DE3836453A1 - Valve arrangement for controlling and monitoring the working pressure of a hydraulic consumer - Google Patents

Abstract

A valve arrangement (10) for controlling and monitoring the working pressure in a hydraulic clamping cylinder (11) of machine tools is proposed which in the event of a voltage loss maintains the actual set pressure value or increases the clamping pressure to the accumulator pressure value. The valve arrangement (10) has a control valve (39), the longitudinal spool (15) of which controls the functions of directional control and pressure control, is driven by a rotary stepping motor (18) via a threaded connection (23) and lies in a pressure-control circuit (37) with an electrohydraulic pressure sensor (33) measuring the clamping pressure. In the event of a voltage loss, the longitudinal spool (15) remains in its instantaneous position and prevents a pressure drop in the clamping cylinder (11). In NC control systems the valve arrangement (10) can be operated as an actuator for controlling speed and pressure programs. <IMAGE>

Description

State of the art

The invention relates to a valve arrangement for controlling and monitoring the working pressure of a hydraulic consumer according to the preamble of the main claim. It is already such a valve arrangement for a consumer, e.g. B the tailstock or the chuck ter of a machine tool, known from DE-OS 32 49 806. In this valve arrangement, the control oil flow flowing from a pilot-controlled Druckmin derventil is passed via a flow indicator which, when the pressure falls below a predetermined working pressure, supplies an electrical signal for switching off a machine. Nachtei lig is that the pressure control is carried out by making an indirect pressure measurement of a single switching point. Furthermore, a constant control oil flow to the tank is required in this valve arrangement, which leads to undesirable energy losses. Above all, the known valve assembly builds relatively expensive because it requires a number of individual devices for the various functions. So a 4/2-way valve is provided for direction control of the consumer, while a pressure regulating, separate pressure reducing valve is arranged to limit the clamping force. Furthermore, an additional pressure switch is required for clamping pressure control, while additional throttle check valves are usually required for speed settings. The large number of such individual devices leads to a high construction cost. Furthermore, such a valve arrangement has the disadvantage that, in the event of a power failure or in the event of a failure of the pressure medium pump, the pressure reducing valve usually opens, so that either the maximum contact pressure is passed to the tensioning cylinder or this is relieved to the tank. While damage to the clamped workpiece can easily occur at maximum contact pressure, an undesirable drop in the clamping pressure leads to loosening of the clamping device and thus to a corresponding accident risk.

Similarly complex valve arrangements for hydraulic clamping devices are from the publications DE-OS 32 16 683, 33 13 381, 34 18 261 known.

Furthermore, DE-OS 22 04 186 is an electrohydraulic step known motor that works with a longitudinal slide. This longitudinal slide is at one end with an electromechanical Rotary drive in connection, while at its other end a Rotating movement into a longitudinal movement converting screw connection is arranged. In this stepper motor, the screw connection forms a mechanical return from an output shaft Axial piston pump. Control and monitoring of the working pressure a hydraulic consumer is not provided here.

Advantages of the invention

The valve arrangement according to the invention for control and monitoring the working pressure of a hydraulic consumer with the ident The drawing features of the main claim have the opposite part that they have an electrically controllable clamping cylinder control enables the high safety requirements in the event of a power failure  is enough. The respective current in the valve arrangement le position of the control element can be maintained. This will result in egg In a number of positions, the pressure setting value is maintained or one maximum pressurization reached, so that a Druckab case avoided and thus an unintentional loosening of the clamping cylinder which is prevented by a drop in pressure. With the help of the pressure sensor the respective pressure is measured continuously and directly and after Required regulated accordingly, so that the valve arrangement FITS suitable for NC controls. Since practically all functions in one control can be realized in a single device short reaction times of pressure changes. This valve also leads arrangement for a particularly space-saving and inexpensive solder solution.

The measures listed in the subclaims provide for partial training and improvements in the main claim specified valve arrangement possible. So with an apprenticeship according to claim 2, the control and monitoring of the working pressure in two consumer connections are made. Extremely useful is moderate training according to claim 3, which is easier and cheaply a sensitive drive of the control element can do that, moreover through the self-locking of the wall of movement lers steplessly lockable in any position. Start claim 4 a particularly compact and simple design. According to An Say 5, the functions for pressure control and Direction control for both consumer connections in one Integrate gene component, both an internal clamping as well an external clamping is controllable. In an education according to claim 6 particularly high security requirements are met.

