BACKGROUND ART Conventional bearings of this type are machined to have a circular cylindrical bearing surface e. g. by using a fine bore. These types of bearings comprise an upper shell and a lower shell defining a bearing surface for supporting a corresponding bearing journal. The hydrodynamic effects of the rotational movement of the journal create a pressure to be build up in the oil film between the bearing surface and the journal with a pressure that lifts the journal from the bearing surface. It is well known, however, that the journals of a crankshaft of a piston engine in operation cannot maintain a perfect alignment with the bearing axis of the main-, crank pin-, or cross head bearings. Bending and torsion of the crankshaft cause a misalignment between the journal axis and the bearing axis that varies during each engine cycle. The misalignment can lead to high edge pressures causing fatigue cracks in the bearing material. High edge pressures are also related with low oil film thickness. Another problem with high pressures in the oil film that is common in these types of bearings is not related to misalignment, but relates to the profile of the oil film pressure in a direction along the main bearing axis.

Modified bearing surfaces of the bearing shells have been proposed to reduce high edge pressures caused by misalignment. DK-U-9600132 discloses one method of obtaining reduced edge pressures by machining at least the bearing surface dose to the bearing edges to be contoured in a direction along the main axis of the bearing to give place for misalignment of the journal with respect to the bearing. Machining this kind of contour in the bearing surface of the shells is, however, relatively expensive and special equipment is required to produce the contoured shape in bearing surface quality.

Another method of obtaining reduced edge pressures presented in DK-U-9600132 is by removing shell material from the outer surface of the shell in the areas near the front and/or rear edge of the shell. These areas of the shell are thus less stiff

because they are not supported by the bearing housing and are supposed to deform dynamically by the pressure applied by the misaligned journal. It has been shown however, that under certain circumstances the edge pressures are high, but act only on a very small area, so that the force exerted (the product of the pressure and the area) on the edge area is insufficient to deform the shell. In this situation, the resulting edge pressure will be too high and that the bearing is likely to be damaged before the end of its expected lifetime. Even if the force exerted on the edge area is large enough to deform the shell, the dynamic deformation of the edge area with each engine cycle can lead to fatigue in the shell material.

DISCLOSURE OF THE INVENTION On this background, it is an object of the present invention to provide a bearing of the kind referred to initially, which overcomes the above-mentioned problem. This object is achieved in accordance with claim 1 by creating at least one weakened area in the shell, which under the tangential load caused by tightening the bearing housing head causes a deformation of the shell. The deformation creates an indentation in the bearing surface of the shell. The indentation reduces locally the oil film pressure and/or increases the oil film thickness.

The area with reduced wall thickness is according to a preferred embodiment formed by removing shell material from the outer surface of the shell. Advantageously, a recess is created in the outer surface of the shell. The area with reduced wall thickness is located according to a preferred embodiment in those areas where high oil film pressures tend to occur with misalignment of the bearing shaft of journal, namely, at the front and/or rear edge of said shell. According to another preferred embodiment, the bearing may further be provided with an area with reduced wall thickness located in a central part of said shell for reducing peak oil film pressures in these areas. For facilitating the production of said recesses with a miller cutter, the recesses are preferably oval or elliptical shaped. Alternatively, the shape of the recesses can be trapezoid with rounded corners.

It is another object of the present invention to provide a shell, which overcomes the above-mentioned problem. This object is achieved in accordance with claim 13 by creating at least one weakened area in the shell, which under the tangential load caused by tightening the bearing housing head causes a deformation of the shell.

The deformation creates an indentation in the bearing surface of the shell. The indentation reduces locally the oil film pressure and/or increases the oil film thickness.

The area with reduced wall thickness is according to a preferred embodiment formed by removing shell material from the outer surface of the shell. Advantageously, a recess is created in the outer surface of the shell. The area with reduced wall thickness is, according to a preferred embodiment located in those areas where high oil film pressures tend to occur with misalignment of the bearing shaft of journal, namely, at the front and/or rear edge of said shell. According to another preferred embodiment, the bearing may further be provided with an area with reduced wall thickness which is located in a central part of said shell for reducing peak oil film pressures in these areas. For facilitating the production of said recesses with a miller cutter, the recesses are preferably trapezoid, oval or elliptical shaped.

It is another object of the present invention to provide a method of producing a shell for a main-, crank pin-or cross head bearing for a piston engine, which overcomes the above-mentioned problem. This object is achieved in accordance with claim 21 by providing a jig with a support surface substantially corresponding in shape to the outer surface of said shell, except for at least one protrusion on said support surface, placing said shell on said support surface, applying a tangential load to said shell high enough to ensure that substantially the complete outer surface of said shell is in contact with said support surface, thereby deforming said shell to match the shape of said support surface, machining a bearing surface on the inner surface of said shell with said shell placed as in the jig under a tangential load, and removing said shell from said support surface of said jig, thereby allowing said shell to return to its shape before step deforming it, whereby at least one indentation in the bearing surface is formed.

