EP1656224A1

EP1656224A1 - Device for controlling the drawing process in a transfer press

Info

Publication number: EP1656224A1
Application number: EP04741376A
Authority: EP
Inventors: Stefan Arns; Helmut Behl
Original assignee: Bosch Rexroth AG
Current assignee: Bosch Rexroth AG
Priority date: 2003-08-07
Filing date: 2004-08-06
Publication date: 2006-05-17
Anticipated expiration: 2024-08-06
Also published as: JP2007507349A; EP1656224B1; US7827843B2; DE502004006732D1; WO2005016571A1; ES2302006T3; DE10336279A1; US20060254337A1; ATE390966T1

Abstract

A device for controlling the drawing process in a transfer press has two tool parts which act in opposition to one another and between which the workpiece to be deformed is held. One tool part is moved between two reversal points by a mechanical crank mechanism driven at a constant rotational speed. The second tool part is connected to the piston of a hydraulic differential cylinder via a piston rod. The movement of the piston is controlled by the supply of pressure medium into a first chamber and by the discharge of pressure medium out of the second chamber of the differential cylinder. During a first time segment within a range delimited by the first and the second reversal point, the rod-side face of the piston is acted upon by a pressure which is sufficiently high to accelerate the second tool part as that, when the two tool parts impinge one onto the other, both tool parts move virtually at the same speed.

Description

description

Device for controlling the drawing process in a transfer press

The invention relates to a device for controlling the drawing process in a ^"" Transfer press according to the preamble of claim 1.

In the case of a press in the form of a transfer press, a workpiece to be deformed is between two counteracting ones. Tool parts held. One of the two tool parts, which is designed in particular as a negative mold, can be moved between an upper and a lower reversal point by a mechanical crank mechanism driven at constant speed. The movement from the upper to the lower reversal point is referred to as the forward stroke and the subsequent movement from the lower to the upper reversal point is referred to as the return stroke. The movement of the tool part driven by the crank mechanism is predetermined by the design of the crank mechanism and by its rotational speed. During a work cycle of the drawing process consisting of a forward stroke and a return stroke, the crank mechanism makes one full rotation. Since the speed of rotation of the crank mechanism is constant, there is a fixed relationship between the crank angle and time. It is thus possible to consider the corresponding times instead of the respective crank angles. This context is also used in the following description. ^' The other tool part, which is designed in particular as a die cushion, is over ^• A piston rod is connected to the piston of a hydraulic differential cylinder. The movement of the piston rod is controlled by supplying pressure medium to a first chamber of the differential cylinder and by removing pressure medium from the other chamber. The movement of the tool part held on the piston rod can be influenced by controlling the pressure medium flow to and from the differential cylinder independently of the movement of the crank mechanism. A work cycle of the press drawing process is divided into a series of successive periods. During a first period, which in the selected example extends within the forward stroke, the rod-side surface of the piston is pressurized in such a way that the differential cylinder accelerates the second tool part to such an extent that when the first tool part strikes the second tool part both tool parts move practically at the same ^'speed; In a second time period, which follows the first time period within the advance stroke and which extends to the lower reversal point, the two tool parts lie against the workpiece from opposite sides and deform it. During the forming process, the two tool parts come closer together. At the lower reversal point, the pressure medium is decompressed in the differential cylinder. With the reversal of the direction of motion of the crank drive, the return stroke begins with a further time period which extends at most until the upper reversal point is reached. In this period, the second tool part can either move into a special removal position or initially move together with the crank mechanism in the direction of the Move the top reversal point. In both cases, the speed of the second tool part driven by the differential cylinder is not greater than the speed of the tool part driven by the crank mechanism. The pump intended to supply the differential cylinder with pressure medium must be designed in such a way that it is able to accelerate the second tool part during the first time period as described above. This time period is the time period with the greatest pressure medium demand during one Since the pump has to be designed for the greatest pressure medium requirement, it is oversized for periods with a lower pressure medium requirement and consumes more energy than required in these time segments. Such devices for controlling the drawing process in a transfer press are of the type

Mannesmann Rexroth AG (now trading as Bosch Rexroth AG) has been offered and distributed.

The invention is based, the above-mentioned device for controlling the drawing process with the task. To improve the goal of reducing energy consumption.

