Bearing structure of crankshaft in internal combustion engine

A bearing structure of a crankshaft in an internal combustion engine supports a crankshaft on a crankcase with rolling bearings includes a play absorbing device for absorbing the play in the radial direction generated between an outer race of the rolling bearing and a bearing hole of the crankcase. The play absorbing device includes a push plug which pushes an outer race of the ball bearing in the direction perpendicular to an axis of the crankshaft. The push plug is arranged in parallel to the crankshaft between the cylinder and the crankshaft. The outer race of the ball bearing is resiliently biased toward a side of the bearing hole of the crankcase which receives an explosion pressure. As a result, the generation of a hitting sound which is attributed to the above-mentioned “play” can be largely reduced even with respect to an internal combustion engine with a high explosion pressure.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims is related to Japanese Patent Application Nos. 2003-339418 and 2003-339416, both filed Sep. 30, 2003, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the structure of a bearing which rotatably supports a crankshaft in a crankcase.

2. Description of Background Art

In general, with respect to a single-cylinder internal combustion engine of a motorcycle or the like, a crankshaft is rotatably and pivotally supported in a bearing hole of a crankcase by way of a pair of rolling bearings, a piston which is slidably fit in a cylinder bore is connected to a crankpin of the crankshaft by way of a connecting rod, and an explosion pressure applied to the piston is received by the crankcase by way of the pair of rolling bearings from the crankshaft. In this case, it is desirable to ensure the smooth and light rotation of the crankshaft by maintaining a desired bearing function in such a manner that outer races and inner races of the above-mentioned pair of rolling bearings are press-fit into a bearing hole of the crankcase and a journal shaft portion of the crankshaft respectively thus preventing the occurrence of a hitting sound attributed to a play between the bearing hole and the journal shaft portion.

However, when the pair of left and right rolling bearings are press-fit into both of the crankshaft and the crankcase as mentioned above, there arises a drawback that the assembling of the crankshaft to the crankcase and the maintenance after the assembling become difficult.

Accordingly, conventionally, to overcome such a drawback, as shown inFIG. 9, at the time of assembling the crankshaft6to the crankcase10,11, before incorporating the crankshaft6into the crankcase10,11, the outer race23of the right-side rolling bearing BBr is preliminarily press-fit into a right-side crankcase two-split body11and, at the same time, the inner race22of the left-side rolling bearing BBl is press-fit into a left-side journal shaft portion6lof the crankshaft6. Further, at the time of assembling the crankshaft6, the right-side journal shaft portion6rof the crankshaft6is press-fit into the inner race26of the right-side rolling bearing Br (the outer race being already press-fit into the right-side crankcase two-split body). Subsequently, the outer race23of the left-side rolling bearing BBl in which the inner race22has been already press-fit into the left-side journal shaft portion6lof the crankshaft6is loosely fit into (with a play) the left-side crankshaft two-split body10so as to facilitate the assembling of the crankshaft6to the crankcase10,11and to facilitate the maintenance thereafter.

However, in such an operation, a phenomenon that some play is generated in the radial direction between the outer race23of the left-side rolling bearing BBl and the bearing hole of the crankcase10,11is unavoidable. Accordingly, at the time of running the internal combustion engine, there arises a drawback that a desired bearing function cannot be obtained besides the occurrence of a hitting sound attributed to the above-mentioned play.

Accordingly, in Patent Document JP-A-2003-83080 (shown herein inFIG. 10of this application), there has been proposed a technique in which an outer surface of either one (left-side) outer race of a pair of ball bearings BBl is pushed in the axial direction by a push plug30having an inclined push surface so as to absorb the above-mentioned “play”.

However, with respect to the technique described in the above-mentioned Patent Document JP-A-2003-83080, the above-mentioned push plug30is arranged at a side opposite to the cylinder with the crankshaft sandwhiched between the push plug30and the cylinder, that is, the push plug is arranged at the crankcase side so as to push the outer race of the rolling bearing BBl toward the cylinder side of the bearing hole of the crankcase (the side opposite to the direction along which the explosion pressure is applied to the piston) and hence, the pushing direction becomes opposite to the explosion load applied to the piston. Accordingly, the absorption of the play by the push plug30becomes insufficient and hence, with respect to a high-output internal combustion engine which generates a large explosion force, the outer race of the rolling bearing BBl hits the side of the bearing hole of the crankcase opposite to the cylinder whereby there arises a drawback that a hitting-sound reduction effect cannot be sufficiently achieved.

