GB2105966A - Load control device for a tractor - Google Patents

Abstract

A device intended to adjust the load applied to a tractor and thus the grip of the wheels of the tractor on the grund when the tractor is in combination with an implement 1 connected to the tractor by a three-point lifting gear and partly supported by a gauge wheel 26. The device includes a hydraulic jack 7 which constitutes the third-point bar of the lifting gear and means for adjusting the length of the jack to thereby adjust the load in response to the vertical load applied to the gauge wheel. <IMAGE>

Description

SPECIFICATION Load control device for a tractor The present invention relates to a device which is intended to improve the grip on the ground of a tractor having an implement, such as a plough provided with a gauge wheel, attached at the front or at the rear on a three-point lifting gear.

At the present time the tendency is to develop implements which, by a single passage of the tractor, permit a largest possible area to be covered.

These implements are of two types: either they are carried, i.e. mounted on the three-point lifting gear of the tractor, or they are semi-carried, i.e. supported at one end by three-point lifting gear of the tractor and supported, at the other end, at an intermediate point along their length, by a wheel.

As the implements become increasingly larger and increasingly heavier, the tractor is unbalanced and, as a consequence, tends to rear or buck if the implement is carried. If the front axle is the driving axle, its adhesion is poor and, consequently, the said axle is inefficient. As all the weight is transferred to a single axle, the working conditions of the tractor are poor; moreover, there is a tendency to pack the earth at the level of the driving wheels, which agriculturally is very bad. On the contrary, the vertical load applied to the tractor is considerable.

These inconveniences are partly overcome by the implements which are provided with a gauge wheel.

In practice, such a wheel permits the variations in level of the terrain to be perfectly followed. On the contrary, since a part of the weight of the machine is supported by the wheel, the vertical load applied to the tractor is diminished and this, in certain cases, is a serious disadvantage. In effect, for a given grip or adhesion, the greater the vertical load on the tractor, the greater is the traction capacity.

The present invention has for an object to provide a device which has, firstly, the advantages of carried implements without the disadvantages thereof, and, secondly, the advantages of semi-carried implements, the disadvantages of these latter being eliminated.

According to the present invention, a device for improving the grip on the ground of a tractor provided with a three-point lifting gear or system comprises two lifting arms and a third-point bar and to which is attached a machine of which the frame comprises soil-working tools, means for its attachment to the three-point gear, the said machine, when operating, being partly supported by a gauge wheel, the.third-point bar being formed by a hydraulic jack, while the gauge wheel is connected to means for controlling the hydraulic jack are so arranged that, in response to a vertical force applied to the said gauge wheel and exceeding a determined or set threshold, the hydraulic jack is shortened and that, in response to a vertical force applied to the said gauge wheel and below a set threshold, the hydraulic jack is lengthened.

Thanks to this arrangement, it will be possible, under all working conditions, to maintain a vertical load applied to the tractor which is a function of the vertical load supported by the gauge wheel and thus to enable the tractor to operate with its maximum capacity as a function of the force or effort to be supplied.

In accordance with one particular form of construction, the gauge wheel is supported by a bar mounted for vertical sliding movement in a sleeve or casing of a support, the said sleeve or casing comprising a lateral lug supporting a guide rod of a compression spring, the said rod extending through a lug integral with the bar, the compression spring being inserted between a stop on the rod and the lug integral with the bar, this latter lug being provided with a first electric contact adapted to co-operate with a fixed electric contact when the force applied on the gauge wheel exceeds a determined threshold, these contacts being included in the electric circuit of a corresponding electromagnetic valve, and a second electric contact adapted to co-operate with a fixed electric contact when the vertical force applied to the gauge wheel is below a determined threshold, these last contacts being included in the electric circuit of a corresponding electromagnetic valve.

The stop against which one of the ends of the compression spring is bearing is preferably adjustable along the rod for modifying the calibration of the said spring.

