GB2082988A

GB2082988A - A Drawbar Trailer

Info

Publication number: GB2082988A
Application number: GB8020998A
Authority: GB
Original assignee: REDMENT ENG Ltd
Current assignee: REDMENT ENG Ltd
Priority date: 1980-06-26
Filing date: 1980-06-26
Publication date: 1982-03-17
Also published as: FR2485450A1; DE3109925A1

Abstract

A drawbar trailer (2) has forward wheels (6), steerable rearward wheels (8) and a drawbar (10) with articulation means (12) for connecting the trailer (2) to a towing vehicle, and operable in response to relative rotation between the towing vehicle and the drawbar, to steer the rearward wheels in accordance with the path of the towing vehicle. <IMAGE>

Description

SPECIFICATION A Drawbar Trailer The present invention relates to a drawbar trailer comprising forward wheels, steerable rearward wheels and means for articulating the trailer to a towing vehicle.

A known close-coupled trailer has wheels which are not steerable and are located towards the centre of the trailer and a drawbar for connection to a towing vehicle which is so fixed to the trailer body as to be immovable relative to the trailer body in a horizontal plane. The drawbar may comprise two drawbar members which converge to an apex where the drawbar is pivotally coupled to the towing vehicle.

Such an arrangement enables the clearance between the trailer and the towing vehicle to be kept to minimum. Also, since the drawbar at the forward end of the trailer is immobile relative to the trailer body in a horizontal plane the trailer has the advantage that it can be reversed by the towing vehicle relatively easily.

Such close-coupled trailers have both legal and practical drawbacks. Under certain regulations they cannot have a load carrying length of more than 7 metres. This restricts the load that can be carried by the trailer. Also under these regulations if, when the trailer is carrying an evenly distributed load, more than 1 5% of the gross weight acts downwardly on the coupling between the trailer and the towing vehicie, the overall length of the towing vehicle and trailer is restricted to 1 5 metres, whereas if the trailer has its wheels at the forward the rearward ends the overall length of the towing vehicle and trailer can normally be up to 18 metres.

Another disadvantage of a close-coupled trailer is that, because its wheel-carrying axles are located near to each other and in the middle region of the trailer, the trailer is not very stable and a seesawing motion may be set up when the towed trailer hits a bump or pothole. This motion may impose a severe strain on the drawbar of the trailer, the towing coupling and the towing vehicle. Also, by having the wheels located in the middle region of the trailer the tyres are subject to excessive wear when turning corners. Spacing the wheels further apart longitudinally of the trailer provides better stability of the trailer but results in greatly increased tyre wear when cornering since the wheels are not steerable. Another disadvantage is that when a loaded trailer is uncoupled from the towing vehicle great care must be taken due to the inherent instability of the trailer.Furthermore, when turning a corner the trailer follows a different path from that of the towing vehicle turning on a smaller circle. If care is not taken the trailer wheels may for example mount a pavement.

Another known type of trailer is sometimes referred to as a full drawbar trailer. The axles carrying the wheels of this trailer are spaced from each other towards the forward and rearwards ends of the trailer, which makes the trailer more stable than a close-coupled trailer. The front wheels and axle together with the drawbar are pivotally mounted to the trailer by means of a free pivot. Such a full drawbar trailer can be 1 2 metres long and can be easily coupled to and uncoupled from a towing vehicle since no downwardly acting load forces are exerted on the drawbar. By suitable design, it can be made to follow exactly the path of the towing vehicle when being towed forwards.

A disadvantage of the full drawbar trailer is that it is pivotable at two places; one where the drawbar is coupled to the towing vehicle. and the other at the free pivot mentioned above connecting the front axle and wheels together with the drawbar to the trailer. This means that the trailer is extremely difficult to reverse and in some circumstances reversing can be impossible.

Since it is pivotable at two places, adequate clearance must be provided betwecn the trailer and the towing vehicle especially for reversing.

This requirement restricts the available load carrying space on the trailer.

According to the present invention there is provided a drawbar trailer comprising forward wheels, steerable rearward wheels, and means for articulating the drawbar of the trailer to a towing vehicle about a single axis which is substantially vertical when the trailer is on a horizontal surface, which articulating means includes a device responsive to relative rotation between the towing vehicle and the drawbar, which device is connected to means for steering the rearward wheels and is operable to cause positive steering of the rearward wheels in accordance with the path of the towing vehicle.

