DE102009031743A1 - Mobile work machine, particularly industrial truck, has pressure chamber which is upstream to brake valve and another pressure chamber which is upstream to another brake valve - Google Patents

Abstract

The mobile work machine has a brake, which is designed as hydraulically detachable spring-loaded brake (3a,3b). The work machine has a pressure chamber (4a,4b) which is upstream to a brake valve (6) and another pressure chamber (9a,9b) which is upstream to another brake valve (11). Both the brake valves are formed as controlled valves, which stand in operative connection with a common operating element.

Description

The Invention relates to a mobile work machine, in particular industrial truck, with at least one brake acting as hydraulically releasable Spring brake is formed and a first, opposite to a spring storage effective pressure chamber and a second, in the same direction Having the spring storage effective pressure chamber.

In the DE 199 03 564 A1 is disclosed a brake for a generic machine. The brake combines the function of a parking brake and the function of a service brake. The parking brake is assigned to the first pressure chamber, wherein the brake is closed in the pressureless state by the action of the spring accumulator and thus the working machine is locked. To release the brake, the first pressure chamber is acted upon by a sufficiently large, counteracting the spring accumulator pressure. To decelerate the moving work machine in normal situations, a controllable brake pressure is generated in the second pressure chamber, which closes the brake against the effect of the pressure in the first pressure chamber. This process is also referred to as "override".

at a failure of the pressure supply of the second pressure chamber, the moving machine after discharge of the first pressure chamber nevertheless with the help of the braking force generating spring accumulator to a standstill to be brought. The parking brake thus also serves as a safety brake. However, it acts a constant, relatively large braking force. If the work machine designed as a forklift, so it can when transporting the load and when the pressure supply fails the first pressure chamber to a sudden collapse of the Brake and thus under unfavorable circumstances too a drop of the load come.

Of the The present invention is based on the object of a work machine to provide the type mentioned above, at the sudden, unexpected closing of the Brake is avoided.

These The object is achieved according to the invention that the first pressure chamber, a first brake valve and the second Pressure chamber is connected upstream of a second brake valve, wherein the two brake valves are designed as controllable valves with at least one common operating element in operative connection.

Of the Thought essential to the invention is therefore, by use suitable valve means not only the pressure in the second pressure chamber, but also be able to control the pressure in the first pressure chamber and while using a common operating element (it can also two common control elements be present, for. B. in shape a so-called "double pedal control", at in each case a drive / brake pedal of the forward direction and a travel / brake pedal of the reverse direction assigned). If one of the two brake circuits fails, can then be the work machine by pressing of the common control element, which acts both on the brake valve of the first pressure chamber and on the brake valve of the second pressure chamber acting, still dosed decelerate.

Conveniently, is the first brake valve as a proportional pressure reducing valve with falling characteristic and the second brake valve as a proportional pressure reducing valve formed with increasing characteristic.

According to one Embodiment of the invention, the common control element is designed as a brake pedal.

Further proves to be useful when the operator starting from a starting position a first operating area in which only the second brake valve actuated is, and a subsequent second operating area, in addition, the first brake valve actuated is. Thus, the brake valves come offset away one after the other Commitment. This is initially metered in the second pressure chamber the brake pressure increases (oversteer). Only in the second operating range of the brake pedal is additionally the first pressure chamber vented.

It However, it is also possible, the two brake valves directly to couple with each other, so that when the common Operating member both brake valves pressure-controlling act. This execution is particularly appropriate if the first and the second Brake valve are integrated into a common structural unit.

In Advantageous development of the invention is proposed that the first pressure chamber is preceded by a parking brake valve, the one electrically actuated, the first pressure chamber with a pressure supply device connecting, first switching position, an electrically operable, the first pressure chamber with a Pressure sink connecting second switch position and in the de-energized Condition effective third switching position, in which the first Pressure chamber with a pressure reducing device or a pressure holding device connected is.

It is thus provided as an electrically operable three-position valve parking brake valve, wherein the two electrically activated switching positions for the pressure build-up and the pressure reduction in the first pressure chamber of the brake and thus responsible for the opening and closing of the parking brake. On the other hand ensures in the de-energized state, ie when switching off or failure of electrical energy, then automatically assumed third switching position of the parking brake valve for maintaining or gradually lowering the hydraulic pressure in the first pressure chamber.

