EP1987255A1

EP1987255A1 - Pneumatic drive system

Info

Publication number: EP1987255A1
Application number: EP06707116A
Authority: EP
Inventors: Ulrich Hirling; Ute Casimir
Original assignee: Festo SE and Co KG
Current assignee: Festo SE and Co KG
Priority date: 2006-02-21
Filing date: 2006-02-21
Publication date: 2008-11-05
Anticipated expiration: 2026-02-21
Also published as: EP1987255B1; CN101384825A; US20100276171A1; NO334223B1; WO2007095964A1; AU2006338713A1; AU2006338713B2; CA2640774C; CN101384825B; CA2640774A1; US7896102B2; NO20083999L

Abstract

Proposed is a pneumatic drive system (1), in which the movement of a drive output unit (7) of a pneumatic drive (2) is controlled by control valve means (25, 26) which are connected to the working chambers (12, 13) of the pneumatic drive (2). The control valve means (25, 26) comprise at least one air-saving position (30) which defines a throttle cross section, and a high-force position (29) which defines a flow cross section which is greater than said throttle cross section. It is provided by actuating means (36, 37) that the control valve means (25, 26) are switched, as a function of the air pressure prevailing in at least one working chamber (12, 13), from the normally-assumed air-saving position (30) into the high-force position (29) when the drive output unit (7) is subjected to an increased movement resistance.

Description

Pneumatic drive system

The invention relates to a pneumatic drive system, comprising at least one pneumatic drive, which has a drive housing and a driven in this regard by compressed air driven output unit, wherein the output unit includes a driven piston, which divides in the drive housing two working chambers of which one or both controlled to Serving compressed air serving control valve means are connected, which are switchable between a plurality of switching positions, among which there is a one NEN throttle section predetermining air-saving.

A known from WO 02/14698 Al pneumatic drive system of this type is used for crust crusher applications in aluminum processing. It contains a pneumatic drive designed as a crusher cylinder, whose output unit can be driven to perform oscillating working movements in which it is temporarily immersed in an aluminum melt bath while puncturing a possibly constructed material crust. Responsible for the respective working movement is a directional presetting valve, which controls the supply and discharge of compressed air in two working chambers divided by the output piston of the output unit. The control of the compressed air supply also has doubly existing control valve means, which are connected in the connection between the directional specification valve and a respective working chamber. are turned on. These control valve means can assume different switching positions, wherein a switching position is responsible for causing the working movement by releasing an air passage. In order to minimize the air consumption, this switching position is designed as air-saving, in that the fluid passage has a throttle cross-section, which allows only a limited flow. Thus, the degree of filling of the connected working chamber remains at the lowest possible level. If the output unit meets an aluminum crust and is therefore opposed to increased resistance to movement, progressively higher actuating pressure gradually builds up in the connected working chamber over the throttle cross section until the required penetration force has been achieved. Upon arrival in a stroke end position causes the output unit finally switching the control valve means in a blocking position to prevent further compressed air supply to the pneumatic drive.

Due to the time required to build up the pressure in the pneumatic drive, when the output unit has to break a melt crust, irregular time delays arise from time to time during the individual work cycles.

A comparable arrangement is described in EP 0771396 B1. Although there is described as an alternative design also the possibility to dispense with a built-in control lines choke. However, this has a constant intensive pressurization of the working chambers result, which adversely affects the compressed air consumption.

An essential object of the present invention is to propose measures which allow a reduction in the allow no excessive consumption of compressed air.

To solve this problem, it is provided that the control valve means as a further switching position have a comparatively s to the throttle cross-section larger Strömungsguerschnitt predetermining high power position, and that the control valve means are assigned actuating means, the switching of connected to this working chamber control valve means during the compressed air supply into a working chamber as a function of the air pressure prevailing in at least one working chamber, such that switching takes place from the normally assumed air-saving position into the high-force position, if and at least as long as the drive unit is exposed to an increased resistance to movement.

