WO2006037167A1

WO2006037167A1 - Steering rack

Info

Publication number: WO2006037167A1
Application number: PCT/AU2005/001520
Authority: WO
Inventors: Arthur Ernest Bishop
Original assignee: Bishop Innovation Limited
Priority date: 2004-10-07
Filing date: 2005-10-06
Publication date: 2006-04-13

Abstract

A method of manufacturing a steering rack (10) for a vehicle rack and pinion steering gear from a solid elongate blank. The steering rack comprising a toothed region (11) and a tubular shank (13). The method comprising performing a machining operation to produce an intermediate form of the tubular shank having a substantially constant wall thickness, followed by performing an upsetting operation on the intermediate form to increase the wall thickness of at least one portion thereof.

Description

STEERING RACK

TECHNICAL FIELD

The present invention relates to steering racks for vehicle rack and pinion steering gears, and in particular to the manufacture of steering racks.

BACKGROUND

Typically, vehicle steering racks are manufactured from round solid bar stock, with the toothed region broached across the bar near one end. This results in the cross section of the toothed region having a 'D' shape and hence these racks are commonly referred to as "D-racks". The toothed region of such a broached D-rack has significantly less bending strength than the round solid shank extending from it. However, to minimise the weight of the steering rack, it is desirable that the toothed region and the shank have similar bending strength. A common approach to this problem is to gun drill the shank over most of its length resulting in a substantially tubular shank.

An alternative method of manufacturing a steering rack from round solid bar stock is to forge the toothed region. US Patents 4,571 ,982 (Bishop et al) and 5,862,701 (Bishop et al) disclose die apparatus for flashless warm forging the toothed region to net shape. "Net shape" means that the forged rack teeth do not require any further machining after forging. An advantage of forging is that the rack teeth may be shaped to have a variable gear ratio. The cross section of the toothed region of racks forged by the type of die disclosed in US Patents 4,571 ,982 and 5,862,701 has a Υ shape and such racks are commonly referred to as "Y-racks". The toothed region of a forged Y-rack has greater bending strength than the toothed region of a D-rack broached from the same diameter solid bar, and so Y-racks can be forged from smaller diameter bar whilst maintaining overall bending strength. However, the shanks of Y-racks are still commonly gun drilled to further reduce weight. Various methods of forging racks having substantially D-shaped toothed regions from solid bar stock are also known. For example WO 2005/053875 (Bishop Innovation Limited) discloses a die apparatus for flashless forging a substantially D-shaped rack. The shanks of these racks are also typically gun drilled.

As an alternative to manufacturing steering racks from solid bar stock, numerous attempts have been made to further reduce weight by the manufacture of hollow steering racks from tube stock. One such method used in limited production is disclosed in US Patent 4,598,451 (Takanosuke) where a series of mandrels is passed through a flattened tube to progressively fill an external tooth die. This method is expensive and time consuming and as such is not well suited to high volume production. Furthermore, the size of teeth that can be produced is limited by the wall thickness of the tube and the method is not well suited to producing racks with variable ratio teeth. A disadvantage of manufacturing steering racks from tube stock is increased cost of material compared with using solid bar stock.

EP 0099311 (Vallourec) discloses another method of manufacturing a steering rack from tube stock where the raw tube stock is cold-swaged and drawn to form a hollow intermediate blank having regions of different thickness. The teeth are then formed during a subsequent step. A disadvantage of this method is that the region of the intermediate blank where the teeth are to be formed is hollow and as such it is not suited to economical tooth forging methods applicable to solid bar stock, such as using the types of die disclosed in US Patents 4,571 ,982 and 5,862,701.

A "composite rack" is defined as a rack made by joining two or more members to each other, A composite rack is typically made by welding a tubular shank to a short solid rack member. Composite racks have the advantages of reduced weight without the limitations of forming the rack teeth onto a tube. Various methods of making composite racks have been proposed or used in limited production. For example, a composite steering rack has been used in Honda Odyssey" mini-van vehicles. This rack is made by welding a tubular shank to a short solid forged D-shaped member. Whilst welded composite steering racks have been commercialised, they have not as yet become widely accepted, and many automotive manufacturers prefer racks that are manufactured from round solid bar stock by gun drilling to form the tubular shank.

