GB2231631A

GB2231631A - Embedded threaded pipe connectors, clutch master and slave cylinders

Info

Publication number: GB2231631A
Application number: GB9007200A
Authority: GB
Inventors: Juergen Schonlau; Ralf Harth; Wolfgang Ritter
Original assignee: Alfred Teves GmbH
Current assignee: Continental Teves AG and Co OHG
Priority date: 1989-04-03
Filing date: 1990-03-30
Publication date: 1990-11-21
Anticipated expiration: 2010-03-30
Also published as: FR2647519B1; GB2231631B; DE3910691C2; GB9007200D0; DE3910691A1; FR2647519A1

Description

J.bclionlau b4-JJ-i 1 t 1 1 FLUID ASSEMBLY The present invention relates

to a fluid assembly, and in particular, to a hydraulic assembly, made of plastic material.

in many technologies, and particularly in automotive vehicle technology, assemblies made of plastic material are being employed to a steadily increasing extent.

The invention has the general object of modifying the mechanical stresses of plastic material bodies incorporated in these assemblies by design provisions which are compatible with the plastic material. In this way, the forces affecting the plastic material body are to be absorbed and transmitted in a manner preserving the construction material.

is In particular, the present invention has the following groups of objects which preferably deal with assemblies making part of the automotive vehicle technology.

Firstly, in vessels of plastic material, especially in pressure vessels of plastic material which are filled with a hydraulic or a pneumatic fluid, such as, for example, housings for use in clutch master cylinders or in clutch slave cylinders, it is necessary to accommodate a threaded bushing of metal in the wall of the plastic material vessel for the connection of a pressure discharge or of a pressure supply line. However, plastics, and metal materials. have, invariably, different coefficients of thermal expansion. In use this can lead to excessive clearance between the metal X 1 2 bushing and the plastic material which can provide a leakage path for the hydraulic fluid.

The invention has the object of substantially alleviating this disadvantage and to create a reliable and tight connection between the threaded metal bushing and the plastic material vessel.

Furthermore, the present invention also seeks to create the preconditions which enable an 0-ring seal, which according to the state of the art was introduced into the mould for the plastic material vessel and embedded in the plastic material by injection moulding, to be omitted.

Secondly the present invention also seeks to provide an improved clutch slave cylinder of the type that is, for example, described on page 123 et seq. of the Brake Handbook, 8th re-edition, by Alfred Teves GmbH. The invention provides a slave cylinder of a plastics material which.is equipped with a low cost electrical and corrosionprotected clutch lining wear sensing device. A clutch slave cylinder according to this aspect of the invention is cheaper than cast iron cylinders and, furthermore, is of light-weight material.

Thirdly, the present invention also seeks to provide an improved clutch master cylinder of the type that is, for example, described in the aforementioned Brake Handbook which may be manufactured of plastic material and which may be secured to a vehicle more easily and at lower cost. In particular, rigid fixation of the master cylinder to the automotive vehicle by means of bolts, screws or equivalent rigid fasteners is to be rendered unnecessary.

Thus, the series assembly of the master cylinder, which is time consuming, may be improved as the complicated state-ofthe-art assembly operations can be simplified.

In known master cylinder assemblies mobility between the piston of the master cylinder and the push rod is required for functional reasons. In practice, it is expensive to realise this required mobility. It forms part of the objects of the present invention to create the 3 preconditions which permit the mobility required for reasons of functioning to be dispensed with.

Fourthly. the stop of a clutch pedal in the position of rest is, usually, not arranged at the pedal pedestal. As the pedal is linked to the piston of the master cylinder, the stop is normally positioned in the master cylinder itself. In very many cases, the stop of the piston of the master cylinder in the housing of the master cylinder also serves as the stop for all other components of the pedal mechanism which are moved in the course of the clutch operation.

When the clutch pedal is allowed to snap back to its rest position, that is, in the event of a sudden release of the clutch pedal, a considerable amount of kinetic energy is generated in the moving components. In addition, the hydraulic pressure in the actuating system acts on the p iston. These factors enhance the forces experienced by the stop member.

It will be appreciated that, if and when the piston is arrested abruptly in the housing of the clutch master cylinder and, in particular, at a stop provided for this purpose in the housing, damage may be caused to the housing. Moreover, the housing and the piston are often made of a duroplastic material which is a construction material having a relatively low impact resistance. The specific group of objects under review, therefore,, also seeks to avoid the described disadvantages. In addition, an improved, cheaper clutch master cylinder is to be created.

