WO1999036719A1

WO1999036719A1 - Valve

Info

Publication number: WO1999036719A1
Application number: PCT/SE1999/000015
Authority: WO
Inventors: Jan HÖLCKE; Mikael Theorin
Original assignee: Hoelcke Jan; Mikael Theorin
Priority date: 1998-01-20
Filing date: 1999-01-08
Publication date: 1999-07-22
Also published as: AU2303799A; SE9800127L; SE9800127D0

Abstract

The present invention relates to a valve (10) for controlling a flow of fluid, comprising: a valve housing (11) having an inlet (12) and an outlet (13); a valve member (20) which is arranged in the valve housing and which is operable between a first and a second position; and valve member operating means (50, 55, 60, 65) which comprise a magnetostrictive or piezoelectric element (50) and which effect a longitudinal change of the element. The invention is characterized in that the valve member (20) is turnable about an axis (30), and that said longitudinal change produces a force which is made to act on the valve member (20) to turn the same between the first and the second position and vice versa.

Description

VALVE

The present invention relates to a valve for controlling a flow of fluid, comprising: a valve housing having an inlet and an outlet; a valve member which is arranged in the valve housing and which is movable between a first and a second position, said valve member having a first portion which is arranged to prevent flow of fluid through the valve in the first position; and valve member operating means which comprise a magnetostrictive or piezoelectric element.

Field of the Invention and Background Art

In recent years, there has been an increasing demand for so-called quick valves. Conventional valves are usually limited to operating frequencies of up to 500 Hz. By using quick valves, one hopes to be able to achieve operating frequencies of up to 1000 Hz and more. Such quick valves could be used, for instance, as pilot valves in servo valve assemblies for high frequency app] ications, as acoustically damping control valves for reducing variations in pressure in a flow of fluid and as control valves for quick control of the supply of cooling water in connection with continuous casting of thin steel wires. A further conceivable application is in connection with mechanical testing and load simulation at high fre- quencies.

A technique of achieving operation of valves at high frequencies is to use piezoelectric or magnetostrictive materials, so-called quick materials, as operating means. Magnetostrictive and piezoelectric materials share the property that their dimensions are dependent on magnetic and electric fields applied to the materials. Thus, the operation can be very well controlled with the aid of electric control means. The dimensional changes have the effect of generating forces which are sufficient for use in an advantageous manner at very high hydraulic pressures. Moreover, the materials react very quickly to changes of said magnetic or electric fields, which is a very important property in connection with high frequency applications.

A drawback of magnetostrictive and piezoelectric materials is that the dimensional changes that can be achieved with these materials are very small in relation to the movement which is often necessary in an operable, closing and opening valve member in a valve. It is therefore necessary to produce some kind of ratio between the magnetostrictive or piezoelectric element and the valve member, so that the limited longitudinal change of the element is converted into a considerably greater movement of the valve member.

An example of such a ratio-producing device is disclosed in European Patent Application EP-A 0 450 078. The device comprises a piezoelectric operating element, the longitudinal change of which is transferred by means of an assembly of gears and/or worm gears to a longitudinal movement of a valve slide. The gear assembly produces the ratio between the piezoelectric element and the valve slide .

A drawback of this device is that the ratio-produc- ing steps correspondingly reinforce the inertial forces that must be overcome to produce a movement of the valve slide. At high frequencies, a very great inertia will act on the piezoelectric or magnetostrictive element.

Moreover, it is important for the design and ope- ration of the valve member to be such that the pressure drop of the flow of fluid across the valve in an open position is as low as possible. At high frequencies, it is also important that the pressure drop be low not only in the open position of the valve, but also in the momen- tary positions which together form the opening and closing procedure of the valve. US Patent 4,158,368 discloses a valve which comprises a magnetostrictive element operating a valve member. The valve member extends and is displaceable transversely of a flow duct through the valve. Moreover, the valve member has an opening which in an open position is aligned with the flow duct and in a closed position is arranged beside the flow duct so that the flow is blocked by the valve member. Thus, the valve is controlled by means of a magnetostrictive element but has no ratio as described above and besides is of a design which in a half open state causes turbulence and eddies in the fluid downstream of the closing and opening member.

