WO2008101291A1

WO2008101291A1 - Rack and pinion steering gear

Info

Publication number: WO2008101291A1
Application number: PCT/AU2008/000229
Authority: WO
Inventors: Andrew James Heathershaw; David Cauchi; John Baxter
Original assignee: Bishop Innovation Pty Ltd
Current assignee: Bishop Innovation Pty Ltd
Priority date: 2007-02-22
Filing date: 2008-02-21
Publication date: 2008-08-28
Anticipated expiration: 2009-08-22

Abstract

A rack and pinion steering gear for a vehicle. The steering gear comprising a rack having rack teeth and a pinion having pinion teeth that mesh with the rack teeth. The rack teeth and the pinion teeth both have a herringbone form. The rack teeth may have a variable ratio form.

Description

RACK AND PINION STEERING GEAR

TECHNICAL FIELD

The present invention relates to rack and pinion steering gears for vehicles.

BACKGROUND

Rack and pinion steering gears for vehicles are well known. This type of steering gear has a pinion with gear teeth that mesh with the gear teeth on a rack. The pinion is connected to a steering wheel and the rack is connected to the steerable wheels of a vehicle, in a well known manner.

Rack and pinion steering gears may be classified as "constant ratio" or "variable ratio". In a constant ratio steering gear, the ratio of the number of turns of the pinion to the displacement of the rack is constant over the range of travel of the rack. In a variable ratio steering gear, the instantaneous ratio of the number of turns of the pinion to the displacement of the rack varies along the travel of the rack

The teeth of a typical pinion for constant ratio and variable ratio steering gears have a helical form for smooth meshing. For a constant ratio steering gear, the rack teeth are straight and they may either be perpendicular or skewed to the axis of the rack depending on the helix angle of the pinion teeth and the angle between the axis of the pinion and the axis of the rack. The form of the rack teeth for a variable ratio steering gear varies along the length of the rack as disclosed in US Patent 3,753,378 (Bishop).

In some applications, it is desirable that the meshing between the pinion and the rack teeth does not apply any axial thrust to the pinion. This problem can be solved by using a pinion with straight teeth instead of helical teeth. However, this degrades the meshing quality and limits the amount of variable ratio that may be used as discussed in US Patent 3,753,378 (Bishop). A herringbone gear is effectively two side by side helical gears of opposite hand. Herringbone gears have the advantage of eliminating axial thrust whilst maintaining the smooth meshing of a helical gear. US Patent 6,912,786 (Jinkins et al) describes herringbone gears in general and discloses a method of making them.

EP 0939249 B1 (Alpha Getriebebau GmbH) and US Patent 4,270,404 (Murakoshi et al) disclose herringbone rack and pinion gearing. However, the use of herringbone gearing for rack and pinion steering gears is not known.

It is an object of the present invention to provide a steering gear that minimises axial thrust applied to the pinion whilst retaining smooth meshing.

SUMMARY OF INVENTION

In a first aspect, the present invention consists of a rack and pinion steering gear for a vehicle, comprising a rack having rack teeth and a pinion having pinion teeth meshing with the rack teeth, characterised in that the rack teeth and the pinion teeth have a herringbone form. Preferably, the rack teeth have a variable ratio form.

In one preferred embodiment, the axis of the pinion is substantially perpendicular to the axis of the rack. In another preferred embodiment, the axis of the pinion is skewed to the axis of the rack.

Preferably, the pinion is supported by bearings that do not axially restrain the pinion. Preferably, the bearings supporting the pinion are needle roller bearings.

Preferably, transition regions blend opposing flanks of the rack teeth and the pinion teeth, the transition regions being shaped such that there is always clearance between the rack teeth and the pinion teeth in the transition regions.

Preferably, the opposing outwards facing flanks of the rack teeth are blended by a first radius and the opposing inwards facing flanks of the pinion teeth are blended by a second radius that is smaller than the first radius, and the opposing outwards facing flanks of the pinion teeth are blended by a third radius and the opposing inwards facing flanks of the rack teeth are blended by a fourth radius that is smaller than the third radius.

In a second aspect, the present invention consists of a rack for a rack and pinion steering gear for a vehicle, the rack having rack teeth, characterised in that the rack teeth have a herringbone form. Preferably, the rack teeth have a variable ratio form.

