Balancer device

Abstract

This balancer device comprises: a pair of balancer shafts 3, 4 provided with balance weights 14-17, respectively; a drive transmission mechanism for transmitting the rotational driving force of an internal combustion engine to the pair of balancer shafts; and a housing 2 in which the pair of balancer shafts are housed in parallel with each other. The housing has: first opening parts 19, 20 formed in an upper housing 8; a plurality of fixing boss parts 18 arranged in parallel with a rotation axis of the balancer shaft at the outside part of the upper housing; and a plurality of connection wall parts 21-22 for connecting the respective fixing boss parts adjacent to each other in the rotation axis direction of the balancer shaft. On the upper surfaces of the connection wall parts, circular arc surface-shaped recessed surfaces 24-25 for avoiding the rotation locus of a crank shaft 01 of the internal combustion engine are formed. With such a configuration, the bottom part of the internal combustion engine and the housing of the balancer device can be disposed sufficiently close to each other, while the configuration increasing the strength of the housing.

Description

Balancer device

[0001] The present invention relates to a balancer device that reduces secondary vibrations in an internal combustion engine.

[0002] A conventional balancer device is described in the following Patent Document 1. Outlined, this balancer device includes a housing attached to the lower part of the crankcase of an internal combustion engine and housed in an oil pan, a drive-side balancer shaft rotatably housed and supported via a plurality of plain bearings provided at predetermined intervals in the internal axial direction of the housing and to which rotational force is transmitted from the crankshaft, and a driven-side balancer shaft rotatably housed and supported by a plurality of plain bearings also provided at predetermined intervals in the internal axial direction of the housing and to which rotational force is transmitted from the drive-side balancer shaft.

[0003] The drive-side balancer shaft is adapted to receive torque from the crankshaft via a drive chain wound around a drive-side sprocket at its front end, and is provided with a pair of balance weights, one at the front and one at the rear, in the axial direction.

[0004] The driven balancer shaft receives rotational force from a drive gear provided on the driving balancer shaft via a driven gear. The driven balancer shaft also has a pair of balance weights, one at the front and one at the rear, integrally provided with the shaft.

[0005] When the engine starts and the crankshaft is driven to rotate, the drive balancer shaft is driven to rotate at twice the rotational speed of the crankshaft via the drive sprocket of the drive balancer shaft wound by the drive chain, and as the drive gear rotates, the driven gear meshing with it rotates in the opposite direction, driving the driven balancer shaft to rotate in the opposite direction to the drive balancer shaft. This rotation of the balance weights effectively suppresses secondary vibrations of the internal combustion engine.

[0006] Japanese Patent Application Laid-Open No. 2008-115917

[0007] In the balancer device, the housing must have sufficient strength to withstand vibrations caused by the rotation of the plurality of balance weights.

[0008] However, in recent years, there has been a demand for a layout in which the balancer device is located closer to the internal combustion engine (crankshaft) in order to reduce the vertical size of the entire internal combustion engine, including the balancer device. However, with this layout, the upper housing of the housing cannot be made thicker at its upper end because there is a risk that it will interfere with the crankshaft during rotation. In other words, if priority is given to ensuring sufficient housing strength, the balancer device cannot be located closer to the crankshaft.

[0009] The present invention was devised in consideration of the technical problems of the conventional balancer devices, and one object of the present invention is to provide a balancer device that can be made compact by increasing the strength of the balancer device housing while arranging the bottom of the internal combustion engine and the balancer device housing sufficiently close to each other.

[0010] The invention described in claim 1 is characterized in that it comprises, inter alia, a pair of balancer shafts, each provided with a balance weight, a drive transmission mechanism that transmits the rotational drive force of an internal combustion engine to the pair of balancer shafts, and a housing that accommodates the pair of balancer shafts in a state where they are parallel to each other, wherein the housing comprises: a first opening formed at an end facing the internal combustion engine and provided so that the balance weights can be visually observed; fixing bosses that are arranged radially outward of the rotational axes of the pair of balancer shafts and parallel to the rotational axes and are fixed to the internal combustion engine; and a connecting portion that connects the fixing bosses that are adjacent in the rotational axis direction of the balancer shafts, extends from an outer portion that connects the adjacent fixing bosses to an opening edge of the first opening, and has a concave surface on a surface facing the bottom of the internal combustion engine that avoids the rotational trajectory of the crankshaft of the internal combustion engine.