Further, with conventional device described in the above-mentioned Patent Document JP-A-2003-83080 (shown hereinFIG. 10of this application), in the midst of assembling the push plug30, the push plate PP which is pushed outwardly due to the coil spring31in a free enlongation state is disengaged from the positioning members PM and hence, the position of the push plug30is not determined whereby the operator has to assemble the push plug30while compressing the coil spring31or pushing the push plate with his hand thus giving rise to a drawback that the assembling is difficult and cumbersome.

SUMMARY AND OBJECTS OF THE INVENTION

The invention has been made in view of such circumstances and it is an object of the invention to provide a novel bearing structure of a crankshaft in an internal combustion engine which can overcome the above-mentioned drawbacks by allowing an outer race of a rolling bearing which supports a crankshaft to push a side of a bearing hole of a crankcase opposite to a cylinder.

To achieve the above-mentioned object, a first aspect of the invention described is directed to a bearing structure of a crankshaft in an internal combustion engine comprising a cylinder block and a crankcase which is integrally connected to the cylinder block, wherein a crankshaft which is connected with a piston slidably fitted in a cylinder of the cylinder block by way of a connecting rod in an interlocking manner is rotatably supported in a bearing hole of the crankcase by way of a rolling bearing and the bearing structure includes play absorbing means for preventing a play in the radial direction between the rolling bearing and the bearing hole, wherein the bearing structure has the first feature that the play absorbing means includes a push plug which pushes an outer race of the rolling bearing in the direction perpendicular to an axis of the crankshaft, the push plug is arranged parallel to the crankshaft between the cylinder and the crankshaft, and the outer race of the rolling bearing is resiliently biased toward a side of the bearing hole of the crankcase which receives an explosion pressure.

Further, to achieve the above-mentioned object, a second aspect of the invention includes a feature whereby the play absorbing means having the push plug is arranged to be more offset to a rotational direction front side of the crankshaft than a cylinder center axis of the cylinder.

Further too achieve the above-mentioned object, a third aspect of the invention is directed to a bearing structure of a crankshaft in an internal combustion engine in which a crankshaft is rotatably supported on a crankcase by way of a rolling bearing, play absorbing means is interposed between an outer race of the rolling bearing and the crankcase, and a play in the radial direction which is generated between the outer race of the rolling bearing and a bearing hole of the crankcase is absorbed by the play absorbing means, wherein the bearing structure has the first feature that the play absorbing means includes a push plug which is movably mounted in the crankcase and pushes an outer race of the rolling bearing in the radial direction of the crankshaft, a spring set plate which is fixed to the crankcase by a bolt, and a spring member which is interposed between the push plug and the spring set plate and resiliently biases the push plug toward the outer race, wherein the spring set plate includes a bent portion which is bent toward the crankcase and the bent portion is, when the spring member is in a free elongation state and the spring set plate is in a temporarily stopped state, engaged with a positioning portion which is formed on the crankcase thus enabling the positioning of the spring set plate.

Further, to achieve the above-mentioned object, a fourth aspect of the invention includes a feature whereby a boss portion which projects outwardly from an outer surface is integrally formed on the crankcase, the push plug is movably mounted on the boss portion, a recessed portion which receives a free end of the spring member is formed in an intermediate portion of the spring set plate in a state that the recessed portion faces the boss portion in an opposed manner, and the boss portion and the recessed portion are fit into each other in a state that the assembling of the spring set plate is completed.

Still further, to achieve the above-mentioned object, a fifth aspect of the invention includes a feature whereby the positioning portion formed on the crankcase is formed in a fork shape which opens toward the outer surface so as to receive the spring set plate from the outside of the crankcase.

