WO2015094091A1

WO2015094091A1 - Crankshaft and method for manufacturing a crankshaft

Info

Publication number: WO2015094091A1
Application number: PCT/SE2014/051491
Authority: WO
Inventors: Per LARSÉN
Original assignee: Scania Cv Ab
Priority date: 2013-12-20
Filing date: 2014-12-12
Publication date: 2015-06-25
Also published as: SE537746C2; EP3084237A1; SE1351557A1; BR112016014268A2; KR20160101171A; EP3084237A4

Abstract

The present invention relates to a crankshaft (4) with a central axis (A), wherein the crankshaft (4) comprises at least two main bearing journals (12), through which the central axis (A) extends, and at least one crankshaft web (10) arranged between the main bearing journals (12), wherein the at least one crankshaft web (10) comprises two crank discs (14, 14a, 14b) connected with each other via a crankpin (16). At least one crank disc (14) has a recess (26) with a plane bottom surface (28), which is substantially at a right angle in relation to the direction of the central axis (A), wherein the recess (26) is adapted in the at least one crank disc (14, 14a, 14b), in such a way that the central axis (A) cuts the plane bottom surface (28). At least one main bearing journal (12) and one crank disc have a hole (30) extending substantially coaxially with the central axis (A). The present invention also relates to a method to manufacture such a crankshaft (4).

Description

CRANKSHAFT AND METHOD FOR MANUFACTURING A

CRANKSHAFT

FIELD OF TECHNOLOGY The present invention relates to a crankshaft, a method to manufacture such a crankshaft, a combustion engine comprising such a crankshaft, and a vehicle comprising such a combustion engine.

BACKGROUND OF THE INVENTION AND PRIOR ART

A piston engine, such as a combustion engine, comprises a crankshaft which converts the backwards and forwards movement of one or more pistons, and thus the crank rods of the engine, into a rotating movement of the crankshaft, or vice versa. The crankshaft comprises one or several crankshaft webs, each of which is connected to at least one crank rod of a cylinder in the engine. For example, a six-cylinder piston engine may thus have a crankshaft with six crankshaft webs. Each crankshaft web comprises two crank discs, which are connected with each other via a crankpin. The piston is connected to the crankpin via the crankpin. Each crankpin has a connecting rod bearing, such as a plain bearing, at which the crank rod rotates. Between each crankshaft web the crankshaft has main bearing journals, which are arranged coaxially with the central axis of the crankshaft, around which the crankshaft rotates. The main bearing journals have main bearings arranged in connection with the crank house, in which the crankshaft is arranged. The crankshaft rotates at the main bearings, which thus absorb the forces arising between the crankshaft and the crank house. Usually there is a main bearing journal comprising a main bearing at each end of the crankshaft and one between each crankshaft web. Today, efforts are made to minimise the weight of vehicles in order to e.g. reduce fuel consumption. Accordingly, it is also desirable to manufacture as light engines as possible. Different ways of reducing the weight of an engine by reducing the weight of the crankshaft have been proposed in prior art.

Document US20120024105 A1 describes a method to manufacture a light crankshaft, wherein the crankshaft is adapted with recesses and holes in its crankpins. However, this method is time-consuming, since holes and recesses are obtained by way of forging and stamping. Similarly, document DE4325722 A1 describes how a crankshaft is adapted with recesses in its crankpins, with the objective of reducing the weight.

SUMMARY OF THE INVENTION

Despite prior art solutions in this field, there remains a need to further develop a light crankshaft and a method to manufacture a light crankshaft that is simple and cost effective.

One objective of the present invention is to achieve a crankshaft with a reduced weight. Another objective of the present invention is to achieve a crankshaft with a good flexural and torsional strength.

Another objective of the present invention is to achieve a crankshaft which facilitates boring of holes through the crankshaft.

Another objective of the present invention is to achieve a method to manufacture a crankshaft, which is simple, time-efficient and cost effective.