[0011] According to this invention, the balancer device can be disposed as close as possible to the crankshaft of the internal combustion engine while ensuring sufficient strength of the housing, thereby enabling the internal combustion engine, including the balancer device, to be made smaller in the vertical direction.

[0012] 5 is a side view showing, in cross section, the positional relationship between the balancer device according to the present invention and the crankshaft of a four-cylinder internal combustion engine, illustrating a state in which each large diameter portion of each connecting rod is inserted into the concave groove of the balance weight. FIG. 6 is a bird's-eye view showing an embodiment of the balancer device according to the present invention. FIG. 7 is a plan view of the balancer device according to this embodiment. FIG. 8 is a side view of the balancer device according to this embodiment. FIG. 9 is a bottom view of the balancer device according to this embodiment. FIG. 10 is a diagram showing a drive-side balancer shaft used in this embodiment, with (a) being a plan view of the drive-side balancer shaft and (b) being a side view of the drive-side balancer shaft. FIG. 11 is a cross-sectional view taken along line A-A in FIG. 5. FIG. 12 is a cross-sectional view taken along line B-B in FIG. 5. FIG. 13 is a cross-sectional view taken along line CC in FIG. 3.

[0013] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which a balancer device according to the present invention is applied to, for example, an in-line four-cylinder internal combustion engine of an automobile will be described below with reference to the drawings.

[0014] FIG. 1 is a side view showing in cross section the positional relationship between the balancer device according to the present invention and the crankshaft of a four-cylinder internal combustion engine, showing the state in which the large diameter portions of the connecting rods are fitted into the recessed grooves of the balance weights.

[0015] As is well known, a bearing cap having a bearing portion that supports the crankshaft is fixed to the lower part of the cylinder block of an internal combustion engine, and an oil pan that is divided into upper and lower parts and stores engine oil inside is attached to the lower part of the bearing cap.

[0016] 1, the crankshaft 01 includes a main shaft 02 that protrudes outside the internal combustion engine, a main journal 03 that is supported by a plurality of bearings, such as bearing caps, that are fastened to the lower part of the cylinder block by bearing bolts, and a crank pin 04 to which a large-diameter portion (not shown) of a connecting rod 05 is rotatably connected. The crankshaft 01 also has a large-diameter crank sprocket (not shown) attached to the axial end of the main shaft 02, and counterweights 06, 06 provided on both sides of the crank pin 04.

[0017] A balancer device 1 for suppressing secondary vibrations of the engine is housed and disposed in a space surrounded by the lower part of the cylinder block, the bearing cap, and the oil pan.

[0018] 2 is a bird's-eye view of the balancer device according to the present invention, FIG. 3 is a plan view of the balancer device according to this embodiment, FIG. 4 is a side view of the balancer device according to this embodiment, FIG. 5 is a bottom view of the balancer device according to this embodiment, FIG. 6 shows a drive side balancer shaft used in this embodiment, (a) is a plan view of the drive side balancer shaft, (b) is a side view of the drive side balancer shaft, FIG. 7 is a cross-sectional view taken along line A-A in FIG. 5, FIG. 8 is a cross-sectional view taken along line B-B in FIG. 5, and FIG. 9 is a cross-sectional view taken along line C-C in FIG. 3.

[0019] As shown in Figures 2 to 9, this balancer device 1 comprises a housing 2 fixed to the underside of a cylinder block, a drive-side balancer shaft 3 and a driven-side balancer shaft 4 rotatably supported in a space 2a inside the housing 2 and arranged parallel to the longitudinal direction of the engine, and a helical drive-side gear 5 and a driven-side gear 6 provided near the approximate centers of the balancer shafts 3, 4, respectively, with their teeth meshing with each other.

[0020] The housing 2 is composed of a lower housing 7 on the oil pan side, and an upper housing 8 disposed above the lower housing 7 and fixed to the underside of the cylinder block.

