CN111347713B

CN111347713B - Hydrostatic drive, in particular for a press or injection molding machine

Info

Publication number: CN111347713B
Application number: CN201911317811.3A
Authority: CN
Inventors: G.亨德里克斯
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2018-12-20
Filing date: 2019-12-19
Publication date: 2024-07-12
Anticipated expiration: 2039-12-19
Also published as: DE102018222425A1; CN111347713A

Abstract

The present invention relates to a hydrostatic drive comprising: a cylinder device on which a piston surface with a hydrostatic action that is not equally large can be acted upon by pressure for a fast feed stroke and for a subsequent pressing stroke; a first hydraulic machine, the pressure fluid supply of which can be varied, which can be operated for a fast stroke in a closed hydraulic circuit with a first cylinder chamber upstream of a first piston surface, which is pressurized in the fast stroke, and with a second cylinder chamber upstream of a second piston surface; a second hydraulic machine, the pressure fluid supply of which can be varied, which can be operated in a closed hydraulic circuit for a pressing stroke with a third cylinder chamber upstream of the third piston surface, which is pressurized during the pressing stroke, and with a fourth cylinder chamber upstream of the fourth piston surface; and a valve that affects the flow of pressurized fluid when transitioning from the fast stroke to the compression stroke. The valve is a passive valve, the state of which is derived from a predetermined value of the delivery for both hydraulic presses.

Description

Hydrostatic drive, in particular for a press or injection molding machine

Technical Field

The present invention relates to a drive, in particular for a hydrostatic drive for a press or injection molding machine or the like. The hydrostatic drive has: a cylinder device on which a piston surface with a hydrostatic action that is not equally large can be acted upon by pressure for a fast feed stroke and for a subsequent pressing stroke; a first hydraulic machine, the pressure fluid supply of which can be varied and which can be operated for a fast stroke in a closed hydraulic circuit with a first cylinder chamber upstream of a first piston face, which is pressurized in the fast stroke, and with a second cylinder chamber upstream of a second piston face; a second hydraulic machine, the pressure fluid supply of which can be varied and which can be operated in a closed hydraulic circuit for a pressing stroke with a third cylinder chamber upstream of the third piston face and pressure-loaded in the pressing stroke and with a fourth cylinder chamber upstream of the fourth piston face; and a valve device which influences the flow of pressurized fluid when switching from the fast stroke to the pressing stroke.

Background

Such hydrostatic drives are known from EP 0 311 779 A2. There, a fast-stroke cylinder, which is designed as a synchronous cylinder, having a piston with a small piston surface, and a compression-stroke cylinder or a power-stroke cylinder, which is also designed as a synchronous cylinder, are mechanically connected to one another by a common piston rod. There are two hydraulic machines which are adjustable in terms of their working volume and which can be driven together by a single electric motor. The transport stream can be reversed by two hydraulic presses. The two hydraulic machines are connected parallel to each other with a first connection to a first cylinder chamber of the quick travel cylinder and with a second connection to a second cylinder chamber of the quick travel cylinder and can be operated in a closed hydraulic circuit with the quick travel cylinder. A reversing valve having two switch positions and four joints, of which a first joint is connected to the first cylinder chamber, a second joint is connected to the second cylinder chamber of the press stroke cylinder, and a third joint is connected to the first joint of the two hydraulic presses, and a fourth joint is connectable to the second joint of the two hydraulic presses, controls the conversion from the quick stroke to the press stroke. In the quick travel, the reversing valve assumes a switching position in which its third and its fourth connection are blocked relative to the first and second connections and the first and second connections are connected to one another. The pressing cylinder is thus blocked with respect to the two hydraulic presses. The two hydraulic machines operate parallel to each other only with the fast-stroke cylinders in a closed hydraulic circuit. The pressure fluid is displaced from one chamber of the compression stroke cylinder into the other chamber via the reversing valve. The switching of the reversing valve to the other switching position results in a transition from the fast stroke to the pressing stroke. The direct connection between the two chambers of the compression stroke cylinder is blocked by a reversing valve, one chamber being connected to the first connection of the two hydraulic presses and the other chamber being connected to the second connection. The quick travel cylinder and the hold-down travel cylinder are now connected in parallel with each other and run in a closed hydraulic circuit with the hydraulic machine.