In a third aspect, the present invention consists of a pinion for a rack and pinion steering gear for a vehicle, the pinion having pinion teeth, characterised in that the pinion teeth have a herringbone form.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 shows a rack and pinion steering gear in accordance with the present invention.

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

Fig. 3 is a sectional view of the steering gear shown in Fig. 1 along the axis of the pinion.

Fig. 4 shows the rack of the steering gear shown Fig. 1.

Fig. 5 is a plan view of the rack shown in Fig. 4.

Fig. 6 is an enlarged plan view of the teeth of the rack shown in Fig. 4.

Fig. 7 shows the pinion of the steering gear shown in Fig. 1.

Fig. 8 is a side view of the pinion shown in Fig. 7. BEST MODE OF CARRYING OUT THE INVENTION

Figs. 1 , 2 and 3 show a rack and pinion steering gear 1 in accordance with the present invention. Steering gear 1 has a configuration that is particularly suited to single seat racing cars. However, the invention is also applicable to steering gears for other vehicles. For simplicity of description, steering gear 1 is a manual steering gear, it being understood that the invention is equally applicable to power assisted steering gears.

Steering gear 1 comprises a rack 2 with rack teeth 3, and a pinion 4 with pinion teeth 5. The rack teeth 3 and pinion teeth 5 mesh together such that rotation of pinion 4 laterally moves rack 2. The axis 6 of rack 2 is perpendicular to the axis 7 of pinion 4. Pinion 4 is connected to a steering wheel (not shown). Each end of rack 2 has an internal thread 13 for the attachment of tie rods (not shown) that are connected to the steerable wheels of a vehicle.

Rack 2 is enclosed in a rack housing 10 with a bush 12 at each end to support rack 2 and allow it to freely move laterally. Pinion 4 is enclosed in a pinion housing 11 and supported by two needle roller bearings 14. Needle roller bearings 14 allow pinion 4 to freely rotate and they do not axially restrain pinion 4. Pinion housing 11 is attached to rack housing 10 by screws 15.

As can be clearly seen in Figs. 7 and 8, pinion teeth 5 have a herringbone form. Pinion teeth 5 comprise two helical regions 18a and 18b joined by a transition region 19. Helical regions 18a and 18b both have the same helix angle with region 18a having a right hand helix and region 18b having a left hand helix. Each tooth 5 has a pair of opposing inwardly facing flanks 23a and 23b, and a pair of opposing outwardly facing flanks 24a and 24b. Transition region 19 has a radiused shape to smoothly blend the opposing flanks of pinion teeth 5, with an internal radius 28 blending the opposing inwardly facing flanks 23a and 23b, and an external radius 29 blending the opposing outwardly facing flanks 24a and 24b. As can be clearly seen in Figs. 4, 5 and 6, rack teeth 3 have a herringbone form. Rack teeth 3 also have a variable ratio form, which in this example provides a higher ratio of pinion turns to rack displacement at the centre of the tooth form than at each end. The variable ratio aspect of teeth 3 can be designed using the principles disclosed in US Patent 3,753,378 (Bishop). Referring to Fig. 6, rack teeth 3 are symmetrical about the rack axis 6. Rack teeth 3 are comprised of two opposite contact regions 20a and 20b joined by a transition region 21. Each tooth 3 has a pair of opposing inwardly facing flanks 25a and 25b, and a pair of opposing outwardly facing flanks 26a and 26b. Transition region 21 has a radiused shape to smoothly blend the opposing flanks of rack teeth 3, with an internal radius 30 blending the opposing inwardly facing flanks 25a and 25b, and an external radius 31 blending the opposing outwardly facing flanks 26a and 26b.

As the pinion teeth 5 mesh with the rack teeth 3, pinion teeth flanks 23a, 23b, 24a and 24b contact rack teeth flanks 26a, 26b, 25a and 25b respectively. Transition regions 19 and 21 are shaped such that there is always clearance between pinion teeth 5 and rack teeth 3 in these regions. This clearance is achieved by making the internal radius 28 blending the opposing inwardly facing flanks 23a, 23b of the pinion teeth 5 smaller than the external radius 31 blending the opposing outwardly facing flanks 26a, 26b of the rack teeth 3, and by making the internal radius 30 blending the opposing inwardly facing flanks 25a, 25b of the rack teeth 3 smaller than the external radius 29 blending the opposing outwardly facing flanks 24a, 24b of the pinion teeth 5. Providing clearance between the transition regions 19 and 21 makes the pinion teeth 5 and rack teeth 3 easier to manufacture and design since it is not necessary to manufacture these regions as accurately as would be required if they were designed to contact each other. The transition regions may have shapes other than simple radiuses whilst still providing clearance between each other.

