intake  3  and the chamber  11 , hydraulic fluid can flow throttled into and out of the chamber  11 , thus controlling the movement of the piston  12 , without any negative effects from additional hydraulic fluid from the hydraulic fluid intake  3 , which could otherwise enter the chamber  11 . The sides  15  of the piston  12  are designed such that the cross-section  16  and the cross-section  17  change nearly evenly with the movement of the piston  12 . The throttled shifting of the piston  12  and the nearly even change in the cross-sections  16  and  17  serve to prevent any shifting shock. If the hydraulic radial piston engine is to be moved in the opposite direction, then the hydraulic fluid return flow  13  and the hydraulic fluid intake  3  are reversed, so that the hydraulic fluid flows into the hydraulic fluid return flow  13 , and flows out via the hydraulic fluid intake  3 . 
     FIG.  2 : 
     Hydraulic fluid flows into the hydraulic engine  18  via the hydraulic fluid intake  3 , and flows back from the hydraulic engine  18  via the hydraulic fluid outlet  13 . Via a valve  19 , a valve  20  is constantly connected to the hydraulic fluid intake, wherein the valve  20  serves to control the level of pressure. Via the line  21 , when there is existing pressure on the hydraulic fluid intake  3 , the brake  22  is lifted and thus disengaged. Via a line  23 , a valve  24  is controlled, and a line  25  is connected to a chamber  11  of a valve  1 , which connects torque-generating pistons to the hydraulic fluid intake  3 , thus controlling the absorption volume of the hydraulic engine  18 . In the valve  24  a throttle  26  is in the intake position and a throttle  27  is in the outlet position. The throttle  26  is preferably larger than the throttle  27 , in order to delay shifting from maximum absorption volume to minimum absorption volume during braking. The valve  1  is controlled such that when the chamber  11  is free from pressure, the spring  4  forces the piston  12  into its initial position, hence all the torque-generating pistons of the hydraulic radial piston engine  18  are connected to the hydraulic fluid intake  3 . An automatic valve may be connected to the line  25 , which, when the hydraulic fluid intake  3  drops below a certain pressure level, supplies the line  25  with hydraulic fluid, thus reversing the valve  1 , so that only a portion of the torque-generating pistons are connected to the hydraulic fluid intake  3 , thereby turning the hydraulic engine  18  at a higher rpm, and generating lower torque; when the level of pressure in the hydraulic fluid intake  3  rises above a certain level, this automatic valve then shifts the line  25  without pressure, in order to reverse the valve  1 , so that all of the torque-generating pistons are connected to the hydraulic fluid intake  3 , hence the hydraulic engine  18  generates a maximum torque, but a lower rpm. It is also possible for the line  25  to be connected to a manual shift device, via which the driver can fill the line  25  with hydraulic fluid, or switch the pressure off. Because the valve  24  is positioned in front of the valve  1 , critical safety situations are prevented. If the vehicle is equipped with an automatic shifting device and the vehicle is traveling uphill at maximum absorption volume for the hydraulic engine  18 , then the valve  24  will be switched such that the line  25  is connected to the valve  1 . As a result of the automatic shifting that takes place when the pressure in the hydraulic fluid intake  3  is above a certain level, the line  25  is held without pressure so that the valve  1  in its initial position connects all torque-generating pistons with the hydraulic fluid intake  3  via the spring  4 . If the vehicle is to be held in a stationary position on an incline, then the flow of hydraulic fluid at the hydraulic fluid intake  3  is reduced until the hydraulic fluid intake  3  is nearly without pressure. Because a drop in pressure of the hydraulic fluid at the hydraulic fluid intake  3  below a certain level causes the automatic shifting to supply the line  25  with hydraulic fluid, then without the valve  24 , the valve  1  would be reversed, and the vehicle would begin to move downhill until the brake  22  was engaged, or until the valve  1  was reversed to interrupt the connection of the hydraulic fluid intake  3  to the hydraulic fluid outlet  13 . Because the valve  24  is positioned in front of the valve  1 , when the pressure at the hydraulic fluid intake  3  drops below a certain level, the valve  24  is reversed to its initial position, and the line  25  to the valve  1  is interrupted, so that the valve  1  remains in its initial position and is not reversed. In this manner, the vehicle can be held on an incline, until the brake  22  has been engaged. If the vehicle is traveling downhill at minimum absorption volume, i.e. with a pressurized valve  1 , and the hydraulic fluid intake  3  is reduced in order to allow the vehicle to come to a stop, then the valve  24  will reverse below a certain pressure level, and the valve  1  will shift into its initial position, so that the vehicle can be braked with the maximum absorption volume i.e. using all of the torque-generating pistons. Critical safety situations are thus eliminated. In order to change the direction of travel of the vehicle, the hydraulic fluid pump, not illustrated here, is connected to the hydraulic fluid return flow  13 , whereby the hydraulic fluid return flow  13  becomes the hydraulic fluid intake, and the hydraulic fluid intake  3  becomes connected to the hydraulic fluid return flow, hence the hydraulic fluid will flow out via the hydraulic fluid intake  3 . Because the throttles  26  and  27  are integrated into the valve  24 , the hydraulic radial piston engine can be realized without critical safety situations, and an undesirable shifting shock can be prevented. A valve  38  is preferably positioned between the line  25  and the line  39 , with this valve connecting the line  25 , unthrottled, with the line  39 , above a certain pressure level. The valve  38  is actuated against the force of a spring by the pressure from the hydraulic fluid intake  3 . The valve  38  is designed such that the level of pressure required to actuate the valve  38  lies below the level of pressure for the automatic switchover valve, not illustrated here, so that if shifting is to take place on a hill at a low rpm and high intake pressure, unthrottled shifting can be achieved. 
