WO2001050029A1

WO2001050029A1 - Connecting rod assembly

Info

Publication number: WO2001050029A1
Application number: PCT/GB2001/000016
Authority: WO
Inventors: Charles Oliver Bryce Carey
Original assignee: Charles Oliver Bryce Carey
Priority date: 2000-01-06
Filing date: 2001-01-03
Publication date: 2001-07-12
Also published as: GB0000170D0; AU2384201A; GB2358057A

Abstract

A connecting rod and bearing assembly is described, the connecting rod having a main part including a stem and an associated little-end and a big-end part wherein a cap portion of the big-end opening is formed from a bent metal strap having a bearing material element associated therewith to co-operate, in use, with a shaft journal.

Description

CONNECTING ROD ASSEMBLY

This invention relates to a connecting rod, in particular a connecting rod for internal combustion piston engines.

A known connecting rod comprises a body with a stem which connects an opening at one end, the small connecting rod eye or little-end, to an opening at the other end, the big-end, and this stem has ribs, either joined with a web or otherwise, which connect the two openings. The big- end opening contains bearing shells which are held in the opening by means of a radial pressure due to an interference between the outer surface of the bearing shell and the inner surface of the opening; the inner surface of the bearing shells provides the profile of the big-end bore in which the crankshaft rotates during engine operation. The big-end opening is also provided with a detachable cap to permit assembly of the connecting rod to the crankshaft. The cap is attached to the main body by means of bolts or screws. The form of the big-end opening is determined by prior machining of the connecting rod big-end bore together with suitable selection of the bearing shells that are fitted inside the big-end opening. The bearing shell may have a no - uniform thickness which determines the inner surface profile and the resulting radial clearance between the crankshaft but this profile is set with respect to the form of the inner surface of the machined big-end bore in the connecting rod. Disadvantages of conventional connecting rod constructions relate to the fact that the rod assembly is relatively very heavy due to the cast or forged construction and to the necessity to allow sufficient cross ^■ sectional area to permit the usual generally axially directed bolts or screws to enable tightening of the cap to the main rod portion. It is a continuing pursuit to reduce weight in vehicles and especially in moving parts of internal combustion engines so as to improve fuel efficiency.

Further disadvantages relate to the extensive machining of conventional connecting rods and their associated bearings which adds to the manufacturing cost thereof.

It is an object of the present invention to lighten the connecting rod particularly in the region of the big-end opening and also reduce the manufacturing cost of the connecting rod and big-end bearing assembly.

According to a first aspect of the present invention there is provided a connecting rod and bearing assembly, the connecting rod having a main part including a stem and an associated little-end and a big-end part wherein a cap portion of the big-end opening is formed from a bent metal strap having a bearing material element associated therewith to co-operate, in use, with a shaft journal.

The term "bent metal strap" is used to denote a big-end cap portion which is formed by bending of a piece of metal into a cap shape to form the cap rather than prior art methods of casting, forging and the like. An example of this would be by bending of a previously substantially flat metal strip into a cap shape having a generally semi-circular portion therein.

The main part of the connecting rod may have an integrally formed little-end.

The main part of the connecting rod may be formed by casting, powder metallurgy techniques, forging, fabrication or by any other suitable method known in the art .

That portion of the connecting rod main part which forms part of the big-end opening may be machined to a predetermined size as is known in the art and may have a bearing element associated therewith. The bearing- element may be formed by a strip of bearing material of any suitable material known in the art affixed or held thereto or the main part may be coated with a suitable bearing material . Where a separate bearing material

" element is affixed or held to the big-end opening of the main part of the connecting rod, it may be in the form of a bearing material such as an aluminium alloy or copper alloy, for example, having a strong backing such as steel, for example. The element may be a segment of a cylindrical bearing and may form an insert in the connecting rod main part . The insert may be held in place by virtue of the geometry of the whole connecting rod assembly; by separate fastening means; or, by being welded in place. In some cases such as in small engines for horticultural equipment, for example, the upper bearing surface may be formed by the material of the connecting rod main part itself. It is common for lawn- mower engines, for example, to have aluminium alloy connecting rods which form the bearing surface per se of the big-end opening.