Preferably, the bearing surface machined when the shell is in the jig has a contour having the shape of a circular arc, and preferably said bearing surface has a geometry corresponding to a half of a right circular cylinder. According to a preferred embodiment, said at least one indentation is formed at the front and/or rear edge of said shell and alternatively or in addition, said indentation formed in a central part of said shell. In order to ensure that the shell will return to its shape as before said deformation, said deformation is preferably substantially exclusively elastic.

According to another embodiment of the invention, the protrusion on the support surface is formed by one or more layers of film material placed on the support surface of a conventional jig, preferably with said support surface having a geometry corresponding to a half of a right circular cylinder. Alternatively, said at least one protrusion is an integral part of the material forming the bearing surface.

It is another object of the present invention to provide another method of producing a shell for a main-, crank pin-or cross head bearing for a piston engine, which overcomes the above-mentioned problem. This object is achieved in accordance with claim 28 by: a) determining the oil film pressure and its distribution over said bearing surface of said shell under load conditions and/or determining the oil film thickness and its distribution over the bearing surface under load conditions, preferably by using a finite element method, b) identifying in which area or areas the oil film pressure exceeds maximum value and/or the oil film thickness is below a minimum value, c) creating a indentation in the bearing surface of said shell in the area or areas identified in step b).

Preferably, the load conditions in step a) are chosen to correspond to the maximum load condition of the internal combustion engine. Advantageously, the certain geometry for the bearing surface is chosen to correspond to a half of a right circular cylinder. In a preferred embodiment, steps a) and b) are repeated with a modified bearing surface geometry before carrying out step c). In order to optimize the bearing design, the indention is preferably created in step c) by removing shell material from the outer surface of said shell, thereby weakening said shell locally so that said indention is formed in the bearing surface of said shell when a tangential pressure is applied to said shell upon mounting of said shell in a bearing housing. Alternatively, said indentation is formed by removing material from the bearing surface of said shell.

It is another object of the invention to provide a jig for supporting a shell of a main-, crank pin-or cross head bearing for a piston engine during production that helps to overcome the above-mentioned problem. This object is achieved in accordance with

claim 34 by providing the support surface with at least one protrusion for deforming the shell when it is pressed onto the support surface.

According to a preferred embodiment, the at least one protrusion is an integral part of the jig. Alternatively, at least one protrusion is formed by one or more layers of film material placed on the support surface of a conventional jig, preferably with said support surface having a geometry corresponding to a half of a right circular cylinder.

Further objects, features, advantages and properties of the bearing, shell and production methods according to the invention will become apparent from the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS In the following detailed portion of the present description, the invention will be explained in more detail with reference to the exemplary embodiments shown in the drawings, in which Figure 1 is a diagrammatic view on a bearing, Figure 2a shows a rolled out graph of the oil film pressure and distribution as determined by using a finite element method, Figure 2b shows a graph of the oil film thickness as determined by using a finite element method, Figure 2c shows the oil film pressure distribution along the main axis of the bearing when the bearing is aligned, Figure 3a is an elevated diagrammatic view on a shell with locally reduced wall thickness, Figure 3b is a view from another angle of the shell shown in Fig. 3a Figure 4 shows the shells of Figs. 3a and 3b mounted in the bearing with a detailed cutout view on the deformation, Figure 5a shows a jig for supporting a shell in production, Figure 5b shows the jig of Fig. 5a with a shell clamped in the jig, Figure 5c shows an alternative embodiment of the jig, Figure 6 shows a shell after being machined on the jig shown in Fig. 5a, and Figure 7 shows the shells of Fig. 6 mounted in a bearing.

DETAILED DESCRIPTION In the following detailed description, the invention will be described by the preferred embodiments. With reference to Fig. 1, the bearing 1 is of the type that is suitable for use as a main-, crank pin-or cross head bearing of a piston engine such as an otto- or diesel engine. The bearing is particularly suited for medium speed four-stroke diesel engines or slow speed two-stroke diesel engines that can be used as prime mover in ships or in stationary power plants such as cross head uniflow scavenged diesel engines, which in the large versions have an output per cylinder in the range of 350 to 6700 kW.

The bearing comprises an upper and a lower shell. The terms"upper"and"lower"as used here are, however, not intended to limit the use of the shells to a certain position with respect to the gravitational field, but are merely used to give a distinct name to each of the shells.

