GB2125672A

GB2125672A - Drawbar

Info

Publication number: GB2125672A
Application number: GB08224595A
Authority: GB
Inventors: Robert Johnson Greatbatch
Original assignee: WEIGHT DISTRIBUTION Ltd
Current assignee: WEIGHT DISTRIBUTION Ltd
Priority date: 1982-08-27
Filing date: 1982-08-27
Publication date: 1984-03-14
Also published as: EP0134213A1; WO1984000936A1

Abstract

A connecting arrangement for connecting a trailer to a tractor. The connecting arrangement has first and second connectors connectible to the tractor and to the trailer respectively. In one embodiment, a drawbar D' constitutes the connecting arrangement, the drawbar having a beam (15), the first connector at one end of the beam, and a third connector at the other end of the beam. The third connector is attached to a weight transfer hitch W attached to the tractor. In another embodiment, the connecting arrangement is used to connect the tractor and trailer of an articulated semi-trailer. In this case, the first connector is a fifth wheel (30) and the second connector is a king pin (40). The fifth wheel (30) is fixed to the end of an arm (33) pivotally mounted on the chassis (36) of the tractor. The king pin is fixed to the front of the trailer. In both embodiments means (such as a hydraulic ram 17 or 37) is provided for moving the first connector laterally with respect to the central longitudinal axis of the tractor.

Description

SPECIFICATION Drawbar This invention relates to a drawbarforconnecting a trailer vehicle 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 ofthese hitches istotransferweightfrom atrailervehicleontothe 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 prime mover and thetrailervehicle 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 primemoverandthetrailervehicle mustbe ableto 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 ofthe composite vehicle. This is generally known as the 'rolling axis'.

3. The two components of the composite vehicle mustbeableto moveindependentlyin a pitching or rearing sense in ordertotraversesurfaceundulations which are perpendicularto the longitudinal centre line ofthe composite vehicle. The axis about which this movement takes place isgenerally known as the 'humping axis'.

Thus, a weighttransfer hitch must consist of a mechanism incorporating thesethree axes, and must be so arranged that movement about a given axis does not inhibit movementaboutthe others. During movement about one or more of these axes, the weighttransfer hitch must form part of a continous beam which is rigid in the vertical plane.

The principal function of a weighttransfer hitch is to improvethetractivecapabilityoftheprime mover, by transferring weight from the trailervehicle (and its payload) onto the driving wheels ofthe prime mover.

The increase in tractive capability reduces the tendencyforthe prime moverwheelsto slip. When the prime mover rear wheels start to slip, there is little or no increase in rear axle torque, and so the application of additional powerto the driving axle results only in increased slippage. If the tractive capability of the prime mover is increased, by the application ofweight onto the driving wheels, additional power to those driving wheels results in increased rearaxletorque, the reaction to which tends to cause the frontwheels of the prime mover to lift.To minimisethis undesireable effect, the point of attachment of the weight transfer hitch to the prime mover is placed as low as possible, and generally somewhat in front of the centre line of the prime mover rear axle. The humping axis of the weight transfer hitch is the effective location of this point of attachment. The location of the humping axis below, and ahead of, the prime mover rear axle poses practical problems of clearance under the prime mover axle, and obstruction between the rear of the prime mover and the hitch when humping takes place.Therefore, it is necessarvthatthe hitch extends rearwards from the humping axis by a distance which is sufficient to ensure clearance of obstructions at the rear of the prime mover 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 axle ofthe prime mover.

The principal advantage of locating the humping axle belowthe rearaxle ofthe prime moveristhatthe momentaboutthe point of contact of the rearwheels and the ground is maintained atthe lowest possible level, so thatthetorque reaction, which would tend to lift the front wheels of the prime mover, is kept ata minimum.

Weight transfer hitches which are mounted below the rear axle of the prime mover and extend rearwards as previously described, have certain common char acteristicswhich, in the past, have placed severe limitations on the vehicle designer. The most important of these characteristics is the transposition ofthe 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 progressivelytakes moreofthe rolling function until, when the prime mover is at 900 to the trailer vehicle, 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 vehicle T by a conventional drawbar D. In Figure 1, Y indicates the yawing axis ofthe hitch W, B and C indicatethe points of contact of the wheels ofthe trailer vehicle Twith the ground, R indicates the centre-line ofthe rear axle of a prime mover (not shown) attached to the hitch, Po indicates the rolling axis of the hitch forthe straight-ahead position ofthe composite vehicle, RsD indicates the rolling axis of the hitch fora 90" leftturn, and Ho indicates the humping axis ofthe hitch forthe straight-ahead position.In the straight-ahead position, the plane of support of the trailer vehicle is an isosceles triangle ABC, having its base at the points of contact B and C of the trailer vehicle wheels with the ground, and its appexA on the point of intersection of the yawing and rolling axes Y and R0 ofthe hitch W. When a turn is initiated, the apex ofthe plane of support moves progressively in the directionoftheturn; and,whentheprimemoveris lying at 900 to the trailer vehicle T, the apex A' of the plane of supportA'BC lies on the original humping axis H, which has become the new rolling axis R90. As the prime mover makes a 90" tu rn,the apex of the plane of support moves along an arc between the points A and A' (A' being the centre point ofthe 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 distributed, the centre of gravity G ofthetrailervehicleTwill lie ata point on the perpendicular of the isosceles triangle ABC. During a turn however, the plane of support The drawing(s) originally filed was/were informal and the print here reproduced is taken from a later filed formal copy.

