WO2008138033A1

WO2008138033A1 - Hollow steering rack

Info

Publication number: WO2008138033A1
Application number: PCT/AU2008/000561
Authority: WO
Inventors: Juergen Dohmann
Original assignee: Bishop Innovation Limited
Priority date: 2007-05-09
Filing date: 2008-04-22
Publication date: 2008-11-20

Abstract

A method of manufacturing a hollow steering rack for a vehicle rack and pinion steering system from a tubular member. The method comprises firstly performing a pre-forming operation on the tubular member to form a pre-formed region on the tubular member. The pre-forming operation may increase the wall thickness of a portion of the tubular member. A first mandrel is then inserted into one end of the tubular member and a second mandrel is inserted into the other end of the tubular member such that the first and second mandrels substantially abut each other. A toothed region is then forged onto the pre-formed region of the tubular member; and the mandrels are removed.

Description

HOLLOW STEERING RACK

TECHNICAL FIELD

The present invention relates to steering racks for vehicle rack and pinion steering gears, and more particularly to a method of manufacturing hollow steering racks.

BACKGROUND

Typically, steering racks for vehicle rack and pinion steering gears 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 'D¹ shape and hence these racks are commonly referred to as "D-racks". The shank of these racks is typically gun drilled to remove weight and provide a path for air to pass from one end of the rack to the other. The disadvantages of gun drilling are that material is wasted and it is a relatively expensive process. As an alternative to broaching the toothed region, WO 2005/053875 A1 (Bishop Innovation) discloses a die apparatus for forging the toothed region of a steering rack. Forging makes it possible for the teeth to have a variable ratio form.

To eliminate gun drilling and minimise weight, it is desirable to form a hollow steering rack from tube stock. WO 2005/053875 A1 (Bishop Innovation) discloses a method of forging a short hollow rack from tube stock. This short rack is intended to be welded to a tubular shank to form the complete rack. The method disclosed uses a single mandrel inserted into the tube stock before forging. However, it is desirable for the complete rack to be formed from a single length of tube stock without welding. The single mandrel method disclosed is not suitable for forging a complete rack from a length of tube stock because it cannot not support the tube stock against collapse during forging in the regions of the tube stock adjacent both ends of the toothed region.

It is an object of the present invention to provide a method of manufacturing a hollow steering rack that ameliorates at least some of the problems of the prior art. SUMMARY OF INVENTION

The present invention consists of a method of manufacturing a hollow steering rack for a vehicle rack and pinion steering system from a tubular member, the method comprising the steps of:

(i) performing a pre-forming operation on the tubular member to form a preformed region on the tubular member;

(ii) inserting a first mandrel into one end of the tubular member and a second mandrel into the other end of the tubular member such that the first and second mandrels substantially abut each other; (iii) forging a toothed region onto the pre-formed region of the tubular member; and (iv) removing the first and second mandrels from the tubular member.

Preferably, the pre-forming operation increases the wall thickness of at least a portion of the tubular member in the pre-formed region. Preferably, the pre-forming operation reduces the outer circumference of the tubular member in the pre-formed region. Preferably, at least a portion of the tubular member is heated prior to the pre-forming operation. Preferably, the pre-forming operation is performed using a die set.

Preferably, the toothed region axially overlaps both first and second mandrels. Preferably, at least one of the first and second mandrels has a free end shaped to correspond to a portion of the bore of the toothed region, and each of the first and second mandrels has a cylindrical region adapted to support a region of the tubular member that is gripped during forging.

Preferably, the free end of at least one of the first and second mandrels that corresponds to the bore of the toothed region is tapered over at least a portion of its length. Preferably, at least one of the first and second mandrels has a transition region that smoothly blends between the cylindrical region and the free end of the mandrel. Preferably, the free end of at least one of the first and second mandrels is substantially D-shaped. Preferably, the tubular member is a length of tube stock. Preferably, the toothed region is forged using a die apparatus adapted to flashless forge a D-rack. Preferably, the toothed region is warm forged.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a longitudinal sectional view of a steering rack made by a method in accordance with the present invention.

