GB2279726A

GB2279726A - Switching valve

Info

Publication number: GB2279726A
Application number: GB9412229A
Authority: GB
Inventors: Bernd Niethammer
Original assignee: Hydraulik Ring GmbH
Current assignee: Hilite Germany GmbH
Priority date: 1993-06-17
Filing date: 1994-06-17
Publication date: 1995-01-11
Also published as: ITMI941238A0; IT1270221B; ITMI941238A1; DE4320017A1; FR2706571A1; GB9412229D0; FR2706571B1

Description

1 Switchinq valve 2279726 The invention relates to switching valves of the

kind having a housing in which at least one switching valve member is displaceable between two end positions in each of which it bears against a respective stop under the action of a force.

In the case of known switching valves of this kind, the switching valve member is pushed into its end position by a disc spring. To actuate the valve, the switching valve member is displaced by the hydraulic medium in opposition to the force of the disc spring. The disc spring has a spring force characteristic curve which represents a cubic function. Initially, the spring force increases as deformation increases, until it falls again on exceeding a maximum, then to increase again on further deformation. The disc spring has a relatively large external dic-qneter so that the switching valve is of a correspondingly large-volume construction. When the disc spring is used the useful stroke distance is relatively small. Moreover, the tensile forces in the disc spring very quickly reach critical values on elastic deformation, so that a 2 so-called relaxation of the disc spring occurs. This reduction in the spring force can be attributed to tension cracks in the disc spring.

Pressure switching valves are also known which have_ three control slide valves, of which one control slide valve serves as the main control slide valve and conveys the hydraulic current from the pump to the storage system or, in an unpressurized state, to the tank. The second control slide valve serves to check a top switching point and to supply the pressure signal to the main control slide valve so that it switches to the tank in an unpressurized state. The third control slide valve has the task of monitoring the bottom switching point and, if the value falls below this switching point, of displacing the main control switching valve back to the charge position with the aid of a control current. The mechanical complexity of a pressure switching valve of this type is great. Three valve member slide valves and the precise valve member bores necessary for these are required. A switching valve of this type is also extremely susceptible to faults.

Finally, a storage charge valve is known which has a pressure sensor which monitors the pressure values of a 1 3 storage system and passes the pressure signals by way of a processing electronics system to a switching magnet which for its part actuates the switching valve member.

Again, the structural complexity of this switching valve is very great.

The object of the invention is to provide a switching valve of the kind initially referred to such that, while being simple structurally, it has a compact design, is less susceptible to faults and has a long service life.

This invention resides in a switching valve having a housing in which at least one switching valve member is displaceable between two end positions in each of which it bears against a respective stop under the action of a force, characterized in that the valve member is held in one of the said end positions by magnetic force.

In the case of the.switching valve according to the invention, the switching valve member is held in the one end position by magnetic force. For this, permanent magnets or electromagnets which have only a very small structural size can be used. The switching valve according to the invention can thus be of extremely compact and small construction. The magnets are a 4 wear-free components which are moreover inexpensive to procure. They ensure problem-free operation of the switching valve even after a long period of use. Since the magnets do not wear, the response behaviour of the switching valve member does not alter either, so that the switching valve according to the invention has constant switching properties throughout its duration of use. The use of magnets does not require any structural expense, so that the switching valve according to the invention can also be made in an extremely inexpensive way.

Further features of the invention are found in the dependent claims, the description and the drawings.

The invention will be explained in more detail with reference to embodiments illustrated in the drawings, which:

Fig. 1 shows, in longitudinal section, a first embodiment of a switching valve according to the invention, Fig. 2 shows a force/distance graph in which the characteristic curves of the switching valve in accordance with Fig. 1 and of a known switching valve are entered, Fig. 3 shows, in an illustration corresponding to Fig. 1, a second embodiment of a switching valve according to the invention, Fig. 4 shows the force/distance graph of the switching valve according to Fig. 3, Fig. 5 shows, in an illustration corresponding to Fig. 1, a third embodiment of a switching valve according to the invention, Fig. 6 shows the force/distance graph of the switching valve according to Fig. 5, Fig. 7 shows a force/distance graph of a switching valve embodying to the invention, having two mutually repelling magnets, Fig. 8 shows, in an illustration corresponding to Fig. 1. a fourth embodimpnt of the switching of the invention, and Fig. 9 shows the force/distance graph of the switching valve according to Fig. 8.

