WO2009052552A1

WO2009052552A1 - Method and apparatus for manufacturing composite steering racks

Info

Publication number: WO2009052552A1
Application number: PCT/AU2008/001513
Authority: WO
Inventors: John Baxter; Andrew William Hassett
Original assignee: Bishop Innovation Limited
Priority date: 2007-10-25
Filing date: 2008-10-14
Publication date: 2009-04-30

Abstract

A method and device for manufacturing a steering rack for a vehicle rack and pinion steering gear. The steering rack comprises a toothed member having a toothed region, and a tubular member having a longitudinal axis. The method comprises performing a welding operation to join the toothed and tubular members together. During the welding operation, the toothed and tubular members are held relative to each other in a manner that maintains a specified offset distance between the toothed region and the longitudinal axis. The device holds the toothed member during the welding operation using the toothed region as a datum.

Description

METHOD AND APPARATUS FOR MANUFACTURING COMPOSITE STEERING RACKS

TECHNICAL FIELD

The present invention relates to steering racks for vehicle rack and pinion steering gears, and more particularly to such racks manufactured by welding a tubular member to a toothed member.

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 strengths. A common approach to this problem is to gun drill the shank over most of its length resulting in a substantially tubular shank. The gun drilled hole also provides a passage for air to pass from one side of the steering gear to the other in operation. The disadvantages of gun drilling are that material is wasted and it is a relatively expensive and slow process.

An alternative method of manufacturing a steering rack from round solid bar stock is to forge the toothed region. WO 2005/053875 A1 (Bishop Innovation Limited) discloses a die apparatus for flashless warm forging the toothed region of a D-rack to net shape. "Net shape" means that the forged teeth do not require any further machining after forging. An advantage of forging is that the teeth may be shaped to have a variable gear ratio. US Patents 4,571 ,982 (Bishop et al) and 5,862,701 (Bishop et al) disclose die apparatus for flashless warm forging racks having a toothed region with a "Y" shaped cross section. These types of racks are known as "Y-racks". Typically, the shanks of both forged D-racks and Y-racks are gun drilled after forging. A "composite rack" is defined as a rack made by joining two or more members together. A typical composite rack is made by joining a short solid toothed member to a tubular member. The tubular member then becomes the shank of the finished rack. The members are typically joined using a welding operation. Friction welding is typically used, but other types of welding may also be used such as magnetic arc, laser, plasma, resistance, TIG or MIG welding. WO 2005/028141 A1 (Bishop Innovation Limited), WO 2006/066309 A1 (Bishop Innovation Limited), EP 1316492 A1 (Neturen Co Ltd), Japanese Patent Abstract 2005-247163 (NTN Corp), and Japanese Patent Abstract 2006-046423 (Hitachi Ltd) disclose various welded composite racks.

Friction welding comprises rotating the members to be welded relative to each other whilst pushing them together. The friction at the interface of the members generates enough heat to locally melt them. US Patent 5,858,142 (TuIIy et al) discloses a typical friction welding machine in which one of the members to be welded is rotated whilst the other member is fixed to a slide that is pushed by a ram towards the rotating member.

The offset distance between the toothed region and the shank axis of a steering rack must typically be tightly controlled. This is particularly the case for steering gears in which the power assistance is provided by an electric motor driving a ball nut that engages a ball screw track formed on the rack shank. EP 1316492 A1 (Neturen Co Ltd) and Japanese Patent Abstract 2005-247163 (NTN Corp) disclose examples of composite racks having a ball screw track formed on the shank. In these steering gears, the smooth operation of the ball nut is very sensitive to the alignment of the rack shank in the assembled steering gear, and this alignment is determined by the offset distance between the toothed region and the shank axis. This offset distance is typically specified as the distance between a precision ball or roller sitting in the teeth and the shank axis.

Controlling the offset distance between the toothed region and the shank axis is particularly difficult during the manufacture of a welded composite rack as there are typically no features on the end faces of the toothed member and tubular member to mechanically align them to each other as they are joined.

It is an object of the invention to improve the manufacture of composite racks.

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, the steering rack comprising a toothed member having a toothed region, and a tubular member having a longitudinal axis, the method comprising performing a welding operation to join the toothed and tubular members together, characterised in that, during the welding operation, the toothed and tubular members are held relative to each other in a manner that maintains a specified offset distance between the toothed region and the longitudinal axis.

Preferably, during the welding operation, the toothed member is held in a clamping device that positions the toothed member using the toothed region as a datum.