Where the surface of the big-end opening of the main part of the connecting rod is coated with a bearing material such coating may be effected by any suitable technique such as sputtering, high-velocity oxy-fuel spraying, electro-plating, plasma spraying or any other suitable coating technique known in the bearing art.

The bent metal strap may have a bearing material directly or integrally associated therewith such as by being formed from a strip or sheet material comprising a steel backing layer, for example, having one or more layers of bearing material bonded thereto. Such metallic material may comprise, for example, aluminium alloy roll-pressure bonded to steel or copper alloy cast onto or sintered to steel.

The bent metal strap may be affixed to the main part of the connecting rod by either welding or by mechanical fastening means . Welding techniques may include projection, spot or laser welding, for example. Mechanical fastening methods may include threaded fasteners; blind rivets; or shear pins, for example. Whatever the method employed for securing the metal strap to the main part of the connecting rod, in use, the forces tending to separate the strap from the main part of the connecting rod will be mainly shear forces.

A significant advantage of the connecting rod construction according to the present invention where, for example, the metal strap is attached to the main part of the connecting rod by screws for example is that since the main forces acting upon the fastening means are shear loads, then screws of much smaller diameter may be used than in conventional connecting rod constructions. In conventional connecting rods where the axes of the tightening screws are generally in the axial direction, i.e. generally parallel to the direction of the axis of the connecting rod stem, the forces tending to separate the rod from the cap are mainly tensile forces. Also, because of the high tightening forces employed, to provide for an interference fit between the separate shell bearing and its housing, the screw diameter is generally relatively large, e.g. 8mm for example, and the screw length relatively long thus, the screw is consequently relatively heavy. In the present invention, the screw diameter and length may be much smaller and successful tests using a screw diameter of only 5mm have been carried out thus, greatly reducing weight. In the present invention, where the big-end bearing strap is provided by being formed from steel backed bearing alloy such high tightening forces are not required as there is no interference fit between separate housing and bearing shell components . According to a feature of a first embodiment of the present invention there is provided a connecting rod which has an inner profile of the big-end opening determined at one or more points by the form of the shaft journal which passes within the big-end opening, in use. The term ^uin use" in this context is intended to denote when the big-end bearing strap is assembled to the connecting rod main part around the shaft journal rather than when the engine is actually running.

With the arrangement described in the preceding paragraph, on assembly of the connecting rod to the crankshaft the closing of the big end opening by the fixing of the bent metal strap is dimensionally controlled by the form and dimension of the crankshaft pin passing within the big-end opening. By utilising the crankshaft journal as a means to determine the profile of the big end opening machining of the connecting rod may be greatly reduced including some or all of the expensive and precise fine boring of the big end opening. As described above, the connecting rod may be made of two pieces, a bent metal cap portion in the form of a strap and a main body part, the two being joined together with screws or bolts or alternatively riveted or welded for a non-reusable assembly after dismantling. The main body may be constructed of a substantially metallic material and preferably may be integral with the bearing element . The cap may also be integral with the bearing element eliminating the need for two components, namely cap and bearing shell. In consequence the number of separate components, namely two bearing shells together with two connecting rod components, namely main body and cap, is reduced. With such a configuration the bearing element may still be manufactured by current processes such as casting, roll-bonding, sintering or sputtering a bearing surface material onto a sheet steel backing strip. The flexibility of such a bearing element requires that the surface of the crankshaft pin to determine the form of the resulting big end opening during assembly and may result in contact between the bearing surface and the crankpin due to the non-circularity of the big end bore. However the el stohydrodynamic forces during engine operation arising from the interaction between the big- end bore, oil film, and crankshaft, may deform the big- end bore resulting in an inner surface profile that gives acceptable engine operation. In addition any severe regions of non-circularity in the big end bore may be alleviated by wear of the bearing surface during an initial running-in period of the engine.

