Power system and method of manufacturing the same

A power system includes an electric motor, a transmission, and a differential gear system. The transmission includes a first gear that is mechanically connected to the electric motor, a second gear that has a rotation axis in common with the first gear and is mechanically connected to a differential gear casing of the differential gear system, a pinion gear that meshes with the first gear and the second gear, and a pinion holder that rotatably supports the pinion gear. The pinion holder has a pinion gear supporting portion which is disposed on a side of the differential gear casing of the differential gear system to support the pinion gear, relative to a meshing portion between the second gear and the pinion gear. An inner diameter of the second gear at the meshing portion is equal to or smaller than an outer diameter of the pinion holder.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority from Japanese Patent Application No. 2016-247120 filed on Dec. 20, 2016, the entire contents of which are incorporated herein by reference.

FIELD

The present invention relates to a power system that is provided in an electric motor vehicle and a method of manufacturing the power system.

BACKGROUND

A power system is disclosed in JP-A-2002-104001 which includes an electric motor for driving a left wheel and a right wheel of a vehicle, a transmission that is disposed on power transmission paths between the electric motor and the left wheel and the electric motor and the right wheel, and a differential gear system that distributes output power shifted by the transmission to the left wheel and the right wheel. This type of power system is provided in an electric motor vehicle such as a hybrid vehicle or an electric vehicle, as a front wheel drive system or a rear wheel drive system, and it is important for appropriate power transmission how to support gears constituting the transmission. Also, it is desired that the power system is minimized.

However, no specific description is given in JP-A-2002-104001 with respect to the support of the gears constituting the transmission and the minimizing of the power system.

SUMMARY

The invention is to provide a power system in which a pinion holder for holding a pinion gear is appropriately disposed to realize appropriate power transmission and the size thereof can be minimized, and a method of manufacturing the power system.

The invention provides following aspects (1) to (6).

(1) A power system (e.g., a power system1in an embodiment to be described below) including:

an electric motor (e.g., an electric motor2in an embodiment) that drives a left wheel and a right wheel of a vehicle;

a transmission (e.g., a transmission5in an embodiment) that is disposed on power transmission paths between the electric motor and the left wheel and between the electric motor and the right wheel; and

a differential gear system (e.g., a differential gear system6in an embodiment) that distributes output power shifted by the transmission to the left wheel and the right wheel, wherein:

the transmission includes

a first gear (e.g., a first gear51in an embodiment) that is mechanically connected to the electric motor,

a second gear (e.g., a second gear52in an embodiment) that has a rotation axis in common with the first gear and is mechanically connected to a differential gear casing (e.g., a differential gear casing61in an embodiment) of the differential gear system,

a pinion gear (e.g., a pinion gear53in an embodiment) that meshes with the first gear and the second gear, and

a pinion holder (e.g., a pinion holder54in an embodiment) that rotatably supports the pinion gear:

the pinion holder has a pinion gear supporting portion (e.g., a pinion gear supporting portion54ain an embodiment) which is disposed on a side of the differential gear casing of the differential gear system to support the pinion gear, relative to a meshing portion (e.g., a meshing portion M in an embodiment) between the second gear and the pinion gear; and

an inner diameter (e.g., an inner diameter R1in an embodiment) of the second gear at the meshing portion is equal to or smaller than an outer diameter (e.g., an outer diameter R2in an embodiment) of the pinion holder.

(2) The power system according to (1), wherein

the pinion gear is configured such that one end of a pinion shaft (e.g., a pinion shaft53cin an embodiment) is rotatably supported on a partition wall portion (e.g., a partition wall43in an embodiment) through a first bearing (e.g., a bearing55in an embodiment) and an other end thereof is rotatably supported on the pinion gear supporting portion of the pinion holder through a second bearing (e.g., a bearing56in an embodiment), and

the pinion holder is fixed to the partition wall portion.

(3) The power system according to (1) or (2), wherein

the pinion gear includes a large diameter gear (e.g., a large diameter gear53ain an embodiment) and a small diameter gear (e.g., a small diameter gear53bin an embodiment), and

the small diameter gear meshes with the second gear.

