EP2115305B1 - Hydraulic drive, in particular for machine tools, and method for controlling the hydraulic drive - Google Patents

Description

The invention relates to a hydraulic drive in particular for machine tools, for example for a press, punching machine, nibbling machine and the like, according to the preamble of claim 1 and a method for controlling the hydraulic drive according to the preamble of claim 12.

Hydraulic drives for machine tools are known in the art. The FIGS. 1 . 2 and 4 refer to previously used arrangements.

The FIG. 1 shows the basic apparatus structure of a conventional hydraulic drive for a punching machine. A

Pressure supply device comprises in the in FIG. 1 1, a constant-displacement pump 1.1 for generating an operating pressure and a first pressure-limiting valve 1.2, which can be adjusted to a desired outlet pressure range, are shown. In the supply network, a first hydraulic accumulator 1.3 is connected, by its use, the volume flow in the supply network can be increased in a short time.

A first pressure chamber 1.8 in the working cylinder 1.5 of the drive is always connected via the first line B to the pressure source, whereas a second pressure chamber 1.7 can be connected in the working cylinder 1.5 by means of the 3-way valve either with the pressure source or a tank 1.9. The two pressure chambers 1.7, 1.8 are separated from each other by the piston 1.6 of the working cylinder 1.5, wherein the surface on the piston 1.6, which is acted upon by pressure from the first pressure chamber 1.8, due to the one-sided piston rod is smaller than the surface of the piston 1.6, with Pressure from the second pressure chamber 1.7 is acted upon.

The extension of the piston 1.6 or connected to this piston rod is effected by connecting the second pressure chamber 1.7 with the pressure source, the maximum extension force is determined by the area ratio of the two mentioned the pressure chambers 1.7, 1.8 limiting and opposing surfaces of the piston 1.6. By extending the piston 1.6, the punching tool (not shown) to the workpiece (not shown) introduced and then moved on.

Due to the resistance that opposes the workpiece of the piston movement, the piston 1.6 is first decelerated, and in the second pressure chamber 1.7 is built according to the required cutting force of the necessary pressure. According to the modulus of elasticity of the hydraulic oil in the second pressure chamber 1.7, a spring energy is stored in the oil column in the second pressure chamber 1.7, that is, the oil column is biased, so to speak.

When the workpiece breaks, there is a sudden release of force from the piston 1.6 and, as a consequence, its explosive acceleration in the extension direction. The oil in the first pressure chamber 1.8 is ejected from the first pressure chamber 1.8 by the outwardly accelerating piston 1.6 and pushed back toward the memory 1.3 via the first line B, while the piston 1.6 accelerating expansion of the oil column in the second pressure chamber 1.7 , At this time, the 3-way valve 1.4 must be reversed as quickly as possible for the return stroke of the piston 1.6, wherein there is an interruption of the oil supply in the second line A, which is connected to the second pressure chamber 1.7 comes. If this interruption coincides in time with the extended as a result of the so-called cutting stroke extension speed of the piston 1.6, it comes in the second line A to suppress, which in turn lead to disturbing noises and cavitation at the control edges or the housing of the 3-way valve 1.4.

The FIG. 2 shows the basic apparatus structure of another conventional hydraulic drive similar to that of the FIG. 1 , however with a 4/3-way valve 2.1 instead of the 3-way valve (3/3-way valve) 1.4 from the FIG. 1 , Also, this 4/3-way valve 2.1, as the 3-way valve 1.4 from the FIG. 1 , Also be designed as a continuous valve or switching valve. Notwithstanding the in the FIG. 1 However, the first pressure chamber 1.8 is always connected to the pressure source when the second pressure chamber 1.7 is connected to the tank 1.9, and with the pressure source, no constant pressure on the first line B and thus the first pressure chamber 1.8 is exercised the tank 1.9 connected when the second pressure chamber 1.7 is connected to the pressure source. In the middle position of the 4/3-way valve 2.1 both pressure chambers 1.7, 1.8 are separated from both the pressure source and the tank 1.9, whereas according to the FIG. 1 only the second pressure chamber 1.7 in the middle position of the 3-way valve 1.4 is separated from both the pressure source and the tank 1.9.

