WEIGHTTRANSFERCONNECTINGARRANGEMENTFORSEMITRAILER-TRACTOR COMBINATIONS

This invention relates to an arrangement for connecting a trailer to a tractor.

The invention has a particular application to a drawbar for connecting a trailer to a weight transfer hitch attached to a prime mover such as a tractor. There are a number of weight transfer hitches currently on the

market. The purpose of these hitches is to transfer

weight from a trailer onto the driving wheels of a prime

mover which is generally an agricultural type tractor.

In order to function effectively, a weight transfer

hitch must be so constructed as to permit relative freedom of movement between the tractor and the trailer in three senses, namely:-

1. The composite vehicle must be able to change

direction (or steer), which requires a vertical axis

generally known as the steering or 'yawing axis'.

2. The tractor and the trailer must be able to

move independently about a horizontal axis to enable the

composite vehicle to traverse terrain where the surface

is inclined at different angles within the wheel base of

the composite vehicle. This is generally known as the

'rolling axis'.

3. The two components of the composite vehicle

must be able to move independently in a pitching or

rearing sense, in order to traverse surface undulations

which are perpendicular to the longitudinal centre line of the composite vehicle. The axis about which this movement takes place is generally known as the 'humping axis'

Thus, a weight transfer hitch must consist of a mechanism incorporating these three axes, and must be so

arranged that movement about a given axis does not inhibit

movement about the others. During movement about one or

more of these axes, the weight transfer hitch must form

part of a continuous beam which is rigid in the vertical plane.

The principal function of a weight transfer hitch

is to improve the tractive capability of the tractor, by

transferring weight from the trailer (and its payload) onto the driving wheels of the tractor. The increase in tractive capability reduces the tendency for the tractor

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wheels to slip. When the tractor rear wheels start to

slip, there is little or no increase in rear axle torque,

and so the application of additional power to the driving

axle results only in increased slippage. If the tractive

capability of the tractor is increased, by the application

of weight onto the driving wheels, additional power to

those driving wheels results in increased rear axle

torque, the reaction to which tends to cause the front wheels of the tractor to lift. To minimise this undesir- able effect, the point of attachment of the weight transfer

hitch to the tractor is placed as low as possible, and generally somewhat in front of the centre line of the

tractor rear axle. The humping axis of the weight transfer

hitch is the effective location of this point of attach-

ment. The location of the humping axis below, and ahead

of, the tractor rear axle poses practical problems of

clearance under the tractor axle, and obstruction between

the rear of the tractor and the hitch when humping takes

place. Therefore, it is necessary that the hitch extends

rearwards from the humping axis by a distance which is sufficient to ensure clearance of obstructions at the rear of the tractor, and that the humping axis is situated low enough to provide a predetermined humping angle before the hitch makes contact with the underside of the rear

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axle of the tractor.

The principal advantage of locating the humping

axle below the rear axle of the tractor is that the

moment about the point of contact of the rear wheels and

the ground is maintained at the lowest possible level, so

that the torque reaction, which would tend to lift the

front wheels of the tractor, is kept at a minimum.

Weight transfer hitches which are mounted below

the rear axle of the tractor, and extend rearwards as previously described, have certain common characteristics which, in the past, have placed severe limitations on the vehicle designer. The most important of these char¬ acteristics is the transposition of the rolling and humping axes during turns, when the rolling axis (which

lies along the vehicle centre line) begins to share the rolling function with the humping axis which progressively

takes more of the rolling function until, when the tractor

is at 90 to the trailer, the original humping axis takes

over the full rolling function.

Figure 1 of the accompanying drawings shows,

schematically, a weight transfer hitch W attached to a

trailer T by a conventional drawbar D. In Figure 1, Y

indicates the yawing axis of the hitch W, B and C indicate

the points of contact of the wheels of the trailer vehicle

T with the ground, R indicates the centre-line of the

rear axle of a tractor (not shown) attached to the hitch,

R indicates the rolling axis of the hitch for the straigh o ^fe ahead position of the composite vehicle, R _Qn indicates the

rolling axis of the hitch for a 90 left turn, and H indicates the humping axis of the hitch for the straight-

ahead position. In the straight-ahead position, the plane

of support of the trailer is an isosceles triangle ABC,

having its base at the points of contact B and C of the

trailer wheels with the ground, and its apex A on the point of intersection of the yawing and rolling axes Y and R of the hitch W. When a turn is initiated, the o apex of the plane of support moves progressively in the

direction of the turn; and, when the tractor is .lying at 90 to the trailer T, the apex A' of the plane of support