(4) The power system according to any one of (1) to (3), wherein

the differential gear casing of the differential gear system has an input plate (e.g., an input plate61bin an embodiment) that extends in a radial direction and is mechanically connected to the second gear, and

the inner diameter (e.g., an inner diameter R3in an embodiment) of the second gear at a connecting portion (e.g., a connecting portion52bin an embodiment) between the second gear and the input plate is larger than the outer diameter of the pinion holder.

(5) The power system according to (4), wherein

the second gear includes a second-gear large diameter portion (e.g., a second-gear large diameter portion52cin an embodiment) constituting the connecting portion, a second-gear small diameter portion (e.g., a second-gear small diameter portion52din an embodiment) constituting the meshing portion, and a second gear connecting portion (e.g., a second gear connecting portion52ein an embodiment) connecting the second-gear large diameter portion with the second-gear small diameter portion, and

an outer diameter of the second-gear small diameter portion is smaller than an outer diameter of the second-gear large diameter portion.

(6) A method of manufacturing a power system (e.g., the power system1in an embodiment), wherein

the power system includes:

an electric motor (e.g., an electric motor2in an embodiment) that drives a left wheel and a right wheel of a vehicle;

a transmission (e.g., a transmission5in an embodiment) that is disposed on power transmission paths between the electric motor and the left wheel and between the electric motor and the right wheel; and

a differential gear system (e.g., a differential gear system6in an embodiment) that distributes output power shifted by the transmission to the left wheel and the right wheel,

the transmission includes:

a first gear (e.g., a first gear51in an embodiment) that is mechanically connected to the electric motor;

a second gear (e.g., a second gear52in an embodiment) that has a rotation axis in common with the first gear and is mechanically connected to a differential gear casing (e.g., a differential gear casing61in an embodiment) of the differential gear system;

a pinion gear (e.g., a pinion gear53in an embodiment) that meshes with the first gear and the second gear; and

a pinion holder (e.g., a pinion holder54in an embodiment) that rotatably supports the pinion gear,

the pinion holder has a pinion gear supporting portion (e.g., a pinion gear supporting portion54ain an embodiment) which is disposed on a side of the differential gear casing of the differential gear system to support the pinion gear, relative to a meshing portion (e.g., a meshing portion M in an embodiment) between the second gear and the pinion gear, and

the method comprises:

assembling the pinion gear, the second gear, the pinion holder, and the differential gear system in this order after assembling the first gear.

According to aspect (1), since the pinion holder has the pinion gear supporting portion which is disposed on the side of differential gear casing of the differential gear system to support the pinion gear, relative to the meshing portion between the second gear mechanically connected to the differential gear casing of the differential gear system and the pinion gear, the pinion gear can be appropriately supported. This enables appropriate power transmission from the pinion gear to the second gear.

In addition, since the inner diameter of the second gear in the meshing portion with the pinion gear is equal to or smaller than the outer diameter of the pinion holder, this can reduce the size in the radial direction around the pinion gear while improving the degree of freedom in layout around the pinion holder.

According to aspect (2), since the pinion holder for supporting the other end of the pinion shaft is fixed to the partition wall for supporting one end of the pinion shaft, the partition wall portion has both the support function of the pinion shaft and the support function of the pinion holder, so that the components can be used in common and thus the number of components can be reduced.

According to aspect (3), since the pinion gear has the large diameter gear and the small diameter gear and the small diameter gear meshes with the second gear, enlargement in dimension in the radial direction can be prevented.

According to aspect (4), since the inner diameter of the second gear at the connecting portion between the second gear and the differential gear casing is larger than the outer diameter of the pinion holder, the pinion holder can be assembled after the second gear is assembled.

According to aspect (5), the second gear includes the second-gear large diameter portion constituting the connecting portion with the differential gear casing, the second-gear small diameter portion constituting the meshing portion with the pinion gear, the outer diameter of the second-gear small diameter portion is smaller than that of the second-gear large diameter portion, and thus it is possible to prevent enlargement in dimension in the radial direction around the pinion gear. Further, the deceleration ratio of the second gear can be increased.