6 The switching valve according to Fig. 1 can be used as a storage charge valve, as a safety valve, as a pressure switching valve or the like. It has a housing 1 which is of anti-magnetic material and has a central longitudinal bore 2 in which a valve member in the.form of a plunger or spool 3 is displaceably mounted. The plunger 3 is provided with an annular groove 4 on the periphery. At one end, the plunger 3 carries a permanent magnet 5 which is for example a circular disc. The permanent magnet 5 is seated in a metal cup 6, which is for example of steel. The cup 6 encloses the permanent magnet 5 at the edge and at the face remote from the plunger 3.

opposite the permanent magnet 5 lies a disc 7 which is of metal material and is preferably a steel disc and is fixed in the housing 1. The disc 7 and the permanent magnet 5 lie in a space 8 in the housing 1 which is enlarged in diameter by comparison with the longitudinal bore 2. The permanent magnet 5 is secured to the plunger 3 such that attraction forces exist between the permanent magnet 5 and the disc 7. So that the permanent magnet 5 does not lie directly on the steel disc 7, a spacer ring 9 of non-magnetic material is provided between both components. It is also fixed in the space 8. The cup 6 has a small radial spacing from 7 the wall 10 of the space 8, so that the plunger 3 can easily be displaced together with the permanent magnet 5 and the cup 6. As a result of the size of the radial spacing, the speed of the plunger can be damped and thus the switching noise can be minimized.

The space 8 is delimited at the end opposite the disc 7 by an adjustment and closure screw 11 which is screwed into the housing 1. Its end 12 facing the plunger 3 forms a stop face for the cup 6 in the one end position of the plunger 3. The end 12 is provided with a central elevated portion 13 so that the cup 6 does not lie flat on the end 12 in the stop position and the hydraulic medium can still reach the base of the cup 6 in this stop position.

On the side of the plunger 3 remote from the adjustment and closure screw, there is provided a further stop 14 a gainst which the plunger 3 comes to bear in the other end position. The stop 14 is formed by an end of an adjustment screw 15 which, like the adjustment and closure screw 11, is screwed into the housing 1. The other end of the adjustment screw 15 projecting out of the housing 1 is provided with a knurled knob 16 so that the adjustment screw 15 can readily be actuated. The 8 latter has a threaded part 17 by means of which the adjustment screw 15 is screwed into a threaded bore 18 in the housing 1. Attached to each end of the threaded part 17 is a respective pushrod 19 and 20, preferably constructed in one piece with the threaded part 17. The pushrod 19 carries the knurled knob 16, while the end of the pushrod 20 forms the stop 14 of the plunger 3. The pushrod 20 is guided over part of its length by the wall 21 of a passage bore 22 in the housing 1, and is sealed off in the region of the bore 22 by a seal ring 23. The threaded bore 18, the bore 22 and the longitudinal bore 2 are coaxial. The longitudinal bore 2 has a larger diameter than the bore 22 but a smaller diameter than the threaded bore 18.

Instead of the adjustment screw 15, in a simple embodiment it is also possible to provide a stop which is fixed to the housing.

The housing 1 is provided with an L.P. connection point or drain port 24 which is connected to the tank 25. A bore 26 leads from the connection point 24 into the longitudinal bore 2.

9 The housing 1 furthermore has an H.P. connection point or supply port 27 for a pump (not illustrated). Adjoining the connection point 27 is a bore 28 from which two bores 29 and 30 branch off. Seated in the bore 29 is a nonreturn valve 31 which opens in the. direction towards the longitudinal bore 2.

The longitudinal bore 2 is furthermore connected, by way of a bore 32, to a connection point 33 of the housing 1 for an accumulator 34. The two bores 30 and 32 are each closed to the outside by a pressed-in ball 35 and 36 respectively.