By the electric controllable parking brake valve leaves the spring brake both open and again shut down. This makes it possible while the operation of the working machine in which both driven and - especially at stationary vehicle - is working, the spring brake automated to operate and thus the function of a to achieve automatic parking brake, eliminating certain work processes be relieved. This allows the operator to stop the working machine to the operation to be performed concentrate without thinking about locking or loosening the Need to waste spring brake.

He follows while driving machine a power failure, for example by a defect or by accidentally pressing an emergency stop button, then by the embodiment of the invention the parking brake valve further prevents the vehicle by abruptly closing the spring-loaded brake is forced braked. A recorded load is therefore not unexpected thrown off by a sudden, unwanted braking.

The Operator of the work machine, instead, the vehicle targeted to a halt by operating the service brake becomes. In this case, in the manner already described, the brake pedal depressed and first through the second brake valve generates a brake pressure in the second pressure chamber of the brake in the closing direction acts, in addition to the Pressure in the first pressure chamber by actuation of the first brake valve is lowered.

alternative this is done in the presence of a pressure reduction device, which in the third shift position of the parking brake valve is effective, a delayed, gradual closing the spring brake and thus a defined deceleration of the Machine achieved. Here, the operator still has the Possibility of braking by pedaling the brake pedal To shorten.

According to one advantageous embodiment of the invention is the pressure reduction device as throttle point, as adjustable and / or controllable throttle valve or designed as a pressure relief valve. Depending on the characteristic of the Throttle, the throttle valve or the pressure relief valve results in a certain time course of the pressure reduction in first pressure chamber of the brake, the full braking torque only at complete pressure reduction works.

In Advantageous development of the invention is proposed that the flow cross-section of the pressure reducing device by the pressure downstream of the first brake valve is controllable. At maximum brake pressure in the line section between the first brake valve and the parking brake valve and Consequently, the first pressure chamber of the brake is the largest possible opening cross-section set the throttle valve, while decreasing Brake pressure reduces the opening cross-section of the throttle valve and thereby delays the pressure reduction in the first pressure chamber of the spring brake becomes. As a result, the further mining is delayed the brake pressure and the closing of the spring brake.

is instead of a pressure reduction device, a pressure holding device present in the third switching position of the parking brake valve is effective, then the pressure-maintaining device, for example be designed as an accumulator.

Around the stationary machine in case of power failure immediately, d. H. without To be able to arrest delay, for example when the stopped vehicle is on a slope, It is advantageous if the parking brake valve with the common Operating member of the first and second brake valves can be brought into operative connection is. This allows the parking brake valve manually in the second Switch position can be brought, in which the first pressure chamber directly connected to the tank and thus the spring brake immediately closes.

alternative To do this, the parking brake valve with a hand lever in mechanical or hydraulic operative connection can be brought.

Instead of an embodiment of the parking brake valve as a directional control valve With three switch positions, it is also possible that the first pressure chamber is connected to a parking brake valve, which is a electrically actuated, the first pressure chamber with a Pressure supply device connecting the first switching position and has a second switching position effective in the de-energized state, in the first pressure chamber with a pressure reduction device or Pressure holding device is connected.

In this case, the pressure reduction device is preferably designed as an electrically actuatable in the opening direction proportional pressure relief valve be.

When Another, not less advantageous alternative is a working machine proposed in the first pressure chamber, a first, a second and a third switching valve are connected upstream, each electrically are actuated, wherein the first switching valve, the between a pressure supply device and the first brake valve connected is, effective in the de-energized state blocking position and a electrically switchable, the input of the first brake valve with the Having pressure supply device connecting pressure build-up position, wherein the second switching valve is effective in the de-energized state Lock position and an electrically switchable, the first pressure chamber having pressure relief connected with a pressure sink and wherein the third switching valve effective in the de-energized state, the first pressure chamber with a pressure reduction device connecting pressure reduction position and has an electrically switchable blocking position.

When Depressurizer can be used elements, as related to the other embodiments the invention have already been mentioned.

Provided the second switching valve remains de-energized and thus the connection to the pressure sink, d. H. to the tank is interrupted, can by energizing the first and third switching valve, the first pressure chamber hydraulically applied and thereby a force opposite to the spring force be generated, so that the spring brake released is (opening circuit of the parking or parking brake).

Vice versa can the parking brake in the sense of an automatic parking brake be closed when the first and the third switching valve is not more are energized and instead the second switching valve electrical is pressed. The first two valves switch while in locked position, while the latter valve for an immediate pressure reduction in the first pressure chamber of the Spring-loaded brakes ensures, so that the spring accumulator the spring-loaded brake closes.

comes it while driving machine to a power failure, so are all switching valves are de-energized. The first and second switching valve thus switch into blocking position and the third switching valve into a pressure reduction position, in the time-delayed pressure reduction in the first pressure chamber the spring brakes this initially with a reduced Braking torque closes.