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 tendancyforthe trailervehicleTto 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 force designersto accept compromises which are less than satisfactory, and, at worst, make the use of weight transfer hitches impossible under certain conditions.

The aim ofthe invention isto provideadrawbarfor connecting a trailer vehicle to a weig httransfer hitch attached to a prime mover, which drawbar is such that the centre of gravity ofthe trailer will lie well within the plane of support even during a 900turn.

The present invention provides a drawbarfor connecting a trailer vehicle to a weight transfer hitch attached to a prime mover, the drawbar being provided with first and second connectors at opposite endsthereoffor attahing the drawbarto a weight transfer hitch and to a trailer vehicle respectively, wherein the drawbar is provided with means for moving the second connector laterally with respect to the central longitudinal axis ofthe prime mover.

Advantageously, said means is such that, as the prime moverexecutesaturn, the second connector is moved into theturn. This causes the front of the trailer vehicle to move into theturn, and results in the centre of gravity of the trailer vehicle following the movement ofthe centre-line ofthe plane of support.

Preferably, said means is such that the movement of the second connector is proportional to the angle through which the prime mover is turned. This ensures thatthe centre ofgravity of the trailer vehicle always lies substantially on the centre-line ofthe plane of support.

In a preferred embodiment, the drawbar is constituted by a beam, and wherein a hydraulic ram constitutes said means. In orderthatthe ram can control the lateral movement ofthetrailer vehicle for both right-hand and left-hand turns, the ram is conveniently a double-acting hydraulic ram.

The ram may be pivotally mounted between the beam and a connection point on the trailer vehicle.

Preferably, however, the beam is pivotally attached to a strutwhich is attachable to the trailer vehicle, 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 ofthe beam between the beam and the strut.

The drawbar mayfurther comprise a sensorfor measuring the angle ofturn and for controlling the amount of pressurised hydraulic fluid supplied to the ram. Alternatively, the drawbar mayfurther comprise a hydraulic master cylinderfor controlling the hyd raulic 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 weighttransfer hitch may constitute the mechanical linkage.

In another preferred embodiment, the drawbar is 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 ofthe cross-piece, and wherein the free ends of the arms are pivotally connectible to the trailer vehicle so that the crosspiece, the two arms, and the trailer vehicle define a parallelogram linkage, the pivotal connection be tweenthe arms and the trailervehicle constituting the second connector. In this case also, a hydraulic ram constitutes said means, the hydraulic ram being pivotallyconnected across a diagonal ofthe parallelogram linkage.

Preferably, the drawbarfurther comprises a fail-safe device for moving the drawbar into a predetermined position in the event of failure of said means.

The invention also provides a tractor-trailercombination comprising a tractor connected to a trailer by means of a weight transfer hitch and a drawbar,the weighttransfer hitch being attached to the rear of the tractor, and the drawbar being attached to the front of the trailer, the trailer being ofthetype having a pair of wheels provided atthe rearthereof,theweight transfer hitch having a rolling axis, a humping axis and a yawing axis, and the trailer having a triangular plane of support defined bythe points of contact of the wheels of the trailer with the ground and the rolling axis ofthe weight transfer hitch, wherein the drawbar is provided with meansfor maintaining the centre of gravity ofthetrailerwithin the plane of support irrespective ofthe relative orientation between the tractor and the trailer.

The invention will now be described in greater detail, by way of example, with reference to Figures 2 to 4 ofthe accompanying drawings, in which:- Figure 2 is a schematic plan view of a tractor-trailer combination incorporating a weight transfer hitch and a firstform of drawbar constructed in accordance with the invention; Figure 3 is a schematic side elevation of a tractortrailer combination incorporating a weight transfer hitch and a second form of drawbar constructed in accordance with the invention; and Figure 4 is a schematic plan view of the arrangement shown in Figure 3.