In this way, the output unit moves so long in the air saving mode, as it is not exposed to increased resistance to movement. Due to the effective throttle cross section, the degree of filling of the connected

2o the working chamber to a minimum, and consequently the

Air consumption. However, as soon as an increased resistance to movement on the part of the output unit is recorded, the control valve means responsible for the pressure feed into the relevant working chamber switch over into the high force position due to the pressure change occurring in the pneumatic drive 5 and allow a faster flow of air and therefore a rapid pressure increase with increased flow cross section. hung in the connected working chamber. This leads to an increase in the force and overcoming the 0 output unit opposite movement resistance. Mach reducing the resistance to movement may return the control valve means back to the air-saving turn. Thus, an additional air consumption occurs only from or during the operating phase, in which a higher actuation pressure is actually required. Otherwise, the air consumption remains at the throttled normal level. At the same time s reduce the cycle times, because the air filling time is significantly shorter in the high power position than in the always maintained in the prior art air-saving.

The described advantages are particularly noteworthy if the pneumatic drive system is used as crustal breaker system in aluminum production or processing. Due to the short operating cycle times, the air saving proves to be immense. At the same time, if required, an increased actuating force is available with only a short time delay in order, for example, to break through an aluminum crust or to strip off solidified aluminum material adhering to the output unit. Due to the pressure-controlled actuation, there is also the advantage that the pressure build-up in the working chamber responsible for the current working movement is taken into account.

20 takes account of the height of the resistance to be overcome variable takes place. It can thus be achieved that only as much compressed air is fed into the pneumatic drive in the high-power phase, as is required to overcome the movement resistance that is just occurring.

Advantageous developments of the invention will become apparent from the dependent claims.

Although the inventive concept can also be used in rotary and rotary actuators, its application proves to be particularly advantageous in linear drives. The at least one linear drive is preferably a pneumatic cylinder with a piston rod which can be used as a crust breaker cylinder. However, the use is not limited to the range of crust crusher applications 5.

The actuating means for the control valve means are in particular designed so that they control the switching operation in dependence on that air pressure prevailing in the working chamber connected to the control valve means lo. When a resistance to movement of this air pressure increases and causes the switch from the air-saving position in the high-power position.

The switching position of the control valve means is expediently predetermined by the currently assumed position of a control valve member of this control valve means. This is expediently switched in the direction of the air-saving of the input side to the control valve means pending input pressure. The output side of the control valve means, ie on the side of the connected Arbeitskam-

2o mer, prevailing outlet pressure acts on the control valve member opposite in the direction of the high power position. At the same time effective spring means are available in this direction. When the force of the spring means and the force resulting from the output pressure are greater overall

25 as the force resulting from the input pressure, the switching takes place in the high-force position. If the actuating force of the spring means can be set variably, it is possible to individually set the switching threshold.

0 The spring means expediently ensure that the

Control valve means in the depressurized state the high pressure take up. If the operating pressure is switched on, in particular through the intermediary of an upstream directional specification valve, a delayed build-up of the actuating force resulting from the outlet pressure can be caused by a throttling point connected to the outlet pressure, so that the control valve means immediately assume the air-saving position.

A further advantage can be achieved if the control valve means have a third switching position, in which the compressed air is made available in comparison with the throttle cross-section smaller flow cross-section. This switching position is referred to as a holding position, because it is effective to securely hold the output unit in its Hubendlage. The holding position of the control valve means becomes effective as a function of the position of the output unit when the latter approaches or has reached the stroke end position. The switching can be caused mechanically, for example due to a cooperating with the output unit ram actuated actuator, but also electrically, using suitable position sensor means. The reduced flow cross-section available in the stop position prevents too much filling of the connected working chamber and at the same time compensates for possible leakage losses, so that the drive unit is held constant and does not perform any oscillatory movements. A design is considered to be optimal in which the flow cross-section released in the holding position has a dimension which, taking into account the pending operating pressure, predetermines a flow which at least substantially corresponds to the leakage occurring in the pneumatic drive. In this way, the degree of air filling in the connected working chamber does not increase or only slightly, although the air connection is not shut off as required by the prior art.

The invention will be explained in more detail with reference to the accompanying drawings. The sole figure (Figure 1) shows the pneumatic drive system as a simplified circuit diagram in a preferred embodiment which is particularly, but not exclusively, suitable for crust crusher applications.