Another objective of the present invention is to achieve a method to manufacture a crankshaft, which allows for good precision and accuracy, and which minimises the need for finishing works and balancing. These objectives are achieved with a crankshaft according to claim 1 and with a method to manufacture a crankshaft according to claim 10. These objectives are also achieved with a combustion engine comprising such a crankshaft according to claim 8, and with a vehicle comprising such a combustion engine according to claim 9.

According to one aspect of the present invention, a crankshaft with a central axis is provided, wherein the crankshaft comprises at least two main bearing journals and at least one crankshaft web arranged between the main bearing journals, wherein the at least one crankshaft web comprises two crank discs connected with each other through a crankpin, where at least one crank disc has a recess with a plane bottom surface, which is substantially at a right angle in relation to the direction of the central axis, wherein the recess in the at least one crank disc is adapted, in such a way that the central axis cuts the plane bottom surface. The central axis will therefore form the normal for the recess's bottom surface.

By adapting at least one crank disc with a recess with a plane bottom surface in such a way that the central axis cuts the plane bottom surface, a crankshaft is achieved, which is well suited for and facilitates drilling of so-called light holes. Light holes are holes in details, adapted with the objective of reducing the weight of the details. Furthermore, through the recess, a reduction of the weight of the at least one crank disc, and thus the crankshaft, is achieved. Since the recess in the at least one crank disc is adapted in such a way that the central axis of the crankshaft cuts the plane bottom surface of the recess, the recess does not impact the crankshaft balancing. This ensures that no imbalance arises due to the recess, and similar recesses may thus be adapted in as many or as few crank discs as required. The recess in the at least one crank disc is preferably adapted in such a way that the flexural and torsional strength of the crank disc, and thus the crankshaft, may be maintained. Preferably, all crank discs in the crankshaft have a recess with a plane bottom surface. In this manner, the weight of the crankshaft is further reduced. Preferably, at least one crank disc in the crankshaft has a counterweight.

Holes may be drilled in the counterweight to balance the crankshaft. Suitably, several crank discs have a counterweight.

Preferably, the crankpin of the at least one crankshaft web has a connecting rod bearing. Suitably, all the crankpins of the crankshaft have connecting rod bearings. Furthermore, the at least two main bearing journals, preferably, have main bearings at which the crankshaft rotates around the central axis. Suitably, all main bearing journals of the crankshaft have main bearings. Suitably, at least one main bearing journal has a hole extending substantially coaxially with the central axis. This hole may be referred to as a light hole.

Prior art provides for adapting holes and recesses in crankshafts to reduce the weight of the crankshaft, e.g. by drilling holes in the crankpins. By adapting at least one main bearing journal with a hole, which extends in a direction along the direction of the central axis, the crankshaft's weight is reduced. Preferably, several main bearing journals or all main bearing journals of the crankshaft have a hole which is substantially coaxial with the central axis. Accordingly, a light crankshaft is provided. The advantages with a low-weight crankshaft in- elude a reduction in weight of the vehicle in which the combustion engine with said crankshaft is arranged. Thus the vehicle's fuel consumption is reduced. Since the hole(s) is/are adapted substantially coaxially with the central axis, the crankshaft balancing is not impacted. Preferably, the at least one crank disc has a hole extending substantially coaxially with the central axis through the plane bottom surface of the recess. Suitably, the hole of the at least one main bearing journal extends through the at least one crank disc and the plane bottom surface of the recess of the at least one crank disc. The bottom surface of the recess preferably has a shape and an area substantially exceeding the cross-sectional shape and surface of the hole. That is to say, the bottom surface of the recess preferably occupies an area, which contains the entire cross-sectional area of the hole in the at least one crank disc.

Suitably, the hole of the at least one main bearing journal and the at least one crank disc, which hole is coaxial with the central axis, is drilled.