The Indian FIG. 2 As shown, the same problems as the embodiment according to the FIG. 1 were presented.

The FIG. 4 shows a path / time graph of the piston 1.6 of the conventional hydraulic drive of Fig.1 , Therein TOS ("top of sheet") refers to the top of the workpiece, which is a sheet metal. Similarly, BOS ("bottom of sheet") refers to the bottom of the workpiece. Initially, the piston is at top dead center TDC ("top dead center"). In the second pressure chamber 1.7, the equilibrium pressure p ₀ prevails. When extending the piston 1.6 with the pre-stroke 4.3, there is a pressure that is slightly greater than the equilibrium pressure.

As soon as the piston 1.6 reaches the workpiece, the piston 1.6, as explained above, braked and finally accelerated very strongly by the prestressed oil column in the second pressure chamber 1.7 when the workpiece breaks, so that a cutting speed 4.4 temporarily sets, which is a multiple of the pre-stroke speed 4.3 or the return stroke speed is 4.5 (cutting stroke). If now the Ölnachfluss by the reversed 3-way valve is interrupted, the pressure prevailing in the second line A, as can be seen by the steep gradient 4.1, down to cavitation, recognizable by the pressures around 0 bar (4.2), from.

The European patent application EP 0 676 547 A1 describes a flow control device and a lowering brake valve with a metering orifice, over which a predetermined pressure difference is maintained and thereby the speed of adjustment of a hydraulic cylinder is to be kept constant. If the pressure difference deviates from its predetermined value, a control piston, which regulates a quantity of hydraulic fluid flowing through, is displaced in a changing manner until the pressure difference across the metering orifice returns to its predetermined value.

Thus, the EP 0 676 547 A1 a flow control device that responds to an undesirable volume flow of hydraulic medium that has been adjusted due to the process, to bring the flow to the desired value again.

The patent DE 196 08 582 B4 describes a hydro-compensator for surge suppression in the return line of a hydraulic system. In the return or tank line a check valve and a nozzle connected in parallel is provided. The closing body of the check valve is pressed by a compression spring against a seat. The force exerted by the compression spring on the closing body, corresponds to a certain pressure in the tank line section between a directional control valve and the check valve. Only when this pressure is exceeded, the check valve opens. The nozzle causes the pressure in said tank line section to drop to atmospheric pressure after a certain time. Thus, here a damper in the tank line is shown, which in turn reacts to changes in the pressure in the tank line.

The European patent application EP 1 484 209 A1 describes a hydraulic actuator, in particular for a convertible roof. The actuator comprises a double-acting cylinder and a throttle device which is selectively switched to a throttled and a non-throttled state. As long as the piston of the cylinder is actively extended by pressurizing a chamber in the cylinder, the throttle device is kept in the non-throttling state. However, when the folding roof pulls due to gravity on the piston of the cylinder and thereby threatens to strike the windshield, the speed of the piston is faster than predetermined by the pressurization, resulting in a pressure drop in the cylinder chamber. A control piston detects this pressure loss and, in response, switches the throttle device into the throttled state. As a result, the extension movement of the piston is braked.

Also in accordance with the EP 1 484 209 A1 described solution of the hydraulic drive reacts to an already occurring pressure drop in a chamber of a double-acting cylinder, which means that an undesirable acceleration of the piston has already taken place. When using said drive in a machine tool, this would mean that the disturbing noise and cavitation still occur.

The invention has for its object to provide a hydraulic drive, in particular for a machine tool, such as punching machine, nibbling machine or a press specify, in which the described disturbing noises and cavitation eliminated or at least reduced.

The object of the invention is achieved by a hydraulic drive having the features of claim 1 and a method having the features of claim 12. Advantageous embodiments of the invention are defined in the dependent claims.