A bore 37 which is also closed to the outside by a ball 38 also opens into the housing space 8. From the bore 37 there branches off a bore 39 into which the bore 26 opens and which is also closed to the outside by a pressed-in ball 40.

In the position ilustrated in Fig. i, the permanent magnet 5 bears against the spacer ring 9 as a result of the magnetic attraction forces between it and the disc 7. The plunger 3 bears against the stop 14. The tank bore 26 is, in this position of the plunger 3, blocked off from the line 32 leading to the accumulator 34, so that no hydraulic medium can be conveyed into the accumulator 34. The housing space 8 is connected to the tank connection point 24 by way of the bores 37 and 39.

If the hydraulic volume in the accumulator 34 decreases, then pressure is built up by way of the pump connection point 27 if the switching plunger 3 is in the left-hand end position illustrated. The hydraulic medium flows through the bore 28 and the bore 29 and the non-return valve 31 into the annular space 41 of the housing 1 which is in front of the plunger 3. The corresponding plunger face is thus acted upon by the hydraulic medium. This force first counteracts the attraction force between the permanent magnet 5 and the disc 7. By increasing the pressure, finally the hydraulic force exerted on the plunger end overcomes the magnetic attraction force, so that the plunger 3 is abruptly displaced to the right in Fig. 1. The hydraulic medium in the space 8 is displaced through the bores 37 and 39 to the tank connection point 24. At the same time, some of the hydraulic medium is displaced out of the space 8 through the annular gap between the cup 6 and the wall 10 into the space between the disc 7 and the permanent magnet 5. Advantageously, the bore 39 has a choke or damping plate 391, by means of which it is possible to influence the switching speed of the switching plunger 3. The plunger 3 is displaced far enough for the cup 6 to come to bear against the elevated portion 13 of the screw 11. This stop position is achieved when the accumulator 34 is again filled with the required 11 hydraulic volume. Then, the bore 30 is connected by way of the annular groove 4 of the plunger 3 to the bore 26 and thus to the tank connection point 24 so that a further build-up of pressure is prevented.

The nonreturn valve 31 prevents the hydraulic medium from being able to flow out of the annular space 41 back to the pump connection point 27.

If the hydraulic volume in the accumulator 34 decreases, then the hydraulic pressure-acting on the plunger 3 is correspondingly lowered. As soon as the attraction forces between the permanent magnet 5 and the disc 7 exceed the hydraulic force, the plunger 3 is once again abruptly displaced to the left in Fig. 1, in dependence on the damping, until the permanent magnet 5 bears against the spacer ring 7, or the plunger 3 bears against the stop 14. The hydraulic medium in the annular space 41 is in this case displaced through the bore 32 into the accumulator 34.

The thickness of the spacer ring 9 determines the level of the hydraulic force which is necessary to displace the plunger 3 out of the one end position illustrated in Fig. 1. The thinner the spacer ring 9, the greater the magnetic attraction forces and the greater the pressure 12 which has to be applied to the plunger face in order to displace the plunger 3 in opposition to the magnetic attraction forces. Conversely, the hydraulic forces required for this purpose are that much smaller, the thicker the spacer ring 9. Thus, by choice of the thickness of the spacer ring 9 the switching point at which the plunger 3 is displaced can very easily be adjusted. As soon as the magnetic forces are overcome by the hydraulic pressure acting on the plunger'3, the plunger 3 will be abruptly put in its one end position, in which the cup 6 bears against the elevated portion 13 of the screw 11.

Fig. 2 illustrates the associated force/distance characteristic curve of the switching valve. Fig. 2 contains two typical characteristic curves 42 and 43 for a switching valve of this type. Depending on the thickness of the spacer ring 9, the characteristic curve 42 or 43 results. The force F is the magnetic attraction force between the permanent magnet 5 and the disc 7, while the air gap is the spacing between the permanent magnet 5 and the disc 7. It can be seen clearly that in the stop position illustrated in Fig. 1 the magnetic attraction force F is at a maximum. As the spacing increases between the permanent magnet 5 and the 13 disc 7, this force decreases.

