WO1995021087A1

WO1995021087A1 - Improvements in or relating to power steering gears

Info

Publication number: WO1995021087A1
Application number: PCT/AU1995/000036
Authority: WO
Inventors: John Baxter; Kelvin John Tiedman
Original assignee: Ae Bishop & Associates Pty Limited
Priority date: 1994-02-03
Filing date: 1995-01-25
Publication date: 1995-08-10
Also published as: AUPM368594A0

Abstract

A power steering gear comprising a housing (3) having a region subjected to hydraulic pressure when in use, the region forming at least a part of the valve portion (7) and/or the cylinder portion (16) of the steering gear. The region comprises a plastic moulding reinforced by a substantially tubular reinforcement element (46, 47), with said plastic moulding fully encapsulating said reinforcement element (46, 47). The reinforcement element (46, 47) is preferably perforated to enable keying between itself and the plastic, and is provided with protrusions (48, 49) to assist with locating and holding of the reinforcement element (46, 47) during formation of said plastic moulding.

Description

IMPROVEMENTS IN OR RELATING TO POWER STEERING GEΞARS

TECHNICAL FIELD

This invention relates to steering gears and more particularly to steering gear housings for hydraulically power assisted steering of vehicles. Whilst, for purposes of clarity, the present invention is described in reference to rack and pinion power steering gears, it will be appreciated by those skilled in the art of power steering technology that embodiments of the present invention are also applicable to other categories of power steering gears, for example "integral" power steering gears.

BACKGROUND

A power rack and pinion steering gear typically consists of a metal housing which encloses rack and valve assemblies.

The rack assembly incorporates a rack bar, with a plurality of teeth, in the form of a rack, at one end. This rack meshes with the pinion of the valve assembly. The other end of the rack bar, termed the "rack rod", usually has a plain diameter and a piston attached thereto. In a conventional power rack and pinion steering gear, tie rods extend from either axial extremity of the rack bar and are connected to the steerable front wheels of the vehicle.

There are many different types of hydraulic valve formats used in automotive valve assemblies, the two main types being rotary valves and reaction valves. A rotary valve assembly will be assumed for the purposes of explanation in the present specification, since the housing requirements are similar in both cases. The typical rotary valve assembly includes an input-shaft, sleeve, torsion bar and pinion. The input-shaft is usually connected to the steering wheel of the vehicle by a flexible joint, and has in its outer periphery a plurality of blind ended, axially extending grooves separated by lands. Journalled on the input-shaft is a sleeve having in its bore an array of axially extending slots matching the grooves in the input-shaft, but in underlap relationship thereto, the slots of the one being wider than the lands of the other so defining a set of axially extending orifices which open and close when relative rotation occurs between the input-shaft and the sleeve. These orifices are ported as a network such that they form sets of hydraulic Wheatstone bridges which act in parallel.

Passages in the input-shaft and sleeve, together with circumferential ly extending annular ports in the periphery of the sleeve, serve to communicate oil between the grooves in the input-shaft and the slots in the sleeve, a high pressure oil supply, and right-hand and left-hand hydraulic assist cylinder chambers incorporated in the steering gear housing on either side of the rack piston.

The sleeve is rotationally connected to the adjacent pinion, while a torsion bar is disposed within the centre of the input-shaft and is rotationally connected to the pinion at one end and to the remote end of the input-shaft at the other end. The torsion bar therefore serves to urge the input-shaft and sleeve towards a neutral, centred condition when no power assistance is required. When input torque is applied by the driver to the steering wheel the torsion bar deflects, causing a relative rotation of the sleeve and input-shaft from the neutral condition, and thereby imbalancing the sets of hydraulic Wheatstone bridges. This causes a differential pressure to be developed between the right-hand and left-hand hydraulic assist cylinder chambers and therefore an assist force to be generated at the rack piston.

The net output force imparted by the steering gear is therefore the sum of the power and manual components acting on the rack assembly, namely the hydraulically generated force acting on the piston and the mechanical force generated at the rack due to the input torque imparted to the meshing pinion by the torsion bar. The general method of operation of such conventional rotary valves is well known in the art of power steering design and so will not be described in any greater detail in this specification. An excellent description of this operation is contained in US Patent 3,022,772 (Zeigler), commonly held as being one of the "original" patents disclosing the rotary valve concept.