With regard to the speed at which a pressure wave propagates in a hydraulic oil, it is also very important at high frequencies that the valve is compact, so that the physical distance between a fluid-controlling element, such as a valve member, and an element actuated by the control, such as a main slide of a servo valve, becomes small. An object of the invention thus is to provide a quick valve of the type mentioned by way of introduction for application at high frequencies.

One more object of the invention is to provide a quick valve of the type mentioned by way of introduction, which produces an efficient ratio between a magnetostrictive or piezoelectric element and a valve member of the valve .

Another object of the invention is to provide a quick valve of the type mentioned by way of introduction, which has a low pressure drop both in the open position and during the entire opening and closing procedure.

An additional object is to provide a compact valve of the type mentioned by way of introduction.

One more object of the invention is to provide a magnetostrictively or piezoelectrically operated three- way valve. Summary of the Invention

The objects of the invention are achieved by means of a valve of the type mentioned by way of introduction, which has the features defined in the appended claims. The invention thus is based on the understanding of the advantage that the valve member is turnable about an axis, and that said longitudinal change produces a force which is made to act, in a preferably essentially linear manner, on the valve member to effect turning thereof between the first and the second position and vice versa. According to one embodiment of the invention, said operating means and the valve member are arranged to make said force act on the valve member in a direction whose perpendicular distance to the axis is small in relation to a distance between the axis and the first portion of the valve member, whereby the motion caused by said turning of the first portion of the valve member becomes great in relation to the longitudinal change of said element . According to one more preferred embodiment of the invention, the ratio between the longitudinal change of said element and the movement of the first portion of the valve member which is achieved with the above design is greater than 1:5. By the fact that the longitudinal change of the magnetostrictive or piezoelectric rod produces a force which is made to act, in a preferably essentially linear manner, on the valve member in a direction whose perpendicular distance to the axis of the valve member is very small, said preferred ratio between the longitudinal change of the rod and the movement of the valve member is produced by the actual turning motion of the valve member.

According to a further embodiment of the invention, the valve member has a second portion which is arranged to form, together with a portion of the valve housing, a duct for a flow of fluid from the inlet to the outlet in the second position, which duct is, at least in the open position, designed as a diffuser.

This embodiment thus has the advantage that a diffuser is formed as an integrated element of the central parts of the valve. This means that the total size of the valve can be reduced compared with a construction in which a valve flap and the diffuser are formed of separate parts. The diffuser shape of the duct also means that the occurrence of turbulence at the valve and pres- sure drops across the valve is counteracted also in the successive or momentary, partly open positions which all in all form the opening and closing procedure of the valve .

Brief Description of the Drawings

The present invention will now be described in more detail by way of exemplifying embodiments with reference to the accompanying drawings, in which

Fig. 1 is a schematic cross-sectional view of a valve according to an embodiment of the invention, the valve being shown in a first, closed position;

Fig. 2 is a schematic cross-sectional view of the valve in Fig. 1 in a second, open position;

Fig. 3A is a schematic cross-sectional view, on a larger scale, of central pivot portions of the valve member in Fig. 1;

Fig. 3B is a schematic cross-sectional view, on a larger scale, of central pivot portions of the valve member in Fig. 2; Fig. 4 illustrates schematically a detail of a valve according to a further embodiment of the invention,

Fig. 5 is a schematic cross-sectional view of a valve according to a further embodiment of the invention, said valve having a further connection; Fig. 6 is schematic cross-sectional view of a further embodiment of the invention; Fig. 7A is a schematic cross-sectional view along line I-I in Fig. 2;

Fig. 7B is a schematic cross-sectional view along line II-II in Fig. 6, and Fig. 8 is a schematic cross-sectional view of a valve assembly according to one more embodiment of the invention.

Detailed Description of Preferred Embodiments Figs 1 and 2 are cross-sectional views of a valve 10 according to an embodiment of the present invention, in a closed and an open position respectively. A section along line I-I in Fig. 2 will be described below with reference to Fig. 7A. The valve 10 comprises a valve housing 11 which has an inlet 12 and an outlet 13. The inlet 12 communicates with the outlet 13 through a duct 16.