The disadvantage here is that, depending on the selected overlap, the cylinder chambers of the pressing stroke cylinders can be closed off from one another or opened both from one another and toward the hydraulic machine in a short time when switching the reversing valve. This results in a stopping of the cylinder or also in a temporary uncontrolled movement. This characteristic strongly interferes with the process flow in many applications.

If the 4/2 directional control valve is divided into three 2/2 directional control valves with their own actuating elements, the individual valves must be switched with precise control over time relative to one another. This is difficult to achieve.

Disclosure of Invention

The object of the present invention is therefore to design a hydrostatic drive having the features mentioned at the outset in such a way that the conversion from the fast run to the pressing run takes place in a controlled manner and the process flow is thereby improved.

This is achieved in that the valve device comprises a plurality of passive valves, the state of which is determined by a predetermined value for the delivery rate of the two hydraulic machines. The switching between a small piston face and a large piston face is thus accomplished by two hydraulic presses, the delivery of which is variable. The passive valve reacts to the pressure resulting from the delivery of the hydraulic machine without external actuation. One hydraulic machine can in this case completely convey a different quantity of liquid than the other hydraulic machine simultaneously.

The passive valve is preferably a check valve, wherein the first check valve is arranged fluidically between the third and fourth cylinder chambers of the cylinder arrangement and opens from the fourth cylinder chamber towards the third cylinder chamber, and the second check valve is arranged between a first line leading from the first hydraulic machine to the first cylinder chamber and a second line leading from the second hydraulic machine to the third cylinder chamber and opens from the second line towards the first line. In this case, preferably no further valves are located in the first line and the second line.

Preferably, the speed of the cylinder device, i.e. the fast travel speed, is predetermined by the delivery quantity of the first hydraulic machine during the fast travel of the feed. During the fast stroke, an amount of liquid corresponding to the fast stroke speed is displaced from the fourth cylinder chamber into the third cylinder chamber. This can in principle be done solely by a non-return valve arranged between the third and fourth cylinder chambers. It appears advantageous, however, that during the fast stroke of the feed, the delivery amount of the second hydraulic machine is greater than zero and that the second hydraulic machine delivers pressurized liquid from the fourth cylinder chamber to the third cylinder chamber. The delivery capacity of the second hydraulic machine is in particular maximum during the fast stroke of the feed. Because the use of an excessively large pump and the operation with an excessively large rotational speed is not intended, the maximum delivery volume of the second hydraulic machine is still smaller than the volume of liquid to be displaced between the fourth and third cylinder chambers, and thus also flows through the first non-return valve.

In the transition from the fast forward travel to the pressing travel, the delivery rate of the second hydraulic machine is first adjusted in such a way that it corresponds to the desired pressing speed. If the delivery rate of the first hydraulic machine becomes smaller, the inflow of pressurized fluid from the first hydraulic machine into the first cylinder chamber results in a velocity of the cylinder device that is less than the pressing velocity, then the second hydraulic machine determines the velocity of the cylinder device and the pressure in the first cylinder chamber drops and the pressure in the third cylinder chamber rises. When the pressure in the third chamber is equal to the pressure in the first chamber, the second hydraulic machine is also fed into the first chamber, wherein its feed becomes larger when the speed is not or should not be reduced. The speed of the cylinder device can be determined by means of a displacement sensor and the delivery rate of the second hydraulic machine can be adjusted to the desired speed profile. The delivery rate of the first hydraulic machine must be smaller than the delivery rate of the second hydraulic machine, at least in the same proportion as the piston surface adjoining the first cylinder chamber is smaller than the piston surface adjoining the third cylinder chamber.

It may be provided that at least one of the two hydraulic machines is adjustable in terms of its working volume. The delivery rate can then be varied relative to one another, even if the two hydraulic machines can be jointly driven by a single motor, which is speed-regulated.

However, two speed-regulated motors may also be present, wherein the first hydraulic machine can be driven by a first of the two speed-regulated motors and the second hydraulic machine can be driven by a second of the two speed-regulated motors. The two hydraulic machines may additionally be adjustable in terms of their working volumes, i.e. in view of the amount of liquid flowing through the hydraulic machine per rotation, whereby the delivery amount of each hydraulic machine may be varied by varying the rotational speed and/or by varying the working volume.