According to the first aspect of the invention, the push plug which pushes the outer race of the rolling bearing in the direction perpendicular to the axis of the crankshaft is arranged in parallel with the crankshaft between the cylinder and the crankshaft, and the outer race of the rolling bearing is resiliently biased such that the outer races are pushed to a side of the bearing hole of the crankcase which receives an explosion force. Accordingly, when a load in a radial direction acts on the rolling bearing due to the explosion combustion of the internal combustion engine, a “play” between the outer race of the rolling bearing and the bearing hole of the crankcase can be surely absorbed and hence, the generation of a hitting sound attributed to the above-mentioned “play” can be largely reduced even in a high-output internal combustion engine which exhibits a high explosion pressure. Further, by arranging the push plug parallel to the crankshaft, not only it is possible to allow a pushing force of the push plug to effectively act on the outer race of the rolling bearing but also the assembling of the “play absorbing means” having the push plug into the crankcase can be easily performed from the outside of the crankcase.

Further, according to the second aspect of the invention, the acting direction of the pushing force which is applied to the outer race of the rolling bearing by the pushing plug can be substantially aligned with the acting direction of the maximum pressure (explosion pressure) which the piston receives and hence, it is possible to allow the pushing force acting on the outer race of the rolling bearing to effectively work as a “play” absorbing load between the rolling bearing and the bearing.

Further, according to the third aspect of the invention, in assembling the “play absorbing means” which is constituted of a push plug, the spring member and the spring set plate into the crankcase, the spring set plate can be positioned in a state that the spring member is held in a free elongation state, that is, in a state that a spring load is not applied. Accordingly, an operator can assemble the “play absorbing means” into the crankcase without compressing the spring member or pushing the spring set plate and hence, the assembling operability can be largely enhanced. Further, the spring set plate includes a bent portion which is bent toward the crankcase side and hence, it is possible to accommodate the bent portion into the positioning portion at the crankcase side without largely projecting the spring set plate from the crankcase whereby a space which the “play absorbing means” occupies for assembling thereof can be reduced.

Further, according to the fourth aspect of the invention, the boss portion formed in the crankcase and the recessed portion of the sprig set plate are fitted into each other in a state that the assembling of the “play absorbing means” is completed and hence, it is possible to accommodate the “play absorbing means” in an outer surface of the crankcase thus preventing portions thereof projecting largely from the crankcase.

Still further, according to the fifth aspect of the invention, the manipulation for positioning the spring set plate with respect to the crankcase is facilitated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First of all, inFIG. 1andFIG. 2, an OHC type 4-cycle single-cylinder internal combustion engine E includes a cylinder block CB, a cylinder head CH which is fixed to a deck surface of the cylinder block CB, a two (left and right)-split crankcase CC which is fixed to a lower portion of the cylinder head CH, and a head cover HC which is mounted on an upper surface of the cylinder head CH so as to cover the cylinder head CH. In a cylinder1having a cylinder sleeve2which is formed at a center portion of the cylinder block CB, a piston3is slidably fit. A combustion chamber4is formed in the cylinder head CH such that the combustion chamber4faces a top surface of the piston3. A small end portion of a connecting rod5is rotatably connected to a piston pin of the piston3, while a large end portion of the connecting rod5is rotatably connected to a crankpin7of the crankshaft6. The crankshaft6is rotatably supported on the crankcase CC by way of left and right rolling bearings BB, BR described later.