Referring to the drawings, Figure 2 shows a tractor-trailercombinationwhich 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 weighttransfer hitch W attached to a trailerT 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 ofthe wheels oftrailerTwith the ground, R indicatesthecentre-lineofthe rear axle of a tractor (not shown) attached to the hitch, R0 indicates the rolling axis of the hitch for the straight-ahead position ofthe composite vehicle, R90 indicates the rolling axis ofthe hitch for a 90" left turn, and Ho indicates the humping axis of the hitch forthe 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 weighttransfer 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 ofthe cross-piece. The opposite ends of the arms 2 are pivotally attached, by pivotjoints 5 and 6, to the front ofthetrailerT. The pivotjoints 3,4,5 and 6 are so positioned that the cross-piece 1, the arms 2, and the frontofthe trailerTdefine 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.Aconstant-displacement, double-acting hydraulic rams has a memberfixed to its piston within the cylindrical working chamber of the ram. This member is of such a size asto compensateforthe presence of the piston rod within the otherworking chamber of the ram, so that the ram can be extended and retracted by substantially the same distance by supplying the sameamountofhydraulicfluidto each of its working chambers.

A sensor (not shown) is provided for sensing (either directly or indirectly) the angle through which the tractorturns. Preferably, the sensor is attached to the weighttransferhitchWso asto detectthe 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 ofthe standard hydraulics ofthetractor. The hydraulic control valve is effective to supply pressurised hydraulic fluid to the ram 7, the amount of pressurised hydraulicfluid supplied being proportional to the angle sensed, and hence to the angle of turn.Obviously, the hydraulic control valve supplies pressu rised hydraulic fluid to one side ofthe piston ofthe ram 7for right-hand turns, and to the other side of the piston for left-hand turns.

In the straight-ahead position ofthetractor-trailer combination,the plane of support of the trailerT is again the isosceles triangle ABC, where A is the point of intersection of the yawing and rolling axes Y and R0 ofthe 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 ofthe ram 7.As the movementofthe ram 7 is proportional to the angle of turn, the degrees of distortion of the drawbar D' is also proportional to the angle ofturn. This distortion is, therefore, such that the front edge ofthe trailerT (the rear member ofthe parallelogram linkage) always lies at right-anglesto the prependicular ofthe plane of support ofthetrailer, which is always an isosceles triangle.Figure 2 shows, in dash lines, the relative positions ofthe hitch W,the drawbarD' and the trailerTfora 90 left-handturn. In this position, the plane of support ofthe trailer T is the isoscelestriangle A'B'C', where A' is the centre point ofthe line which was the original humping axis Ho (nowthe rolling axis Rio), and B' and C' arethe points of ground contact of the rear wheels of the trailer. The two triangles ABC and A'B'C' are indentical, except that the prependicular height ofthe triangle A'B'C' is slightly less than that of triangle ABC, this height difference corresponding to the reduction in the "length" of the parallelogram caused by the distortion thereof.

The use ofthe drawbar D' thus ensuresthatthe stability of the trailerT is unaffected by the relative attitudes of the tractor and thetrailerwithout detracting from the effectiveness of the weig httransfer hitch W.

Figures 3 and 4 show a tractor-trailer combination similar two that of Figure 2. Here, however, the parallelogram-linkagetype 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 rearwheels 12 of which only are shown in in Figures 3 and 4) about the humping axis H,. As shown, the humping axis Hb is positioned slightlyforwards of the centre-lineR ofthe rear axle of the tractor.This ensures that the weight transferred from the trailerT by the hitch W acts within the wheel base of the tractor, so that there is no tendency for the front wheels ofthe tractor to lift clear ofthe ground. The arms 11 support a pair of pivot bearings 13a and 1 3b by means of four inclined struts 14.The pivot bearing 1 3a defines the rolling axis R,, and the pivot bearing 13b defines the yawing axis Y.

This type of weight transfer hitch is described in greater detail in our British Patent Specifications 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 Wfor pivotal movement about the yawing axis Y. The other end ofthe beam is pivotally attached to a tubular strut 16 which is fastened to the front edge ofthe trailerT. A constant-displacement, double-acting hydraulic ram 17 is pivotally connected, by pivot joints 18 and 19 respectively, 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 1 3b (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 ofthetractor-trailer combination, the plane of support ofthetrailerT is again the isocelestriangle 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 ofthe trailer Talways lies at right-angles to the perpendicular of the plane of support ofthetrailer, which is always an isoceles triangle. Thus, the centre of gravity G ofthe trailer always lies substantially on the perpendicular ofthe plane of support. Fig. 4shows, in dash lines, the relative positions of the hitch W, the drawbar D", the rearwheels 12 ofthetractor, andthetrailerTfora 90" left-hand turn. In this position, the plane of support of thetrailerTis again the isoscelestriangleA'B'C', and the centre of gravity of the trailerT 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 ofthe drawbar D" ensures that the stability of the trailer T is unaffected bythe relative attitudes ofthe tractor and the trailer.