According to a modified constructional form, the gauge wheel is carried by an arm which is movable on a shaft extending substantially perpendicularly of the direction of advance of the machine, the said arm being movable against the action of resilient means, the said control means of the hydraulic jack comprising a switch which is fast with the frame and which is so arranged that, when the arm rocks in a direction corresponding to a vertical force applied to the wheel and exceeding a determined threshold, it closes an electric control circuit so as to shorten the jack and, on the contrary, when the force applied to the gauge wheel falls below a determined threshold, it will lengthen the jack.

The resilient means are preferably formed by rubber blocks.

In order to be able to regulate the thresholds from which the jack has to be shortened or lengthened, the position bf the rubber blocks is adjustable.

In the case of a reversible plough comprising a frame and ploughshares and mouldboards on both sides of the said frame, and since the control means are electrical and it is necessary to provide wires and cables, it is desirabie that the electrical members are fixed. One constructional feature is concerned with an application to a reversible plough.To this end, the device comprises a lug which is fast with the frame of the plough and which supports two rubber blocks disposed symmetrically with respect to a horizontal plane passing through the longitudinal axis of the frame of the plough, two switches which are likewise symmetrically disposed and arranged for closing a first control circuit when they receive a pressure which exceeds a determined threshold and for closing a second control circuit when this pressure falls below a determined threshold, a stop provided on the arm for co-operating with the corresponding switch in the ploughing position and a contact-mak erfor opening, on the return, the general circuit of the switch which was used at the time of the preceding ploughing operation and for closing the general circuit on the other switch which will be used at the time of the next ploughing operation.

According to yet another feature, for an adaptation to a reversible plough, the arm supporting the gauge wheel is fitted into a forked-joint with two wings or flanges, and a web which comprises, inside the said wings or flanges, the resilient buffers which co-operate with the said arm and form the resilient means, while the web supports the switch, the control of which is assured by a cam pivoted on the web and connected to the said arm, the gauge wheel being mounted for rocking movement on a support mounted on the arm in such a manner as to be able to selectively occupy two positions offset by 1800 and means being provided for securing it in the said two positions. In this way, an arrangement is provided in which the electrical members are simplified.

In certain cases, the ploughs are supported by the gauge wheel in the position for transport, the frame occupying an intermediate angular position between these two working positions, that is to say, it is held pivoted by 90" relatively to the said working positions. Another feature visualises the possibility of using the gauge wheel for transporting purposes. To this end, the forked-joint is mounted for pivotal movement so as to be able to be retracted, while the support of the gauge wheel comprises means for wedging the gauge wheel in two positions offset angularly by 90".

In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure lisa diagrammatic elevational view of a tractor-plough assembly with a device according to the invention, Figure 2 is a diagrammatic sectional view, showing the mounting of the gauge wheel, Figure 3 shows the control means of the jack, Figure 4 is a section along the line IV-IV of Figure 2, Figure 5 shows diagrammatically one construc tionalform of the invention adaptable to a reversible plough, Figure 6 is a view to a larger scale of a dead-centre overrun switch, Figure 7 is a sectional view of a control switch, Figure 8 shows the hydraulic diagram of the device, Figure 9 shows the control diagram of the device, Figure 10 is a perspective view of a modified form of the device according to the invention, Figure 11 shows the electrical control diagram, Figure 12 shows the control cam of the switch, Figure 13 is a sectional view of a detail to a larger scale, Figure 14 is a perspective view of the gauge wheel in the transporting position, and Figure 15 shows a detail of the mounting of the gauge wheel, particularly intended for reversible ploughs disposed at the front of the tractor.

Represented in Figure lisa plough 1,which is coupled to the three-point lifting gear 3 of a tractor 2.

In this example, the three-point lifting gear 3 is located at the front of the tractor 2 and the plough is thus pushed when in use. It would obviously be possible to use the device according to the invention with a plough which is towed by a tractor and also with an assembly comprising two ploughs, one at the front and the other at the rear of the same tractor.

The three-point lifting gear 3 comprises two lifting arms 4, on which is fixed a coupling means 5 provided on the head of the plough, the said coupling including a shaft6forthefixing ofthe third-point arm 7. In principle, the third-point arm is formed by a plain bar of adjustable length. In the arrangement according to the invention, the third -point arm is formed by a single-acting hydraulic jack, which is connected by way of a suitable conduit 8 to the distributor 9.