Particularly high demands on security can be relatively simple means if, according to claims 11 to 14 Ma solenoid valve means are provided, which in the event of a power failure  Also block the clamping cylinder so that even with a control valve til, the longitudinal slider of which just one the power failure Pressure-reducing position assumes an unwanted loosening of the Clamping cylinder is prevented.

It is also advantageous, according to claim 15, this high security safety requirements by using a small battery Control movement from a pressure reduction position to the neighboring, blocking zero position is achieved.

It is also particularly cheap and simple, according to claim 16 a special concept of pressure control to ensure that a Pressure reduction position in the clamping pressure rule as a rule is not controlled, but the leakage oil that occurs in the control valve til is used to lower the pressure.

Further advantageous configurations result from the others Claims, the description and the drawing.

drawing

An embodiment of the invention is shown in the drawing represents and explained in more detail in the following description. It shows

Fig. 1 shows a longitudinal section through a valve assembly for controlling and monitoring the working pressure of a hydraulic consumer, and Fig. 2 is a corresponding diagram of the Ventilan order of Fig. 1. Fig. 3 shows a second valve arrangement with an intermediate plate valve partially in longitudinal section and Fig. 4 shows the associated circuit diagram. Fig. 5 shows the circuit diagram of a two-th intermediate plate valve. Fig. 6 shows the circuit diagram of a third valve arrangement with an auxiliary power source and Fig. 7 shows the circuit diagram of a fourth valve arrangement with a special pressure control concept.

Description of the embodiments

Fig. 1 shows a valve assembly 10 for controlling and monitoring the working pressure of a hydraulic consumer, as shown in Fig. 2 as a clamping cylinder 11 .

The valve arrangement 10 has a valve housing 12 which receives a valve sleeve 14 in a continuous longitudinal bore 13 . In the valve sleeve 14 a longitudinal slide 15 is tightly and slidably guided as a control member, which can perform both rotary and longitudinal movements.

The valve housing 12 is covered on its one end face 16 by a clutch housing 17 , to which a stepping motor 18 is flanged coaxially to the longitudinal axis of the longitudinal slide 15 . The stepper motor 18 is designed as an electromechanical rotary drive, the output-side output shaft 19 forms part of a clutch 21 which is arranged within the clutch housing 17 and transmits rotational movements of the output shaft 19 without play to the longitudinal slide 15 , the longitudinal slide 15 relative to the output shaft 19 axial movements can perform. The longitudinal slide 15 is thus with its protruding into the clutch housing 17 end via a rotation movements only transmitting clutch 21 with the stepper motor 18 in operative connection.

On an opposite end 20 of the clutch housing 17 , the longitudinal bore 13 in the valve housing is closed by a cover 22 ver. An inwardly projecting end of the cover 22 forms an extension of the longitudinal slide 15, a threaded connection 23, which converts a rotational movement at the longitudinal slide 15 in a to ent speaking longitudinal movement. Instead of this particularly a simple threaded connection 23 , another suitable device for converting motion can also be used.

In the valve housing 12 , all work connections are led out in the form of a standardized hole pattern in a lower flange surface 24 , so that there is a first consumer connection 27 and a second consumer connection 28 in addition to an inlet connection 25 and a tank connection 26 . In the upper area of the valve housing 12 , a shuttle valve 29 is connected between the two consumer connections 27 , 28 , with an additional test connection 31 and 32 being provided on each of the consumer connections 27 , 28 for mounting a manometer. The pressure from a center connection 30 of the shuttle valve 29 is fed to the input of an electrohydraulic pressure sensor 33 , which forwards its electrical signals to an electronic control device 34 . The pressure sensor 33 and the control device 34 are thus mounted on an upper side 35 of the valve housing 12 and protected against external influences by a cover 36 .