     FIG.  3 : 
     The line  28  represents the change in the cross-section  16  in FIG. 1, which changes over the stroke of the piston  12 . The line  29  represents the change in the cross-section  17  in FIG. 1, which changes over the stroke of the piston  12 . The line  30  represents the change in rpm of the hydraulic engine with a change in the cross-sections  16  and  17  caused by the shifting of the piston  12 . The line  31  represents the output torque of the hydraulic engine shown in FIG. 1, for the torque-generating pistons over the change in the cross-sections  16  and  17  caused by a shifting of the piston  12 . The line  32  represents the pressure in the second portion  8  of torque-generating pistons shown in FIG. 1, over the change in the cross-sections  16  and  17  caused by a shifting of the piston  12 . The line  33  represents the pressure at the hydraulic fluid intake  3  over the change in the cross-sections  16  and  17  caused by a shifting of the piston  12 . The line  34  represents the pressure at the hydraulic fluid return flow  13  over the change in the cross-sections  16  and  17  caused by a shifting of the piston  12 . At the start of the shift, the piston  12  is in its initial position; it is then pressed, throttled, against the tension of the spring  4 . The cross-section  16 , the change in which is indicated by the line  28 , is completely open at the start of the shift. The cross-section  17 , the change in which is indicated by the line  29 , is completely closed at the start of the shift. Starting with the shifting path  35 , the cross-section  16  is reduced, and the cross-section  17  is expanded, so that the second portion  8  of the torque-generating pistons is connected almost evenly to the hydraulic fluid return flow. This causes the torque of the line  32  to drop to the line  34 . Starting with the shifting path  36 , the second portion  8  of torque-generating pistons is connected completely to the hydraulic fluid return flow  13 , and is separated from the hydraulic fluid intake  3 . 
     FIG.  4 : 
     The line  28  represents the change in the cross-section  16  shown in FIG. 1, which changes over the stroke of the piston  12 . The line  29  represents the change in the cross-section  17  shown in FIG. 1, which changes over the stroke of the piston  12 . The line  30  represents the change in rpm of the hydraulic engine with a change in the cross-sections  16  and  17  caused by a shifting of the piston  1 . The line  37  represents the output torque of the hydraulic engine over the change in the cross-sections  16  and  17  caused by a shifting of the piston  12 . The line  33  represents the pressure at the hydraulic fluid intake  3  in the hydraulic engine, over the change in the cross-sections  16  and  17  caused by the shifting of the piston  12 . The line  34  represents the pressure at the hydraulic fluid return flow  13  in the hydraulic engine, over the change in the cross-sections  16  and  17  caused by a shifting of the piston  12 . At the start of the shift, the piston  12  is in its final position; it is then shifted, throttled, by the tension of the spring  4 . The cross-section  16 , the change in which is indicated by the line  28 , is completely closed at the start of the shift. The cross-section  17 , the change in which is indicated by the line  29 , is completely open at the start of the shift. Starting with the shifting path  35 , the cross-section  17  decreases, while the cross-section  16  increases, so that the second portion  8  of the torque-generating pistons is connected nearly evenly to the hydraulic fluid intake. This causes the torque of the line  32  to drop to the line  34 . Starting with the shifting path  36 , the second portion  8  of the torque-generating pistons is completely connected to the hydraulic fluid intake  3 , and is separated from the hydraulic fluid return flow  13 . 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 Reference Numbers 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                  1 valve 
                 21 line 
               
               
                   
                  2 torque-generating piston 
                 22 brake 
               
               
                   
                  3 hydraulic fluid intake 
                 23 line 
               
               
                   
                  4 spring 
                 24 valve 
               
               
                   
                  5 recess 
                 25 line 
               
               
                   
                  6 hydraulic fluid divider 
                 26 throttle 
               
               
                   
                  7 first portion 
                 27 throttle 
               
               
                   
                  8 second portion 
                 28 line 
               
               
                   
                  9 piston carrier 
                 29 line 
               
               
                   
                 10 brake 
                 30 line 
               
               
                   
                 11 chamber 
                 31 line 
               
               
                   
                 12 piston 
                 32 line 
               
               
                   
                 13 hydraulic fluid return flow/outlet 
                 33 line 
               
               
                   
                 14 sealing point 
                 34 line 
               
               
                   
                 15 side 
                 35 shifting path 
               
               
                   
                 16 cross-section 
                 36 shifting path 
               
               
                   
                 17 cross-section 
                 37 line 
               
               
                   
                 18 hydraulic engine 
                 38 valve 
               
               
                   
                 19 hydraulic engine 
                 39 line 
               
               
                   
                 19 valve 
               
               
                   
                 20 valve