The invention relates to a crankshaft for a two¬ stroke internal combustion engine, having throws comprising a crank pin with two crank arms each of which arms has a hole and a hollow chamfer positioned at the transition between the pin and the hole, which throws are joined together by means of main bearing pins with a central journal section and two cylindrical end sections which are positioned adjacent to the journal section and are inserted in the hole of the associated arm.

Such a crankshaft is very well-known from large two-stroke internal combustion engines. The hollow chamfer at the transition between the crank pin and the hole reduces the stress concentrations in this area to a permissible level.

British patent No. 1 095 370 describes a crank¬ shaft, the main bearing pins of which are joined together with the associated arm by means of a shrink or press connection. The cylindrical fitting surface of the joint is stepped in at least two sections having different diameters. The section of the fitting surface of the bearing pin which is closest to the journal surface of the crank pin has the smallest diameter. The invention of said patent is to the effect that the second section of the fitting surface is designed with a larger overmeasure than the smaller section, which increases the stresses in the material around the larger section. The enlarged diameter of the larger section involves an enlarged diameter of the adjacent journal section of the bearing pin, which may result in a shorter journal without reduction of the bearing area.

The object of the present invention is to provide an alternative method of obtaining a larger journal diameter and thus a crankshaft of short length, without thereby causing a weakening of the arm. With this object in view, the crankshaft according to the invention is characterized in that the two end sections of the main bearing pin have a smaller diameter than the adjacent journal section and have a centre axis which is radially displaced in relation to the centre axis of the journal section.

The invention renders it possible to produce a crankshaft of a very short length in relation to the engine power, as the jump in diameters between the journal and end sections of the main bearing pin permits a design of the journal section with a large diameter without the hole in the arm needing a larger diameter, that is, the arm maintains its strength, and the hollow chamfer is not affected either.

The radial displacement between the centre axes of the sections causes the end sections to be positioned eccentrically on the journal section. This may be utilized to position the hole in the arm at a larger distance from the centre axis of the crank pin, which in turn makes it possible either to reduce the stress level in the arm or to increase the diameter of the crank pin journal so that the shaft length may be further reduced.

If the increased journal diameter is utilized to increase the area of the journal so that the shaft is able to transmit larger forces and moments, the joint, the arms and the end sections inserted in the holes must be made capable of receiving a larger moment, which is conventionally done by increasing the diameter or length of the end sections. Instead, according to the inven- tion, the cylindrical outer side of the end section

and/or the inner side of the hole is/are preferably provided with a friction-increasing material, which renders it possible for the joint to transmit larger moments. The invention also relates to a method of use in a crankshaft of the above type, characterized in that the stroke length of the engine is altered by rotating the main bearing pins about their centre axis in relation to the associated crank arms, whereby the centre distance between the main bearing journals and the crank pin journals is altered. With standardized components for the crankshaft, it is thus possible to produce shafts with a mutually different distance between the axes of the journals. An alteration of this centre axis distance results in the double alteration of the stroke length of the engine. The same engine type may thus be delivered with a stroke length which is carefully adapted to the intended use of the engine. With the invention, it is also possible to change the stroke length in a delivered engine by rotating the crank throws in relation to the main bearing pins by means of special tools, which gives great liberty in, for example, uprating an engine, if its operational conditions are permanently changed. An embodiment of the invention will now be ex¬ plained in further detail below with reference to the schematic drawings, in which

Figs. 1 and 2 show a side view and a cross-sec¬ tional view along the line II-II in Fig. 1, respec- tively, of part of a known crankshaft,

Figs. 3 and 4, on a slightly smaller scale, show corresponding views of a shaft according to the in¬ vention, and

Figs. 5-7, on an even smaller scale, show cross- sectional views through the shaft according to the in-

vention, to illustrate how the stroke length may be altered by rotating the journals in relation to each other.

Fig. 1 shows a forged or cast throw 1 with two arms 2 made integrally with a crank pin 3. Each arm has an axially extending hole 4 positioned at such a radial distance from the crank pin that in the transitional area between the periphery of the pin and the edge of the hole 4 there is room for forming a hollow chamfer 5 in the arm. The shape of the hollow chamfer 5 is adapted in a known manner so that the stress concentra¬ tions in the arm become as small as possible. The individual throws are interconnected by main bearing pins 6 having a central journal section 7 which substan- tially smoothly passes into an end section 8 which with a suitable overmeasure has been shrunk into the cylin¬ drical hole 4 in the arm 2 . In this known crankshaft, the holes 4 extend coaxially with the journal sections 7, and the stroke length of the engine is fixed to be double the centre distance c between the holes 4 and the crank pin 3.