[0021] The drive-side balancer shaft 3 has a helical-tooth driven gear 9 attached to its tip shaft portion 3a, which protrudes forward from the housing 2. This driven gear 9 is meshed with a helical-tooth drive gear 10 located on its side, and a drive sprocket 11, which serves as a transmission mechanism, is attached to the tip of the drive gear 10. The rotational force from the crankshaft 01 is transmitted to the drive-side balancer shaft 3 via a drive chain (not shown) wound between the drive sprocket 11 and the crank sprocket. This causes the balancer shafts 3 and 4 to rotate in opposite directions via the drive-side gear 5 and the driven-side gear 6. The balancer shafts 3 and 4 are set to rotate twice per rotation of the crankshaft 01. The tension of the drive chain is adjusted by a chain tensioner (not shown).

[0022] As shown in FIG. 7, the tip shaft portion 3a is rotatably supported by a half-split bearing portion 50 formed between the lower housing 7 and the upper housing 8, and this bearing portion 50 is fixed vertically by a pair of fastening bolts 12 that fasten the lower housing 7 and the upper housing 8 together.

[0023] As shown in Figures 6(a), 6(b) and 7, the drive-side balancer shaft 3 has three cylindrical journal surfaces 3b to 3d formed respectively at the front end, central portion and rear end in the axial direction. These three journal surfaces 3b to 3d are rotatably supported by three plain bearings 51, 52, 53 provided in first to third bearing grooves formed respectively at positions between the first to third cross beam deck portions 13a to 13c of the upper housing 8 and the corresponding first to third cross beam deck portions 13d to 13f of the lower housing 7, as shown in Figures 1 and 7.

[0024] As shown in Figures 6(a), 6(b), and 7, the drive-side balancer shaft 3 is integrally provided with two semicircular first and second balance weights 14, 15 at symmetrical positions on either side of the second journal surface 3c. These balance weights 14, 15 are formed to have substantially the same shape and weight, but are circumferentially offset by a predetermined angle. Furthermore, at the axial centers of the balance weights 14, 15, recessed grooves 14a, 15a are formed as constricted portions to avoid interference during rotation of the crankshaft 01. Each recessed groove 14a, 15a is formed in a substantially arc-shaped shape, with its central portion being the deepest recess.

[0025] The drive-side gear 5 is fixed to the drive-side balancer shaft 3 by press-fitting or the like, and is sandwiched between thrust walls (not shown) so that its movement in the axial direction is restricted.

[0026] As shown in Figures 2 and 3, the driven side balancer shaft 4 has an axial length that is shorter than that of the drive side balancer shaft 3, and like the drive side balancer shaft 3, journal surfaces (not shown) formed at three locations in the axial direction are rotatably supported via plain bearings in three semicircular arc-shaped bearing grooves formed in the first cross beam deck section 13a to the third cross beam deck section 13c.

[0027] 3 and 8, the driven-side balancer shaft 4 is integrally provided with two semicircular arc-shaped first and second balance weights 16, 17 at symmetrical positions in the front and rear with the second cross beam deck 13b in between. These balance weights 16, 17 are set to have approximately the same shape and the same weight, and also have grooves 16a, 17a, which are constricted portions formed at the axial center of the drive-side balancer shaft 3 to avoid interference with the crankshaft 01. Each of these grooves 16a, 17a is formed in an approximately arc-shaped shape with the center portion being maximally recessed.

[0028] The driven gear 6 is fixed to the driven balancer shaft 4 by press fitting or the like, and, like the drive gear 5, is sandwiched within a groove formed in the lower housing 7, i.e., between opposing thrust walls of the groove, so that its axial movement is restricted.

[0029] The lower housing 7 and the upper housing 8 are made of aluminum alloy material to reduce weight, and are fastened and fixed from above and below by the above-mentioned plurality of fastening bolts 12 .