Unlike a conventional helical pinion, pinion 4 is axially located by the meshing of its herringbone form teeth 5 with the herringbone form teeth 3 of rack 2. This means that the meshing between the pinion teeth 5 and rack teeth 3 does not apply any axial thrust to the pinion 4, which is desirable in some applications as discussed above. Furthermore, the herringbone gearing provides smooth meshing with scope for a significant degree of variable ratio thus overcoming the problem of using a pinion with straight teeth for these applications. Using needle bearings 14 that do not provide any axial restraint means that the axial location provided by the herringbone form teeth does not fight with the bearings supporting pinion 4.

For racing car applications, preferably rack teeth 3 and pinion teeth 5 are manufactured by milling them directly from a solid material using a numerically controlled milling machine with a ball end cutter. However, in other applications the teeth may be forged or cold formed. Die apparatus such as disclosed in US Patent 5,862,701 (Bishop et al) or WO 2005/053875 A1 (Bishop Innovation) may be used to forge herringbone form rack teeth.

Herringbone form rack teeth 3 have a variable ratio form but in other not shown embodiments of the invention the rack teeth may have a constant ratio form. Also, in other not shown embodiments of the invention the axis of the pinion may be skewed to the axis of the rack, in which case either the herringbone form of the rack teeth, pinion teeth, or both will no longer be symmetrical. More specifically, the possible configurations if the pinion axis is skewed to the rack axis are a pinion with symmetric herringbone teeth meshing with a rack with asymmetric herringbone teeth, a pinion with asymmetric herringbone teeth meshing with a rack with symmetric herringbone teeth, and a pinion with asymmetric herringbone teeth meshing with a rack with asymmetric herringbone teeth. In other not shown embodiments of the invention, the pinion teeth and rack teeth may both have an asymmetric herringbone form with the pinion axis being perpendicular to the rack axis.

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 rack and pinion steering gear for a vehicle, comprising a rack having rack teeth and a pinion having pinion teeth meshing with the rack teeth, characterised in that the rack teeth and the pinion teeth have a herringbone form.

2. A rack and pinion steering gear for a vehicle as claimed in claim 1 wherein the rack teeth have a variable ratio form.

3. A rack and pinion steering gear for a vehicle as claimed in claim 1 wherein the axis of the pinion is substantially perpendicular to the axis of the rack.

4. A rack and pinion steering gear for a vehicle as claimed in claim 1 wherein the axis of the pinion is skewed to the axis of the rack.

5. A rack and pinion steering gear for a vehicle as claimed in claim 1 wherein the pinion is supported by bearings that do not axially restrain the pinion.

6. A rack and pinion steering gear for a vehicle as claimed in claim 5 wherein the bearings supporting the pinion are needle roller bearings.

7. A rack and pinion steering gear for a vehicle as claimed in claim 1 wherein transition regions blend opposing flanks of the rack teeth and the pinion teeth, the transition regions being shaped such that there is always clearance between the rack teeth and the pinion teeth in the transition regions.

8. A rack and pinion steering gear for a vehicle as claimed in claim 1 wherein the opposing outwards facing flanks of the rack teeth are blended by a first radius and the opposing inwards facing flanks of the pinion teeth are blended by a second radius that is smaller than the first radius, and the opposing outwards facing flanks of the pinion teeth are blended by a third radius and the opposing inwards facing flanks of the rack teeth are blended by a fourth radius that is smaller than the third radius.

9. A rack for a rack and pinion steering gear for a vehicle, the rack having rack teeth, characterised in that the rack teeth have a herringbone form.

10. A rack as claimed in claim 10 wherein the rack teeth have a variable ratio form.

11. A pinion for a rack and pinion steering gear for a vehicle, the pinion having pinion teeth, characterised in that the pinion teeth have a herringbone form.