Preferably, the clamping device comprises at least one clamp unit, the clamp unit comprising a support member adapted to support at least a portion of the toothed region and a clamp member adapted to urge the portion of the toothed region into contact with the support member.

Preferably, the clamping device further comprises an end stop adapted to support an end of the toothed member during the welding operation.

Preferably, the clamping device further comprises a base, the end stop being fixed to the base and the clamp unit being movable relative to the base in an axial direction parallel to the longitudinal axis.

Preferably, a spring means resists movement of the clamp unit in the axial direction, relative to the base. Preferably, the at least one clamp unit comprises at least two clamp units adapted to be spaced apart along the toothed region.

In one preferred embodiment, the toothed region is comprised of a plurality of gear teeth each having two opposite flanks, and the clamping device is adapted to position the toothed member using the flanks as a datum. In another preferred embodiment, the toothed region is comprised of a plurality of gear teeth each having a tip, and the clamping device is adapted to position the toothed member using the tips as a datum.

Preferably, the welding operation comprises friction welding by means of rotating the toothed and tubular members relative to each other whilst pushing them together.

In a second aspect, the present invention consists of a clamping device for use in the manufacture of a steering rack for a vehicle rack and pinion steering gear, the steering rack comprising a toothed member having a toothed region, and a tubular member having a longitudinal axis, the toothed member and the tubular member being welded together by a welding operation, the clamping device being adapted to hold the toothed member during the welding operation using the toothed region as a datum, the clamping device comprising at least one clamp unit, the clamp unit comprising a support member adapted to support at least a portion of the toothed region, and a clamp member adapted to urge the toothed member into contact with the support member.

Preferably, the welding operation comprises friction welding and the clamping device is adapted to be mounted onto a friction welding machine.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 shows a finished steering rack manufactured in accordance with the present invention.

Fig. 2 is a sectional view of the steering rack shown in Fig. 1.

Fig. 3 is an enlarged sectional view of the steering rack shown in Fig. 1.

Fig. 4 shows a forged toothed member used to manufacture the steering rack shown in Fig. 1.

Fig. 5 shows the toothed member of Fig. 4 after machining.

Fig. 6 shows a tubular member used to manufacture the steering rack shown in Fig. 1.

Fig. 7 shows the toothed member of Fig. 5 welded to the tubular member of Fig. 6 to form a partially finished steering rack. Fig. 8 shows a friction welding machine with a first preferred embodiment of a clamping device in accordance with the present invention.

Fig. 9 shows the friction welding machine of Fig. 8 loaded with the toothed member of Fig. 5 and the tubular member of Fig. 6.

Fig. 10 shows the friction welding machine of Fig. 9 after the toothed member of Fig. 5 and the tubular member of Fig. 6 have been welded together to produce the partially finished steering rack of Fig. 7.

Fig. 11 is an isometric view of the clamping device shown in Fig. 8.

Fig. 12 shows the clamping device of Fig. 11 with the toothed member of Fig. 5 loaded into it.

Fig. 13 is a sectional view of the clamping device of Fig. 11 with the toothed member of Fig. 5 loaded into it.

Fig. 14 shows a second preferred embodiment of a clamping device in accordance with the present invention, with the toothed member of Fig. 5 loaded into it.

Fig. 15 is a sectional view of the clamping device of Fig. 14.

BEST MODE OF CARRYING OUT THE INVENTION

The present invention will be described using a constant ratio D-rack for an hydraulic power steering gear as an example, it being understood that the invention is equally applicable to other types of steering racks, including Y-racks, variable ratio racks, and steering racks having a ball screw track formed on the rack shank.

Figs. 1 , 2 and 3 show a finished steering rack 1 manufactured in accordance with the present invention. Steering rack 1 is D-rack comprising a toothed member 2 and a tubular member 3. A welded joint 8 rigidly joins an end of the toothed member 2 to an end of the tubular member 3, such that the tubular member 3 forms the shank 18 of the finished rack 1. The end of the toothed member 2 that is welded to the tubular member 3 has a short tubular portion 17 that has the same inside and outside diameters as the tubular member 3. As can be seen in Fig. 1 , an air bypass hole 11 is drilled through one side of the rack 1 , into the tubular portion 17, to provide a passage for air to flow through the rack shank 18 when the steering gear is in operation. Each end of rack 1 has an internal thread 9 for attaching a tie rod. Two circumferential grooves 10 are machined on the outside of the tubular member 3 to locate a piston.