If the two hydraulic machines are hydraulic machines with a constant working volume, it is advantageous if there are two speed-regulated motors, and one of the two hydraulic machines can be driven by the speed-regulated first motor and the second of the two hydraulic machines can be driven by the speed-regulated second motor. The delivery or throughput, i.e. the amount of fluid that passes through the hydraulic machine per unit of time, can then be varied only by varying the rotational speed of the motor and thus the hydraulic machine.

The two hydraulic machines may be in fluid communication with the second cylinder chamber in common, wherein a valve is arranged between the hydraulic machine and the fourth cylinder chamber, with which valve a flow of liquid from the hydraulic machine to the fourth cylinder chamber can be stopped, and wherein a switching valve is arranged between the third cylinder chamber and the fourth cylinder chamber, which switching valve can be operated arbitrarily and has a first switching position in which the third cylinder chamber and the fourth cylinder chamber are separated from each other, and a second switching position in which the third cylinder chamber and the fourth cylinder chamber are in fluid communication with each other. In the closed position of the switching valve arranged between the third and fourth cylinder chambers, pressure fluid can be caused to flow from the fourth cylinder chamber into the third cylinder chamber during the fast stroke of the feed via the non-return valve arranged between the two said cylinder chambers. For return in the fast stroke, the switching valve needs to be brought into its open switching position.

The valve arranged between the hydraulic machine and the fourth cylinder chamber may be a check valve which is closed off from the fourth cylinder chamber towards the hydraulic machine.

The first cylinder chamber is in fluid communication with the low pressure reservoir through a check valve opening toward the first cylinder chamber, the second cylinder chamber is in fluid communication with the low pressure reservoir through a check valve opening toward the second cylinder chamber, and the third cylinder chamber is in fluid communication with the low pressure reservoir through a check valve opening toward the third cylinder chamber, and the fourth cylinder chamber is in direct fluid communication with the low pressure reservoir.

Drawings

Two embodiments of the hydrostatic drive according to the invention are shown in the drawings. The invention will now be explained in detail with the aid of these figures. In the accompanying drawings:

Fig. 1 shows a first embodiment in which two hydraulic machines are adjustable with respect to their respective working volumes and driven by a single motor with rotational speed adjustment; and

Fig. 2 shows a second embodiment, in which two hydraulic machines each have a constant working volume and are driven separately from each other by a respective motor that is speed-regulated.

Detailed Description

The hydrostatic drives shown in the figures each comprise a quick travel cylinder 10 configured as a synchronous cylinder and a compression travel cylinder or power travel cylinder 11 also configured as a synchronous cylinder. It is also possible to provide a plurality of quick travel cylinders in the hydraulic circuit in parallel with each other. It is likewise possible to provide a plurality of compression stroke cylinders which are situated parallel to one another in the hydraulic circuit.

The quick travel cylinder contains in the cylinder housing 12 a piston 13 which separates a first cylinder chamber 14 from a second cylinder chamber 15 and which is firmly connected to a piston rod 16 which passes through the cylinder housing and which has the same diameter on one side of the piston 13 as on the other side.

The piston rods of the two cylinders 10 and 11 are mechanically connected to each other by means of a cross bar 17, which is for example part of a powder press, so that they always move together with each other. The position of the crossbar can be detected by means of the displacement sensor 18. From this, the velocity and acceleration of the cross bar can also be deduced.

The compression stroke cylinder 11 likewise contains a piston 23 in the cylinder housing 22, which separates a third cylinder chamber 24 from a fourth cylinder chamber 25 and which is firmly connected to a piston rod 26 which passes through the cylinder housing and has the same diameter on one side of the piston 23 as on the other side. The annular faces A1 and A2 of the piston 23 facing the cylinder chambers 24 and 25 are much larger than, for example ten times larger than, the annular faces A3 and A4 of the piston 13 facing the cylinder chambers 14 and 15.

Each of the hydrostatic drives shown has a first hydraulic machine 30 and a second hydraulic machine 32. Each of the hydraulic machines 30 and 32 may be flown through by pressurized fluid in two opposite directions and operate as both a pump and a hydraulic motor in two flow directions.

