Hydraulic control of a roll gap for a roll stand

Abstract

A roll stand with at least one working roll for rolling strip material and with a hydraulic arrangement for controlling a roll gap of the roll stand, comprises at least one hydraulic adjustment unit for adjusting the roll gap, the hydraulic adjustment unit comprising a cylinder and an adjusting piston dividing the cylinder into a first chamber and a second chamber; a first double-acting valve arrangement and a second double-acting valve arrangement which are each connected to the first chamber and the second chamber for variable control of the adjusting unit, wherein the second double-acting valve arrangement is connected in parallel with the first valve arrangement and has a larger nominal volume flow than the latter; and wherein the adjusting unit can be pressurized with an working pressure of more than 200 bar.

Claims

1. A roll stand with at least two working rolls for rolling strip material and with a hydraulic arrangement for controlling a roll gap of the roll stand, comprising: at least one hydraulic adjusting unit for adjusting the roll gap, the hydraulic adjusting unit comprising a cylinder and an adjusting piston, the adjusting piston dividing the cylinder into a first chamber and a second chamber; a first valve arrangement for variably controlling the adjusting unit; a second valve arrangement for variably controlling the adjusting unit, which is arranged in parallel to the first valve arrangement; wherein the first valve arrangement has a smaller nominal volume flow than the second valve arrangement; wherein the first valve arrangement is hydraulically connected to the first chamber and the second chamber for double-acting actuation of the adjusting unit such that the piston can be both extended and retracted by the first valve arrangement, wherein the second valve arrangement is hydraulically connected to the first chamber and the second chamber for double-acting actuation of the adjusting unit such that the piston can be both extended and retracted by the first valve arrangement, wherein the hydraulic adjusting unit is controllable such that a first stroke section of a stroke of the cylinder is implemented by both the first valve arrangement and the second valve arrangement, and a second stroke section of the stroke of the cylinder is implemented only by the first valve arrangement that has a smaller nominal volume flow than the second valve arrangement, and wherein the hydraulic adjusting unit is capable of being subjected to a working pressure greater than 200 bar from a pressure medium source.

2. The roll stand of claim 1, wherein at least one of the first valve arrangement and the second valve arrangement comprises at least one continuous valve for controlling the hydraulic adjusting unit.

3. The roll stand of claim 1, wherein the hydraulic adjusting unit can be pressurized by a working pressure greater than 250 bar, wherein the pressure medium source comprises a pump or a pressure medium reservoir with a connected pump.

4. The roll stand of claim 1, wherein the second valve arrangement has a nominal volume flow which is greater than twice the nominal volume flow of the first valve arrangement.

5. The roll stand of claim 1, wherein a main tank is provided which is hydraulically connected to the first valve arrangement and the second valve arrangement, and at least one intermediate tank is arranged between the main tank and at least one of the first valve arrangement and the second valve arrangement, wherein the intermediate tank is arranged at a distance of less than three meters from at least one of the first valve arrangement and the second valve arrangement.

6. The roll stand of claim 5, wherein the intermediate tank is arranged in a return line between at least one of the first valve arrangement and the second valve arrangement, and the main tank.

7. Roll stand according claim 1, wherein the adjusting unit comprises exactly one adjusting piston, so that only two chambers are formed in the cylinder; whereby the only two chambers are the first chamber and the second chamber.

8. The roll stand of claim 1, wherein the first valve arrangement controls a volume flow of a first pressure medium path, which is connected to the first chamber, and a volume flow of a second pressure medium path, which is connected to the second chamber.

9. The roll stand of claim 1, wherein the second valve arrangement controls a volume flow of a third pressure medium path, which is connected to the first chamber, and a volume flow of a fourth pressure medium path, which is connected to the second chamber.

10. The roll stand of claim 1, wherein a pulsation damper is arranged in a return line between at least one of the first valve arrangement and the second valve arrangement, and the main tank.