The hydraulic drive, which is provided in particular for a press, a punching or nibbling machine, has a double-acting working cylinder, the piston having an effective in the retraction direction first working surface and an effective in the extension direction second working surface. The work surfaces each delimit a pressure chamber. The two work surfaces are in particular the two single working surfaces of the piston, that is, the working cylinder then has exactly two pressure chambers. In order to retract the piston in the working cylinder or extend from this or to reverse the Kolbenhubrichtung, at least two different pressures can be switched by means of an actuator on one or both working surfaces. In particular, this is an embodiment, as initially described with reference to the FIGS. 1 and 2 has been explained, see there in particular the integration of the 3/3-way valve between the pressure source, the tank and the second pressure chamber of the working cylinder in the FIG. 1 or the 4/3-way valve between the pressure source, the tank and both pressure chambers of the working cylinder in the FIG. 2 ,

According to the invention, however, an adjusting unit is additionally provided in the hydraulic drive, which allows a throttled backflow of hydraulic medium from the first pressure chamber bounded by the first working surface. The drive also has control means, with which before an abrupt acceleration of the piston due to the sudden release of force, the actuator is controlled such that effected by the control unit effected by throttling a hydraulic damping of the movement of the piston.

The hydraulic drive according to the invention or the method according to the invention thus prevent the unwanted sudden acceleration of the piston by measures that are already taken as a precaution, that is, the actuator causes a throttling of the return flow of hydraulic fluid from the first pressure chamber before a sudden acceleration of the piston has been used. The Anrieb reacts not advantageous to a pressure drop in the limited by the second working surface second pressure chamber, but switches the actuator so that it causes a throttled back flow of hydraulic medium from the limited of the first working surface first pressure chamber, in response to a pressure increase in the second limited by the second working surface pressure chamber. Namely, such an increase in pressure takes place even before the undesired sudden acceleration of the piston due to its sudden release of force, so that the control of the actuator according to the invention takes precaution or anticipation, in contrast to a subsequent deceleration of an already accelerated accelerated piston.

According to the invention, it is thus possible to avoid noise and cavitation before a situation has occurred which involves the risk of such noises and cavitations.

According to a first embodiment, the adjusting unit can directly effect the throttling of the hydraulic medium flowing out of the first pressure chamber, that is to say have a throttle point - with a constant or adjustable flow cross-section - which is selectively switched into and out of the flow path or permanently in the flow path Flow path is arranged and whose flow cross section is selectively changed between a throttling position and a non or substantially non-throttling position or is variable.

According to a second embodiment, the actuator causes the throttling indirectly, that is, it is connected in parallel to a throttle - with a constant or adjustable flow cross-section with respect to the hydraulic medium flow and selectively releases an additional flow cross-section in a bypass line to the throttle or blocks (partially or completely).

The outflow of hydraulic medium from the first pressure chamber, which can be throttled directly or indirectly by the actuating unit, that is to say the flow due to displacement by the extending piston, takes place, for example, in the direction of the pressure supply, in particular by means of a pump and / or an accumulator, by means of which the first pressure chamber advantageous permanently, in particular as in the FIG. 1 , or optionally, in particular as in the FIG. 2 , connected is.

The actuator can basically be designed in any conceivable, suitable manner. For example, however, the actuator comprises a flow valve (also flow valve or throttle valve), which is arranged in a first line or in a bypass line parallel to this, can flow through the hydraulic medium from the first pressure chamber.

The flow control valve is, for example, a control valve, a directional control valve or a throttle point with a constant cross section. Of course, however, in principle other suitable flow control valves come into consideration.

For example, the actuator is a 2/2-way valve, which - when arranged in said bypass line to a throttle point in the first line - is continuous in a first switching position and locks in a second switching position.

Alternatively, a 2/2-way valve directly in said first line, via which hydraulic medium can flow from the first pressure chamber, be arranged and have two switching positions, wherein the flow resistance through the 2/2-way valve in the first switching position less than in the second switching position is.

According to one embodiment of the invention, the adjusting unit (in the first line or parallel to a throttle point in the first line) is designed so that it depends on the ratio of the supply pressure to the pressure p _A , the in the second working surface limited second pressure chamber prevails, can be brought into the passage position or blocking position or in non-throttled position or throttle position. In one embodiment with alternately switched pressures in both pressure chambers of the working cylinder, such as in the FIG. 2 is shown, the switching of the actuator can also be done in dependence on the ratio of the pressures in the two pressure chambers.