Figs. 3 and 4 illustrate a throw 11 of a shaft according to the invention, where the two arms 12 of the throw are forged or cast integrally with a crank pin journal 13. Each arm 12 is fastened to an end section 18 of a main bearing pin 16, which end section 18 is designed with an overmeasure in relation to a hole 14 in the arm. The bearing pin 16 has a central journal section 17 with a substantially larger diameter than the end section 18. As best seen from Fig. 4, where the periphery of the journal section 17 is shown by a dashed line, the end section 18 is positioned eccentrically in relation to the journal section 17. This has the effect that the distance between the centre axis 19 of the crank pin journal and the centre axis 20 of the main

bearing journal is different from the distance between the centre axis 19 and the centre axis 21 of the hole 14. The main bearing pin 16 is symmetrical about a middle plane 22 at right angles to the centre axis 20. In the radial direction, the crank pin journal 13 and the main bearing journal 17 are mutually overlap¬ ping, but as a result of the eccentric position of the end section 18, there is still such a minimum distance between the crank pin journal 13 and the section 18 that there is room to shape the hollow chamfers 15 in the usual manner.

With the mounting of the main bearing pin in the arm 12 shown in Fig. 3, the eccentric journalling of the end section 18 on the journal section 17 has the effect that the hole 14 is a distance e, corresponding to the eccentricity of the end section, further away from the centre axis 19 of the crank pin journal than the hole would be in a shaft with the same centre distance c and with an end section extending coaxially with the adjacent journal section. This displacement of the hole 14 in a direction away from the crank pin journal is utilized to increase the diameter of the crank pin journal in the embodiment of the shaft shown.

It may be seen from Figs. 3 and 4 that the crankshaft has a very large journal diameter in relation to the distance c between the centre axes of the journals, as the diameter of the main bearing is d,,, = 1.33 c, while the diameter of the crank pin journal is d _c = 0.95 c. As there has to be room for the hollow chamfer 5, the sum of the diameters of the main bearing journal and the crank pin journal in known shafts is less than the stroke length of the engine multiplied by 0.9, while the sum of the diameters of the two journals in the shaft according to the invention may be at least 8 per cent,

and suitably at least 12 per cent, larger than the stroke length of the engine. Depending on the engine type chosen, the total diameter of the two journals may be distributed evenly on both journals or with an overweight to one or the other journal. The large diameters of the journals render it possible to transmit a large power in relation to the length of the shaft, which makes the shaft particularly applicable as the crankshaft of a V engine, and especially a large two- stroke V engine of the crosshead type. Such an engine may have two pistons connected to the same crank pin journal, which results in a large load on both this journal and the main bearing journal.

One or both of the associated surfaces on the inner side of the hole 14 and the outer side of the end section 18 may be provided with a friction-increasing material, such as a coating of nickel and/or particles substantially harder than the basic material of the crankshaft arm and the journal element, such as carbor- undum powder (SiC). The friction-increasing material improves the grip between the end section and the arm so that the joint may transmit larger moments.

It will now be explained how the stroke length of the engine depends on the mutual angular position between the main bearing pins and the associated arms in which the end sections of the pin are inserted. First looking at Fig. 3 it is clear that all the centre axes 20 of the bearing pins are coaxial and coincide with the centre axis of the main bearings. The individual throws 11 are arranged in the usual manner in mutual angular positions depending on the number of cylinders of the engine.

Figs. 5-7 show the effect of rotating the throw 11 in relation to the centre axes of the end sections 18 of the associated bearing pins 16. In Fig. 5, the arm

12 is fastened in relation to the bearing pin 16 so that the eccentric distance e of the end section is posi¬ tioned on the connecting line between the centre axes 19 and 20 of the journals, and so that the distance c between these axes and thus also the stroke length of the engine is minimal.

In comparison with Fig. 5, it can be seen that in Figs. 6 and 7, the arm 12 has been rotated about 60° and about 120°, respectively, in relation to the bearing pin 16, which, in Fig. 6, results in a distance c' between the centre axes 19 and 20 which is larger than the di¬ stance c i Fig. 5, but smaller than the distance c' ' in Fig. 7. The distance c may be altered by up to double the eccentric distance e, which again means that the stroke length of the engine may be varied by up to e multiplied by 4. In the embodiment shown in the draw¬ ings, the stroke length of the engine may be varied by up to 18.3 per cent, which, given the same engine compo¬ nents, results in considerable liberty in modifying the engine parameters.