In cases where it is desirable to drill holes in several or all of the main bearing journals of a crankshaft, it is also necessary to drill through the crankshaft web/webs arranged between each main bearing journal. In cases where holes are to be drilled through several main bearing journals and crankshaft webs, a long drill is required, which is weak and unstable as a result. Furthermore, drilling through several main bearing journals and at least one crankshaft web means that the drill will not drill through any material between the crankshaft web's crank discs. That is to say, the drill will periodically pass through the space between the crankshaft web's crank discs. The drill will thus intermit- tently hit a new surface in a subsequent crank disc. Currently, crank discs are designed with different chamfering and levels, and have oblique surfaces where the central axis cuts the crank discs. In connection with drilling holes, substantially coaxially with the central axis, through several main bearing journals, the surface that the drill intermittently hits would be oblique or uneven. Accordingly, the drill's entry and/or exit into/out of the crank discs would have an oblique surface. This may entail that the drilled hole becomes oblique and may also lead to increased friction, wear or even failure of the drill. At further drilling there is also a risk that the subsequent holes become increasingly oblique. Oblique holes in main bearing journals and crankshaft webs entail an imbalance in the crankshaft, which in turn leads to an increased need for balancing the crankshaft. If the drill drills obliquely and passes the surface at an oblique angle, the occurrence of burrs also increases, entailing an increased need for after-processing. By adapting recesses with a plane bottom surface, which is substantially at a right angle against the direction of the central axis, in such a way that the central axis cuts the plane bottom surface in at least one crank disc facilitates the drilling of holes through this crank disc. The plane bot- torn surface of the recess therefore constitutes the surface through which the drill is drilled, when holes must be drilled through two or more main bearing journals. Since the bottom surface is plane and substantially at a right angle in relation to the direction of the central axis, the drilling method for the main bearing journals in the crankshaft is facilitated, and the risk of the drilled hole becoming oblique is minimised.

The crankshaft preferably comprises six crankshaft webs. The crankshaft thus suitably comprises seven main bearing journals, of which one main bearing journal is arranged at each end of the crankshaft and one main bearing journal is arranged between each crankshaft web. The crankshaft is preferably adapted for an engine with six cylinders. Suitably, all of the twelve crank discs have recesses with a plane bottom surface, which is substantially at a right angle in relation to the direction of the central axis, wherein the recess is adapted in such a way that the central axis cuts the plane bottom surface. Suitably, all twelve crank discs and seven main bearing journals have holes extending coaxially with the central axis.

The crankshaft may be adapted to a straight engine with three, four, five or six cylinders, and thus comprises three, four, five or six crankshaft webs. Alterna- tively, the crankshaft may be adapted to a so-called V engine with six, eight, or twelve cylinders, and thus comprises three, four, or six crankshaft webs.

At least one main bearing journal in the crankshaft preferably comprises an oil conduit hole for oil supply. The oil conduit hole is suitably connected with the crankpin of a crankshaft web arranged next to the at least one main bearing journal. The hole made in the at least one main bearing journal coaxially with the central axis, is preferably adapted in such a way that it is separated from the oil conduit hole.

According to embodiments of the invention, all crank discs may comprise a recess with a plane bottom surface, which is substantially at a right angle in relation to the direction of the central axis, wherein the recess is adapted in such a way that the central axis cuts the plane bottom surface.

According to embodiments of the invention, all crank discs and main bearing journals may have holes extending substantially coaxially with the central axis.

According to the embodiments of the invention, two crankshaft webs arranged next to each other may be offset around the central axis in relation to each other.

Sealing elements are suitably arranged at each end of the crankshaft. When a main bearing journal at the crankshaft end has a hole for weight reduction, dirt may penetrate into the main bearing journal. By arranging a seal at the end of the crankshaft, the risk of dirt penetrating into the main bearing journal is mini- mised. Furthermore, the seal entails that no lubricant leaks out through the crank house through the hole in the main bearing journal.

