MXPA99010333A

MXPA99010333A - Tractor hitch lower link lifting device

Info

Publication number: MXPA99010333A
Application number: MXPA/A/1999/010333A
Authority: MX
Inventors: Coenen Herbert
Original assignee: Gkn Walterscheid Gmbh
Priority date: 1999-01-09
Filing date: 1999-11-10
Publication date: 2000-10-01

Abstract

The way valve (33) of each bottom rod cylinder unit (11) has two connections (31,32) to the cylinder notably to piston (23,23a) and rod (24,24a) spaces and a third connection (34,34a) to pressure (P) and a fourth (36,36a) to the return (37,37a). All connections are closed off in the first valve (33) position and in the second position, the cylinder piston side (23,23a) connects to pressure (P) as opposed to the rod side (24,24a) connected to return. In the third position of the valve, the rod side (24,24a) is pressurized and the piston side (23,23a) joins to return. A circuited switching unit (28,28a) contains one input fed with value (F) corresponding to the ideal bottom rod setting, and a second input fed with the position pickup (25) signal showing the current rod position. The unit (28,28a) uses its comparator (27,27a) to compare value (F) with the current position signal and from this switches the valve into one of its three settings accordingly.

Description

ELEVATION MECHANISM FOR THE LOWER CONDUCTOR BAR OF A TRACTOR MOUNTED DEVICE FIELD OF THE INVENTION The invention relates to a lifting mechanism for the lower guide rods of a mounted device of a tractor or self-propelled work machine.

BACKGROUND OF THE INVENTION The tractors, or also, the self-propelled work machines are provided in the rear part and, if necessary, in the front, of a mounted device, which includes the lower conductive bars, which are arranged in a displaced manner. in the opposite direction to the longitudinal axis of the tractor or the self-propelled work machine. They are located in the rear part, or, in the front, hinged at one end, in particular, they can pivot about an essentially horizontal axis. In addition, there is a possibility of slightly lateral pivoting. The conductive bars have at their free ends coupling means for connection with an apparatus to be assembled. A lifting mechanism is used to turn the bus bars. By means of the same, therefore, an apparatus placed on the lower conductor bars is supported and raised or lowered with respect to the floor. Also, depending on the apparatus to be assembled, or the type of work, the lifting mechanism must allow the conductor bars to move freely to some extent laterally. In other types of work a possibility of free movement in a vertical sense is required. Mainly used lifting mechanisms in the tractor or work machine and including two parallel lifting arms, which are fixed to the ends of a lifting tree. This is rotated by a drive, so that the lifting arms move correspondingly describing a circular arc. At the ends of the lifting arms are fixed lifting supports, which are attached to the lower conductor bars. In addition, the lifting shaft ensures synchronization of the two lifting arms. To achieve a better possibility of adjustment, especially in the case of a lifting mechanism arranged in the rear part, a longitudinal fine adjustment of the lifting supports is provided. Likewise, a device is usually provided which, optionally, provides a fixed connection with the lower conductor bars or, upon unlocking, allows a free vertical movement. This device makes possible a vertical movement path for the yaw of the apparatus, that is, a compensation of the transverse inclination to allow work without stresses. In the case of the front lifting mechanisms, the lower bus bars are usually connected to each other as an oscillating link, and the drive, in the form of two hydraulic lifting supports, is directly connected to the bus bars. The vertical degree of freedom for the compensation of the transverse inclination is given by a longitudinal perforation in the coupling element. A device such as this is described, for example, in WO 96/03024. The two hydraulic lifting brackets can be connected through a common hydraulic line, with a pump or a pressure supply source. This means that they are loaded with the same pressure and that they receive pressure agent in the same amount. The lifting mechanisms described above have in common that, in spite of the different embodiments in terms of construction, a synchronized movement of the two lower conductor bars is provided and a vertical degree of freedom is available. In the lifting mechanisms it is disadvantageous that an exact mechanical coupling is not guaranteed, since, for example, losses due to leakage can cause a different displacement despite the common load.