This object is achieved by the features specified in claim 1. The invention makes use of the consideration that a high pressure is only required during the first period of the drawing process and that a pressure which is lower than this pressure is sufficient for the movement of the second tool part in at least one further period of a working cycle. The use of a low-pressure ^pump required for this increases the initial cost of the Press, however, these additional costs are more than offset by savings in operating costs, so that energy savings outweigh the press over the entire service life.

5 Advantageous further developments of the invention are characterized in the subclaims. ^' They relate to measures that lead to further energy savings and details of _. such facilities. Due to these ^measures' smaller size can be used, inter alia, a cylinder. In addition, 10. the cooling capacity required decreases. A tank with smaller dimensions can be used for the pressure medium.

The invention is explained in more detail below with its further details on the basis of three exemplary embodiments shown in the drawings. Show it

FIG. 1 shows a schematic representation of a first device designed according to the invention for controlling the drawing process in a transfer press,

Figure ^' 2 is a diagram in which the movement of the. two tool parts of the transfer press shown in FIG. 1 are shown during the individual periods of a working cycle,

3 shows the hydraulic part of a second inventive design means for controlling ^'of the 5 • drawing operation in a transfer press, FIG. 4 shows the hydraulic part of a third device designed according to the invention for controlling the drawing process in a transfer press,

5 shows the cylinder used in FIG. 4 in an enlarged view,

6 ^'shows the rod-side annular surface of the cylinder shown in FIG

7 shows the pressure on the bottom surfaces of the cylinder shown in FIG. 5.

Figure 1 shows a schematic representation of a transfer press and a first device for controlling the drawing process according to the invention. An excessively deforming workpiece 10 is held between two counteracting tool parts 11 and 12, one of which is formed Who kzeugteil ^'11 as a negative mold and the mold part 12 as the die cushion. A mechanical crank mechanism 13 driven by a motor (not shown in FIG. 1) at a constant rotational speed moves the tool part 11 between an upper reversal point OT and a lower reversal point UT, the lower limit of the tool part 11 being designated as the reference position s _s . A differential hydraulic cylinder 15 with a piston 16 and a force acting on the tool part 12 piston rod 17 will move the tool part 12 within ^'delimited by ^• reversal points OT and UT region. The upper limit of the tool part 12 is here as a reference Position S _k denotes. A rotary angle transmitter 20 converts the angular position φ of the crank mechanism 13, which is a measure of the position s _{s of} the tool part 11, into an electrical voltage signal U _φ . A symbolized by a ruler. 5-position encoder 21 converts the position S of the tool part 12 into a further voltage signal u _sk . The voltage signals U _φ and u _sk are fed to a computing circuit 22 as input signals. The arithmetic circuit 22 combines the input signals into control signals 10 according to predetermined algorithms. ^u _s t _b ^und du _s s, which control the pressure medium supply to the chambers of the differential cylinder 15 provided with the reference numerals 15s and 15b ^' .

A first pump 25 designed as a constant pump conveys pressure medium from a tank 26 and loads a pressure accumulator 27

15. to a pressure p _s π, the level of which is limited by a pressure shut-off valve 28. Another pump 30, also designed as a constant pump, delivers pressure medium from the tank 26 and charges another pressure accumulator 31 to a pressure p _SN , the level of which is limited by a further pressure shutoff valve 0 32. The pressure p _SH is chosen so large that the tool part 12 can be moved with the maximum acceleration required during operation. The pressure p _SN is significantly smaller than the pressure p _SH - In one embodiment, is p _sN in the order of a quarter of p _SH - 5 A Proportionalveήtil 35 and a switching valve 36 controlling the supply of pressure medium from the pressure accumulators 27 and 31 ^• to the chambers 15s and 15b- of the differential cylinder 15 in accordance with the control signals output by the arithmetic circuit 22 u _s tb and u _s t _s - The pressure accumulator 31 is connected to the rod-side chamber 15s of the differential cylinder 15 via a check valve 39 and via hydraulic lines 40 and 41. In the rest position of the valve 35 shown in FIG. 1, one of the two end positions of this valve, the chamber 15b is connected to the tank 26 via a further hydraulic line 42. The connection between the check valve 39 and the chamber 15b is blocked in the rest position of the valve 35. If the valve 36 is also in the rest position shown in FIG. 5, the connection between the pressure accumulator 27 and the line 41 is blocked, the chamber 15s is only pressurized with the pressure p _{SN of} the pressure accumulator 31. In the other end position of the valve 35, which corresponds to the maximum value of the control signal u _s t _b , the pressure p _{SN is also} applied to the chamber 15b in addition to the chamber 15s. At values of the control signal ^u _{stb r} which lie between zero and its maximum value, the comb 15b is connected both to the tank 26 and to the line 40, the size of the respective passage cross sections being determined by the respective size of the control signal u _sb is.