The largely circular cylindrical bearing surface 20 of the bearing is formed by the inner surfaces of two shells 3,4 abutting each other at two partition faces 5,6. The shells 3,4 are made of steel supporting material, on the inner side of which bearing metal has been deposited. In the drawing, the transition between the carrying material and the bearing material is not shown, but it should be understood that the two bearing materials cohere like one unit. If a tin-aluminum alloy is used as a bearing material, it may be deposited by rolling, and if the bearing material is white metal or lead-bronze, it may be cast onto the carrying material by centrifugal casting.

The shells 3,4 have a front edge 17 and a rear edge 18.

In this embodiment, the shells 3,4 are of the so-called"thick"type, which means a bearing where the thickness of the shell is more than 4.5 percent of the diameter of the shaft 40 to be supported by the bearing 1. The shells 3,4 in the thick shell bearing is sufficiently thick and thus, rigid to support the bearing surface 20 against the occurring dynamic influences, also even though the shells 3,4 project beyond the bearing housing or the shells 3,4 and are not supported by the bearing housing along their entire circumference of their outer surface 14.

A lower bearing housing part 9 is shaped directly in the engine bedplate (not shown) and the lower shell 4 is placed directly in the bearing boring of the bedplate. An upper

bearing housing part 11 is tightened towards the bedplate (not shown) by means of bearing studs 12, which may be bolts or threaded rods with nuts. The lower ends of the bearing studs 12 are screwed onto the upward threaded holes in the bedplate, and the bearing studs produce a holding force sufficiently large to pre-stress the shells 3,4 to the desired level. The radial pressure is provided by tightening the upper bearing housing part 11 towards the bedplate (not shown) by means of the bearing studs 12 so that the upper shell 3 is pressed down towards the lower shell 4 via the associated partition faces 5,6 creating a tangential load on the shells 3,4.

A journal or shaft 40 rotates in the mainly circular cylindrical space formed by the two shells 3,4. An oil film between the bearing surface 20 and the journal surface carries the journal 40 and prevents substantially any direct contact between the journal surface and the inside surface of the shells 3,4 and provides lubrication.

Fig. 2a is a graph showing the magnitude and distribution of the oil film pressure as calculated with a finite element method for a shell 3,4 with a circular cylindrical bearing surface 20. Alternatively, any other suitable numerical method may be used.

In Fig. 2a, the circular bearing surface has been rolled out to form a flat surface for illustration purposes only. For the calculations with the so-called finite element method, the bearing load has been chosen to correspond to the maximum load condition of the running engine. The bending and torsion of the crankshaft and the resulting misalignment of the journal has been taken into account in the calculation of the oil film pressure. On the graph of Fig. 2a, it is possible to identify areas in which the pressure in the oil film pressure exceeds a maximum value. This maximum value is illustrated by the plane A-A'-A".

Fig. 2b is a graph showing the. oil film thickness determined in a manner analogous to the determination of the oil film pressure. From the graph the areas with little oil film thickness can easily be recognized. Both the oil film thickness and the oil film pressure can be used as a criterion for determining in which areas of the bearing indentations in the bearing surface 20 could improve the bearing performance.

Pressures in the oil film are also often too high in the middle of the bearing. The pressure profile from the one edge to the other in a direction along in the main bearing axis usually has a shape as shown in Fig. 2c. The oil film pressure has its peak in the middle of the bearing. In order to increase the overall bearing capacity, it

can therefore be desirable to reduce the peak pressure in the most highly loaded areas in the middle of the bearing to thereby distribute the pressure in the oil film more evenly over the bearing surface. By using e. g. the finite element method, it can be determined in which areas of the bearing surface the oil film pressure exceeds a preset maximum threshold value as illustrated by the dotted line B-B'.

By using the same finite element method the oil film thickness can be determined and displayed in a graph (not shown) or other visual medium and compared with a minimum value for the oil film thickness that is e. g. required for sufficient lubrication.

The first calculation with the finite element method will usually start with a geometry for the bearing surface corresponding largely to a right circular cylinder. The bearing surface geometry can be modified for further calculations with the finite element method to determine the effect of bearing surface geometry modifications e. g. indentations in the area or areas that were identified as having an oil film pressure exceeding the maximum value. The indentations depth, shape and location may be repeatedly adjusted after each recalculation of the oil film pressure to optimize the bearing properties.

When the correct shape, depth, and location of the indentation (s) in the bearing surface 20 is established, the indentation (s) can be created in accordance with one of the preferred embodiments.