The pneumatic drive system designated in its entirety by reference numeral 1 comprises at least one pneumatic drive 2, which is expediently a linear drive. Associated with it is a control device serving its operational control, designated overall by reference numeral 3.

The type of pneumatic drive 2 is in principle arbitrary. For example, it could be a rodless linear drive. As an example, it is designed as equipped with a piston rod 4 pneumatic cylinder.

The pneumatic drive 2 contains a housing designated as a drive housing 5, having a certain longitudinal extent, in the interior of which there is a linearly displaceable output piston 6 which is combined with the already mentioned piston rod 4 to form a movement unit designated as output unit 7. This output unit 7 is under the execution of either an extending or a retracting working movement 8a, 8b relative to the drive housing 5 linearly movable.

By the output piston 6, the interior of the drive housing 5 in a back-side first working chamber 12 and a front, penetrated by the piston rod 4 second working chamber 13 divided.

The first working chamber 12 is connected to a first fluidic control line 14, the second working chamber 13 to a second fluidic control line 15. These two

Control lines 14, 15 are also part of the control device 3 as a directional default valve 16, to which the two control lines 14, 15 are connected with their pneumatic drive 2 opposite ends.

About the directional default valve 16, the pressurization of the two working chambers 12, 13 are controlled to cause the currently desired working movement 8a, 8b of the output unit 7. The directional default valve 16 may connect either one (14) or the other (15) control line to a compressed air source 17, while simultaneously bleeding the other control line 15, 14 to the atmosphere 18, depending on the switch position taken by it. The compressed air source 17 provides pressurized air under a certain operating pressure.

The directional specification valve is exemplified by a 5/2 way valve. It is biased by a spring device 22 in an apparent from Figure 1 basic position in which the second control line 15 is connected to the compressed air source 17 and the first control line 14 is vented. By an electric or electromagnetic actuator 23, the directional specification valve 16 can be switched to the opposite switching position.

The directional default valve 11 may be a directly actuated or pilot operated valve. to Realization of the desired functionality can also consist of several functionally linked individual valves, for example, two 3/2 way valves.

In a preferred application, the pneumatic drive 2 is designed as Krustenbrecherzylinder. In this case, an impact element 24, which is suitable for penetrating the crust formed on the surface of an aluminum melt bath or of another molten metal bath, is arranged on the end region of the piston rod 4 located outside the drive housing 5. Here, the pneumatic drive 2 is typically installed with a vertical longitudinal direction and downwardly projecting piston rod 4. When the driven unit 7 is retracted - this condition is evident from FIG. 1 - the push element 24 occupies a position at a distance above the material crust. To pierce the crust, the output unit 7 is driven to its extending working movement 8a, wherein it dives with the shock element 24 ahead, piercing the possibly existing crust in the molten aluminum bath.

The first and second control valve means 25, 26 operating independently of one another are connected to the two working chambers 12, 13. The first control valve means 25 are turned on in the course of the first control line 14, the second control valve means 26 in the course of the second control line 15. They allow, in addition to the directional specification valve 16, a particularly controlled pressurized air admission of the respectively connected working chamber 12, 13.

The control valve means 25, 26 each have a valve inlet 27 connected to the directional specification valve 16 and a valve outlet 28 connected to the working chamber 12, 13 to be controlled. Both control valve means 25, 26 are switchable between different switching positions. In this case, both control valve means 25, 26 alternatively occupy a high force position 29, an air-saving position 30 and a stop position 31. Shown is an operating state in which the first control valve means 25 are in the high-force position and the second control valve means 26 are in the air-saving position.

Conveniently, the two control valve means 25, 26 each formed by a control valve which has an optionally positionable in one of three positions control valve member 32, which is illustrated purely symbolically in the drawing. The control valve member 32 may be, for example, a piston valve.

All three switch positions have in common that they release a compressed air connection between the directional specification valve 16 and the connected working chamber 12, 13. The only difference is the size of the shared flow cross section. In no switch position, the air passage is completely shut off.

