ENGINE

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The present invention relates to methods used for producing camshafts of internal-combustion engines, of the type in which a metal tubular element is expanded by introducing a high-pressure fluid therein. A method of this type is, for example, described and illustrated in the document No. WO 2003/033189. According to the above known technique, the camshaft is obtained by mounting a series of annular elements that are to define the cams on a tubular element and by expanding the tubular element with a hydroforming operation in order to secure the aforesaid annular elements to the tubular element. There are on the other hand known alternative methods of production in which the annular elements defining the cams are secured to the tubular element by expanding the latter by means of a mechanical action, with the aid of an expansion mandrel. The known methods described above have been proposed and used as an alternative to more conventional methods, which envisage obtaining a camshaft in a single piece, by means of a conventional forging operation.

A method as set forth in the preamble of claim 1 is known from WO 99/20414 Al . Similar methods are known from JP 2000 192805 A, EP 0 906 800 Al, US 2003/221514 Al, JP S59 113944 A.

The object of the present invention is to provide a camshaft for an internal-combustion engine by means of a forming operation using high-pressure fluid, in particular by hydroforming or by forming with gas at a high pressure to obtain at the same time a camshaft structure in a single piece.

With a view to achieving the above purpose, the subject of the invention is a method having the features of claim 1.

The forming operation using high-pressure fluid may be performed cold or hot, with liquid (for example, water or oil) or with gas. For instance, the forming operation using high-pressure fluid expansion mandrel be performed by means of gas, such as for example nitrogen, at a pressure of between 400 and 800 bar and at a temperature of between 800°C and 950°C.

As an alternative, the forming operation using high-pressure fluid may be a hydroforming operation performed by means of water or oil at a pressure of between 2000 and 3000 bar, at room temperature or higher .

In a first embodiment, the portions of enlarged thickness of the tubular element that is set in the die are obtained by subjecting a tubular element initially of uniform thickness to a preliminary operation of permanent deformation by means of flow-forming, which is performed by setting the tubular element in rotation and by applying around it rotating forming rollers that are displaced along the tubular element keeping them pressed against the tubular element.

In one variant, in the case where the tubular element is made of aluminium or its alloys, it is obtained with contiguous portions of different thickness by means of an extrusion process.

In another embodiment, said tubular element is obtained with contiguous portions of different thickness by means of bending and welding of a metal sheet having portions of different thickness.

The tubular element may, for example, be made of case-hardening steel, such as 22MnB5 boron steel.

Preferably, each of the widened portions of the cavity of the die has opposite ends having a diameter that varies progressively between a minimum value and a maximum value so that the cams of the camshaft are obtained with a corresponding conformation, including opposite ends having a diameter that varies progressively between a minimum value and a maximum value .

There may be envisaged the use of a composite die, with radially mobile parts in regions corresponding to the portions of the tubular element that are to form the cams. In this case, it may be envisaged that during the forming operation using high-pressure fluid the aforesaid mobile parts are displaced radially outwards in successive steps, one after another, so as to form said cams one at a time.

It may also be envisaged that the die is provided with heating means in positions corresponding to the portions of the cavity of the die that are to form the cams of the camshaft so as to obtain a localized case- hardening of the cams following upon the forming operation by means of pressurized fluid.

In general, if the forming operation using high- pressure fluid is performed hot, the piece obtained can be already case-hardened. Otherwise, following upon the forming operation using high-pressure fluid, an operation of thermal treatment is carried out.

According to a further characteristic, the piece obtained with the method described above is finally subjected to a grinding operation.

Further characteristics and advantages of the invention will emerge from the ensuing description with reference to the annexed drawings, which are provided purely by way of non-limiting example and in which:

- Figure 1 is a cross-sectional view of a tubular element that must be subjected to the method according to the invention, according to an example of embodiment ;

Figure 2 is a schematic illustration of the device used for the preliminary operation of flow- forming of the tubular element;

- Figure 3 is a cross-sectional view of the tubular element after the preliminary flow-forming operation;

- Figure 4 is a perspective view of the tubular element illustrated in Figure 3;

- Figures 5A-5F are schematic illustrations of the various steps of the forming operation using high- pressure fluid; and

- Figures 6 and 7 are perspective views that show, respectively, a camshaft according to the conventional technique and a camshaft obtained with the method according to the invention.

