BACKGROUND TO THE INVENTION, AND STATE OF THE ART

The present invention relates to a crankshaft and a method for making such a crankshaft according to the preambles of claims 1 and 11.

Crankshafts of combustion engines are usually made by drop forging, resulting in crankshafts with a fine-grained crystal structure where pores and voids in the initial material close up during the forging process. The crankshafts are also of uniform quality, with good notch toughness and excellent fatigue characteristics, making it possible for them to be of relatively low weight.

Crankshafts comprise journals, crankpins and webs which connect the crankpins to the journals. When a crankshaft is in operation, the angled transition region between the webs and the crankpins and the angled transition region between the webs and the journals are subject to bending loads which result inter alia in stress concentration at a centrally situated portion of the transition region. Fatigue damage on crankshafts may often derive, from cracks which begin in the centrally situated portion. The greatest bending loads_pn a crankshaft occur in the webs situated outermost. It is therefore usual for these webs to be of larger dimensions than other webs in order to improve the cranksnan s fatigue characteristics. Such a solution results in a crankshaft which is heavier agjusually also longer.

A less well-known second method for improving a crankshaft's fatigue characteristics is to provide the webs with centrally situated recesses. These recesses make the central structure of the webs weaker, with the result that the bending load transmitted between webs and crankpins and between webs and journals will to a larger extent be transmitted via the flanks of the webs. This leads to a more even stress distribution in the transition region between webs and crankpins and in the transition region between • webs and journals. There is thus less risk of the crankshaft suffering fatigue damage, while at the same time the recesses reduce the weight of the crankshaft.

CH 383 083 refers to a crankshaft whose webs are provided with recesses. After the journals, crankpins and webs of the crankshaft have been formed by a forging process, said recesses are made by milling and drilling. The forming of the crankshaft's parts thus involves a number of manufacturing steps and different methods. The cost of making the crankshaft will therefore be high.

DE 43 25 722 refers to a crankshaft provided with trough-shaped recesses in the transition regions between webs and crankpins and in the transition regions between webs and journals. It indicates a number of different ways of forming crankshafts and recesses. It states inter alia that an initially formed crankshaft with journals, crankpins and webs can be provided with recesses by means of a forging method. Thus, here again, forming the crankshaft involves a number of manufacturing steps. The cost of making the crankshaft will therefore here again be high.

US 4 356 741 refers to a crankshaft made by a casting process. The crankshaft is provided with recesses close to the portions which are subject to stress of the transition regions between webs and crankpins and between webs and journals, hi this case the crankshaft's journals, crankpins, webs and recesses are made in a single manufacturing step by a casting process. The cost of making the crankshaft will therefore be relatively low, but the strength characteristics of a cast crankshaft are not as good as those of a forged crankshaft. The crankshaft is therefore likely to have to be of larger dimensions and will therefore inevitably be heavier than a corresponding forged crankshaft.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a crankshaft which is relatively inexpensive to make while at the same time being of a low weight and having good strength characteristics.

The object indicated above is achieved with the crankshaft mentioned in the introduction which is characterised by what is indicated in the characterising part of claim 1. In this case at least one web is provided with a recess which has an extent close to a central portion of a transition region between the web and a crankpin or of a transition region between the web and a journal. The concentration of stresses at the central portion of said transition region can be considerably reduced by such a recess. Such a material recess also renders the crankshaft lighter. In such cases the recess may be of a shape which can be created in a drop forging process. It is thus possible to form the crankshaft's journals, crankpins, webs and said recess by a common drop forging process in one step. The result is a crankshaft which is relatively inexpensive to make while at the same time being of low weight and having good strength characteristics.

