Rotatable shafts are used in many technical connections for transmitting a torque. In certain cases it is necessary to use shafts which provide a certain length variation in order for a torque to be able to be transmitted in a satisfactory manner. Such length- variable shafts may comprise a first shaft part comprising outwardly directed elongated ribs and a second shaft part comprising a hub with corresponding inwardly directed elongated grooves, i. e. the shaft comprises a spline connection between the first and the second shaft part. The outwardly directed ribs of the first shaft part are here allowed to be displaced in a axial direction in relation to the inwardly directed grooves of the second shaft part such that a length variation of the shaft is obtained.

Usually, cardan shafts require a certain length variation in order to be able to take up motions from, for example, motor, gear box or rear shaft of a motor-driven vehicle. A cardan shaft usually comprises a plurality of parts such as two forged end parts, which are provided with coupling members for allowing a connection of the ends of the cardan shaft to connecting components. The main extension of the cardan shaft consists of a main tub, which, at one end, is welded together with an end piece and, at the second end, with a sleeve comprising outwardly directed splines. The second forged end piece comprises a hub with internal splines, which are arranged to be mounted in engagement with the external splines of the sleeve. A conventional cardan

shaft constitutes a relatively complex construction with usually five included components, which are to be joined together. The cost for manufacturing a conventional cardan shaft is therefore relatively high.

SUMMARY OF THE INVENTION The object of the present invention is to provide a rotatable and length-variable shaft which is very functional at the same time as it has a simple construction with few included parts such that it can be manufactured to a relatively low cost.

The above mentioned object is achieved with the initially mentioned rotatable and length-variable shaft which is characterised by that which is mentioned in the characterising part of claims 1 and 11. The main tube is thus manufactured of a substantially plane sheet billet. The sheet billet is profiled such that it between two opposite end edges comprises a number of elongated areas, arranged in parallel, having an extension at different height levels. Such an elongated sheet billet is initially successively roll formed until a substantially tubular elongated body is formed, where said end edges achieve a position in connection to each other. The sheet billet is roll formed in a direction such that the elongated areas obtain an extension in the longitudinal direction of the tubular body. The end edges of the tubular body are after the roll forming in contact with each other or located at a very small distance from each other. Thereafter, the end edges are connected, for example, by welding such that a completely tubular body is formed. Thereafter, the tubular body may be cut in parts for forming a plurality of main tubes. Main tubes, manufactured of a profiled sheet, disclose elongated areas with wall surfaces, which alternatively are located at different radial distances from a centre axis extending through the main tube. Hereby, a main tube is obtained in a simple manner with a shape, which may be used externally as well as internally in a spline connection. A separate manufactured sleeve, which comprises external splines, does not here need be fixedly connected to the main tub, for example, by welding. Thereby, the number of separate parts, of which such a shaft is manufactured, is reduced and thereby also the manufacturing cost of the shaft.

According to a preferred embodiment of the present invention, the main tube comprises a hardened material. In case the shaft is used for transmitting torque of large values, it is suitable that the shaft comprises a hardened material. Hereby, relatively thin sheet billets may be used as starting material for manufacturing the main tube.

Advantageously, the hardening of the main tube is performed after that it has been roll formed and welded together to a complete tube shape. Preferably, the main tube comprises an induction hardened material. Induction hardening is an advantageous hardening method especially in connection with an automated manufacturing of elongated profile elements such as tubes.

According to another preferred embodiment of the present invention, the first end piece and the second end piece comprise a tubular part with a cross section profile which allows for a mounting on the inside or the outside of the main tube. Hereby, the first end piece provides, in a mounted state, an axial mobility in relation to the main tube. The second end piece may be displaced on or in the main tube a distance before a fixed connection is established with the main tube by, for example, welding.

