Patent Description:
Hitch devices are used in tractors to attach different types of implements to the tractor. Hitch devices comprise a plurality of different arms with the common feature that they comprise a first attachment link and a second attachment link, wherein the length between the attachment links is adjustable in order to render the arms suitable for use with different tractors and with different implements.

A common hitch device is a three-point hitch device which can be arranged in the rear of the tractor and which includes two drawing arms and one pushing arm, which are generally operated hydraulically. The pushing arm is arranged above the drawing arms and centred between the latter in a horizontal direction. The function of the pushing arm is to control the angle of inclination of the connected implement parallel to the tractor. The drawing arms regulate the height of the connected implement. Lateral stabilizers are also employed between the drawing arms and the tractor by means of which the lateral movement of the implement is limited. By means of the adjustable lifting bar of the drawing arm, it is possible to adjust the height difference between the respective drawing arms and thus the angle of transverse inclination of the connected implement.

It is also possible to use a two-point hitch device, which lacks the pushing arm of the three-point hitch device, to attach implements to the tractor. The drawing arms forming the two points regulate the height of the implement and limit its lateral freedom.

The length adjustment of the arm can be provided by means of a thread or hydraulically according to the prior art. In particular turnbuckle solutions are common. For the length adjustment of the arm between the first attachment link and the second attachment link, a rotatable adjustment tube is frequently used that has two threads in a turnbuckle solution: first threads at a first end of the adjustment screw and second, inverse threads at a second end of the adjustment tube. The adjustment tube is thus freely rotatable relative to the first attachment link and the second attachment link. The arms of the tractor hitch device are subjected to pushing and pulling loads, so it is necessary to make the structure of the arms very robust. In the solutions according to the prior art, in particular the rotary joints of the adjustment tube represent weak points.

The patent publication <CIT> represents known prior art, in which a threadless rotary joint is provided in an arm of a tractor hitch device between the adjustment tube and an axle attached to an attachment link. The arm disclosed in the publication is manufactured by machining a ridge into the axle, partially inserting the axle with the ridge first into the adjustment tube, and then welding a separate sleeve onto the end of the adjustment tube, the sleeve blocking the ridge in the adjustment tube. The attachment link is then welded to the end of the axle opposite the ridge. A problem with known arms is the poor durability of the welded joints.

Further prior art can be found in documents <CIT>, <CIT>, and <CIT>.

It is an object of the invention to provide an improved arm of a tractor hitch device that can withstand the pushing and pulling loads better than before. The characteristic features of this invention are indicated in the attached patent claim <NUM>. A further object of the invention is to provide a new method for manufacturing an arm of a tractor hitch device in order for the arm to withstand the pushing and pulling loads better than before. The characteristic features of this invention are indicated in the attached patent claim <NUM>.

An arm of a tractor hitch device according to the invention comprises a first attachment link and a second attachment link, wherein a first axle is attached to the first attachment link and a second axle is attached to the second attachment link, and wherein an adjustment tube with a sleeve-like first end and with a second end is provided between the first attachment link and the second attachment link, and wherein the first axle is partially inserted coaxially into the sleeve-like first end of the adjustment tube and wherein the second axle is attached to the second end of the adjustment tube, and wherein a rotary joint is provided between the first axle and the adjustment tube that allows the first axle and the adjustment tube to rotate freely relative to each other, wherein, for the formation of the rotary joint, the first axle includes a ridge and a thinner axle section between the ridge and the first attachment link, and the sleeve-like first end of the adjustment tube includes a narrowing that extends to the thinner axle section, the narrowing being configured around the thinner axle section. The rotary joint is a crimped reduction joint, wherein the first end of the adjustment tube is reduced so as to render the diameter of the inner edge of the narrowing smaller than the diameter of the ridge while leaving the diameter of the inner edge of the narrowing larger than the diameter of the thinner axle section.

It is thereby possible to form a threadless rotary joint between the first attachment link and the adjustment tube that can withstand pushing and pulling loads better than the prior art. In addition, the threadless rotary joint according to the invention requires less space compared to a turnbuckle solution, i.e. the arm can be made shorter than before. The arm according to the invention is also advantageous to manufacture in terms of its cost, as less labour and manufacturing materials are required than before.

In the arm according to the invention, the narrowing of the sleeve-like first end of the adjustment tube, i.e. the diameter of the inner edge of the ridge on the inner surface, is rendered smaller than the diameter of the ridge of the first axle by reducing the first end of the adjustment tube in a manufacturing step of the arm. The narrowing of the first end of the adjustment tube is configured loosely around the thinner axle section of the first axle, which allows the adjustment tube to rotate freely around the first axle while the edges of the thinner axle section limit the longitudinal movement of the first axle when they hit the edges of the narrowing of the adjustment tube.