The flow cross-section released in the air-saving division 30 is referred to as the throttle cross-section. It is less than the nominal cross section of the respectively connected control line 14, 15 and causes a throttling of the flowing compressed air. If the output unit 7 can move unhindered, thus at the valve output 28 is compared to the operating pressure fed lower output pressure, which is present as the instantaneous working pressure in the connected working chamber 12, 13.

The flow cross section released in the high force position 29 is greater than the throttle cross section. He allows in particular an unthrottled air passage and suitably corresponds to the nominal cross section of the control lines 14, 15th

The smallest flow cross section is provided in the holding position 31. This is even much smaller than the effective in the air-saving 30 throttle area, which will be discussed later.

Both control valve means 25, 26 are assigned independently operating first and second actuating means 36, 37. These are responsible for whether the associated control valve means 25, 26 occupy the high-force position 29 or 30 Luftsparstellung. Switching to the holding position 31, however, they can not cause.

For switching to the holding position 31, first and second further actuating means 38, 39 are responsible, which unlike the purely pressure-dependent first and second operating means 36, 37 are activated or deactivated expediently purely as a function of the linear position of the output unit 7, with respect to The first and second actuating means 36, 37 have priority. If the output unit 7 reaches a position relevant for switching to the holding position 31, the switching operation takes place irrespective of whether the control valve means 25, 26 had previously taken up the high-force position 29 or the air-saving division 30.

The first and second actuation means 36, 37 are capable of controlling the switching of the associated control valve means 25, 26 in response to the air pressure prevailing in at least one working chamber. The control is based in particular on the pressure that is currently in the Compressed air fed working chamber 12, 13 prevails and which in this case coincides with the prevailing at the valve outlet 28 outlet pressure. The design is expediently such that normally, when the output unit 7 can move without interruption, the air-purging 30 is present and that, starting therefrom, a switch to the high-power position 29 is caused when the output unit 7 during its working movement 8a, 8b is exposed to an increased resistance to movement and thereby increases in the currently supplied with compressed air working chamber 12, 13 prevailing working pressure up to a predetermined switching threshold.

To enable this switching operation in a particularly simple manner, a respective control valve member 32 in the embodiment, two oppositely oriented first and second Luftbeaufschlagungsflächen 42, 43 assigned. Actuation of the first air-admission surface 42 leads to a restoring force in the direction of the air-saving position 30, an admission of the second air-admission surface 43 has an effective in the direction of the high-force position 29 actuating force.

The first Luftbeaufschlagungsfläche 42 is supplied via a first supply passage 44 of the pending on the valve inlet 27 inlet pressure. Via a second admission channel 45, the second air admission surface 43 is acted upon by the outlet pressure prevailing at the valve outlet 28. In addition, spring means 46 are present, which exert a force acting also in the direction of the high force position on the control valve member 32. The actuating force of the pedestrian means 46 is expediently adjustable, which is illustrated symbolically by an oblique arrow.

In the course of the second Beaufschlagungskanals 45 a throttle point 47 is suitably turned on, which causes a time-delayed pressure force build-up on the second Luftbeaufschlagungsfläche 43.

If the holding position 31 is initially disregarded, in particular the operating sequence of the pneumatic drive system 1 explained below is possible.

The explanation begins with a basic position as far as possible in the drive housing 5 retracted output unit 7 and pressureless system. Here, the two control valve means 25, 26 - if the further actuating means 38, 39 would not be present - held by the force of the spring means 46 in the maximum flow permitting high force position 29.

Based on this, the directional specification valve 16 is switched with the compressed air source 17 in the second switching position, not shown, so that the first control line 14 supplied under operating pressure compressed air and at the same time the second control line 15 is vented. The compressed air flowing in via the first control line 14 flows through the first control valve means 25 located in the high force position and acts on the output unit 7 in the direction of extension, so that it is driven to the extending working movement 8a. The compressed air which has been ejected from the second working chamber 13 by the output piston 6 passes through the high-force position which is also enabled by the spring means 46 in the likewise full passage. Since the control line 15 is at atmospheric pressure, the switching position of the second control valve means 26 is not affected during the venting phase.