11. A method for controlling a roll gap of a roll stand with the roll stand comprising at least two working rolls for rolling strip material, a hydraulic arrangement for controlling the roll gap, at least one hydraulic adjusting unit for adjusting the roll gap, the hydraulic adjusting unit comprising a cylinder and an adjusting piston, the adjusting piston dividing the cylinder into a first chamber and a second chamber, a first valve arrangement for variably controlling the adjusting unit, a second valve arrangement for variably controlling the adjusting unit, which is arranged parallel to the first valve arrangement, wherein the first valve arrangement has a smaller nominal volume flow than the second valve arrangement, wherein the first valve arrangement is hydraulically connected to the first chamber and the second chamber for double-acting actuation of the adjusting unit, wherein the second valve arrangement is hydraulically connected to the first chamber and the second chamber for double-acting actuation of the adjusting unit, and wherein the hydraulic adjusting unit is capable of being subjected to a working pressure greater than 200 bar from a pressure medium source; the method comprising: determining an actual roll position of a working roll of the at least two working rolls; determining a nominal roll position of the working roll; and controlling an opening degree of the first valve arrangement and an opening degree of the second valve arrangement depending on the actual roll position and the nominal roll position, wherein the roll gap is changed during rolling, wherein for at least one thickness change of a thickness profile to be rolled, the adjusting unit is controlled in a first section of a stroke of the cylinder between the actual roll position and the nominal roll position by the first valve arrangement and the second valve arrangement, and in a second section of the stroke of the cylinder only by the first valve arrangement that has a smaller nominal volume flow than the second valve arrangement.

12. The method of claim 11, wherein, for roll gap changes of more than 10% of the roll gap value, the adjusting unit is controlled in the first section of the stroke by the first valve arrangement and the second valve arrangement.

13. The method of claim 11, wherein at least one setting variable for controlling the opening degree of the first valve arrangement and at least one setting variable for controlling the opening degree of the second valve arrangement are output with a time offset.

14. The method of claim 11, wherein, in order to control the opening degree of the first valve arrangement, a first setting variable for a first valve of the first valve arrangement and a second setting variable for a second valve of the first valve arrangement are output with a time offset, and for controlling the opening degree of the second valve arrangement, a first setting variable for a first valve of the second valve arrangement and a second setting variable for a second valve of the second valve arrangement are output with a time offset.

15. The method of claim 11, wherein the nominal roll position is determined depending on a nominal thickness profile and at least one of a thickness measurement on an inlet side of the working roll and a profile thickness measurement on an outlet side of the working roll.

Description

BRIEF SUMMARY OF THE DRAWINGS

(1) In the following figure representations, example embodiments the disclosed hydraulic arrangement and method are described. Thereby

(2) FIG. 1 schematically shows a hydraulic arrangement and a roll stand with a hydraulic arrangement according to a first embodiment;

(3) FIG. 2a shows a section of the hydraulic arrangement from FIG. 1 in a first switching position of valve arrangements;

(4) FIG. 2b shows a section of the hydraulic arrangement from FIG. 1 in a second switching position of the valve arrangements;

(5) FIG. 2c shows a section of the hydraulic arrangement from FIG. 1 in a third switching position of the valve arrangements;

(6) FIG. 2d shows a section of the hydraulic arrangement from FIG. 1 in a fourth switching position of the valve arrangements;

(7) FIG. 3 schematically shows a hydraulic arrangement respectively a roll stand according to the disclosure with a hydraulic arrangement according to a second embodiment;

(8) FIG. 4a shows a section of the hydraulic arrangement from FIG. 3 in a first switching position of the valve arrangements 9, 10′;

(9) FIG. 4b shows a section of the hydraulic arrangement from FIG. 3 in a second switching position of the valve arrangements 9, 10′;

(10) FIG. 4c shows a section of the hydraulic arrangement from FIG. 3 in a third switching position of the valve arrangements 9, 10′;

(11) FIG. 4d shows a section of the hydraulic arrangement from FIG. 3 in a fourth switching position of the valve arrangements 9, 10′;

(12) FIG. 4e shows a section of the hydraulic arrangement from FIG. 3 in a fifth position of the valve arrangements 9, 10′; and

(13) FIG. 5 illustrates a method according to the disclosure for controlling a roll gap of a roll stand in a flow diagram.

DESCRIPTION

(14) FIGS. 1 and 2a to 2d, which are described together in the following, schematically show a roll stand 32 and a hydraulic arrangement for controlling a roll gap 19 of the roll stand according to a first embodiment. In the rolling process, incoming strip material 18 is rolled through the roll gap 19 from a constant nominal thickness to a variable thickness profile of the outgoing strip material 18′. The roll gap 19 is formed by two working rolls 6, 6′ of roll stand 32, which is designed as a four-high roll stand. A four-high roll stand, as the name suggests, comprises four rolls, two working rolls 6, 6′ and two supporting rolls 5, 5′, although it is understood that rolling stands with a different number of rolls can also be used, for example two-high or three-high roll stands.