Preferably, each ofthe drawbars D' and D" is provided with a fail-safe device for returning the drawbarto 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 hydraulics failed during a turn. Thus, if the hydraulics failed with the tractor and trailer at 900,the centrifugal forces acting on the system would force the drawbar D' or D" over its central, "straight-ahead" position to the opposite side; andthis could carry the centre of gravity ofthetrailer 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 ofturn. In this case, a sectored pinion could be attached to the movable partofthe 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, orto control the lateral movement ofthetrailer by a direct mechanical linkage, a screw jack or an electric motor.

The drawbars described above could also be used to connect the tractor and trailer parts of an articulated lorry. In this case, the lateral movement of the rear end ofthe draw barwould help preventthe articulated lorryjack-knifing.

Claims

1. A drawbarforconnecting a trailervehicleto a weighttransfer hitch attached to a prime mover, the drawbar being provided with first and second connectors at opposite ends thereof for attaching the drawbar to a weighttransfer hitch and to a trailer vehicle respectively, wherein the drawbar is provided with means for moving the second connector laterally with respecttothe central longitudinal axis of the prime mover.

2. A drawbar as claimed in claim 1, wherein said means is such that, as the prime mover executes a turn, the second connector is moved into the turn.

3. A drawbar as claimed in claim 2,wherein said means is such that the movement of the second connector is proportional to the angle through which the prime mover is turned.

4. A drawbar as claimed in any one of claims 1 to 3, wherein the drawbar is constituted by a beam, and wherein a hydraulic ram constitutes said means.

5. A drawbar as claimed in claim 4, wherein the ram is a double-acting hydraulic ram.

6. A drawbar as claimed in claim 4 or claim 5, wherein the hydraulic ram is pivotally mounted between the beam and a connection point on the trailer vehicle.

7. A drawbar as claimed in claim 4 or claim 5, wherein the beam is pivotal ly attached to a strut which is attachable to the trailer vehicle, the strut extending laterally with respectto the longitudinal axis ofthe 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.

8. A drawbar as claimed in any one ofclaims 4to 7 when appendantto claim 3, further comprising a sensorfor measuring the angle ofturn and for controlling the amount of pressurised hydrauliefiuid supplied to the ram.

9. Adrawbarasclaimed in any one of claims 4to7 when appendantto claim 3, further comprising a hydraulic mastercylinderfor controlling the hydraulic ram, the master cylinder being actuated by a mechanical linkage which is moved in dependence upon the angle ofthe turn.

10. A drawbar as claimed in claim 9, wherein a rack-and-pinion associated with the weight transfer hitch constitutes the mechanical linkage.

11. A drawbar as claimed in any one of claims 1 to 3, wherein the drawbar is constituted by a cross-piece and a pair of parallel arms, the cross-piece being pivotally attachable to the weighttransfer hitch, and the two arms extending rearwardlyfrom opposite ends of the cross-piece, and wherein the free ends of the arms are pivotally connectible to thetrailervehicle so that the cross-piece, the two arms, and the trailer vehicle define a parallelogram linkage, the pivotal connection between the arms and the trailer vehicle constituting the second connector.

12. A drawbar as claimed in claim 11, wherein a hydraulic ram constitutes said means, the hydraulic ram being pivotallyconnected across a diagonal ofthe parallelogram linkage.

13. A drawbaras claimed in any one of claims 1 to 12, further comprising a fail-safe device for moving the drawbar into a predetermined position in the event offailure of said means.

14. Atractor-trailercombination comprising a tractor connected to a trailer by means of a weight transfer hitch and a drawbar, the weighttransfer hitch being attached to the rear of the tractor, and the drawbar being attached to the front of the trailer, the trailer being ofthetype having a pairofwheels provided atthe rear thereof, the weighttransfer 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 ofthe wheels ofthetrailerwith the ground and the rolling axis 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.

15. Adrawbarsubstantiallyas hereinbeforedescribed with reference to, and as illustrated by, Figure 2, or Figures 3 and 4 of the accompanying drawings.

16. Adrawbarforconnecting a trailer vehicle to a prime mover, the drawbar being provided with first and second connectors at opposite ends thereof for attaching the drawbarto the prime mover and to the trailer vehicle respectively, wherein the drawbar is provided with means for moving the second connector laterally with respectto the central longitudinal axis of the prime mover.