If the valve 36 is in the working position, the pressure p _s π is applied to the chamber 15s and the pressure p _SH acts on the area A _r - the check valve 39 blocks, since - as described above - p _{SH is} greater than p _SN , If the valve 35 is in the rest position, the chamber 15b is relieved to the tank 26. At these positions of the valves 35 and 36, the greatest downward force acts on the piston 16. When the control signal u _s t _b is increased, the Connection to tank 26 throttled. An upward force, determined by the size of the control signal u _s t _b , now acts on the surface A of the base of the piston 16, which counteracts the downward force and thus reduces the resulting downward force.

The operation of a transfer press with that in the figure

I shown control device is described below with reference to Figure 2. FIG. 2 shows the position s _{s of} the tool part 11 (curve 45) and the position S _{k of} the tool part 12 (curve 46) during a working cycle of the transfer press. Since the rotational speed of the crank mechanism 13 is constant, there is a fixed relationship between the crank angle φ, which is a measure of the position s _s , and the time t. This makes it possible, the respective spa instead belwinkel .phi..sub.i these respective times t th to betrach- ^■. The work cycle described below ^■ .beginnt at the time to a pre-stroke, in 'which the tool part

II moved from the upper reversal point OT to the lower reversal point UT. This turning point was reached at time ^' t ₃ . The forward stroke is followed by the return stroke, in which the tool part 11 moves back from the lower reversal point UT to the upper reversal point OT. This reversal point is reached at time t _ß . Because of the constant rotary movement of the crank drive, a new work cycle begins immediately at time tξ, which proceeds in the same way as the work cycle between times to and t _ß . In contrast to the movement of the tool part 11, the movement of which is predetermined by the crank mechanism 13, the movement of the tool part 12 can be effected by acting on the chambers 15b and Ferentialzylinders control 15s of Dif ^'15 with hydraulic pressure medium. For this purpose, a program is stored in the arithmetic circuit 22, which forms control signals u _s t _b and u _s t _s for the valves 35 and 36 from the signals u _φ and u _sk such that the position S _{k of} the tool part 12 corresponds to the Curve 46 corresponds ^• . At time to, valve 36 is in its working position, ie chamber 15s is pressurized with pressure p _SH . Up to the point in time ti, the valve 35 is controlled in such a way that the tool part 12 maintains its initial position, denoted S _O '. In this case, in the chamber ^'15b, a pressure at which the forces acting from opposite sides on the piston 16 forces cancel (taking account of the dead weight of the tool part 12 and the workpiece 10) straight. Due to the movement of the tool part 11, the distance between the tool parts 11 and 12 decreases in the time interval Δt between to and ti. From the time ti, the arithmetic circuit 22 controls the valve 35 such that the distance between the tool parts 11 and 12 further reduced until tool parts 11 and 12 meet at time t ₂ . At time t ₂ , the arithmetic circuit 22 switches the valve 36 back into its rest position. This reduces the energy consumption of the pump 25, since only the pressure p _{SH of} the pressure accumulator 27 is maintained without 27 pressure medium being removed from the pressure accumulator. For the remaining time of the advance stroke, ie in the time interval Δt ₃ between the times t ₂ and t ₃ , and during a first part of the return stroke, for. B. during the periods Δt ₄ and Δts between the times t3 and t5, the valve 36 maintains its rest position. During this time, the chambers 15b and 15s of the differential cylinder 15 only pressurized from the pressure accumulator 31. The computing circuit 22 controls the valve 35 again in such a way that a pressure which acts on the surface A of the piston 16 is established in the chamber 15b and, in conjunction with the other forces acting on the piston 16, the tool part 12 moved according to the course of the curve 46. The curve 46 applies in the event that the tool parts 11 and 12 move together with the workpiece 10 located between them up to the point in time t. In the time period Δts, which extends to the time ts, the tool parts 11 and 12 separate from one another and release the workpiece 10 for removal. At the time ts, the tool part 12 has reached its initial position S _kO , while the tool part 11 is still traveling to the upper turning point OT, which it reaches at the time tς. At time t _ß , the arithmetic circuit 22 switches the valve 36 back into its working position, in which the pressure p _SH is supplied to the chambers of the differential cylinder 15 via the lines 40 and 41 ^' . In principle, the switchover of the valve 36 ^' into its working position can also take place at a later point in time, but at the latest by the point in time t. As an alternative to the curve 46, the dashed line 47 shows that the tool part 12 first moves from a point in time t ₃ into a special removal position for the workpiece 10 and only reaches its starting position S _O again between the times ts and tζ.