Preferably, the first end of the adjustment tube in the reduction joint is cold-formed so as to have a smaller diameter. In the cold forming, the sleeve-like first end of the adjustment tube is crimped to a smaller diameter at room temperature, which causes a plastic deformation of the first end of the prefabricated adjustment tube, in the course of which the narrowing is left loosely in the groove formed by the thinner axle section and its edges when the inner diameters of the first end of the adjustment tube are reduced in the crimping step. When a metal article is cold-formed, its grains are flattened in the direction of working. The relative proportion of the grain boundary in the volume of the article and the dislocation density increase. The article becomes stronger; its yield strength increases while its elongation decreases. Crimping does not leave any pronounced points of discontinuity in the first end of the adjustment tube because the working of the metal brought about by the crimping increases towards the end of the article.

The reduction according to the invention can also be realized by other known metal working techniques such as other cold-working techniques or hot working at elevated temperatures.

Preferably, the first end of the adjustment tube is essentially structurally homogeneous with the exception of the changes in its crystal structure caused by the reduction. Structural homogeneity here means that the article, here the first end of the adjustment tube, is manufactured from a single piece of manufacturing material. In other words, the article thus does not contain any welded joints or other structural joints that compromise the mechanical strength of the structure.

By means of the reduction in the manufacturing phase and resulting reduction joint in the first end of the adjustment tube of the finished arm, the first end of the adjustment tube can be rendered structurally homogeneous, which makes welded joints or other joints that compromise mechanical strength unnecessary in the first end of the adjustment tube. This additionally allows the arm to be produced in a simpler manner than before, which expedites the manufacture of the arm and increases its mechanical strength.

The adjustment tube can be structurally homogeneous as a whole. It is thus possible for the adjustment tube to be formed entirely from a single piece of manufacturing material, preferably of metal pipe, most preferably made of steel, so that selected narrowings, reductions and threads or other features according to a selected criterion are machined into the adjustment tube. It is thereby possible to render the arm structurally very robust.

Preferably, the first attachment link and the first axle are structurally homogeneous. The first attachment link and the first axle can thus be machined from a single piece of manufacturing material, preferably steel, which renders the first attachment link and the first axle structurally very robust. The reduction of the first end of the adjustment tube of the arm makes it possible to attach the whole formed by the structurally homogeneous attachment link and the first axle to the adjustment tube by means of a rotary joint.

Preferably, the arm is a screw-adjustable lateral stabilizer or an automatic lateral stabilizer or a lifting bar or a pushing arm of a three-point hitch device. These applications require a rotary joint that can withstand in particular pulling and pushing loads. The arm can also be the arm of a two-point hitch device or of some other tractor hitch device.

Preferably, the inner surface at the second end of the adjustment tube has an internal thread and the outer surface of the second axle has a corresponding external thread so that the second axle is configured so as to be partially rotatable into the adjustment tube by a selected distance in order to form the length adjustment of the arm. This allows a length adjustment of the arm to be provided in a mechanically simple manner. The adjustment tube thus only has one threaded joint at the second end of the adjustment tube with the second axle, which increases the precision of a length adjustment of the arm vis-à-vis a turnbuckle solution, since an adjustment of a single rotation of the adjustment tube halves the distance of movement vis-à-vis a turnbuckle using the same threads. Alternatively, the length adjustment of the arm can be provided, for example, hydraulically, whereby the arm can accommodate a hydraulic piston by means of which it is possible to adjust the length of the arm by controlling the pressure of a hydraulic fluid with a hydraulic pump.

Preferably, the diameter of the outer surface of the first end of the adjustment tube is smaller than the diameter of the thickest point of the outer surface of the adjustment tube. The adjustment tube can thus be manufactured from a metal tube of uniform material thickness in its initial condition and the rotary joint is produced by reducing the first end of the adjustment tube.

Preferably, the diameter of the outer surface of the second end of the adjustment tube is smaller than the diameter of the thickest point of the outer surface of the adjustment tube. An attachment for the second axle can thus be made in the second end of the adjustment tube by means of reduction.

The diameter of the inner surface of the narrowing can be <NUM> - <NUM>, preferably <NUM> - <NUM>, larger than the diameter of the thinner axle section. This yields a joint which can withstand a load very well structurally, yet which still has sufficient clearance for a rotational degree of freedom.