The distributor 9 is designed for permitting the supply to the jack 7, for maintaining the jack 7 at a certain pressure and for connecting the supply to the jack to atmosphere.

At its free end, the plough 1 has a gauge wheel 26 on its beam 10.

The gauge wheel 26 (see Figures 2 and 4) is carried by a support 11 and is mounted freely on an axle 12 carried by a forked-joint 13, the web of which is fast with a square-section bar 14.

The plough beam 10 is fast laterally with a horizontal bearing 15 which extends transversely of the longitudinal axis of the beam 10 and which is integral with an annular flange 16 having two notches 17 and 18 offset by 1800.

Mounted freely in the bearing 15 is a shaft 19 which is integral with a square-section casing or sleeve 20, in which is located the bar 14, this latter being able to slide in the said casing. On one side 21, the casing 20 carries a forked-joint 22 through which extends a pin 23 on which is povoted a latch 24 arranged to co-operate selectively with the notches 17 and 18.

On its face opposite to the face 21, the casing 20 carries a lug 28 having a hole 29 in which is engaged a rod 30. At its upper end, the rod 30 has a square-section member 31 and two stops 32 and 33, arranged such that it is able to turn freely but it is fixed axially.

The rod 30 is formed with a screw-thread 35, on to which is screwed a nut 36 having one edge 37 bearing against the face 27.

Fixed on the bar 14 is a lug 38 which passes through an elongated opening 39 in the casing 20, the said lug having a hole 46 which is aligned with the hole 29 and through which the rod 30 extends.

Inserted between the lugs 38 and the nut 36 is a compression spring 47 fitted on the rod 30, of which the bottom end forms a stop 48.

The upper side of the lug 38 carries a first contact 40 which is intended to co-operate with a contact 41 carried by the casing 20 and the lower side of the lug carries a second contact 42 which is intended to co-operate with a contact 43 carried by the said casing 20.

By turning the member 31, it is possible to modify the calibration of the spring 47.

The distributor 9 is controlled electrically and comprises a three-position slide valve, in an intermediate position A of which, oil is drawn from a tank 45 and returned directly into the latter, while, in a position B of the valve, oil drawn from the tank 45 is sent into the jack 7, and, in the position C, the jack 7 is directly connected to the tank 45. The distributor is so arranged as to return normally into the position A, that is to say, the jack 7 is maintained a determined length which corresponds to normal working.If the vertical force applied to the wheel 26 exceeds a predetermined threshold, this being defined by the calibration of the spring 47, the bar 14 is displaced upwardly and the contact 40 comes to bear against the contact 41, the corresponding electric circuit is closed and the slide valve 9 is then controlled in such a way as to be placed in the position B and the jack receives oil under pressure, so that it is shortened. As soon as the working conditions return to normal, under the action of the spring 47, the contacts 40 and 41 are separated and the slide valve returns to the position A.On the other hand, if the vertical thrust applied to the wheel 26 becomes lower than a threshold value determined by the spring 47, the bar 14 pushed by the spring 47 is displaced downwardly and the contacts 42, 43 close the electric control circuit of the slide valve 9 which is then moved into position C. In this case, the jack 7 tends to be lengthened, since its cylinder is directly connected to the return to the tank 45.

Represented in the figures is an application of the device to a reversible plough. At the moment of turning round, the latch 24 is disengaged from the notch 17 and the shaft 19 is able to turn freely in the bearing 15, so that, due to its own weight, the support 11 pivots through 1 80". When the plough has executed a rotational movement, the notch 18 comes into a position facing upwardly, while the notch 17 is turned to face downwardly and the latch 24 comes to be positioned in the notch 18. When the plough is next pivoted, it will return into the initial position which is shown in Figure 2.

Because of this arrangement, the adjustment of calibration of the spring 47 will be carried out once, whatever are the ploughshares being used.

Represented in Figures 5 - 9 is a constructional form in which the gauge wheel, which here bears the reference 50, is carried by a pivoted arm 51, the mounting being particularly suitable for reversible ploughs.