In Figure 1, the reference number 1 designates as a whole a tubular element made, for example, of 22MnB5 boron steel. In a concrete example of embodiment, the tubular element 1 had a length L = 500 mm and internal and external diameters d = 27 mm and D = 30 mm.

In the embodiment illustrated, the semi-finished product 1 is first of all subjected to a flow-forming operation by means of an apparatus such as the one illustrated schematically in Figure 2. The tubular element 1 is mounted on a mandrel 2, which is set in rotation about its axis 2a by motor means (not visible in the drawing) . Whilst the tubular element 1 is set in rotation, a plurality of forming rollers 3 are displaced axially around it. Each forming roller 3 thus presents both a movement of rotation about its axis and a movement of translation in a direction parallel to the axis 2a. Moreover, each support 3a of each forming roller 3 is pushed radially in such a way that each roller 3 exerts a high pressure against the wall of the tubular element 1. Figure 2 does not illustrate the motor means associated to the shaft 3b of each forming roller 3.

As a result of the aforesaid preliminary flow- forming operation, the tubular element 1 assumes the shape illustrated in Figures 3 and 4, including a plurality of portions la of enlarged thickness that are to define the cams of the camshaft. The portions la are set at an axial distance apart from one another by portions lb having a smaller outer diameter. The inner diameter of the tubular element 1 is, instead, constant throughout its length, as may be seen in Figure 3.

In the aforesaid concrete example of embodiment, the inner diameter of the tubular element 1 obtained after the preliminary flow-forming operation was d = 22 mm, whereas the outer diameter of the portions la and lb was Di = 30 mm and D ₂ = 25 mm.

As may be seen in Figure 4, the portions la of the tubular element 1 obtained after the preliminary flow- forming operation may have axial lengths different from one another, according to the desired conformation of the camshaft.

Figures 5A-5F are schematic illustrations of the various steps of the forming operation using high- pressure fluid, to which the tubular element 1 obtained following upon the preliminary flow-forming operation is subjected. The figures show schematically the method with reference to a theoretical piece of small length including a single portion that is to define a single cam. It is evident that in the concrete embodiment, both the piece and, consequently, the die have a larger axial dimension. Likewise, the die is illustrated in the annexed drawings as including a main cavity with a single widened portion that is to define a single cam of the camshaft. It is evident that in the concrete embodiment the cavity of the die has a more extensive axial dimension, and its cavity has a plurality of widened portions set axially apart from one another that are to define the various cams of the camshaft.

Figure 5A shows the preliminary step, in which the tubular element 1 obtained following upon the preliminary flow-forming operation is positioned in the die. The die for hydroforming is illustrated in the annexed drawings in simplified form as including a top die element SI and a bottom die element S2 that can displace vertically with respect to one another between an open condition, where they are set apart, which enables introduction of the piece to be formed or extraction of the piece that has been formed (see Figures 5A and 5F) , and a closed condition where they are set close to one another for carrying out the forming operation (see Figures 5B- 5E) .

The annexed drawings do not show the hydroforming machine within which the die is positioned either. Said machine may be obtained in any known way. In particular, it is possible to make use of a hydroforming machine such as the one forming the subject of the document No. EP 1 992 427 Al filed in the name of the present applicant.

In the initial step illustrated in Figure 5A, the two die elements SI, S2 are in the open condition to enable positioning of the tubular element 1 within the die. In the drawings of Figures 5A-5F, the tubular element is illustrated as theoretical tubular element, with constant outer diameter and constant inner diameter. However, as has already been mentioned, according to the invention the tubular element is inserted into the forming die after being previously subjected to the flow-forming operation described above, following upon which it has the portions 1A of enlarged thickness that are to constitute the cams of the camshaft.

Figure 5B shows the step in which the die elements SI, S2 are closed after introduction of the element 1 between them. In this condition, the two elements SI, S2 define a substantially cylindrical axial cavity, which gives out at its two opposite ends onto the opposite ends of the die elements SI, S2. In areas corresponding to each portion la of the tubular element 1 that is to form a cam, the cavity of the die has a widened portion CI. In the concrete embodiment, the cavity of the die has a plurality of widened portions CI set axially apart from one another by cylindrical portions C2.