Also An emergency brake function is possible in the case of driving Working machine and de-energized first and second switching valve (released parking brake) the third switching valve, which normally energized is brought by deliberately switching off in pressure reduction position becomes.

Provided the switching valves are designed as seat valves, the reliability the machine according to the invention increases, because poppet valves are less sensitive to soiling and have a lower probability of failure than slide valves.

Further Advantages and details of the invention will be apparent from the in the schematic figures illustrated embodiments explained in more detail. It shows

1 a detail of a hydraulic circuit diagram of a working machine according to the invention,

2 a first variant of the invention,

3 a second variant of the invention,

4 a third variant of the invention and

5 a fourth variant of the invention.

The inventive machine can, for example be designed as a battery-electric forklift.

Two wheel shafts 1a and 1b , each by a wheel motor 2a respectively. 2 B can be driven, can be locked by a respective brake, as a hydraulically releasable spring brake 3a respectively. 3b (preferably oil-cooled multi-disc brake) is formed. The spring-loaded brakes 3a . 3b each have a first pressure chamber 4a respectively. 4b on, connected to a common hydraulic pipe 5 connected. Into the line 5 is a first brake valve 6 connected, which is formed in the present embodiment as a controllable proportional pressure reducing valve with decreasing characteristic.

To the hydraulic line 5 is upstream of the first brake valve 6 connected to a pressure supply device not shown in the figures. Furthermore, there may be a memory 7 The one towards the brake valve 6 opening check valve 8th is subordinate.

If the first brake valve 6 is not operated (and the pressure supply device is working properly) and to be described, electrically actuated parking brake valve 13 is in a first switching position I, is in the line 5 and thus in the first pressure chamber 4a respectively. 4b a hydraulic pressure, which the spring brake 3a respectively. 3b solves. The work machine ne can therefore through the wheel motors 2a and 2 B be set in motion.

The spring-loaded brakes 3a . 3b have in addition to the first pressure chambers 4a respectively. 4b each with a second pressure chamber 9a respectively. 9b , in which a spring assembly in the closing direction is effective and in a line 5 parallel line 10 a second brake valve 11 upstream, which is formed in the present embodiment as a proportional pressure reducing valve with increasing characteristic. The first brake valve 6 and the second brake valve 11 are thus connected in parallel. However, it is also possible to connect in series.

The through the second brake valve 11 controllable, in the second pressure chamber 9a respectively. 9b the applied pressure acts in each case on the release pressure in the first pressure chamber 4a respectively. 4b opposite. This allows the released spring brakes 3a . 3b be operated in the closing direction. This process is also referred to as "oversteer".

The two brake valves 6 and 11 can by a common operating element, in the present embodiment as a brake pedal 12 is formed, operated. This is where the brake pedal points 12 a first operating area, in which only the second brake valve 11 is actuated, and then, ie on further passage of the brake pedal 12 , a second operating area, in addition to the first brake valve 6 is pressed.

To bring the moving work machine to a halt, the operator operates the brake pedal 12 and thereby generates by means of the brake valve 11 one the release pressure in the pressure chamber 4a respectively. 4b oversteering hydraulic pressure in the pressure chamber 9a respectively. 9b , whereby the spring brake is moved in the closing direction and the vehicle doses braked to a standstill. Depending on how far along the brake pedal 12 is depressed, it also comes to an actuation of the first brake valve 6 and thus to a pressure reduction in the first pressure chamber 4a respectively. 4b ,

Is the brake circuit, which is the second brake valve 11 is assigned, defective, so the operator by appropriate passage of the brake pedal 12 the first brake valve 6 actuate and metered pressure reduction in the first pressure chambers 4a and 4b bring the work machine to a standstill. Conversely, even if the first brake valve 6 is not operable, only with the second brake valve 11 be slowed down.

In order to lock the stationary work machine, is a the first brake valve 6 downstream parking brake valve 13 intended. The parking brake valve 13 has three switch positions I, II and III, of which the first shift position I and the second shift position II are electrically actuated. In the first switching position I (pressure supply position) is the line 5 unlocked and are thus at unoperated first brake valve 6 the first pressure chambers 4a respectively. 4b connected to the pressure supply device. In the second switching position II (pressure reduction position) is the line 5 with one as a tank 14 trained pressure sink connected. The pressure chambers 4a and 4b are thus relieved. In a third shift position III (shown in the figure) of the parking brake valve 13 are the pressure chambers 4a . 4b the spring-loaded brakes 3a and 3b with a throttle point 15 trained depressurizer connected.