In one embodiment of the invention, the length of the thinner axle section of the first axle is <NUM> - <NUM>, preferably <NUM> - <NUM>, longer than the length of the narrowing of the adjustment tube, which makes it possible to form a joint that is rotary yet still fixed in the longitudinal direction of the arm. By means of a joint that only has just enough clearance in both the longitudinal and radial directions, a robust and sturdy yet freely rotatable structure is provided, which can be used, for example, in an automatic lateral stabilizer, a lifting bar or a pushing arm of a three-point hitch device of a tractor.

In another embodiment of the invention, the length of the thinner axle section of the first axle is significantly longer, preferably <NUM> - <NUM>, more preferably <NUM> - <NUM> longer, than the length of the narrowing of the adjustment tube, thus allowing a longitudinal movement of the first axle and the adjustment tube with respect to each other. This yields an arm that also has, in addition to the rotary joint, a longitudinal degree of freedom so that the structure is suitable for use, for example, in a screw-adjustable lateral stabilizer or a lifting bar of a three-point hitch device of a tractor for which a longitudinal freedom of movement of the arm can also be necessary depending on the implement.

In the method according to the invention for manufacturing an arm of a tractor hitch device, the arm comprises a first attachment link and a second attachment link, wherein a first axle is attached to the first attachment link and a second axle is attached to the second attachment link, and wherein an adjustment tube with a sleeve-like first end and with a second end is provided between the first attachment link and the second attachment link, and wherein the first axle is partially inserted coaxially into the sleeve-like first end of the adjustment tube and wherein the second axle is attached to the second end of the adjustment tube, and wherein a rotary joint is formed between the first axle and the adjustment tube that allows the first axle and the adjustment tube to rotate freely relative to each other, a ridge is machined into the first axle and a thinner axle section is machined between the ridge and the first attachment link, and a narrowing is machined into the sleeve-like first end of the adjustment tube. The first axle is partially inserted coaxially into the adjustment tube so that the thinner axle section and the narrowing are aligned, after which the sleeve-like first end of the adjustment tube is reduced by crimping so as to render the diameter of the inner edge of the narrowing smaller than the diameter of the ridge while leaving the diameter of the inner edge of the narrowing larger than the diameter of the thinner axle section.

It is thereby possible to manufacture a rotary joint between the first attachment link and the adjustment tube that can withstand pushing and pulling loads better than the prior art. In addition, the rotary joint manufactured by means of the method according to the invention requires less space compared to a turnbuckle solution. The method according to the invention also requires less labour and manufacturing materials to be employed for the manufacture of an arm than before.

Preferably, the first attachment link and the first axle are joined together before the reduction, or the first attachment link and the first axle are machined from a single piece of manufacturing material. This facilitates and expedites the manufacture of the arm.

Preferably, an internal thread is machined into the inner surface of the second end of the adjustment tube and a corresponding external thread is machined into the outer surface of the second axle, and the second axle is partially rotated into the adjustment tube by a selected distance in order to form the length adjustment of the arm. It is thereby possible to provide the length adjustment of the arm in a mechanically simple manner. Only one threaded joint is thus manufactured in the adjustment tube at the second end of the adjustment tube, which increases the precision of a length adjustment of the arm vis-à-vis a turnbuckle solution, inasmuch as an adjustment of a single rotation of the adjustment tube halves the distance of movement vis-à-vis a turnbuckle using the same threads.

The diameter of the inner surface of the narrowing can remain <NUM> - <NUM>, preferably <NUM> - <NUM>, larger than the diameter of the thinner axle section. This yields a joint that can withstand a load very well structurally, yet which still has sufficient clearance for a rotational degree of freedom.

Preferably, the second end of the adjustment tube is also reduced prior to the machining of the inner thread of the inner surface so that the inner thread has the same diameter as the external thread on the outer surface of the second axle. The thread can thus only be machined over a short distance and the second axle can be easily rotated into the adjustment tube, in part into the wider, i.e. unreduced, section of the adjustment tube.

Preferably, the first end and/or the second end of the adjustment tube are reduced by means of a crimping machine with radially moving jaws. It is thereby possible to reduce the adjustment tube uniformly so that the narrowing of the adjustment tube retains its circular shape in cross-section well. Furthermore, it must be possible with the crimping machine for the first attachment link at the end of the first axle to pass through the jaws of the crimping machine, since the first axle must be partly inside the adjustment tube when the reduction is carried out.

The first end and/or the second end of the adjustment tube can be reduced by means of a crimping machine comprising <NUM> - <NUM> jaws, preferably <NUM> jaws. This makes it possible to reduce the adjustment tube uniformly in different directions.