According to first preferred embodiment shell material is removed from the outer surface of the shell as shown in Figs. 3a and 3b. A half-elliptical shaped cavity 30 is created in the outer surface 14 of the shell 3,4 by using a cutter miller. The cavity 30 opens to the front edge 17 of the shell 3,4. Further, an elliptical cavity 31 is created in a central part of the outer surface 14 of the shell by using a cutter miller. The cavities 30,31 in the outer surface 14 of the shell 3,4 reduce the wall thickness of the shell thereby weakening it locally. The tangential load on the shell 3,4 created by tightening the upper housing part 11 towards the bedplate (not shown) causes the shell 3,4 to deform as shown in Fig. 4 (the magnitude of the deformations has been strongly exaggerated in this figure for illustrative purposes). Smoothly shaped indentations are formed in the bearing surface 20. The bottom of the cavities 30,31 does not come into contact with the housing. The oil film pressure peaks in a bearing 1 with the indentations in the bearing surface 20 are reduced. Lower maximum pressures in the oil film increase the durability and/or load capacity of the bearing.

The recesses 30 or 31 do not have to be elliptical, any shape that results in the desired deformation and thus the correct indentation is suitable. Oval or trapezoid shapes (with rounded-off corners) can be used. The recesses 30,31 may stretch out over a small portion of the circumference or over the complete circumference of the shell. The recesses 30,31 may stretch out over a small portion of the width or over the complete width of said shell. The method of forming the indentations according to the above embodiment of the invention is found to be particularly suitable for so- called thick shells.

According to another embodiment of the invention (cf. Figs. 5a, 5b, 6,7), the shells are of the so-called"thin shell"type. A thin shell bearing usually means a bearing in which thickness of the shell 3,4 is from 1.5 to 3 percent of the diameter of the shaft 40 to be supported by the bearing. The conventional characteristics of the thin-shell bearing are that the shell 3,4 has to be supported by the bearing housing over the full extend of the shell 3,4, as the thickness of the shell 3,4 itself is not sufficient for the shell 3,4 to support the bearing metal and thus, the bearing surface 20 against the dynamic influences occurring during operation of the engine. The upper bearing housing part 11 is positioned during tightening by wedging it into precisely machined inclined surfaces 50,51 on the engine bedplate (not shown). The upper housing part 9 is tightened toward the bedplate (not shown) by means of bearing studs 12, which may be bolts or threaded rods with nuts. The lower ends of the bearing studs are screwed into upward threaded holes in the bedplate, and the bearing studs produce a holding force which is sufficiently large to pre-stress the shells 3,4 to the desired level.

The areas of the bearing surface 20 of the thin shells 3,4 in which the oil film is too thin and/or in which the pressure in the oil film is too high are calculated in the same way as explained above for the embodiment with the thick shells.

The indentations in the thin shells 3,4 are created by producing them in the following manner. After a layer of bearing metal is applied (not shown), the shell 3,4 is in a conventional production method placed in a jig provided with a circular cylindrical support face for precision machining the bearing surface e. g. with a fine bore.

According to the invention however, the shell 3,4 is placed in a jig 45 in which the support surface 55 is not fully circular cylindrical, because the support surface 55 has protrusions 57 placed in areas corresponding to the areas of the bearing surface 20

identified with the finite element method as having too high pressures in the oil film and/or too little oil film thickness. The shell 3,4 is pressed into the jig 45 by clamping tools 36,37 with a force sufficient to press the outer surface 14 of the shell 3,4 substantially completely into contact with the support surface 55. The shell 3,4 is thereby (substantially elastically) deformed as shown in Fig. 5b (the magnitude of the deformations has been strongly exaggerated in this figure for illustrative purposes).

Next, an as such conventional precision boring of the shell is carried out with e. g. a fine bore to obtain a circular cylindrical bearing surface on the shell 3,4 in its deformed state clamped in the jig 45. There, where the shell has been forced upward by the protrusions 57 on the support surface 55 of the jig 45 more material has been removed be the fine machining operation (cf. dotted line in Fig. 5b). When the clamps 36, 37 are removed and the shell is taken out of the jig 45, the shell 3,4 returns to its shape as before pressing it into the jig 45, and an indentation is formed in the bearing surface as shown in Fig. 6 The bearing surface 20 machined when the shell 3,4 as in the jig 45 has a contour having the shape of a circular arc, and preferably has a geometry corresponding to a half of a right circular cylinder. According to a preferred embodiment, the indentations are formed at the front edge 17 and/or rear edge 18 of said shell 3,4 and alternatively or in addition, an indentation formed in a central part of said shell 3,4. In order to ensure that the shell 3,4 will return to its shape as before the deformation in the jig 45, the extend of the deformation is preferably substantially exclusively elastic.

In a preferred embodiment, which is particularly suitable for an engine in which central bearing axes of the bearings of a crankshaft have been deliberately displaced with respect to one another, the bearing surface 20 is machined to have a contour in the shape of an oblique circular cylinder.

According to a preferred embodiment, the protrusions 57 on the support surface may be formed by a film or layers of film 58 placed on the support surface 55 of an otherwise conventional jig 45 as shown in Figure 5c. Alternatively, the protrusions 57 are an integral part of the material forming the bearing surface 55.