A plate-shaped valve member or turning slide 20 is arranged in the valve housing 11. The valve member is turnable about a stationary axis 30 between a first, closed position as shown in Fig. 1 and a second, open position as shown in Fig. 2 and vice versa.

More specifically, the valve member is turnable on a pivot 31 which has a cut-off portion defining a plane sliding surface which slidingly engages a corresponding surface of a wedge 32 coacting with the pivot 31. The wedge 32 is arranged in a recess in the valve member 20 in connection with the pivot 31. Moreover, the wedge 32 is turnable in relation to the valve member 20 and dis- placeable in relation to the pivot 31 along the surface thereof.

The valve member 20 has a first edge portion 21, which is arranged to block or prevent a flow of fluid through the inlet 12 in the closed position, and an essentially flat second portion 22, which extends from the portion 21 and is designed to form, together with an opposite, essentially flat portion 15 of the valve hous- ing 11, the duct 16 from the inlet 12 to the outlet 13 in the open position.

The second portion 22 of the valve member 20 and the wall portion 15 of the valve housing 11 are designed such that the duct 16 forms a diffuser in the open position, which preferably has an opening angle of between 4° and 10°, preferably 8°. It will, however, be appreciated, that the opening angle of the duct will vary during the actual opening and closing procedure. When the fluid flows past the first portion 21 of the valve member 20 in an open or partly open position, as shown in Fig. 2, the flow of fluid partly changes into a jet or into a more turbulent flow. In this connection, a pressure of the fluid at the inlet 12 is partially converted into kinetic energy of the jet, and thus a decrease of pressure arises. The diffuser, which is formed of the second portion 22 of the valve member 20 and the portion 15 of the valve housing 11, then has the effect of again converting the kinetic energy of the jet into a pressure of the fluid at the outlet 13. Thus the pressure drops across the valve becomes low.

The valve member is operated by a magnetostrictive or piezoelectric rod 50. In the case as shown in the Figures, the rod 50 is a magnetostrictive rod, the longi- tudinal change of which thus is controlled by an electromagnetic field generated by an electromagnetic coil 65, which in turn is supplied with voltage from a voltage source/control unit 60. The electromagnetic coil and the magnetostrictive rod 50 are arranged in a casing 17 in connection with the valve housing 11.

Since this type of rods is very sensitive to flexu- ral stress, the rod 50 is hingedly fixed to the casing 7 and to a link arm 55 by means of some sort of friction- less, articulated connections, which in the Figures are exemplified in the form of balls 51 and 52. The link arm 55 transfers the longitudinal extension of the rod 50 in a linear manner to the valve member 20 without any ratio. The link arm 55 and the magnetostrictive rod 50 are axially aligned with each other and extend through an opening in the wall of the valve housing 11 and a bore in the valve member 20 to the wedge 32. According to this embodiment, operation of the valve member is effected by the voltage source/control unit 60 feeding a voltage to the coil 65 which then generates a magnetic field causing a longitudinal change of the rod 50. The longitudinal change of the rod 50 produces a force which via the end of the link arm 55 is, in an essentially linear manner, made to act with slidable engagement on the curved surface of the wedge 32 which is arranged in the valve member 20. The sliding movement of the wedge along the plane sliding surface of the pivot 31 makes the pivot 31 and the valve member 20 turn as desired about the axis 30 in the valve housing 11.

As is evident from the Figures, the axis 30 is directed essentially perpendicular to the direction of flow through the valve. Some magnetostrictive or piezoelectric materials have brittle properties, which is a problem at high frequencies when the accelerating power acting on the rods in such applications is particularly great. The rods can therefore easily break when subjected to tensile load. Depending on the choice of material, it is therefore preferable that the rod 50 is prestressed. The prestress can be achieved either mechanically, with different types of springs, and/or hydraulically. In the embodiment in Figs 1 and 2, the valve 10 comprises a torsion spring (not shown) which acts to turn the pivot 31 with a view to turning the valve member counterclockwise in the Figure, whereby the wedge 32 is continuously pressed against the link arm 50 which in turn transfers this compressive load to the rod 50. In the valve housing, one or more seals 40, 41 are arranged between the valve member 20 and the valve housing 11 to prevent the fluid from flowing another way through the valve 10 than through the duct 16 and to protect the magnetostrictive or piezoelectric rod. It will here be appreciated that the valve preferably also comprises seals between the valve housing and the valve me - ber surfaces which are plane-parallel with the cross-section as shown in the Figures.