A'BC lies on the original humping axis H which has

become the new rolling axis R _qπ . AS the tractor makes a

90 turn, the apex of the plane of support moves along

an arc between the points*A and A' (A' being the centre

point of the line which was the original humping axis H ). When the composite vehicle is in the straight-ahead

position, and assuming that the load is uniformly distri¬ buted, the centre of gravity G of the trailer T will lie at a point on the perpendicular of the isosceles triangle

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A3C. During a turn, how ^τ ever, the plane of support becomes

distorted, its boundary approaching the centre of gravity

G on the side opposite to the direction of turn. This

distortion reaches its maximum when the 90 position is

reached, where the magnitude of distortion is equal to the

distance AA*. The result is an increased tendency for the

trailer T to overturn in a direction opposite to that in

which the turn is being made. ^■ This effect, together with

the centrifugal forces acting in the same direction,

frequently forces designers to accept compromises which are less than satisfactory; and, at worst, make the use of weight transfer hitches impossible under certain conditions.

One aim of the invention is to provide a drawbar for connecting a trailer to a weight transfer hitch

attached to a tractor, which drawbar is such that the

centre of gravity of the trailer will lie well within the

plane of support even during a 90 turn.

The invention also has a particular application

to goods vehicles for highway operation, generally known

as 'articulated semi-trailers'. Such a vehicle consists of first and second vehicles, the first vehicle being a

purpose-designed road tractor, and the second vehicle

being a trailer pivotally connected to the tractor by

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means of a lockable coupling generally known as a 'fifth

wheel'. The fifth wheel is so constructed as to permit relative movement, in the horizontal plane, of up to 90°

on either side. The trailer may have one or more axles so arranged as to transfer a large part of the weight of

the trailer onto the tractor through the fifth wheel.

Articulated semi-trailers are subject to a phen¬

omenon known as 'jack-knifing'. This occurs, while the

vehicle is in motion, when the longitudinal centre lines of the tractor and trailer are horizontally inclined to

each other, the change of direction taking place about the

vertical centre line of the fifth wheel coupling. Jack-

knifing is initiated by a number of happenings, and is usually accelerated by road conditions which give less than optimum adhesion. Jack-knifing occurs when the kinetic energy in the trailer is applied as a force to the tractor at such an angle, and in such a position, as to cause the rear of the tractor to rotate about (or near) the point of contact of the outside front wheel of the tractor.

The precise point at which jack-knifing commences

is a matter for conjecture, as it is subject to the

relative positions of the tractor and the trailer, the

magnitude of the force applied by the kinetic energy from

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of action of the force approaches the point of contact

of the outside front wheel of the tractor, so does the

tendency to jack-knifing increase; and, once initiated, the action becomes increasingly violent and rapidly uncontrollable. Jack-knifing is the cause of many road accidents with, loss of lives, much injury and damage.

Known anti-jack-knifing systems incorporate frictional

retarding devices. Unfortunately, because of the big

loads involved and the high forces to be absorbed, these

devices are not very effective.

Another aim of the invention is, therefore, to

reduce the incidence of jack-knifing by providing improved

connecting means for an articulated semi-trailer. The present invention provides an arrangement for

connecting a trailer to a tractor, the connecting arrange¬

ment comprising first and second connectors connectible to

the tractor and to the trailer respectively, wherein means are provided for moving the first connector laterally with respect to the central longitudinal axis of the tractor.

Advantageously, said means is such that, as the tractor executes a turn, the first connector is moved into

the turn. This causes the front of the trailer to move

into the turn. Where the tractor and trailer constitute

an articulated semi- railer, this lateral movement of th

first connector diverts the line of action of the kineti

force applied from the trailer to the tractor in such a

way that the tendency of the tractor to rotate in respons

to this force is reduced, thereby reducing the possibilit

of jack-knifing. Moreover, where the trailer is connecte

to the tractor via a weight transfer hitch, this lateral movement of the first connector results in the centre of

gravity of the trailer following the movement of the centre-line of the plane of support.