According to aspect (6), since the pinion gear, the second gear, the pinion holder, and the differential gear system are assembled in this order after the first gear is assembled, it is possible to improve the degree of freedom in layout around the pinion holder while making it easy to enlarge the bearing and secure a lubrication path around the pinion holder.

DETAILED DESCRIPTION

A power system1according to an embodiment of the invention will be described with reference toFIGS. 1 to 7andFIGS. 8A to 8D.

The power system1of the embodiment includes an electric motor2that is a drive source for driving axles, and is provided in an electric motor vehicle such as a hybrid vehicle or an electric vehicle as a front wheel drive system or a rear wheel drive system.

InFIG. 1, reference numerals3A and3B indicate left and right axles, which are coaxially disposed in a vehicle width direction. In the power system1, a housing4has the entire shape formed into a substantially cylindrical shape, and includes therein the electric motor2for driving the axles, a transmission5that decelerates driving rotation of the electric motor2, and a differential gear system6that distributes the driving rotation decelerated by the transmission5to the axles3A and3B.

The housing4includes a first casing41for accommodating the electric motor2and a second casing42for accommodating the transmission5and the differential gear system6. A partition wall43is provided at a boundary between the first casing41and the second casing42, and an internal space of the first casing41is partitioned from an internal space of the second casing42by the partition wall43. The partition wall43is fastened to a step portion41bprovided on an outer peripheral part of the first casing41with a bolt47. Therefore, a mating surface A1of the first casing41and the partition wall43is located closer to the first casing41relative to a mating surface A2of the first casing41and the second casing42. A bottom of the housing4functions as a storage portion44for storing lubricating oil (liquid medium), and the lubricating oil is stored up to a stationary oil level L illustrated inFIG. 6. The stationary oil level L is set lower than an air gap G of the electric motor2(a clearance secured between an inner periphery of a stator21and an outer periphery of a rotor22to be described below) so as to reduce an agitation loss of the lubricating oil in the electric motor2. As also illustrated inFIG. 2, a communication port43ais formed in a lower part of the partition wall43to allow the lubricating oil to flow therethrough.

The electric motor2includes the stator21fixed to an inner peripheral part of the first casing41and the rotor22disposed rotatably on the inner periphery of the stator21. A rotor shaft23is coupled to an inner peripheral part of the rotor22so as to surround an outer periphery of one axle3A, and the rotor shaft23is supported on an end wall41aof the first casing41and the partition wall43through bearings24and25so as to be relatively rotable coaxially with the axle3A. In addition, one end of the axle3A and the rotor shaft23penetrates the partition wall43and extends into the second casing42, and the other end of the axle3A penetrates the end wall41aof the first casing41and extends to the outside of the housing4.

The transmission5includes a first gear51mechanically connected to the electric motor2, a second gear52having the same rotation axis as that of the first gear51and mechanically connected to a differential gear casing61of the differential gear system6, a plurality of pinion gears53meshing with the first gear51and the second gear52, and a pinion holder54for supporting the plurality of pinion gears53to be rotatable and not to revolve, and the decelerated driving rotation is output to the differential gear casing61of the differential gear system6through the pinion gear53and the second gear52when the driving rotation of the electric motor2is input from the first gear51.

The first gear51is an external gear, and is formed integrally with the rotor shaft23. The pinion gear53includes a large diameter gear53awhich is an external gear, a small diameter gear53bwhich is an external gear, and an pinion shaft53cwhich supports the large diameter gear53aand the small diameter gear53bin an integrally rotatable manner. The large diameter gear53ais coupled to the pinion shaft53con a side of the electric motor2, and meshes with the first gear51. In addition, the small diameter gear53bis formed integrally with the pinion shaft53con a side of the differential gear system6, and meshes with the second gear52. An end of the pinion shaft53con the side of the electric motor2is rotatably supported on the partition wall43through the bearing55, and an end thereof on the side of the differential gear system6is rotatably supported on a pinion gear supporting portion54aof the pinion holder54through the bearing56.