According to one embodiment of the invention, a first control surface of the actuating unit with the supply pressure or the pressure from the first pressure chamber can be acted upon, while a second control surface of the actuating unit with the pressure p _A in the second pressure chamber can be acted upon, wherein the actuating unit in dependence on the ratio of the on the first pressure applied to the pressure applied to the second control surface in the passage position or blocking position or in unthrottled position or throttle position is brought.

For example, the working cylinder is designed as a differential cylinder, that is, only one side of the piston is provided with a piston rod, whereby the piston has two different sized effective surfaces, which are acted upon by the pressure from a respective pressure chamber.

According to one embodiment of the invention, the second pressure chamber for extending or retracting the piston is optionally acted upon by means of the actuator with supply pressure or tank pressure, while the first pressure chamber is constantly acted upon by supply pressure.

According to an alternative embodiment, the second pressure chamber can be connected to a low-pressure source, a high-pressure source or a tank, while the first pressure chamber is always connected to the low-pressure source.

According to a further embodiment, the first pressure chamber and the second pressure chamber are each selectively connectable to a pressure source or tank and in particular separable from both, wherein in one embodiment with a 4/3-way valve, as shown in the FIG. 2 is shown, the first pressure chamber is always connected to the pressure source when the second pressure chamber is connected to the tank and vice versa.

The actuator is, for example, a continuously variable valve, a servo valve with electrical actuation or a linear amplifier or copying valve with mechanical feedback of the position of the piston in the working cylinder.

Thanks to the invention, the noise during punching or the like of the workpiece (cutting impact) can be significantly attenuated. Furthermore, the risk of cavitation is significantly reduced.

• Figur 1 den prinzipiellen apparativen Aufbau eines hydraulischen Antriebs gemäß dem Stand der Technik;
• Figur 2 den prinzipiellen apparativen Aufbau eines weiteren herkömmlichen hydraulischen Antriebs;
• Figur 3 den prinzipiellen apparativen Aufbau eines erfindungsgemäßen hydraulischen Antriebs;
• Figur 4 einen Weg/Zeit-Graphen des Kolbens des herkömmlichen hydraulischen Antriebs von Figur 1; und
• Figur 5 einen Weg/Zeit-Graphen des Kolbens des erfindungsgemäßen hydraulischen Antriebs aus der Figur 3.

In the following, an embodiment of the invention will be explained in more detail with reference to the accompanying drawings. Show it:

• FIG. 1 the basic apparatus design of a hydraulic drive according to the prior art;
• FIG. 2 the basic apparatus structure of another conventional hydraulic drive;
• FIG. 3 the basic apparatus structure of a hydraulic drive according to the invention;
• FIG. 4 a path / time graph of the piston of the conventional hydraulic drive of FIG. 1 ; and
• FIG. 5 a path / time graph of the piston of the hydraulic drive according to the invention from the FIG. 3 ,

The Indian FIG. 3 in principle and schematically shown hydraulic drive according to the invention comprises in the FIGS. 1 and 2 a constant pump 3.1 shown for generating an operating pressure as well as an adjustable pressure to a desired output pressure relief valve 3.2. In turn, a hydraulic accumulator 3.3 is connected in the supply network, by means of which the volume flow in the supply network can be increased in a short time. The pressure supply may also have a storage charge at the location of the pressure limiting valve 3.2. Further, instead of the constant pump 3.1 or in addition to this, a variable displacement pump with controllable pump power could be provided.

The operating pressure acts via the first line B on the annular first working surface 3.15 of the piston 3.6 and exerts on the latter acting in the retraction time constant force. By appropriate control of the directional control valve 3.4, in particular continuous valve, the first working surface 3.15 opposite second working surface 3.16 of the piston 3.6 switch to tank so that there is a force acting on the piston resulting force in retraction and thus the piston moves back 3.6. If the connection P is connected to the supply pressure of the pressure source via the second pressure chamber 3.7 to the second working surface 3.16, the piston 3.6 extends. The maximum force is known to be defined by the ratio of the second working surface 3.16 to the first working surface 3.15.

The maximum extension force can be increased while maintaining a high overall efficiency, for example, by providing an additional supply pressure (high pressure "HD") which is higher than the primary applied supply pressure (low pressure "ND"). This high pressure can be switched on in different ways, for example load-dependent. For example, in this case, a high-pressure connection, as they are from the documents DE 10 2004 024 126 A1 and EP 1 138 958 B1 is known to be resorted to.