The above-mentioned crankcase CC is formed of a cast product made of light alloy of iron, aluminum or the like and is constituted by integrally coupling a left-side crankcase two-split body10and a right-side crankcase two-split body11using a plurality of bolts14. An outer surface of an opening of the left-side crankcase two-split body10is covered with a left-side cover12and the left-side cover12is fixed to the outer surface by bolts15, while an outer surface of an opening of the right-side crankcase two-split body11is covered with a right-side cover13and the right-side cover13is fixed to the outer surface by bolts16. On the left-side and right-side crankcase two-split bodies10,11, left and right journal walls10j,11jwhich face each other with a space therebetween are respectively integrally formed. In these left and right journal walls10j,11j, left and right bearing holes18,19(the left bearing hole18having a diameter larger than a diameter of the right bearing hole19) are respectively opened or formed coaxially. Bearing surfaces of these left and right bearing holes18,19are formed by cast rings made of Fe which are cast at the time of casting the crankcase. In the left and right bearing holes18,19, left and right journal shaft portions6jl,6jrof the crankshaft6are rotatably supported by way of left-side and right-side rolling bearings BB, BR. As explicitly shown inFIG. 4, the left-side rolling bearing BB is constituted of a ball bearing and the right-side rolling bearing BR is constituted of a roller bearing. An inner race22of the above-mentioned left-side rolling bearing, that is, the ball bearing BB is press-fit in the left journal shaft portion6jlof the crankshaft6and, at the same time, an outer race23of the ball bearing BB is loosely fit in the bearing hole18of the left journal wall10j(a slight play in the radial direction). The outer race of the ball bearing BB is more extended outwardly in the axial direction (the direction away from the cylinder axis) than the inner race22, an engaging groove41having a recessed cross-section which constitutes “movement suppressing means S” described later is formed on an inner peripheral surface of an extending portion23e, and a hook portion40fof the movement suppressing member40is detachably engaged with the engaging groove41. As shown inFIG. 4, between the above-mentioned engaging groove41and the hook portion40f, a fine gap is defined so as to allow a slight movement of the outer race23in the axial direction with respect to the hook portion40f.

Further, the inner race26of the above-mentioned right-side rolling bearing, that is, the roller bearing BR is inserted into the right journal shaft portion6jrof the crankshaft6, while an outer race27of the roller bearing BR is press-fit into the bearing hole19of the right journal wall11j. Here, after inserting the inner race26into the right journal shaft portion6jrof the crankshaft6, the fixing of the inner race26to the right journal shaft portion6jrof the crankshaft6is performed by collectively fastening together with a gear train62, a centrifugal filter63(seeFIG. 1) using a nut from a shaft end of the crankshaft6.

As shown inFIG. 1andFIG. 3, between the crankshaft6and the cylinder1, on the left journal wall10jof the left-side crankcase two-split body10, “play absorbing means A” which is provided for absorbing a “play” in the radial direction between the outer race23of the ball bearing BB and the bearing hole18is provided along the axial direction of the crankshaft6in a state that the play absorbing means A is arranged close to the outer side of an upper portion of the above-mentioned ball bearing BB.

The above-mentioned “play absorbing means A” is, as explicitly shown inFIG. 3,FIG. 4andFIG. 6, constituted of a push plug30, a coil spring31and a spring set plate32. the push plug30is formed in a bottomed hollow cylindrical shape and an inclined surface30sis formed on a corner portion of the bottomed-side end wall. Further, the push plug30is slidably fit in the inside of a cylindrical hole35which is formed in a boss portion34disposed at an oblique upper position with respect to the ball bearing BB of the left journal wall10jand has an axis thereof in the axial direction of the crankshaft6(the direction perpendicular to the cylinder axis).

As shown inFIG. 4, a corner portion23chaving a circular cross section of the outer race23of the ball bearing BB projects into the inside of the cylindrical hall35and the inclined surface30sof the above-mentioned push plug30is brought into contact with the corner23c. The above-mentioned coil spring31is housed in the inside of the push plug30and has an outer end thereof projected more outwardly than an end surface of the opening of the push plug30. The above-mentioned spring set plate32is formed in an elongated manner in the radial direction of the crankshaft6, and includes a proximal portion32bwhich forms a bolt hole33, an intermediate portion32nhaving a recessed portion d for receiving spring in an inner surface thereof, and a free end portion32fhaving a bent portion b which is folded inwardly, wherein the proximal portion32bis fixed to an outer surface of the left-side crankcase two-split body10using a mounting bolt38, the recessed portion d for receiving spring formed in the intermediate portion32nreceives an outer end of the above-mentioned coil spring31, and the above-mentioned free end32fis formed to have a width narrower than a width of the intermediate portion32nand is engaged with the inside of a positioning portion36having a fork-like cross section (seeFIG. 5) which is integrally formed with the left-side crankcase two-split body and is formed in a chevron shape in a side view (seeFIG. 2) whereby the rotation of the spring set plate32is stopped, that is, the spring set plate32can be positioned.