[0030] The upper housing 8 is integrally provided with a plurality of fixing bosses 18 on both sides thereof, which are provided along the rotational axis direction of the drive-side balancer shaft 3 and the driven-side balancer shaft 4. Each fixing boss 18 is formed in a cylindrical shape that is long in the vertical direction, and has insertion holes 18a formed therein that penetrate vertically, into which bolts (not shown) are inserted to attach and fix the housing 2 to the cylinder block 2. The upper housing 8 also has a front end wall 8a at one longitudinal end (near the drive-side gear 5), and has the above-mentioned first cross beam deck portion 13a to third cross beam deck portion 13c behind it.

[0031] Furthermore, the upper housing 8 is formed with two first openings 19, 20 at the positions where the pairs of front and rear balance weights 14, 15, 16, 17 of the drive-side balancer shaft 3 and the driven-side balancer shaft 4 are disposed. As shown in Fig. 3, the first openings 19, 20 are formed in a rectangular shape that is long in the front-rear direction, and allow the balance weights 14 to 17 to be visually observed. The first openings 19, 20 also communicate with second openings 27a to 27d (described later) formed in the bottom wall of the lower housing 7.

[0032] The upper housing 8 is provided with a plurality of first and second connecting walls 21, 22 which are connecting portions that connect the fixing bosses 18 adjacent to each other in the rotational axis direction of the drive-side and driven-side balancer shafts 3, 4. The front first connecting walls 21, 21 are integrally connected to the front end wall 8a via a pair of fixing bosses 18, 18 provided near the front end of the drive-side and driven-side gears 5, 6. The rear second connecting walls 22, 22 are integrally connected to the third cross beam deck portion 13c via a pair of fixing bosses 18, 18 on the rear end side.

[0033] As shown in Figures 2, 3 and 9, each connecting wall portion 21, 22 has a height H that is set from the lower surface of the upper housing 8 to near the upper end surface of each fixing boss portion 18, and has a width W that is set from the outer opening edges 19a, 20a of the first openings 19, 20 to near the outer surfaces of the fixing boss portions 18. This width W is set to be larger than the maximum vertical length H. This increases the rigidity of the upper housing 8. Also, as shown in Figures 2, 3 and 7 to 9, each connecting wall portion 21, 22 has a concave surface 24, 25 that avoids the rotation locus X of the crankshaft 01 (including the large-diameter portion of the connecting rod 05) at the center of the upper surface facing the bottom surface of the cylinder block.

[0034] In addition, each connecting wall portion 21, 22 is formed so that both side portions on the side of the adjacent fixing boss portions 18, 18 in the direction of the rotational axis of the balancer shafts 2, 3 are gradually formed higher from the centrally located concave surface 24, 25, and is positioned close to the fastening surface to the internal combustion engine, which is the upper surface of the fixing boss portions 18, 18.

[0035] Each of the concave surfaces 24, 25 is formed in a direction perpendicular to the longitudinal direction of the upper housing 8, and is formed in a downward slope from the outer surface of each of the connecting walls 21, 22 toward the outer opening edges 19a, 20a of the first and second openings 19, 20. Specifically, as shown in Figure 9, each of the concave surfaces 24, 25 is formed in an arc shape along the rotation trajectory X of the crankshaft 01, thereby suppressing interference with the crankshaft 01 during rotation.

[0036] Furthermore, the outer ends 24a, 25a of each of these concave surfaces 24, 25 are open to the outside, while the tip ends 24b, 25b are directed toward the inner bottom surface of the lower housing 7 via the concave grooves 14a, 15a of each of the balance weights 14, 15.

[0037] 3, the upper housing 8 has a third opening 23 formed in its front end wall 8a, and third connecting walls 26, 26 are integrally joined between the front fixing bosses 18, 18 and the front end wall 8a. Structurally, the third connecting walls 26, 26 are substantially the same as the first and second connecting walls 21, 22, and each has a concave surface 26a, 26a formed in its longitudinal center that avoids the rotation locus X of the crankshaft 01 (including the large-diameter portion of the connecting rod 05). Since the third connecting walls 26, 26 are joined to the first connecting walls 21, 21 via the front fixing bosses 18, 18, the rigidity (strength) of the entire upper housing 8 is high.