Preferably, said means is such that the movement of the first connector is proportional to the angle throu

which the tractor is turned. Where the trailer is cormec ted to the tractor by a weight transfer hitch, this ensures that the centre of gravity of the trailer always

lies substantially on the centre-line of the plane of support.

In a preferred embodiment, a drawbar constitutes the connecting arrangement, the drawbar having the first

connector at one end thereof, the other end of the drawba

being provided with a third connector for attachment to

a weight transfer hitch attached to the tractor. Advan¬

tageously, the drawbar is constituted by a beam, and a

hydraulic ram constitutes said means. In order that the

ram can control the lateral movement of the trailer for both right-hand and left-hand turns, the ram is conven¬ iently a double-acting hydraulic ram.

The ram may be pivotally attached to the beam and

pivotally attachable to a connection point on the trailer.

Preferably, how ^τ ever, the beam is pivotally attached to a

strut which is attachable to the trailer, the strut

extending laterally with respect to the longitudinal axis of the beam and constituting the second connector,

and the hydraulic ram being pivotally mounted at an

angle to the longitudinal axis of the beam between the

beam and the strut.

The drawbar may further comprise a sensor for measuring the angle of turn and for controlling the amount of pressurised hydraulic fluid supplied to the ram.

Alternatively, the drawbar may further comprise a hydraulic master cylinder for controlling the hydraulic ram, the master cylinder being actuated by a mechanical linkage which is moved in dependence upon the angle of the turn.

In this case, a rack-and-pinion associated with the

weight transfer hitch may constitute the mechanical

linkage.

In another preferred embodiment, the drawbar is

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constituted by a cross-piece and a pair of parallel arms,

the cross-piece being pivotally attachable to the weight

transfer hitch, and the two arms extending rearwardly

from the opposite ends of the cross-piece, and wherein

the free ends of the arms are pivotally connectible to

the trailer so that the cross-piece, the two arms, and

the trailer define a parallelogram linkage, the pivotal

connection between the arms and the trailer constituting

the second connector. In this case also, a hydraulic ram constitutes said means, the hydraulic ram being pivotally connected across a diagonal of the parallelogram linkage.

Preferably, the drawbar further comprises a fail¬ safe device for moving the drawbar into a predetermined

position in the event of failure of said means.

In a preferred form of connecting arrangement to be incorporated in an articulated semi-trailer as a jack- knife preventing device, the first connector is a fifth wheel attachable to the tractor, and the second connector is a king pin attachable to the trailer. Advantageously, the fifth wheel is fixed to a frame which is pivotally

attachable to the chassis of the tractor by means of an

arm and a vertical pivot joint.

Preferably, the fifth wheel is movable in an arc

about the axis of the pivot joint. In this case, the

arrangement may further comprise an arcuate track which

is fixable to the chassis of the tractor, the frame being

provided with roller means which engage with the track.

Advantageously, a double-acting hydraulic ram

constitutes said means, the hydraulic ram being pivotally

attached to the arm and being pivotally attachable to

the chassis of the tractor, the hydraulic ram being

angled relative to the longitudinal axis of the arm. Preferably, the arm is connected to a cross-member by the pivot joint, the cross-member being fixable to the

chassis of the tractor transversely with respect to the

longitudinal axis of the tractor, and the hydraulic ram

is pivotally attached to the cross-member adjacent to

one end thereof.

The arrangement may further comprise sensing means

for sensing the angle of turn of the trailer relative to

the tractor, and for controlling the amount of pressurised hydraulic fluid supplied to the hydraulic ram. Advantage- ously, the sensing means is constituted by a cam fixable to the front of the trailer, and a cam follower moun¬

ted on the fifth wheel. Preferably, the sensing means is such that the angular movement of the cam is converted into linear movement of the cam follower, and

the linear movement of the cam follower is transmitted, via a rack-and-pinion drive, to a hydraulic control unit

for controlling the amount of pressurised hydraulic fluid

supplied to the hydraulic ram. Conveniently, the hydrauli

control unit comprises a diverter valve and a proportional

valve.