Typically, steering racks for hydraulic power steering gears have at least one external circumferential groove approximately mid way along the shank for locating a hydraulic piston. A problem with conventional gun drilled steering racks and other racks having a tubular shank of constant wall thickness, such as typical composite racks, is that the constant wall thickness must be thick enough to machine these grooves without weakening the rack. This means that the wall thickness of the shank must be thicker than it would otherwise need to be if these grooves were not required. In a similar manner, the end of the tubular shank must have sufficient wall thickness to machine an internal thread for attaching a tie rod end.

The present invention seeks to ameliorate at least some of the disadvantages of prior art racks that are manufactured from round solid bar stock.

SUMMARY OF INVENTION

In a first aspect, the present invention consists of a method of manufacturing a steering rack for a vehicle rack and pinion steering gear from a solid elongate blank, said steering rack comprising a toothed region and a tubular shank, characterised in that said method comprises performing a first operation to produce an intermediate form of said tubular shank having a substantially constant wall thickness, followed by performing an upsetting operation on said intermediate form to increase the wall thickness of at least one portion thereof.

In one preferred embodiment, said first operation comprises a machining operation. Preferably, said machining operation comprises gun drilling. In another preferred embodiment, said first operation comprises a forming operation.

Preferably, said portion of increased wall thickness is located remote from the free end of said shank. Preferably, at least one circumferential groove adapted to locate a hydraulic piston is machined on the outside diameter of said portion of increased wall thickness.

Preferably, said portion of increased wall thickness is located at the free end of said shank. Preferably, a thread is machined in the bore of said portion of increased wall thickness.

Preferably, said machining operation and said upsetting operation are performed after said toothed region is formed.

Preferably, said solid elongate blank is cylindrical. Preferably, said toothed region is formed by a forging operation.

In a second aspect, the present invention consists of a steering rack for a vehicle rack and pinion steering gear, said steering rack being of a single piece construction manufactured from a solid elongate blank, and having a toothed region and a tubular shank, characterised in that said tubular shank is manufactured by a method comprising a first operation to produce an intermediate form of said tubular shank having a substantially constant wall thickness, followed by an upsetting operation performed on said intermediate form to produce at least one portion of increased wall thickness thereon.

Preferably, said portion of increased wall thickness is located remote from the free end of said shank. Preferably, at least one circumferential groove adapted to locate a hydraulic piston is machined on the outside diameter of said portion of increased wall thickness. Preferably, said portion of increased wall thickness is located at the free end of said shank. Preferably, a thread is machined in the bore of said portion of increased wall thickness.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 shows a steering rack manufactured in accordance with a first embodiment of the present invention.

Fig. 2 is a cross sectional view of the steering rack of Fig. 1 through H-II.

Fig. 3 shows the blank used to manufacture the steering rack shown in Fig. 1.

Fig. 4 shows a first intermediate form of the steering rack of Fig. 1 during its manufacture.

Fig. 5 shows a second intermediate form of the steering rack of Fig. 1 during its manufacture.

Fig. 6 shows a third intermediate form of the steering rack of Fig. 1 during its manufacture.

Fig. 7 is a cross sectional view of an intermediate form of a steering rack manufactured in accordance with a second embodiment of the present invention.

BEST MODE OF CARRYING OUT THE INVENTION

Figs. 1 and 2 show a steering rack 10 manufactured in accordance with a first embodiment of the present invention, comprising a toothed region 11 and a tubular shank 13 extending from an end of steering rack 10 towards toothed region 11. As shown in Fig. 2, toothed region 11 has a substantially D-shaped cross section, and as such steering rack 10 is considered to be a D-rack. Toothed region 11 has constant ratio teeth 12.

Tubular shank 13 has a relatively thin wall 19 of substantially constant thickness, except for portions 15 and 16 of increased wall thickness. Thickened portion 16 is remote from the free end 20 of shank 13, and is approximately mid-way along the length of shank 13. Portion 16 has two circumferential grooves 18 on its outside diameter, adapted to locate a hydraulic piston (not shown) by means of circlips, swaging, or a combination of both. Thickened portion 15 is located at the free end 20 of shank 13, and has a thread 17 in its bore for attaching a tie rod end (not shown).

Steering rack 10 is manufactured by a method comprising steps 1 to 6 as follows, and described in detail below. Step 1 : Provide a solid elongate blank.

Step 2: Form toothed region.

Step 3: Gun drilling.

Step 4: Upsetting.