Furthermore. it belongs to this group of objects that an improved transmission of the forces exerted by the piston rod onto the piston is achieved in the body of the piston.

In addition, it is to be possible to dispense with the piston return spring if this is desired. Finallyr the amount of material required and the space required for fitting of the clutch master cylinder are to be reduced.

According to the invention, the objects are 4 attained in that components made of metal and or of a non metallic material other than plastic are disposed in the plastic material body or bodies of the assembly as functional elements.

In vessels made of plastic material, in particular in pressure vessels, for fluids, preferably for hydraulic fluid, it is proposed that a connecting element for a connecting line is envisaged whose material, in particular metal, has a coefficient of thermal expansion higher than that of the plastic material of the assembly, in particular of the vessel, surrounding the connecting element, that the connecting element is provided with a first and a second sealing surface, such that with a rise in temperature of the assembly, the plastic material body comes to be sealingly abutted against the first sealing surface, and with a fall in temperature of the assembly the plastic material body comes to be sealingly abutted against the second sealing surface.

A further development of this embodiment consists in that the connecting element is configurated to accommodate a ring- shaped part of the plastic material body.

In particular, the first and second sealing surfaces are of cylindrical shape.

In this context, it has proved especially appropriate that the first and second sealing surfaces of cylindrical shape are disposed coaxially with each other and arranged to face each other so as to accommodate therebetween the ring-shaped art of the plastic material body.

when applying the said embodiment to a pressure vessel of plastic material it may be envisaged that a threaded bushing for a connecting line is provided which is arranged at the periphery of the plastic material body and which is furnished with at least one, preferably with two coaxially disposed cylindrical projections extending into the plastic material body and presenting at least the first and second sealing surfaces.

The basic inventive thought may also be utilised in a particularly advantageous manner for the configuration of a clutch slave cylinder, and in particular. for a clutch slave cylinder with a clutch disc wear indicating device. It is proposed for this purpose that a reed contact is positioned in a base portion of the cylinder, that a magnet is positioned in an end portion of a piston for the cylinder, for activating the reed contact and enabling a visual or an acoustic indicator for indicating wear of a clutch disc.

Along the lines of a further development of this embodiment it can be envisaged that the cylinder comprises an inner surface composed of a first and a second part. the first part being configured as a guide member for the piston and comprising a surface having a relatively low coefficient of friction, and the second part being configured in the shape.of a pressure absorbing part and comprising a material suitable for the pressure load of the cylinder.

In the interest of protecting the piston, it is proposed that the piston is furnished with a metal element of plate-type shape for transmitting clutch actuation forces to a clutch actuation member.

Furthermore, the basic inventive thought may also be utilised in an advantageous manner to provide a clutch master cylinder. It can be envisaged that the master cylinder is arranged for suspension from two points which jointly constitute an axis of rotation for the master cylinder. The two points may form the ends of a preferably elastic bracket element for accommodating the master cylinder.

In view of its rotatability, the master cylinder is connected to a flexible hose line. The piston and the push rod of the master cylinder may rigidly be connected to each other.

A highly favourable solution with regard to manufacturing costs consists in that the master cylinder being preferably made of plastic material presents recesses 6 with undercuts for enabling snap fixing of the master cylinder to the bracket element.

It is proposed for the protection of the clutch master cylinder that at the pedal-side end in the bore of 5 the cylinder an energy cancelling element, preferably a compressible body, is positioned against which the piston strikes, in particular in the event of a sudden release of the clutch pedal and to which the piston transmits its kinetic energy.

The energy cancelling element may consist of a damping ring which is, in particular, made of rubber.

In order to achieve a conveyance of power into the piston of the clutch master cylinder which is compatible with the material of construction, it is proposed according to a special embodiment that a captivation of the piston rod in the piston is envisaged which is constituted by a metal element which preferably has the shape of a disc and which is embedded by injection moulding in the plastic material piston and makes catch behind the piston-side end of the piston rod having the form, in particular, of a ball end.

The following advantages are attained by the invention:

A reliable and tight connection is obtained between the threaded metal bushing and the plastic material vessel. The 0-ring which according to the state of the art was introduced into the mould for the plastic material vessel and which was embedded in the plastic material by injection moulding has become superfluous.