(15) The working rolls 6, 6′ are each supported by a supporting roll 5, 5′ to reduce the deflection of the working rolls 6, 6′. The working rolls 6, 6′ and the supporting rolls 5, 5′ are each rotatably mounted in chocks not shown in the figures. The chocks are in turn accommodated in a roll column of roll stand 32. It is provided that the rolls 5, 6; 5′, 6′ are each rotatably mounted at their ends in an associated chock, i.e. two chocks are provided per roll, which together support the roll rotatably. In the present embodiment, the chocks of the lower working roll 6′ and the lower supporting roll 5′ are firmly held in the roll column, and the chocks of the upper working roll 6 and the upper supporting roll 5 are mounted or guided in the roll stand 32 so as to be vertically displaceable. To change the roll gap 19, only the upper supporting roll 5 and the upper working roll 6 are moved vertically, while the lower supporting roll 5′ and lower working roll 6′ are kept stationary. However, arrangements are also possible in which only the lower rolls 5′, 6′ are moved and the upper rolls 5, 6 are kept stationary, or in which both the upper rolls 5, 6 and the lower rolls 5′, 6′ are vertically movable against each other.

(16) At least one adjusting unit 1, which acts at least indirectly on a working roll, is provided for adjusting or changing the roll gap 19. For each chock of an adjustable roll, an associated adjusting unit 1 can be provided, i.e. the adjustable roll is set via two adjusting units. The adjustable roll can be a working roll, for example in a two-high roll stand, i.e. in this case the adjusting unit 1 acts on the chocks of the working roll. The adjustable roll can also be a supporting roll, for example in a four-high roll stand, in which case the adjusting unit 1 acts on the chocks of the supporting roll 5, which in turn adjusts the working roll 6.

(17) In the present embodiment, it is provided in particular that the chocks of the upper supporting roll 5 are each positioned vertically by means of one adjusting unit 1. The adjusting units 1 can exert a vertical force on the respective chock so that the upper supporting roll 5 applies a rolling force to the upper working roll 6. The adjusting unit 1 comprises an adjusting piston 2 which divides an adjusting cylinder 17 movably into a first chamber 3 and a second chamber 4. If the first chamber 3 is pressurized with a higher pressure than the second chamber 4, the adjusting piston 2 moves towards the second chamber 4 and the roll gap 19 is reduced. If the second chamber 4 is subjected to a higher pressure than the first chamber 3, the adjusting piston 2 moves towards the first chamber 3 and the roll gap 19 is increased.

(18) The pressurization of the two chambers 3, 4 is controlled by two valve arrangements 9, 10. The first valve arrangement 9 and the second valve arrangement 10 each comprise exactly one valve 11, 12, whereby the valve 12 of the second valve arrangement 10 has a larger nominal volume flow than the valve 11 of the first valve arrangement 9.

(19) As can be seen in FIGS. 2a to 2b, the valve 11 in the first embodiment of the hydraulic arrangement is designed as a 5/3-way valve, which controls a first pressure medium path 13 and a second pressure medium path 14, with a control element 20. The valve 11 is connected with a port A to the first chamber 3 via the first pressure medium path 13 and with a port B to the second chamber 4 via the second pressure medium path 14. In addition, valve 11 is connected with two ports P to a pressure medium source 27 and with a port T to a tank 28, which are only shown in FIG. 1.

(20) Valve 12 of the second valve arrangement 10 is also designed in the first embodiment of the hydraulic arrangement as a 5/3-way valve, which controls a third pressure medium path 15 and a fourth pressure medium path 16, with a control element 21. Valve 12 is connected with a port A to the first chamber 3 via the third pressure medium path 15 and with a port B to the second chamber 4 via the fourth pressure medium path 16. In addition, valve 12 is connected with two ports P to a pressure medium source 27 and with a port T to a tank 28, which are only shown in FIG. 1.