According to one aspect of the present invention, a method is provided for manufacture of a crankshaft with a central axis, wherein the crankshaft com- prises at least two main bearing journals and at least one crankshaft web arranged between the main bearing journals, wherein the at least one crankshaft web comprises two crank discs connected with each other through a crankpin. The method suitably comprises the step of forming a recess with a plane bottom surface, which is substantially at a right angle in relation to the central axis, in at least one crank disc, wherein the recess is adapted in such a way that the central axis cuts the plane bottom surface. The recess is preferably formed by providing a crankshaft forging tool comprising two tool halves, wherein one of the tool halves is adapted with at least one protrusion. A raw material is applied between the tool halves and the tool halves are pressed together, so that the at least one protrusion of one of the tool halves forms the recess of the at least one crank disc in the crankshaft. The raw material of which the crankshaft is manufactured suitably consists of forged steel. Preferably, the tool halves are adapted with a clearance angle. Accordingly, the bottom surface of the formed recess may potentially need to be finished, so that the bottom surface becomes plane and substantially at a right angle in relation to the central axis.

By forging the crankshaft in one piece and, while forging, forming the recess of the at least one crank disc, a method is achieved to manufacture a crankshaft which is simple, time-efficient and cost effective. Furthermore, a crankshaft with good strength is achieved. The protrusion in the one tool half suitably has a shape corresponding to the recess with the plane bottom surface. When the tool halves are pressed together, the protrusion displaces the raw material and as a result the recess is formed. The forging is preferably carried out in two steps, in accordance with prior art. After the forging, the crankshaft is suitably finished.

By using a crankshaft-forging tool comprising two tool halves, a splice is achieved between the tool halves, a so-called parting line. At the splice, the raw material may be pressed out, which entails a need for finishing in order to remove superfluous material. At the finishing, the area at the parting line obtains a somewhat impaired surface finish. The area around the parting line is also allowed to have larger measurement deviations. Achieving the recess of the at least one crank disc with a protrusion in one of the tool halves, ensures that the recess is formed in the crank disc at a position separate from the splice between the tool halves. Accordingly, a protrusion with a plane bottom surface with a good surface finish is achieved. The crankshaft-forging tool is preferably adapted with several protrusions, so that the protrusions form recesses with a plane bottom surface in several crank discs, when the tool halves are pressed together. Suitably, only one of the tool halves has a protrusion. Alternatively, both tool halves have protrusions. Pref- erably, the tool halves are adapted with a clearance angle in relation to the forging direction.

Alternatively, the recess is formed by casting the crankshaft with a crankshaft mould, comprising at least one protrusion, which at the casting forms the re- cess in the at least one crank disc of the crankshaft. At casting, raw material is added to the crankshaft mould via an opening in the mould. The crankshaft mould suitably consists of a sand mould. In case the recess is adapted with a clearance angle at casting, the bottom surface of the recess formed is finished in such a way that the bottom surface becomes plane and substantially at a right angle in relation to the central axis.

Alternatively, the recess is shaped by first forging the crankshaft with a crankshaft-forging tool and, retrospectively, forming the recess with the plane bottom surface of the at least one crank disc by way of machining. Machining may e.g. relate to drilling, turning, planing, chipping, grinding or milling.

Preferably, the method also comprises to drill a hole coaxially with the central axis through at least one main bearing journal and through the at least one crank disc with the recess, so that the drilled hole extends through the plane bottom surface of the recess. The drilled hole is suitably adapted in such a way that its cross-sectional surface fits within the bottom surface of the recess.

Suitably, the method comprises drilling a hole coaxially with the central axis through several main bearing journals and crankshaft webs of the crankshaft. The drilling is suitably carried out starting from one end of the crankshaft, in a direction towards the other end. Alternatively, the drilling is carried out starting from both ends of the crankshaft, in a direction towards each opposite end. By drilling a hole through one or several main bearing journals and at least one crank disc, the weight of the crankshaft is reduced. Since the at least one crank disc has a recess with a plane bottom surface, which is substantially at a right angle in relation to the direction of the central axis, the drilling through the at least one crank disc is facilitated.

Preferably, drilling is carried out through several main bearing journals and crank discs, wherein all of said several crank discs have recesses with a plane bottom surface. The drill will then intermittently hit the plane bottom surface of the respective crank disc, entailing that the risk of the hole becoming oblique is minimised. Furthermore, drill bit wear and the occurrence of burrs are minimised.