Immediately after the air feed into the first control line 14, the first control valve means 25 switch to the air-saving position 30. This is due to the fact that the operating pressure hitherto prevailing in the entire first control line 14 can act on the first air-admission surface 42 without restriction, but due to the intermediate throttle restriction 47, an initially only low activation pressure is applied to the second air-admission surface 43. The design is such that the 42 imposed on the first air i5 impact surface, in the direction of

Air-pressure acting pressure force is greater than the sum of the pressure applied to the second Luftbeaufschlagungsfläche 43 and the force of the spring means 46th

After switching to the air-purging 30 2o arises due to the now effective throttle cross-section compared to the input pressure lower output pressure, which is also effective in the connected first working chamber 12 and there for the propulsion of the output unit 7.

Even if a constant actuating pressure prevails in the entire second admission channel 45 after a certain time, the first control valve means 25 remain in the air-saving position, because the design of the first and second actuating means 36, 37 is selected such that the above-mentioned

30 te, maximum actuation pressure corresponding to the valve output pressure together with the spring means 46 a maximum Actuate force that is below the on the valve input pressure based, opposite actuating force.

As long as the output unit 7 encounters no obstacle, it is now extended with the reduced output pressure of the first control valve means 25, wherein the degree of filling of the first working chamber 12 corresponding to the low output pressure is also relatively low.

If the output unit 7 reaches its maximum extended stroke end position undisturbed, a reversed sequence of motions can be initiated by switching the directional specification valve 16, the second control valve means 26 behaving as the first control valve means 25 and the first control valve means 25 behave like the second control valve means 26 before ,

The operating behavior changes, however, if the output unit 7 is opposed to an increased resistance to movement during one or the other working movement 8a, 8b. During extension, this may be due to the output unit 7 impinging with its impact element 24 on a material crust to be pierced. During retraction, such a resistance can be caused for example by solidified materials from the melting pot, which are deposited on the extended end portion of the piston rod 4.

In such an operating state, the working pressure prevailing in the working chamber 12 or 13 which is currently pressurized with compressed air increases. The speed of the pressure increase depends on the Querschnittsgrδße of the released in the air-saving throttle area. Since the working pressure increasing in the applied working chamber 12 or 13 also acts on the control valve member 32 via the second admission passage 45, the operating force 5 acting in the direction of the high force position eventually exceeds the opposite operating force effective over the first air impingement face 42. The relevant for the changeover switching threshold can be influenced and set by mutual coordination of the surface dimensions of the two Luftbeaufschlagungsflächen 42, 43 and the restoring force lo of the spring means 46.

In a typical application, an operating pressure of 6 bar is applied, from which a working chamber pressure of 2 bar results in the air-purging, wherein the switching threshold value for switching to the high-force position is at a working chamber pressure of approximately 2.5 bar.

By switching to the high force position of the injected compressed air is a much larger flow area available. Consequently, the prevailing in the connected working chamber 12 or 13 working pressure increases in

20 short time to maximally fed to the control valve means operating pressure, so that the output unit 7 is subjected to a greatly increased fluidic force, due to which it is able to overcome the resistance to movement, i. in the present case, for example, the mate-

25 rial crust to break.

As soon as the output unit 7 can be moved again with less resistance, as a rule the working chamber pressure drops again due to the system dynamics, so that a resultant operating force is again applied to the control valve member 32 in favor of switching to the air gap. ment and a corresponding switching back into the air-saving position 30 takes place.

Even if the control valve means 25, 26 can not switch back to the air saving after switching to the high power position during the ongoing work movement due to the dynamics of the system, a significant air consumption advantage remains, because switching to the high power position in the individual working movements always only occurs when an increased resistance to movement occurs. In many cases, this will not be the case, so that then a operation under full utilization of the air-saving function is possible.

Further advantages arise when the control valve means 25, 26 allow the already mentioned additional switchover to a holding position 31.

In this connection, the further actuation means 38, 39 are designed such that they bring about the holding position currently enabling compressed air to be fed into a working chamber 12 or 13 into the holding flow which enables an only greatly reduced flow throughput, if the output unit 7 reached a stroke end position or a position just before the Hubendlage. This position-dependent switching ensures that in the Hubendlagen when the output unit 7 can not move on, the compressed air can flow with a further reduced flow rate in the connected working chamber 12 or 13, as long as the directional specification valve 16 is not switched.