As FIG. 2 shows in more detail, the pressure sensor 33 and the stepping motor 18 are in an electro-hydraulic control circuit 37, which serves as actual values of the pressure signals with the electrical on an input compares 38 PRE-setpoint that signals in the electronic controller 34 and, depending on the Stepper motor 18 controls so that the control deviation becomes zero. As can be seen from Fig. 2 in more detail, the part of a control valve 39, the longitudinal slide 15 taking at least five positions 41 to 45 , of which in a middle zero position 41 all work connections 25 to 28 are hydraulically blocked. The longitudinal slide 15 is also designed such that it has a first ( 42 ) or second control position 43 on both sides of the zero position 41 for controlling the clamping pressure in the clamping cylinder 11 . Subsequent to the re gel positions 42 , 43 each follow a first ( 44 ) or second working position 45 to the outside, which serve for the actual directional control of the piston 46 in the clamping cylinder 11 . While in the two working positions 44 , 45 the connection from the inlet connection 25 to that consumer connection in which the pressure is to be regulated is turned on, in the respectively adjacent control position 42 or 43 this connection to the inlet connection 25 is continuous and thus proportional from each steering of the longitudinal slide 15 more or less throttled, this connection finally being completely blocked in a control position near the zero position. The control positions 42 , 43 thus extend over a certain stroke range, in which the longitudinal slide 15 works like a proportional throttle valve.

As is also apparent from FIG. 2, the inlet connection 25 is connected via a check valve 47 to a pump 48 , which is additionally assigned a hydraulic accumulator 49 .

The operation of the valve arrangement 10 is explained as follows, reference being made to FIGS. 1 and 2 at the same time.

In the valve arrangement 10 , the control valve 39 driven by the stepper motor 18 can take over both the functions of the directional control and the pressure control for the tensioning cylinder 11 in both directions. By entering corresponding setpoint signals according to size and direction at the electrical input 38 , the longitudinal slide 15 is optionally adjustable in its outer working positions 44 or 45 , whereby in the first working position 44 the piston rod 51 of the tensioning cylinder 11 can be extended while it is shown in the two shown Working position 45 is retractable. The geometry of the longitudinal slide bers 15 is designed such that, in addition to this direction control, the speed function is also taken over. The working positions 44 , 45 thus extend over a limited stroke range of the longitudinal slide 15 , in which the control valve 39 works proportionally, so that the clamping cylinder 11 is continuously adjustable in both directions at different speeds.

For pressure control in opposite rooms of the clamping cylinder 11 , the control valve 39 can be moved into its control positions 42 or 43 by corresponding electrical signal input. If, for example, the clamping pressure in the second consumer connection 28 is to be regulated, the longitudinal slide 15 is adjusted from its drawn second working position 45 in the direction of the second control position 43 , the relief of the first consumer connection 27 to the tank connection 26 remaining open. however, the connection between the second consumer port 28 and inlet port 25 is more or less throttled or in the limit case also controls or in position 42 an excessive pressure in the tensioning cylinder 11 is reduced. The pressure prevailing in the second consumer connection 28 is conducted via the shuttle valve 29 to the hydraulic input of the pressure sensor 33 . This pressure sensor 33 gives a corresponding actual value signal via the control circuit 37 to the electronic control device 34 , where it is compared with the predetermined setpoint signal and the control valve 39 is controlled as a function thereof. Depending on the size and polarity of the electrical input signal to the stepper motor 18 , this will execute a motion that is proportional to the direction of rotation and angle of rotation at the output shaft 19 , which is transmitted by the coupling 21 to the longitudinal slide 15 , which due to its threaded connection 23 is one of them will perform proportional adjusting movement in its longitudinal axis. If the pressure sensor 33 reports a pressure that is too high compared to the target value 38 , the longitudinal slide 15 will throttle the connection to the inlet connection 25 until the control deviation becomes zero. Conversely, the connection mentioned is opened accordingly if the pressure is too low. This actuation of the longitudinal slide 15 with the aid of a stepping motor 18 and a threaded connection 23 allows a sensitive longitudinal adjustment, the longitudinal slide 15 being practically infinitely lockable in each position, so that it always pulses into its regulating positions 42 and 43, respectively, to the electrical signals in electrical mode maintains a proportional position on the stepper motor 18 .

If the clamping pressure in the first consumer connection 27 is regulated, the control valve 39 is moved in a corresponding manner into its first control position 42 , the connection to the inlet connection 25 also being able to be throttled accordingly.