The slave cylinder is provided with a cheap electrical and corrosion-protected clutch lining wear sensing device.

The rigid fixation of the master cylinder to the automotive vehicle by means of screws or bolts is replaced by a very simple-design and cheap bracket structure.

Damages to the housing by allowing the clutch pedal to snap back are avoided. Nevertheless,, the extent of material required and the volume necessary for the assembly 7 of the clutch master cylinder are reduced.

In order that the invention may be clearly understood and readily carried into effect, some embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a cross sectional representation of a threaded bushing; Figure 2 shows a cross sectional representation of a clutch slave cylinder; and Figures 6 to 8 show details of a clutch master cylinder.

In Figure 1, a section of a wall of a vessel made of plastic material is shown. The vessel wall bears the reference numeral 1. A threaded bushing 2, having an internal thread 3 is embedded in the vessel wall 1, such as by injection moulding. The threaded bushing 2 is made of metal.and has a different coefficient of thermal expansion in respect of the plastic material surrounding it. The threaded bushing 2 is formed with axially extending projections 4 and 5.

The outer projection 4 presents a sealing surface 6. The inner projection 5 is furnished with a sealing surface 7 being positioned to face the sealing surface 6 of the outer projection 4. A connecting duct 8 for hydraulic fluid is accommodated in the inner projection 5.

A part 9 of the plastic material body 1 and having a ring-shaped configuration is positioned between the sealing surface 6 and the sealing surface 7.

As the assembly rises in temperature the ring- shaped part 9 of the plastic material will expand, the degree of expansion being dependant on the coefficient of thermal expansion of the plastic material. As a consequence, the plastic material is pressed against the sealing surface 6, in the direction of the arrows 10. In this way, pressure-tightness between the vessel 1 and bushing 2 will be safeguarded even in the event of heating of the assembly.

8 When the assembly cools down the plastic material ring 9 will contract, the degree of contraction being dependant on the coefficient of contraction of the plastics material. As a result, the plastic material is pressed against the sealing surface 7, in the direction of the arrows 11. In other words, the plastic material will contract like a ring around the projection 5 of the threaded bushing. In this way, pressure tightness will also be safeguarded in the vicinity of the sealing surface 7 in the event of the overall assembly cooling down.

It can be seen, therefore, that the shape of the threaded bushing has obviated the need for an 0-ring seal, which is customary in such assemblies.

It will be appreciated forthwith that by the universally applicable principle described here, many problems of sealing between metal elements, being embedded by injection moulding on one hand, and plastic material, on the other hand, which in prior assemblies were resolved by means of an 0-ring, can be resolved in a cost-saving manner with the aid of the described sealing surfaces.

Figure 2 shows a cross-sectional representation of a clutch slave cylinder of plastic material comprising a housing 12 including a cylinder part 13. and a guide part 14.

A piston 15 is located in the housing which upon operation of the clutch is moved to the right in the direction of the arrow 16, so as to actuate the clutch through a transmission member 17.

As wear of the clutch occurs, the piston 15 will, in its position of rest, be displaced to an increasing extent in the direction of the arrow 18. into the cylinder part 13. At an end 19 of the piston 15, a magnet 20 is embedded by injection moulding which. in the event of a wear of the clutch, approaches a reed contact 21 positioned in a bottom portion 22 of the cylinder part 13 of the clutch housing 1, as indicated by the arrow 18. The reed contact 21 is arranged to be triggered at an exactly defined 9 distance between the magnet 20 and the reed contact 21. Upon triggering, the reed contact is arranged to activate an electric contact which results in a visual or audible indication. So as to inform a driver that the wear of the 5 clutch discs has reached a critical stage.

The embodiment of the invention described above creates in an advantageous manner the conditions for the application of a reed contact. In state-of-the-art slave cylinders it is not possible to incorporate a reed contact because existing metal elements effectively shield the reed contact.

Advantageously, in the clutch slave cylinder according to Figure 2, a division of functions is realised. The guide part 14 assumes the function of guiding the piston 15 and is, for this reason, made of a material whose surface has good sliding properties, i.e. a low coefficient of friction.

on the other hand, the cylinder part 13 is made of a material which is particularly suited to withstand pressure loads arising from the build-up of pressure on the cylinder part 13.