Step 5: Machine features. Step 6: Finishing operations.

Step 1 comprises providing a solid elongate blank 22 as shown in Fig. 3, preferably made from steel. In this embodiment, blank 22 is substantially cylindrical, except for a "necked" region 23 of reduced outside diameter. The length of necked region 23 is substantially the same as the length of toothed region 11.

Step 2 comprises forming toothed region 11 on blank 22 to produce a first intermediate form 10a, as shown in Fig.4. In this embodiment, toothed region 11 is formed by warm forging the necked region 23 of blank 22 in a die apparatus such as disclosed in WO 2005/053875 (Bishop Innovation Limited). Teeth 12 are preferably forged to net shape. The necked region 23 of blank 22 has reduced cross sectional area compared with the rest of blank 22 so that toothed region 11 can be forged without flash. Step 3 comprises performing a gun drilling machining operation on first intermediate form 10a to produce a second intermediate form 10b having an intermediate tubular shank 13b, as shown in Fig. 5. Intermediate tubular shank 13b has a constant wall thickness.

Step 4 comprises performing an upsetting operation on the intermediate tubular shank 13b of intermediate form 10b to produce a third intermediate form 10c with its shank 13 having portions 15 and 16 of increased wall thickness, as shown in Fig. 6. The upsetting operation is performed by locally heating intermediate shank 13b, then axially compressing it whilst its outside diameter is diametrically restrained by rollers or the like. By restraining the outside diameter, the tube is upset inwardly thereby increasing the wall thickness and reducing the inside diameter of portions 15 and 16. The outside diameter of portions 15 and 16 is not significantly changed by the upsetting operation. Note that blank 22 must be longer than finished rack 10 to accommodate the reduction of length during the upsetting operation.

Step 5 comprises machining third intermediate form 10c to form circumferential grooves 18 on the outside diameter of thickened portion 16, and thread 17 in the bore of thickened portion 15. A hole 24 with an internal thread for attaching a tie rod end, is also typically machined at the other end of steering rack 10.

Step 6 comprises finishing operations to produce finished steering rack 10, such as machining the outside diameter of tubular shank 13, and hardening toothed region 11.

The localised thickening of portions 15 and 16 provided by the upsetting operation clearly allows tubular shank 13 to be manufactured with a thinner wall than would otherwise be required if the tubular shank was of constant wall thickness. For example, a rack having a shank outside diameter of approximately 26mm may be made in accordance with the present invention by firstly gun drilling to produce an intermediate shank having a constant wall thickness of approximately 4mm. This intermediate shank may then be upset to increase the wall thickness to, for example, approximately 5.5mm in thickened portion 16 to provide sufficient strength once grooves 18 are machined. Portion 16 must be sufficiently thick to minimise notch sensitivity caused by grooves 18 when bending moments are applied to the rack by forces transmitted through the tie rods. A conventional shank made without upsetting would require a wall thickness of at least 5.5mm along its length to provide the same strength in the vicinity of grooves 18, and hence such as rack would be significantly heavier than one made in accordance with the present invention.

Fig. 7 is a cross sectional view of an intermediate form 1Od of a steering rack manufactured in accordance with a second embodiment of the present invention. Intermediate form 10d is at a similar stage of manufacture as intermediate form 10c shown in Fig. 6. The only difference between this embodiment and the first embodiment described above, is that during the upsetting operation the outside diameter of the gun drilled intermediate tubular shank is not restrained thereby producing an intermediate shank 13d with thickened portions 15d and 16d that bulge outwardly, as well as inwardly. In a further step, the outward bulges of thickened portions 15d and 16d must be machined off to produce an intermediate form similar to 10c shown in Fig. 6. Grooves 18 and thread 17 can then be machined in a similar manner to the first embodiment.

In other not shown embodiments of the present invention, the toothed region may be forged in a die apparatus such as disclosed US Patents 4,571,982 (Bishop et al) and 5,862,701 (Bishop et al). In this case, the cross section of the toothed region will have a Y-shape and the elongate blank used will preferably be cylindrical over its entire length. In any of the embodiments of the present invention where the toothed region is forged, the teeth may be either constant ratio or variable ratio.

Alternatively, in other not shown embodiments, the toothed region may be formed by broaching. In this case the elongate blank used will also preferably be cylindrical over its entire length.