It thus always requires the action of an electrically generated actuating force to the parking brake valve 13 from the pressure reduction position (second shift position II) to the pressure supply position (first shift position I) and - vice versa - to move from the pressure supply position to the pressure reduction position. The activation of the parking brake valve 13 takes place by means of an electronic control unit, not shown in the figures. By detecting the actual speed of the work machine and processing this signal in the electronic control unit, it is possible to provide the work machine with the function of a parking brake automatically incident when the vehicle is stationary.

Are in the machine according to the invention, the spring-loaded brakes 3a and 3b solved, with the parking brake valve 13 is in the first switching position I (pressure supply position) and thus a sufficiently large release pressure in the pressure chambers 4a and 4b is pending, and drives the work machine, then a power failure initially has no effect on the spring brakes 3a . 3b because the parking brake valve 13 switches to the third switching position III, in which the pressure in the pressure chambers 4a . 4b by means of the throttle point 15 will be reduced only delayed.

The Parking brake is thus not sudden in case of power failure and activated unintentionally. One transported by forklift on the fork Load is therefore not dropped by abrupt deceleration.

In order to tow the stationary work machine in the de-energized state, is in a tank 14 with the line 5 connecting suction line 16 a manually operated hand pump 17 between two to the tank 14 closing check valves 18 . 19 arranged.

Partially redundant existing pressure sensors P allow during operation of Ar Beitsmaschine plausibility checks, for example, the correct operation of the pressure supply device.

At the in 2 illustrated variant of the machine according to the invention is used instead of the throttle point 15 a throttle valve 20 provided by the pressure downstream of the first brake valve 6 is controllable.

In case of power failure, the parking brake valve switches 13 in the starting position (switch position III) and connects the pressure chambers 4a and 4b with the inlet of the throttle valve 20 that is due to the effect of in the line 5 downstream of the first brake valve 6 pending (still maximum) brake pressure is in a setting with maximum opening width. By connecting the throttle valve 20 with the pressure sink (tank 14 ) the pressure in the pressure chambers decreases 4a and 4b and the spring brakes 3a . 3b begin to close. The gradually decreasing release pressure in the line 5 causes an increasingly smaller opening cross-section of the throttle valve 20 , so that the further pressure reduction in the pressure chambers 4a and 4b delayed. Depending on the vote of the throttle valve 20 results in a certain braking effect until the machine is brought to a standstill.

The spring-loaded brakes 3a and 3b are thus forcibly closed in case of power failure, but with a defined delay, so that there is a braking effect below the maximum possible braking effect.

If the braking effect is to be increased in the emergency braking, the operator can by pressing the brake pedal 12 the defined reduction of the release pressure in the line 5 by pressure build-up in the pipe 10 Override and bring the work machine, if necessary, with maximum braking faster to a halt.

To lock the vehicle at standstill immediately, is the parking brake valve 13 with an additional, non-electrical actuator 13a Mistake. This can be in mechanical or hydraulic operative connection (dash-dotted line W) with the brake pedal 12 are brought, expediently at the full passage of the brake pedal 12 , Thus, the parking brake valve 13 by stepping on the brake pedal 12 in the second switching position II (pressure reduction position) switchable, in which the pressure chambers 4a and 4b immediately with the tank 14 be connected and the spring brakes 3a . 3b close immediately. Alternatively to the operative connection with the brake pedal 12 it is also possible, the actuator 13a the parking brake valve 13 to bring into operative connection with a hand lever.

The according to the variant 2 explained active compound W can of course also in the variant according to 1 to be available.

At the in 3 illustrated embodiment of the invention are the first brake valve 6 and the second brake valve 11 together with the parking brake valve 13 summarized in a common unit B. Furthermore, the brake valves 6 and 11 not offset by the brake pedal 12 pressed, but are positively coupled.

To tow the stationary machine, a pressure-maintaining valve must be used 21 closed to prevent the manual from the hand pump 17 for opening the spring brakes 3a . 3b pressure generated again not immediately on the located in the de-energized third shift position III parking brake valve 13 and a throttle point 22 as well as a parallel pressure valve 23 to the tank 17 is reduced.