With this arrangement, expensive machining of the connecting rod is avoided including the big end bore and the joint faces between the main body and cap; the less expensive method of fracture splitting the cap from the main body is also avoided with significant savings in cost.

According to a feature of a second embodiment of a connecting rod according to the present invention, the connecting rod cap portion may be made from a bent metal strap having a bearing element as described before, i.e. either directly bonded to the steel strap or otherwise associated therewith. However, .in this embodiment the bore of the big-end is effected either by conventional machining techniques, such as by boring for example, prior to the connecting rod being assembled to the crankshaft or, by the big-end bearing strap being precision formed by bending, pressing and coining, for example, to have a predetermined bore to suit a shaft journal, in use. In this latter method a composite bearing material having a steel backing with a bearing alloy material layer thereon may be either produced with a predetermined thickness of bearing alloy such that no further machining thereof is required or the bearing alloy layer may be machined to a desired thickness in the flat state, for example, prior to precision forming.

In all embodiments where the strap portion is fixed to the connecting rod main portion by mechanical fixing means, it is preferred that a joint face region between the strap portion and the co-operating connecting rod main portion is provided with co-operating mating features . Such features may comprise a roughened surface or serrations on each face, for example, which interlock and are held in engagement so as to increase the resistance to shear forces tending to separate the strap portion from the connecting rod.

Where the big-end bearing strap is formed from bearing material strip comprising a strong backing layer such as steel and a having a bearing alloy bonded thereon, it is preferred that the bearing alloy in the joint face region between the side faces of the connecting rod main part and the corresponding faces of the big-end bearing strap is removed. Thus, the mating joint faces may comprise the steel of the connecting rod main part and the steel backing of the bearing material .

The bent metal strap big-end portion may be generally "U- shaped" with the legs of the "U" being fixable to side faces of the connecting rod main part. In order to achieve the majority of the benefits accruing from the connecting rod assembly of the present invention it is not necessary that the big-end strap is precisely "U- shaped" in that some divergence or convergence of the legs of the "ϋ" is permissible and, in some cases, desirable. Divergence enables a larger big-end bearing surface area on the connecting rod main part to be achieved. A larger bearing area in this region may be beneficial in accommodating engine combustion loads. Convergence of the legs of the "U" may enable still lighter connecting rods to be formed and may also provide a more beneficial stress pattern during operation.

The bent metal strap forming the big-end cap portion may be made from steel strip typically ranging in thickness from about 1mm up to about 6mm. However, it should be borne in mind that connecting rods according to the present invention may be used for engines as small as strimmer motors for horticultural equipment at one extreme and marine diesel engines at the other extreme. Thus, it is possible that the thickness of the bearing strap for connecting rods around these are likely to lie outside of this range. A particular advantage of connecting rods made according to the present invention is that they are considerably narrower in radial width than conventional connecting rods capable of accommodating a given crank-pin size, Thus, the swept volume of a connecting rod according to the present invention, rotating on a crankshaft is less than for a conventional connecting rod. Consequently, an engine designed ah ini tio to utilise connecting rods according to the present invention may have a crankcase, for example, of less volume and less weight than an engine employing conventional connecting rods.

A further particular advantage of connecting rods according to the present invention where the big-end straps employ steel backed bearing material made by conventional techniques of roll-pressure bonding of an aluminium alloy to steel or casting or sintering of copper alloys to^' steel resides in the flexibility of these known production processes. Continuous strip of virtually unlimited length, in a practical sense, of any desired width and of virtually any_ desired combination of steel thickness and bearing alloy composition and thickness may be produced at will. Thus, the technology to make the material for big-end bearing straps for connecting rods according to the preferred embodiments of the present invention already exists.