The valve assembly and the rack assembly are located within the housing in a defined geometric relationship so as to maintain correct meshing of the rack and pinion teeth through the full axial movement of the rack bar.

The valve assembly is usually supported in bearings in the housing, one on either side of the pinion and one on the input-shaft adjacent the connection for the flexible joint. These bearings allow rotational motion, and also provide axial location, of the assembly within the housing. The pinion engages with the rack teeth where its rotational motion is converted to axial motion of the rack assembly.

The rack assembly is located at one end by a rack pad and the pinion, the rack pad arranged to urge the rack towards the pinion and thereby maintain the correct meshing geometry. The rack rod portion of the rack assembly, remote from the rack teeth, is journalled in the housing by a rack bush.

As well as locating the rack and valve assemblies in the correct relative orientation, and permitting the required degrees of motion of these components, the housing must withstand the hydraulic pressure generated by the valve assembly and imparted to the cylinder chambers. These functional portions of the housing subject to hydraulic pressure, henceforth termed the "valve portion" and the "cylinder portion", together with the remaining "mechanical portion" of the housing, will now be described. The valve portion of the housing encloses the valve and can be considered to be axially terminated at one end by a pinion seal adjacent the inner pinion journal and at the other end by an input seal, the latter allowing rotational inputs to be imparted to the steering gear via the input-shaft. The chamber so formed surrounding the valve is always oil filled, however four axially disposed circumferential ring seals surrounding the outside diameter of the sleeve serve to divide this chamber into five sub- chambers. The axially remote first and fifth sub-chambers are normally hydraulically communicated via porting in the input-shaft and provide a return oil flow path back to the pump via a radially disposed return port in the wall of the valve portion of the housing. The remaining second, third and fourth sub-chambers are normally communicated to the left-hand cylinder chamber, the feed from the pump, and the right-hand cylinder chamber respectively (or vice versa) via radially disposed cylinder and feed ports in the wall of the valve portion of the housing. Maximum pressures are normally generated by the valve during vehicle parking manoeuvres when, typically, pressures up to 10 MPa may be applied to the second and third sub-chambers or the third and fourth sub-chambers depending on whether a clockwise or counterclockwise input torque is applied to the input-shaft. The pressure in these annular sub- chambers acts directly on the wall of the valve portion of the housing.

The cylinder portion of the housing encloses the rack rod and piston, and can be considered to be axially terminated at each end by rod seals mounted in the housing, the outer rod seal normally being incorporated into the earlier mentioned rack bush. A circumferential ring seal on the outside diameter of the piston serves to divide the cylinder into left-hand and right-hand cylinder chambers. External hydraulic tubes serve to communicate hydraulic fluid between left-hand and right-hand ports in the wall of the cylinder portion of the housing and the appropriate ports in the valve portion of the housing. The above mentioned maximum parking pressure is either applied to the left-hand or right-hand cylinder chamber, depending on whether a clockwise or counterclockwise input-torque is applied to the input-shaft, and therefore acts directly on the wall of the cylinder portion of the housing.

The four ports in the valve portion and the two ports in the cylinder portion of the housing usually have profiled seats or external protrusions to locate the formed ends of the respective hydraulic tubes, these ends usually fitted with elastomeric seals, such as O rings, to achieve sealing. Another less common method of hydraulic tube attachment is 'banjo' fittings. However all these methods of attachment require accurately machined surfaces and/or threads on the housing in the region of the ports in order that satisfactory sealing occurs over the life of the steering gear.

The mechanical portion of the housing encloses the pinion, the rack end of the rack bar and rack pad. This area usually contains no pressurised hydraulic fluid, however does contain grease or oil to lubricate the meshing pinion and rack teeth, and also lubricate the sliding action of the rack on the rack pad.