As illustrated inFIG. 2, the transmission5of the embodiment includes three pinion gears53. The three pinion gears53are disposed at equal intervals (intervals of 120°) in a circumferential direction around the first gear51. At least one of the three pinion gears53is partially or entirely located in the storage portion44described above, and functions as a rotating body that splashes up the lubricating oil stored in the storage portion44with the rotation caused by the driving of the electric motor2. In the example illustrated inFIG. 2, the lowermost pinion gear53disposed immediately below the first gear51functions as a rotating body for splashing the lubricating oil, and thus the splashed lubricating oil is supplied to two upper pinion gears53. Here, assuming that the pinion gear53rotates in the counterclockwise direction inFIG. 2, the lubricating oil splashed by the rotation of the lowermost pinion gear53is mainly supplied to the pinion gear located at an upper left, and then the lubricating oil scattered by the rotation of the pinion gear located at the upper left is mainly further supplied to the pinion gear located at an upper right.

In the second gear52, a gear portion52ais an internal gear and meshes with the small diameter gear53bof the pinion gear53. The second gear52includes a connecting portion52bextending from the gear portion52atoward the differential gear system6across the outer peripheral side of the pinion holder54(pinion gear supporting portion54a), and the connecting portion52bis mechanically connected to the differential gear casing61of the differential gear system6through a spline. In other words, the second gear52includes a second-gear large diameter portion52cconstituting the connecting portion52bwith the differential gear casing61, a second-gear small diameter portion52dconstituting the gear portion52ameshing with the pinion gear53, and a second gear connecting portion52efor connecting the second-gear large diameter portion52cand the second-gear small diameter portion52d, and an outer diameter of the second-gear small diameter portion52dis smaller than that of the second-gear large diameter portion52c. Further, the second gear52has a lower end located in the storage portion44described above, and also functions as a rotating body for splashing the lubricating oil stored in the storage portion44with the rotation caused by the driving of the electric motor2.

As illustrated inFIGS. 3 to 5, the pinion holder54includes three pinion gear supporting portions54aeach of which rotatably supports the pinion shaft53cof the pinion gear53through the bearing56, three fixed portions54bthat are fixed to the partition wall43, and a bottomed cylindrical cup portion54cformed on a center (on an inner diameter side of the pinion gear supporting portion54aand the fixed portion54b) of the pinion holder54.

The pinion gear supporting portion54ais disposed on the side of the differential gear casing61of the differential gear system6relative to a meshing portion M between the second gear52mechanically connected to the differential gear casing61of the differential gear system6and the small diameter gear53bof the pinion gear53. Thus, the other end of the pinion shaft53c, in which one end thereof is supported by the partition wall43through the bearing55, is supported by the pinion gear supporting portion54athrough the bearing56, and thus the pinion gear53can be appropriately supported in a state of being held at both sides.

The three fixed portions54bare located at intermediate portions between the pinion gear supporting portions54aadjacent to each other in the circumferential direction, and are fastened to the partition wall43with bolts57, respectively. Thereby, the partition wall43serves as a support member of the pinion holder54as well as a support member of the pinion shaft53c.

The cup portion54csurrounds the outer periphery of one axle3A through the space portion S from one end side to the other end side of the meshing portion M in the axial direction and on the inner peripheral side of the meshing portion M in the radial direction, and the bottom54don one end side is provided with a through-hole54ethrough which the axle3A penetrates. In addition, the inner peripheral part on the other end side of the cup portion54crotatably supports one end side of the differential gear casing61through the bearing65. Thereby, the pinion holder54serves as a support member of the differential gear casing61as well as a support member of the pinion gear53.

The differential gear system6includes the differential gear casing61, a differential pinion shaft62, a differential pinion gear63, and left and right side gears64A and64B so as to allow difference in rotation of the left and right axles3A and3B while distributing the driving rotation, which is input to the differential gear casing61from the second gear52, to the left and right axles3A and3B.