In order that the present invention may be more fully understood, embodiments of the invention are illustrated below by way of example only with reference to the accompanying drawings, of which:

Figures 1 and 2 show a side view of a connecting rod, crankpin and cap;

Figures 3 , 4 and 5 show an alternative connecting rod assembly to that shown in Figures 1 and 2;

Figures 6A and 6B show a schematic big-end of a connecting rod according to the present invention assembled to a shaft journal by projection welding;

Figures 7 to 9 show views similar to Figure 6 but for alternative welding methods;

Figures 10A and 10B show a portion of a connecting rod main part with a bearing element being fixed in place;

Figures 11 to 14 show alternative forms of connecting rod main parts and big-end portions;

Figure 15 shows an alternative geometry of connecting rod main part and big-end portion;

Figure 16 shows three corresponding traces of engine speed, oil temperature and oil pressure taken from an engine test using connecting rod assemblies according to the present invention; and Figure 17 which shows a further embodiment of a connecting rod assembly according to the present invention having an alternative geometry.

Referring now to the drawings_/ Figure 1 shows a connecting rod 1 comprising a little end 2 and a bearing surface 3 to form part of the big end opening. The cap 4 has also a bearing surface 5 which comprises the other part of the big end opening. A substantially cylindrical crankpin 6 passes between the connecting rod 1 and cap 4. On. assembly of the components the connecting rod 1, with or without the piston assembly attached, is drawn over the crankpin 6; the cap 4 is similarly drawn over the crankpin 6 and joined to the connecting rod at the faces 7 shown in Figure 2 by means of a bolt, screws, welding or otherwise. The form of the big end opening comprising the bearing surfaces 3 and 5 is determined by the form of the crankshaft at points a and b shown in Figure 2, either by direct contact at these points or by means of a controlled spacer or spacing mechanism.

Another embodiment of the invention is shown in Figure 3 where a single bearing element 8 with a big end opening surface 9 is attached to the connecting rod 1 by means of welding or otherwise. The complete assembly is brought over the crankpin 6 in Figure 4 and then closed around the pin in Figure 5 by either elastically and/or plastically deforming the protruding arms of the bearing element 8 around the surface of the crankpin. In this embodiment the flanges 10 are brought together to form a joint face 11 which might be closed by means of a bolt, welding or otherwise. Whereas the internal perimeter of the big end bore bearing surface is determined by the length of the bearing element 8 and position of the flanges 10, the form of the crankshaft has substantially determined the shape of the resulting big end opening. Before closing the bearing element 8 around the crankpin 6 the points c and d on the crankshaft are determining positions of the bearing surface 9 whereas during closure points e, f and g are points which may control the big end bore bearing surface. There may or may not be contact at these points between the bearing surface 9 and crankpin 6 prior to and during engine operation.

Figures 6A and 6B show the big-end region 20 of a connecting rod main part 22. The side faces 24 of the connecting rod main part are provided with projections 26 which form the weld regions 28 as shown in Figure 6A. In this welded embodiment, the welding step per se is effected with the shaft journal 30 in situ and constitutes a non-reusable construction after disassembly. The legs 32 of the U-shaped strap 34 are permanently fixed to he side faces.24. The bore 36 of the big-end strap 34 may be determined by forming around the journal 30 or by pre-boring of a bearing lining (not shown) on the strap 34 in a jig for example prior to placing in position for welding.

The embodiment shown in Figure 6A could be formed by spot welding without the prior formation of projections 26 as shown in Figure 6B . Figures 7 to 9 show similar views to Figure 6 but with different welding methods employed in construction. Figure 7 shows TIG welded butt joints 50 and is a non- preferred construction. Figure 8 shows TIG fillet welds 52 whereas Figure 9 shows laser staked lap welds 54. Figures 10A and 10B show the big-end region 50 of a connecting rod main part which has a separate bearing element 60 affixed thereto in the bore portion 62. The element 60 comprises a bimetal material having a steel backing and a lining (not shown) of an aluminium alloy roll pressure bonded thereto. The element back face 64 is pressed into intimate contact with the bore 62 of the connecting rod main part and is tack welded 66 into position.