In the first line B, via the hydraulic medium displaced by the extending piston 3.6 can flow out of the first pressure chamber 3.8 bounded by the first working surface 3.15, a controllable throttle valve or, as in this case, a constant throttle 3.13 is arranged. The throttle 3.13 is bypassed by a bypass line 3.14, which opens on both sides of the throttle 3.13 in the first line B. In the bypass line 3.14, a 2/2-way valve 3.10 is arranged as a proper actuator, with which the flow of the hydraulic medium through the bypass line 3.14 can either be passed or blocked.

The 2/2-way valve 3.10 is operated hydraulically in the present case. For this purpose, a first control surface 3.12 of the 2/2-way valve 3.10 is acted upon by the supply pressure p and a second control surface 3.11 of the 2/2-way valve 3.10 with the pressure p _A prevailing in the second pressure chamber 3.7 or in the second line A behind the directional control valve 3.4 , In operation with low load forces, the pressure in the second line A or in the second pressure chamber 3.7 corresponding to the area ratio of the two working surfaces 3.15 and 3.16 always significantly lower than the pressure in the first line B and in the first pressure chamber 3.8 or in the illustrated connection of the control pressure line for the actuator (2/2-way valve 3.10) before the throttle 3.13 as the supply pressure p, which prevails there. It follows that the 2/2-way valve 3.10 is always in the passage position in operation with low load forces, so that in this case the first pressure chamber 3.8 on the sum cross section (sum of the cross sections of the throttle 3.13 and the 2/2-way valve 3.10 ) is in flow communication with the pressure source (supply pressure p).

Now, when the piston 3.6 or an associated with this punching tool hits the workpiece and is thereby braked, increases, as above under FIG. 1 addressed, the pressure in the limited of the second working surface 3.16 second pressure chamber 3.7 at. The ratio of the second control surface 3.11 to the first control surface 3.12 of the 2/2-way valve 3.10 can now be set so that the 2/2-way valve 3.10 is brought into the blocking position when a selectable critical maximum value of the pressure prevailing in the second pressure chamber 3.7 is exceeded in which the displaced by the extending piston 3.6 hydraulic medium can now flow out of the first pressure chamber 3.8 via the throttle 3.13. The acceleration of the piston 3.6 during the cutting stroke is now damped by the throttle effect by the throttle 3.13, whereby the maximum achievable piston extension speed is reduced. As a result, the directional control valve 3.4 can be reversed without the risk of the occurrence of a negative pressure in the second line A for reversing the piston movement direction. As soon as the opposing force acting on the piston drops after the workpiece has broken, the pressure in the second pressure chamber 3.7 or in the second line A, to which the control pressure line of the 2/2-way valve 3.10 is connected, also drops, so that the 2/2 Directional valve 3.10 is automatically switched to the passage position again.

By the constructive choice of the area ratio of the control surfaces 3.11 and 3.12 of the actuator, the switching threshold between the throttled position and non-throttled position can be set to any pressure ratio between the pressures in the lines A and B and thus to any working force of the piston. In systems with multiple operating pressures (especially HD and ND), this switching threshold is advantageously set to a value just below the maximum value of the worker in the first pressure stage (ND).

Instead of a constant throttle 3.13 in the first line B and a controllable throttle valve may be provided. Alternatively, the throttle 13 can be replaced by a directional control valve, in particular 2/2-way valve, which in the first switching position unthrottled and in the second switching position a has throttled passage. This 2/2-way valve (not shown), corresponding to the 2/2-way valve 3.10 from the FIG. 3 be controlled to allow a throttled backflow of hydraulic medium according to the invention from the limited first of the first working surface 3.15 first pressure chamber 3.8 optionally, always at or before a sudden acceleration of the piston 3.6 due to its sudden power relief. In the other operating states, a comparatively unthrottled return flow of hydraulic medium from the first pressure chamber 3.8 or in the first pressure chamber 3.8, the latter at a retraction movement of the piston 3.6, be provided, in particular by automatic switching or holding the 2/2-way valve 3.10 according to FIG. 3 or a 2/2-way valve with throttled passage position instead of the throttle 3.13, if in the second pressure chamber 3.7 and the second line A, a lower pressure than in the first pressure chamber 3.8 or the first line B, or, in the illustrated connection of the control pressure line to the control surface 3.12 of the 2/2-way valve 3.10 (seen from the pressure source) in front of the throttle 3.13 when the pressure in the second pressure chamber 3.7 or in the second line A exceeds the supply pressure p.