FIG. 3 shows only the hydraulic part of a second device designed according to the invention for controlling the drawing process in a transfer press. This facility corresponds in many parts with the device shown in Figure 1. Components that are shown in the Figure 1 above a dash-dot line 50, ^'namely, the tool parts 11 and 12, the crank mechanism 13 and the re chenschaltung 22 are also not shown for clarity in Figure 3 again. The piston rod 17 of the differential cylinder 15 ending at line 50 in FIG. 3 leads to the tool part 12. The output signal u _{Sk of} the displacement sensor 21 is supplied to the arithmetic circuit 22 as an input signal. The output signal u _{φ of} the angle encoder 20 is fed to the computing circuit 22 as a further input signal. The arithmetic circuit 22 uses these signals to form the control signal u _s t _b for a hydraulic valve 51 and the control signal u _sts ' for a further hydra.ul valve 52. The valves 51 and 52 are designed as proportional valves. This measure allows sensitive control of the pressure medium flow. The valve 51, which is connected via a hydraulic line 53 to the chamber 15b, controls the flow of pressure medium to the bottom-side chamber ^'15b. The valve 52 controls the pressure medium flow to the rod-side chamber 15s. As in FIG. 1, two pumps 25 and 30, two pressure cut-off valves 28 and 32, two pressure accumulators 27 and 31 and a check valve 39 are provided in FIG. The pressure accumulator 31 is connected to the chamber 15s via the check valve 39 and the lines 40 and 41.

The valve 51 can be steplessly controlled between two end positions by the control signal u _stb . In the end position shown in FIG. 3, the chamber 15b is relieved of the load on the tank 26. In the other end position of the valve 51 is the Pressure p _{SH is} applied to chamber 15b. At values of the control signal u _s tb - which lie between zero and its maximum value, the valve 51 assumes an intermediate position in which the chamber 15b is connected both to the tank 26 and to the pressure accumulator 27, the size of the respective one Passage cross sections is determined by the respective value of the control signal u _s t _b . The valve 52 can also be steplessly controlled between two end positions by the control signal u _s t _s . In the end position shown in FIG. 3, the pressure p _s π is applied to the chamber 15s. Since in this position of the valve 52 the pressure p _{SH is} greater than the pressure p _SN , the check valve 39 closes. In its other end position, the valve 52 closes and the chamber 15s is acted upon by the pressure p _SN . In the intermediate positions of the valve 52 raises the pressure in the chamber ¹ to a 15s lying between p _s and p π _SN value that is dependent on the size of the control signal .. u _s t _^s'.

The computing device 22 controls the valves 51 and 52 so that the tool part 12 connected to the piston rod 12 follows the curve 46 shown in FIG. The working cycle starts at time to with a prestroke, ^'in which the tool part 11 moves from the top dead center TDC to the bottom reversal point UT. In the time period Δt ₂ between the times ti and t ₂ , the valves 51 and 52 are located in FIG. in the rest position shown in FIG. 3, in which the chamber 15s is subjected to the pressure p _SH and the chamber 15b is relieved to the tank 26. With this valve position combination, the greatest possible force acts on the piston 16. At the time t ₂ , in which the tool part 11 on the tool Tool part 12 hits, the valve 52 closes. The chamber 15s is acted upon by the pressure accumulator 31 via the check valve 39 and the lines 40 and 41 with pressure medium. The tool part 11 driven by the crank mechanism 13