The walls of the cylinder and valve portions of the housing must be capable of withstanding the large radial forces exerted outwardly on their respective bores due to the hydrostatic pressure acting in the adjacent annular chambers during valve pressurisation. As stated earlier, this pressure can typically reach 10 MPa during parking for a standard power steering gear. For certain less common classes of power steering gears incorporating a "high pressure" hydraulic system, this pressure can be as high as 14 MPa. Such higher pressures will be more common in the future as the trend continues towards power steering systems with minimum energy loss, and hence least negative impact on vehicle fuel consumption. The housing and related components must also maintain their dimensional integrity not only over a large operating pressure range, but also over large fluctuations in operating temperature as great as -40°C to 135°C in some climates. For this reason metals such as cast iron, steel and aluminium alloys have been the favoured material for such housings. Cast iron and steel are relatively low cost materials and have been found to provide the necessary strength and stiffness to resist the hydraulically and mechanically induced forces earlier referred to. In recent times aluminium alloys, even though relatively more expensive compared to cast iron and steel, have become more popular due to the weight reduction demands in the industry.

As expected there are many different housing configurations to suit different vehicles and operating requirements. Consequently typical practice varies from the use of one-piece housings where all three functional areas are catered for in a single . machined casting, through two-piece housings where either the valve and mechanical portions or the mechanical and cylinder portions are combined, to three-piece housings in which all three functional portions are housed within separately produced components. For the two-and three-piece housings, the separate components are assembled using a variety of joining methods. Some of these jointing methods allow repeated assembly and disassembly such as a bolted flange arrangement between the valve and mechanical portions. Other permanent and semi permanent joining techniques can also be used including interference fits, interference fits with subsequent cold swaging, interference fits with radially disposed pins, cast (or moulded) in situ components and many more.

Substituting plastics for metals in automotive components is one way of achieving a reduction in total weight. Also, generally speaking, manufacturing costs can also be reduced as many machining operations can be eliminated and reliance placed on the accuracy imparted by the as-moulded component. However while industrial plastics have progressed in recent years as structural materials in terms of ultimate tensile strength, the stiffness of even the most "exotic" fibre-filled plastics does not approach that of aluminium, let alone cast-iron or steel. Whilst proposals have been disclosed for the use of all-plastic housings for manual rack and pinion steering gears, as shown in US Patent 4,008,627 (Bradshaw et al.), this approach can be shown as not suitable for power steering gears as the portions which contain pressurised hydraulic fluid are far too flexible to offer satisfactory dimensional control. This excessive flexibility is exacerbated at elevated temperatures since the Modulus of Elasticity for most plastics degrades significantly with increase in temperature (unlike most metals). The stiffness of fibre-filled plastics is additionally very sensitive to the orientation of the fibres which in turn relates to the direction of the plastic flow in the die during moulding. For tubular elements such as the valve or cylinder portions of the housing, this plastic flow direction will inevitably be orientated axially and hence so will be the general orientation of the fibres. Whilst maximising the axial stiffness of such tubular elements this direction of fibre orientation minimises the stiffness of the tubular plastic elements circumferentially, reducing hoop stiffness and hence the diametral stiffness of these housing portions under the action of internal hydrostatic pressure.

The prior art which is most closely related to that of the present invention is as disclosed in US Patent 4,809,806. (Pietrzak et al.) which shows a power rack and pinion gear with a housing comprising a shell of engineering plastic forming the outer surface of the mechanical portion, with a steel cylinder portion permanently attached during the plastic moulding operation. Additionally an extruded & machined metal sleeve constituting the valve portion, is later inserted into a moulded bore in the mechanical portion and retained by a locknut.

However such an arrangement still requires machining of the various metallic surfaces in the valve and cylinder portions of the housing, including the accurate bores and porting. Also substantial amounts of metal is still used in the construction of this prior art housing with attendant penalties in terms of weight. The essence of the present invention is the provision of a reinforced plastic housing in which internal and external features can be accurately moulded, whilst eliminating the exposure of any plastic/metal interface to pressurised hydraulic fluid.

DISCLOSURE OF INVENTION

In one aspect the present invention is a power steering gear comprising a housing having a region subjected to hydraulic pressure when in use, characterised in that said region comprises a plastic moulding reinforced by a substantially tubular reinforcement element, the plastic moulding fully encapsulating said element where said region is subjected to hydraulic pressure.

It is preferred that the said region of the housing forms at least a part of the valve portion partially surrounding the valve assembly and/or, in the case of a rack and pinion steering gear, the cylinder portion partially surrounding the rack assembly.