According to an embodiment of the present invention, the recess has an extent in a radial direction from a peripheral opening in the web to an internal surface which is situated close to the central portion of the transition region. Drop forging is effected by two dies being pressed towards one another from opposite sides of an initial material. By a drop forging process it is possible to form recesses which have an extent parallel with the direction of movement of the dies. To this end, the dies are provided with suitably shaped portions which successively move into the initial material so that webs with recesses are formed during the drop forging process. With advantage, the recess is provided with a bottom surface which has an extent between the peripheral opening and the internal surface. Said recess can thus be formed in the web in a region situated substantially immediately radially externally about the central portion of the angled transition region. Such a recess renders the web structure definitely weaker in the region close to the central portion. The concentration of stresses at the central portion is thus effectively reduced. The internal surface preferably has a rounded shape, thereby preventing stresses from being concentrated at a certain portion of the internal surface.

According to another embodiment of the present invention, the recess has a substantially constant or at least partly decreasing cross-sectional area along its radial

extent between the peripheral opening and the internal surface. It is perfectly possible to make recesses with such cross-sectional areas in a drop forging process. With advantage, the plane defined in the characterising part of claim 1 divides the recess into two substantially similar portions. The recess can thus be positioned quite centrally on the web. Loads in the web can therefore be distributed substantially uniformly to the flanks of the web.

According to another embodiment of the present invention, the crankshaft comprises at least two webs provided with recesses, said webs being parallel or oriented 180° relative to one another. A recess formed by a first of said dies has an extent which is parallel with and runs in the same direction as the direction of movement of the first die during the drop forging process. A recess formed by a second of said dies has an extent which is parallel with and runs in the same direction as the direction of movement of the second die during the drop forging process. A recess formed by the second die thus has an opposite extent to that of a recess formed by the first die. In this case the webs which are to be provided with recesses must also be parallel or oriented 180° relative to one another when they are formed with recesses during a drop forging process.

According to another embodiment of the present invention, a web has a peripheral surface which comprises connecting means for fitting a counterweight. It is usually not possible to provide a web with both a recess and a counterweight in a drop forging process. It is possible, however, to provide the web with a peripheral surface which comprises connecting means, e.g. in the form of a hole to make it possible to fit a counterweight subsequently by means of screws or the like. With advantage, the crankshaft comprises a number of webs, whereby at least the webs situated outermost on the crankshaft are provided with recesses. When a crankshaft is in operation, the greatest bending loads substantially always occur in the outermost webs. In many cases it may therefore be advantageous that only the crankshaft's outermost webs be provided with recesses. This is particularly advantageous in the case of crankshafts which have webs oriented at many different angles to one another. In such cases the

outermost webs maybe parallel or oriented 180° relative to one another so that at least these webs can be provided with recesses during the drop forging process.

According to another embodiment of the present invention, the crankshaft comprises journals with a first radius and crankpins with a second radius, whereby the aggregate value of said first radius and said second radius minus 10 mm is greater than or equal to the perpendicular distance between said first centreline and said second centreline. Experiments in which the stress-reducing function of the recess was measured for different types of crankshafts with varying radial overlaps between journals and crankpins led to the insight that the stress-reducing effect imparted by a recess to a web is relatively closely related to the magnitude of the radial overlap. The smaller the overlap of journals and crankpins on a crankshaft, the greater the stress-reducing effect imparted by a recess. There is a clear stress-reducing effect in the case of crankshafts which have an overlap of 10 mm or less.

The object indicated above is also achieved with the method mentioned in the introduction which is characterised by what is indicated in the characterising part of claim 11. To this end, it is necessary that the recess be of a shape which can be created in a drop forging process. With a suitably shaped recess, the crankshaft's journals, crankpins, webs and said recess can be formed in a common drop forging process in one step. The result is a crankshaft which is relatively inexpensive to make while at the same time being of low weight and having good strength characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention is described below by way of example with reference to the attached drawings, in which: Fig. 1 depicts part of a crankshaft according to a first embodiment of the invention,

Fig. 2 depicts a cross-sectional view of the crankshaft in Fig. 1, Fig. 3 depicts part of a crankshaft according to a second embodiment of the invention, Fig. 4 depicts a crankshaft for a six-cylinder combustion engine and