Preferably, the diverter valve is a flow control

valve having two outputs connected to the opposite ends

of the cylinder of the hydraulic ram, two inputs connected

to the proportional valve, an input leading from a source

of hydraulic fluid, and an input leading back to said source, the hydraulic control unit being such that hyd¬ raulic fluid passes to the output side of the diverter valve only after passing through the proportional valve.

The diverter valve and the proportional valve may be controlled by a mechanical linkage driven by the rack- and-pinion drive.

The invention also provides a tractor-trailer

combination comprising a tractor connected to a trailer

by means of a weight transfer hitch and a drawbar, the weight transfer hitch being attached to the rear of the

tractor, and the drawbar being attached to the front of

the trailer, the trailer being of the type having a

pair of wheels provided at the rear thereof, the weight

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transfer hitch having a rolling axis, a humping axis

and a yawing axis, and the trailer having a triangular

plane of support defined by the points of contact of

the wheels of the trailer with the ground and the point

of intersection of the rolling and yawing axes of the

weight transfer hitch, wherein the drawbar is provided with means for maintaining the centre of gravity of the

trailer within the plane of support irrespective of the relative orientation between the tractor and the trailer. The invention further provides an articulated semi-trailer comprising a tractor connected to a trailer by means of a connecting arrangement, the connecting arrangement having a fifth wheel mounted on the chassis

of the tractor, and a king pin attached to the front

of the trailer, wherein means are provided for moving the engaged fifth wheel and king pin laterally with

respect to the central longitudinal axis of the tractor.

The invention will now be described in greater

detail, by way of example, with reference, to Figures

2 to 11 of the accompanying drawings, in which:-

Figure 2 is a schematic plan view of a tractor-

trailer combination incorporating a weight transfer hitch and a first form

of drawbar constructed in accordance

with the invention; Figure 3 is a schematic side elevation of a tractor- trailer combination incorporating a weight transfer hitch and a second form of drawbar constructed in accordance

with the invention;

Figure 4 is a schematic plan view of the arrangement

shown in Figure 3;

Figure 5 is a schematic plan view of an articulated

semi-trailer incorporating a simplified

form of connecting arrangement constructed

in accordance with the invention; Figure 6 is a schematic partial plan view of the

articulated semi-trailer of Figure 5, and

shows the forces which tend to lead to jack-knifing;

Figure 7 is a plan view, on a larger scale, of a

practical form of connecting arrangement

constructed in accordance with the inven¬

tion; Figure 8 is a side elevation of the connecting

arrangement shown in Figure 7;

Figure 9 is a partial plan view showing the fifth

wheel and associated control apparatus of

the control arrangement of Figures 7 and 8;

Figure 10 is a side elevation corresponding to

Figure 9; and

Figure 11 is a hydraulic circuit diagram.

Referring to the drawings , Figure 2 shows a tractor-

trailer combination which is similar to that shown in

Figure 1, except that the conventional drawbar D of

Figure 1 has been replaced by a drawbar D' constructed

in accordance with the present invention. Thus, Figure

2 shows a weight transfer hitch W attached to a trailer

T by the drawbar D*. As with the arrangement shown in

Figure 1, Y indicates the yawing axis of the hitch W,

B and C indicate the points of contact of the wheels of

trailer T with the ground, R indicates the centre-line of

the rear axle of a tractor (not shown) attached to the

hitch, R indicates the rolling axis of the hitch for o ^e the straight-ahead position of the composite vehicle, R _qn

indicates the rolling axis of the hitch for a 90 left

turn, and H indicates the humping axis of the hitch

for the straight-ahead position.

The drawbar D' is constituted by a cross-piece 1

and a pair of arms 2. The cross-piece 1 is mounted on

the weight transfer hitch W for pivotal movement about

the yawing axis Y, and the two arms 2 are pivotally

attached, by pivot joints 3 and 4, to the opposite ends of the cross-piece. The opposite ends of the arms 2 are pivotally attached, by pivot joints 5 and 6, to the front of the trailer T. The pivot joints 3, 4, 5 and 6 are so positioned that the cross-piece 1, the arms 2, and the front of the trailer T define a parallelogram linkage.