The arm 51 is mounted on a shaft 52 supported by the frame 53 of the plough, the frame having resilient stops 54 and 54a and switches 55 and 55a.

The resilient stops 54 and 54a are supported by a bar 57 having tapped holes 58 and 59, into which are respectively screwed screws 60 and 61, these screws being fast with the said stops 54 and 54a, which are formed by simple blocks of rubber.

Each switch 55 and 55a (see Figure 7) comprises a body 62 having, at one end, a contact 63 against which is able to abut a contact 64 fast with a rod 65 which traverses the body 62 through a hole 66 in order to project externally, the said rod supporting a second contact 67 capable of bearing against a corresponding contact 68. A compression spring 70 is interposed between a stop 71 and the contact 67 in such a manner that the contacts 67 and 68 tend to be in engagement.

Provided on the arm 51 is a stop 73, which is so disposed that, in one working direction of the plough, it encounters the rod 65 of the switch 55, whereas, when the the plough is turned round, the said stop 73 encounters the rod 65 of the switch 55a.

The hydraulic diagram of the arrangement shown in Figures 5 - 7 is represented in Figure 8. A tank 76 is provided, from which the oil can be drawn by a pump 77, preferably actuated by the power of the tractor.

The pump 77 is connected by a conduit 78 to a distributor indicated as a whole by the reference 80, a pressure-limiting device 81 being included in the circuit for protecting the operating members. The distributor 80 comprises a first position "I", in which the oil drawn in by the pump 77 returns to the tank 76 through a pipe 82. In a position "II", a jack 85 is supplied through a flow regulator 86, while, in a position "Ill", the jack 85 is directly connected to the tank 76.

Provided on the supply circuit of the jack 85 is a piloted non-return valve 87 which permits the jack to be held in position, thus avoiding any leakages. This non-return valve is freed when the distributor is situated in the position "III".

In the three-point lifting system, the jack 85 replaces the third-point bar.

The distributor 80 is controlled by an electric circuit which is shown diagrammatically in Figure 9.

An electrical supply 90 is provided, to which are connected the switches 55 and 5a, with interposition of two contact-makers 91 and 91 a which rock on the overrunning of the dead-centre position.

Represented in detail in Figure 6 is a contact-maker 91 or 91a comprising a fork 92, the ends of which carry contacts 93 and 94 adapted to co-operate with corresponding contacts 95 and 96. The fork is conductive, pivoted on an insulating shaft 98 and fast with an insulating arm 97. A member 99 is provided, which tends to hold the contacts 94 and 93 applied respectively to the contacts 96 and 95, starting from an established angular position, when the arm 97 is urged in the direction of the arrow "f" and, on the contrary, for causing the fork 92 to rock against a stop 100 when the said arm is urged in the direction of the arrow g The arm 51 carries a finger 102 which is so disposed that, when the plough is raised and the return movement is initiated, it comes against the arm 97 of the contact-maker 91 for opening the supply circuit of the switch 55 and, on completing the return travel, for coming against the arm 98 of the contact-maker 91 a for closing the supply circuit of the switch 55a, and vice versa, when the plough is disposed in the initial position.

The manner in which this embodiment operates is as follows: The wheel 50 is resting in the ground, it being possible for the reversible plough to be pulled or pushed, and it constitutes a gauge wheel.

The plough is connected to a three-point lifting gear of a tractor, of which the third-point bar is formed by the hydraulic jack 85.

The position of the resilient stops 54 and 54a is regulated by tightening or slackening offthe screws 60 and 61 in such a manner that the closing of the contacts 63 and 64 of the switches 55 and 55a correspond to an established force.

The distributor 80 normally resides in the position "I". If the force applied to the wheel 50 exceeds a predetermined or setthreshold, which is defined by the position of the stop 54, the stop 73 pushes the rod 65, which closes the contacts 63 and 64, so that the distributor 80 is controlled and its slide valve comes to accupy the position "II", oil under pressure being admitted into the jack 85 from the piston rod side, so that the third-point bar, which is formed by the said jack, is shortened. In this way, a part of the weight of the plough is automatically transferred to the tractor.