Due to the constantly maintained air supply can be compared to a complete shut off the big advantage be achieved that occurring in the system leaks are compensated and the prevailing in the applied working chamber air pressure normally never falls below a threshold, the relative movements of the output unit 7 with respect to 5 of the drive housing 5 permits.

This is particularly relevant when used as Krustenbrecherzy- linder when it comes to securely retract the retracted and therefore highly powered output unit 7 and to prevent even a minimal decrease.

It is expedient to select the flow cross-section of the control valve means 25, 26 released in the holding position 31 in such a way that, based on the pending operating pressure, the permitted flow corresponds at least substantially to the leakage occurring in the pneumatic drive 2. Preferably, the permitted flow rate is at least equal to or slightly greater than the leakage flow which occurs, for example, between the output piston 6 and the drive housing 5.

To detect that axial position of the output unit 7, 2o in which the switching of the control valve means 25, 26 is to be triggered in the holding position 31, the further actuating means 38, 39 are equipped with suitable response means 48, 49. These response means 48, 49 are expediently on or in the drive housing 5, wherein they 5 are formed in the embodiment in order to produce a purely mechanical switching of the control valve means 25, 26.

For the purpose of mechanical activation, they expediently each comprise at least one stelike actuator 0 48a, 49a, which protrudes into the stroke of the output unit 7, that it is acted upon by these on reaching the desired switching position and relocated.

Conveniently, the response means 48, 49 are direct components of the control valve means 25, 26. This makes it possible in a particularly advantageous manner, the control valve means 25, 26 to install directly on or in the drive housing 5, as indicated by dash-dotted lines in Figure 1. For reasons of better clarity, the control valve means 25, 26 are shown separately in FIG. 1 from the drive housing 5 and it is made clear by respectively assigning the reference numerals 48, 49 which response means 48, 49 belong to which control valve means 25, 26.

A purely mechanical switching has the advantage that can be dispensed with an electrical equipment. However, it would readily be possible to provide responsive sensors 48, 49 without contact with the position of the output unit 7 to be detected, which output an electrical sensor signal upon activation, by means of which electrical switching of the control valves 25, 26 into the Haltment 31 is caused.

At this point, it should be mentioned that, in principle, the switching between the high-force position 29 and the air-purging position 30 can also be caused by electrical signals when the relevant pressure parameters are picked up by pressure switches or pressure sensors.

In conjunction with the further actuating means 38, 39 also results in the start of the above-described operation, a change to the effect that the output unit 7 initially briefly moves at a reduced speed, because the switched on the ventilation Arbeitsamam- mer associated control valve means are held by the further actuating means in the holding position 31 until the output unit has left the response range of the addressing means 48 and 49 respectively.

5 As long as the output unit 7 is in the response range of the response means 48 or 49, take the associated control valve means 25, 26, the holding position 31 irrespective of the prevailing in the working chambers 12, 13 working pressures. The switching position is here specified depending on the position of the positio on the output unit 7. Only outside this response range is the switching position of the control valve means 25, 26 controlled pressure-dependent between the air-saving position 30 and the high-power position 29.

As already indicated, at least the two control valves 5, 25 can be designed as a structural unit with the pneumatic drive 2. The directional specification valve 16 may also be a component of this structural unit.

The pneumatic drive system 1 may comprise more than just a pneumatic drive 2, in which case each pneumatic drive 20 is expediently assigned its own first and second control valve means 25, 26. The directional specification valve 16, in contrast, can in principle serve for the simultaneous control of several parallel-connected pneumatic drives 2.

Deviating from the exemplary embodiment, the pneumatic kantrieb 2 associated control valve means 25, 26 may be present only simply. They are then suitably switched on either in the first control line 14 or in the second control line 15, depending on which stroke direction the associated functionality is desired.