In the present valve arrangement 10 , the functions of direction control, speed control and pressure control function in the tension cylinder 11 for both cylinder sides 27 , 28 are thus fulfilled with one and the same control valve 39 . The pressure to be checked and regulated is measured directly and continuously by the pressure transmitter 33 and its signals are evaluated electronically. A large number of individual hydraulic components can therefore be dispensed with. With the aid of the pressure sensor 33 and the stepping motor 18 , the valve arrangement 10 is particularly suitable for use in NC controls. It can be used as an actuator in a pressure control loop and for setting the direction and speed. Programmed pressure setpoints can also be kept constant in a controlled manner. This makes it particularly easy to implement pressure programs if, for example, a two-pressure clamping with pressure change within a clamping cycle is desired or different clamping and expansion pressures are to be used. Functional requirements that were previously shown hydraulically and mechanically, in particular different pressures for tension pressure monitoring and tension pressure adjustment, can now be represented electronically. At the same time, programmed setpoints for the cylinder driving speed and for a speed program are realized in an NC control with the aid of the valve arrangement 10 , so that, for example, a slow closing and a fast opening stroke can be carried out. In addition, the control valve 39 is suitable for universal use for internal and external clamping.

A particular advantage of the valve arrangement 10 can be seen in the fact that the electrical stepping motor 18 in conjunction with the movement transducer 23 for adjusting the longitudinal slide 15 ensures that the current slide position is retained in the event of a power failure. In the event of a power failure, the control valve 39 remains in its respective position within the control positions 42 and 43 , so that the respectively controlled throttle cross section does not change and the charged hydraulic accumulator 49 maintains the respective clamping force.

If the control valve 39 is in the event of a power failure, for example in the working position 44 , with tensioning being assumed when the tensioning cylinder 11 extends, the accumulator pressure, which is greater than the tensioning pressure, builds up via the open control valve 39 and the first consumer connection 27 in the clamping cylinder 11 . A clamped workpiece definitely remains clamped; only the tool could be damaged by the higher storage pressure, but this is harmless in terms of safety.

This behavior also applies to intermediate positions in pressure control between working position 44 , control position 42 and zero position 41 . A smaller opening cross-section from the inlet connection 25 to the first consumer connection 27 only leads to a slower pressure build-up. The security requirement is still guaranteed. If the control valve 39 is in the zero position 41 in the event of a power failure, the clamping pressure in the clamping cylinder 11 is maintained approximately at least until the spindle comes to a standstill with the clamped workpiece. This ensures the necessary security in this position.

The same applies if a control with retracting clamping cylinder 11 is controlled with the control valve 39 .

The valve assembly 10 can be made compact as a single device so that it can be mounted directly on a clamping cylinder 11 . This results in short reaction times for pressure changes even when working. In addition, the connections 26 to 28 of the Ven valve arrangement can be adapted to standardized hole patterns, so that easy mounting on previous control blocks or cylinders or also conventional connecting plates is possible.

Of course, changes to the embodiment shown are possible without departing from the spirit of the invention. In this way, the valve arrangement can also be used advantageously if a clamping pressure control is only desired on one cylinder side and one of the control positions 42 or 43 could thus be omitted. The valve arrangement shown in which a pressure sensor with a longitudinal slide control valve driven by a rotary stepping motor is used in an electrohydraulic pressure control circuit is a particularly advantageous combination.

Although the zero position 41 shown, in which all connections are blocked, is particularly advantageous since it ensures the greatest degree of safety in the event of a power failure, it may also be expedient in certain applications if the two working connections 27 , 28 are in a zero position are connected to the tank port 26 while the inlet port 25 is blocked.

FIG. 3 shows a second valve arrangement 60 and FIG. 4 the associated circuit diagram. The second valve arrangement 60 differs from the first valve arrangement 10 according to FIGS. 1 and 2 primarily by an additional intermediate plate valve 61 , in which two individual 2/2-way solenoid valves 62 , 63 are arranged as solenoid valve means. Both solenoid valves 62 , 63 are designed as magnetically actuated against spring force, 64 double-density Ven tile in the spring-centered rest position and are each connected to the consumer connections 27 , 28 . The clamping pressure tap to the shuttle valve 29 is carried out by means of pressure channels 65 , 66 which branch off from consumer-side sections 67 , 68 of the connections 27 , 28 . Furthermore, the longitudinal slide 15 has a central zero position 69 , in which the two working connections 27 , 28 are connected to one another and to the tank connection 26 .