A trailer embodying the present invention can have a length of up to 12 metres or possibly more, depending on the legally allowed maximum. Also it is stable both when loaded and unloaded since it has forward the rearward wheels.

Furthermore by so articulating the trailer to the towing vehicle that relative rotation between the trailer and the towing vehicle can take place only about the one substantially vertical axis, the trailer can be relatively easily reversed by the towing vehicle.

The articulating means may comprise a coupling king pin on the drawbar for retention in a coupling device provided on the towing vehicle, a pinion rotatable about the axis of the king pin, and a peg engageable with the coupling device and rotatable with the pinion on relative rotation between the trailer and the towing vehicle, which pinion meshes with a rack which is fixed to an hydraulic cylinder which is slidable by movement of the rack over a fixed piston, and hydraulic lines connect outlets from the ends of the cylinder to hydraulic steering means for the rearward wheels.

The use of hydraulic devices for transmitting the steering power has the advantages that they can be robustly constructed and are not particularly susceptible to dirt which inevitable accummulates from the road surface.

Preferably the hydraulic steering means for the rearward wheels comprises an hydraulic steering cylinder located adjacent the rearward wheels, a steering piston slidable within the cylinder and two piston rods extending from opposite ends of the cylinder and respectively connected to steering arms for the rearward wheels, the hydraulic lines being connected to the opposite ends of the steering cylinder to move the steering piston in response to movement of the rack when the towing vehicle changes direction.

The use of an hydraulic piston and cylinder enables a direct hydraulic connection to be effected between the articulation means and the steering means so that no separate source of steering power is required.

Although hydraulic means are preferred it may be possible to use other means such as a mechanical linkage or electrical devices which respond to the relative rotation between the trailer and the towing vehicle. If electrical devices are used, however, a separate source of power would be required to steer the rearward wheels.

Although it is preferable to steer each rearward wheel about its own pivot to enable Ackerman steering to be provided and thereby minimise tyre wear, it is possible to steer the rearward wheels by mounting an axle carrying the rearward wheels on a turntable which is pivotable about a vertical axis intersecting the longitudinal axis of the trailer.

This system does not have the Ackerman effect and would therefore cause accelerated tyre wear.

The forward wheels of the trailer may also be steerable by connecting them into the means for positively steering the rearward wheels so that the forward wheels also steer in accordance with the path of the towing vehicle.

Alternatively the forward wheels of the trailer may be pivotally mounted to follow approximately the path of the towing vehicle by caster action.

In which case in order to facilitate reversing of the trailer by the towing vehicle means for locking the pivotal mounting of the forward wheels may be provided for use when the trailer is reversed by the towing vehicle.

An embodiment of the present invention will now be described by way of example, with reference to the accompanying drawings in which: Figure 1 shows schematically a side elevation of a drawbar trailer embodying the present invention, Figure 2 shows a plan view of the trailer shown in Fig. 1, Figure 3 shows a perspective view of the articulation means of the trailer shown in Figures 1 and 2, and Figure 4 shows a rearward axle of the trailer shown in Figures 1 and 2, illustrating hydraulic steering means for the rearward wheels.

Referring to the accompanying drawings, Figures 1 and 2 show a drawbar trailer 2 comprising a load carrying body 4, forward wheels 6, steerable rearward wheels 8 and a drawbar 10. The drawbar 10 is provided with articulation means 12 for connecting the trailer 2 to a towing vehicle (not shown). The drawbar is pivotally connected to the forward end of the trailer body 4 about a horizontal axis only so that it is immovable relative to the trailer body 4 in a horizontal plane when the trailer is on a horizontal surface. This enables the trailer to be articulated only about articulation means 12 when it is being towed around a.corner or curve and provides for relatively easy reversing of the trailer by the towing vehicle.

The drawbar 10 can swing about its horizontal axis to accommodate holes and bumps in the road surface.

The articulation means 12 is shown in greater detail in Figure 3 and comprises a king pin 14 which is welded to the underside of a trailer rubbing plate 1 5 of the drawbar 10 and is adapted to be retained by jaws of a coupling device such as a "Fifth Wheel" (not shown) mounted on the towing vehicle. The trailer rubbing plate is provided with a further pin (not shown) which extends upwardly from the plate 1 5 and on which is mounted a carrier member 17 which is free to rotate relative to this further pin.

