This invention relates to a tractor/trailer combination comprising a tractor unit (e g a truck) and a trailer unit

Coupling an undπven trailer to a truck or tractor causes an increase in the resistance to travel, without any compensation increase in tractive force This can significantly reduce the ability of the tractor to traverse adverse grades or soft ground It would therefore be desirable to be able to improve this ability by the addition of the trailer, rather than detracting from it

The present invention provides a tractor/ trailer combination comprising a tractor unit and a trailer unit, characterised by means for alternately forcibly increasing and forcibly decreasing the distance between the units

In one aspect the invention provides a tractor'trailer combination comprising a tractor unit, a trailer unit, and an extensible and retractable linear drive connecting the units, and control means for alternately extending and retracting the linear drive so as to alternately increase and decrease the distance between the units

In another aspect the invention provides a method of assisting the progress of a tractor/trailer combination comprising a tractor unit and a trailer unit, the method comprising alternately forcibly increasing and forcibly decreasing the distance between the units

The inventor has found that increasing and decreasing the distance between a tractor unit and a trailer unit in a series of pulses can have the advantageous effect that motion in the direction of travel (forward or reverse) is maintained, where it might otherwise be halted owing to the terrain Alternativ ely, it is possible to increase the stroke length and move step-wise, the tractor unit and the trailer unit being stationary alternately

The invention will be described further, by way of example, with reference to the accompanying drawings, in which

Figure 1 is a side view of a tractor/trailer combination, with a linear drive fully retracted,

Figure 2 is a side view of the combination, with the linear drive fully extended,

Figure 3 is a side view of the tractor unit separated from the trailer unit, and

Figure 4 is a side view of the trailer unit separated from the tractor unit.

The tractor/trailer combination illustrated comprises a tractor unit (truck) 1 with all-wheel drive from an internal combustion engine, and a trailer unit (trailer) 2 with an extensible drawbar 3 which is coupled to a variable-height hook 4 (pivotally mounted on the rear of the frame of the truck 1)

The drawbar 3 is extended and retracted by means of a linear drive in the form of a hydraulic double-acting piston-and-cylinder device 5 pin-jointed to the drawbar 3 at its rod end 6 and to the frame of the trailer 2 at its cylinder end 7 A control unit 1 1 including a valve controls delivery of hydraulic oil from an engine-driven hydraulic pump 10 From the control unit 1 1 the hydraulic fluid is delivered to the piston-and- cylinder device 5 through two flexible hydraulic pipes 12, 13

The control unit 1 1 is operated from the driver's cab of the truck 1 The driver selects the amplitude of the drawbar extension-retraction cycle, typically within a range from 50 mm to 4000 mm, and the control unit 1 1 automatically controls the flow through the pipes 12, 13 accordingly The frequency of the cycle will normally depend on the amplitude, since extension and retraction of the hydraulic piston-and-cylinder device proceed at substantially constant speeds, extension normally being slower than retraction

In conditions where the truck provides (or appears likely to provide) insufficient tractive force to propel the tractor/trailer combination, the driver selects a pulsing short stroke For example, assuming that it takes 12 seconds to extend the drawbar 3 by 4000 mm and 8 seconds to retract the drawbar, if the stroke length (amplitude) is selected to be 50 mm. the frequency of pulsing will be about 4 cycles per second If the stroke length is 200 mm, the frequency is about 1 cycle per second At such frequencies, the push-pull effect will be virtually indistinguishable from the background vibrations of the truck due to the terrain

The trailer 2 momentarily pushes the truck 1 and is then pulled by the truck 1, which slows down but does not stop The trailer brakes are automatically applied during the extension phase, using the hydraulic pressure exerted in extending the linear drive device 5, under control of the control unit 1 1 Similarly, the truck brakes may optionally be applied during the retraction phase Braking of the truck 1 is usually only required if a long stroke (e g more than one metre) has to be selected and the truck is unable to proceed without assistance from the trailer To further extend the tractive capability, in extremely low coefficient of adhesion ground conditions, a ground anchor 14 on the trailer 2 and a ground anchor 15 on the truck 1 (each comprising a pivotally- mounted rearwardly directed strut or struts) can be lowered to resist backward motion Each anchor 14, 15 drags over the ground during the forward motion In these conditions, using the maximum stroke length of e g 4 m, with an extension time of 12 s and a retraction time of 8 s, it is possible to progress at a speed of at least 12 m/min

Although the above description is directed to forward travel, it will be apparent that the tractor/trailer combination can be driven in reverse, with appropriate arrangement of ground anchors and truck or trailer braking if required

To detach the trailer 2, the drawbar 3 is extended (Figure 4) and a support 8 pivotably mounted on it is lowered The hook 4 on the truck 1 is then lowered until it disengages from an eye 9 at the end of the drawbar 3 (Figure 3)

Thus, as described above, a tractor and undriven trailer are provided in a combination in which the distance between the tractor and trailer wheels is alternately increased and decreased forcibly, by a hydraulically extendible drawbar, alternately increasing and decreasing the tractive requirement of the tractor in such a way that forward (or rearward) motion is maintained, where it might otherwise be halted. A long-stroke feature is also provided, which alternately stops the tractor and the trailer, for even worse tractive conditions, assisted where necessary by land anchors.