Preferably, the first end and/or the second end of the adjustment tube are reduced by a crimping machine with a range of jaw motion in the radial direction of at least <NUM>, preferably at least <NUM>. The first attachment link of the arm can thus pass between the jaws when the jaws are open in an end position so that the first attachment link and the first axle can be joined together and/or machined into a completed piece prior to the formation of the rotary joint.

The first end of the adjustment tube can be reduced in such a manner that the diameter of the inner edge of the narrowing is reduced by <NUM> - <NUM>, preferably <NUM> - <NUM>, from its original condition. The diameter of the inner edge of the narrowing is thus large enough prior to the reduction to allow the first axle to be partly inserted into the adjustment tube while the adjustment tube is still reduced enough to yield a rotary joint that is structurally very resistant to pulling and pushing loads.

Preferably, a conical recess is machined into the inner surface of the adjustment tube prior to reduction in order to form a retaining groove and a narrowing. A retaining groove and a narrowing can thus be produced easily in the inner surface of the adjustment tube for the ridge of the first axle while the adjustment tube remains structurally durable.

The invention is illustrated in the following in detail with reference to the attached drawings illustrating embodiments of the invention, wherein.

<FIG> show a three-point hitch device <NUM> arranged on the rear part <NUM> of a tractor and comprising arms according to the invention. The three-point hitch device <NUM> is provided with drawing arms <NUM>, which are not the subject of the present invention, as well as a screw-adjustable lateral stabilizer <NUM>, an automatic lateral stabilizer <NUM>, a lifting bar <NUM> and a pushing arm <NUM> according to the invention. All arms according to the invention have a first attachment link <NUM> and a second attachment link <NUM>, which can be forked heads or eyelet heads depending on the intended use. In all embodiments, a threadless rotary joint <NUM> is configured between the first attachment link <NUM> and the second attachment link <NUM>. The arms according to the invention can also be used in other tractor hitch devices, such as a two-point hitch device.

<FIG> show a screw-adjustable lateral stabilizer <NUM> according to the invention with a first attachment link <NUM> and a second attachment link <NUM>. A first axle <NUM> is attached to the first attachment link <NUM>, and a second axle <NUM> is attached to the second attachment link <NUM>. An adjustment tube <NUM> with a sleeve-like first end <NUM> and with a second end <NUM> is provided between the first attachment link <NUM> and the second attachment link <NUM>. The first axle <NUM> is attached to the sleeve-like first end <NUM> of the adjustment tube <NUM> and the second axle <NUM> is attached to the second end <NUM> of the adjustment tube <NUM>. A threadless rotary joint <NUM> is provided between the first axle <NUM> and the adjustment tube <NUM> that allows the first axle <NUM> and the adjustment tube <NUM> to rotate freely relative to each other. The first attachment link <NUM> and the second attachment link <NUM> can consequently also rotate relative to each other. The adjustment tube <NUM> is used to adjust the length of the screw-adjustable lateral stabilizer <NUM> so as to provide a length adjustment at the second end <NUM> of the adjustment tube <NUM>. In order to form the length adjustment, the outer surface of the second axle <NUM> has an external thread and the inner surface of the second end <NUM> of the adjustment tube <NUM> has a corresponding internal thread so that the second axle <NUM> is configured so as to be partially rotatable into the adjustment tube <NUM>. The screw-adjustable lateral stabilizer <NUM> also has a quick-release clamp <NUM> with which the second axle <NUM> can be locked in any position in the adjustment tube <NUM> in a continuous manner.

In order to form the threadless rotary joint <NUM>, the first axle <NUM> includes a ridge <NUM> and a thinner axle section <NUM> between the ridge <NUM> and the first attachment link <NUM>, while the sleeve-like first end <NUM> of the adjustment tube <NUM> includes a narrowing <NUM>, i.e. a ridge on the inner surface of the adjustment tube <NUM>, that extends to the thinner axle section <NUM>. The first axle <NUM> is partially inserted coaxially into the sleeve-like first end <NUM> of the adjustment tube <NUM>. The first end <NUM> of the adjustment tube <NUM> is structurally homogeneous and comprises a reduction at the point of the narrowing <NUM>. Specifically, the entire adjustment tube <NUM> in this embodiment is structurally homogeneous, i.e. the adjustment tube does not include any welded joints or other joints inasmuch as the adjustment tube <NUM> is manufactured from a single piece of manufacturing material. The adjustment tube <NUM> is manufactured from a straight metal tube of uniform thickness here. The narrowing <NUM> is configured around the thinner axle <NUM> by reducing the sleeve-like first end <NUM> of the adjustment tube <NUM> so as to render the diameter D1 of the inner edge of the narrowing <NUM> smaller than the diameter D2 of the ridge <NUM> while leaving the diameter D1 of the inner edge of the narrowing <NUM> larger than the diameter D3 of the thinner axle section <NUM>. In this embodiment, the adjustment tube is manufactured starting from a straight metal pipe of uniform thickness so that the reduction is visible on the outer surface of the adjustment tube, i.e. the diameter D4 of the outer surface of the first end <NUM> of the adjustment tube <NUM> is smaller than the diameter D5 of the thickest point of the outer surface of the adjustment tube <NUM> after the reduction. The formation of the rotary joint <NUM> is addressed in greater detail in <FIG> and later on in the description.