[0038] 4 and 5, the lower housing 7 is formed in a rectangular shape with approximately the same outer shape as the upper housing 8, and its top surface is formed in a flat shape that can abut against the bottom surface of the upper housing 8. Furthermore, as shown in Figures 7 and 8, the lower housing 7 is provided with the first to third cross beam deck portions 13d to 13f described above at positions corresponding to the first to third cross beam deck portions 13a to 13c of the upper housing 8, and the fastening bolts 12 that vertically join the lower housing 7 and the upper housing 8 are fastened from below to each of these cross beam deck portions 13a to 13f.

[0039] 7, the lower housing 7 has protrusions 7a, 7b formed on the inner bottom surface of the bottom wall along the outer surfaces of the grooves 14a, 15a of the balance weights 14, 15, with a predetermined gap between them so as not to interfere with the bottom surfaces of the grooves 14a, 15a. These protrusions 7a, 7b increase the rigidity of the lower housing 7.

[0040] As shown in Fig. 5, four second openings 27a to 27d are formed through the bottom wall of the lower housing 7. These openings 27a to 27d are disposed at approximately equal intervals along the length of the lower housing 7 and are shaped like elongated rectangles extending along the width of the lower housing 7. A fourth opening 39 is formed at the front end of the lower housing 7, communicating with the third opening 23 of the upper housing 8, as shown in Fig. 8.

[0041] The lower housing 7 has a pair of protruding portions, namely, first and second vertical ribs 28, 29, on both sides of each of the second openings 27a-27d on the outer bottom surface of the bottom wall. The first and second vertical ribs 28, 29 have the same uniform predetermined width W1 and extend parallel to each other along the longitudinal direction of the outer bottom surface of the lower housing 7, with one longitudinal end 28a, 29a connected to the underside of the first cross beam deck portion 13d. Meanwhile, the other end 28b, 29b extends further beyond the third cross beam deck portion 13f and is connected to a first cross rib 30 provided along the width direction of the lower housing 7.

[0042] 5 and 8, the first and second vertical ribs 28, 29 are joined to the first horizontal rib 30 at the other ends 28b, 29b, as well as to eight second to eighth horizontal ribs 31-38 that are arranged at predetermined spans from the first horizontal rib 30 toward the one ends 28a, 29a on the underside of the lower housing 7. Therefore, the first and second vertical ribs 28, 29 and the first to eighth horizontal ribs 30-38 form a lattice pattern, surrounding the second openings 27a-27d and sufficiently reinforcing the lower housing 7.

[0043] [Operations and Effects of the Balancer Device According to the Present Embodiment] The operations and effects of the balancer device according to the present embodiment will be described below. In this embodiment, the first and second connecting walls 21, 22, in particular, that connect adjacent fixing bosses 18 are characterized in that the connecting walls 21, 22 have a large wall thickness between their outer sides and their inner sides up to the first openings 19, 20, and the width W of each connecting wall is made larger than the maximum length H in the up-down direction, thereby ensuring sufficient strength of the housing 2, in particular the upper housing 8.

[0044] Moreover, by providing the concave surfaces 24, 25 at the center positions of the first and second connecting wall portions 21, 22, respectively, interference with the crankshaft 01 during rotation can be avoided.

[0045] Therefore, in order to withstand the vibrations caused by the rotation of the balance weights 14 to 17, the balancer device 1 can be placed as close as possible to the internal combustion engine (crankshaft 01) while ensuring sufficient strength of the upper housing 8. As a result, the overall size of the internal combustion engine, including the balancer device 1, can be reduced in the vertical direction.

[0046] In particular, the concave surfaces 24, 25 are not simply inclined surfaces but are formed as arcuate surfaces that follow the arcuate locus X of the crankshaft 01. Therefore, the balancer device 1 can be disposed sufficiently close to the crankshaft 01, further facilitating miniaturization in the vertical direction.

[0047] The third connecting wall portion 26 of the upper housing 8 also contributes to the strength of the upper housing 8 .

[0048] The housing 2 can be made lighter overall by providing the first to third openings 19, 20, 23, and 27a to 27d.

[0049] The oil that flows into the upper end side of the upper housing 8 and is received by the concave surfaces 24, 25 of each connecting wall portion 21, 22 is guided toward the inner bottom surface of the lower housing 7 without coming into contact with the balance weights 14, 16, thereby suppressing the generation of rotational resistance of the balancer shafts 3, 4 due to the oil.