In another aspect of the present invention is a rack and pinion power steering gear comprising a housing having a valve portion, a mechanical portion and a cylinder portion, said valve portion having a valve assembly substantially housed therein, said mechanical portion having a rack of a rack assembly substantially housed therein and said cylinder portion having a rack rod and piston of said rack assembly substantially housed therein, characterised in that the valve portion and/or cylinder portion of said housing comprises a plastic moulding reinforced by a tubular reinforcement element, said reinforcement element being fully encapsulated by plastic in the region of the valve portion and/or cylinder portion subjected to hydraulic pressure when in use.

In a first preferred embodiment for all aspects of the invention the tubular reinforcement element is of a metal such as steel or an aluminium alloy. However other high stiffness materials such as carbon fibre can also be used. In a second preferred embodiment for all aspects of the invention the ports formed in the regions or portions subjected to hydraulic pressure when in use, such as in a valve portion or a cylinder portion are formed as plain (unthreaded) circular bores with seats at their open ends. The ports are adapted to connect with hydraulic tubes, such as for interconnection of the valve portion to an external hydraulic pump and, in the case of rack and pinion power steering gears, for interconnection of the valve portion and cylinder portion. Each port is adapted to receive a free end of a hydraulic tube. The tube is provided with a flange near its free end which, when connected to its respective port, abuts the seat about the open end of the port. The port/tube interconnection is sealed by an elastomeric seal such as an O-ring which surrounds the tube beneath the tube flange and seals against the inner portion of the seat. The tube flange is seated by means of a retaining bolt and bracket. The bracket may be in the form of a bridging piece holding a pair of tubes within a pair of respective ports situated closely together, as is typical in the valve portion of the power steering gear, the retaining bolt being threadedly connected to a nut. The nut or head of the retaining bolt can be seated in a tunnel or cavity formed in the outer surface of the housing and accessible therefrom. Alternatively, the retaining bolt, nut and bracket can be replaced by a spring clip.

In a third preferred embodiment for all aspects of the invention the plastic moulding is of an engineering plastic.

In a fourth embodiment of all aspects of the invention the tubular reinforcement element is preferrably cold formed from a length of cylindrical tube or cold drawn from a disk of sheet steel. Alternatively, the reinforcement element may be constructed from cold rolled metal plate and seam welded or keyed to form a tubular shape. In further not shown embodiments the reinforcing element may be manufactured by other known methods, such as die casting or laminated from carbon fibre.

In all embodiments of the present invention it is preferable that the reinforcing element be provided with a plurality of holes (perforations) to allow for keying between the element and plastic during manufacture of the housing.

It is also further preferred that the reinforcement element be provided with at least one flanged protrusion in order to assist with locating and holding of the reinforcement element during the plastic moulding operation. Alternatively, the flanged protrusions could be replaced by threaded studs welded to the reinforcement element, which in addition to being of use for locating and holding of the element during the plastic moulding operation, can be used in lieu of the retaining bolt for holding the bracket or bridging piece as discussed in the second preferred embodiment.

In preferred embodiments of all aspects of the invention the housing may be of a one, two or three-piece construction, the individual pieces being separately manufactured.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will now be described by way of example with reference to the accompanying drawings, in which:

Fig. 1 is a sectional view of a rack and pinion power steering gear having a one-piece reinforced moulded plastic housing according to the present invention;

Fig. 2 is a sectional view of a rack and pinion power steering gear, according to another embodiment of the present invention, having a two-piece housing incorporating integral reinforced moulded plastic mechanical and valve portions and a separate pre-manufactured metal cylinder portion;

Fig. 3 is a sectional view of a rack and pinion power steering gear, according to still another embodiment of the present invention, having a two-piece housing incorporating integral reinforced moulded plastic mechanical and cylinder portions and a separate reinforced moulded plastic valve portion;

Fig. 4 is an enlarged sectional view of a valve portion of a housing similar but not identical to that shown in Fig. 3 with the hydraulic tubes retained thereto;

Fig. 5 is an isometric view of the metallic tubular reinforcement element used to reinforce the valve portion of the housing shown in Fig. 4;

Fig. 6 is an enlarged sectional view similar to Fig. 4, but with a different configuration of the metallic tubular reinforcement element;

Fig. 7 is an enlarged sectional view similar to Fig. 4, but with a different method of retaining the hydraulic tubes;

Fig. 8 is an enlarged sectional view similar to Figs. 4 and 6, but with still another different configuration of the metallic tubular reinforcement element; and

Fig. 9 is an isometric view of the metallic tubular reinforcement element used to reinforce the valve portion of the housing shown in Fig. 8. MODE FOR CARRYING OUT INVENTION

Fig. 1 shows a power rack and pinion steering gear comprising valve assembly 1 and rack assembly 2 located in the correct geometrical relationship by housing 3.