(21) Any arrangement which can provide a substantially constant working pressure greater than 200 bar, in particular greater than 250 bar, in particular greater than 300 bar, at a defined volume flow at ports P of the valve arrangements 9, 10 is conceivable as a pressure medium source 27. Thus, a direct connection of one or more pumps to ports P of valve arrangements 9, 10 is possible or one or more pressure medium reservoirs can be arranged between the valve arrangements 9, 10 and a pump. In the shown embodiment, ports P of valve arrangements 9, 10 are fed from a common pressure medium source. However, it is also conceivable that at least some of the ports P of the valve arrangements 9, 10 are connected to a separate pressure medium source.

(22) Any arrangement is conceivable as tank 28, which enables the hydraulic fluid flowing out of the adjusting unit 1 to be collected and the pumps of the pressure medium source 27 to be supplied with hydraulic fluid. The arrangement can be designed in such a way that the hydraulic fluid flowing out can drain off as quickly as possible. For this purpose, it is conceivable that the outflowing hydraulic fluid could reach an intermediate tank 29, which is positioned near the valve arrangements 9, 10 and in particular has a distance of less than 3 m from the valve arrangements 9, 10, and from there be conveyed to a main tank 28. Pulsation dampers 30 can be arranged in the return line 31 between the valve arrangements 9, 10 and the main tank 28, especially before the intermediate tank 29, to dampen pulsations of the hydraulic fluid flowing out quickly from the valve arrangements 9, 10.

(23) The valve 11 of the first valve arrangement 9 and the valve 12 of the second valve arrangement 10 are shown in FIG. 2a in a first switching position in which the first chamber 3 and the second chamber 4 are not pressurized with working pressure from the pressure medium source and the adjusting piston 2 remains in closed position. This is achieved by positioning the control elements 20, 21 each in such a way that the two chambers 3, 4 of the adjusting cylinder 17 are hydraulically isolated from both the pressure medium source and the tank, thus preventing the hydraulic fluid from flowing into and out of one of the two chambers 3, 4. If unintentional leakages between the adjusting cylinder 17 and the adjusting piston 2 respectively at the valve arrangements 9, 10 are neglected, the adjusting piston 2 cannot move either towards the first chamber 3 or towards the second chamber 4 due to the substantial incompressibility of the hydraulic fluid.

(24) FIG. 2b shows valve 11 of the first valve arrangement 9 and valve 12 of the second valve arrangement 10 in a second switching position, in which the second chamber 4 is pressurized with the working pressure of the pressure medium source. In this switching position the adjusting piston 2 moves towards the first chamber 3 and the roll gap 19 is increased. This is achieved by positioning the control elements 20, 21 each in such a way that the first chamber 3 of the adjusting cylinder 17 is hydraulically connected to the tank and thus hydraulic fluid can flow from the first chamber 3 to the tank. The outflow of the hydraulic fluid is represented in the figures by white arrows with black borders. In addition, the second chamber 4 of the adjusting cylinder 17 is hydraulically connected to the pressure medium source via valves 11, 12 and the hydraulic fluid is fed into the second chamber 4 at the working pressure. The inflow of the hydraulic fluid is shown in the figures by filled arrows.

(25) FIG. 2c shows valve 11 of the first valve arrangement 9 and valve 12 of the second valve arrangement 10 in a third switching position, in which the first chamber 3 is pressurized with the working pressure of the pressure medium source. In this switching position, the adjusting piston 2 moves towards the second chamber 4 and the roll gap is reduced or the roll force is increased. This is achieved by positioning the control elements 20, 21 each in such a way that the second chamber 4 is hydraulically connected to the tank and thus hydraulic fluid can flow from the second chamber 4 to the tank. In addition, the first chamber 3 of the adjusting cylinder 17 is hydraulically connected to the pressure medium source via valves 11, 12, and the hydraulic fluid under the working pressure flows into the first chamber 3.

(26) For a desired displacement of the adjusting piston 2 by the stroke ΔX, as shown between FIGS. 2a and 2b, a stroke volume corresponding to the product of the effective cross-sectional area of the adjusting cylinder 17 and the stroke ΔX must be conveyed into the second chamber 4 and simultaneously out of the first chamber 3. It has to be noted that the effective cross-sectional area of the second chamber 4 is annular due to the piston rod and is smaller than the effective cross-sectional area of the first chamber 3. The second valve arrangement 10 has a larger nominal volume flow than the first valve arrangement 9. When valve arrangements 9, 10 are fully open, a larger part of the stroke volume is thus conveyed into the second chamber by the second valve arrangement 10 than by the first valve arrangement. It is therefore possible to divide the stroke ΔX into a first section, in which the roll position should be changed as quickly as possible, and at least one subsequent second section, in which the nominal position should be approached as accurately as possible. The control of valve arrangements 9, 10 can be designed in such a way that in the first section of the stroke ΔX both valve arrangements 9, 10 are open to allow the largest possible volume flow, as shown in FIGS. 2b and 2c. In the second section, the second valve arrangement 10 is closed and the resulting volume flow corresponds to the nominal volume flow of the first valve arrangement 9.