Preferably, the end pieces are manufactured in a substantially corresponding manner each by a profiled sheet billet, which between two opposite end edges comprises at least two different elongated areas. The elongated areas have a substantially parallel extension and surfaces located at different levels. The respective sheet billets are roll formed to a substantially tubular body where the end edges have obtained a position in connection to each other. Hereby, the sheet billets are orientated during the roll forming process such that the end edges obtain an extension in the longitudinal direction of the formed tubular body. Thereafter, the end edges are welded together such that a circumferentially completely closed tubular body is formed. A part of the tubular body is formed plastically, for example, by thermoforming or pressing to a connecting part, which allows a connection of the respective end piece with connecting components. The remaining part of the end pieces consists of a tubular part with a cross sectional profile, which allows a connection with the main tube on its inside or outside. Consequently, the end pieces are manufactured of sheet billets with a substantially corresponding profiling as the sheet billets from which the main tubes are manufactured. Thereby, the tubular parts of the end pieces obtain a corresponding cross section shape as the main tube but with a somewhat larger or lesser diameter for allowing a mounting inside or outside the main tub.

According to another preferred embodiment of the present invention, the main tube comprises a third area, which is located between the first and the second elongated areas and comprises a surface with an inclination such that only transmission of a torque up to a certain level only is allowed via the displaceable connection between the main tube and the first end piece. If a torque, which exceeds said level, is to be

transmitted in such a shaped spline connection, which comprises inclined contact surfaces, the relatively thin wall material in the spline connection obtains an elastic deformation. The protruding portions and grooves of the spline connection are thereby turned out of engagement with each other with a clattering sound when a too large torque attempts to be transmitted via the shaft. With such a design, it is guaranteed that torques, above a determined level, are not transmitted via the shaft. Thereby, damages are prevented from arising on sensible components, which may be included in a motion transmission mechanism in which said shaft is included as a component.

According to another preferred embodiment of the present invention, the shaft is comprised in a vehicle. Motion transmission shafts are used in vehicles in different connections, which require a length variation. A cardan shaft of a vehicle, which is included as a part of a power train, usually requires such a length variation. Such a length variation of the cardan shaft is required such that it can take up movements from different components in the power train of the vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS In the following, preferred embodiments of the invention are described as examples with reference to the attached drawings, in which: Fig. 1 shows a cardan shaft according to the present invention, Fig. 2 shows a main tube of the cardan shaft, Fig. 3 shows a sectional view of the main tub, Fig. 4 shows a first end piece, Fig. 5 shows a second end piece and Fig. 6 shows a method for manufacturing a cardan shaft.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Fig. 1 shows a cardan shaft 1, which is arranged to transmit a torque in a power train in a vehicle. The cardan shaft 1 has a construction, which allows a length variation such that it can take up longitudinal movements in the power train from, for example, a motor, a gearbox, or a rear shaft of a vehicle. The cardan shaft 1 therefore consists of a first shaft part and a second shaft part displaceably arranged in relation to the first shaft

part. The first shaft part comprises a first end piece 2 and the second shaft part comprises a main tube 3 and a second end piece 4. The main tube 3 and the second end piece 4 are thus fixedly connected to each other. The length variation of the cardan shafts 1 is allowed by a spline connection 5 which is arranged in the connection between the first end piece 2 and the main tube 3.