If, on the contrary, the force applied to the wheel 50 falls below the set value, the stop 73 tends to be spaced from the rod 65 and the contacts 67 and 68 close the circuit on the distributor, so that the latter comes into the position "III", the oil contained in the jack 85 returning to the tank 76. In this case, the jack 85 is extended and a part of the weight of the plough is transferred to the gauge wheel 50.

As soon as the working conditions return to normal, the electric circuits are opened and the distributor 80 returns to its initial position "I".

When the plough is caused to pivot at the end of the field for the return travel, the arm 51 rocks on its shaft 52, so as the bear against the resilient stop 54a.

With this movement of the said arm 51, the finger 102 comes up against the arm 97 of the contact-maker 91, so that the circuit of the switch 55 is opened, the said switch coming into a position in which its fork 92 bears against the stop 100.

With this rocking movement, the arm 51 comes up against the bar or arm 97 of the contact-maker 91 a, which is then brought into a position in which the circuit of the switch 55a is closed (see Figure 9).

During operation, the same procedure as that which has just been described in continued and it is to be understood that, when the arm 51 returns to its initial position (Figure 5), it will control the opening of the circuit of the switch 55a and the closing of the circuit of the switch 55.

Yet another modification which can be adapted to a reversible plough is shown in Figures 10-14.

In this embodiment, a shaft 111 journalled in a bearing 112 and integral with an arm 113 is provided atthe free end of the frame 110 ofthe plough.

At its free end, the arm 113 carries a shaft 114, the free end of which supports a sleeve or bushing 115, in which is mounted a bar 116, the free end of which supports a gauge wheel 117.

The bar 116 has drilled therein two holes which are offset at an angle of 90 , while the bushing 115 has a hole 118, which is arranged to coincide selectively with the holes of the bar 116 in such a way that, by selective engagement of a spindle or pin, it is possible to secure the said bar in two positions which are offset by 90". The shaft 114 is able to turn freely in a bearing 120 of the arm 113, a ring 121 provided with two notches 122, 123, offset by 1800, is fast with the arm 113. Provided on the shaft 114 is a latch 124 pivoted on the shaft of a forked-joint 125 and capable of co-operating selectively with the notches 122, 123.

The frame 110 supports a casing on which is pivoted a forked-joint 128 comprising two side plates or cheeks 129 and 130 connected by a plate 131. The cheek 129 is fast with a collar 132, which can be brought into angular positions for coinciding with sockets or bushings 135 and 133 which are fast with the frame 170, so as to be able, by introduction of a pin 134, to permit the securing of the forked-joint 128 in the chosen position.

The cheek 129 has a groove 139 through which extends a screw 140, on to which is screwed a nut 141 and which carries a rubber block 142.

The cheek 130 has a groove 143 which permits the fixing of a rubber block 144 by means of a screw and nut.

Fixed on the plate 131 is a switch 150 of the same type as that described in connection with Figure 7 and comprising a member 151 with contacts 152 and 153, a rod 154 supporting contacts 155 and 156 and on which is engaged a spring 157 interposed between a stop 158 and the contact 155, the spring urging the contact 155 into engagement with the contact 153.

Pivoted on the plate 131 and on the shaft 161 is a cam 160 extended by a finger 162, the free end of which carries a small roller 163 which can be moved in a rail 164caried by the arm 113.

The hydraulic installation controlling the lengthening or shortening of the third-point arm, for example, the jack 85, is the same as that represented in Figure 8, but nevertheless the control installation can be much more simplified (see Figure 11), since the same switch is able to function in the two angular positions of the plough when operating.

It will be noted that the cam 160 is symmetrical with respect to a plane passing through the finger 162, so that the control of the switch 150 is identical in the two angular working positions of the plough.

In operation, the arm 113 is fitted between the blocks 142 and 144, of which the position along the slots 139 and 143 determines the adjustment of the thresholds from which the switch 150 has to actuate a distributor, such as the distributor 80.

If the force exerted on the arm 113 exceeds preset threshold value, the cam 160 controls the closure of the supply circuit of the jack by the closure of the contacts 152 and 156, so that the third-point arm (jack 85) is shortened.