Suitably, the method comprises finishing of the crankshaft and balancing of the crankshaft. Balancing of the crankshaft may be achieved by drilling holes in one or several crank discs. Alternatively, the balancing may be achieved by adding material to one or several crank discs.

The crankshaft according to the present invention may be arranged in an engine, which may e.g. be used as an industrial engine, in motor-powered indus- trial robots or in different types of power plants, such as an electric power plant comprising a diesel generator.

Additional objectives, advantages and novel features of the present invention will be apparent to one skilled in the art from the following details, and through exercising the invention. While the invention is described below, it should be apparent that the invention is not limited to the specifically described details. One skilled in the art, having access to the teachings herein, will recognise additional applications, modifications and incorporations in other areas, which are within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Below is a description, as an example, of preferred embodiments of the invention with reference to the enclosed drawings, in which:

Fig. 1 shows a schematic side view of a vehicle;

Fig. 2 schematically shows a crankshaft according to one embodiment of the present invention;

Fig. 3 schematically shows two crank discs according to one embodiment of the present invention;

Fig. 4a schematically shows a crankshaft-forging tool to achieve a crankshaft according to one embodiment of the present invention;

Fig. 4b schematically shows a crankshaft mould to achieve a crankshaft according to one embodiment of the present invention;

Fig. 5a shows a flow chart for a method to manufacture a crankshaft, according to one embodiment of the present invention; and

Fig. 5b shows a flow chart for a method to manufacture a crankshaft, according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

For the sake of clarity, certain reference numerals have been omitted from the drawings, regarding details displayed several times in one and the same figure, and/or in several different figures.

Figure 1 shows a schematic side view of a vehicle 1 . The vehicle 1 comprises a combustion engine 2, which comprises a crankshaft 4 according to the present invention. The combustion engine 2 is connected to a gearbox 6 via the crankshaft 4, which is further connected to the vehicle's driving wheels 8 via a transmission. The vehicle 1 may be a heavy goods vehicle, such as a truck, bus, wheel loader, forest machine, dumper, mining machine, tracked vehicle or tank. The vehicle 1 may also be a car.

Figure 2 schematically shows a crankshaft 4 according to one embodiment of the present invention. The crankshaft 4 comprises six crankshaft webs 10 arranged next to each other along a central axis A, around which the crankshaft 4 rotates. Two crankshaft webs 10 arranged next to each other may be offset around the central axis A in relation to each other. For example, two crankshaft webs 10 arranged next to each other may be offset120 degrees around the central axis A in relation to each other. Below, the invention is described in connection with a crankshaft with such a twisting of crankshaft webs 10. The crankshaft 4 has a main bearing journal 12 at each end and a main bearing journal 12 between each crankshaft web 10. Each crankshaft web 10 comprises two crank discs 14, which are connected with each other via a crankpin 16. The crankpin 16 in each crankshaft web 10 is parallel to and separated from the central axis A. Around each crankpin 16, a connecting rod bearing 18 is arranged in the form of a plain bearing, on which the crank rods (not displayed) of the combustion engine 2 are mounted. Around each main bearing journal 12, a main bearing 20 is arranged, on which the crankshaft 4 is mounted in the engine block (not displayed). The crankshaft webs 10 may comprise crank discs 14 with counterweights 22, in order to counteract imbalance of the crankshaft 4. Furthermore, the crankpins 16 and the main bearing journals 12 have oil conduit holes 24 for supply of oil and lubrication. The crank discs 14 have recesses 26 with a plane bottom surface 28 (see Fig. 3). The recesses 26 of the crank discs 14 in a crankshaft web 10 are adapted so that they face each other. That is to say, the recesses 26 are adapted on the side of a crank disc 14, which faces the other crank disc 14 in the same crankshaft web 10. The recesses are described in further detail in Fig. 3. Figure 3 schematically shows a crank disc 14a without any counterweight 22, and a crank disc 14b with a counterweight 22, according to one embodiment of the present invention. The crank discs 14a, 14b each have a recess 26 with a plane bottom surface 28, which is substantially at a right angle to the direction of the central axis A. The recesses 26 are adapted in the crank discs 14a, 14b in such a way that the central axis A cuts the plane bottom surface 28 of the recess 26. The crank discs 14a, 14b have holes 30 extending substantially coaxially with the central axis A through each plane bottom surface 28, and through each crank disc 14a, 14b. The hole 30 in the crank disc 14a without any counterweight 22 extends through the entire crank disc 14a and further through the main bearing journals 12 arranged in connection with the crank disc 14a. The holes 30 in each crank disc 14a, 14b are suitably adapted in such a way that their cross-sectional area fits in the bottom surface 28 of each recess 26. At the crank disc 14a without a counterweight, a crankpin 16, cut in the figure, is displayed with an oil conduit hole 24.