A disc-shaped metal element 23 is arranged in the piston 15. When the piston moves in the direction of the arrow 16, that is to say, to the right, the piston will exert a force on the transmission member 17. An unprotected plastic material piston would be damaged or possibly destroyed by the forces exerted. The metal element 23 ensures that in the course of their transmission from the piston to the thrust member 17 the forces occurring are distributed in the plastic material body of the piston. By this provision, the risk of damage of the piston is considerably reduced and hence, the piston itself is capable of transmitting greater forces.

In contrast to state-of-the-art master cylinders.

the clutch master cylinder illustrated in Figures 3 to 5. which is made of plastic material, is arranged so as to be rotatable about an axis of rotation 24. Moreover,, the clutch master cylinder is not connected to a rigid pipe line but to a flexible hose line 25, as shown in Figure 3.

The axis of rotation is provided by bent ends 26,27 of a bracket 28, only the arms of the bracket being shown in part in Figures 3 to 5. The master cylinder is inserted into the bracket by way of two mutually opposed recesses 29, one of which only is visible in Figures 3 to 5.

Figure 5 is a partial cross-sectional representation along the line V-V in Figure 3, only one half of the sectional view being shown in Figure 5, in view of the symmetrical shape of the master cylinder.

It will be appreciated from Figures 3 to 5 that the recess 29 is formed with an undercut 30. The ends 26,27 of the bracket 28 are inserted into the recesses 29 and then pressed, or, rather, snapped into their positions shown in Figures 3 to 5, having overcome the elastically yielding projections of the undercut 30.

As already mentioned above, the master cylinder is rotatable, about the axis 24, the rotatability being indicated by the double arrow 31 in Figure 3.

Conventional master cylinders are rigidly fixed to the automotive vehicle by means of screws, bolts or similar fastenings. With modern assembly line manufacture, such rigid fixing is relatively time consuming and cumbersome because of the number of operations involved. Furthermore on account of the rigid coupling, a degree of mobility between the piston and the push rod of the clutch pedal is, in addition, required for functional reasons. The realisation of that required mobility also gives rise to increased manufacturing costs.

These disadvantages are alleviated by the rotatable mounting of the master cylinder. In addition, there is the easy and rapid insertion of the master cylinder onto its support bracket.

In lieu of the relatively expensive realisation of a degree of mobility between the piston and the push rod as is required in prior art assemblies, in the subject matter

11 of the embodiment in accordance with Figures 3 to 5, a rigid connection can be tolerated between a plastic material piston 32 and a push rod 51. The rigid connection can be achieved by a simple injection mould embedding of the push 5 rod in the piston.

Advantageously. the embodiment according to Figures 3 to 5 is distinguished in that as compared to the state of the art it represents a quick-fixing means for the clutch master cylinder. This results in considerable savings during assembly of the master cylinder.

The bracket 28 being described in Figures 3 to 5 may be a wire bracket which engages the plastic material cylinder housing 33. Alternatively, any other suspension device can be envisaged which is comprised of two suspension points for the master cylinder, the two points forming an axis of rotation for the master cylinder.

In Figure 6 a clutch master cylinder is shown whose housing 34 and piston 35 are made of plastic material. Reference numeral 36 denotes a piston rod, that is to say, the connection between the piston and the pedal mechanism of the clutch.

Usually, the stop of a clutch pedal in the position of rest is not in the range of the pedal pedestal. The clutch actuating pedal is captivated in many cases to the piston of the master cylinder. As a result, the stop of the piston simultaneously forms the stop for the pedal and for all components of the pedal mechanism which are moved together with the pedal. The stop of the piston is disposed within the housing of the master cylinder. Therefore, a considerable amount of kinetic energy needs to be cancelled at the stop of the master cylinder housing in the event that the clutch pedal is allowed to snap back to its rest position.

indeed. the energy to be absorbed not only relates to the kinetic energy of the moving piston but also to the kinetic energy of all moving components of the pedal and of the pedal mechanism. In addition, there is the hydraulic 12 pressure in the system which assists the movement of the piston.

This abrupt cancellation of a kinetic energy which, in total, is relatively high, can damage the housing of the master cylinder. Such damage can appear more readily if the piston and the housing are made of duroplastic material, that is to say, of a material having a relatively poor impact resistance.