A pinion 18 is fixed on the carrier member 17 and a peg 1 9 is held in a slot in the carrier member 1 7. The peg 1 9 extends downwardly and is located in a V-shaped recess in the coupling device. As the direction of the towing vehicle changes the peg 1 9 rotates with the coupling device 1 6 in accordance with the change in direction of the towing vehicle. This rotation of the peg 19 cause the pinion 18 amd the carrier member 17 to rotate. The pinion 1 9 meshes with a movable rack 20 which carries an hydraulic cylinder 21.A piston 22 is located within the cylinder and is connected at opposite sides to piston rods 24 which protrude through the sealed ends of the hydraulic cylinder 20 and which each have an end fixed to a member 26 of the drawbar 10.

The rotation of the pinion 1 6 causes the rack 18 together with the cylinder 21 to slide longitudinally over the fixed piston 22. Hydraulic fluid in the cylinder is forced by this movement of the cylinder to flow through one of two outlets 28 and 30 and through one of two hydraulic lines 32 and 34 towards an hydraulic cylinder 36 for steering the rearward wheels, shown in Figure 4.

Hydraulic fluid returns through the other line. The lines 32 and 34 through which the fluid flows depends on the direction in which the cylinder 21 moves which in turn depends on the direction of rotation of the pinion and therefore the change in direction of the vehicle.

A piston is located inside the cylinder 36 and has opposite sides connected to piston rods 38 and 40. Each piston rod 38, 40 is connected to a respective steering arm 42, 44. The steering arms 42, 44 are connected to the hubs 46, 48 of the rearward wheels which pivot about king pins 50, 52 which are fixed to the suspension system for the rearward wheels which is of known kind and is indicated generally at 54, 56. Fluid flowing into the cylinder from either line 32 or 34 will displace the piston and the piston rods 38 and 40 in the appropriate direction in response to movement of the rack when the towing vehicle changes direction. The steering arms 42 and 44 pivot on the ends of the piston rods 38 and 40 and cause positive steering of the rearward wheels during towing in accordance with the path of the towing vehicle. Tyre wear is thereby considerably reduced.

Although in the above described embodiment only the rearward wheels are positively steered it is possible to connect the articulation means to similar steering means for the forward wheels as well, so that both forward the rearward wheels are steerable in accordance with the path of the towing vehicle.

Such an hydraulic system has the advantage of being relatively robust and free from attack by dirt from the road surface and also does not require a separate source of power for steering the rearward wheels since steering is effected by the flow of hydraulic fluid between the two cylinders.

Instead of using an hydraulic system for steering, it is possible to use mechanical linkages or electrical sensing devices altough electrical devices would require the use of a separate power source for steering.

Claims

1. A drawbar trailer comprising forward wheels, steerable rearward wheels, and means for articulating the drawbar of the trailer to a towing vehicle about a single axis which is substantially vertical when the trailer is on a horizontal surface, characterised in that the articulation means includes a device responsive to relative rotation between the towing vehicle and the drawbar, which device is connected to means for steering the rearward wheels and is operable to cause positive steering of the rearward wheels in accordance with the path of the towing vehicle.

2. A trailer according to claim 1, characterised in that the articulation means comprises a coupling king pin on the drawbar for retention in a coupling device providedon the towing vehicle, a pinion rotatable about the axis of the king pin, and a peg engageable with the coupling device and rotatable with the pinion on relative rotation between the trailer and the towing vehicle, which pinion meshes with a rack which is fixed to an hydraulic cylinder which is slidable by movement of the rack over a fixed piston, and hydraulic lines connect outlets from the ends of the cylinder to hydraulic steering means for the rearward wheels.

3. A trailer according to claim 2, characterised in that the hydraulic steering means for the rearward wheels comprises an hydraulic steering cylinder located adjacent the rearward wheels, a steering piston slidable within the cylinder and two piston rods extending from opposite ends of the cylinder and respectively connected to steering arms for the rearward wheels, the hydraulic lines being connected to the opposite ends of the steering cylinder to move the steering piston in response to movement of the rack when the towing vehicle changes direction.

4. A trailer according to any one of the preceding claims, characterised in that the forward wheels are pivotally mounted to follow approximately the path of the towing vehicle by caster action.

5. A trailer according to claim 4, characterised by means for locking the pivotal mounting of the forward wheels for use when the trailer is reversed by the towing vehicle.

6. A drawbar trailer substantially as hereinbefore described with reference to the accompanying drawings.