FIELD OF THE INVENTION

The present invention relates generally to heavy transportation vehicles and more particularly to a steering and suspension system for such vehicles.

The invention has been developed primarily for use in relation to heavy road transport vehicles such as trucks and semi-trailers and will be described hereinafter with reference to this application. It will be appreciated, however, that the invention is not limited to this particular field of use.

BACKGROUND OF THE INVENTION

Heavy road transport vehicles such as trucks and semi-trailers typically incorporate two or more spaced apart axles, often grouped together in sets of two or three to increase load bearing capacity. Each axle

supports corresponding pairs of road wheels and the axles of each group or set are usually spaced apart by a distance marginally greater than the wheel diameter. The axles are normally mounted to the vehicle chassis by suspension assemblies which prevent relative movement in horizontal planes, but allow a limited degree of controlled movement in vertical planes to accommodate variations in the road surface. Throughout this suspension movement, however, the axles when viewed from above remain substantially parallel and steering is normally only effected by the front wheels of the prime mover.

Because the vertical planes defined by the non-steering axles are effectively fixed in parallel relationship, when cornering the axes of rotation of the wheels cannot all be aligned with the centre of the vehicle's turning circle. Consequently, the associated road wheels must inevitably "scrub" across the road surface whenever the vehicle turns. This occurs irrespective of the turning radius and vehicle speed, although it becomes less obvious as the turning radius increases.

This scrubbing effect is a major cause of tyre wear, which is a substantial component of heavy vehicle running costs. The scrubbing phenomenon also decreases rolling efficiency thereby increasing fuel costs, impedes manoeuvrability particularly at low

speeds, and makes the vehicle generally more difficult to drive. Worst still, the resultant loadings in the vehicle suspension and chassis have been known to crack axles, break centre bolts and permanently bend suspension and chassis components. Furthermore, the tyre scrubbing causes premature wear and damage to the road over which the vehicle passes.

In an attempt to ameliorate these problems, various steering mechanisms have been proposed. In one such mechanism each axle or axle group is supported on a sub-frame which is pivoted about a vertical axis in concert with the main steering wheels by a linked steering mechanism. Such mechanisms have, however, proven to be excessively complex, expensive, and unacceptably prone to failure.

In another known system, a sub-frame associated with each axle is supported for rotation about a vertical axis disposed forwardly of the central axle, such that the sub-frame operates as a castor mechanism tending to align the associated trailing axle with the centre of the vehicle's turning circle. These axles are sometimes referred to as "self-steering" axles. The major problem with such axles, however, is that the system of forces tending to steer each trailing axle with the vehicle travelling in the forward direction becomes unstable when the vehicle is reversed, tending to rotate the entire sub-frame and

axle assembly through an angle of 180°. In order to control this, it is necessary to lock the sub-frame to the vehicle chassis in a centred orientation and thereby deactivate the inherent self-steering mechanism whenever the vehicle is reversed. However, this immediately re-introduces the problem of tyre scrubbing whenever the vehicle is reversed. This system also poses other problems because of the necessity to centre each sub-frame before being locked in the reverse configuration. These problems make low speed manoeuvring particularly difficult.

It is an object of the present invention to overcome or substantially ameliorate at least some of these disadvantages of the prior art.

DISCLOSURE OF THE INVENTION

Accordingly, the invention as presently contemplated consists in a self-steering axle assembly for a vehicle, said assembly comprising: a sub-frame supporting an axle; first pivot means engageable selectively to permit relative rotation between the sub-frame and the vehicle body about a first substantially vertical pivot axis disposed forwardly of said axle; second pivot means engageable selectively to permit relative rotation between the sub-frame and the vehicle body about a second substantially vertical

pivot axis disposed rearwardly of said axle; and selector means operable to engage the first pivot means and to disengage the second pivot means during forward movement of the vehicle, and operable to engage the second pivot means and to disengage the first pivot means during reverse movement of the vehicle; such that resultant castoring forces tend to align the axle with the centre of the vehicle's turning circle during both forward and reverse movement of the vehicle.

Preferably, a turntable is disposed intermediate the sub-frame and the vehicle chassis and said first and second pivot means define respective pivot axes of the turntable. The turntable preferably comprises a pair of substantially flat plates disposed in overlapping relationship for sliding contact during relative rotation about the respective pivot axes. The plates are preferably formed from a tough relatively rigid plastics material such as polyester or polycarbonate. A particularly preferred material is marketed in Australia under the trade mark "RALLOY".