As is evident from Fig. 2, the valve member 20 and the rod/link arm 50, 55 are designed so that the perpendicular distance a between the direction of the force produced by the longitudinal extension of the rod 50 and the axis 30 is relatively small relative to the distance A between the axis 30 and the first portion 21 of the valve member 20. This means that the longitudinal change of the rod 50 is small relative to the displacement, pro- duced by the turning motion, of the first portion 21 of the valve member 20. For instance, the ratio in the prototypes that have been developed so far by the inventors has been between 1:5 and 1:20, i.e. a longitudinal change of the magnetostrictive rod of 0.1 mm has resulted in a peripheral motion of the first portion 21 of the valve member 20 of between 0.5 and 2 mm.

With the aid of the wedge construction in the Figures above, it is ensured that the distance a is kept constant during the entire turning motion, which thus prevents the link arm 55 from being laterally displaced during the turning motion. Such lateral displacement of the link arm 55 would result in a change of the ratio during the turning motion, which may cause certain construction difficulties. Figs 3A and 3b are schematic cross-sectional views on a larger scale of the central pivot portions of the valve member 20 in the position shown in Fig. 1 and Fig. 2, respectively. For better understanding of the movement of the wedge 32 relative to the pivot 31 and the valve member 20, the change of position shown in the Figures is slightly exaggerated. The valve member in Figs 3A and 3B is turnable on the pivot 31 which has a cut-off portion defining a plane sliding surface which slidably engages the corresponding surface of the wedge 32. The wedge 32 is arranged in a recess in the valve member 20 in connection with the pivot 31. As mentioned above, the wedge 32 is turnable relative to the valve member 20 and displaceable relative to the pivot 31 along the surface thereof.

Fig. 4 is a cross-sectional view of a detail of a valve according to a further embodiment of the invention, in which prestressing of the magnetostrictive rod is effected hydraulically .

In Fig. 4, the valve housing 11 and a portion 23 of the circumference of the valve member 20 are designed such that a distance B between the axis 30 and the edge portion 21 of the valve member 20, which edge portion is adapted to engage, in the closed position, the surface 15 of the valve housing 11, is greater than a distance b between the axis 30 and the seal 41, as is shown in the Figure. The fluid pressure from the inlet 12 then acts on a portion 23 of the valve member 20 between the edge portion 21 and the seal 41. Owing to the difference in distance, the pressure acting upon the portion 23 produces on the whole a moment that wants to turn the valve body 20 counterclockwise in the Figure. This turning moment acts via the valve member on the link arm 55 which, in turn, transfers this compressive load to the magnetostrictive or piezoelectric rod, whereby the above-mentioned prestress of the rod 55 is produced by the fluid pressure and the actual design of the valve member.

Fig. 5 is a cross-sectional view of a valve according to one more embodiment of the invention, said valve comprising a connection 14.

The connection 14 is formed in the valve housing 11 in or at the portion 15 immediately downstream of the first portion 21 of the valve member 20. When, in the open position shown in Fig. 5, the fluid flows from the inlet 12 past the connection 14 and through the diffuser- shaped duct 16 to the outlet 13, an ejector effect depending on the flow of fluid arises at the connection 14. As long as the flow of fluid continues, this ejector effect prevents fluid from flowing from the inlet 12 to the connection 14 in the open position. In the closed position, a flow of fluid is possible from the connection 14 through the duct 16 to the outlet 13 and vice versa. This ejector effect, which in the open position is caused by the flow past the connection 14, produces at the connection 14 a negative pressure, which can be used to suck in fluid or material through the connection 14 to the duct 16.

This valve thus is a three-way valve which has the special property that the connection 14 communicates with the outlet 13 both in the closed and in the open position, but nevertheless a flow of fluid to and/or from the connection 14 can occur in the closed position only.