.5 actively urges the tool part 12 held on the piston rod 17 downwards. The arithmetic circuit 22 controls the valve 51 so that the desired counterforce of the tool part 12 is set

10 chamber 15b and the tank 26 increases the counterforce of the tool part 12. The valve 51 acts as a controllable throttle, which determines the pressure in the bottom chamber 15b. At time t ₃ , the tool part 12 reaches the lower reversal point UT. Now, control circuit 22 controls

15 the valves 51 and 52 so that both the chamber 15b and the chamber 15s are acted upon by the pressure p _s π. D at the valves 51 and 52 are controlled in detail so that - the tool part 12 follows the curve 46. Here too, the differential cylinder 15 only applies in the time interval Δt ₂

-0 is supplied with pressure medium from the pressure accumulator 31 charged to the lower pressure p _SN . This means that in this exemplary embodiment, too, the energy consumption of the pump 25 in the time period Δt _{2 is} reduced compared to the other time periods of a work cycle. 5 A further reduction in the energy consumed during a working cycle of the transfer press is made possible by the embodiment examples described with reference to FIGS. 4 to 7. FIG. 4 shows a control device in a representation corresponding to FIGS. 1 and 3. As far as in Figures 1, 3 and 4 identical components are used, they are provided with the same reference numerals. 4, a differential cylinder 55 is used to drive the tool part 12, which has a different construction than the differential cylinder 15 used in FIGS. 1 and 3. As already shown in the figure

3, the tool parts 11 and 12 and the crank mechanism 13 i are not shown again in FIG. 4. The differential cylinder 55 is shown in Figure 5 on an enlarged scale ^•. Such a differential cylinder is, z ^' . B. in connection with a commercial vehicle, known from de ^' r US-PS 6,145,307. The differential cylinder 55 has a piston 56 which is provided with a bore 57. A housing-fixed piston 58, which engages in the bore 57, forms, together with the bore 57, an inner, bottom-side chamber 55bj_. The pressure medium is supplied to the chamber 55bi via a channel 59 'in the piston 58. Furthermore, the differential cylinder 55 has an outer bottom-side chamber 55b _a and a rod-side chamber 55s. The lines 41 (coming from valve 52) and 53 (coming from valve 51) are connected to the chambers 55s and 55b _a . The pressurized areas of the

Pistons 56 are labeled A _r , A i and A _b a. FIG. 6 shows the ring surface A _{r of} the rod-side chamber 55s. The ^'Figure 7 shows the annular area A _B A of the outer bottom-side chamber 55b _a and the circular area A _b i of the inner bottom side chamber 55bi, wherein the circular area A _b a is formed larger than the' _b annular surface A driving i- An electric motor 62 a flywheel 64 and a variable displacement pump 6.5 on a shaft 63. The delivery volume of the variable displacement pump 65 can be adjusted between a minimum value and a maximum value by a control signal u _{s H.} A second shaft 66 is via a clutch 67 connected to the shaft 63. The shaft 66 drives a hydraulic machine 70, which is _s u _M in response to a control signal continuously from pump operation to engine operating controllable, and which is designed as a fixed displacement pump pump 30. The hydraulic machine 70 is a hydraulic 'line ^■ 73 with the in the chamber 55bi leading channel 59 in the housing-fixed piston 58 of the differential cylinder 55 connected. A non-return valve 75 is arranged between the pressure accumulator 31 and the line 73, which blocks whenever the pressure in the line 73 is greater than p _SN .