In the FIG. 5 relating to the inventive hydraulic drive of FIG. 3 relates, the time course of the path of the piston 3.6 and the prevailing in the second pressure chamber 3.7 pressure p _{A is shown} graphically. Due to the pressure increase in the second pressure chamber 3.7, the 2/2-way valve 3.10 is brought into the blocking position. After breaking the workpiece, the piston 3.6 is accelerated due to the throttling effect only to a relatively low speed 5.4. The pressure gradient 5.1 is compared with Fig. 4 significantly flatter, and the pressure p _A remains above the range of cavitation.

Claims (16)

1. Hydraulic drive, in particular for a press, a punching or nibbling machine,

   1.1 having a double-acting working cylinder (3.5), the piston (3.6) of which has a first working surface (3.15) effective in retraction direction and a second working surface (3.16) effective in the extension direction, which each define a pressure chamber (3.7, 3.8), wherein

   1.2 for retracting and extending the piston (3.6) into and out of the working cylinder (3.5) at least two different pressures can be switched on at least one of the working surfaces (3.15, 3.16) by means of an actuator, wherein

   1.3 the hydraulic drive has an actuating unit, which alternately enables a throttled return flow or a comparatively unthrottled return flow of hydraulic medium from the first pressure chamber (3.8) delimited by the first working surface (3.15), characterized in that

   1.4 the drive has control means with which before an abrupt acceleration of the piston (3.6) due to its sudden power relief, the actuating unit is triggered such that as a result of a throttling, which is caused directly or indirectly by the actuating unit, of the previously relatively unthrottled return flow of hydraulic medium from the first pressure chamber (3.8) a hydraulic damping of the movement of the piston (3.6) takes place in that