Preferably also, the turntable incorporates abutment means disposed to limit the maximum rotational displacement of the sub-frame with respect to the vehicle chassis. One preferred form of abutment means comprises a locating pin extending from

one plate of the turntable into a corresponding oversized aperture or slot formed in the other. The upper plate of the turntable is preferably attached to the vehicle chassis and the lower plate attached to the sub-frame, both by counter-sunk screws or other suitable means such that the overlapping surfaces of the plates are substantially smooth and unobstructed.

The sub-frame is preferably fabricated from steel or aluminium and supports the axle by means of an hydraulic suspension system comprising a series of double acting hydraulic rams and associated accumulators or reservoirs. The sub-frame may support a group of two or more axles, and two or more such sub-frames may be provided if desired.

Each pivot assembly preferably comprises a first vertically extending bush disposed on a bridge member supported below the vehicle chassis. A second support bush coaxial with the first is disposed on an upper frame assembly above the turntable plates. Each pivot assembly further comprises a retractable pivot pin adapted in the operative position to extend through the top and bottom bushes and through corresponding aligned holes formed in the intermediate turntable plates to define the respective first and second pivot axes.

The selector preferably comprises a rocker beam or other linkage mechanism whereby the respective

pivot pins are alternately raised and lowered in concert, such that when one pin is raised the other is lowered and vice versa. In another embodiment, a pair of independently actuable selectors may be provided whereby both pivot pins can may be engaged simultaneously to lock the turntable in a central position.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:-

Figure 1 is a diagrammatic plan view showing a triaxle suspension assembly for a semi-trailer embodying the self-steering mechanism of the present invention;

Figure 2 is a diagrammatic side elevation showing the suspension assembly of Figure 1;

Figure 3 is an enlarged side elevation showing one of the axle assemblies of Figures 1 and 2 in more detail;

Figure 4 is a front elevation of the suspension assembly; and

Figure 5 is a diagrammatic side elevation showing an alternative selector mechanism.

PREFERRED EMBODIMENTS OF THE INVENTION

Referring to Figures 1 to 4 , wherein corresponding features are denoted by corresponding reference numerals, the invention provides a self-steering axle assembly 1 adapted for mounting to the chassis 2 of a vehicle. In Figure 1, three such axle assemblies are shown in a triaxle configuration of the type used in many trucks and semi-trailers.

Each assembly 1 comprises an independent sub-frame 5 supporting an axle 6. Each axle in turn supports a set of road wheels 7. As best seen in Figure 3 , each sub-frame 5 comprises a pair of vertically extending support arms 8 and a corresponding pair of triangulated tie rods 9. The tie rods are connected centrally to the axle by means of an axle mounting bracket 10. The axle is further located by drag links 11 which run parallel to the chassis rails and an hydraulic suspension units 12. Each suspension unit consists of double-acting hydraulic ram interconnected with one or more associated accumulators or reservoirs. Separate reservoirs can be provided for both compression and rebound damping and in the preferred embodiment, these accumulators or reservoirs are built into the hydraulic rams. Each ram is connected to the axle by means of a spherical bearing, "rose joint" or other suitable form of rod end. In the preferred

embodiment, rose joints manufactured substantially from "RALLOY" are used.

Each sub-frame 5 is connected to the vehicle chassis 2 by means of an intermediate turntable 15. The turntable comprises an upper turntable plate 16 mounted to the vehicle chassis rails by a support frame and a corresponding lower turntable plate 17 mounted to the sub-frame. The turntable plates are disposed in overlapping relationship for sliding contact during relative rotation of the sub-frame with respect to the vehicle chassis, as described in more detail below. The plates may be formed from any suitable material but are preferably formed from a tough relatively rigid plastics material such as polyester or polycarbonate. The particularly preferred material is marketed in Australia under the trade mark "RALLOY" . All mounting screws and other fittings are countersunk or recessed as appropriate into the respective turntable plates such that the mating surfaces are smooth and unobstructed. As best seen in Figure 3 , the turntable incorporates respective first and second pivot assemblies 20 and 21. Each pivot assembly comprises a lower vertically extending bush 22 supported by an underslung bridge rail 23 fixedly mounted to the chassis rails 2. An upper support bush 24 coaxial with its lower counterpart is disposed on a fixed

upper frame assembly 25 associated with the vehicle chassis. The upper and lower turntable plates 16 and 17 incorporate corresponding holes alignable with the top and bottom bushes 22 and 23.