The opposite ends of the cavity of the die are closed by two cylindrical closing elements T, each having an axial channel Tl connected to a circuit for supply of high-pressure fluid.

As has already been mentioned above, the method according to the invention envisages both use of a liquid at high pressure in a hydroforming process (for example, water or hot oil) and, as an alternative, use of a gas (inert gas) at a high pressure (for example, nitrogen) .

Figure 5C shows the subsequent step in which the internal cavity of the die is filled with fluid (water W in the typical example of embodiment) , getting it to flow through channels Wl, W2 made through the die elements SI , S2.

Figure 5D shows a subsequent step in which the closing elements T are made to advance axially until they come into contact with the end of the tubular element 1, so as to isolate the chamber inside the tubular element 1 from the channels Wl, W2.

Figure 5E shows the subsequent step in which the cavity inside the tubular element 1 is connected to the high-pressure circuit P in such a way that the tubular element 1 is forced to expand within the widened cavity CI by the water at high pressure contained therein.

As may be seen in the example of embodiment illustrated, each widened portion CI of the cavity of the die has opposite ends having a diameter that varies progressively between a minimum value and a maximum value so that also the deformed portion of the tubular element 1 that defines each cam assumes a corresponding configuration, with axially opposite ends having a diameter that varies progressively between a minimum value and a maximum value.

Figure 5F shows the step in which, after the end of forming the tubular element, the die is opened, and the tubular element is extracted to be subjected to an operation of thermal treatment (case-hardening) and to a subsequent grinding operation.

Figure 7 shows an example of camshaft obtained with the method according to the invention compared with a functionally equivalent conventional shaft. In the camshaft of the invention the portions of the tubular element functioning as cam are designated by the reference la, whereas designated by lb are the portions set between them, at least some of which are used for support in rotation of the camshaft by means of rolling bearings.

As already mentioned above, the forming process using high-pressure fluid may be carried out cold or else hot. In the case of the hot process, gas or oil is preferably used. The heating temperature is preferably comprised between 850°C and 950°C, and is preferably around 900°C. With the above hot process, it is possible to obtain that the piece is already completely case-hardened following upon the forming operation. As an alternative, the process can be carried out cold. It may also be envisaged that the die is provided with heating means (for example, induction heating means) in positions corresponding to the portions of the cavity of the die that are to form the cams so as to obtain an induction case-hardening of the cams during the forming operation.

Otherwise, the piece obtained is subjected to an operation of thermal treatment after the forming operation.

Finally, as has been said, the piece is subjected to a grinding operation.

There may also be envisaged a composite die, with portions that can displace radially in positions corresponding to the portions of the cavity of the die that are to form the cams. Dies of this type are known and have been proposed, for example, in the document No. EP 1 579 931 Al filed in the name of the present applicant for forming tubular elements with branchings. In this case, it is preferably envisaged that during the forming operation using high-pressure fluid the aforesaid mobile parts are displaced radially outwards in successive steps, one after another so as to form said cams one at a time.

Thanks to the characteristics referred to above, the method according to the invention enables a camshaft to be obtained in a single piece, with relatively simple and fast operations. The camshaft obtained with the method according to the invention presents advantages of greater lightness and greater stiffness as compared to camshafts of a conventional type. Moreover, the camshaft thus obtained is more reliable than camshafts in which the cams are constituted by separate annular elements associated to a tubular shaft that is expanded by means of hydroforming .

Of course, without prejudice to the principle of the invention, the details of construction and the embodiments may vary widely with respect to what has been described and illustrated herein purely by way of example, without thereby departing from the scope of the present invention.

In particular, as already mentioned above, as an alternative to the preliminary step of flow-forming, a tubular element may be provided with contiguous portions of different thickness, obtained, for example, by extrusion (in the case of aluminium) or by bending and welding of a metal sheet made up of portions of different thicknesses. Nor is there ruled out the use of a tubular element of uniform thickness for carrying out the forming operation using high-pressure fluid.