The first working connection 33 of the first hydraulic machine 30 is in fluid communication with the cylinder chamber 14 of the quick travel cylinder 10 permanently and without an intermediate connecting valve by way of a line 34. The second working connection 35 of the first hydraulic machine 30 is in fluid communication with the cylinder chamber 15 of the quick travel cylinder 10 permanently and without an intermediate connecting valve by way of a line 36. The hydraulic machine 30 and the quick travel cylinder 10 thus operate together in a closed hydraulic circuit.

The first working connection 37 of the second hydraulic machine 32 is in fluid communication with the cylinder chamber 24 of the press stroke cylinder 11 permanently and without an intermediate connecting valve via a line 38. The second working connection 39 of the second hydraulic machine 32 is coupled to the second working connection 35 of the first hydraulic machine 30 and is therefore connected to the line 36.

The two lines 34 and 38 are in fluid communication with each other through a check valve 40 that opens from line 38 toward line 34.

The fourth cylinder chamber 25 is in fluid communication with the line 36 and thus with the two working joints 35 and 39 of the two hydraulic machines 30 and 32 by means of a line 45. A check valve 46 is arranged in the line 45, which check valve opens from the cylinder chamber 25 towards the line 36. Between the non-return valve 46 and the cylinder chamber 25, a low-pressure reservoir 47 is connected directly to the line 45, in which there is a pressure in the range of, for example, 2 to 3 bar.

Between the two cylinder chambers 24 and 25 of the compression stroke cylinder 11, a check valve 50 is arranged, which opens from the cylinder chamber 25 toward the cylinder chamber 24 and closes in the opposite direction. Pressurized fluid may be displaced from chamber 25 into chamber 24 by check valve 50. Parallel to the non-return valve 50, a 2/2-way valve 51 is arranged, which under the action of a spring takes up a closed rest position and can be switched into a through position by actuating the switching magnet 52. The two valves 50 and 51 can also be combined to form a single valve, for example, a check valve that can be shut off or a logic valve with a check valve function.

Lines 34, 36 and 38 are each in fluid communication via a check valve 53 opening towards them with a low-pressure reservoir 54 in which there is a pressure in the range of 2 to 3 bar as in low-pressure reservoir 47.

The two embodiments are configured differently only with respect to the two hydraulic presses 30 and 32 and their drives. In the exemplary embodiment according to fig. 1, two hydraulic machines, which may be, for example, swash plate type axial piston machines, are adjustable with respect to their working volume. The two hydraulic machines are driven in common by a motor 60, which is speed regulated. In the exemplary embodiment according to fig. 2, the two hydraulic machines are so-called metering machines, and therefore have a fixed, unchangeable working volume. Hydraulic machine 30 is driven by one motor 61 that is speed regulated and hydraulic machine 32 is driven by another motor 62 that is speed regulated. By varying the rotational speed of the associated motor, the delivery of one hydraulic machine can thus be varied independently of the delivery of the other hydraulic machine.

It is assumed that the punch, not shown, which is fastened to the cross bar in fig. 1 is in the initial position at the beginning of the pressing cycle. At the beginning of the press cycle, the punch should be moved or fed toward the die in a fast stroke. The switching valve 51 is in its closed position. Hydraulic machines 30 and 32 are driven by motor 60. The direction of rotation of the motor 60 and thus of the hydraulic machine 30 is selected in such a way that the hydraulic machine 30 removes the pressurized fluid from the cylinder chamber 15 and feeds it into the cylinder chamber 14. In which the pressure required for the movement of the crossbar is built up. The check valve 40 prevents the pressurized liquid fed by the hydraulic machine 30 from flowing into the line 38 and thus into the cylinder chamber 24 of the pressing cylinder 11. The rotational speed and working volume of hydraulic machine 30 are adjusted such that the amount of pressurized fluid flowing to cylinder chamber 14 results in the desired fast stroke speed of fast stroke cylinder 10. The pressing cylinder 11 is entrained by the fast-stroke cylinder 10 via the crossbar 17. For this purpose, it must be possible to squeeze out the pressure liquid from the cylinder chamber 25 of the pressing cylinder 11. This occurs once through the check valve 50, through which the pressure fluid flows from the cylinder chamber 25 into the cylinder chamber 24. Furthermore, the working volume of the hydraulic machine 32 is adjusted in such a way that a part of the pressurized fluid to be displaced from the cylinder chamber 25 is fed to the cylinder chamber 24 via the non-return valve 46 and the hydraulic machine 32. Because the piston faces A3 and A4 of the hold-down cylinder 11 are extremely large compared to the piston faces A1 and A2 of the quick travel cylinder, on the other hand, because the two hydraulic machines 30 and 32 are as large or about large, the amount of liquid flowing through the hydraulic machine 32 is small compared to the amount of liquid flowing through the check valve 50. But a smaller volume flow through the check valve 50 is advantageous.