Fig. 2 shows the main tube 3, which has a profiled wall surface with a substantially constant wall thickness. The main tube 3 comprises here, around its circumference, four symmetrically arranged first elongated areas Al each of which has a parallel extension along the whole length of the main tube 3. The main tube 3 also comprises, around its circumference, four symmetrically arranged second elongated areas A2, which also has a parallel extension along the whole length of the main tube 3. The first areas Al and the second areas A2 have a parallel extension and are alternatively arranged around the circumference of the main tube 3. The internal surface of the first area Al is located at a first substantially constant radial distance rli from a centre axis line 6 extending through the main tube 3. The internal surface of the second area A2 is located at a second substantially constant radial distance r2i from den centre axis line 6 extending through the main tube 3. The first internal radial distance r, is larger than the second internal radial distance r2i. Since the main tube 3 has a substantially constant wall thickness, the external surface of the first area A1 is located at a first external radial distance ruz which is larger than the corresponding radial distance r2u of the external surface of the second area A2. Between each of the first areas Al and the second areas A2, a third area A3 extends. The third area A3 comprises an inclined internal transition surface, which connects the internal surface of the first area A1 with the internal surface of the second area A2. The internal surface of the third area A3 forms a larger angle v ; than 90° with the internal surface of the first area Al. The third area A3 comprises, in a corresponding manner, an inclined external transition surface, which connects the external surface of the first area A1 with the external surface of the second area A2. The external surface of the third area A3 forms a larger angle Vu than 90° with the external surface of the second area A2.

Fig. 4 shows the first end piece 2. The first end piece 2 comprises a connecting part 2a having a rounded shape with recesses 7 for receiving, for example, a universal joint cross for allowing an articulated connection of the cardan shaft 1 with a connecting component in the power train of the vehicle. The first end piece 2 comprises also a tubular part 2b, which comprises four symmetrically arranged first elongated areas Bu

which each has a parallel extension along the whole tubular part 2b. The first elongated areas B, also extend over the connecting part. Den tubular part 2b also comprises four symmetrically arranged second elongated areas B2 which each has a parallel extension along the whole tubular part 2b. The first areas Bi and the second areas B2 have a parallel extension but are located at different distances from a centre axis extending through the tubular part 2b. A third area B3 extends between each of the first areas B I and the second areas B2. The third area B3 comprises an inclined transition surface, which connects the first area B, with the second area B2. The elongated areas Bi, B2, B3 of the first end piece 2 here have an outer cross sectional profile which corresponds to the inner cross sectional profile of elongated areas Al A2, A3 of the main tube 3, i. e. a corresponding cross sectional profile as shown in Fig. 3.

Thereby, the tubular part 2b may be displaced into the main tube 3. Thereby, an axial movable connection 5 is obtained between the first end piece 2 and the main tube 3.

The tubular part 2b of the first end piece 2 comprises a slit 8. By such a slit 8 an elastic deformation inwardly of the protruding areas B1 of the tubular part 2b, which is in engagement on the inside of the protruding areas Al of the main tube, is facilitated. By a suitable inclination of the areas B3 of the tubular part 2b and the inclined areas A3 of the main tube 3, only transmission of a torque up to a certain level is here obtained.

Hereby, the angle v ; ought to be obtuse and thus be larger than 90°. Thereby, damages of sensitive components in power train of the vehicle are avoided.

Fig. 5 shows the second end piece 4. The second end piece 4 comprises a connecting part 4a, which has a rounded shape with recesses 7 for receiving, for example, a universal joint cross for allowing an articulated connection of the cardan shaft 1 with a connecting component in the power train of the vehicle. The second end piece 4 also comprises a tubular part 4b, which comprises four symmetrically arranged first elongated areas C1 which each has a parallel extension along the whole tubular part 4b.

The tubular part 4b also comprises four symmetrically arranged second elongated areas C2 which each has a parallel extension along the whole tubular part 4b. The first areas Ci and the second areas C2 have a parallel extension but are located at different radial distances from a centre axis extending through the tubular part 4b. A third area C3 extends between each of the first areas Ci and the second areas C2. The third area C3 comprises an inclined transition surface, which connects the first area Cl with the second area C2. The elongated areas Cl C2, C3 of the second end piece 4 have here an inner cross sectional profile which substantially corresponds to the outer cross sectional profile of the elongated areas Al, A2, A3 of the main tube 3. The tubular part

4b of the second end piece 4 may thereby be mounted on the outside of the main tube 3. In this mounted state, the tubular part 4b of the second end piece 4 is fixedly connected to the main tube 3. Advantageously, this is performed by welding but also other fastening methods may be used. A suitably shaped locking ring may, for example, here be used.