The first and second pivot assemblies further comprise respective pivot pins 28 and 29. Each pivot pin is adapted in its operative position to extend through the corresponding top and bottom support bushes 22 and 24 and through the aligned holes formed in the intermediate turntable plates 16 and 17. In this way, the first pivot 20 when engaged defines a first substantially vertical pivot axis 30 about which the turntable 15 and hence the lower sub-frame assembly 5 can rotate. Similarly, the second pivot 21 when engaged defines a second substantially vertical pivot axis 31 for the turntable.

The assembly further comprises selector means 35. In the embodiment of Figures 1 to 4 , the selector means comprises a pair of independently operable pneumatically assisted hydraulic brake units or "maxi-brakes" 36 operable via intermediate bellcrank linkages 37 to withdraw the respective pivot pins 28 and 29. Thus, as best seen in Figure 3, as the actuating arm of the rear maxi-brake is extended, the associated bellcrank rotates anti-clockwise (when viewing the drawing) to extract the pivot pin 29 associated with the second pivot assembly 21.

Similarly, with reference to the first pivot assembly 20, retraction of the actuating arm of the associated maxi-brake engages the pivot pin 28 whereby the first pivot axis 30 becomes the operative rotational axis of the turntable. Conversely, when the second pivot assembly is engaged and the first pivot disengaged, the turntable rotates about the second rotational axis 31.

Figure 5 shows an alternative form of selector mechanism, whereby the pivot pins 28 and 29 of the respective first and second pivot assemblies 20 and 21 are engaged and retracted by a single intermediate rocker arm 38. With this system, when one pivot pin is raised, the other is correspondingly lowered and vice versa. This is a simpler mechanical arrangement operable by a single actuator and can replace the dual maxi-brake and bellcrank assemblies of the previously described embodiment. It will be appreciated, however, that any other suitable mechanism including cams, pneumatic or hydraulic cylinders, motors, rockers, or other linkages may also be used.

Turning now to describe the operation of the invention, during normal forward movement of the vehicle, the first pivot 20 is engaged whilst the second pivot 21 is retracted. In this configuration, the upper and lower turntable plates permit relative rotation between the sub-frame 5 and the vehicle

chassis about the corresponding first pivot axis 30. Importantly, this axis is disposed forwardly of the axle 6 such that as the vehicle begins to turn in direction T, a castoring force indicated as F in Figure 1 causes the sub-frame 5 to rotate, tending to align the axle with the centre of the vehicle's turning circle. The extent of rotation will depend upon the relative position of the axle on the vehicle, and on the radius of the turning circle. In all cases, however, the castoring action operates to align each trailing axle with the centre of the turning circle.

When the vehicle is to be reversed, the first pivot assembly 20 is retracted and the second pivot 21 engaged, whereby the axis of rotation 31 of the turntable is effectively displaced to a corresponding position behind the axle. In this configuration, as the vehicle moves in reverse, the axle similarly trails the effective pivot axis of the sub-frame to produce an analogous system of castoring forces.

The selector mechanisms actuating the first and second pivots may be linked to the vehicle's gear selection mechanism such that the first pivot is automatically operative whenever a forward gear is selected and the second pivot operative as appropriate upon selection of any reverse gear. Alternatively, the selection mechanism may be manually operable,

controlled in response to the direction of wheel rotation, or be controlled by any other suitable means.

Advantageously, the embodiment of Figures 1 to 4 permits both pivot assemblies to be engaged simultaneously, thereby locking the turntable and the sub-frame 5 in a central position if desired.

It will be appreciated that by providing a self-steering mechanism which permits each of the normally fixed axles to align itself automatically with the centre of the vehicle's turning circle, the invention virtually eliminates tyre scrubbing and the associated problems of excessive tyre wear, excessive road wear, increased fuel costs, damage to suspension components, reduced manoeuvrability, and the like. Moreover, the invention provides a simple, efficient and effective mechanism whereby these advantages are achieved throughout all forward and reverse movements of the vehicle. Thus, the invention represents a commercially significant improvement over the prior art.

Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.