If, on the contrary, the force is lower than the set threshold value, the cam 160 permits the closure of the return circuit to the tank for the oil from the jack 85 by the closing of the electric circuit corresponding to the control of the distributor 80 by the contacts 153 and 155 which are applied to one another.

When the plough is caused to pivot, for the return travel, the latch 124 is withdrawn from the notched 122 and the shaft 114 is then freed and is able to turn freely under the weight of the wheel 117 until the end of the return movement, when the latch 124 is then engaged in the notch 123. In this position, the system functions in the same manner as that which has just been described. The construction form which has just been described is of interest, because it is capable of being used just as well for a plough which is towed and disposed at the rear as for a plough which is pushed and disposed at the front. In the first case, the shaft 111 is positioned in front of the point at which the wheel 117 is bearing on the ground, while, in the second case, it is behind.In the case where the point of support of the wheel on the ground is in front of the shaft 111, there is obtained the additional advantage that the resistances to the rolling movement are compensated for by greater reaction on the ground, so that it is not produced by rotational torque with respect to the shaft 111, with which torque there would be the danger of causing an incorrect controlling of the switches.

The constructional form which has just been described may be of additional interest for the transporting of the plough. In actual fact, for the transporting position, the plough is caused to pivot through 90 , the gauge wheel having to support the said plough.

When the plough has to be brought into a position for being transported, the forked-joint 128 is caused to rock so as to disengage the arm 113, the latter then being able to swing or rock freely on its shaft 111. At the moment of turning round, the shaft 114 comes into a position vertically underneath the arm 113 and for placing the wheel 117 in a position which is suitable for it to support the-plough, it is sufficient to withdraw the pin 119 and, after having caused the bar 116to turn in the bushing 115, to resecurethe latter so that the wheel assumes the position which is shown in Figure 14. Thus, the wheel 117 which had been disengaged with respect to the arm 116 is re-centered, the latch 124 ensuring the locking effect by co-operation with the notch 122.The wheel thus constitutes a follower wheel, the bar 113 being held in a direction perpendicularto the arm 116.

In the case where the plough is a pushed plough, the turning round is effected in the opposite direction, that is to say, the shares pass above the plough, whereas, with pulled or towed ploughs, they pass beneath. As regards the pushed ploughs, it is, therefore, necessary to arrange for the bearing 112 to be fast with one end of a bar 170, the other end of which is fast with a section member 171. The bar 170 is so disposed as to form an obtuse angle with the section member 171 on the side of the latter which is adjacent to the frame 110 and is in two parts mounted to be pivotable coaxially.

Figure 15 shows diagrammatically a pushed plough in the position for being transported. For greater clarity, the forked-joint 128 has not been shown. It will be pointed out that, because of the arrangement which has just been described, the frame is horizontal in the position for transport.

In the various embodiments which have been described, a single-acting jack has been used, but it would be possible to provide a double-acting jack, the distributor then being designed accordingly.

In certain cases, it is possible to provide two ploughs, one mounted at the front and the other at the rear of the same tractor. Each plough has a device such as those which have just been described, such device making it possible to stabilise the assembly of tractor and ploughs by balancing the transfers of load at the front and at the rear.

It is obvious that the invention is not limited to the embodiments which have just been described and illustrated. It will be possible to apply thereto numerous modifications as regards details, without thereby departing from the scope of the invention.

Thus, it would, for example, be possible to replace the electric pick-ups by equivalent means, such as piezo-resistive pick-ups, pick-ups functioning as a result of unbalance of a Wheatstone bridge or pick-ups functioning by measurement of the hydraulic pressure.

Claims (11)

1. Device for improving the grip on the ground of a tractor provided with a three-point lifting gear or system comprising two lifting arms and a third-point bar and to which is attached a machine of which the frame comprises soil-working tools, means for its attachment to the three-point lifting gear, the said machine, when operating, being partly supported by a gauge wheel, the third-point bar being formed by a hydraulic jack, while the gauge wheel is connected to means for controlling the hydraulic jack are so arranged that, in response to a vertical force applied to the said gauge wheel and exceeding a determined or set threshold, the hydraulic jack is shortened and that, in response to a vertical force applied to the said gauge wheel and below a set threshold, the hydraulic jack is lengthened.