Valve assembly 1 is rotationally supported and axially located by upper pinion bearing 4, lower pinion bearing 5 and input bearing 6. Valve portion 7 of housing 3 is axially terminated at either end by pinion seal 8 and input seal 9 to create valve chamber 10.

Rack assembly 2 is supported in housing 3 by rack bush 12 at rack rod end 13 and by the combination of pinion 14 and the rack pad (not shown) at rack end 15. Cylinder portion 16 of housing 3 is axially terminated by rod seals 17 and 18 and contains cylinder 19 radially lying between the outside diameter of rack rod 13 and cylinder bore 20. Piston 21 , fixed to rack rod 13 and sliding sealingly in bore 20, divides cylinder 19 into left and right hand cylinder chambers 22 and 23. Left and right hand cylinder ports 24 and 25 in cylinder portion 16 enable communication of hydraulic fluid from appropriate cylinder ports 26 and 27 in valve portion 7, via cylinder tubes 28 and 29 respectively.

During operation of the power rack and pinion steering gear, hydraulic fluid supplied from pump 90 enters valve assembly 1 via feed port 30 in valve portion 7, and is distributed to or from left and right hand cylinder chambers 22 and 23 via cylinder ports 26 and 27 depending on the relative angular rotation of input-shaft 33 and sleeve 34 of valve assembly 1. Hydraulic fluid from valve assembly 1 is returned to pump reservoir 91 via return port 35 in valve portion 7.

As explained earlier, although valve chamber 10 is always oil filled, four axially disposed circumferential ring seals 36-39 surrounding the outside diameter of sleeve 34 serve to divide this chamber into five sub-chambers 40-44. As can be seen, axially remote sub-chambers 40 and 44 mutually communicate to return port 35, sub- chambers 41 and 43 communicate to left and right hand cylinder ports 26 and 27 and central sub-chamber 42 communicates to feed port 30.

During operating of the rotary valve, and particularly during low speed and parking manoeuvres, large hydrostatic pressures exist in either sub-chambers 41 and 42 or sub-chambers 42 and 43 depending on the direction of input torque application to input-shaft 33. The pressure in these annular sub-chambers acts directly on the adjacent bore 45 of valve portion 7. Similarly these same large hydrostatic pressures are communicated to either left hand cylinder chamber 22 or right hand cylinder chamber 23, and act on adjacent cylinder bore 20.

Housing 3, in the form shown in Fig. 1 , is moulded to net shape from a filled engineering plastic such as a liquid crystal polymer marketed under the trade mark VECTRA or PPS (Poly Phenylene Sulfide) marketed under the trade mark FORTRON both available from Hoechst Celanese. Valve portion 7 and cylinder portion 16 are strengthened by tubular metallic reinforcement elements 46 and 47 respectively. Valve portion reinforcement element 46 and cylinder portion reinforcement element 47 are located and clamped in the moulding die by their respective protrusions 48 and 49. These protrusions locate the reinforcement elements both axially and radially so as to maintain the required thickness of plastic between the moulded surfaces and the reinforcement elements. Full encapsulation of the reinforcement element in the region of the plastic moulding subject to high pressure hydraulic fluid is essential to the operation of a power steering housing according to the present invention. By "encapsulation" it is meant that the reinforcing element is closed in or surrounded by the plastic material, for example where ports 24 and 25 pass through cylinder portion reinforcement element 47 and where ports 26, 27, 30 and 35 pass through valve portion reinforcement element 46. Full encapsulation is essential as any plastic metal interface exposed to the high pressure hydraulic fluid will allow the penetration of the fluid along the interface. In the above example, a lack of full encapsulation would allow hydraulic fluid to eventually externally leak from the plastic/metal interface adjacent to protrusions 48 and 49.