OBJECTIVES AND ADVANTAGES OF THE INVENTION The invention is based on the objective, on the one hand, to simplify in comparison with the solutions of the prior art, the configuration of the lifting mechanism with respect to the main functions, and in addition, to guarantee an exact synchronization for the displacement, as well as to foresee other possibilities of displacement. This object is achieved according to the invention by means of a lifting mechanism for the driving rods of a mounted device of a tractor, which are placed with one end on the tractor, so that they can rotate, and each bar including a sensor position, which generates a position signal representing the corresponding rotational position of the lower bus bar, a double action piston-cylinder unit, which can be hydraulically loaded and is placed with one end on the tractor and connected with the another end with the lower bus bar, either directly or indirectly through an intermediate lifting gear, and having a cylinder space on the piston side and a cylinder space on the piston rod side, each piston unit having cylinder a circuit, which includes a distribution valve with four connections, which can be placed in at least three positions of switching, using two connections for the connection with the piston-cylinder unit, of which the first connection is connected through a line with the cylinder space on the side of the piston, and the second connection, with the cylinder space on the side of the piston rod, a third connection being used for connection with a supply of pressure agent and the fourth connection for the reflux connection, and all the connections of the distribution valve in the first position thereof being blocked. and the cylinder space of the piston side with the supply of pressure agent and the cylinder space of the piston rod side with the reflux being joined in the second position, and in the third position, the cylinder space of the piston side with the reflux, and in the third position, the cylinder space of the piston rod. piston rod with the supply of pressure agent and the cylinder space with the reflow, and the circuit presenting an electric switching unit, which has a input to enter a guiding dimension corresponding to the theoretical position of the lower bus bar, an input to input a position signal coming from the position sensor and representing the respective position of the lower bus bar, and a comparator that compares the guide dimension with the position signal, where the switching unit, depending on the result of the comparison, changes the distribution valve to assume one of the at least three positions.

In this configuration, it is advantageous that with a simplified shaping there are additional functions. Likewise, an exact displacement is guaranteed, since the corresponding real position of the lower bus bars is compared with the predetermined theoretical position and a subsequent adjustment is carried out so that each time the theoretical position is sought to be reached. The foregoing also implies that, in principle, the two lower bus bars assume the same position, or the predetermined position. Furthermore, by means of the individual control of the piston-cylinder units, it is possible to independently position the two lower bus bars, for example, to compensate for a transverse inclination. In one embodiment of the invention, it is proposed that the distribution valve provided by each lower bus bar be brought to a fourth switching position in which both the cylinder space on the piston side and that on the rod side thereof are connected with reflux. Thus, for example, when a reflux switch is given in the piston-cylinder units, that is, when the length of the piston-cylinder units can be freely adjusted, an unforced decoupling of the device is also possible. Likewise, it is also possible to adjust a movement path without the operator having to leave the driver's seat, as is the case in mechanical solutions. For a synchronized movement of the lower bus bars, it is provided that the two switching units are supplied with guide dimensions with the same values or guide dimensions with different values. To complement the solution according to the invention, a selection switch must be provided, which allows the choice of various working modes and acts on the two switching units. Thus, they can receive a specification, in the sense that the control of the two lower bus bars is carried out according to the same guiding dimensions or different guiding dimensions. In addition, it is envisaged that the two switching units are connected to a common selection switch, through which the switching units of a distribution valve can be predefined to a specific position or of the distribution valves of the switching devices. two conductive bars lower than the fourth position. There, an exchange of pressure agent takes place between the cylinder spaces of the piston rod side and the piston side of the two piston-cylinder units with each other and with the reflow. Preferably, the guide dimensions can be predetermined, namely, in particular, manually predetermined by means of an adjusting member. Thus, the operator can specify the desired position of the lower bus bars. With different guide dimensions for them, a transversal inclination can be compensated. To achieve a certain orientation of the device towards the tractor in the case of a transverse inclination thereof, an inclination sensor can also be used, which registers, for example, the transversal inclination of the tractor. In addition, means can be provided, through which a signal for the superimposition of the guide dimensions, which represents the transverse inclination, can be fed to at least one of the two switching units. The driver himself can carry out a compensation, based on a visual evaluation of the transversal inclination. For example, it can be determined that the lower bus bar that is closest to the floor is controlled according to the predetermined guide dimension, while the adjustment of the other lower bus bar can only be made in the direction of a higher elevation. Therefore, in addition, it can also be foreseen that the selection switch also includes a switching position, in which the inclination sensor is activated and the signal generated by it can be superimposed on the guide dimensions fed to the two control units. commutation. You can also specify maximum differences for the guide dimensions, to avoid too high a stress on the components and the assembled device. It can also be envisaged that when raising an apparatus to the transport position, straightening takes place from a certain elevation position. In order to adjust the position of the two lower bus bars, for example, a device with which the corresponding movement position of the piston rod exit of the piston-cylinder unit can be registered. However, it is also possible to carry out the position sensor by means of a rotation angle sensor, which registers the respective angular position of the corresponding lower bus bar in relation to the tractor.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawing several embodiments of the invention are shown. In principle, the modalities can be used both in a mounting device in the rear part as well as in a front mounting device. They show: Figure 1, a first embodiment of the lifting mechanism for a mounting device on a tractor, which is placed at the rear thereof. Figure 2, a circuit for the displacement of the piston-cylinder units, to drive the lower conductor bars, the piston-cylinder units of a length meter being provided as a position sensor. Figure 3, schematically, a mode in which the position sensors are made by rotation angle sensors. And Figure 4, a mode in which an indirect load of the lower bus bars is carried out with the piston-cylinder units, by means of the intermediate connection of a lever gear.