Fig. 6 is a cross-sectional view of a valve accord- ing to a further embodiment of the invention. In this embodiment, the link arm 55 is hingedly connected, for instance, by means of a ball 53 to an engaging point 25 on the circumference of the valve member 20. The magnetostrictive rod, the link arm 55 and the engaging point 25 are aligned with each other so that the force exerted by the rod/link arm is directed in a direction whose perpendicular distance to the axis 30 is small in relation to the distance between the axis 30 and the portion 21 of the valve member 20, thereby achieving the desired ratio of the longitudinal change of the rod.

In this embodiment, the wedge 32 in the above embodiments is not employed, but the valve member is turnable on a circular-cylindrical pin 33, preferably using a bearing, such as a needle bearing 36. Owing to this arrangement, the perpendicular distance a between the direction of action of the force and the axis 30 is not constant during the turning motion, but will be slightly changed during the turning motion of the valve member. The essential thing is, however, that the "momentary" distance is small relative to the distance between the axis 30 and the portion 21 of the valve member 20 to achieve the necessary ratio.

In the embodiment shown in Fig. 6, the valve member is formed with through portions 27 that have been removed. These cavities are designed with a view to reducing, on the one hand, the inertia of the valve members and, on the other hand, the friction of the valve members against the valve housing, thereby facilitating turning of the valve members. It goes without saying that this feature is not limited to the embodiment in Fig. 6 and can advantageously be used in the other embodiments. Fig. 7A is a cross-sectional view along line I-I in Fig. 2. The arrangement in the form of the plate-shaped valve member 20, the pivot 31 and the wedge 32 is turnable relative to the axis 30 in the valve housing 11. The pivot 31 extends into the valve housing 11 and is turn- ably connected to the valve housing 11 via bearings, such as needle bearings 36. Besides, the valve member 20, the pivot 31 and the wedge 32 are slidable relative to each other or turnably arranged relative to each other as described above with reference to Figs 1 and 2. Fig. 7B is a cross-sectional view along line II-II in Fig. 6. The wedge 32 is missing in this embodiment, and the pin 33 thus is a "full" circular-cylindrical pin. The valve member 20 is turnable in the valve housing 11 on the circular-cylindrical pin 33 by means of the needle bearing 36. The circular-cylindrical pin 33 is in turn connected to the valve housing 11.

Fig. 8 is a cross-sectional view of a valve assembly according to a further embodiment of the invention. In this embodiment use is made of two oppositely arranged valves 10 and 10' according to Fig. 5 to achieve the control of a flow of fluid from a common inlet 12" to two or more outlets 13, 13'; for instance, in the Figure use is made of two valves as described above as pilot valves in a servo valve assembly.

In Fig. 8, the inlets 12, 12' of the valves 10 and 10' are connected to the common inlet 12", and the out- lets 13 and 13' of the valves are connected to a working loop 70, 70', in which a working means 80, which can be, for instance, a cylinder piston or a main slide of a servo valve as shown in the Figure, is arranged to be actuated and preferably displaced by the pressure exerted by the flow of fluid through the valves.

In the embodiment in Fig. 8, the connections 14 and 14 ' are further connected to a return connection 14" which is common to the valve assembly.

A common control unit 60", which replaces or com- prises the control units 60 and 60', respectively, of the individual valves 10 and 10', is arranged for common synchronisation and operation of the valve members 20 and 20' .

Furthermore, the assembly in Fig. 8 comprises one or more detectors 61 which preferably are arranged downstream of the valves and preferably in the vicinity of the main slide. The detector 61 can be a pressure sensor or a mechanical detector which senses the pressure downstream of the valves or the position of the main slide and which feeds this information to the control unit 60". This results in the feed-back coupling to the control unit which is frequently necessary for correctly synchronised control and operation of the valve assembly.

An example of the mode of function of the valve assembly in Fig. 8 will now be described.

When the valve member 20 of the first valve 10 is moved to its second, open position and the valve member 20' of the second valve 10' is moved to its first, closed position, the fluid flows from the inlet 12" and 12 through the first valve 10 to the outlet 13 and further to the working loop 70 and makes the main slide 80 move to the right in the Figure. Fluid in the working loop 70' on the right-hand side of the main slide 80 is then pressed by the same towards the outlet 13' of the second valve 10' and through the connection 14' to the common return connection 14" or further through the connection 14 into the flow of fluid through the first valve 10.