The embodiment of the power rack and pinion steering gear shown in Fig. 2 has identical internal components, the only difference being that the housing 3 is of two- piece construction. Valve portion 7 and mechanical portion 50 are identical to those shown in Fig. 1 with cylinder portion 16 being constructed as a pre-manufactured steel cylinder tube 51 with cylinder ports 24 and 25 contained in external cylinder tube connections 52 and 53 attached by welding. The connection between steel cylinder tube 51 and mechanical portion 50 is achieved by locating the steel cylinder tube in the moulding die during the plastic injection moulding process. The plastic fills the die to the cavity shape surrounding the tube and also flows through the radially positioned rows of holes 54, the latter providing a keying action.

In the embodiment shown in Fig. 3, the power rack and pinion steering gear has a housing 3 with an integral mechanical portion 50 and cylinder portion 16, these being identical to those in Fig. 1. However valve portion 7 is a separate component which is attached to mechanical portion 50 using a flange connection which is clamped by two bolts 55 and 56. Valve portion 7 is strengthened with tubular metallic reinforcement element 57. Reinforcement element 57 is held by its protrusions 58 in the moulding die during moulding. Protrusions 58 serve to position reinforcement element 57 relative to valve portion 7 both axially and radially, and also provide a hard metallic seat for bolts 55 and 56 when tightened during securing of valve portion 7 to mechanical portion 50.

Several embodiments of the valve portion are shown in Figs. 4-9 showing possible methods of securing the hydraulic tubes. Similar methods could of course be used to secure the cylinder tubes to the cylinder portion. The embodiment shown in Fig. 4 is similar to that shown in Fig. 3 except protrusions 59 of reinforcement element 60 extend to the opposite side of the flange connection, providing a more accurate metal datum between valve portion 7 and mechanical portion 50. The free ends 81 and 82 of the feed tube 61 and return tube 62, are respectively fitted into the feed port 30 and return port 35 of the valve portion 7. Ports 30 and 35 are plain circular bores (unthreaded) formed in valve portion 7 during the plastic moulding operation. Ports 30 and 35 have seats 84 at their open ends and are adapted to each accommodate O-rings 83, which seal the connections between tubes 61 and 62 and ports 30 and 35, with flanges 70 of each tube abutted against seats 84 at the respective port openings. O-rings 83 are positioned within the ports 30 and 35, against the inner portions of seats 84, and beneath flanges 70 of the respective tubes 61 and 62. Feed tube 61 and return tube 62 ,are retained in valve portion 7 by bridge piece 63 which is loaded onto tube flanges 70 by retaining bolt 64 and nut 65. The cylinder tubes 28 and 29 are similarly connected and retained in ports 26 and 27 on valve portion 7 by bridge piece 66 which is loaded onto tube flanges 70 by retaining bolt 67 and nut 68.

Cylinder tubes 28 and 29 are also connected to cylinder portion 16 in a like manner, as shown in Figs. 1 and 3. Flanges 71 of cylinder tubes 28 and 29 are held in place by bracket 85, retaining bolt 86 and nut 87, instead of the bridge piece arrangement used on the valve portion port connections. This aspect of the invention eliminates the need for machining threads into the ports located in the housing as is typically required in the prior art.

Fig. 5 shows an isometric view of reinforcement element 60 which is preferably cold formed from a short length of cylindrical steel tube or cold drawn from a disk of sheet steel. If steel is used for the manufacture of reinforcement element 60, a radial thickness of 1-2 mm has been found to provide adequate stiffness to valve portion 7 (or indeed cylinder portion 16). Of course other metals such as aluminium could also be employed depending on weight requirements and metal forming complexities. Additional perforated holes 69, not necessarily associated with the feed, return or cylinder ports, may be incorporated in reinforcement element 60 to provide keying between the plastic and the reinforcement element.

The embodiment shown in Fig. 6 is similar to that shown in Fig. 4 except that reinforcement element 60 is shortened so that protrusions 59 are at the top face of the flange. As stated in reference to Fig. 3, this arrangement may be preferred depending on the properties of the plastic used to manufacture the valve portion, since it provides a hard metallic seat for the mounting bolts.