Fig. 2 is a cross section along H-II of the steering rack shown in Fig. 1.

Fig. 3 shows a length of tube stock used to make the steering rack shown in Fig. 1.

Fig. 4 is a longitudinal sectional view of a pre-formed tubular member made from the tube stock of Fig. 3.

Fig. 5 is a cross section along V-V of the pre-formed tubular member shown in Fig. 4.

Fig. 6 shows a portion of the tube stock of Fig. 3 that is heated prior to pre-forming the tube stock.

Fig. 7 is a cross section along VII-VII of the heated tube stock shown in Fig. 6.

Fig. 8 is a longitudinal sectional view of a pre-forming die set in its open position with the tube stock of Fig. 3 placed in it.

Fig. 9 is a cross section along IX-IX of the pre-forming die set shown in Fig. 8, in its open position.

Fig. 10 is a longitudinal sectional view of the pre-forming die set of Fig. 8, in its closed position. Fig. 11 is a cross section along XI-XI of the pre-forming die set shown in Fig. 10, in its closed position.

Fig. 12 shows two mandrels used in the manufacture of the steering rack shown in Fig. 1.

Fig. 13 is an end view along XIII-XHI of one of the mandrels shown in Fig. 12.

Fig. 14 shows the mandrels of Fig. 12 inserted into the pre-formed tubular member of Fig. 4.

Fig. 15 is a longitudinal sectional view of a forging die apparatus used to manufacture the steering rack shown in Fig. 1 from the pre-formed tubular member shown in Fig. 4.

Fig. 16 is a cross section along XVI-XVI of the forging die apparatus shown in Fig. 15.

Fig. 17 is a longitudinal sectional view of the forging die apparatus of Fig. 15, in a partially closed position.

Fig. 18 is a longitudinal sectional view of the forging die apparatus of Fig. 15, in its fully closed position.

Fig. 19 is a cross section along XIX-XIX of the forging die apparatus shown in Fig. 18, in its fully closed position.

Fig. 20 is a longitudinal sectional view of an alternative pre-formed tubular member made from the tube stock of Fig. 3.

Fig. 21 is a cross section along XXI-XXI of the pre-formed tubular member shown in Fig. 20. BEST MODE OF CARRYING OUT THE INVENTION

Figs. 1 and 2 show a finished steering rack 1 made in accordance with the present invention. Rack 1 is hollow along its entire length with a D-shaped toothed region 2 towards one end. Teeth 6 are formed on one side of the toothed region 2. The back 7 of the toothed region 2, opposite the teeth 6, has a semi-circular shape. The rack shank 3 and the region 4 at the other end of the rack 1 are tubular with a substantially constant wall thickness. The back 7 of the toothed region 2 has substantially the same radius as the shank 3 and the end region 4. Transition regions 5 smoothly blend the toothed region 2 with the shank 3 and the end region 4. The bore 10 of the rack 1 is substantially D-shaped in the toothed region 2. The shank 3 has two external circumferential grooves 8 for locating a hydraulic piston, and each end of the rack 1 has an internal thread 9 for attaching a tie rod end.

Fig. 3 shows a length of steel tube stock 12 used to make steering rack 1. As an example only, tube stock 12 may have an outside diameter in the range 24 to 32mm, and a constant wall thickness 33 in the range of 4 to 6mm. In this example, the length of tube stock 12 is the same as the length of rack 1.

The first step in a method of manufacturing rack 1 in accordance with the present invention is to perform a pre-forming operation on the tube stock 12 to form a preformed tubular member 28 as shown in Figs. 4 and 5. The pre-formed tubular member 28 has a pre-formed region 29. In this embodiment, the pre-formed region 29 has an upper portion 31 with a wall thickness that varies but is always greater than the constant wall thickness 33 of the tube stock 12. For example, the wall thickness 32 near the ends of the portion 31 is significantly larger than the constant wall thickness 33, and the wall thickness 34 at the middle of the portion 31 is only slightly larger than the constant wall thickness 33.