A computing circuit 77 forms according to predetermined algorithms from ^'the input signals u _φ and u _s, the control signals u _s t _b and u _s t _s (for the valves 51 and 52, respectively) as well as other control signals u _s t _H (for the variable displacement pump 65) and u _s t _M (for the hydraulic machine 70). For the sake of clarity, the individual electrical lines between the computing circuit 77 and the actuators (valves 51 and 52, variable pump 65, hydraulic machine 70) are not shown in FIG. The arithmetic circuit 77 controls the actuators such that the position S _{k of} the tool part 12 also corresponds to the curve 46 shown in FIG. 2 in this exemplary embodiment. The work cycle starts again at the time to with a preliminary stroke in which the tool part 11 moves from the upper reversal point OT to the lower reversal point UT. In the time interval Δt ₂ between the times ti and t ₂ , the valves 51 and 52 are in the rest position shown in FIG. 3, in which the pressure p _{SH is} applied to the chamber 55s and the chamber 55b _a to the tank 26 is relieved. The hydraulic machine 70 is in this Time period set to approx. 50% tank funding. With this combination, the greatest possible force acts on the piston 56. At the time t ₂ , in which the tool part 11 hits the tool part 12, the valve 52 closes. During the time interval Δt3, the chamber 55s from the pressure accumulator 31 via the Check valve 39 and lines 40 and 41 are pressurized with pressure medium. The tool part 12 held on the piston 56 is actively displaced downward by the crank mechanism 13 via the tool part 11 and the workpiece 10 located between the tool parts 11 and 12. In this period, the arithmetic circuit 77 controls the valve 51 in such a way that the desired counterforce of the tool part 12 is established. It applies here that a reduction in the passage cross section of the connection between the chamber 55b _a and the tank 26 increases the counter-holding force of the tool part 12. The hydraulic machine 70 operates as a motor and delivers mechanical energy to the flywheel 64. ^'' The variable pump 65 swivels to 100% delivery volume. The pressure control in the chamber 55b via _a valve 51 and the hydraulic ^'metallic machine 70. At time t _3, the tool part 12 reaches the lower reversal point UT. Now the arithmetic circuit 77 controls the valves 51 and 52 in such a way that both the chamber 55b _a and the chamber 55s are acted upon by the pressure p _SH . In addition, the chamber 55bi is filled via the check valve 75 and the hydraulic machine 70 operated as a pump by the computing circuit 77 for this purpose. The actuators (valves 51 and 52, variable pump 65, hydraulic machine 70) are controlled in detail so that the tool part 12 follows the curve 46. It also applies here that the differential cylinder 55 does not open up in the time interval Δt ₂ the higher pressure. p _s π charged pressure accumulator 27 is supplied with pressure medium. This means that in this exemplary embodiment, too, the energy consumption of the pump 25 in the time period Δt _{2 is} reduced compared to the other time periods of a work cycle, with the use of the hydraulic machine 70 giving an even better utilization of the energy used to supply the electric motor 62 ,

Claims

Patent claims

1. Device for controlling the drawing process in a transfer press with two mutually acting tool parts, between which a workpiece to be deformed is ^held , of which one tool part, in particular a negative mold, is of a constant _. Rotational speed driven mechanical crank mechanism can be moved between two reversal ^points , the first of which is assigned to the start of a working cycle, and of which the second tool part, in particular a die cushion, is connected via a piston rod to the piston of a hydraulic differential cylinder, whereby the movement of the piston is controlled by supplying pressure medium into a first chamber and by discharging pressure medium from a second chamber of the differential cylinder, and in which the rod-side surface of the piston during a first period of time which extends within an area delimited by the first and second reversal points , is subjected to a pressure that is sufficiently large to accelerate the ^second tool part in such a way that when the first tool part and the second tool part meet, both tool parts move at practically the same speed, and one between a bottom chamber of the differential cylinder and a controllable throttle arranged in a tank determines the pressure in the bottom chamber, characterized in that in a second time period (Δt3) following the first time period (Δt ₂ ), which extends until the second reversal point (UT) is reached, the rod- side surface (A _r ) of the piston (16; 56) is subjected to a second pressure (P _S N) which is smaller than the pressure (P _S H) during the first time period (Δt2).

2. Device according to claim 1. characterized in that the rod-side surface (A _r ) of the piston (16; 56) begins in a third time period ^• (Δt ₄ + Δts) of the working cycle starting with the reversal of the direction of movement of the crank mechanism (13), which is at the latest at that point in time (tö) is ended when the crank mechanism (13) reaches the first reversal point (OT) and is again subjected to the first pressure (P _S H).

3. Device according to claim 1, characterized in that the rod-side surface (A _r ) of the piston (16; 56) in one with the reversal of its direction of movement. The second pressure (P _S N) continues to be applied to the beginning of the third time period (Δt4 + Δts) of the working cycle, which ends at the latest at the time (tζ) in which the crank mechanism (13) reaches the first reversal point (OT).