Fig. 5 depicts a crankshaft blank for a V-8 engine.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Figs. 1 and 2 depict part of a crankshaft 1 of a combustion engine. The crankshaft 1 comprises in a conventional manner a number of journals 2 which in the crankshaft's fitted state are intended to be arranged in undepicted main bearings. The journals 2 are cylindrical in shape with a shell surface which extends round a centreline 3 which constitutes the crankshaft's axis of rotation. The crankshaft 1 also comprises a number of crankpins 4, one of which is depicted in Fig. 1. The crankpins 4 are cylindrical in shape with a shell surface which extends round a second centreline 5. Webs 6 connect the end surfaces of the crankpins 4 to adjacent journals 2.

During operation, crankshafts 1 are subject to great loads. The loads tend inter alia to bend the crankshaft 1, resulting in stresses in the angled transition region 7 between crankpins 4 and webs 6. These stresses are concentrated at a centrally situated portion 7a in the case of conventional crankshafts. The centrally situated portion 7a is in a region where it is intersected by a central plane 8 which extends through the centreline 3 of the journals 2 and the centreline 5 of the respective crankpin 4. To reduce the stress in the centrally situated portion 7a, the web 6 according to the present invention has a central recess 9 situated close to the centrally situated portion 7a. The presence of such a central recess 9 in the web 6 results in weakening of the central portion of the web. The loads transmitted between crankpins 4 and journals 2 during operation of the crankshaft 1 will therefore to a large extent be transmitted via the flanks of the web 6. The stress concentration at the centrally situated portion 7a of the transition region 7 between web and crankpin 4 will be reduced.

The crankshaft 1 comprises journals 2 with a first radius T ₁ and crankpins 4 with a second radius r ₂ . The centreline 3 of the journals and the centreline 5 of the depicted crankpin are at a perpendicular distance d from one another. If the sum of the radius T ₁ of the journals and the radius r ₂ of the crankpins 4 is greater than the perpendicular

distance d between the centrelines 3, 5, the journals 2 and the crankpins 4 overlap one another to a greater or lesser extent. The amount of such overlap can be determined. If journals 2 and crankpins 4 do not overlap one another, the result is a zero or negative overlap. The stress-reducing effect due to providing a web 6 with a central recess 9 is related to the amount of overlap. Experiments have shown a clear stress reduction in the centrally situated portion 7a of crankshafts 1 which have an overlap of 10 mm or less. In the case of the crankshaft 1 depicted in Fig. 2, the journals 2 and the crankpins 4 have a negative overlap. In this case the stress concentration at the centrally situated portion 7a can thus be considerably reduced by the recesses 9 formed in the webs 6.

The crankshaft 1 is made by drop forging. Drop forging results in a crankshaft 1 with very good strength characteristics. Drop forging is effected by a hot initial material being deformed between two dies (swages) provided with concavities (impressions) which give the forged product its shape. Products with relatively complex shapes can be made by drop forging. As the dies are pressed towards one another from opposite sides of the initial material, the shape of the forged product is inevitably subject to certain limitations, particularly where drop forging is to be effected in one step without any intermediate reorientation of the initial material. Making a crankshaft by drop forging in one step without any intermediate reorientation of the material is of course more cost-efficient.

Journals 2, crankpins 4 and webs 6 with recesses 9 are thus formed on the crankshaft 1 during a drop forging process without any intermediate reorientation of the initial material. The crankshaft 1 is made by hot initial material being deformed by two dies being pressed towards one another. Each of the dies comprises impressions which during the drop forging process substantially form half of the crankshaft 1. Forming a crankshaft 1 which comprises webs 6 with recesses 9 by a drop forging process in one step involves a large number of insights. One such insight is that only recesses 9 which have mutually parallel extents can be forged by the two dies being pressed towards one another. This means that the recesses may have an extent in the same direction or in mutually exactly opposite directions. Thus only webs which are in parallel or oriented 180° relative to one another can be provided with recesses. It is