Before the pressing speed is reached at the end of the fast stroke of the feed, the hydraulic machine is adjusted to such a conveying quantity that this conveying quantity corresponds to the desired pressing speed taking into account the size of the faces A3 and A4 of the pressing cylinder 11. The delivery of the hydraulic machine 30 is now reduced to the desired pressing speed and possibly below it. The ratio between the delivery of hydraulic machine 30 and the delivery of hydraulic machine 32 is highest as large as the ratio between faces A1 and A3. The pressure in the cylinder chamber 14 thus drops and a pressure builds up in the cylinder chamber 24. The passive check valve 50 and the on-off valve 51 in its off position prevent a pressure drop in the low pressure branch of the actuator. The pressure in the line 38 and in the cylinder chamber 24 increases further until finally the passive check valve 40 opens and a pressure equalization in the direction of the cylinder chamber 14 is achieved. And then pressed by two hydraulic presses through two faces A1 and A3. The pressing speed is regulated here by means of the displacement sensor 18 by adjusting the feed rate of the hydraulic machine 32.

For depressurization, the direction of conveyance of the two hydraulic presses is reversed compared to the direction during pressing. The on-off valve 51 is operated after the depressurization. The return stroke now takes place, so that the hydraulic machine 30 feeds the pressure fluid removed from the cylinder chamber 14 into the cylinder chamber 15 of the quick travel cylinder 10, wherein the check valve 46 closes off the line 36 and the cylinder chamber 15 to the low-pressure accumulator 47 and the cylinder chamber 25 of the pressing cylinder 11 and builds up pressure in the cylinder chamber 15 of the quick travel cylinder 10 and a quick travel is achieved. The on-off valve 51 is operated so that pressure liquid can overflow from the cylinder chamber 24 into the cylinder chamber 25 of the pressing cylinder. The hydraulic machine 32 adjusts the working volume to zero.

List of reference numerals:

10. Quick travel cylinder

11. Compression stroke or power stroke cylinder

12 10 Cylinder housing

13 10 Piston

14 10, A first cylinder chamber in the cylinder 10

15 10, A second cylinder chamber in the cylinder block

16 10, Piston rod

17. Cross bar

18. Displacement sensor

22 11, Cylinder housing

2311, Piston

24 11, A third cylinder chamber in 11

25 11, Fourth cylinder chamber in 11

26 11, Piston rod of 11

30. First hydraulic press

32. Second hydraulic press

33 30, First working joint

34. Between 33 and 14

35 30, Second working joint

36. Between 35 and 15

37 32, A first working joint

38. Between 37 and 24

39 32, A second working joint

40. Check valve

45. Pipeline

46. Check valve

47. Low voltage memory

50. Check valve

51 2/2 Reversing seat valve

52. Switch magnet

53. Check valve

54. Low voltage memory

60. Motor with a motor housing having a motor housing with a motor housing

61. Motor with a motor housing having a motor housing with a motor housing

62. Motor with a motor housing having a motor housing with a motor housing

A1 10, face in

A2 10, face in

A3 11, face in 11

A4 11.

Claims

1. A hydrostatic drive for a press or injection molding machine, the hydrostatic drive having: cylinder devices (10, 11) on which differently large, hydrostatically acting piston surfaces (A1, A3) can be acted upon by pressure for the fast feed stroke and for the following compression stroke; a first hydraulic machine (30) whose pressure fluid supply can be varied and which can be operated for a fast stroke in a closed hydraulic circuit with a first cylinder chamber (14) upstream of the first piston face (A1) and pressure-loaded in the fast stroke and with a second cylinder chamber (15) upstream of the second piston face (A2); a second hydraulic machine (32) whose pressure fluid supply can be varied and which can be operated in a closed hydraulic circuit for a pressing stroke with a third cylinder chamber (24) upstream of the third piston face (A3) and pressure-loaded in the pressing stroke and with a fourth cylinder chamber (25) upstream of the fourth piston face (A4); and valve means which influence the flow of pressurized liquid when switching from the fast stroke to the pressing stroke,