2. Device according to claim 1, characterised in that the gauge wheel is supported by a bar mounted for sliding movement in a sleeve or casing of a support, the said casing comprising a lateral lug supporting a guide rod of a compression spring, the said rod extending through a lug integral with the bar, the compression spring being fitted between a stop of the rod and the lug integral with the bar, this latter lug being provided with a first electric contact arranged to co-operate with a fixed electric contact when the force applied to the gauge wheel exceeds a set threshold value, these contacts being included in the electric circuit of a corresponding electromagnetic valve, and a second electric contact adapted to co-operate with a fixed electric contact when the vertical force applied to the gauge wheel is below a determined or set threshold value, these last contacts being included in the electric circuit of a corresponding electromagnetic valve.

3. Device according to claim 2, characterised in that the stop against which one of the ends of the compression spring is bearing is adjustable along the rod for modifying the calibration of the said spring.

4. Device according to claim 1, characterised in that the gauge wheel is carried by an arm movable on a shaft extending substantially perpendicularly of the direction of advance of the machine, the said arm being movable against the action of resilient means, the said means for controlling the hydraulic jack comprising a switch which is fast with a frame and which is arranged so that, when the arm rocks in a direction corresponding to a vertical force applied to the wheel and exceeding a set threshold, it closes an electric control circuit in such a way as to shorten the jack and, on the contrary, when the force supplied to the gauge wheel falls below a set threshold value, to lengthen the jack.

5. Device according to claims 1 and 4, characterised in that the resilient means are formed by rubber blocks.

6. Device according to claims 1,4 and 5, characterised in that the position of the rubber blocks is adjustable.

7. Device according to claims 1,4, and 6, and a dapted to a reversible plough comprising a frame and having ploughshares and mouldboards on both sides of the latter, characterised in that it comprises a lug or arm fast with the frame of the plough and supporting two rubber blocks disposed symmetricalliy with respect to a horizontal plane passing through the longitudinal axis of the frame of the plough, two switches which are also symmetrically disposed and arranged for closing a first control circuit when they receive an impulse which exceeds a set threshold and for closing a second control circuit when this impulse falls below a set threshold, a stop provided on the arm for co-operating with the corresponding switch in the ploughing position and a contact-mak erforopening, the time of return, the general circuit of the switch which was used at the time of the preceding ploughing operation and for closing the general circuit on the other switch which will be used at the time of the next ploughing operation.

8. Device according to claims 1,4 and 5, and adaptable to a reversible plough comprising a frame and having ploughshares and mouldboards on both sides of the latter, characterised in that the arm supporting the gauge wheel is fitted or inserted in a forked-joint with two side-pieces or wings and a web and comprising, inside its wings, the resilient buffers co-operating with the said arm and forming the resilient means, while the web supports the switch, the controlling of which is assured by a cam pivoted on the web and connected to the said arm, the gauge wheel being mounted for rocking movement on a support mounted on the arm so as to be able selectively to occupy two positions offset by 1800 and means being provided for wedging or securing it in the said two positions.

9. Device according to claim 8, and adaptable to a reversible plough, comprising a frame and having pioughshares and mouldboards on both sides of the said frame, characterised in that the forked-joint is pivotally mounted so as to be able to be retracted, while the support of the gauge wheel comprises means for wedging or securing the gauge wheel in two positions offset angularly by 90".

10. Device for adjusting the load applied to a tractor by an implement having soil-working tools which is connected to the tractor by a three-point lifting gear and is partly supported by a gauge wheel spaced from the tractor, the device including a hydraulic jack which constitutes the third-point bar of the lifting gear and means for controlling the jack so that, in response to a vertical force applied to the gauge wheel in excess of a predetermined value, the length of the jack is changed to increase the load on the tractor, and, in response to a vertical force applied to the gauge wheel below a predetermined value, the length of the jack is changed to reduced the load on the tractor.

11. Device for adjusting the load applied to a tractor substantially as hereinbefore described with reference to the accompanying drawings.