The mode of operation of the second valve arrangement 60 largely corresponds to that of the first valve arrangement 10 , but achieves an even higher degree of safety through the intermediate plate valve 61 in the event when the longitudinal slide 15 is in the second control position 43, for example to reduce pressure, in the event of a power failure. A drop in pressure could release the tool clamping before the spindle comes to a standstill. This is prevented by the solenoid valves 62 , 63 , which assume their spring-centered rest positions 64 in the event of a power failure and block the clamping cylinder 11 , so that an unwanted loosening of the clamped workpiece is also prevented in this case.

Fig. 5 shows a circuit diagram of a second intermediate plate valve 70 , which differs from that according to Fig. 3 or 4 det in that instead of two tight solenoid valves, a single 4/2-Ma solenoid valve 71 is used in slide design. The 4/2 solenoid valve 71 is connected to both consumer connections 27 , 28 and blocks them in the event of a voltage / power failure. In the case of this solenoid valve 71 with a longitudinal slide, there is a slow pressure reduction in the tensioning cylinder 11 in the spring-centered rest position 72 . The minimum he required clamping pressure is not undercut before the machine comes to a standstill, so that unintentional loosening of the workpiece is avoided when the spindle rotates. The 4/2 solenoid valve 71 allows a simple, inexpensive solution.

Fig. 6 shows a third valve arrangement 80, which mainly characterized by det failed from the first valve arrangement 10 of FIG. 2 that an auxiliary power source 81 is provided which is connected to the electronic controller 34. This auxiliary energy source 81 can be designed as a relatively weak battery, the energy of which is just sufficient to adjust the longitudinal slide 15 from a pressure-reducing control position 43 or 42 into the adjacent zero position 41 . This auxiliary power source 81 causes the aforementioned adjustment of the longitudinal slide 15 in the blocking zero position 41 in the event of a voltage / power failure, so that the clamping cylinder of the 11 is hydraulically blocked and the clamping pressure is maintained until the spindle has come to a standstill. When the tensioning cylinder 11 is extended in the direction of the extension of the piston rod 51, the second control position 43 is used for reducing the pressure, while the first control position 42 is used in the case of a cylinder 11 which is exciting in the direction of the insertion rod 51 . This means that the auxiliary power source 81 can also achieve the highest level of safety without additional intermediate plate valves.

FIG. 7 shows a circuit diagram of a fourth valve arrangement 90 , which differs from that according to FIG. 2 only by another electronic control device 91 , which operates according to a special pressure control concept. When controlling to constant clamping pressure, the control valve 39 only works in the outermost ( 44, 45 ) and the adjacent positions 42 , 43 . Opening the respective clamping pressure side of the clamping cylinder 11 to the tank connection 26 is therefore not possible. If the pressure in the tensioning cylinder 11 is somewhat higher than the target value, for example due to overshoot, it is reduced again by leakage oil at the control valve 39 . Only in the case of a two-pressure clamping, for example, is the control position 43 or 42 activated to relieve pressure; however, since the spindle is stationary, a power failure in this phase is harmless.

The electronic control device 91 has a first input 92 for setting the pressure setpoint, a second input 93 for setting the pressure range and thus the maximum and minimum pressure, a third input 94 for controlling the directions left and right, respectively fourth input 95 for the control of the functions internal and external clamping, a fifth input 96 for the speed setting, a first output 97 for pressure monitoring and a second output 98 for specifying the pressure greater than maximum pressure or less than minimum pressure Control, for example, of a machine stop function.

Of course, from 3 to 7 changes to the embodiments shown in FIGS. Possible without zuweichen from the spirit of the invention from.

Claims (17)