The above-mentioned ejector effect, which depending on the flow of fluid through the duct 16 of the first valve 10 arises at the connection 14, facilitates this return flow of fluid from the working loop 70' by acting in a "sucking" manner at the connection 14, thereby facilitating the movement of the main slide 80 to the right.

When correspondingly the valve member 20 of the first valve 10 is moved to its closed position and the valve member 20' of the second valve 10' is moved to its open position, the fluid flows from the inlet 12" and 12' through the valve 10' to the outlet 13' and further to the working loop 70' and makes the main slide 80 move to the left in the Figure. Fluid in the working loop 70 on the left-hand side of the main slide 80 is then pressed by the same towards the outlet 13 of the first valve 10 and through the connection 14 to the common return connection 14" or further through the connection 14' into the flow of fluid through the second valve 10'.

Also in this case, the above-mentioned ejector effect, which now depending on the flow of fluid through the duct 16' of the valve 10' arises at the connection 14', will facilitate this return flow of fluid from the working loop 70 by acting in a "sucking" manner at the connection 14', thereby facilitating the movement of the main slide 80 to the left.

It is obvious to a person skilled in the art that the features which in this specification have been presented in connection with specific embodiments are not necessarily restricted to the respective embodiments, and that these can advantageously be combined in different ways depending on the field of application and the desired mode of function of the valve or valve assembly. Although the valve member according to the various embodiments is turnable about a stationary axis, it will be appreciated that the turning motion may consist of a combined turning and translational motion, the pivot being moved successively or momentarily relative to the valve member or valve housing during the turning and translational motion.

Although an open and a closed position are mentioned in the various embodiments, it will be appreciated that the open position may comprise several positions with different degrees of opening, which, to different extents, prevent the flow through the valve, and that the closed position need not necessarily imply that the duct through the valve is completely closed, and that the closed position can be a blocking position in which the flow through the valve is prevented to a greater degree than in the open position.

Claims

1. A valve (10) for controlling a flow of fluid, comprising: a valve housing (11) having an inlet (12) and an outlet (13); a valve member (20) which is arranged in the valve housing and which is operable between a first and a second position; and valve member operating means (50, 55, 60, 65) which comprise a magnetostrictive or piezoelectric element (50) and means (60, 65) for effecting a valve-member-affecting longitudinal change of said element (50) , c h a r a c t e r i s e d in that the valve member (20) is turnable about an axis (30) between a first and a second position, the valve member having a first portion (21) which is arranged to prevent fluid from flowing through the valve (10) in the first position and allowing fluid to flow in the second position; and that said longitudinal change produces a force which is made to act in a preferably essentially linear manner on the valve member (20) to produce a turning motion of the valve member (20) between the first, closed position and the second, open position and vice versa.

2. A valve as claimed in claim 1, c h a r a c t e r i s e d in that the axis (30) is a pivot (31) , and that the axis (30) extends through the valve member (20) .

3. A valve as claimed in claim 1 or 2, c h a r a c t e r i s e d in that said operating means (50, 55, 60, 65) and the valve member (20) are arranged to make said force act on the valve member in a direction whose perpendicular distance (a) to the axis (30) is essential- ly smaller than a distance (A) between the axis and the first portion (21) of the valve member, whereby the motion, caused by said turning, of the first portion (21) of the valve member (20) becomes great in relation to the longitudinal change of said element (50).

4. A valve as claimed in claim 3, c h a r a c t e r i s e d in that said operating means (50, 55, 60, 65) and the valve member (20) are designed such that a ratio between the longitudinal change of said element (50) and the movement of the first portion (21) of the valve member (20) is greater than 1:5.

5. A valve as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that said force is made to act in an engaging point (25) on the valve member (20) .

6. A valve as claimed in claim 1, 2, 3 or 4, c h a r a c t e r i s e d in that said force is transmit- ted to the valve member (20) in close connection with the axis, preferably with the aid of movable wedge means (32), such that the perpendicular distance (a) between the direction of the force produced by said operating means (50, 55, 60, 65) and the axis (30) is constant during turning of the valve member.