If you forget after towing the machine, the pressure relief valve 21 can be opened again by pressing the brake pedal 12 via an operative connection W between the brake pedal 12 and the pressure-holding valve 21 the latter will be opened again.

In the embodiment according to 4 is the parking brake valve 13 designed as an electromagnetic two-position valve, which is an electrically actuated, the first pressure chamber 4a respectively. 4b with the output of the first brake valve 6 Ver binding first switching position I and has an effective in the de-energized state second switching position II. In the second switching position II, the first pressure chamber 4a respectively. 4b connected to a pressure reduction device, which can be actuated as an electrically in the opening direction proportional pressure relief valve 24 is trained.

To release the spring brakes 3a . 3b becomes the parking brake valve 13 switched to the switching position I. By appropriate electrical control of the parking brake valve 13 and the proportional pressure relief valve 24 the function of a parking brake automatically incident upon vehicle standstill can be achieved by switching the parking brake valve 13 in the switching position II and by electrically actuating the proportional pressure relief valve 24 in the open position, the first pressure chambers 4a . 4b the spring-loaded brakes 3a . 3b be vented suddenly. Conversely, the parking brake valve 13 be switched in the presence of a movement command in the first switching position I, which in the first pressure chambers 4a . 4b a pressure is built up, which the spring-loaded brakes 3a . 3b releases again. In this context, is through the memory 7 facilitates a quick pressure build-up.

In case of power failure, the parking brake valve switches 13 in the second switching position II, in which the first pressure chambers 4a . 4b the spring-loaded brakes 3a . 3b with the proportional pressure relief valve 24 connected, and the proportional pressure relief valve 24 is closed. Due to the effect of in the pressure chambers 4a . 4b Pending pressure opens the proportional pressure relief valve 24 again and the pressure starts to fall. As a result, the spring brakes begin 3a . 3b close and create a braking torque. With decreasing pressure in the pressure chambers 4a . 4b becomes the proportional pressure relief valve 24 increasingly closed and thus the pressure drop in the first pressure chambers 4a . 4b the spring-loaded brakes 3a . 3b and thus limits the braking torque.

Through this hydraulic circuit, the first pressure chambers 4a . 4b the spring-loaded brakes 3a . 3b thus not relieved suddenly, but it results in a brake force curve, the valve characteristic of the proportional pressure relief valve 24 depends. The braking process can also be done by pressing the brake pedal 12 be shortened

For towing the machine, the parking brake valve 13 mechanically in the first switching position 1 be brought to the drain to the tank 7 to block and a pressure build-up in the first pressure chambers 4a . 4b by means of the hand pump 17 to enable.

At the in 5 variant shown are the first pressure chamber 4a respectively. 4b three electrically actuated switching valves S1, S2, S3 upstream, which are preferably designed as seat valves. Here, the first switching valve S1 is not directly but indirectly connected upstream, since it is between the pressure supply device and the first brake valve 6 is arranged.

The first switching valve S1 is in the de-energized state in a blocking position and has an electrically switchable, the input of the first brake valve 6 with the pressure supply device connecting pressure build-up position. The second switching valve S2 is in the de-energized state also in a blocking position and has an electrically switchable, the first pressure chamber 4a respectively. 4b depressurization connected to a pressure sink. The third switching valve S3 connects in the de-energized state (pressure reduction position) the first pressure chamber 4a respectively. 4b with a pressure reduction device, for example, as a pressure relief valve 23 is formed, and can be electrically switched to a blocking position.

When de-energized second switching valve S2 is the connection of the first pressure chamber 4a respectively. 4b to the pressure sink, ie the tank interrupted and it can by energizing the first and third switching valve S1, S3, the first pressure chamber 4a respectively. 4b acted upon hydraulically and thereby one of the spring force opposite force can be generated. This will cause the spring brakes 3a . 3b solved, which corresponds to an open parking brake.

The parking brake can be closed in the sense of an automatic parking brake when the first and third switching valve S1, S3 are no longer energized and instead the second switching valve S2 is energized. The switching valve S2 ensures an immediate, so instantaneous pressure reduction in the first pressure chamber 4a respectively. 4b the spring-loaded brakes 3a . 3b , so that the spring accumulator the spring-loaded brakes 3a . 3b shut down.

If there is a power failure while the machine is moving, all switching valves S1, S2, S3 are de-energized. The first and second switching valve S1, S2 thus switch in the blocking position and the third switching valve S3 in the pressure reduction position, in the time-delayed pressure reduction in the first pressure chamber 4a respectively. 4b the spring-loaded brakes 3a . 3b close them first with a reduced braking torque.