Fig. 6 shows schematically a method for manufacturing a cardan shaft 1. The manufacturing of the main tube 3 comprises, at 10, that a profiled sheet billet, which, between two opposite end edges comprises first elongated areas Al and second elongated areas A2, which have a substantially parallel extension and surfaces located at different levels, is roll formed such that said end edges obtain a position in connection to each other. Thereby, a substantially elongated tubular body is formed.

The end edges are here in contact with each other or are located at a very small distance from each other. At 11, the end edges are welded together such that an elongated tubular body with a circumferencially closed tube shape is obtained. At 12, the elongated tubular body is cut in determined lengths, which correspond to desired lengths of the main tube 3 to be manufactured. At 13, the main tubes are 3 induction hardened such that the material obtains a sufficient hardness. At 14, a finished manufactured main tube 3 is shown.

In parallel with the manufacturing of the main tube 3, the manufacturing of the first end pieces 2 and the second end pieces 4 are performed. At 15, the manufacturing of the first end piece 2 starts. Here, a profiled plane sheet billet is roll formed, which between two opposite end edges comprises four first elongated areas B1 and four second elongated areas B2, which have substantially parallel extensions and surfaces located at different levels, to a substantially tubular body. The end edges are here in contact with each other or located at a very small distance from each other. The end edges have here thus an extension along the whole length of the formed tubular body.

At 16, the end edges are welded together such that a circumferencially closed tubular body is formed. At 17, a cutting of the elongated tubular body is performed in lengths suited for individual first end parts 2 to be manufactured. At 18, a part of the cut tubular body obtains a plastic deformation by, for example, pressing such that a connecting part 2b is formed which allows for a connection of the first end piece 2 with connecting components. The remaining part of the end piece 2 constitutes of a tubular part 2b. At 19, a hardening of the first end piece 2 is performed with a suitable method. At 20, a finished manufactured first end piece 2 is shown.

At 21, the manufacturing of the second end piece 4 starts. Here, an originally plane profiled sheet billet is roll formed, which between two opposite end edges comprises four first elongated areas C1 and four second elongated areas C2, which have substantially parallel extensions and surfaces located at different levels, to a substantially elongated tubular body. After the roll forming, the end edges obtain contact with each other or they are located at a very small distance from each other. At 22, the end edges are welded together such that a circumferencially closed tubular body is formed. At 23, a cutting of the tubular body is performed in lengths suited after the individual second end parts 2 to be manufactured. At 24, a part of the tubular body is formed to a connecting part 4b. The forming is performed by plastic deformation by, for example, pressing. The connecting part 4b is given a shape, which allow for a connection with connecting components. The remaining part forms a tubular part 4b.

Finally, the second end pieces 4 are hardened, at 25, with a suitable hardening method.

A finished manufactured second end piece 4 is shown, at 26.

At 27, the first end piece 2 is mounted together with the main tube 3 by displacing the tubular part 2b of the first end piece 2 into the main tube 3, such that a axially movable connection between the first end piece 2 and the main tube 3 is established. At 28, the tubular part 4b of the second end piece 4 is displaced onto the main tube 3, whereupon the tubular part 4b and the main tube are welded together such that a fixed connection between the second end piece 4 and the main tube 3 is established. A finished manufactured cardan shaft 1 is finally shown, at 29.

The invention is not in any way restricted to the described embodiment but may be varied freely within the scope of the claims. The sub-components of the shaft may, for example, be manufactured of sheet billets with a substantially arbitrary number of profiled portions located at different high levels. In the showed embodiment, the first end piece is mounted on the inside of the main tube and the second end piece on the outside of the main tube. Obviously, the first end piece may be mounted on the outside of the main tube and the second end piece on the inside of the main tube. Alternatively, both end pieces may be mounted on the inside or the outside of the main tube.