Here, in assembling the above-mentioned “play absorbing means A” into the left-side crankcase two-split body10, as shown inFIG. 8, the spring set plate32is loosely and temporarily fastened to the left-side crankcase two-split body10using the mounting bolt38. Then, in a state that an end portion of the coil spring31having a free elongation length is received by the spring receiving recessed portion d of the intermediate portion32n, the bent portion b of the free end32fof the spring set plate32is engaged with the fork-shaped positioning portion36of the left-side journal wall10jand hence, it is possible to position the spring set plate32while preventing the rotation of the spring set plate32. Accordingly, an operator can perform the assembling of the “play absorbing means A” without performing cumbersome manipulations such as compressing the coil spring31or pushing the spring set plate32with his hand to prevent the rotation of the spring set plate32. Further, by further fastening the mounting bolt38, as indicated by a chain line shown inFIG. 8, the assembling of the “play absorbing means A” is completed In this assembling completion state, it is possible to allow the boss portion34of the left-side crankcase two-split body10and the recessed portion d of the intermediate portion32nof the spring set plate32to fit into each other whereby the “play absorbing means A” can be housed in an outer surface of the crankcase10in a compact manner and the partial protrusion of the “play absorbing means A” can be prevented. Further, as mentioned previously, since the assembling of the “play absorbing means A” can be performed from the outside of the left-side crankcase two-split body10, it is possible to obtain the extremely favorable assembling operability.

As shown inFIG. 4, in the assembling completion state of the “play absorbing means A”, the spring set plate32pushes the push plug30inwardly by way of the coil spring31and pushes the outer race23of the ball bearing BB with an inclined downward pushing force F due to the inclination surface30sof the push plug30. Due to a vertical-direction component force Fr of the pushing force F, that is, due to the component force Fr in the radial direction of the crankshaft6, the outer race23is pushed in the radial direction and it is possible to absorb or eliminate the above-mentioned “play” in the radial direction between the outer peripheral surface of the outer race23and the bearing hole18due to the pushing force whereby it is possible to properly support the crankshaft6without a play with respect to the load in the radial direction of the crankshaft6due to an explosion force applied to the piston3and the occurrence of a hitting sound can be preliminarily prevented.

Further, the acting direction of the pushing force which the above-mentioned “play absorbing means A” applies to the ball bearing BB is substantially aligned with the acting direction of the maximum pressure (explosion pressure) which the piston3receives. That is, in this internal combustion engine, the position of the piston which receives the maximum explosion pressure is a position slightly delayed from a top dead center and hence, as shown inFIG. 2, the “play absorbing means A” is disposed at a position slightly offset to the rotational direction (the arrow R direction inFIG. 2, the counterclockwise direction) side than the cylinder axis L—L as viewed from the axial direction of the crankshaft6and is disposed in the direction along the cylinder axis L—L. Accordingly, it is possible to make the direction of the pushing force which the “play absorbing means A” applies to the ball bearing BB aligned with the direction of the maximum pressure which the crankshaft6receives from the piston3thus allowing the ball bearing BB to properly support the crankshaft6.

Here, the push plug30of the above-mentioned “play absorbing member A” is arranged parallel to the crankshaft6between the cylinder1and the crankshaft6and resiliently biases the ball bearing BB such that the ball bearing BB is pushed to the side of the bearing hole18of the crankcase10which receives the explosion pressure. Accordingly, when the excessive load in the radial direction acts on the ball bearing BB along with the explosion combustion of the internal combustion engine, the “play” between the outer race23and the bearing hole18of the crankcase10can be surely absorbed and hence, it is possible to largely reduce the generation of the hitting sound attributed to the above-mentioned “play” even in the high-output internal combustion engine having the high explosion pressure. Further, by arranging the push plug30in parallel to the crankshaft6, not only it is possible to allow a pushing force of the push plug30to effectively act on the outer race23of the ball bearing BB but also the assembling of the “play absorbing means A” having the push plug30into the crankcase10can be easily performed from the outside of the crankcase10.