In this embodiment, the first attachment link <NUM> and the first axle <NUM> are structurally homogeneous, i.e. the first attachment link <NUM> and the first axle <NUM> are machined from a single piece of manufacturing material, which yields a mechanically very robust structure. On the whole, this is rendered possible by the rotary joint <NUM> according to the invention, which is produced by reducing the first end <NUM> of the adjustment tube <NUM>, as the attachment of the first axle <NUM> to the adjustment tube <NUM> would not be feasible without the reduction of the adjustment tube <NUM>. The invention, however, does not exclude the possibility that the first axle and the first attachment link are machined from separate pieces, which are joined together, for example by welding, in a selected manufacturing step.

In the screw-adjustable lateral stabilizer <NUM> shown in <FIG>, the length L2 of the thinner axle section <NUM> of the first axle <NUM> is significantly longer than the length L1 of the narrowing <NUM> of the adjustment tube <NUM> (<FIG>). This allows a longitudinal movement of the first axle <NUM> and the adjustment tube <NUM> relative to each other, and thus also of the first attachment link <NUM> and the second attachment link <NUM> relative to each other, along the length of the thinner axle section <NUM>. The longitudinal movement between the first axle <NUM> and the adjustment tube <NUM> can optionally be locked or released. In order to allow or lock the longitudinal movement of the first axle <NUM> and the adjustment tube <NUM> relative to each other, the adjustment tube <NUM> of the screw-adjustable lateral stabilizer <NUM> has a first hole <NUM> and a second hole <NUM> together with a pin <NUM>. The first axle <NUM> also has a hole, which, in the terminal position of movement of the first axle <NUM> in which the first axle <NUM> is as deep as possible inside the adjustment tube <NUM>, is aligned with the first hole <NUM> of the adjustment tube <NUM> so that, by inserting the pin <NUM> into both the first hole <NUM> of the adjustment tube <NUM> and the corresponding hole of the first axle <NUM>, it is possible to block both a longitudinal movement between the first attachment link <NUM> and the second attachment link <NUM> as well as a rotational movement. The movement can be released by placing the pin <NUM> in the second hole <NUM> of the adjustment tube <NUM>. The position of the pin and thereby the locking/release of the longitudinal movement is selected according to the implement to be attached to the tractor.

<FIG> show an automatic lateral stabilizer <NUM> according to the invention. The automatic lateral stabilizer <NUM> corresponds in many respects to the screw-adjustable lateral stabilizer <NUM> described in the foregoing. The automatic lateral stabilizer <NUM> has a first attachment link <NUM> and a second attachment link <NUM>, and a first axle <NUM> is attached to the first attachment link <NUM> and a second axle <NUM> is attached to the second attachment link <NUM>. Provided between the first attachment link <NUM> and the second attachment link <NUM> is an adjustment tube <NUM>, to the second end <NUM> of which a second axle <NUM> is attached by means of threads in this embodiment as well, while a quick-release clamp <NUM> can be used for locking the length adjustment. The first axle <NUM> is attached to the sleeve-like first end <NUM> of the adjustment tube <NUM> by means of a rotary joint <NUM> that allows the first axle <NUM> and the adjustment tube <NUM> to rotate freely relative to each other. In contrast to the previous embodiment, the length L2 of the thinner axle section <NUM> of the first axle <NUM> is only slightly longer here, by approximately <NUM>, than the length L1 of the narrowing <NUM> of the adjustment tube <NUM> (<FIG>). The longitudinal movement between the first attachment link <NUM> and the second attachment link <NUM> is instead provided by a damping element provided inside the automatic lateral stabilizer <NUM> between the second attachment link <NUM> and the second axle <NUM>. In particular in an automatic lateral stabilizer <NUM>, the rotary joint <NUM> according to the invention allows the adjustment tube <NUM> to be made shorter than before compared to a turnbuckle solution so that there is more space for the damping element. In the automatic lateral stabilizer <NUM>, the longitudinal movement of the first attachment link <NUM> and second attachment link <NUM> can optionally be locked and released by means of a catch <NUM>. The catch <NUM> can be attached, for example, to a mudguard of a tractor so that the longitudinal movement between the first attachment link <NUM> and the second attachment link <NUM> is released and locked automatically according to the height position of the automatic lateral stabilizers <NUM> and drawing arms <NUM>. The locking mechanism of the automatic lateral stabilizer <NUM> is described in greater detail in the patent publication <CIT>.