Figures 11 to 14 show alternative constructions of connecting rods according to the present invention, only the lower portion of the connecting rod main part 70 being shown. Figure 11 shows a solid connecting rod main part construction having a bearing element 72 affixed in the bore 74 thereof. The bearing element 72 may be tack welded in place similarly to Figure 10. The big-end bearing strap 76 is made of a steel-backed bearing material and is held to the side faces 78 of the connecting rod main part by small bolts 80. Figure 12 shows a similar construction to Figure 11 but wherein the bearing element 72 is located against rotation in use by recesses 84 cut into the inner face of the big-end bearing strap 76. Figure 13 shows a connecting rod having a forked lower end comprising two prongs 90 to which the legs 92 of the U-shaped big-end strap 76 are affixed either by welding or by bolts for example. The bearing element 72 is supported and fixed by welding or otherwise on the machined ends of the prongs 90. The connecting rod assembly of Figure 14 is similar to that of Figure 13 but with the lower ends of the prongs being formed with a bridging piece 96. The method of fixing the strap 76 to the connecting rod main part 70 may either by welding or mechanical fixing as desired.

Figure 15 shows a ' construction wherein the legs 100 of the U-shaped bearing strap 102 are divergent. The legs 104 of the strap are held on the sloping side faces 106 by small bolts 108. An advantage of this construction is that the big-end region 110 of the connecting rod main part 112 has greater bearing area to receive the firing combustion loads in an engine than a construction where the big-end bearing strap has substantially parallel legs . The bearing surfaces per se may be provided in any suitable manner such as by a coating or by a separate bearing element insert on the connecting rod main part. The big-end bearing strap 102 may be formed from steel backed bearing material strip.

An engine test was carried out on a 1.4 litre Ford Zetec SE (trade name) gasoline engine utilising connecting rods according to the present invention. The connecting rod assemblies according to the present invention each weighed 23Og compared with the standard connecting rod assembly which weighs 42Og. The construction of the rod was similar to that shown in Figure 14 above. The big-end crankpin diameter was 40mm and the distance between small-end and big-end centres was 136mm. The bolts 80 holding the big-end strap onto the main part side faces were M5. The big-end bearing strap 76 overall thickness was 3.2mm including 0.2mm of a bearing alloy. The upper bearing element 72 was 1.5mm overall thickness including 0.2mm of bearing alloy and was held in place by tack welds in the manner indicated in Figure 10 above. The joint faces between the main part side faces 78 and the corresponding faces 82 of the big-end bearing strap were provided with matching serrations to enhance resistance to shear loads imposed during running. The big-end bore was provided by offering the bearing strap up to the connecting rod main part (with the bearing insert 72 held in place) in its final operational position, clamping the main part and the strap together and boring the big-end to the desired size for the journal to be used. The big- end strap was then removed and the connecting rod assembled to the crankshaft.

The engine test was 6.5 hours long. Figure 16 shows the engine test log: the upper trace being engine speed; the middle trace being oil temperature; and, the lower trace being oil pressure. The horizontal scale is in seconds. After the test, the big-end bearing strap showed no signs of fatigue and the bearing material layer showed no signs of scuffing or seizure.

Figure 17 shows a connecting rod assembly 130 according to the present invention wherein the legs 132 of the bearing strap 134 converge slightly and are fixed to the connecting rod main part 136 by screws 138. The bearing strap 134 is flexible in the sense that its elasticity enables the ends 140 of the legs 132 to be sprung apart to go over the shaft journal 142 before being fixed to the side faces of the connecting rod main part 136. Stresses generated during operation tend to pull the legs 132 of the bearing strap more tightly against the side faces of the connecting rod main part .

In the embodiments of the connecting rod assembly exemplified above, the various features of the examples may be interchanged therebetween where appropriate or desirable .

Claims

1. A connecting rod and bearing assembly, the connecting rod having a main part including a stem and an associated little-end and a big-end part wherein a cap portion of the big-end opening is formed from a bent metal strap having a bearing material element associated therewith to co-operate, in use , with a shaft j ournal .