Figure 4a shows a crankshaft-forging tool 40 to achieve a crankshaft 4, ac- cording to one embodiment of the present invention. The crankshaft-forging tool 40 comprises two tool halves 42, 44, of which one of the tool halves 44 has at least one protrusion 46. At the manufacture of a crankshaft 4 through forging, the raw material 48, such as forging steel, is placed , between the tool halves 42, 44. Subsequently, the tool halves 42, 44 are pressed together and the raw material 48 is shaped according to the design of the tool halves 42, 44. The method to manufacture the crankshaft 4 with the crankshaft-forging tool 40 is further described below, with reference to Figure 5b. The at least one protrusion 46 of one of the tool halves 44 is adapted in such a way that it entails achieving a recess 26 in the at least one crank disc 14 of the crankshaft 4. Furthermore, the protrusion 46 is adapted in such a way that the recess 26 formed has a plane bottom surface 28, substantially at a right angle in relation to the direction of the central axis A of the crankshaft 4, and in such a way that the central axis A cuts the plane bottom surface 28 of the formed recess 26. Accordingly, the central axis A will form a normal to plane bottom surface 28 of the recess 26. Figure 4b shows a crankshaft mould 50 for achieving a crankshaft 4 according to one embodiment of the present invention. The crankshaft mould 50 comprises an opening 52 and a hollow 54, wherein raw material is supplied to the crankshaft mould via the opening 52, into the hollow 54. The crankshaft mould 50 comprises at least one protrusion 56, which at the casting forms a recess 26 in at least one crank disc 14 of the crankshaft 4. The protrusion 56 is adapted in such a way that the formed recess 26 has a plane bottom surface 28 substantially at a right angle in relation to the direction of the central axis A of the crankshaft 4, and in such a way that the central axis A cuts the plane bottom surface 28 of the formed recess 26 (see also Fig. 5b).

Figure 5a shows a flow chart for a method to manufacture a crankshaft 4, according to one embodiment of the present invention. The flow chart shows a method for manufacture of a crankshaft 4 with a central axis A, wherein the crankshaft 4 comprises at least two main bearing journals 12 and at least one crankshaft web 10 arranged between the main bearing journals 12, wherein the at least one crankshaft web 10 comprises two crank discs 14 connected with each other through a crankpin 16. The method suitably comprises the step of a) forming a recess 26 with a plane bottom surface 28, which is sub- stantially at a right angle in relation to the central axis A, in at least one crank disc 14, wherein the recess 26 is adapted in such a way that the central axis A cuts the plane bottom surface 28. The recess 26 may be formed with different manufacturing methods, of which some examples are described in Fig. 5b. Figure 5b shows a flow chart for a method for manufacture of a crankshaft 4 with a central axis A, wherein the crankshaft 4 comprises at least two main bearing journals 12 and at least one crankshaft web 10 arranged between the main bearing journals 12, wherein the at least one crankshaft web 10 comprises two crank discs 14 connected with each other through a crankpin 16.