The valve arrangement is characterized in that the valve arrangement comprises a plurality of passive valves, the state of which is derived from a predetermined value of the delivery quantity for the two hydraulic machines (30, 32), wherein the valves are check valves, and wherein a first check valve is arranged between the third cylinder chamber (24) and the fourth cylinder chamber (25) and opens from the fourth cylinder chamber (25) towards the third cylinder chamber (24), and a second check valve is arranged between a first line (34) leading from the first hydraulic machine (30) to the first cylinder chamber (14) and a second line (38) leading from the second hydraulic machine (32) to the third cylinder chamber (24) and opens from the second line (38) towards the first line (34).

2. Hydrostatic drive according to claim 1, wherein the speed of the cylinder arrangement (10, 11) is predetermined by the delivery of the first hydraulic machine (30) during the fast stroke of the feed.

3. Hydrostatic drive according to claim 2, wherein during a fast stroke of feed the second hydraulic machine (32) has a delivery volume greater than zero and the second hydraulic machine (32) delivers pressurized liquid from the fourth cylinder chamber (25) to the third cylinder chamber (24).

4. A hydrostatic drive according to claim 3, wherein the second hydraulic machine (32) has a maximum delivery during the fast stroke of feed.

5. A hydrostatic drive according to claim 3, wherein the delivery of the first hydraulic machine (30) is so small that the inflow of pressurized liquid from the first hydraulic machine (30) to the first cylinder chamber (14) gives rise to a speed of the cylinder arrangement (10, 11) which is smaller than the pressing speed, when switching from the fast stroke of feed to the pressing stroke, and wherein the delivery of the second hydraulic machine (32) is so adjusted that the pressing speed is given.

6. Hydrostatic drive according to claim 5, wherein at least one of the two hydraulic machines (30, 32) is adjustable in terms of its working volume.

7. Hydrostatic drive according to claim 6, wherein there is a rotation-speed-regulated motor (60) by which the two hydraulic machines (30, 32) can be driven jointly.

8. A hydrostatic drive according to any one of claims 1 to 6, wherein there are two rotation speed regulated motors (61, 62), and wherein the first hydraulic machine (30) is drivable by a first motor (61) of the two motors (61, 62) and the second hydraulic machine (32) is drivable by a second motor (62) of the two motors (61, 62).

9. Hydrostatic drive according to any one of claims 1 to 5, wherein there are two rotation-speed-regulated motors (61, 62), wherein the first hydraulic machine (30) is drivable by a first motor (61) of the two rotation-speed-regulated motors (61, 62) and the second hydraulic machine (32) is drivable by a second motor (62) of the two rotation-speed-regulated motors (61, 62), and wherein each of the two hydraulic machines (30, 32) is a hydraulic machine with a constant working volume.

10. Hydrostatic drive according to claim 9, wherein the two hydraulic machines (30, 32) are in fluid communication with the second cylinder chamber (15) in common, wherein a valve is arranged between the two hydraulic machines (30, 32) and the fourth cylinder chamber (25), with which valve a flow of liquid from the two hydraulic machines (30, 32) to the fourth cylinder chamber (25) can be stopped, and wherein a switching valve (51) is arranged between the third cylinder chamber (24) and the fourth cylinder chamber (25), which switching valve can be actuated at will and has a first switching position in which the third cylinder chamber (24) and the fourth cylinder chamber (25) are separated from each other, and a second switching position in which the third cylinder chamber (24) and the fourth cylinder chamber (25) are in fluid communication with each other.

11. Hydrostatic drive according to claim 10, wherein the valve arranged between the two hydraulic machines (30, 32) and the fourth cylinder chamber (25) is a check valve.

12. Hydrostatic drive according to claim 11, wherein the first cylinder chamber (14) is fluidly connectable to the low pressure reservoir (54) via a check valve (53) opening towards the first cylinder chamber, the second cylinder chamber (15) via a check valve (53) opening towards the second cylinder chamber and the third cylinder chamber (24) via a check valve (53) opening towards the third cylinder chamber.

13. Hydrostatic drive according to claim 12, wherein the fourth cylinder chamber (25) is in direct fluid communication with a low pressure reservoir (47).