The pre-formed region 29 has an arcuate lower portion 35 that is not significantly deformed by the pre-forming operation and hence its wall thickness 33 is substantially the same as the tube stock 12. The pre-formed region 29 also has two flattened portions 36 on either side, between the upper portion 31 and the lower portion 35. The wall thickness of the flattened portions 36 varies between being slightly larger and slightly smaller than the wall thickness 33 of the tube stock 12. The outer circumference of the pre-formed region 29 is less than the outer circumference of the tube stock 12.

Figs. 8 to 11 show a pre-forming die set 38 used to form the pre-formed region 29 onto the tube stock 12, thus forming the pre-formed tubular member 28. A portion 18 of the tube stock 12 is locally heated prior to the pre-forming operation, as indicated by the shaded areas in Figs. 6 and 7. The portion 18 is the portion of the tube stock 12 that is deformed by the pre-forming operation, and heating portion 18 softens it to allow it deform more readily with less residual stress. Preferably portion 18 is heated to a temperature in the range 500⁰C to 750⁰C, and preferably portion 18 is heated by induction.

The heated tube stock 12 is then placed in the pre-forming die set 38 in its open position, as shown in Figs. 8 and 9. The die set 38 is placed in a press (not shown) that opens and closes it. The die set 38 comprises a mandrel 39, an upper die 40, a lower die 41 , and two side punches 42. The upper die 40 has a concave surface 44 shaped to match the upper portion 31 of the pre-formed region 29, and the lower die 41 has a concave surface 45 shaped to match the lower portion 35 of the pre-formed region 29. The side punches 42 have flat faces 46 corresponding to the flattened portions 36 of the pre-formed region 29. The mandrel 39 is approximately D-shaped and is inserted into the bore of the heated tube stock 12.

The die set 38 then closes to form the pre-formed region 29, as shown in Figs. 10 and 11. As the die set 38 closes, the lower die 41 and the mandrel 39 remain stationary with respect to the heated tube stock 12, whilst the upper die 40 and the side punches 42 converge onto the heated tube stock 12. As the die-set 38 closes, the wall thickness of the heated portion 18 of the tube stock 12 increases as it is pushed inwards, thereby reducing the outer circumference of the pre-formed region 29. In its closed position, there is clearance between the upper die 41 and the side punches 42, and between the bore of the pre-formed region 29 and the mandrel 39. This clearance is permissible because it is not necessary to generate a very high pressure in the tube stock 12 to form the pre-formed region 29. The die set 38 then opens and the preformed tubular member 28 is removed.

Figs. 12 and 13 show two mandrels 13a and 13b used in the manufacture of steering rack 1. At the free end of each mandrel 13a, 13b is a region 14 with a cross sectional shape corresponding to that of the bore 10 of the toothed region 2 of rack 1. The cross section of each region 14 is approximately D-shaped, and the length of each region 14 is approximately equal to half the length of toothed region 2. Each mandrel 13a, 13b has a transition region 15 that smoothly blends region 14 into a cylindrical region 16. Region 16 has a constant circular cross section that is sized to be a sliding fit in the bore of the tube stock 12. The shape of mandrel transition regions 15 corresponds to the shape of the bore of the rack transition regions 5. To enable mandrels 13a and 13b to be inserted into and removed from the tubular member 28, the cross section of regions 14 must lie within the diameter of regions 16, as shown in Fig. 13. Each region 14 has a short tapered portion 48 adjacent the transition region 15 of its respective mandrel 13a, 13b. Each tapered portion 48 has a slightly larger cross sectional area where it meets the transition region 15 than at the other end of the tapered portion 48 that meets the remainder of the region 14. The tapered portions 48 assist the end teeth of the rack 1 (the end teeth being the two or three teeth 6 closest to the transition regions 5 of the rack 1) to fill properly during forging, and may assist in removing the mandrels 13a, 13b after forging.