DETAILED DESCRIPTION OF THE INVENTION In Figure 1 the rear part of a tractor with a device mounted on the rear side is shown. Two lower guide bars 2, 3 are provided. In view of the rear part of the tractor 1, the lower right-hand guide bar 2 is arranged to the right, and the lower-left guide bar 3, to the left, of the axis Longitudinal of the tractor. Both lower busbars 2, 3 are mounted rotatably around a transverse axis 4, by means of connection ends 5, 6, in the corresponding trestles 7, 8 of the rear part of the tractor, so that the ends provided with the coupling hooks 9, 10 can move more or less close to the floor in which the tractor 1 is located, or, moving upwards from that one. . To move the two lower guide bars 2, 3 around the axis of rotation 4, these are respectively subordinated to a piston-cylinder unit 11, or 12. The piston-cylinder units 11, 12 come into contact by means of a connecting end. 13, 14 in the form of a fork with the corresponding lower bus bar. With their other connecting end 15 or 16, they are supported by trestles 17 or 18, which are fixed to the rear of the tractor. The configuration is carried out, for example, by intermediately connecting a joint element, which is connected to the housing of the corresponding piston-cylinder unit 11 or 12 so that it can rotate about a first axis and which, moreover, is joined with the respective ridge 17 or 18, so that it can rotate about an axis extending parallel to the axis of rotation 4, to thereby allow a free adjustment from all sides of the plunger-cylinder units 11 or 12 to the displacing the lower bus bars 2, 3. Furthermore, in the area of the connecting ends 5, 6, the two lower bus bars 2, 3 can be tilted slightly about their longitudinal axis. To complement the assembled device, there is also an upper bus bar 19, which serves to engage the upper connection means of the apparatus to be assembled. The two coupling hooks 9, 10 serve to engage the lower connecting elements of the apparatus to be assembled. The function of the lifting mechanism, which includes the two piston-cylinder units 11, 12, is described in more detail with the aid of Figure 2. The two piston-cylinder units 11, 12 for driving the lower right-hand bus bar, or Well, left 2, 3 and their respective switching means are configured in an identical manner, so that a description of the drive is made only based on the piston-cylinder unit 12 assigned to the lower left bus 3. unit has a housing 20, which displaceably encompasses the plunger 21 with the plunger rod 22 exiting the housing 20. The plunger rod 22 carries the connecting end 14 for joining with the lower left bus bar and the housing 20 presents the connection end 16 for joining with the stand provided on the side of the tractor. For the other description, the cylinder space on the piston side is indicated as 23, while the cylinder space on the rod side of the cylinder carries the reference 24. The piston-cylinder unit is shown in Figure 2 in the fully introduced position, ie , the cylinder space on the side of the piston 23 adopted a minimum, while the cylinder space on the side of the piston rod 24 assumed its maximum. In addition, a position sensor 25 is integrated in the piston-cylinder unit 12. Said sensor generates a signal, which represents the position of insertion, or expulsion, of the piston rod 22 in the housing 20, and thereby, indicates the length between the points of articulation of the connecting ends 14, 16. The position sensor 25 is connected by a signal line 26 with a comparator 27 of a switching unit 28. The plunger-cylinder unit 12 is attached to pressure agent lines 29, 30 with the two connections 31, 32 of a distribution valve 33. In this way, the pressure agent line 29 establishes a connection between the cylinder space of the piston side 23 and the first connection 31, while the second pressure agent line 30 establishes a connection between the cylinder space of the rod side of the piston 24 and the second connection 32 of the distribution valve 33. This presents two other cores. nexions, namely, a third connection 34, which is attached to a pressure line 35, which is fed by a pump P or other supply of pressure agent. The fourth connection 36 is connected without pressure to the tank 38 by the reflux line 37. The distribution valve 33 is an electromagnetically adjustable valve, which can adopt the four positions and which, for the different positions, establishes or blocks different connections between the connections 31, 32, 34, 36. For this, it can be moved either in the direction A or in the direction B., showing the basic position, in which all the connections 31, 32, 34, 36 are separated each. The movement of the distribution valve 33 to adopt one of the possible switching positions is carried out by the corresponding control lines 39, 40, which receive their switching pulses from the switching unit 28. To the comparator 27 of the switching unit 28 can be specified by means of a control line 42, a guide dimension F through a manual adjustment member 41. Thus, with said member it is possible to adjust the lifting position of the lower guide bar corresponding to the piston-cylinder unit 12. The distribution valve 33 is in the first position, ie the switching position "0", all the connections 31, 32, 34, 36 being blocked. In the switching position "I" , the pressure line 35 is connected through the connection 34 and the connection 31 with the pressure agent line 39, and through it, with the cylinder space of the piston side 23, so that the piston rod 22 leaves the housing 20. In the position "II" of the distribution valve 33, the pressure line 35 is connected through the connection 34 to the connection 32 and through the corresponding line of agent pressure 30, with the cylinder space on the rod side of the plunger 24, so that it is inserted into the housing 20. The above is equivalent to raising the respective lower bus bar. In the "III" position of the distribution valve 33, the connection of the pressure line 35 and the connection 34 is blocked. The pressure agent lines 29 and 30 are connected to each other through the corresponding connections 31 and 32, and through the connection 36, with the reflux 37 towards the tank 38, so that a free exchange of the agent of pressure, that is to say, the piston rod 22 can freely enter and exit in and out of the housing 20. The configuration described in relation to the piston-cylinder unit 12 is carried out in the same way in the case of the piston-cylinder unit 11. this reason, the description of the unit 12 can be read in that of the unit 11, and in it, for the equivalent components in the piston-cylinder unit 12 the same references are used, which to differentiate have been provided only with a hairline. The adjustment member 41 also acts via a control line 43 on the switching unit 28 'belonging to the piston-cylinder unit 11. This means that also the switching unit 28' receives the same guide dimension F or a different, which is fed through the control line 42 of the switching unit 28. Additionally, there is a selection switch 44, which also acts on the two switching units 28 and 28 ', by means of the control lines 45, 46. The selection switch 44 allows the adjustment of various working modes. Thus, for example, in a first selection position a working mode can be achieved, in which a synchronized displacement of the two lower buss is desired. This means that an identical change in the length of the two piston-cylinder units 11, 12 is desired. This also means that the specified guide dimension F to the two switching units 28, 28 'via the adjusting member 41 it is the same for the two piston-cylinder units 11, 12, so that a variation of these of the guide dimension, depending on the size of the variation, leads to a drive of the respective distribution valves 33, 33 ', in order to reach the corresponding length adjustment. Said adjustment is achieved when the control signal, emitted by the position sensor 25, through the signal line 26, to the comparator 27, is reproduced with a size that agrees with the guide dimension F. In this case, adopts switching position "0". In variations of the above, which require an exit of the plunger rod 22 or an introduction thereto, the passage of the distribution valve 33 to one of the other two switching positions "I" or "II" takes place. For the case in which the selection switch 44 passes to a position, in which a floating movement of the two lower buss is to be allowed, the two distribution valves 33 pass to the "III" position. If the same guide dimensions are specified via the adjustment member 41 for driving the two lower guide bars, the two pass into identical positions. This system can be supplemented by additionally using a transverse inclination sensor, by means of which an overlap can also take place depending on the tilting position of the tractor. The selection switch 44 can also provide for a switching possibility such as this and correspondingly affect the two switching units 28, 28 '. The signal provided additionally by the inclination sensor to the switching unit 28 or 28 ', can lead to the overlap of the guide dimension F, and thus, make one of the piston-cylinder units 11, 12 adopt a greater or lesser length. Figure 3 shows a modified modality with respect to that according to Figures 1 and 2. The two lower conductive bars 2 ', 3' and the upper conductive bar 19 'placed on top can also be observed. Also, it can be recognized that the two lower bus bars 2 ', 3' are assigned respectively a piston-cylinder unit 11 ', 12'. Unlike the embodiment of the piston-cylinder units 11, 12 according to Figure 2, the piston-cylinder units 11 ', 12' according to Figure 3 do not have any length measuring device as a position sensor. As a position sensor 25 ', angle sensors are provided, which are positioned on the axis of rotation 4' of the two lower bus bars 2 ', 3' and register the angular position of the two lower bus bars 2 ', 3 ', which represents the rotation position of the same. The signal generated by the position sensors 25"is fed to the respective switching units. Figure 4 shows another embodiment in which the two lower conductor bars 2, 2 ', 3' 'are joined by simple lifting brackets 47, 48, which can not be modified in their length, with two lever arms 49, 50, which can rotate about a common axis of rotation 51, but without being synchronized with each other. The two lever arms 49, 50 are loaded by two piston-cylinder units 11"and 12", so that a driving of the two lower conductor bars 2"6 3" takes place, with the intermediate connection of the lever gear of the two lifting brackets 47, 48 and the corresponding lever arms 49, 50. The two piston-cylinder units 11"and 12" may have a configuration that agrees with that of Figure 2. With regard to With respect to the position record for the lower bus bars 2, 2 ', a configuration as described in relation to Figure 3 is also possible.