For the towing operation, the third switching valve S3 closed by hand become.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - DE 19903564 A1 [0002]

Claims (15)

Mobile working machine, in particular industrial truck, with at least one brake acting as hydraulically releasable spring-loaded brake ( 3a respectively. 3b ) is formed and a first, opposite to a spring-loaded pressure chamber ( 4a respectively. 4b ) and a second, in the same direction to the spring-loaded pressure chamber ( 9a respectively. 9b ), characterized in that the first pressure chamber ( 4a respectively. 4b ) a first brake valve ( 6 ) and the second pressure chamber ( 9a respectively. 9b ) a second brake valve ( 11 ), wherein the two brake valves ( 6 . 11 ) are designed as controllable valves with at least one common operating element (brake pedal 12 ) are in operative connection. Work machine according to claim 1, characterized in that the first brake valve ( 6 ) as a proportional pressure reducing valve with falling characteristic and the second brake valve ( 11 ) is designed as a proportional pressure reducing valve with increasing characteristic. Work machine according to claim 1 or 2, characterized in that the common operating member as a brake pedal ( 12 ) is trained. Work machine according to one of claims 1 to 3, characterized in that the operating element (brake pedal 12 ) has, starting from a starting position, a first actuating region, in which only the second brake valve ( 11 ) is operable, and an adjoining second operating area, in addition to the first brake valve ( 6 ) is operable. Work machine according to one of claims 1 to 4, characterized in that the first and the second brake valve ( 6 . 11 ) are integrated in a common structural unit (B). Work machine according to one of claims 1 to 5, characterized in that the first pressure chamber ( 4a respectively. 4b ) a parking brake valve ( 13 ), which is an electrically operable, the first pressure chamber ( 4a respectively. 4b ) connected to a pressure supply device, first switching position (I), an electrically actuated, the first pressure chamber ( 4a respectively. 4b ) with a pressure sink (tank 14 ) connecting the second switching position (II) and an effective in the de-energized state third switching position (III), in which the first pressure chamber ( 4a respectively. 4b ) is connected to a pressure reducing device or a pressure holding device. Work machine according to claim 6, characterized in that the pressure reduction device as throttle point ( 15 ), as an adjustable and / or controllable throttle valve ( 20 ) or designed as a pressure relief valve. Working machine according to claim 7, characterized in that the flow cross section of the pressure reducing device by the pressure downstream of the first brake valve ( 6 ) is controllable. Work machine according to claim 6, characterized that the pressure-retaining device is designed as a pressure accumulator. Work machine according to one of claims 6 to 9, characterized in that the parking brake valve ( 13 ) with the common control (brake pedal 12 ) of the first and second brake valves ( 6 . 11 ) in mechanical or hydraulic operative connection (W) can be brought. Work machine according to one of claims 6 to 9, characterized in that the parking brake valve ( 13 ) can be brought with a hand lever in mechanical or hydraulic operative connection. Work machine according to one of claims 1 to 5, characterized in that the first pressure chamber ( 4a respectively. 4b ) a parking brake valve ( 13 ), which is an electrically operable, the first pressure chamber ( 4a respectively. 4b ) has a first switching position (I) which connects to a pressure supply device and a second switching position (II) which operates in the currentless state and in which the first pressure chamber (11) 4a respectively. 4b ) is connected to a pressure reducing device or pressure holding device. Work machine according to claim 12, characterized in that the pressure reduction device as electrically operable in the opening direction proportional pressure relief valve ( 24 ) is trained. Work machine according to one of claims 1 to 5, characterized in that the first pressure chamber ( 4a respectively. 4b ) a first, a second and a third switching valve (S1, S2, S3) are connected upstream, which are each electrically actuated, wherein the first switching valve (S1) connected between a pressure supply device and the first brake valve ( 6 ), an effective in the de-energized state blocking position and an electrically switchable, the input of the first brake valve ( 6 ) having the pressure supply device connecting pressure build-up position, wherein the second switching valve (S2) has an effective in the de-energized state blocking position and an electrically switchable, the first pressure chamber ( 4a respectively. 4b ) with a pressure sink (tank 14 ) and the third switching valve (S3) has an effective in the de-energized state, the first pressure chamber ( 4a respectively. 4b ) with a pressure relief device (pressure relief valve 23 ) has connecting pressure reduction position and an electrically switchable blocking position. Work machine according to claim 14, characterized the switching valves (S1, S2, S3) are designed as seat valves are