The next step in the method of manufacturing the rack 1 is to heat the entire preformed region 29 of the pre-formed tubular member 28 to a temperature in the range 500⁰C and 900⁰C suitable for warm forging. As shown in Fig. 14, the mandrels 13a and 13b are then inserted into the ends of the heated pre-formed tubular member 28 such that they abut against each other at an axial location 19 along the pre-formed tubular member 28 corresponding approximately to the middle of toothed region 2. Mandrel 13b is longer than mandrel 13a because mandrel 13b is inserted into the end of the pre-formed tubular member 28 that forms the shank 3 of rack 1.

The pre-formed tubular member 28, with the mandrels 13a and 13b inserted, is then placed in a die apparatus 20 as shown in Figs. 15 and 16. Die apparatus 20 is similar to that disclosed in WO 2005/053875 A1 (Bishop Innovation), and it is designed to flash less forge a D-rack. Figs. 15 and 16 show die apparatus 20 in its open position. Die apparatus 20 comprises an upper die element 21 , a lower die element 22, two side punches 23, and a pair of grippers 26 at each end. The die apparatus 20 is mounted in a press (not shown). The mandrels 13a, 13b are oriented such that the flat faces of their regions 14 face the upper die element 21.

The upper die element 21 has a toothed recess 24 corresponding in shape to the teeth 6 of rack 1 , and the lower die element 22 has a semi-circular recess 25 corresponding to the back 7 of the toothed region 2 of rack 1. The side punches 23 move sideways and are fixed vertically with respect to the lower die element 22. The grippers 26 each have a semi-circular recess 27 of the same radius as the outside of the tube stock 12. The grippers 26 move vertically independently of the die elements 21 , 22.

The die apparatus 20 is then closed to forge the toothed region 2 and transition regions 5 of rack 1 to net shape, transforming the pre-formed tubular member 28 into a partially finished steering rack 1a. Fig. 17 shows the die apparatus 20 in a partially closed position. In this position, the grippers 26 surround and strongly grip the outside of the cylindrical regions of the pre-formed tubular member 28, but the die elements 21 , 22 have not yet contacted the pre-formed region 29.

Figs. 18 and 19 show the die apparatus 20 in its fully closed position. As the die apparatus further closes from the position shown in Fig. 17, the die elements 21 and 22 firstly fully close to form a closed die cavity and then the side punches 23 move radially inwards to complete the forging process. The side punches 23 form longitudinal indents 11 along the sides of the toothed portion 2 of rack 1 , as shown in Fig. 2. The wall of the pre-formed tubular member 28 is forced inward by the forging operation such that it surrounds the mandrels 13a, 13b without any gaps. The resulting forged toothed region 2 axially overlaps both mandrels 13a and 13b. The cylindrical regions 16 of the mandrels 13a, 13b prevent the pre-formed tubular member 28 from collapsing under the gripping and forging forces. The cylindrical regions 16 also prevent the pre-formed tubular member 28 from being axially squeezed away from the toothed region 2 during forging.

The die apparatus 20 is then opened, and the mandrels 13a, 13b are extracted from the partially finished rack 1a. The mandrels 13a, 13b are extracted immediately after the die apparatus 20 opens to minimise the shrinkage of the toothed region 2 onto the mandrels 13a, 13b as it cools. The partially finished rack 1a is then hardened and machined in the usual manner to produce the finished steering rack 1.

The advantage of forging the rack 1a from the pre-formed tubular member 28, rather than directly from the tube stock 12, is that the pre-forming assists in minimising folding of the wall of the tubular member 28 during forging. The pre-forming also assists in preventing flash from occurring during forging.

The mandrels 13a, 13b are reusable, and a single set of mandrels may be used to forge many racks. Various methods may be used to insert and extract the mandrels 13a, 13b. For example, the mandrels may be manipulated by a robot. Alternatively, one or both of the mandrels may be built into the die apparatus with appropriate actuators to move them.

As shown in Fig. 12, the D-shaped regions 14 at the free ends of both mandrels 13a, 13b have approximately the same length. However, in other not shown embodiments of the invention the D-shaped regions of the two mandrels may have different lengths, providing that the total length of the two D-shaped regions is approximately equal to the length of the toothed region of the rack being forged. Furthermore, the D-shaped region of one mandrel may extend the whole length of the toothed region, with the free end of the other mandrel having a shape corresponding to the bore of the transition region of a rack. In other not shown embodiments of the invention, the D- shaped regions may be tapered along all of their length or not tapered at all.