A constant-displacement,, double-acting hydraulic ram 7

is pivotally mounted between the pivot joints 3 and 6,

that is to say across one diagonal of the parallelogram linkage. The constant-displacement, double-acting hydrauli ram 7 has a member fixed to its piston within the cylindric

working chamber of the ram. This member is .of such a size as to compensate for the presence of the piston rod within the other working chamber of the ram, so that the ram can be extended and retracted by substantially the same distance by supplying the same amount of hydraulic

fluid to each of Its working chambers.

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A sensor (not shown) is provided for sensing

(either directly or indirectly) the angle, through which

the tractor turns. Preferably, the sensor is attached

to the weight transfer hitch W so as to detect the angle

of movement about the yawing axis Y (this being equal to

the angle of turn). The sensor is used to actuate a

hydraulic control valve (not shown) associated with a

hydraulic pump which forms part of the standard hydraulics

of the tractor. The hydraulic control valve is effective to supply pressurised hydraulic fluid to the ram 7, the amount of pressurised hydraulic fluid supplied being proportional to the angle sensed, and hence to the angle of turn. Obviously, the hydraulic control valve supplies pressurised hydraulic fluid to one side of the piston of the ram 7 for right-hand turns, and to the other side of the piston for left-hand turns.

In the straight-ahead position of the tractor-

trailer combination, the plane of support of the trailer

T is again the isosceles triangle ABC, w ^τ here A is the

point of intersection of the yawing and rolling axes Y

and R of the hitch W. In this position, the drawbar D'

defines a rectangle. When a turn is initiated, however,

the drawbar D' is distorted to define a parallelogram, this distortion being caused by the extension or retraction

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of the ram 7. As the movement of the ram 7 is proportiona

to the angle of turn, the degree of distortion of the drawbar D' is also proportional to the angle of turn.

This distortion is, therefore, such that the front edge

of the trailer T (the rear member of the parallelogram

linkage) always lies at right-angles to the perpendicular

of the plane of support of the trailer, which is always

an isosceles triangle. Figure 2 shows, in dash lines,

the relative positions of the hitch W, the drawbar D'

and the trailer T for a 90 left-hand turn. In this

position, the plane of support of the trailer T is the

isosceles triangle A'B'C* , where A ^f is the centre point

of the line which was the original humping axis H (now the rolling axis R _qn ) _> and B ¹ and C ^f are the points of ground contact of the rear wheels of the trailer. The two triangles ABC and A'B'C' are identical, except that

the perpendicular height of the triangle A'B'C' is slightly

less than that of the triangle ABC, this height difference corresponding to the reduction in the "length" of the

parallelogram caused by the distortion thereof.

The use of the drawbar D ¹ thus ensures that the

stability of the trailer T is unaffected by the relative

attitudes of the tractor and the trailer without detracting

from the effectiveness of the weight transfer hitch W.

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Figures 3 and 4 show a tractor-trailer combination

similar to that of Figure 2. Here, however, the parallel¬

ogram-linkage type drawbar D' has been replaced by a simple beam-like drawbar D' ' , and the weight transfer hitch W and the tractor are shown in more detail. The weight transfer hitch W has a pair of parallel, rigid arms 11, one end of each of which is pivotally attached to the tractor (the rear wheels 12 of which only are

shown In Figures 3 and 4) about the humping axis H . As

shown, the humping axis H is positioned slightly forwards

of the centre-line R of the rear axle of the tractor. This ensures that the weight transferred from the trailer

T by the hitch W acts within the wheel base of the tractor

so that there is no tendency for the front wheels of the

tractor to lift clear of the ground, The arms 11 support

a pair of pivot bearings 13a and 131) by means of four

inclined struts 14. The pivot bearing 13a defines the rolling axis R , and the pivot bearing 13_b defines the yawing axis Y. This type of weight transfer hitch is described in greater detail in our British Patent Spec¬ ifications Nos. 1,602,735 and 2,006,595.

The drawbar D' ' comprises a beam 15, one end of which is attached to the weight transfer hitch W for

pivotal movement about the yawing axis Y. The other

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end of the beam is pivotally attached to a tubular strut

16 which is fastened to the front edge of the trailer T. A constant-displacement, double-acting hydraulic ram 17

is pivotally connected, by pivot joints 18 and 19 respec- tively, to the beam 15 and to one arm of the strut 16.