(27) FIG. 2d shows valve 11 of the first valve arrangement 9 and valve 12 of the second valve arrangement 10 in a fourth switching position, in which—as previously in FIG. 2c—the first chamber 3 is pressurized with the working pressure of the pressure medium source. In this switching position, the adjusting piston 2 moves towards the second chamber 4 and the roll gap is reduced or the roll force is increased. In this fourth switching position, the control element 21 of valve 10 is in a closed position, so that hydraulic fluid flows into the first chamber 3 and hydraulic fluid flows out of the second chamber 4 only via the first valve 9. The resulting volume flow and thus also the actuating speed of adjusting piston 2 is thus reduced compared to the third switching position. This enables a more precise positioning of the adjusting unit respectively the working roll 6.

(28) The valve arrangements 9, 10 are each controlled by a setting variable which is output by a controller 25. The valves 11, 12 are each designed as continuous valves, in particular as servo valves or servo valves with pilot control, so that the two valves 11, 12 can be set continuously between an open position with nominal volume flow and a closed position without volume flow via the setting variable. By varying the opening degrees of valves 11, 12, the resulting volume flow and thus the stroke speed of adjusting piston 2 can be adjusted specifically via the stroke ΔX.

(29) To determine the setting variables, the actual roll position can be fed to controller 25 as a controlled variable and the nominal roll position from a process controller as a reference variable. The nominal roll position can be specified by the process control system depending on a nominal thickness profile. It is also conceivable that the nominal roll position is determined depending on an actual thickness profile of the outgoing strip material 18′ recorded by the measuring unit 8 and/or a thickness profile of the incoming strip material 18′ recorded by the measuring unit 7.

(30) FIG. 3 schematically shows a hydraulic arrangement and a roll stand with such a hydraulic arrangement according to a second embodiment, which differs from the hydraulic arrangement in FIG. 1 only in the alternative design of the second valve arrangement 10′. Identical elements of the hydraulic arrangements are marked with identical reference numbers. For the similarities, therefore, it is referred to the explanations to FIGS. 1 and 2a to 2d.

(31) The second valve arrangement 10′ according to the second embodiment comprises a first valve 12′ and a second valve 12″. The first valve 12′ controls with a control element 21′ the third pressure medium path 15′ and an additional fifth pressure medium path 22. For this purpose, the valve 12′ is connected with a port A to the first chamber 3 via the third pressure medium path 15′ and with a port B to the second chamber 4 via the fifth pressure medium path 22. In addition, the valve 12′ is hydraulically connected with a port P to a pressure medium source not shown and with a port T to a tank. The second valve 12″ controls the fourth pressure medium path 16′ and an additional sixth pressure medium path 23 with a control element 21″. The second valve 12″ is hydraulically connected with a port B to the second chamber 4 via the fourth pressure medium path 16′ and with a port A to the first chamber 3 via the sixth pressure medium path 23. In addition, the valve 12′ is hydraulically connected with a port P to a pressure medium source not shown and with a port T to a tank.

(32) The two control elements 21′, 21″ of the valves 12′, 12″ are kinematically decoupled, so that the valve 12′ and the valve 12″ can be adjusted independently of each other by a controller 25. In particular, the two valves 12′, 12″ are designed identically as 5/3-way valves and have a total nominal volume flow which is greater than the nominal volume flow of the first valve arrangement 9. It is understood that the individual valves can also be designed or controlled differently, for example as valves acting on adjusting unit 1 in one direction only, which together form the double-acting valve arrangement for actuating adjusting unit 1 in both directions. In a further modified embodiment, the first valve arrangement 9 may alternatively or additionally comprise two or more valves, which may be designed analogously to the two valves 12′, 12″ of the second valve arrangement 10′, as described above.