List of references Tractor 2, 2 ', 2"Lower right conductive bar 3, 3 ', 3"Conductor bar lower left 4, 4 'Spindle 5 Connection end 6 Connection end 7 Stand 8 Stand 9 Coupling hook 10 Coupling hook 11, 11', 11 '', 11 Plunger-cylinder unit 12, 12 ', 12' 'Unit piston-cylinder 13 Connection end 14, 14 'Connection end 15 Connection end 16, 16' Connection end 17 Ridge 18 Ridge 19, 19 'Top guide bar 20, 20' Housing 21, 21 'Plunger 22, 22' Piston cup 23, 23 ', 23"Cylinder space of piston side 24, 24 Cylinder space of piston rod side 25, 25', 25 '' Position sensor 26, 26 'Signal line 27, 27 'Comparator 28, 28' Switching unit 29, 29 'Pressure agent line 30, 30' Pressure agent line 31, 31 ', 31' 'First connection 32, 32' Second connection 33, 33 ', 33' 'Distribution valve 34, 34', 34 '' 'Third connection 35, 35' Line of pressure 36, 36 ', 36' "Fourth connection 37, 37 'Reflow 38 Tank 39, 39 'Control line 40, 40' Control line 41 Adjusting device 42 Control line 43 Control line 44 Selection switch 45 Control line 46 Control line 47 Lifting bracket 48 Lifting bracket 49 Lever arm 50 Brake arm lever 51 Spindle F Guide dimension P Supply of pressure agent