<FIG> show a lifting bar <NUM> according to the invention, which is structurally very similar to a screw-adjustable lateral stabilizer <NUM>. The lifting bar <NUM> has a first attachment link <NUM> and a second attachment link <NUM>, and a first axle <NUM> is attached to the first attachment link <NUM> and a second axle <NUM> is attached to the second attachment link <NUM>. Provided between the first attachment link <NUM> and the second attachment link <NUM> is an adjustment tube <NUM>, to the sleeve-like first end <NUM> of which the first axle <NUM> is attached by means of a rotary joint <NUM> and to the second end <NUM> of which the second axle <NUM> is attached by means of threads while a quick-release clamp <NUM> can be used to lock the length adjustment. In this embodiment, there is no degree of freedom at all between the first attachment link <NUM> and the second attachment link <NUM> in the longitudinal direction of the lifting bar <NUM>, inasmuch as the rotary joint <NUM> is configured to be fixed in the longitudinal direction in such a manner that the length L2 of the thinner axle section <NUM> of the first axle <NUM> is only slightly longer, by approximately <NUM>, than the length L1 of the narrowing <NUM> of the adjustment tube <NUM> in order to provide sufficient clearance to allow the rotation of the rotary joint <NUM> (<FIG>).

<FIG> show a further lifting bar <NUM> according to the invention, which has a longitudinal degree of freedom corresponding to that of the screw-adjustable lateral stabilizer <NUM>. Here too, the lifting bar <NUM> has a first attachment link <NUM> and a second attachment link <NUM>, and a first axle <NUM> is attached to the first attachment link <NUM> and a second axle <NUM> is attached to the second attachment link <NUM>. Provided between the first attachment link <NUM> and the second attachment link <NUM> is an adjustment tube <NUM>, to the sleeve-like first end <NUM> of which the first axle <NUM> is attached by means of a rotary joint and to the second end <NUM> of which the second axle <NUM> is attached by means of threads while a quick-release clamp <NUM> can be used to lock the length adjustment. In this embodiment, the length L2 of the thinner axle section <NUM> of the first axle <NUM> is significantly longer than the length L1 of the narrowing <NUM> of the adjustment tube <NUM>, thus allowing a longitudinal movement of the first axle <NUM> and the adjustment tube <NUM> relative to each other along the length of the thinner axle section <NUM> (<FIG>). The longitudinal movement between the first axle <NUM> and the adjustment tube <NUM> can optionally be locked or released. In order to allow or lock the longitudinal movement of the first axle <NUM> and the adjustment tube <NUM> relative to each other, the adjustment tube <NUM> of the lifting bar <NUM> has a first hole <NUM> and a second hole <NUM> as well as a pin <NUM>. In this embodiment, the first axle <NUM> does not have a hole, the pin <NUM> rather being placed in the first hole <NUM> of the adjustment tube <NUM> in order to lock the longitudinal movement, which locks the movement of the lifting bar <NUM> in a terminal position in which the distance between the first attachment link <NUM> and the second attachment link <NUM> of the lifting bar <NUM> is at a maximum, by pressing the edge of the ridge <NUM> of the first axle <NUM> against the edge of the narrowing <NUM> of the adjustment tube <NUM> (<FIG>). The pin <NUM> thus does not lock the rotary joint <NUM> of the lifting bar <NUM> inasmuch as there is still a rotational degree of freedom between the first attachment link <NUM> and the adjustment tube <NUM>. Correspondingly, the pin <NUM> can be moved into the second hole <NUM> of the adjustment tube <NUM> so that the longitudinal degree of freedom of the lifting bar <NUM> is freed, thus allowing a rotation and a longitudinal movement between the first attachment link <NUM> and the adjustment tube <NUM> (<FIG>). The position of the pin and thereby the locking/release of the longitudinal movement is selected according to the implement to be attached to the tractor.