The toothed member 2 has a toothed region 6 comprising a plurality of gear teeth 7. Each tooth 7 has a tip 13 and two opposite flanks 12, as shown in Fig. 3. The tubular member 3 has a longitudinal axis 5. The positional relationship between the toothed region 6 and the axis 5 is specified as an offset distance 15 between the toothed region 6 and the axis 5. In this example, the offset distance 15 is measured as the distance from the axis 5 to the top of a precision ball 16 contacting the teeth flanks 12. A reference line 14 for this toothed region 6 is defined as parallel to the axis 5 and tangent to the top of the ball 16. In other words, the offset distance 15 is the distance between the reference line 14 and the axis 5. The offset distance 15 is often specified in this manner because it directly measures to flanks 12, which are the critical features of the toothed region 6. In the example shown in Fig. 3, the precision ball 16 is near the end of the toothed region 6 that is closest to the tubular member 3. The advantage of this is that it minimises the effect of any bend in the toothed member 2 on the alignment at the weld 8 between the tubular member 3 and the toothed member 2. In other not shown embodiments of the invention, the precision ball 16 may be at other locations along the toothed region 6, such as in the middle of the toothed region 6.

The method of manufacturing the steering rack 1 , in accordance with the present invention, will now be described. Firstly, a solid, partially finished toothed member 2a is formed. In this example, partially finished toothed member 2a, as shown in Fig. 4, is formed by forging a solid steel bar using a method and die apparatus such as disclosed in WO 2005/053875 A1 (Bishop Innovation Limited) that forges the toothed region 6 to net shape. In other not shown embodiments of the invention, the toothed member may be machined, rather than forged, by a process such as broaching or milling the toothed region onto a solid bar.

Next, the ends of the partially finished toothed member 2a are machined to produce toothed member 2, as shown in Fig. 5. The short tubular portion 17 is formed by drilling the end of the toothed member 2 that is to be joined to the tubular member 3. The internal thread 9 is drilled and tapped into the other end of the toothed member 2. It is not necessary that the internal thread 9 be machined at this stage, and instead it can be machined after the toothed member 2 and tubular member 3 are welded together. Also, the air bypass hole 11 may be drilled at this stage, or after welding. The toothed region 6 may be hardened either before or after the toothed member 2 and tubular member 3 are welded together. An advantage of hardening the toothed region 6 before welding is that the teeth 7 are less likely to be damaged due to handling and clamping occurring during welding.

Tubular member 3 is made from a length of steel tube as shown in Fig. 6. Typically the tube outside diameter will between 25mm and 32mm, and the wall thickness will be between 3mm and 5mm, but this will vary depending on the rack design. The steel tube may be welded or seamless. In this example, the tubular member has a constant diameter and wall thickness but in other not shown embodiments the tubular member may be locally thickened as described in WO 2005/028141 A1 (Bishop Innovation Limited) and WO 2006/066309 A1 (Bishop Innovation Limited).

Next, the toothed member 2 and tubular member 3 are joined together by a welding operation to produce a partially finished rack 1a, as shown in Fig. 7. In this example, the welding operation comprises friction welding, but in other not shown embodiments of the invention other welding methods may be used, such as those listed in the background.

The welding operation is performed by a friction welding machine 20, as shown in Fig. 8. The friction welding machine 20 is a conventional friction welding machine. It has a rotating chuck 21 that can hold and rotate a work piece about an axis 23, and a table 22 that is movable on a slideway (not shown) such that it can move towards and away from the chuck 21 along an axis (not shown) parallel to the chuck axis 23. As the table 22 moves, the distance 26 between the top surface 27 of the table 22 and the chuck axis 23 remains constant. The table 22 is moved by a hydraulic ram or servo motor (not shown). A first preferred embodiment of a clamping device 25 in accordance with the present invention is mounted on the top surface 27 of the table 22.

Referring to Fig. 9, the tubular member 3 is loaded into and held by the rotating chuck 21 such that the axis 5 of the tubular member 3 and the chuck axis 23 are co-linear. The toothed member 2 is loaded into and held by the clamping device 25. The arrangement is such that the toothed member 2 and tubular member 3 are held relative to each other with the specified offset distance 15 between the tubular member axis 5 and the reference line 14 of the toothed region 6. This is achieved by the clamping device 25 positioning the toothed member 2 using the toothed region 6 as a datum, in a manner described in detail below, such that the distance 28 between the toothed region reference line 14 and the top surface 27 of the table 22 is the same for each individual toothed member 2 that is held in the clamping device 25.