Claims

NOVELTY OF THE INVENTION Having described the above invention, it is considered as a novelty, and therefore, the content of the following is claimed as property: CLAIMS

1. A lifting mechanism for the lower guide bars of a tractor mounted device, which are rotatably mounted with one end on the tractor, which includes for each lower bus bar (2, 2 ', 2", 3, 3 ', 3") a position sensor (25, 25', 25"), which generates a position signal representing the corresponding rotational position of the lower bus bar (2, 2 ', 2", 3 , 3 ', 3"), for each lower conductive bar (2, 2', 2 '', 3, 3 ', 3") a piston-cylinder unit (11, 11', 11", 12, 12 ', 12") with double action, which can be hydraulically loaded and is placed with one end (5, 6, 13 , 14, 14 ', 15, 16, 16') on the tractor (1) and connected with its other end (5, 6, 13, 14, 14 ', 15, 16, 16') with the lower guide bar (2, 2). ', 2", 3, 3', 3"), either directly or indirectly through an intermediate lifting gear (47, 48, 49, 50), and having a cylinder space on the piston side (23 , 23 ') and a cylinder space on the side of the plunger rod (24, 24'), each plunger-cylinder unit (11, 11 ', 11", 12, 12', 12") having a circuit, which it includes a distribution valve (33, 33 ') with four connections (31, 31', 32, 32 ', 34, 34', 35, 35 ', 36, 36'), which can be placed in at least three switching positions, using two connections (31, 31 ', 32, 32') for the connection with the piston-cylinder unit (11, 11 ', 11", 12, 12', 12"), of which the first connection (31, 31 ') is connected through a line with the cylinder space of the cylinder. side of the plunger (23, 23 '), and the second connection (32, 32'), with the cylinder space of the rod side of the plunger (24, 24 '), a third connection (34, 34 ') for the connection with a supply of pressure agent (P) and the fourth connection (36, 36') for the connection with a reflux, and all the connections (31, 31 ', 32, 32', being blocked), 34, 34 ', 36, 36') of the distribution valve (33, 33 ') in the first position thereof and the cylinder space of the piston side being connected in the second position. (23, 23 ') with the supply of pressure agent (P) and the cylinder space of the rod side of the plunger (24, 24') with the reflux (37, 37 '), and in the third position, the cylinder space of the piston rod side (24, 24 ') with the supply of pressure agent (P) and the cylinder space of the piston side (23, 23') with the reflux (37, 37 '), and the circuit also having an electrical switching unit (28, 28 '), which has an input for entering a guide dimension (F) corresponding to the theoretical position of the lower bus bar (2, 2', 2). '', 3, 3 ', 3' '), an input for inputting a position signal coming from the position sensor (25, 25', 25") and representing the respective position of the lower bus bar (2, 2 ', 2", 3, 3', 3"), and a comparator (27, 27 ') comparing the guide dimension (F) with the position signal, wherein the switching unit (28, 28') , depending on the result of the comparison n, change the distribution valve (33, 33 ') to assume one of the at least three positions.