It should be understood that in other not shown embodiments, the gun-drilling operation and subsequent upsetting operation to form the tubular shank may be carried out on the blank before the toothed region is formed. Alternatively, the toothed region may be formed in between the gun drilling and upsetting operations.

Whilst all of the abovementioned embodiments utilise gun drilling, in other not shown embodiments the machining operation to form the intermediate tubular shank 13b may comprise twist drilling, boring, or any other suitable machining method. If the inside diameter of the tubular shank is bored, this can be carried out in multiple stages.

It should also be understood, that during the upsetting operation of abovementioned embodiments, the inside diameter of the tubular shank may be axially constrained by a mandrel to control the axial position of the thickened portions.

Also, in another not shown embodiment, the upsetting operation can be carried out in such a manner that the thickened portion at the free end of the shank has both a reduced inside diameter and a reduced outside diameter. A thread is still subsequently machined in the bore of this thickened portion for attaching a tie rod end.

In other not shown embodiments of the present invention, the intermediate tubular shank 13b, having a substantially constant wall thickness, may be produced by a forming operation instead of a machining operation. Suitable types of forming operation include well known processes such as reverse extrusion and spin extrusion applied to an end of the solid elongate blank, or to an end of an intermediate form of the steering rack having the toothed region already formed.

The term "comprising" as used herein is used in the inclusive sense of "including" or "having" and not in the exclusive sense of "consisting only of.

Claims

1. A method of manufacturing a steering rack for a vehicle rack and pinion steering gear from a solid elongate blank, said steering rack comprising a toothed region and a tubular shank, characterised in that said method comprises performing a first operation to produce an intermediate form of said tubular shank having a substantially constant wall thickness, followed by performing an upsetting operation on said intermediate form to increase the wall thickness of at least one portion thereof.

2. A method of manufacturing a steering rack as claimed in claim 1, wherein said first operation comprises a machining operation.

3. A method of manufacturing a steering rack as claimed in claim 2, wherein said machining operation comprises gun drilling.

4. A method of manufacturing a steering rack as claimed in claim 1 , wherein said first operation comprises a forming operation.

5. A method of manufacturing a steering rack as claimed in claim 1 , wherein said portion of increased wall thickness is located remote from the free end of said shank.

6. A method of manufacturing a steering rack as claimed in claim 5, wherein at least one circumferential groove adapted to locate a hydraulic piston is machined on the outside diameter of said portion of increased wall thickness.

7. A method of manufacturing a steering rack as claimed in claim 1 , wherein said portion of increased wall thickness is located at the free end of said shank.

8. A method of manufacturing a steering rack as claimed in claim 7, wherein a thread is machined in the bore of said portion of increased wall thickness.

9. A method of manufacturing a steering rack as claimed in claim 1 , wherein said machining operation and said upsetting operation are performed after said toothed region is formed.

10. A method of manufacturing a steering rack as claimed in claim 1 , wherein said solid elongate blank is cylindrical.

11. A method of manufacturing a steering rack as claimed in claim 1 , wherein said toothed region is formed by a forging operation.

12. A steering rack for a vehicle rack and pinion steering gear, said steering rack being of a single piece construction manufactured from a solid elongate blank, and having a toothed region and a tubular shank, characterised in that said tubular shank is manufactured by a method comprising a first operation to produce an intermediate form of said tubular shank having a substantially constant wall thickness, followed by an upsetting operation performed on said intermediate form to produce at least one portion of increased wall thickness thereon.

13. A steering rack as claimed in claim 12, wherein said first operation comprises a machining operation.

14. A steering rack as claimed in claim 13, wherein said machining operation comprises gun drilling.

15. A steering rack as claimed in claim 12, wherein said first operation comprises a forming operation.

16. A steering rack as claimed in claim 12, wherein said portion of increased wall thickness is located remote from the free end of said shank.

17. A steering rack as claimed in claim 16, wherein at least one circumferential groove adapted to )ocate a hydraulic piston is machined on the outside diameter of said portion of increased wall thickness.

18. A steering rack as claimed in claim 12, wherein said portion of increased wall thickness is located at the free end of said shank.

19. A steering rack as claimed in claim 18, wherein a thread is machined in the bore of said portion of increased wall thickness.

20. A steering rack as claimed in claim 12, wherein said solid elongate blank is cylindrical.

21. A steering rack as claimed in claim 12, wherein said toothed region is formed by a forging operation.