[0050] Furthermore, the oil that has flowed into the inner bottom surface of the lower housing 7 is effectively discharged into the oil pan, which is located outside, through the second openings 27a to 27d of the lower housing 7. This makes it possible to more effectively suppress the generation of rotational resistance in the balancer shafts 3, 4.

[0051] In this embodiment, the balance weights 14, 15, 16, and 17 are provided with grooves 14a to 17a, which are constricted portions, thereby suppressing interference with the crankshaft 01 during rotation. Furthermore, oil that flows onto the concave surfaces 24 to 25 and flows down from the tip ends 24b to 25b of the concave surfaces 24 to 25 passes through the grooves 14a to 17a, making it less likely that the oil will come into direct contact with the balance weights 14 to 17. This also helps to suppress an increase in the rotational resistance of the balancer shafts 3 and 4 due to oil.

[0052] Each of the connecting walls 21, 22 can collect oil from both raised side portions to the center portion, which is each of the concave surfaces 24, 25. This makes it difficult for oil to hit the balance weights 14 to 17.

[0053] Furthermore, in this embodiment, the first and second vertical ribs 28, 29 surrounding the second openings 27a to 27d are provided on the underside of the lower housing 7, thereby improving the strength of the lower housing 7. As a result, as described above, it is possible to effectively suppress the rotational vibration of each of the balance weights 14 to 17 together with the upper housing 8.

[0054] Furthermore, the first and second vertical ribs 28, 29 can prevent oil stored in an oil pan provided at the bottom of the lower housing 7 from entering the inside of the housing 2 through the second openings 27a to 27d. In other words, the first and second vertical ribs 28, 29 function as a blocking wall for the second openings 27a to 27d, making it possible to restrict the flow of oil that has swayed in the oil pan from the second openings 27a to 27d into the housing 2, for example.

[0055] By providing two ribs, the first vertical rib 28 and the second vertical rib 29, the reinforcing effect of the lower housing 7 is further increased and oil penetration into each of the second openings 27a to 27d can be more effectively suppressed.

[0056] Furthermore, in this embodiment, by providing multiple horizontal ribs 30 to 38 between the two vertical ribs 28, 29 to form a ladder-shaped rib overall, the strength of the lower housing 7 can be further improved and oil can be effectively prevented from entering the housing interior through the second openings 27a to 27d.

[0057] In addition, by providing convex portions 7a, 7b on the inner bottom surface of the lower housing 7 that are inclined toward each of the second openings 27a to 27d, oil that flows into the housing 2 from the first openings 19, 20 can be efficiently discharged from the second openings 27a to 27d into the oil pan by utilizing the inclined surfaces of the convex portions 7a, 7b.

[0058] The present invention is not limited to the configuration of the above embodiment, and the internal combustion engine may have a number of cylinders other than four, and the structure of each connecting wall portion 21 to 23, etc. may be further modified.

[0059] 01... crankshaft, 02... main shaft, 03... main journal, 04... crank pin, 05... connecting rod, 06... counterweight, 1... balancer device, 2... housing, 3... driving side balancer shaft, 4... driven side balancer shaft, 5... driving side gear, 6... driven side gear, 7... lower housing, 7a... convex portion, 8... upper housing 8, 13a to 13c... cross beam deck portion, 14 17...balance weight, 14a...17a...groove (constricted portion), 18...fixing boss portion, 18a...insertion hole, 19, 20...first opening, 19a, 20a...outer opening edge, 21, 22...connecting wall portion (connecting portion), 23...third opening, 24, 25...concave surface, 27a...27d...second opening, 28...first vertical rib (protruding portion), 29...second vertical rib (protruding portion), 30...first horizontal rib, 31...38...second to eighth horizontal ribs.