The method may comprise the step of b) providing a crankshaft-forging tool 40, comprising two tool halves 42, 44 (see Fig. 4a), wherein one of the tool halves 44 is adapted with at least one protrusion 46. After step b), step c) is carried out, at which a raw material 48 is applied between both tool halves 42, 44 and, subsequently, step d) is carried out, at which the two halves 42, 44 are pressed together, so that the protrusion 46 forms a recess 26 in at least one crank disc 14 of the crankshaft 4. Herein the recess 26 has a plane bottom surface 28, which is substantially at a right angle in relation to the central axis A, and herein the recess 26 is adapted in such a way that the central axis A of the crankshaft 4 cuts the plane bottom surface 28. In case the tool halves 42, 44 are adapted with a clearance angle, the bottom surface 28 of the formed recess 26 may potentially need to be finished, so that the bottom surface 28 becomes plane, and substantially at a right angle in relation to the central axis A.

Alternatively, the method comprises the step of e) casting the crankshaft 4 with a crankshaft mould 50 (see Fig. 4b, which comprises at least one protrusion 56, which at the casting forms a recess 26 in at least one crank disc 14 of the crankshaft 4. Herein the recess 26 has a plane bottom surface 28, which is substantially at a right angle in relation to the central axis A, and herein the recess 26 is adapted in such a way that the central axis A of the crankshaft 4 cuts the plane bottom surface 28. If the recess 26 is formed with a clearance angle at the casting, the bottom surface 28 of the recess 26 is finished, so that the bottom surface 28 becomes plane and substantially at a right angle in relation to the central axis A. Alternatively, the method comprises the steps of f) forging the crankshaft 4 with a crankshaft-forging tool, and the step of g) forming a recess 26 in at least one crank disc 14 of the crankshaft 4 by means of machining. Herein the recess 26 has a plane bottom surface 28, which is substantially at a right angle in relation to the central axis A, and herein the recess 26 is adapted in such a way that the central axis A of the crankshaft 4 cuts the plane bottom surface 28. Machining may e.g. relate to drilling, turning, planing, chipping, grinding or milling. After step d), e) or f) the step h) is carried out, drilling a hole 30, extending substantially coaxially with the central axis A, through at least one main bearing journal 12 and the at least one crank disc 14 with the recess 26, so that the drilled hole 30 extends through the plane bottom surface 28. By drilling a hole 30 through at least one main bearing journal 12 and the at least one crank disc 14, a reduction of the weight of the crankshaft 4 is achieved. Since the recess 26 has a plane bottom surface 28, substantially at a right angle in relation to the central axis A, the risk of the drilled hole 30 becoming oblique is minimised, as is any drill bit wear. Furthermore, the need for finishing is reduced, since the occurrence of burrs is minimised. The bottom surface 28 of the recess 26 preferably has a shape and an area substantially exceeding the cross-sectional shape and surface of the hole 30 or of the bore. That is to say, the bottom surface 28 of the recess 26 occupies an area in the at least one crank disc 14, into which the entire cross-sectional area of the hole 30 fits. The method is completed after step h).

According to the embodiments of the invention, the step h) may include drilling of a hole 30, substantially coaxially with the central axis A, through all main bearing journals 12 and all crank discs 14, 14a, 14b of the crankshaft 4.

The foregoing description of the preferred embodiments of the present invention has been furnished for illustrative and descriptive purposes. It is not intended to be exhaustive, or to limit the invention to the variants described. Many modifications and variations will obviously be apparent to one skilled in the art. The embodiments have been chosen and described in order to best explicate the principles of the invention and its practical applications, and to thereby enable one skilled in the art to understand the invention in terms of its various embodiments and with the various modifications that are applicable to its intended use. The components and features specified above may, within the framework of the invention, be combined between different embodiments specified.