1. Valve arrangement for controlling and monitoring the working pressure of a hydraulic consumer, for example the chuck or tailstock of a machine tool, the two consumer connections of which are connected via the direction of the consumer piston and control valve means and via pressure control means to a pressure medium source or a tank are and sense a pressure with the electrohydraulic component, characterized in that the electrohydraulic component is a substantially continuously operating pressure sensor ( 33 ) which measures the working pressure in at least one of the consumer connections ( 27 , 28 ) that the means controlling the pressure and the direction are designed as a single Regelven valve ( 39 ), the control member ( 15 ) of which can be actuated by a stepper motor ( 18 ) which is arranged with the pressure sensor ( 33 ) in an electrical control circuit ( 37 ). 2. Valve arrangement according to claim 1, characterized in that the pressure sensor ( 33 ) with its hydraulic input to the Mittelan circuit ( 30 ) of a maximum pressure selection device, in particular egg nes shuttle valve ( 29 ) is connected, the consumer connections between the two United ( 27 , 28 ) is switched. 3. Valve arrangement according to claim 1 or 2, characterized in that the control valve ( 39 ) as a control member has a longitudinal slide ( 15 ) and the electromechanical stepper motor ( 18 ) at its mechanical output ( 19 ) emits a rotary movement and that the rotary movement of the stepper motor ( 18 ) in a longitudinal movement of the control member ( 15 ) converting device, in particular a threaded connection ( 23 ) is arranged. 4. Valve arrangement according to claim 3, characterized in that the threaded connection ( 23 ) and a rotary transmission transmitting coupling ( 21 ) at opposite ends of the longitudinal slide ( 15 ) are arranged. 5. Valve arrangement according to one of claims 3 or 4, characterized in that the longitudinal slide ( 15 ) has at least five positions ( 41 to 45 ), of which on both sides of a middle zero position ( 41 ) each have a pressure control position ( 42 , 43 ) for one of the two consumer connections ( 27 , 28 ) and to the outside one of the direction control assigned working position ( 44 , 45 ) connects. 6. Valve arrangement according to claim 5, characterized in that in the zero position ( 41 ) all four connections ( 25 to 28 ) are hydraulically blocked. 7. Valve arrangement according to one or more of claims 1 to 6, characterized in that the control valve ( 39 ) has a valve housing ( 12 ), on one end face ( 16 ) of which the stepping motor ( 18 ) is mounted and on another side, in particular the upper side ( 35 ), the pressure sensor ( 33 ) and an electronic Steuerein direction ( 34 ). 8. Valve arrangement according to claim 5, characterized in that in a zero position both working connections with each other and with the Tank are connected.   9. Valve arrangement according to one or more of claims 1 to 8, characterized in that a hydraulic accumulator ( 49 ) is connected to the inlet connection ( 25 ) of the control valve ( 39 ). 10. Valve arrangement according to one or more of claims 3 to 9, characterized in that the longitudinal slide ( 15 ), the step motor ( 18 ), the coupling ( 21 ) and the threaded connection ( 23 ) are arranged coaxially to each other. 11. Valve arrangement according to one or more of claims 1 to 10, characterized in that in at least one of the consumer connections ( 27 , 28 ) solenoid valve means ( 62 , 63 ; 71 ) are connected, which in the event of a power failure, the connection to the consumer ( 11th ) block and that the working pressure to the pressure sensor ( 33 ) is tapped in a section ( 67 , 68 ) lying between the solenoid valve means ( 62 , 63 ; 71 ) and the consumer ( 11 ). 12. Valve arrangement according to claim 11, characterized in that in each consumer connection ( 27 , 28 ) as a solenoid valve means a fe derzentrierter, double-tight 2/2-solenoid valve ( 62 , 63 ) is arranged. 13. Valve arrangement according to claim 11, characterized in that the solenoid valve means as a two consumer connections ( 27 , 28 ) associated 4/2 solenoid valve ( 71 ) are formed in a slide construction, which has a spring-centered rest position ( 72 ). 14. Valve arrangement according to one of claims 11 to 13, characterized in that the solenoid valve means ( 62 , 63 ; 71 ) as an on the housing ( 12 ) of the control valve ( 39 ) attachable intermediate plate valve ( 61 , 70 ) are formed. 15. Valve arrangement according to one or more of claims 1 to 7 and 9 to 10, characterized in that an auxiliary power source ( 81 ) is provided which, in the event of a power failure, the longitudinal slide ( 15 ) of the control valve ( 39 ) from a pressure reduction associated control position ( 43 , 42 ) in its adjacent, blocking zero position ( 41 ) adjusted ( Fig. 6). 16. Valve arrangement according to one or more of claims 1 to 10, characterized in that the electronic control device ( 91 ) is designed so that the working pressure during clamping only by means of pressure-increasing positions ( 42 , 44 ; 43 , 45 ) on a single side of the Zero position ( 41 ) is controlled while a pressure reduction is carried out via leak oil in the control valve ( 39 ). 17. Valve arrangement according to claim 16, characterized in that the pressure-reducing position ( 43 , 45 ; 42 , 44 ) on the other side of the zero position ( 41 ) is approached during two-pressure clamping.