The embodiment of Figure 6 seeks to alleviate the problems associated with relatively poor impact resistance materials. As can be seen from Figure 6 a rubber ring disc 37 is provided that absorbs and cancels, which means transforms into heat. the kinetic energy, that is to say, the impact energy.

If the clutch pedal is allowed to snap back to its rest position the rod 36 and the plastic material piston 35 will move in the direction of the arrow 38. The plastic material piston 35 will strike against a stop 39 of the plastic material housing 34. The above-described energy is cancelled by the rubber ring disc 37 by virtue of its damping effect.

The piston rod 36 is provided with a ball shaped end and Figure 7 shows how, in a state of the art device. a ball shaped end 40 of a piston rod is embedded in a plastic material piston by injection moulding.

It will be appreciated from Figure 7 that, when the piston rod 36 moves the piston to the right. as shown by the arrow 41, and, in particular, when the piston abuts against the stop 39, of the housing 34 as described in connection with Figure 6, a force being illustrated by the arrow 42 in Figure 6 will manifest itself at the ball-shaped end of the piston rod 36. As a result of the shaping of the ball shaped end 40 and the piston 35 in the region 43, a comparatively elevated resultant force will be conveyed into the plastic material body of the piston 35. This force is illustrated by the arrow 44. It will be appreciated that a very high load will act on the piston 35 in the region 43 I- 13 which can cause damage to the piston in that region.

As can be seen from the embodiment shown in Figures 6 and 8, a metal ring 45 is embedded in the body of the piston 35 by injection moulding. The force 42 which is illustrated in Figure 7 can now be transmitted by the ballshaped head 46 of the push rod 36,, in particular to the metal ring 45. as indicated by the arrows 47,48. Prom the metal ring the forces will be transmitted into the piston body 35,, the forces manifesting themselves in the piston being illustrated in Figure 8 by the arrows 49,50.

As the metal ring 45 absorbs the relatively large radially directed forces which arise as a result of the ball shape, only compressive forces will, advantageously, continue to act on the plastic material piston which the plastic material is more easily capable of withstanding.

In view of the improved connection between the piston, on one side, and the piston rod, on the other side, it is possible to omit the piston return spring, since the return of the pedal is brought about by the hydraulic pressure in the master cylinder. The omission of the piston return spring enables the overall length of the master cylinder to be reduced. Moreover, less material is required, which leads to a manufacture at a correspondingly lower cost. Finally, the assembly space required is reduced.

A relatively expensive protecting cap as is needed in prior art assemblies also becomes superfluous thanks to the incorporation of the metal ring 45.

Although the present invention has been described with respect to specific embodiments, modifications may be effected whilst remaining within the scope of the invention.

For example, the arrangement for sealing between the vessel 1 and the threaded bushing 2 shown in Figure 1 may equally be incorporated in the slave cylinder of Figure 2 and or the master cylinder arrangements illustrated in Figures 3 to 5 or Figures 6 and 8.

Additionally. the rubber ring disc 37, illustrated 14 in Figure 6, may be used alone or in conjunction with the metal ring 45. Furthermore, the rubber ring disc may also be incorporated into the master cylinder arrangement shown in Figures 3 to 5.

:Q i t a

Claims

CLAIMS:

1. A fluid assembly comprising a body of plastic material, characterised in that components made of metal and or of a non metallic material other than plastic are disposed in the plastic material body of the assembly as functional elements.

2. An assembly as claimed in claim 1 characterised in that a functional element comprises a connecting element for a connecting line. the connecting element comprising a material having a coefficient of thermal expansion higher than that of the plastic material and further comprising a first and a second sealing surface such that, with a rise in temperature of the assembly, the plastic material body comes to be sealingly abutted against the first sealing surface, and with a fall in temperature of the assembly, the plastic material body comes to be sealingly abutted against the second sealing surface.

3. An assembly as claimed in claim 2 characterised in that the connecting element is configured to accommodate a ring-shaped part of the plastic material body.

4. An assembly as claimed in claim 2 or claim 3 characterised in that the first and second sealing surfaces are of cylindrical shape.

5. An assembly as claimed in claim 4 characterised in that the first and second sealing surfaces of cylindrical shape are disposed coaxially with each other and arranged to face each other so as to accommodate therebetween the ringshaped part of the plastic material body.