Claims

1 . Crankshaft (4) with a central axis (A), wherein the crankshaft (4) comprises at least two main bearing journals (12), through which the central axis (A) ex- tends, and at least one crankshaft web (10) arranged between the main bearing journals (12), wherein the at least one crankshaft web (10) comprises two crank discs (14) connected with each other via a crankpin (16), wherein at least one crank disc (14, 14a, 14b) has a recess (26) with a plane bottom surface (28), wherein the recess (26) is adapted in the at least one crank disc (14, 14a, 14b) in such a way that the central axis (A) cuts the plane bottom surface (28), characterised in that the plane bottom surface (28) is substantially at a right angle in relation to the direction of the central axis (A), wherein at last one main bearing journal (12) has a hole (30) that extends substantially coaxially with the central axis (A), and wherein the at least one crank disc (14, 14a, 14b) has a hole (30) extending substantially coaxially with the central axis (A), through the plane bottom surface (28) of the recess (26).

2. Crankshaft according to claim 1 , wherein the recess (26) is adapted in one side of the at least one crank disc (14), facing the other crank disc (14) of the same crankshaft web (10).

3. Crankshaft according to any of the previous claims, wherein the crankshaft (4) comprises six crankshaft webs (10).

4. Crankshaft according to claim 3, wherein all the crank discs (14) comprise a recess (26) with a plane bottom surface (28), which is substantially at a right angle in relation to the direction of the central axis (A), wherein the recess (26) is adapted in such a way that the central axis (A) cuts through the plane bottom surface (28).

5. Crankshaft according to claim 4, wherein all the crank discs (14a, 14b) and the main bearing journals have holes (30) extending substantially coaxially with the central axis (A).

6. Crankshaft according to any of claims 3-5, wherein two crankshaft webs (10) arranged next to each other are offset around the central axis (A) in relation to each other.

7. Crankshaft according to any of the previous claims, wherein sealing ele- ments are arranged at each end of the crankshaft (4).

8. Combustion engine (2) comprising a crankshaft (4) according to any of claims 1 -7.

9. Vehicle (1 ) comprising a combustion engine (2) according to claim 8.

10. Method to manufacture a crankshaft (4) with a central axis (A), wherein the crankshaft (4) comprises at least two main bearing journals (12), through which the central axis (A) extends, and at least one crankshaft web (10) arranged between the main bearing journals (12), wherein the at least one crankshaft web (10) comprises two crank discs (14) connected with each other through a crankpin (16), characterised by the steps of:

a) forming a recess (26) with a plane bottom surface (28), which is substantially at a right angle in relation to the central axis (A), in at least one crank disc (14, 14a, 14b), wherein the recess (26) is adapted in such a way that the central axis (A) cuts the plane bottom surface (28), and

h) drilling a hole (30), extending substantially coaxially with the central axis (A), through at least one main bearing journal (12) and the at least one crank disc (14, 14a, 14b) with the recess (26), so that the drilled hole (30) extends through the plane bottom surface (28) of the recess (26).

1 1 . Method according to claim 10, wherein the recess (26) is formed by the steps of:

b) providing a crankshaft-forging tool (40) comprising two tool halves (42, 44), wherein one of the tool halves (42, 44) is adapted with at least one protrusion (46);

c) applying a raw material (48) between both tool halves (42, 44); and d) pressing together the tool halves (42, 44), so that the protrusion (46) forms the recess (26) in the at least one crank disc (14, 14a, 14b) of the crankshaft (4).

12. Method according to claim 10, wherein the recess (26) is formed by the step of:

e) casting the crankshaft (4) with a crankshaft mould (50), comprising at least one protrusion (56), which at the casting forms the recess (26) in the at least one crank disc (14, 14a, 14b) of the crankshaft (4).

13. Method according to claim 10, wherein the recess (26) is formed by the steps of:

f) forging the crankshaft (4) with a crankshaft-forging tool; and

g) forming the recess (26) in the at least one crank disc (14, 14a, 14b) of the crankshaft (4) by way of machining.

14. Method according to any of claims 10-13, wherein step h) includes drilling a hole (30), substantially coaxially with the central axis (A), through all main bearing journals (12) and all crank discs (14, 14a, 14b) of the crankshaft (4).