A sensor (not shown) is associated with the pivot

bearing 13b, this sensor being provided for measuring

the angle of relative movement between the parts of the

pivot bearing 13b (and hence the angle of turn). As

with the embodiment of Figure 2, the sensor is used to

control the supply of pressurised hydraulic fluid to

the ram 17 via a hydraulic control valve (not shown).

In the straight-ahead position of the tractor- trailer combination, the plane of support of the trailer

T is again the isosceles triangle ABC. In this position,

the beam 15 of the drawbar D' ' lies along the longitudinal

central axis of the tractor-trailer combination. When a turn is Initiated, however, the rear end of the beam 15 is moved laterally with respect to this axis by means of the ram 17, the amount of this lateral movement being

dependent upon the angle of turn. As with the arrangement

shown in Figure 2, the lateral movement of the beam 15

is such that the front edge of the trailer T always lies at right-angles to the perpendicular of the plane of

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support of the trailer, which is always an isosceles

triangle. Thus, the centre of gravity G of the trailer

always lies substantially on the perpendicular of the plane of support. Fig. 4 shows, in dash lines, the

relative positions of the hitch W, the drawbar D' ^! , the rear wheels 12 of the tractor, and the trailer T for a left-hand turn. In this position, the plane of support of the trailer T is again the isosceles triangle

A'B'C', and the centre of gravity of the trailer T is

indicated by the reference G'. Figure 4 also shows,

within the line X, the positions of the ram 17 and the strut 16 for a 90 right-hand turn.

Here again, therefore, the use of the drawbar

D ¹ ' ensures that the stability of the trailer T is unaffec-

ted by the relative attitudes of the tractor and the

trailer T.

Preferably, each of the drawbars D' and D ^{, !} is

provided with a fail-safe device for returning the drawbar

to its "straight-ahead" position in the event of hydraulic failure. If such a device were not present, there would be nothing to prevent a trailer overturning if the hydrau¬ lics failed during a turn. Thus, if the hydraulics failed with the tractor and trailer at 90°, the centrifugal forces acting on the system would force the drawbar D ¹

or D' ' over its central, "straight-ahead" position to

the opposite side; and this could carry the centre of

gravity of the trailer outside the plane of support,

which would lead to the trailer overturning. In the embodiment of Figures 3 and 4, the fail-safe device

would, conveniently, be pivotally mounted to the beam 15 and to the strut 16, with the fail-safe device on

the opposite side of the beam to the ram 17.

Obviously, a number of modifications could be made to the drawbars described above. For example,

the rams 7 and 17 could be controlled by a master cylinder which is controlled in dependence upon the angle of turn.

In this case, a sectored pinion could be attached to the

movable part of the pivot bearing 13b_ defining the yawing

axis, and the master cylinder could be controlled by a rack meshing with the pinion. It would also be possible

to replace the hydraulic rams 7 and 17 by pneumatic rams, or to control the lateral movement of the trailer by a direct mechanical linkage, a screw jack or an electric motor.

Figs. 5 and 6 show an articulated semi-trailer incorporating an anti-jack-knifing mechanism constituted

by a simplified connecting arrangement constructed in

accordance with the invention. This form of connecting

arrangement is a simplified form of a more practical

arrangement described below with reference to Figs. 7

to 11, and illustrates the principles of operation

involved. The articulated semi-trailer has a tractor

20, a trailer 21, and a connecting arrangement indicated

generally by the reference numeral 22. The tractor 20

has front wheels 20a. and rear wheels 20b. The connec¬

ting arrangement 22 (which is shown only schematically

in Figs. 5 and 6) has a fifth wheel 23 supported at

the end of an arm 24. The other end of the arm 24 is

pivotally mounted on the tractor chassis by means of

a vertical pivot joint 25. A king pin 26, which depends

from the front of the trailer 21, engages within the

slotted fifth wheel 23, and is held therein by any suitable

locking means (not shown). An arcuate track 27 is provided

on the chassis of the tractor 20, the fifth wheel 23 being

movable along the track by a powered device (not shown) .