6. An assembly according to any one of claims 2 to 5 characterised in that the connecting element comprises a threaded bushing for a connecting line, arranged at a periphery of the plastic material body, including two coaxially disposed cylindrical projections extending into the plastic material body and presenting at least the first and second sealing surfaces.

7. An assembly as claimed in any one of the preceding claims characterised in that the plastic material 1)k 16 body comprises a clutch slave cylinder.

8. An assembly as claimed in claim 7 characterised by a reed contact positioned in a base portion of the cylinder and by a magnet, positioned in an end portion of a piston for the cylinder, for activating the reed contact and enabling a visual or an acoustic indicator for indicating wear of a clutch disc.

9. An assembly as claimed in claim 8 characterised in that the cylinder comprises an inner surface composed of a first and a second part, the first part being configured as a guide member for the piston and comprising a surface having a relatively low coefficient of friction, and the second part being configured in the shape of a pressure absorbing part and comprising a material suitable for the pressure load of the cylinder.

10. An assembly as claimed in claim 8 or claim 9 characterised in that the piston is furnished with a metal element of plate-type shape for transmitting clu.tch actuation forces to a clutch actuation member.

11. An assembly as claimed in any one of claims 1 to 6 characterised in that the plastic material body compr.ises a clutch master cylinder.

12. An assembly as claimed in claim 11 characterised in that the clutch master cylinder is arranged for suspension from two points which jointly constitute an axis of rotation for the master cylinder.

13. An assembly as claimed in claim 12 characterised in that the two points comprise the ends of a bracket element of elastic material for accommodating the master cylinder.

14. An assembly as claimed in claim 13 characterised in that the master cylinder comprises recesses with undercuts for enabling snap fixing of the master cylinder to the bracket element.

15. An assembly as claimed in any one of claims 11 to 14 characterised in that the master cylinder is connected to a flexible hose line.

17

16. An assembly as claimed in any one of claims 11 to 15 characterised in that the master cylinder comprises a piston rigidly connected to a push rod.

17. An assembly as claimed in claim 16 characterised in that the push rod comprises an end in the shape of a ball, and in that the piston comprises a moulding of plastic material having a socket for accommodating the ball shape end of the push rod, the piston further comprising a metal element embedded therein during moulding and arranged between the ball shaped end of the push rod and an end of the piston for enhancing the retention of the push rod end in the piston.

18. An assembly as claimed in claim 17 characterised in that the metal element comprises a metal disc.

19. An assembly as claimed in any one of claims 11 to 18 characterised by an energy cancelling element arranged within the cylinder body for absorbing kinetic energy caused by a sudden release of a clutch pedal coupled to the master cylinder.

20. An assembly as claimed in claim 19 when appendant to any one of claims 16 to 18, characterised in that the energy cancelling element is arranged at an end of a bore in the cylinder body for accommodating the piston and positioned to act as a stop member for the piston.

21. An assembly as claimed in claim 19 or claim 20y characterised in that the energy cancelling element comprises a damping-ring disc.

22. An assembly as claimed in any one of claims 19 to 21. characterised in that the energy cancelling element comprises a rubber material.

k. a 4.

18

23. An assembly substantially as hereinbefore described with reference to Figures 1 to 6 and Figure 8 of the accompanying drawings.

24. An assembly substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

25. An assembly substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

26. An assembly substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

27. An assembly substantially as hereinbefore described with reference to Figures 3r4 and 5 of the accompanying drawings.

28. An assembly substantially as hereinbefore described with reference to Figures 1,3,4 and 5 of the accompanying drawings.

29. An assembly substantially as hereinbefore described with reference to Figure 6 of the accompanying drawings.

30. An assembly substantially as hereinbefore described with reference to Figure 8 of the accompanying drawings.

31. An assembly substantially ashereinbefore described with reference to Figures 1 and 6 of the accompanying drawings.

32. An assembly substantially as hereinbefore described with reference to Figures 1 and 8 of the accompanying drawings.

33. An assembly substantially as hereinbefore described with reference to Figures 1,6 and 8 of the accompanying drawings.

Published 1990 atThe Patent Office. State Rouse. 6671 High Holborn. LondonWC1R4TP. Further copies maybe obtainedfrom The Patent office. Sales Branch. St Mary Cray, OrpIngton, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con. 1187