2. A connecting rod according to claim 1 wherein the bent metal strap has a bearing material directly or integrally associated therewith.

3. A connecting rod according to claim 2 wherein the bent metal strap is formed from a strip or sheet material comprising a steel backing layer having one or more layers of bearing material bonded thereto.

4. A connecting rod according to either claim 2 or claim 3 wherein the material for the bent metal strap is selected from the group comprising: aluminium alloy roll-pressure bonded to steel; copper alloy cast onto steel; and, copper alloy sintered to steel .

5. A connecting rod according to any one preceding claim wherein the bent metal strap is affixed to the main part of the connecting rod by either welding or by mechanical fastening means.

6. A connecting rod according to claim 5 wherein the welding technique is selected from the group comprising: projection; spot; and laser welding.

7. A connecting rod according to claim 5 wherein the mechanical fastening means is selected from: threaded fasteners; blind rivets; and, shear pins.

8. A connecting rod according to any one preceding claim wherein, in use, the forces tending to separate the main part of the connecting rod and the big-end bearing strap are predominantly shear forces .

9. A connecting rod according to any one preceding claim wherein, in use, an inner profile of. the big- end opening of the bent metal bearing strap is determined at one or more points by the form of the shaft journal which passes within the big-end opening .

10. A connecting rod according to claim 9 wherein on assembly of the connecting rod to the crankshaft the closing of the big-end opening by the fixing of the bent metal strap is dimensionally controlled by the form and dimension of the crankshaft pin passing within the big-end opening.

11. A connecting rod according to any one of preceding claims 1 to 8 wherein a bearing lining associated with the bent metal big-end bearing strap is provided with a predetermined bore dimension prior to assembly to a shaft journal, in use.

12. A connecting rod according to claim 11 wherein the bore of the big-end opening of the big-end bearing strap portion is produced by machining.

13. A connecting rod according to claim 11 wherein the bore of the big-end opening of the big-end bearing strap is produced by precision forming.

14. A connecting rod according to any one preceding claim wherein corresponding joint face regions between side faces of the connecting rod main part and the big-end bearing strap are provided with matching roughened areas.

15. A connecting rod according to any one preceding claim wherein bearing alloy in the joint face region between the side faces of the connecting rod main part and the corresponding faces of the big-end bearing strap is removed.

16. A connecting rod according to any one preceding claim wherein the big-end bearing strap is generally "U-shaped" with the legs of the "U" being fixable to side faces of the connecting rod main part.

17. A connecting rod according to any one of preceding claims 1 to 15 wherein the big-end bearing strap is generally "U-shaped" but with the legs of the "U" being slightly divergent .

18. A connecting rod according to any one of preceding claims 1 to 15 wherein the big-end bearing strap is generally "U-shaped" but with the legs of the "U" being slightly convergent .

19. A connecting rod according to any one preceding claim wherein the steel thickness of the big-end bearing strap lies in the range from 1 to 6mm.

20. A connecting rod according to any one preceding claim wherein the main part of the connecting rod is formed by a technique selected from the group comprising: casting; powder metallurgy techniques; forging; and, fabrication.

21. A connecting rod according to any one preceding claim wherein a portion of the connecting rod main part which forms part of the big-end opening has a bearing element associated therewith.

22. A connecting rod according to claim 21 wherein the bearing element is formed by a strip of bearing material having a strong backing.

23. A connecting rod according to either claim 21 or claim 22 wherein the bearing element is welded in place.

24. A connecting rod according to claim 21 wherein the bearing element is provided by the surface of the big-end opening of the main part of the connecting rod being coated with a bearing material .

25. A connecting rod according to claim 23 wherein the coating technique is selected from the group comprising: sputtering; high-velocity oxy-fuel spraying; electro-plating; and, plasma spraying.

26. A connecting rod according to any one of preceding claims 1 to 20 wherein the material forming the connecting rod main part forms a part of the big-end opening bearing sur ace .