(33) FIGS. 4a to 4d show the switching positions of the second embodiment analogous to the switching positions of the first embodiment in FIGS. 2a to 2d, wherein the respective volume flows belonging to the switching positions are additionally realized by the two pressure medium paths 22, 23. Reference should therefore be made at this point to the explanations of FIGS. 2a to 2d.

(34) FIG. 4e shows an intermediate switching position between the switching positions of FIG. 4c, in which a high stroke speed is realized by a maximum volume flow, and the switching position of FIG. 4d, in which a low stroke speed is realized by a low volume flow for exact positioning. The second valve 12″ of the second valve arrangement 10′ is closed, so that a volume flow into the first chamber 3 and out of the second chamber 4 is only realized by the valves 11 and 12′. The resulting volume flow is smaller than the volume flow realized in FIG. 4c and larger than the volume flow realized in FIG. 4d. It is thus evident that by dividing the second valve arrangement 10′ into two valves 12′, 12″ with kinematically decoupled control elements 21′, 21″, a higher variability in the control of the adjusting unit is achieved. The second valve arrangement can be represented by any number of valves connected in series and/or in parallel, which together enable double-acting actuation of the adjusting cylinder 17 with a larger nominal volume flow than the nominal volume flow of the first valve arrangement. In particular, 2/2, 3/2, 3/3, 4/2, 4/3 or 5/3-way valves can be used for this purpose. It is understood that the first valve arrangement 9 can also be composed analogously by several valves and thus the variability of the control of the adjusting unit 1 can be further increased.

(35) FIG. 5 shows a flow diagram showing a method according to the disclosure for controlling a roll gap 19 of a roll stand 32. In a process step V10 a start roll position of a working roll 6 is determined. Subsequently, in a process step V20, a nominal roll position of working roll 6 is determined, so that a stroke ΔX between the start roll position and the nominal roll position can be determined. This can be done by a control unit 24 depending on a nominal thickness profile as well as a thickness measurement 7 on the infeed side of the working roll 6 and a profile thickness measurement 8 on the outfeed side of the working roll. In a process step V30 an actual roll position is measured by a position sensor 26.

(36) In a process decision VE10 it is subsequently checked whether the actual roll position corresponds to the nominal roll position. If the actual roll position matches the nominal roll position, the process is stopped in a process step S and the roll position is maintained. If the actual roll position and the nominal roll position differ from each other, an opening degree of a first valve arrangement 9 and an opening degree of a second valve arrangement 10, 10′ is controlled in a process step V40 in order to control an adjusting unit, which is operatively connected to the working roll 6, depending on the actual roll position and the nominal roll position. The second valve arrangement 10, 10′ has a larger nominal volume flow than the first valve arrangement 9.

(37) The controlling of the adjusting unit in a first section of a stroke between the actual roll position and the nominal roll position is performed by the second valve arrangement. In this first section, the controlling of the adjusting unit can thus be performed by the second adjusting unit alone or by the second valve arrangement together with the first valve arrangement. Thus, large volume flows can be set, which leads to a high actuating speed of the adjusting unit.

(38) The controlling of the adjusting unit in a second section of the stroke between the actual roll position and the nominal roll position, which includes the nominal roll position, is performed solely by means of the first valve arrangement. Due to the smaller nominal volume flow of the first valve arrangement, the adjusting unit can be positioned more precisely, wherein lower actuating speeds are achieved.

LIST OF REFERENCE NUMBERS

(39) 1 adjusting unit 2 adjusting piston 3 first chamber 4 second chamber 5, 5′ supporting roll 6, 6′ working roller 7 thickness measuring system 8 thickness measuring system 9 first valve arrangement 10, 10′ second valve arrangement 11 valve 12, 12′, 12″ valve 13′ first pressure medium path 14 second pressure medium path 15.15′ third pressure medium path 16, 16′ fourth pressure medium path 17 adjusting cylinder 18, 18′ strip material 19 roll gap 20 control element 21, 21′, 21″ control element 22 fifth pressure medium path 23 sixth pressure medium path 24 control unit 25 controller 26 position sensor 27 pressure medium source 28 main tank 29 intermediate tank 30 pulsation damper 31 return line 32 roll stand A valve port for first chamber B valve port for second chamber P valve port for pressure medium source T valve port for tank ΔX stroke