therefore possible by means of suitably shaped impressions to provide the webs 6 with recesses 9 which have an extent from an opening 9a in a peripheral surface of the web 6 to an internal surface 9b which is situated close to the central portion 7a of the angled transition region between the web 6 and the crankpin 4. The recess 9 is provided with a planar bottom surface 9c which has an extent between the peripheral opening 9a and the internal surface 9b. The recess 9 in Figs. 1 and 2 has a substantially constant cross-sectional area in the direction from the opening 9a to the internal surface 9b. The internal surface 9b has a rounded shape to prevent stresses from being concentrated at any portion of the internal surface 9b.

Fig. 3 depicts part of a crankshaft 1 which has an alternative configuration. In this case the crankshaft 1 likewise comprises in a conventional manner a number of journals 2 which are cylindrical in shape with a shell surface which extends round a first centreline 3 which constitutes the crankshaft's axis of rotation. The crankshaft 1 comprises one or more crankpins 4, one of which is depicted in Fig. 3. The crankpins 4 are cylindrical in shape with a shell surface which extends round a second centreline 5. The crankshaft 1 comprises webs 6 which connect the end surfaces of the crankpin 4 to adjacent end surfaces of the journals 2. The two depicted webs 6 are provided with correspondingly shaped recesses 9. The webs 6 comprise a substantially planar peripheral surface 10 which has an opening 9a for the recess. In this case the recesses 9 have a cross-sectional area which decreases in a direction from the peripheral opening 9a to the internal surface 9b which is situated close to the central portion 7a of the transition region 7 of the web 6 to the crankpin 4. The peripheral surfaces 10 of the webs 6 are also provided with holes 11 adapted to making it possible to fit a counterweight subsequently to each of the respective webs 6 by means of screws. It is difficult both to provide webs 6 with functional recesses 9 and to fasten counterweights in a common drop forging process. It is therefore advantageous if the shape of the web 6 can be such as to allow counterweights to be fitted subsequently.

Fig. 4 depicts a crankshaft 1 for a six-cylinder combustion engine. The greatest bending loads during operation of crankshafts 1 occur in the outermost webs 6' which support the outermost crankpins 4'. The greatest stress concentration in conventional

crankshafts occurs in the central portions 7a of the transition regions 7 between the outermost webs 6' and crankpins 4'. In this case, the outermost pairs of webs 6' have therefore been provided with central recesses 9, but the outermost webs 6' have not been provided with counterweights 12. The intermediate webs 6 are of conventional configuration with integral counterweights 12 which are formed during the common drop forging process. The outermost webs 6' have a parallel orientation, making it possible to form all the recesses 9 in a common drop forging process.

Fig. 5 depicts a crankshaft blank 1' intended for a V8 engine and formed during a drop forging process in one step, hi this case the forging process provides all the webs 6 with central recesses 9, which is made possible by the webs 6 being either parallel or oriented 180° relative to one another, hi this case the outermost pairs of webs 6 have a peripheral surface 10 with holes 11 for fitting of counterweights. Any desired number of webs may of course be provided with holes 11 for fitting of counterweights. To enable the crankshaft blank 1 ' to have a final shape and constitute a crankshaft in a V8 engine, an intermediate portion 1 'a of the crankshaft blank may be turned 90° relative to the outer end portions 1 'b of the crankshaft blank 1. Such reorientation of the crankshaft blank 1' is perfectly possible immediately after the drop forging process, when the crankshaft blank is still hot enough to be formable.

The invention is in no way limited to the embodiment described but may be varied freely within the scopes of the claims, hi the embodiment described, webs are provided with recesses close to a central portion of a transition region between the web and the crankpin. Alternatively or in combination, correspondingly shaped recesses may be made in the webs close to a central portion of a transition region between the web and the journal.