<FIG> show a pushing arm <NUM> according to the invention, which is structurally very similar to the lifting bar <NUM> shown in <FIG>. The pushing arm <NUM> has a first attachment link <NUM> and a second attachment link <NUM>, and a first axle <NUM> is attached to the first attachment link <NUM> and a second axle <NUM> is attached to the second attachment link <NUM>. Provided between the first attachment link <NUM> and the second attachment link <NUM> is an adjustment tube <NUM>, to the sleeve-like first end <NUM> of which the first axle <NUM> is attached by means of a rotary joint <NUM> and to the second end <NUM> of which the second axle <NUM> is attached by means of threads while a quick-release clamp <NUM> can be used to lock the length adjustment. In this embodiment, there is no degree of freedom in the longitudinal direction of the pushing arm <NUM> between the first attachment link <NUM> and the second attachment link <NUM>, inasmuch as the rotary joint <NUM> is configured to be fixed in the longitudinal direction in such a manner that the length L2 of the thinner axle section <NUM> of the first axle <NUM> is only slightly longer, by approximately <NUM>, than the length L1 of the narrowing <NUM> of the adjustment tube <NUM> in order to provide sufficient clearance to allow the rotation of the rotary joint <NUM> (<FIG>).

<FIG> shows a step of the method according to the invention in which the first axle <NUM> and the adjustment tube <NUM> are unattached. In <FIG>, the first axle <NUM> is partially inserted into the adjustment tube <NUM> prior to the reduction. In <FIG>, the first axle <NUM> and the adjustment tube <NUM> are joined together in such a manner that there is a threadless rotary joint <NUM> between them, which constitutes the reduction joint.

Before the first axle <NUM> and the adjustment tube <NUM> are joined together by means of the threadless rotary joint <NUM>, a ridge <NUM> of a selected diameter D2 is machined into the first axle <NUM>, and a thinner axle section <NUM> of a selected length L2 and diameter D3 is machined between the ridge <NUM> and the first attachment link <NUM>. In <FIG>, the first axle <NUM> has the same diameter on either side of the thinner axle section <NUM>, although it can also have diameters of different sizes on either side of the thinner axle section <NUM>. In any case, the diameter of the first axle <NUM> on either side of the thinner axle section <NUM> is greater than the diameter D3 of the thinner axle section <NUM>.

A narrowing <NUM> of a selected length L1 and with a selected diameter D1 of its inner edge is machined into the sleeve-like first end <NUM> of the adjustment tube <NUM>. The first end <NUM> of the adjustment tube <NUM> is structurally homogeneous, i.e. there are no welded joints or other structural joints near the point comprising the narrowing <NUM> of the adjustment tube <NUM>. The length L1 of the narrowing <NUM> is always smaller than the length L2 of the thinner axle section <NUM> of the first axle <NUM>. The length L2 of the thinner axle section <NUM> of the first axle <NUM> can be machined so as to be <NUM> - <NUM>, preferably <NUM> - <NUM>, longer than the length L1 of the narrowing <NUM> of the adjustment tube <NUM>, which yields a rotary yet robust joint that is fixed in the longitudinal direction of the arm. Alternatively, the length L2 of the thinner axle section <NUM> of the first axle <NUM> can be machined so as to be significantly longer, by several centimetres, even tens of centimetres, than the length L1 of the narrowing <NUM> of the adjustment tube <NUM> so as to allow a longitudinal movement of the first axle <NUM> and the adjustment tube <NUM> relative to each other in addition to a rotation. At this step, the diameter D1 of the narrowing <NUM> is larger than the diameter D2 of the ridge <NUM>.

Next, the first axle <NUM> is partially inserted with its first end <NUM> first coaxially into the sleeve-like first end <NUM> of the adjustment tube <NUM> in such a manner that the thinner axle section <NUM> and the narrowing <NUM> are aligned. The first end <NUM> of the first axle <NUM> is the end opposite the end attached to the first attachment link <NUM>. The sleeve-like first end <NUM> of the adjustment tube <NUM> is then preferably reduced at room temperature so as to cause a plastic deformation, in the course of which the diameter D1 of the inner edge of the narrowing <NUM> is rendered smaller than a diameter D2 of the ridge <NUM> of the first axle <NUM>. In other words, the sleeve-like first end <NUM> of the adjustment tube <NUM> is reduced so as to render the diameter D1 of the inner edge of the narrowing <NUM> smaller than the diameter of the first axle <NUM> on either side of the thinner axle section <NUM>, i.e. the narrowing <NUM> is configured loosely around the thinner axle section <NUM>. The edges of the thinner axle section <NUM> limit the longitudinal movement of the narrowing <NUM> on both sides and thus the longitudinal movement of the adjustment tube <NUM>. The diameter D1 of the inner edge of the narrowing <NUM>, however, is left slightly larger than the diameter D3 of the thinner axle section <NUM>. The diameter D1 of the inner surface of the narrowing <NUM> after the reduction can be <NUM> - <NUM>, preferably <NUM> - <NUM>, larger than the diameter D3 of the thinner axle section <NUM>. In the course of the reduction, the diameter D1 of the inner edge of the narrowing <NUM> of the sleeve-like first end <NUM> of the adjustment tube <NUM> can be reduced from its initial condition by <NUM> - <NUM>, preferably by <NUM> - <NUM>.