   1.5 the actuating unit, in response to a pressure increase in the second pressure chamber (3.7) delimited by the second working surface (3.16) switches in such a way that said actuating unit produces the increasingly throttled return flow of hydraulic fluid from the first pressure chamber (3.8) delimited by the first working surface (3.15) at a pressure increase in the second pressure chamber (3.7).
2. Hydraulic drive according to claim 1, characterized in that in a first line (B), via which hydraulic medium can flow out of the first pressure chamber (8), a throttle (3.13) is arranged, and the actuating unit, in particular in the form of a directional control valve, is provided in a bypass line (3.14) which is arranged in parallel to the throttle (3.13) to selectively open and close the bypass line (3.14).
3. Hydraulic drive according to claim 1, characterized in that the actuating unit comprises a throttle valve, which is arranged in a first line (B), via which the hydraulic medium can flow off from the first pressure chamber (3.8), and which has at least two switching positions, in which in a first switching position the throttling of the return flow of hydraulic medium from the first pressure chamber (3.8) is achieved and in the second switching position a comparatively lower throttling or substantially throttling-free return flow or inflow of hydraulic medium from the first pressure chamber (3.8) or into the first pressure chamber (3.8) is achieved.
4. Hydraulic drive according to claim 3, characterized in that the actuating unit, which is designed in particular as a 2/2-way valve, is integrated into the hydraulic drive in such a way that it is brought into its first switching position with throttling or into its second switching position with reduced throttling depending on the ratio of the supply pressure of a pressure source, in particular a pump (3.1) or a hydraulic accumulator (3.3), to the pressure p_A of the second pressure chamber (3.7) delimited by the second working surface (3.16).
5. Hydraulic drive according to claim 2, characterized in that the actuating unit is designed as a 2/2-way valve (3.10).
6. Hydraulic drive according to claim 2 or 5, characterized in that the actuating unit is integrated into the drive in such a way that it is brought into the passage position or blocking position depending on the ratio of the supply pressure of a pressure source, in particular a pump (3.1) or a hydraulic accumulator (3.3), to the pressure p_A which prevails in the second pressure chamber (3.7) delimited by the second working surface (3.16).
7. Hydraulic drive according to one of the claims 2 to 6, characterized in that a first control surface (3.12) of the actuating unit is subjected to the pressure of the first pressure chamber (3.8), a first line (B) connected thereto or the supply pressure of a pressure source, in particular a pump (3.1) or a hydraulic accumulator (3.3), and a second control surface (3.11) of the actuating unit to the pressure from the second pressure chamber (3.7) or a second line (A) connected thereto, wherein the actuating unit is brought into the throttle position or reduced throttle position or passage position or blocking position depending on the ratio of the pressure exerted on the first control surface (3.12) and the pressure exerted on the second control surface (3.11).
8. Hydraulic drive according to one of the claims 1 to 7, characterized in that the working cylinder (3.5) is designed as a differential cylinder.
9. Hydraulic drive according to one of the claims 1 to 8, characterized in that the second pressure chamber (3.7) can be subjected to supply pressure or tank pressure for extending or retracting the piston (3.6) by means of the actuator, and that the first pressure chamber (3.8), when the return flow of hydraulic medium is not throttled therefrom, is constantly subjected therefrom to supply pressure.
10. Hydraulic drive according to one of claims 1 to 8, characterized in that the second pressure chamber (3.7) is connectable to a low pressure source, a high pressure source or a tank, and that the first pressure chamber (3.8) is always connected to the low pressure source.
11. Hydraulic drive according to one of the claims 1 to 8, characterized in that the second pressure chamber (3.7) and the first pressure chamber (3.8) are alternately subjected to supply pressure or tank pressure for extending or retracting the piston (3.6) by means of the actuator.
12. Method for controlling a hydraulic drive with a double-acting working cylinder (3.5), the piston (3.6) of which has a first working surface (3.15) effective in retraction direction and a second working surface (3.16) effective in the extension direction, which each define a pressure chamber (3.7, 3.8), wherein for retracting and extending the piston (3.6) at least two different pressures can be switched on at least one working surface (3.15, 3.16) by means of an actuator, characterized in that
    before a sudden acceleration of the piston (3.6) due to its sudden power relief a previously comparatively unthrottled return flow of hydraulic medium from the first pressure chamber (3.8) delimited by the first working surface is throttled more strongly, so that the extension movement of the piston (3.6) is hydraulically damped in that the actuating unit switches in such a way depending on a pressure increase in the second pressure chamber (3.7) delimited by the second working surface (3.16) that the actuating unit produces the increasingly throttled return flow of hydraulic medium from the first pressure chamber (3.8) delimited by the first working surface (3.15) at a pressure increase in the second pressure chamber (3.7).
13. Method according to claim 12, characterized in that the increased throttling is effected by closing a bypass line (3.14), which is arranged parallel to a throttle (3.13) in a first line (B), via which the hydraulic medium can flow off from the first pressure chamber (3.8), and bypasses the throttle (3.13).
14. Method according to claim 13, characterized in that the closing of the bypass line (3.14) is effected by means of an actuating unit, in particular in the form of a 2/2-way valve (3.10), which is switched or controlled in particular depending on the pressure ratio or the pressure difference between the pressure in the second pressure chamber (3.7) or in a second line (A) connected thereto and the pressure in the first pressure chamber (3.8), a first line (B) connected thereto or the supply pressure.
15. A method according to claim 12, characterized in that the increased throttling is effected by the triggering or switching of an actuating unit, which is arranged in a first line (B), via which the hydraulic medium is removed from the first pressure chamber (3.8), and which is designed in particular as a 2/2-way valve with a first switching position with a comparatively stronger throttling and a second switching position with a comparatively reduced throttling, and the increased throttling is effected by switching to the first switching position.
16. Method according to claim 15, characterized in that the actuating unit is switched depending on the pressure ratio or the pressure difference between a pressure in the second pressure chamber (3.7) or a second line (A) connected thereto and the pressure in the first pressure chamber (3.8) in a first line (B) connected thereto or the supply pressure.

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