Claims

claims

1. Pneumatic drive system, comprising at least one pneumatic drive (2), the drive housing (5) and a movable in this regard by compressed air Ab 5 drive unit (7), wherein the output unit (7) includes a driven piston (6) in the Drive housing (5) two working chambers (12, 13) from one another divides, one or both of which are connected to the controlled compressed air control valve means (25, 26) are connected, o which are switchable between a plurality of switching positions, under which a throttle cross-section predetermining air-saving ( 30), characterized in that. the control valve means (25, 26) as a further switching position has a greater compared to the throttle cross section s flow cross-section high-force position (29), and in that the control valve means (25, 26) actuating means (36, 37) are assigned, during the compressed air supply in a working chamber (12, 13) to control the switching of the control valve means (25, 26) connected to this working chamber (12, 13) as a function of the air pressure prevailing in at least one working chamber (12, 13) in such a way that switching over is possible the normally assumed air-saving position (30) takes place in the high-force position (29) when and at least as long as the output unit (7) is subjected to an increased resistance to movement.

2. Drive system according to claim 1, characterized in that the at least one pneumatic drive (2) is a linear drive.

3. Drive system according to claim 1 or 2, characterized

5 shows that the at least one pneumatic drive (2) is a pneumatic cylinder whose output unit (7) contains a piston rod (4) projecting from the drive housing (5) at the end face.

4. Drive system according to claim 3, characterized in that the pneumatic cylinder is a crust breaker cylinder, on the piston rod (4) of which a push-through element (24) suitable for piercing the crust of a molten metal bath is arranged.

Drive system according to one of Claims 1 to 4, characterized in that the actuating means (36, 37) are designed to switch over the control valve means (25, 26) as a function of that in the control valve means (25, 26). 26) connected working chamber (12, 13) control the prevailing air pressure.

6. Drive system according to claim 5, characterized in that the actuating means (36, 37) are designed such that they switch the control valve means (25, 26) from the hitherto assumed air-saving position (30) in the high-force position (29) in the at this control valve means

25 (25, 26) connected working chamber (12, 13) prevailing air pressure has risen to a predetermined switching threshold.

7. Drive system according to claim 5 or 6, characterized in that the control valve means (25, 26) by a se- ne currently occupied position the switching position of the control valve means (25, 26) defining the control valve member

(32), the inlet pressure acting in the direction of air-saving (30), and the outlet pressure of the control valve means acting in the direction of the high-pressure position (29)

(25, 26) is switched on and is additionally acted upon in the direction of high force position by spring means (46).

8. Drive system according to claim 7, characterized in that the spring means (46) are formed in their actuating force adjustable lo.

9. Drive system according to claim 7 or 8, characterized in that in the control valve member (32) the output pressure aufschaltenden admission channel (45) a time-delayed pressure force build-up on the control valve member i5 (32) inducing throttle point (47) is turned on.

10. Drive system according to one of claims 1 to 9, characterized in that the control valve means (25, 26) on the input side with a compressed air source (17) connectable or connected and in particular as 5/2 way valve ausge-

20 upstream Richtungsvorgabeventil (16) is connected upstream, which is able to alternately feed or vent the two working chambers (12, 13) in opposite directions with compressed air.

11. Drive system according to one of claims 1 to 10, characterized in that the control valve means (25, 26)

25 as another switching position a in comparison to the

Throttle cross-section smaller flow cross-section have predetermined holding position (31) and that the control valve means (25, 26) in dependence on the position of the output unit (7) activatable further actuating means (38, 39) are assigned net, the switching to the holding position (31 ) ken, when the output unit (7) reaches a stroke end position or a position shortly before the stroke end position during their working movement.

12. Drive system according to claim 11, characterized in that the further actuating means (38, 39) to a predetermined position of the output unit (7) responsive and thereby switching to the holding position (31) causing response means (48, 49) included.

13. Drive system according to claim 12, characterized in that the response means (48, 49) comprise at least one in the stroke of the output unit (7) projecting, preferably impact-shaped actuator (48 a, 49 a).

14. Drive system according to one of claims 11 to 13, characterized in that the control valve means (25, 26) i5 to specify their switching positions include a positionable in either one of three positions control valve member (32).

15. Drive system according to one of claims 11 to 14, characterized in that in the holding position (31) of the

2o control valve means (25, 26) released flow cross-section has a degree that predetermines a flow, which corresponds at least to the occurring in the pneumatic drive leakage and is expediently in the range of this leakage.