The embodiment shown in Fig. 7 is identical to that shown in Fig. 4 except that retaining bolt 64 and nut 65 (securing bridge piece 63), and retaining bolt 67 and nut 68 (securing bridge piece 66), are respectively replaced by spring clips 88 and 89. This arrangement provides quick assembly of the hydraulic tubes to the valve portion.

The embodiment shown in Figs. 8 and 9 incorporates a reinforcement element 74 (which differs to that earlier shown in Fig. 4) having threaded studs 72 and 73 welded thereto. When moulded into the valve portion 7 of the housing, the threaded studs 72 and 73 of element 74 provide anchorage means to which bridge pieces 63 and 66 may be secured by means of nuts 75 and 76 to retain the hydraulic tubes 61 , 62, 28 and 29 in connection with ports 30, 35, 26 and 27 respectively (similar to those shown in Fig. 4). Studs 72 and 73 also act as a means of accurately locating reinforcement element 74 in the die during moulding and hence the earlier referred to flange protrusions are no longer necessary, at least from this view point. In this case, reinforcement element 74 could be manufactured by cold rolling a rectangular metal plate and joined by seam welding or by a tongue and key method. The latter style of keying 77 is shown in Fig. 9. It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without parting from the spirit or scope of the invention as broadly described. Present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

1. A power steering gear comprising a housing having a region subjected to hydraulic pressure when in use, characterised in that said region comprises a plastic moulding reinforced by a substantially tubular reinforcement element, said plastic moulding fully encapsulating said element where said region is subjected to hydraulic pressure.

2. A power steering gear as claimed in claim 1 wherein said region of the housing forms at least a part of a valve portion partially surrounding a valve assembly.

A power steering gear as claimed in claim 2 wherein at least one port is formed in said valve portion as a circular bore.

A power steering gear as claimed in claim 3 wherein said port is provided with a seat at its open end.

5. A power steering gear as claimed in claim 1 wherein said power steering gear is a rack and pinion power steering gear and said region of the housing forms at least a part of a cylinder portion partially surrounding a rack assembly.

6. A power steering gear as claimed in claim 5 wherein at least one port is formed in said cylinder portion as a circular bore.

7. A power steering gear as claimed in claim 6 wherein said port is provided with a seat at its open end.

8. A power steering gear as claimed in claim 1 wherein said tubular reinforcement element is of metal such as steel or an aluminium alloy.

9. A power steering gear as claimed in claim 8 wherein said tubular reinforcement element is cold formed from a length of cylindrical tube or cold drawn from a disk of sheet metal.

10. A power steering gear as claimed in claim 8 wherein said tubular reinforcement element is constructed from cold rolled metal plate and seam welded or keyed to form its tubular shape.

11. A power steering gear as claimed in claim 1 wherein said tubular reinforcement element is of a non-metal material having high stiffness properties.

12. A power steering gear as claimed in claim 1 wherein the tubular reinforcement element is provided with at least one protrusion to assist with locating and holding of said tubular reinforcement element during formation of said plastic moulding.

13. A power steering gear as claimed in claim 1 wherein the plastic moulding is of an engineering plastic.

1 . A power steering gear as claimed in claim 1 wherein said tubular reinforcement element is perforated.

15. A power steering gear comprising a housing having a region subjected to hydraulic pressure when in use, characterised in that said region comprises a plastic moulding with at least one port formed therein as a circular bore, said plastic moulding reinforced by a substantially tubular reinforcement element, said plastic moulding fully encapsulating said element where said region is subjected to hydraulic pressure.

16. A power steering gear as claimed in claim 15 wherein said port is provided with a seat at its open end.

17. A rack and pinion steering gear comprising a housing having a valve portion, a mechanical portion and a cylinder portion, said valve portion having a valve assembly substantially housed therein, said mechanical portion having a rack of a rack assembly substantially housed therein and said cylinder portion having a rack rod and piston of said rack assembly substantially housed therein, characterised in that said valve portion and/or said cylinder portion of said housing comprises a plastic moulding reinforced by a substatially tubular reinforcement element, said reinforcement element being fully encapsulated by plastic in the region of the valve portion and/or cylinder portion subjected to hydraulic pressure when in use.