Fig. 2 also illustrates the characteristic curve 44, such as occurs in the case of conventional switching valves with a disc spring. This characteristic curve differs entirely from the characteristic curve of the switching valve in accordance with Fig. 1.

The falling characteristic curve has the advantageous effect that when the force upper limit which has been set is exceeded the permanent magnet 5 and the plunger 3 secured,thereto move abruptly into their maximum set end position. This effect can be designated a digitally switching valve. If the value of the lower set force minimum, which can be set using the screw 11, is not reached, then the reverse switching effect occurs. the lower force limit is not reached by only a minimal I f amount, the permanent magnet 5 and thus also the switching plunger 3 are pushed in the direction of the disc again and thus towards the larger force limit. This switching procedure also takes place in digitally switching manner. The falling force/distance characteristic curve 42, 43 has the effect, by comparison with a force/distance characteristic curve which rises over distance, as occurs for example in the case of a helical pressure spring, that the switching valve can switch over to its end position in one movement. This effect cannot be achieved in the case of 14 a rising force/distance characteristic curve. In this case, as the force increases, new distance positions repeatedly arise in linear manner.

Instead of or in addition to selection of the thickness of the spacer ring 9, the material of the permanent. magnet 5 and/or of the disc 7 can also be used to adjust the characteristic curve.

Since the permanent magnet 5 is enclosed by the cup 6, the magnetic effect is increased. The magnetic flux lines only emerge at the free face of the permanent magnet 5. As a result of the cup 6, the magnetic field can easily be controlled. Disruptive influences from the outside can be prevented by the cup 6. Moreover, the cup 6 has the advantage that any magnetic dirt particles are not attracted. The housing element and the further mechanically moving parts are made of a non-magnetic material, for example aluminium or plastics material. The switching valve according to Fig. 1 is distinguished by a structurally simple design with a low susceptibility to faults. Wear of the switching valve is small, with in particular the attraction forces between the permanent magnet 5 and the disc 7 remaining unchanged even after a long period of use of the switching valve. Using the screws 11 and 15 (Fig. 1) is the two end positions of the switching plunger 3 can be finely adjusted so that the switching valve can be optionally adapted to the desired switching task.

The top switch point can be precisely set using the screw 15, the bottom switch point using the screw 11.

The embodiment according to Fig. 3 differs from fig. 1 in that one end of a helical compression spring 45 acts on the base of the cup 6a and is accommodated in a recess 46 in the end of the_ screw 11a. Otherwise, this switching valve is of the same construction as the embodiment in accordance with Fig. 1. As a result of the helical compression spring 45, the force/distance characteristic curve of the switching valve can additionally be altered. The spring 45 acts in the same direction as the attraction forces between the permanent magnet Sa and the disc 7a. Using the screw 11a, the pre-loading force of the spring 45 can be altered. This makes it possible to alter the slope of the characteristic curve.

In Fig. 4, the falling characteristic curve 47 of the magnet Sa and the rising characteristic curve 48 of the spring 45 are shown. While the force of the spring 45 increases as the spacing between the permanent magnet Sa 16 and the disc 7a increases, the attraction force decreases as the spacing between the permanent magnet 5a and the disc 7a increases. The two characteristic curves 47 and 48 give a resultant characteristic curve 49 which is characteristic of the switching valve and of which the curve can be altered by altering the pre-loading of the helical pressure spring 45. The greater the pre-loading of the spring 45, the flatter the profile of the characteristic curve 49. If the pre-loading force of the spring 45 is correspondingly set to be weak, the resultant characteristic curve 49 is close to the magnetic force characteristic curve 47. By varying the preloading of the spring 45, the characteristic curve 49 desired for the respective application of the switching valve according to Fig. 3 can very easily be set.

As in the embodiment according to Fig. 1, the thickness of the spacer rtng 9a and/or the material of the permanent magnet 5a.and/or of the disc 7a can also be used to adjust the characteristic curve. The cup 4a can - as in the embodiment according to Fig. 1 - be left out without the mode of operation of the switching valve changing.