Claims

1. A balancer device to be attached to the bottom of an internal combustion engine, comprising: a pair of balancer shafts, each equipped with a balance weight; a drive transmission mechanism that transmits the rotational drive force of the internal combustion engine to the pair of balancer shafts; and a housing in which the pair of balancer shafts are contained in a state parallel to each other, wherein the housing comprises: a first opening formed at an end on the internal combustion engine side and provided so that each of the balance weights can be visually observed; fixing boss portions arranged radially outwardly of the rotational shafts of each of the pair of balancer shafts and parallel to the rotational shafts, and fixed to the internal combustion engine; and a connecting portion that connects each of the fixing boss portions adjacent to each other in the rotational axis direction of the balancer shafts, extends from an outer portion connecting the adjacent fixing boss portions to the opening edge of the first opening, and has a concave surface on the surface facing the bottom of the internal combustion engine that avoids the rotation trajectory of the crankshaft of the internal combustion engine.

2. A balancer device as described in claim 1, characterized in that at least a part of the tip of the concave surface on the balancer shaft side of the housing at the connecting portion is formed in a position that avoids the balance weight during rotation.

3. A balancer device as described in claim 2, characterized in that the balance weight is provided with narrowed portions recessed from both sides in the axial direction of the balancer shaft toward the center, and the tip of the concave surface is positioned opposite the narrowed portions.

4. A balancer device as described in claim 1, characterized in that the connecting portion is formed so that both side portions adjacent to the fixing boss portion in the direction of the rotational axis of the balancer shaft are gradually made higher from the concave surface located in the center, and is positioned close to the fastening surface of the fixing boss portion for the internal combustion engine.

5. A balancer device as described in claim 1, wherein the housing comprises an upper housing attached to the bottom of the internal combustion engine and having the first opening formed therein, and a lower housing attached to the lower part of the upper housing, the lower housing having a second opening formed in its bottom wall which communicates with the first opening, and a protrusion provided on the outer bottom surface of the bottom wall of the lower housing which surrounds at least a portion of the second opening.

6. A balancer device as described in claim 1, wherein the housing comprises an upper housing attached to the bottom of the internal combustion engine and having the first opening formed therein, and a lower housing attached to the lower part of the upper housing, wherein the lower housing has a bottom wall formed with a plurality of second openings communicating with the first openings, and a first vertical rib is provided along one side opening edge of each of the second openings on the outer bottom surface of the bottom wall, continuously surrounding the one side opening edge.

7. A balancer device as described in claim 6, characterized in that the lower housing further has a second vertical rib located on the opposite side of the plurality of second openings from the first vertical rib, the second vertical rib being parallel to the first vertical rib and continuously surrounding the other side opening edge of each of the second openings.

8. A balancer device according to claim 7, characterized in that the outer bottom surface of the lower housing has a plurality of horizontal ribs provided between the plurality of second openings and connecting the first vertical rib and the second vertical rib.

9. A balancer device according to claim 5, wherein said lower housing has a convex portion on its inner bottom surface that is inclined toward said second opening.

10. A balancer device as described in claim 1, characterized in that the connecting portion is formed so that the width length from the tip of the concave surface to the outer surface of the outer portion connecting the adjacent fixing boss portions is longer than the vertical length of the thickest part vertically upward from the bottom surface.

11. A balancer device according to claim 1, wherein the concave surface is formed in an arc shape along the rotation trajectory of the crankshaft.

12. A balancer device attached to the bottom of an internal combustion engine, comprising: a pair of balancer shafts, each having a balance weight; a drive transmission mechanism for transmitting the rotational drive force of the internal combustion engine to the pair of balancer shafts; and a housing in which the pair of balancer shafts are accommodated in a state parallel to each other, wherein the housing has: a first opening formed at an end on the internal combustion engine side and provided so that the balance weights can be visually observed; a second opening formed at a position opposite to the first opening and for discharging oil that has flowed into the housing from the first opening to the outside; and fixing boss portions arranged radially outside the respective rotation shafts of the pair of balancer shafts and parallel to the rotation shafts, and fixed to the internal combustion engine. a connecting portion that connects adjacent fixing boss portions in the direction of the rotation axis of the balancer shaft, extends from an outer portion that connects the adjacent fixing boss portions to an opening edge of the first opening, and has an inclined concave surface on a surface facing a bottom of the internal combustion engine, and at least a portion of a tip of the concave surface on the balancer shaft side is formed in a position that avoids the balance weight during rotation.

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