Referring to Figs. 11, 12 and 13, the clamping device 25 comprises a base 34, two clamp units 30, and an end stop 33. The clamp units 30 are positioned such that they are spaced apart along the toothed region 6 of a toothed member 2 held by the clamping device 25. Each clamp unit 30 comprises a frame 38, a support member 31, a clamp member 32, and a cylinder 39. Each support member 31 sits in its respective frame 38 and can be changed for different rack designs. In this example, each support member 31 has two teeth 44 adapted to contact the toothed region 6 of a held toothed member 2 on the flanks 12 of its teeth 7. The tips of the support member teeth 44 have the same radius as the precision ball 16 used to define the offset distance 15. Therefore, each support member 31 positions the toothed member 2 using the flanks 12 of the teeth 7 as a datum, and since the radius of the tips of the support member teeth 44 are the same as the precision ball 16, the reference line 14 of each toothed member 2 held by the clamping device 25 will be at the same distance 28 to the bottom of the base 34. The clamp member 32 of each clamp unit 30 urges the toothed member 2 into contact with support members 31 by means of a cylinder 39 that moves the clamp member 32 up and down relative to its respective frame 38. When the clamp members 32 are raised as shown in Fig. 11 a toothed member 2 can be loaded into and unloaded from the clamping device 25. The clamp members 32 have a semi-circular cut-out 45 to match the shape of the back of the toothed member 2. If, in other not shown embodiments of the invention, the rack being welded was a Y-rack, then the clamp members would have a Y-shaped cut-out to match. The clamp members 32 are guided by the frames 38 to move vertically only and as such the clamp members 32 determine the sideways location of a held toothed member 2.

Each clamp unit 30 is supported by two bearing blocks 40 that are guided by bearing rails 41 attached to the base 34. Each clamp unit 30 can move in an axial direction relative to the base 34, as indicated by arrows 35 in Fig. 13, that is parallel to the axis 5 of a tubular member 3 held in the chuck 21. Springs 36 resist axial movement of the clamp units 30. The arrangement is such that the springs 36 ensure that that clamp units 30 are in the correct axial position for loading a toothed member 2, but when the axial load of friction welding is applied to the toothed member 2, the clamp units 30 can move axially such that almost all of the axial load is transmitted to the end stop 33, which is fixed to the base 34 by a bracket 42. This prevents excessive axial load from being transmitted through the toothed region 6, which may otherwise damage the teeth flanks 12. The clamp units 30 can move axially independently of each other to take up any variation in the spacing of the teeth 7 between individual toothed members 2.

In other not shown embodiments of the invention, the clamping device may have only one clamp unit, or it may have more than two clamp units spaced apart along the toothed region. Furthermore, the support member of each clamp unit may have only one tooth, or more than two teeth. In the case of an embodiment having only one clamp unit, the support member may have the same number of teeth as the toothed region of a supported toothed member. The shape of the support member teeth may vary depending of the type of toothed region. If the teeth of the toothed region are skewed then the support member teeth will be skewed to match. If the toothed region has a variable ratio form, then the support member teeth may be curved to suit. In other not shown embodiments of the invention, the shape of the support member teeth may be the same as the teeth of a pinion gear designed to mesh with the toothed region.

Referring to Fig. 10, the friction welding operation is performed by rotating the tubular member 3 in the chuck 21 whilst the toothed member 2 is pushed towards the tubular member 3 by moving the table 22. Throughout the welding operation, the offset distance 15 is maintained between the toothed region 6 and the tubular member axis 5. Referring to Fig. 7, the welded joint 8 of the partially finished rack 1a extends radially inwards and outwards from the tubular member 3. This is typical of a friction welded joint and it is due to the metal squeezing out as the toothed member 2 and tubular member 3 are pushed axially together.

Next, the partially finished rack 1a is finish machined to form the finished rack 1 shown in Figs. 1 , 2 and 3. This includes machining the circumferential grooves 10 on the outside of the tubular member 3 and machining the internal thread 9 in the end of the tubular member 3. The portion of the welded joint 8 that extends radially outside of the tubular member 3 is machined off. The outside of the tubular member 3 is hardened and ground to final size.