17 In the embodiment according to Fig. 5, the permanent magnet 5b is surrounded only on the periphery by a ring 50, while the face of the permanent magnet 5b remote from the disc 7b is exposed. This face 51 is spaced opposite a further permanent magnet 52 which is secured to the screw 11b. Advantageously, the permanent magnet 52 is surrounded by a cup 53 which surrounds the permanent magnet 52 at the edge and at the underside facing the screw 11b. The permanent magnet 52 can readily be secured to the side of the screw 11b via the cup 53. The two permanent magnets 5b and 52 are so arranged that magnetic repelling forces exist between them. Thus, the north or south poles of both magnets 5b and 52 face one another. Between the disc 7b and the permanent magnet 5b there exist the magnetic attraction forces already described with reference to the embodiments above. In other respects, this switching valve is of the same construction as the switching valve according to Fig. 1. In the end position illustrated in Fig. 5, the switching plunger 3b is held by the attraction forces existing between the permanent magnet 5b and the disc 7b. The level of the attraction forces is again determined by the thickness of the spacer ring 9b. Additionally, it is also possible to adjust the size of the attraction force using the material of the permanent magnet 5b and/or of the disc 7b. Moreover, 18 the repelling force between the two permanent magnets 5b and 52 also acts in this end position.

If the switching plunger 3b is to be displaced out of the end position illustrated into its other end position, the hydraulic pressure has to overcome not only the attraction forces between the disc 7b and the permanent magnet 5b but also the repelling forces between the two permanent magnets 5b and 52. The size of these repelling forces is once again dependent on the spacing between the two permanent magnets. The closer the permanent magnet 5b is pushed towards the permanent magnet 52, the greater become the repelling forces acting between them. This repelling action can be used to influence the switching speed. Thus, switching noise can be eliminated. For this reason, the corresponding force/distance characteristic curve 54 (Fig. 6) increases as the distance s increases. This distance s is the respective spacing between the two permanent magnets 5b and 52. On the other hand, the characteristic curve 55 characterizes the relationships between the permanent magnet 5b and the disc 7b. The characteristic curve 55 falls as the spacing between the disc 7b and the permanent magnet 5b increases. Overlaying the characteristic curves 54 and 55 gives the 19 resultant force/distance characteristic curve 56 which is characteristic of the switching valve according to Fig. 5. It can be adjusted so that it runs horizontally, that is to say the force required to displace the switching plunger 3b remains constant over the distance. This characteristic curve 56 can also have a falling or rising profile. This can be achieved by appropriately choosing the permanent magnet 5b and/or 52 and also by the spacing between the two permanent magnets in the one end position illustrated in Fig. 5. By means of the adjustment screw 11b, the spacing between the two permanent magnets 5b and 52 can be adjusted precisely and continuously variably. Assisted by the repelling forces between the permanent magnets 5b and 52 and by the attraction forces between the permanent magnet 5b and the disc 7b, the plunger 3b once again moves abruptly back into its end position illustrated in Fig. 5, in which it bears against the adjustment screw 15b.

In the embodiments according to Figs. 1 to 4, it is also possible to use two permanent magnets which are arranged so that they repel one another. In this case, the disc 7, 7a is replaced by a corresponding permanent magnet. Then, the plunger 3, 3a is held in abutment against the adjustment screw 11, 11a because of the repelling forces between the permanent magnet. In order to be able operate using this switching valve, for this reason this magnetic repelling force must be overcome by hydraulic force so that the plunger 3, 3a is moved into its other end position.

The characteristic curve associated with a switching valve of this type is illustrated in Fig. 7. This characteristic curve 57 runs, for example, mirror-symmetrically in respect of the force axis to the characteristic curves 42, 43 (Fig. 2). Apart from the different arrangement of the magnets, a switching valve of this type operates in the same way as a conventional valve with a compression spring. In this case, the switching plunger 3b is not displaced abruptly. Rather, it is displaced proportionally to the prevailing hydraulic pressure.