When the articulated semi-trailer is turned, the powered

device is arranged to move the fifth wheel 23 laterally

with respect to the longitudinal axis of the tractor 20

and into the turn.

Fig. 5 shows the trailer 21 in the straight-ahead

position (in full lines). Fig. 5 also shows (in dashed

lines) the trailer 21 in the position in which jack-knifing

is likely to occur for a standard fixed fifth wheel,

and (in broken lines) the equivalent position taken up

by the trailer when the movable fifth wheel 23 is used.

In this position, the fifth wheel 23 occupies a position

23a (shown in dashed lines). Fig. 6 also shows these

two equivalent positions. When using a fixed fifth

wheel, the line of action of the force I (see Fig. 6)

which results from the inertia of the trailer 21 acts

along the line JK (see Fig. 5), where J is the centre

of support of the trailer and K is the centre of the fifth wheel. A force S (the retarding force of the tractor 20) also acts at the point K. The resultant L of these two forces acts as shown in Fig. 6. As the

line of action JK meets the axis of the tractor front wheels 20a. at a point M which lies outboard of the front

wheels, the resultant L tends to rotate the rear wheels

20b of the tractor 20 about the point M, and this tends

to initiate jack-knifing. It will be noted that, in this critical position, the trailer 21 lies at an angle of

25 to the straight-ahead position (see Fig. 5).

When using the movable fifth wheel 23, however,

the line of action of the force I. which results from

the inertia of the trailer 21 acts along the line JK ₁ ,

where K_. is the centre of the fifth wheel in the position

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23a. This force I., acts at the point 1- where the line of action JK ₁ meets the axis of the front wheels 20a of the tractor 20. A force S_. (the retarding force of the tractor 20) also acts at the point M_, . The resultant L.

of these two forces acts along the axis of the front

axle of the tractor 20, and so cannot cause rotation of

the rear wheels 20b_ (because there is no moment arm).

Consequently jack-knifing is extremely unlikely to occur.

It should be noted that, in the position shown, the

trailer 21 lies at an angle of 20 to the straight-ahead

position (see Fig. 5). This is because the front of

the trailer 21 has been moved into the turn by the con¬

necting arrangement. It should also be noted that the

point M_ _j will not move outside the front wheels 20a of

the tractor 20 until the trailer lies at an angle of

41.5 to the straight-ahead position. In other words, a critical jack-knifing situation is not reached when using the movable fifth wheel 23 until the trailer 21 has turned through a considerably larger angle than that through which a critical jack-knifing situation is reached when using a fixed fifth wheel.

Figs. 7 and 8 show a practical form of connecting

arrangement which utilises the principles of operation

described above with reference to Figs. 5 and 6. This

connecting arrangement has a fifth wheel 30 attached to

a frame 31. The frame 31 carries a pair of rollers 32,

and is attached to one end of an arm 33. The other end

of the arm 33 is pivotally mounted on a cross-member 34

by means of a vertical pivot joint 35. The cross-member 34 is fixed to the chassis 36 of a tractor (not shown, but similar to the tractor 20 of Figs. 5 and 6). A double-acting hydraulic ram 37 is pivotally connected

between one end of the cross-member 34 and a bracket 38

fixed to the underneath of the arm 33 at a position

adjacent to the frame 31. An arcuate track 39 is fixed

to the chassis 36, the arcuate track being such that

its centre of curvature coincides with the axis of the

pivot joint 35.

A king pin 40 is detachably fixed to the fifth

wheel 30, the king pin being fixed to, and depending

downwardly from, the front end of a trailer (not shown,

but similar to the trailer 21 of Figs. 5 and 6).

A pair of cams 41 (see Figs. 8 and 9) are fixed to the trailer chassis (shown schematically by the dashed line 42 in Fig. 8) and are positioned above the fifth wheel 30. As shown in Fig. 9, the two cams 41 are positioned symmetrically on opposite sides of the central longitud¬

inal axis N of the fifth wheel 30 when the articulated

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semi-trailer is in the straight-ahead position, that is

to say as shown in Fig. 7 and 9. A respective control

device 43 is associated with each of the cams 41 (see

Fig. 9). Figs. 7 and 8 show only one of the control

devices 43 for reasons of clarity. Each control device

43 has a roller (cam follower) 44 which engages the

associated cam 41. The control devices 43 and the assoc¬

iated cam followers 44 are fixed to the fifth wheel 30.