The second end <NUM> of the adjustment tube <NUM> can also be reduced, and internal threads can be machined into the inner surface of the adjustment tube <NUM> in the reduced area for the attachment of the second axle <NUM>. If the manufacture of the adjustment tube starts with a straight pipe of uniform thickness, the diameter D4 of the outer surface of the sleeve-like first end <NUM> of the adjustment tube <NUM> after the reduction is smaller than the diameter D5 of the thickest point of the outer surface of the adjustment tube <NUM>. Correspondingly, the diameter D6 of the outer surface of the second end <NUM> of the adjustment tube <NUM> after the reduction is smaller than the diameter D5 of the thickest point of the outer surface of the adjustment tube <NUM>.

A diameter D5 of the thickest point of the outer surface of the adjustment tube <NUM> thus remains in the central part of the adjustment tube <NUM>. In an alternative embodiment, it is possible to use a pipe with a variable diameter for the manufacture of the adjustment tube <NUM> in such a manner that, after the reduction and manufacture of the rotary joint <NUM>, the outer surface of the pipe has a constant diameter, so that the reduction in the finished arm is observable on the inner surface of the adjustment tube <NUM>.

The radial reduction of the first end <NUM> and/or second end <NUM> of the adjustment tube <NUM> can be carried out, for example, using a Finn-Power FP120 crimping machine. The crimping machine in question has radially movable jaws <NUM> whose path of movement can be adjusted with great precision, as precise as <NUM>. The radially movable jaws <NUM> of the crimping machine open wide enough to allow the first attachment link <NUM> of the arm to pass between the jaws <NUM> when the jaws <NUM> are open in a terminal position. The first attachment link <NUM> and the first axle <NUM> can thus be joined together and/or machined into a finished piece prior to the formation of the rotary joint <NUM>. The invention can evidently be reproduced using other corresponding crimping machines. Reducing machines for reducing the ends of tubes are known in the prior art; in these machines, however, the tube is pressed with the end to be reduced first into a conical cup. The use of this technique in the manufacture of the arm according to the invention is not possible if the first axle <NUM> and the first attachment link <NUM> attached to the same are partially inserted into the adjustment tube <NUM> prior to the reduction of the first end <NUM> of the adjustment tube <NUM>. The reduction can, however, be provided with the crimping machine with radially movable jaws <NUM> described in the foregoing, the use of which for this purpose is not generally known in the sector.

Claim 1:
An arm of a tractor hitch device, the arm comprising a first attachment link (<NUM>) and a second attachment link (<NUM>), wherein a first axle (<NUM>) is attached to the first attachment link (<NUM>) and a second axle (<NUM>) is attached to the second attachment link (<NUM>), and wherein an adjustment tube (<NUM>) with a sleeve-like first end (<NUM>) and with a second end (<NUM>) is provided between the first attachment link (<NUM>) and the second attachment link (<NUM>), and wherein the first axle (<NUM>) is partially inserted coaxially into the sleeve-like first end (<NUM>) of the adjustment tube (<NUM>) and wherein the second axle (<NUM>) is attached to the second end (<NUM>) of the adjustment tube (<NUM>), and wherein a rotary joint (<NUM>) is provided between the first axle (<NUM>) and the adjustment tube (<NUM>) that allows the first axle (<NUM>) and the adjustment tube (<NUM>) to rotate freely relative to each other, wherein, for the formation of the rotary joint, the first axle (<NUM>) includes a ridge (<NUM>) and a thinner axle section (<NUM>) between the ridge (<NUM>) and the first attachment link (<NUM>), and the sleeve-like first end (<NUM>) of the adjustment tube (<NUM>) includes a narrowing (<NUM>) that extends to the thinner axle section (<NUM>), the narrowing (<NUM>) being configured around the thinner axle section (<NUM>), characterized in that said rotary joint (<NUM>) is a crimped reduction joint, wherein the first end (<NUM>) of the adjustment tube (<NUM>) is reduced so as to render the diameter (D1) of the inner edge of the narrowing (<NUM>) smaller than the diameter (D2) of the ridge (<NUM>) while leaving the diameter (D1) of the inner edge of the narrowing (<NUM>) larger than the diameter (D3) of the thinner axle part (<NUM>).