With the switching valve according to Fig. 8, position-dependent switching is provided by displacing a wedge 58 which is provided instead of the adjustment screw 15. The wedge 58 is displaceable perpendicular to the direction of displacing the switching plunger 3c, in a transverse bore 59 in the housing lc. The wedge 58 bears by means of a wedge face 60 against a side wall 61 of the transverse bore 59, which is correspondingly 21 inclined. An auxiliary plunger 63 which is guided in sealed manner in the longitudinal bore 2c of the housing lc bears against the planar side face 62 of the wedge 58 which is opposite the wedge face 60 and which extends perpendicular to the direction of displacement of the plunger 3c. It bears against the side face 62 of the wedge 58 under the force of a helical compression spring 64. The spring 64 is supported against the plunger 3c, which carries the permanent magnet Sc. Otherwise, the switching valve is of the same construction as the embodiment in accordance with Fig. 1.

In the position illustrated, plunger 3c adopts its one end position, in which the permanent magnet 5c bears against the spacer ring 9c under the magnetic attraction force. In this position, the pre-loading of the spring 64 is smaller than the attraction forces between the disc 7c and the permanent magnet 5c.

As the force/distance graph according to Fig. 9 shows, the permanent magnet Sc has the falling characteristic curve 65 and the spring 64 has the rising characteristic curve 66. In the end position of the plunger 3c illustrated in Fig. 8, the magnetic attraction force FM is great, while the spring force F F is 22 substantially smaller. If the wedge 58 in Fig. 8 is displaced downwardly, then as a result of the wedge face 60 and the side wall 61 of the transverse bore 59 it will also be displaced within the transverse bore towards the plunger 3c. As a result, the auxiliary. plunger 63 is displaced in opposition to the force of the pressure spring 64, towards the plunger 3c. The spring 64 is in this case compressed so that the spring force F F increases, as Fig. 9 shows, as long asthe magnetic force F m is greater than the spring force F F' the plunger 3c remains in its end position illustrated. If the spring force F F exerted by the spring 64 becomes greater than the opposing magnetic force F m as the wedge 58 is displaced, the switching plunger 3c will be displaced abruptly into its other end position, in which it bears against the adjustment screw 11c. In this case, the spring 64 relaxes somewhat. However, its spring force F F is still larger than the magnetic force FM, so that the switching plunger 3c is held reliably in its end position.

If the wedge 58 is pushed back again, then the pressure spring 64 relaxes enough for its spring force F F to become smaller than the magnetic force F M' Then, the switching plunger 3c is again pushed abruptly back into 23 its initial position in accordance with Fig. 8. At the end of this backward movement, the pressure spring 64 is once again pre-tensioned to a certain extent.

As a result of the pre-loading force of the spring 64 and/or the wedge angle of the wedge face 60 of the wedge 58 and/or the thickness of the spacer ring 9c and/or the material of the magnet 5c and/or of the disc 7c, the point in time at which the valve switches can very easily be established. Tn contrast to the embodiments described above, no hydraulic medium is used and thus no differential pressure is used to displace the plunger 3c. Rather, it is displaced in dependence on the displacement distance of the wedge 58, which generates a differential force by way of the spring 64 in the manner described. The sealing ring 67 used to seal the auxiliary plunger 63 is of a low-friction material in order not to impair the described sequence of the switching procedure.

Instead of the wedge 58, it is also possible to use control cams, link guides and the like. It is also possible to use a sliding part which lies coaxially with respect to the auxiliary plunger 63.

24 In the different switching valves which have been described and illustrated, the axes of the permanent magnet and of the disc lie coaxially with respect to one another, so that a symmetrical distribution of the magnetic flux lines is achieved. As a result of this, the plunger is also loaded evenly, so that no tilting forces can act thereon. In this way, it can be displaced in the wall of the housing without disruption. The mutually facing sides of the disc and the permanent magnet are in each case planar and lie perpendicular to the axis of the plunger.

In the embodiments illustrated, the attraction forces are generated by theuse of a permanent magnet. These magnetic attraction forces can also be generated by an electromagnet. Thus, there may be secured to the switching plunger an armature which is surrounded by a coil. By applying current to the coil, a magnetic field which shows the same characteristic curve as the permanent magnet and which cooperates with the discs 7, 7a, 7b is generated in known manner. It is also possible to use an electromagnet instead of the permanent magnet 52 in accordance with Fig. 5.