In use, as the articulated semi-trailer turns,

the ram 37 is pressurised to force the fifth wheel 30

and the king pin 40 (and hence the front of the trailer)

into the turn. As the trailer turns, the angular movement

of the cams 41 about the king pin 40 is sensed by the

cam followers 44. This angular movement of the cams 41 (which is proportional to the relative angular movement

between the tractor and the trailer) is converted into

linear movement of the cam followers 44. This linear

movement of the cam followers is transmitted, via res¬

pective rack-and-pinion drives (shown schematically at

45 in Figs. 9 and 11) to a hydraulic control unit 46

(see Fig. 11).

As shown in Fig. 11, the linear movement of each

of the cam followers 44 is converted Into rotary movement

by the corresponding rack-and-pinion drive 45. This

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rotary movement (whose sense is determined by the directio

_ of turn of the articulated semi-trailer) Is used to drive

a mechanical linkage 47 as indicated by the arrows 48.

The linkage 47 controls a diverter valve 49 and a propor-

tional valve 50, these valves forming part of the hydrauli

control unit 46. The input side of the diverter valve 49

is supplied with hydraulic fluid from a reservoir 51 by

means of a pump 52, and the output side of the diverter valve has hydraulic lines 53a and 53t> leading to the opposite ends of the ram 37. The input side of the diverter valve 49 is also connected to the proportional valve 50 in such a manner that hydraulic fluid passes to the output side of the diverter valve only after it

has passed through the proportional valve. In use, rotation of the linkage 47 actuates the

diverter valve 49 to permit fluid flow to one of the

lines 53a or 53b. At the same time, the proportional valve 50 is actuated to pass a volume of hydraulic fluid that is dependent upon the rotation of the linkage 47 (and hence upon the degree of turn of the articulated

semi-trailer). Consequently, the length of the working stroke of the ram 37 is dependent upon the degree of

turn of the articulated semi-trailer.

Each of the control devices 43 is used to pressurise

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the ram 37, each of the devices acting as a back-up

for the other device should one of the devices fail. It

is, therefore, possible to provide only one such control

device 43, in which case only one cam 41 w ^τ ould be required.

Alternatively, two rams 37 would be provided. In this

case, each ram 37 could be pressurised by a respective

control device 43, or both control devices could be

used to pressurise both rams.

The profile of each of the cams 41 is chosen to balance the working stroke of the ram 37 with the angular

movement sensed. This balancing is necessary, because

the angular movement and the ram working stroke do not

have a linear correlation. If tw*o rams are used, the

cam profiles will require further adjustment to compensate

for variations in the effective areas on opposite sides

of the pistons of double-acting rams.

As with the embodiment of Figs. 2 to 4, it will

be apparent that the hydraulic ram(s) 37 could be replaced

by pneumatic ram means. It would also be possible to control the lateral movement of the fifth wheel 30 by a direct mechanical linkage, a screw jack or an electric motor.

The embodiment of Figs. 7 to 11 could also be

modified by replacing the arm 33 by a parallelogram-type

linkage similar to that shown in Fig. 2. It would also

be possible to use a trapezoidal linkage instead of

such a parallelogram linkage.

It would also be possible to replace the arcuate

track of the embodiment of Figs. 7 to 11 with a straight

track mounted transversely across the tractor chassis

36. In this case, the fifth wheel 30 would have integral

guides, and would be powered hydraulically, pneumatically,

mechanically or electrically by a rotational sensor.

It should be noted that jack-knifing can occur at smaller angles than that shown in Figs. 5 and 6. This illustrated position shows a critical situation in which jack-knifing is probable - though this depends on

other factors such as road speed and road surface con- ditions (in particular tire adhesion). Thus it is possible

for jack-knifing to occur once the line of action of

the force I moves away from the centre of the tractor

front axle. Indeed, if conditions are really bad (for

example if the road is icy), jack-knifing can occur during very small angle turns.

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