4. Device according to one of claims 1 to 3, characterized in that two pressure accumulators. (27, 31), one of which (27) is charged to the first pressure (P _S H) • and the second (31) to the second pressure (p _SN _, and that the loading of the rod side Chamber (15s; 55s) of the differential cylinder (15; 55) is supplied with pressure medium from the pressure accumulator (27, 31) which is at the pressure (p _SH/ -) intended for the respective time period (Δt ₂ , Δt3, Δt ₄ + Δts). P _S N) is charged.

5. Device according to claim 4, characterized in that the second pressure accumulator (31). is connected to the rod-side chamber (15s; 55s) of the differential cylinder (15; 55) via a check valve (39).

6. Device according to claim 5, characterized in that in the line (42; 53) leading to the bottom chamber (15b; 55b _a ) of the differential cylinder (15; 55) there is a proportional valve (35; 51) serving as a controllable throttle ) is arranged, which on the one hand ^' controls the pressure medium flow from one of the pressure accumulators (27, 31) to the bottom chamber (15b; 55b _a ) of the differential cylinder (15; 55) and from this chamber to the tank (26).

7. Device according to one of claims 4 to 6, characterized in that a first pump (25; 65) maintains the pressure (P _S H) in the first pressure accumulator (27) and that a second pump (30) maintains the pressure ( P _S N) is maintained in the second pressure accumulator (31).

8. Device according to claim 7, characterized in that the pumps (25, 30) are fixed displacement pumps and that a pressure shut-off valve (28, 32) is arranged between ^a pump (25, 30) and the corresponding pressure accumulator (27, 31). is.

9. Device according to claim 7, characterized in that the pumps (65) are variable displacement pumps.

10. Device according to one of claims 4 to 9, characterized in that between the first pressure accumulator (27) and the rod-side chamber (15s; 55s) of the differential cylinder (15'; 55) there is a valve (36; 52) is arranged, the output connection of which opens into the line (40, 41) leading from the check valve (39) to the rod-side chamber (15s; 55s).

11. Device according to claim 10, characterized in that the valve arranged between the first pressure accumulator (27) and the rod-side chamber (15s; 55s) of the differential cylinder (15; 55) is a switching valve (36).

12. Device according to claim 10, characterized in that the valve arranged between the first pressure accumulator (27) and the rod-side chamber (15s; 55s) of the differential cylinder (15; 55) is a proportional valve (52).

13. Device according to claim 6, characterized in that the bottom surface of the piston (56) of the differential cylinder (55) is divided into two different sized partial surfaces (A a A i), which are subject to different pressures _. (p _b a, P _b i) are applied, that the pressure .Pba) ^m t to which the larger partial area (Aba) is acted upon is controlled by the proportional valve (51) and that the pressure (p _b i) with which the smaller part area (A _b i) is acted upon by a hydraulic machine (70) which can be continuously controlled from pump operation to motor operation.

14. Device according to claim 13, characterized in that the piston (56) of the differential cylinder (.55) is provided with a bore (57) into which a piston (58) fixed to the housing engages, and that the pressure medium supply to the from the bore (57) and the inner bottom chamber (55bi) formed by the piston (58) fixed to the housing via a channel (59) in the piston (58) fixed to the housing.

15. Device according to claim 13 or claim 14, characterized in that an ^electric motor (62) drives the pumps (30, 65) and the hydraulic machine (70) via a common shaft (63, 66) and that a flywheel (64 ) is connected to the shaft (63).

16. Device according to one of claims 13 to 15, characterized in that the pressure (p _b i) ι with which the smaller partial area (A _b i) is applied is controlled so that it is smaller in the first time period (Δt2). than the first pressure (p _SH ) and in the second time period (Δt3) is equal to the second pressure (P _S N).

1.7. Device according to claim 16, characterized in that the pressure (P _b i) _r with which the smaller partial area (A _b i) is applied is controlled so that it is equal to the first in the third time period (Δt4 + Δts). Pressure

18. Device according to one of claims 13 to 17, characterized in that the hydraulic machine (70) is between that assigned to the start (to) of the working cycle Reversal point (OT) and the beginning (ti) of the first time period (Δt ₂ ) is controlled for tank delivery.

19. Device according to claim 14 or one of the following claims, characterized in that between the second pressure accumulator (31) and the line (73) leading from the hydraulic machine (70) to the inner bottom chamber (55bi) of the differential cylinder (55) a further check valve (75) is arranged.