Further, the acting direction of the pushing force which the above-mentioned push plug30applies to the ball bearing BB is substantially aligned with the acting direction of the maximum pressure (explosion pressure) which the piston3receives. That is, in this internal combustion engine, the position of the piston3which receives the maximum explosion pressure is a position slightly delayed from a top dead center and hence, as shown inFIG. 2, the “play absorbing means A ” is disposed at a position slightly offset to the crankshaft6rotational direction (the arrow R direction inFIG. 2, the counterclockwise direction) side from the cylinder axis L—L as viewed from the axial direction of the crankshaft6and is disposed in the direction along the cylinder axis L—L. Accordingly, it is possible to make the direction of the pushing force which the “play absorbing means A” applies to the ball bearing BB aligned with the direction of the maximum pressure which the crankshaft6receives from the piston3and hence, it is possible to make the pushing force which acts on the outer race23of the ball bearing BB effectively and efficiently function as the “play” absorbing load between the bearing BB and the bearing hole18.

Here, as shown inFIG. 4, due to the pushing force F which is applied to the outer race23by the push plug30of the above-mentioned “play absorbing means A”, the component force Fs in the horizontal direction, that is, in the axial direction of the crankshaft6is generated. Accordingly, due to this component force Fs, a thrust force in the axial direction is inevitably generated on the outer race23and this thrust force acts as a force which moves the outer race23of the ball bearing BB in the axial direction.

Here, according to this embodiment, due to the provision of the “movement suppressing means S” described hereinafter, it is possible to receive the thrust force and suppress the movement of the outer race23in the axial direction.

Next, to explain the “movement suppressing means S”, the “movement suppressing means S” is constituted of a plurality of (three in this embodiment) movement suppressing members40and engaging grooves41which are loosely engaged with hook portions40fof the movement suppressing members40and are formed on an inner peripheral surface of the outer race23of the ball bearing BB. The movement suppressing member40is, as shown inFIG. 7, formed of an elongated plate shape and the hook portion40fwhich is bent in a C shape in cross section is integrally formed on a distal end portion of the elongated plate shape. As shown inFIG. 2andFIG. 3, the above-mentioned three movement suppressing members40are arranged radially in an equidistant manner in the circumferential direction outside the left-side crankcase two-split body10. The movement suppressing members40have outer ends thereof fixed to an outer surface of the left-side crankcase two-split body concentrically with the ball bearing BB using the mounting bolts43. Three movement suppressing members40are directed to the center of the ball bearing BB and the hook portions40fformed on inner ends of the movement suppressing members40are bent toward an inner surface of the outer race of the ball bearing BB and are respectively engaged with the engaging grooves41formed in the inner surface. Between each hook portion40fand the engaging groove41, a slight gap is defined in the axial direction thereof. Due to the engagement of the hook portion40fand the engaging groove41, it is possible to suppress the movement of the outer race23of the ball bearing BB in the axial direction. As a result, it is no more necessary to make the right-side ball bearing receive the thrust force (the technique described in the above-mentioned Patent Document 1) whereby, as in the case of this embodiment, it is possible to adopt the roller bearing BR having a small diameter (compared with the ball bearing BB) as the right-side rolling bearing. Further, as shown inFIG. 2andFIG. 3, by arranging the above-mentioned three movement suppressing members40in a spaced-apart manner in the circumferential direction of the ball bearing BB, it is possible to prevent the inclination of the ball bearing BB.

The above-mentioned “movement suppressing means S” is constituted of the movement suppressing members40which are fixed to the outer surface of the left-side crankcase two-split body10and the engaging grooves41which are formed on the inner peripheral surface of the outer race23of the ball bearing BB and hence, the “movement suppressing means S” has the simple structure whereby in mounting the “movement suppressing means S” on the left-side crankcase two-split body10, it is unnecessary to adopt mounting shapes such as notches in the left-side crankcase two-split body10thus eliminating the possibility that the strength of the left-side crankcase two-split body10is decreased. Further, since this “movement suppressing means S” can be mounted from the outside of the left-side crankcase two-split body10, the assembling can be performed easily and no special facilities and jigs are necessary to perform the assembling.