7. A valve as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the valve member (20) has a second portion (22) which is arranged to form, together with a portion (15) of the valve housing (11), a duct (16) for a flow of fluid from the inlet (12) to the outlet (13) in the second position.

8. A valve as claimed in claim 7, c h a r a c t e r i s e d in that the duct (16) , which is formed of the second portion of the valve member (20) and the por- tion (15) of the valve housing (11), forms a diffuser.

9. A valve as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the axis (30) is directed essentially perpendicular to the direction of flow of the fluid through the valve housing (11) .

10. A valve as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the valve (10) comprises prestressing means for producing prestress of said element.

11. A valve as claimed in claim 10, c h a r a c t e r i s e d in that the valve housing (11) and the valve member (20) are designed to make the flow of fluid from the inlet (12) produce a pressure on a portion (23) of the circumference of the valve member (20) , said pressure constituting said prestressing means by acting on the whole in a turning manner on the valve member (20) in a direction which is prestressing to the element (50) .

12. A valve as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the valve housing (11) has a connection (14) which is arranged downstream of the first portion (21) of the valve member (20), the second portion (22) of the valve member and the portion (15) of the valve housing (11) forming a duct for a flow of fluid between the connection (14) and the outlet (13) and vice versa when the valve member (20) is in the first position, and the flow of fluid from the inlet (12) to the outlet (13) flowing past the connection (14) and producing an ejector effect, depending on the flow of fluid, at the connection (14) when the valve member (20) is in the second position.

13. A valve assembly for controlling a flow of fluid, c h a r a c t e r i s e d in that the assembly comprises a first valve (10) according to any one of the preceding claims, and a second valve (10') according to any one of the preceding claims, the inlet (12) of the first valve (10) and the inlet (12') of the second valve (10') being connected to an inlet (12") which is common to the valve assembly.

14. A valve assembly for controlling a flow of fluid, c h a r a c t e r i s e d in that the assembly comprises : a first (10) and a second (10') valve according to claim 12; a working loop (70, 70') connected between the outlet (13) of the first valve (10) and the outlet (13') of the second valve (10'); working means (80) which is arranged in the work- ing loop (70, 70') and which is actuated by said flow of fluid; a control unit (60") for controlling the operating means (50, 55, 60, 65, 50', 55', 60', 65') of the first and the second valve for producing a flow of fluid through the first valve (10) and/or the second valve

(10') to the working loop (70, 70'), said flow of fluid producing a pressure acting on said working means (80) .

15. A valve assembly as claimed in claim 14, c h a r a c t e r i s e d in that the control unit (60") is arranged either to move the valve member (20) of the first valve (10) to its first position and the valve member (20') of the second valve (10') to its second position, which results in said flow of fluid being made to flow essentially through the second valve (10') to the working loop (70') and forming said flow of fluid which produces a pressure acting in a first direction on said working means (80) , or to move the valve member (20) of the first valve (10) to its second position and the valve member (20') of the second valve (10') to its first posi- tion, which results in said flow of fluid being made to flow essentially through the first valve (10) to the working loop (70) and forming said flow of fluid which produces a pressure acting in a second direction on said working means (80) .

16. A valve assembly as claimed in claim 14 or 15, c h a r a c t e r i s e d in that the inlet (12) of the first valve and the inlet (12') of the second valve are connected to an inlet (12") which is common to the valve assembly.

17. A valve assembly as claimed in claim 14, 15 or 16, c h a r a c t e r i s e d in that the connections (14, 14') of the first and the second valve constitute the respective return outlets from the valve assembly.

18. A valve assembly as claimed in any one of claims 14, 15, 16 or 17, c h a r a c t e r i s e d in that the connection (14) of the first valve and the connection

(14') of the second valve are connected to a return connection (14") which is common to the valve assembly, whereby said flow-dependent ejector effect, which arises from said flow of fluid through the first valve (10) at the connection (14) of the first valve, acts via said common return connection (14") and produces or facilitates a return flow of fluid from the working loop (70') via the outlet (13') of the second valve (10') to the return connection (14', 14") or to the connection (14) of the first valve (10) when the valve member (20) of the first valve is in the second position and the valve member (20') of the second valve is in the first position and vice versa.