In other not shown embodiments of the invention, the pre-formed region of the preformed tubular member may have a cross sectional shape different than that shown in Fig. 5. Furthermore, the pre-forming operation may pre-form the tubular member in a manner that does not increase its wall thickness or reduce its circumference. For example, Figs. 20 and 21 show a pre-formed tubular member 28a, made from the tube stock 12, having an alternatively shaped pre-formed region 29a. The sides of region 29a are deformed radially inwards and the top is deformed radially outwards, but the wall thickness and circumference of the pre-formed region 29a remains the same as the tube stock 12. As a further example, the pre-forming operation may simply involve flattening one side of the tubular member. Also, the pre-forming operation may be done without heating the tube stock.

In other not shown embodiments of the invention, the methods of manufacturing steering racks described above may also be used to manufacture a short rack that is subsequently joined to another tubular member to form a complete rack.

The teeth 6 of the steering rack 1 are entirely radially within the diameter of the shank 3, as can be seen in Fig. 2. However, the method of the present invention may also be used to manufacture racks where the teeth extend beyond the diameter of the shank, providing that the mandrels can still be shaped such that the regions of the mandrels corresponding to the bore of the toothed region still lie radially within the cylindrical regions of the mandrels.

The method of the present invention may be used to manufacture racks with both constant ratio and variable ratio tooth forms.

The embodiment described in detail above has the free ends of the mandrels 13a and 13b abutting against each other such that there is no clearance between them.

However, a small gap is permissible between the ends of the mandrels providing that a significant amount of the tube stock cannot flow into this gap during forging. If there is a small gap between the ends of the mandrels, the mandrels are still considered to substantially abut each other.

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 hollow steering rack for a vehicle rack and pinion steering system from a tubular member, the method comprising the steps of: (i) performing a pre-forming operation on the tubular member to form a preformed region on the tubular member;

2. A method of manufacturing a hollow steering rack as claimed in claim 1 wherein the pre-forming operation increases the wall thickness of at least a portion of the tubular member in the pre-formed region.

3. A method of manufacturing a hollow steering rack as claimed in claim 1 wherein the pre-forming operation reduces the outer circumference of the tubular member in the pre-formed region.

4. A method of manufacturing a hollow steering rack as claimed in claim 1 wherein at least a portion of the tubular member is heated prior to the pre-forming operation.

5. A method of manufacturing a hollow steering rack as claimed in claim 1 wherein the pre-forming operation is performed using a die set.

6. A method of manufacturing a hollow steering rack as claimed in claim 1 wherein the toothed region axially overlaps both first and second mandrels.

7. A method of manufacturing a hollow steering rack as claimed in claim 1 wherein at least one of the first and second mandrels has a free end shaped to correspond to a portion of the bore of the toothed region, and each of the first and second mandrels has a cylindrical region adapted to support a region of the tubular member that is gripped during forging.

8. A method of manufacturing a hollow steering rack as claimed in claim 7 wherein the free end of at least one of the first and second mandrels that corresponds to the bore of the toothed region is tapered over at least a portion of its length.

9. A method of manufacturing a hollow steering rack as claimed in claim 7 wherein at least one of the first and second mandrels has a transition region that smoothly blends between the cylindrical region and the free end of the mandrel.

10. A method of manufacturing a hollow steering rack as claimed in claim 7 wherein the free end of at least one of the first and second mandrels is substantially D-shaped.

11. A method of manufacturing a hollow steering rack as claimed in claim 1 wherein the tubular member is a length of tube stock.

12. A method of manufacturing a hollow steering rack as claimed in claim 1 wherein the toothed region is forged using a die apparatus adapted to flashless forge a

D-rack.

13. A method of manufacturing a hollow steering rack as claimed in claim 1 wherein the toothed region is warm forged.