As shown inFIG. 1andFIG. 2, to the crankshaft6disposed outside the ball bearing BB, a drive sprocket wheel51for timing driving of a valve-operating cam shaft50is fixed. The drive sprocket wheel51is connected with a driven sprocket wheel53which is fixed to the valve-operating cam shaft50which is rotatably and pivotally supported on the cylinder head CH by way of an endless chain52. The rotation of the crankshaft6is transmitted to the valve-operating cam shaft50via the drive sprocket wheel51, the endless chain52and the driven sprocket wheel53. Below the crankshaft6, a chain falling prevention plate54which prevents the falling of the chain52is fixed to the left-side crankcase two-split body10using a plurality of bolts55. The chain falling prevention plate54has, as shown inFIG. 2, an arcuate portion which is disposed close to and faces an arcuate lower surface of the chain52wound around the drive sprocket wheel51, wherein the falling of the above-mentioned chain52is prevented by this arcuate portion.

InFIG. 1andFIG. 2, a flywheel61which fixes an outer rotor60of an ACG thereto is fixed to a left-side end portion of the crankshaft6, while a gear train62and a centrifugal oil filter63which are connected with a transmission, a balancer and the like in an interlocking manner are fixed to a right-side end portion of the crankshaft6.

Next, the manner of operation of this embodiment is explained.

Here, when the internal combustion engine E is operated, the crankshaft6which is supported on the crankcase6by way of the ball bearing BB and the roller bearing BR is rotatably driven. In such an operation, the “play” in the radial direction which is generated between the outer case23of the ball bearing BB and the bearing hole18of the left-side crankcase two-split body10can be absorbed by the above-mentioned “play absorbing means A”. Particularly, the push plug30is arranged parallel to the crankshaft6between the cylinder1and the crankshaft6and the outer race23of the ball bearing BB is resiliently biased such that the outer race23is pushed to the side of the bearing hole18of the left-side crankcase two-split body10which receives the explosion force and hence, when the load in the radial direction acts on the ball bearing BB due to the explosion combustion of the internal combustion engine, the “play” between the outer race23of the bearing BB and the bearing hole18of the left-side crankcase two-split body10can be surely absorbed. Accordingly, it is possible to largely reduce the generation of the hitting sound attributed to the above-mentioned “play” also with respect to the high-output internal combustion engine with the high explosion pressure. Further, the movement of the outer race23of the ball bearing BB in the axial direction which is inevitably present due to the mounting of the above-mentioned “play absorbing means A” can be suppressed by the above-mentioned “movement suppressing means S” and hence, it is possible to form one of the pair of rolling bearings which support the crankshaft6using the roller bearing (having the diameter smaller than the diameter of the ball bearing and having high rigidity) whereby the internal combustion engine E can have a compact configuration.

In other words, when the internal combustion engine E is operated, the crankshaft6which is supported on the crankcase6by way of the ball bearing BB and the roller bearing BR is rotatably driven. In such an operation, the “play” in the radial direction which is generated between the outer race23of the ball bearing BB and the bearing hole18of the left-side crankcase two-split body10can be absorbed by the above-mentioned “play absorbing means A”. Accordingly, it is possible to preliminarily prevent the generation of the hitting sound attributed to the above-mentioned “play” and, at the same time, it is possible to ensure the smooth and light rotation of the crankshaft6by properly supporting the crankshaft6using the above-mentioned bearings BB, BR.

Further, the movement of the outer race23of the ball bearing BB in the axial direction which is inevitably present due to the mounting of the above-mentioned “play absorbing means A” can be suppressed by the above-mentioned “movement suppressing means S” and hence, it is possible to form one of the pair of rolling bearings which support the crankshaft6using the roller bearing (having the diameter smaller than the diameter of the ball bearing and having high rigidity) whereby the internal combustion engine E can have a compact configuration.

Although the embodiment of the invention has been explained heretofore, the invention is not limited to the embodiment and various embodiments are conceivable within the scope of the invention.

For example, a roller bearing or other rolling bearing may be used in prace of the left-side ball bearing or a needle bearing or other rolling bearing may be used in prace of the right-side roller bearing.