2. A lifting mechanism in accordance with claim 1, characterized in that the distribution valve (33, 33 ') provided by each lower bus bar (2, 2', 2", 3, 3 ', 3" ) it is possible to pass to a fourth switching position in which both the piston space of the piston side (23, 23 ') and the cylinder space of the piston rod side (24, 24') are connected to the reflux ( 37, 37 ').

3. A lifting mechanism according to claim 1, characterized in that the guiding dimension (F) of the two switching units (28, 28 ') can previously receive the same or different values.

4. A lifting mechanism in accordance with claim 3, characterized in that the two switching units (28, 28 ') are connected to a common selection switch (44), through which a control of the two bars can be specified for the switching units (28, 28'). lower conductors (2, 2 ', 2", 3, 3', 3") according to the same guide dimension (s) (F) or different guide dimensions.

5. A lifting mechanism in accordance with claim 1, characterized in that the two switching units (28, 28 ') are linked to a common selection switch, through which, they can be specified the step to the fourth position of a distribution valve (33, 33 ') for a given lower bus bar or the distribution valves (33, 33') of both lower bus bars (2, 2 ', 2", 3, 3', 3").

6. A lifting mechanism according to claim 3, characterized in that the guide dimension (F) can be specified manually by means of an adjustment member (41).

7. A lifting mechanism according to claim 1, characterized in that means are provided, through which at least one of the two switching units (28, 28 ') can be fed a signal representing a transversal inclination to overcome the guide dimension (F).

8. A lifting mechanism according to claim 7, characterized in that there is an inclination sensor, which registers the transverse inclination of the tractor (1).

9. A lifting mechanism according to claim 8, characterized in that the selection switch (44) includes a switching position, in which the inclination sensor is activated, which generates a signal that can be superimposed to the guide dimensions (F) fed to the two switching units (28, 28 ').

10. A lifting mechanism according to claim 1, characterized in that the position sensor (25, 25 ', 25") is made by means of a device, with which the respective one can be registered. exit position of the piston rod (22, 22") of the piston-cylinder unit (11, 11 ', 11")

11. A lifting mechanism in accordance with claim 1 of any of claims 1 to 9, characterized in that the position sensor (25, 25 ', 25") is made by a rotation angle sensor, which registers the respective angular position of the corresponding lower conductor bar (2, 2', 2", 3). , 3 ', 3' ') with respect to the tractor (1). SUMMARY OF THE INVENTION The invention relates to a lifting mechanism for the lower guide rods of a tractor. For each lower guide bar 2, 3 a position sensor 25 is provided, which generates a position signal representing its rotational position, a piston-cylinder unit 11, 12, which makes contact in the tractor and in the bar lower conductor 2, 3. Each piston-cylinder unit 11, 12 has a circuit, which includes a distribution valve 33, 33 'with four connections and four switching positions, in which first position all the connections are blocked, in whose second position the cylinder space of the piston side is attached to the supply of pressure agent and the cylinder space of the rod side of the piston is connected to the reflux and in whose third position, the cylinder space of the piston rod side it is connected to the supply of pressure medium and the cylinder space of the piston side is connected to the reflux, and in the fourth position of which the two cylinder spaces are connected to the reflux. To a switching unit 28, 28 'a guide dimension F corresponding to the theoretical position of the lower bus bar 2, 3 is fed, a position signal coming from the position sensor 25, 25' and representing the respective position of the lower conductor bar. A comparator compares the guide dimension F with the position signal. The switching unit 28, 28 ', depending on the result of the comparison, causes the distribution valve 33, 33' to assume one of the four positions.