Figs. 14 and 15 show a second preferred embodiment of a clamping device 25a in accordance with the present invention. Clamping device 25a is adapted to be mounted on a friction welding machine 20 like clamping device 25 described above and operates in a similar manner to clamping device 25. The main difference in function between clamping device 25 and clamping device 25a is that clamping device 25a positions a toothed member 2 using the tips 13 of the teeth 7 as a datum, rather than the flanks 12. This achieved by clamping device 25a having a single support member 31a that extends the length of the toothed region 6. Instead of having teeth, support member 31a has a flat surface 47 parallel to the bottom surface of the base 34, and the teeth tips 13 contact the flat surface 47. The clamp members 32 urge the toothed member 2 into contact with the flat surface 47. Clamp members 32 are guided by frames 38a and are raised and lowered by cylinders 39. Unlike clamping device 25, the support member 31a is fixed to the base 34. Support member 31a does not need to be movable axially because the teeth tips 13 can slide on the flat surface 47 such that the axial load of friction welding is still mainly supported by the end stop 33.

Clamping device 25a may be used in preference to clamping device 25 in cases where contacting the tooth flanks 12 with support members 31 may damage the flanks 12, and where the position of the tooth tips 13 is accurately controlled with respect to the flanks 12.

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, the steering rack comprising a toothed member having a toothed region, and a tubular member having a longitudinal axis, the method comprising performing a welding operation to join the toothed and tubular members together, characterised in that, during the welding operation, the toothed and tubular members are held relative to each other in a manner that maintains a specified offset distance between the toothed region and the longitudinal axis.

2. A method of manufacturing a steering rack as claimed in claim 1 , wherein, during the welding operation, the toothed member is held in a clamping device that positions the toothed member using the toothed region as a datum.

3. A method of manufacturing a steering rack as claimed in claim 2, wherein the clamping device comprises at least one clamp unit, the clamp unit comprising a support member adapted to support at least a portion of the toothed region and a clamp member adapted to urge the portion of the toothed region into contact with the support member.

4. A method of manufacturing a steering rack as claimed in claim 3, wherein the clamping device further comprises an end stop adapted to support an end of the toothed member during the welding operation.

5. A method of manufacturing a steering rack as claimed in claim 4, wherein the clamping device further comprises a base, the end stop being fixed to the base and the clamp unit being movable relative to the base in an axial direction parallel to the longitudinal axis.

6. A method of manufacturing a steering rack as claimed in claim 5, wherein a spring means resists movement of the clamp unit in the axial direction, relative to the base.

7. A method of manufacturing a steering rack as claimed in claim 3, wherein the at least one clamp unit comprises at least two clamp units adapted to be spaced apart along the toothed region.

8. A method of manufacturing a steering rack as claimed in claim 2, wherein the toothed region is comprised of a plurality of gear teeth each having two opposite flanks, and the clamping device is adapted to position the toothed member using the flanks as a datum.

9. A method of manufacturing a steering rack as claimed in claim 2, wherein the toothed region is comprised of a plurality of gear teeth each having a tip, and the clamping device is adapted to position the toothed member using the tips as a datum.

10. A method of manufacturing a steering rack as claimed in claim 1 , wherein the welding operation comprises friction welding by means of rotating the toothed and tubular members relative to each other whilst pushing them together.

11. A clamping device for use in the manufacture of a steering rack for a vehicle rack and pinion steering gear, the steering rack comprising a toothed member having a toothed region, and a tubular member having a longitudinal axis, the toothed member and the tubular member being welded together by a welding operation, the clamping device being adapted to hold the toothed member during the welding operation using the toothed region as a datum, the clamping device comprising at least one clamp unit, the clamp unit comprising a support member adapted to support at least a portion of the toothed region, and a clamp member adapted to urge the toothed member into contact with the support member.

12. A clamping device as claimed in claim 11, wherein the clamping device further comprises an end stop adapted to support an end of the toothed member during the welding operation.

13. A clamping device as claimed in claim 12, wherein the clamping device further comprises a base, the end stop being fixed to the base and the clamp unit being movable relative to the base in an axial direction parallel to the longitudinal axis.

14. A clamping device as claimed in claim 13, wherein a spring means resists movement of the clamp unit in the axial direction, relative to the base.

15. A clamping device as claimed in claim 11 , wherein the at least one clamp unit comprises at least two clamp units adapted to be spaced apart along the toothed region.

16. A clamping device as claimed in claim 11 , wherein the toothed region is comprised of a plurality of gear teeth each having two opposite flanks, and the clamping device is adapted to position the toothed member using the flanks as a datum.

17. A clamping device as claimed in claim 11 , wherein the toothed region is comprised of a plurality of gear teeth each having a tip, and the clamping device is adapted to position the toothed member using the tips as a datum.

18. A clamping device as claimed in claim 11, wherein the welding operation comprises friction welding and the clamping device is adapted to be mounted onto a friction welding machine.