BACKGROUND ART In an internal combustion engine having six cylinders arranged in a V formation, the overall dimensions of the engine are largely determined by the dimensions and structure of the crankshaft, particularly as regards the overall dimension of the crankshaft along the axis of rotation.

It has been found that there is a requirement to provide a crankshaft which will enable an engine with a relatively small overall axial dimension to be produced, and which consists of a limited number of components and is extremely simple to fit on the corresponding engine.

DISCLOSURE OF INVENTION The object of the present invention is to meet the above requirement in a simple and economical way.

According to the present invention, a crankshaft is provided for an internal combustion engine comprising a crankcase and a plurality of piston rods moved by corresponding pistons, the crankshaft comprising: a plurality of crank cheeks which can be

coupled to the said crankcase to allow the crankshaft to rotate about an axis; a plurality of crankpins which are eccentric with respect to the said crank cheeks and which can be journalled in the corresponding said piston rods; and a plurality of arms, each connecting the centre of a corresponding said crank cheek to the centre of an adjacent said crankpin; characterized in that it is made in one piece, and in that the said arms are delimited by corresponding cylindrical lateral surfaces which can be coupled with a sliding fit directly to the said crankcase in such a way that they are rotatable about the said axis, to form the said crank cheeks.

The present invention also relates to an internal combustion engine.

According to the present invention, an internal combustion engine is provided, comprising a crankcase, a plurality of piston rods actuated by corresponding pistons, and a crankshaft comprising a plurality of crank cheeks coupled to the said crankcase to allow the crankshaft to rotate about an axis, a plurality of crankpins which are eccentric with respect to the said crank cheeks and are journalled in the corresponding said piston rods, and a plurality of arms, each connecting the centre of a corresponding said crank

cheek to the centre of an adjacent said crankpin, characterized in that the said crankshaft is made in one piece, and in that the said arms are delimited by corresponding cylindrical lateral surfaces coupled with a sliding fit directly to the said crankcase in such a way that they are rotatable about the said axis to form the said crank cheeks.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described with reference to the attached drawings, which illustrate a non- restrictive example of embodiment, in which : Figure 1 is a side view of a preferred embodiment of the crankshaft of an internal combustion engine according to the present invention; Figure 2 is a perspective view of the crankshaft of Figure'1, shown in longitudinal section along a plane on which the axis of rotation of the crankshaft lies; Figure 3 is a sectional perspective view of a crankcase of the internal combustion engine for the crankshaft of Figures 1 and 2 ; Figure 4 is a longitudinal section through the internal combustion engine provided with the crankshaft of Figures 1 and 2, taken through the plane IV-IV of Figure 3; and Figure 5 is a section through the internal combustion engine, taken along the line V-V of Figure 4.

BEST MODE FOR CARRYING OUT THE INVENTION In the attached figures, the number 1 indicates a crankshaft of an internal combustion engine 2, which comprises a crankcase 3 provided with six cylinders 4 aligned with each other in two rows 4a arranged in a V formation (only one of these rows is shown in Figure 4).

With reference to Figures 3 to 5, the crankcase 3 forms a through tunnel 5 and has four inner circular surfaces 6, which are coaxial and spaced apart from each other along an axis 7, radially delimit the tunnel 5, and support the shaft 1.

The chambers of the cylinders 4 house corresponding pistons 9 which actuate corresponding piston rods 10, each of which comprises a corresponding connecting rod shank 11 and a corresponding big end 12 journalled on the shaft 1. With reference to Figure 5, the big end 12 consists of a pair of half rings 13,14, of which the half ring 13 is made in one piece with the connecting rod shank 11, while the half ring 14 is fixed securely to the half ring 13, preferably by means of screws.

At the positions of the half-rings 14, the crankcase 3 has a plurality of radial apertures 15 which are formed in intermediate axial positions between the surfaces 6 and which allow the radial insertion and fitting of the said half rings 14.

With reference to Figures 1 and 2, the shaft 1 is

made in one piece and comprises six crankpins 16, which are eccentric with respect to the axis 7, and each of which is journalled in a corresponding big end 12.

The shaft 1 also comprises four crank cheeks 17, whose outer cylindrical surfaces are indicated by 18 and are coaxial with each other along the axis 7, each of these crank cheeks being coupled to a corresponding surface 6 in such a way that it can rotate about the axis 7, by means of a sliding coupling suitably lubricated by an oil duct indicated by 19 (Figure 3).

The surfaces 18 and 6 have substantially the same diameter, so that they lie on a common theoretical cylindrical coupling surface 20 between the crankcase 3 and the shaft 1, this surface completely circumscribing the crankpins 16 (Figure 1).

The crankpins 16 extend axially directly from the crank cheeks 17, since the radial arms or cranks commonly present between the crank cheeks 17 and the crankpins 16 (generally known as"links") are incorporated directly in the crank cheeks 17 themselves, in such a way that the axial length of the shaft 1 is kept to a minimum.

In other words, the cranks, each of which connects the centre of a corresponding crank cheek to the centre of an adjacent crankpin, are themselves delimited by the surfaces 18 to form the crank cheeks 17.

In the illustrated example, two crankpins are interposed between each pair of adjacent crank cheeks 17, these crankpins being indicated for the sake of clarity by the reference numbers 16a and 16b in Figure 1, and are coupled to pistons 9 associated with different rows 4a.

Between each pair of crankpins 16a, 16b there is axially interposed a corresponding suitably shaped body 23 for balancing the rotating masses, this body being circumscribed completely by the surface 20 and made in one piece with the shaft 1.

With reference to Figures 1 and 4, the only portion of the shaft 1 which projects radially from the surface 20 is an annular projection 25 carried integrally by an axial end 26 of the said shaft 1. The end 26 constitutes the extension of one of the crank cheeks 17, while the projection 25 forms a shoulder 27 facing the surface 18 of this crank cheek and interacting with a surface 28 of the crankcase 3 by bearing on it axially and sliding on it (Figure 4), this shoulder being retained axially by means of an axially perforated cover 29 joined integrally to the crankcase 3.

Because of its constructional characteristics, the shaft 1 can be fitted in the engine 2 simply by axially inserting the said shaft 1 into the tunnel 5 until the projection 25 bears against the surface 28, in other

words up to a position such that the surfaces 6 and 18 are joined together. The shaft 1 is then locked axially by means of the cover 29 and the piston rods 10 are fitted, the apertures 15 being used to introduce the half rings 14 into the tunnel 5 and to connect the said half rings 14 to the half rings 13 around the crankpins 16.

It is clear from the above that the shaft 1 and, consequently, the engine 2 have a relatively limited axial length and consist of a relatively small number of components, since the shaft 1 is made in one piece and incorporates the cranks directly in the crank cheeks 17.

Additionally, the shaft 1 can be coupled to the crankcase 3 simply by means of an operation of axial insertion into the tunnel 5, because of the surface 20 which circumscribes the crank cheeks 17 and crankpins 16, and because of the fact that the surfaces 18 are coupled directly to the surfaces 6, without the interposition of other supports.

Additionally, the fact that shaft 1 is made in one piece makes it possible to limit the machining and fitting times and costs, as well as the possible play in the coupling, which is normally present in a crankshaft consisting of a plurality of detachable parts.

Finally, the apertures 15 and the half rings 13,14 facilitate the coupling of the piston rods 10 to the

shaft 1.

It is clear from the above, finally, that the shaft 1 described with reference to the attached figures can be modified and varied in ways which fall within the protective scope of the present invention.

In particular, the shaft 1 could be applied to a different engine, in which, for example, there is a different number of cylinders, and/or the cylinders are arranged in a configuration different from what has been described.