In an embodiment which is not illustrated, instead of the adjustment screw 11 it is possible to provide an electrical switch which is actuated by the switching plunger. As a result, the hydraulic pressure signal is converted to an electrical signal which may for example be used for a sequential circuit. - 26

Claims

Claims

1. A switching valve having a housing in which at least one switching valve member is displaceable between two end positions in each of which it bears against a respective stop under the action of a force, characterized in that the valve member is held in one of the said end positions by magnetic force.

2.A switching valve according to Claim 1, characterized in that at least one permanent magnet is provided to generate the magnetic force.

3. A switching valve according to Claim 1, characterized in that at least one electromagnet is provided to generate the magnetic force.

4. A switching valve according to Claim 2 or 3, characterized in that the magnet is connected to the valve member.

5. A switching valve according to Claim 2 or 3, characterized in that the magnet is fixed to the housing.

27 6. A switching valve according to any one of Claims 2 to 5, characterized in that there is associated with the magnet at least one magnetizable part.

7. A switching valve as claimed in claim 6 in which the said magnetisable part is a disk.

8. A switching valve as claimed in claim 6 or 7 in which the magnetisable part is connected to the switching valve member.

9. A switching valve according to any one of Claims 2 to 8, characterized in that the permanent magnet is partially surrounded by a flux shield.

10. A switching valve according to any one of Claims 1 to 9, characterized in that the magnet bears in the said one end position against at least one spacer part.

11. A switching va.1ve as claimed in claim 10 in which the spacer part is of non-magnetic material.

12. A switching valve according to any one of Claims 1 to 11, characterized in that the magnetic force is overlaid by a spring force.

13. A switching valve as claimed in claim 12 in which the spring force assists the magnetic force.

28 14. A switching valve according to Claim 12 or 13 when dependent directly or indirectly on claim 6, characterized in that the magnet is acted upon by the spring force on the side remote from the magnetizable part.

15. A switching valve as claimed in claim 12, 13 or 14 in the spring force is provided by a pressure spring which is supported against an adjustment member.

16. A switching valve as claimed in claim 15 in which the adjustment member is an adjustment screw.

17. A switching valve according to claim 6 or any one of Claims 7 to 16 when dependent thereon, characterized in that in the said one end position attraction forces act between the magnetizable part and the magnet.

18. A switching valve according to claim 4 or any one of Claims 5 to 17 when dependent thereon characterized in that at least one magnet on the housing side is associated with the magnet on the piston side, and in that the two magnets face one another with their like poles.

19. A switching valve according to any one of Claims 1 to 18, characterized in that in the one end position the magnetic attraction force is assisted by a magnetic repelling force.

29 20. A switching valve as claimed in claim 19 when dependent directly or indirectly on claim 6 in which the magnet lies between the magnetizable part and a repelling magnet fixed to the housing.

21. A switching valve as claimed in claim 19 or 20 when dependent directly or indirectly on claim 15 in which the repelling magnet is fixed on the adjustment member.

22. A switching valve according to any one of Claims 1 to 21, characterized in that the switching valve member is displaceable in position-dependent manner.

23. A switching valve as claimed in claim 22 in which the valve member is displaceable by at least one sliding part.

24. A switching valve according to Claim 23 characterized in that the sliding part is a wedge which is movable transversly to the direction of displacement of the valve member.

25. A switching valve according to Claim 23 or 24, characterized in that an auxiliary member bears against the sliding part under force.

26. A switching valve as claimed in claim 25 in which the auxiliary member bears against the sliding part under the force of at least one pressure spring.

27 A switching valve as claimed in claim 26 in which the pressure spring is supported against the switching valve member.

28. A switching valve according to claim 26 or 27 characterized in that the auxiliary member, the pressure spring is supported against the switching valve member.

29. A switching valve substantially as herein described with reference to Figures 1 and 2, Figures 3 and 4, Figures 5 and 6, Figure 7, or Figures 8 and 9 of the accompanying drawings.