The differential gear casing61includes a spherical differential gear casing body61athat accommodates the differential pinion shaft62, the differential pinion gear63, and the left and right side gears64A and64B, an input plate61bthat extends in the radial direction from the outer periphery of the differential gear casing body61aand is mechanically connected to the second gear52, and left and right extension portions61cand61dthat extend in the axial direction from both sides of the differential gear casing body61a. One extension portion61crotatably supports the one axle3A at an inner peripheral part thereof, and an outer peripheral part thereof is rotatably supported by the pinion holder54through the bearing65. Further, the other extension portion61drotatably supports the other axle3B at an inner peripheral part thereof, and an outer peripheral part thereof is rotatably supported by the end wall42aof the second casing42through the bearing66.

The differential pinion shaft62is supported by the differential gear casing body61aso as to be directed in a direction orthogonal to the axles3A and3B, and rotatably supports two differential pinion gears63, which are bevel gears, inside the differential gear casing body61a. That is, the differential pinion shaft62allows the differential pinion gears63to rotate while revolving with the rotation of the differential gear casing61.

The left and right side gears64A and64B are bevel gears, are rotatably supported inside the differential gear casing body61aso as to mesh with the differential pinion gears63from both sides, and are mechanically connected to the left and right axels3A and3B through the connecting unit such as the spline. In a state where the differential pinion gears63revolve without rotating, for example, during straight running, the left and right side gears64A and64B rotate at a constant speed, and the driving rotation is transmitted to the left and right axles3A and3B. Furthermore, during curve running or left or right turning, the differential pinion gears63rotate, so that the left and right side gears64A and64B rotate relative to each other and the difference in rotation between the left and right axles3A and3B is allowed.

[Lubrication Function of Pinion Holder]

Next, a lubrication function of the pinion holder54will be described.

The pinion holder54has a storage space to store lubricating oil splashed from the storage portion44of the housing4by the second gear52or the pinion gear53. The storage space is the above-described space portion S formed by the cup portion54cand the one axle3A, and the lubricating oil splashed by the second gear52or the pinion gear53flows into the space portion S through communication holes54fand54g(which will be described below) communicating with the space portion S.

The lubricating oil flowing into the space portion S is supplied to the above-described bearing65which is disposed adjacent to the space portion S for rotatably supporting one end side of the differential gear casing61, whereby the bearing65is properly lubricated. In addition, the lubricating oil is also distributed from the space portion S to the inside of the differential gear system6requiring lubrication and the electric motor2that needs to be cooled by the lubricating oil. More specifically, the lubricating oil is supplied from the space portion S through the clearance between the axle3A and the extension portion61cof the differential gear casing61to the inside of the differential gear system6, and is supplied from the space portion S to the electric motor2through the clearance between the axle3A and the rotor shaft23.

As illustrated inFIG. 5, the pinion holder54includes first guide portions54ifor receiving the lubricating oil, which is splashed by the second gear52or the pinion gear53, on a first face54hopposed to the pinion gear53. The first guide portions54iare protrusions that are formed on both sides of the cup portion54cand extend linearly toward the cup portion54c, and guide the received lubricating oil to the cup portion54c. The communication hole54fis formed in a connecting portion between the first guide portion54iand the cup portion54c, so that the lubricating oil received by the first guide portion54iis stored in the space portion S through the communication hole54f.

As illustrated inFIG. 3, the pinion holder54includes a second guide portion54kfor receiving the lubricating oil, which is splashed by the second gear52or the pinion gear53, on a second face54jopposed to the differential gear casing61of the differential gear system6. The second guide portion54kis a protrusion that is formed above the cup portion54cand extends in an arc shape below the uppermost fixed portion54b, and guides the received lubricating oil to the cup portion54c. The communication hole54gis formed in a connecting portion between the second guide portion54kand the cup portion54c, so that the lubricating oil received by the second guide portion54kis stored in the space portion S through the communication hole54g. The communication hole54galso communicates with the first face54hof the pinion holder54.

As illustrated inFIGS. 3 and 4, among the pinion gear supporting portions54aof the pinion holder54, the pinion gear supporting portions54asupporting the two upper pinion gears53respectively include pockets54mthat store the lubricating oil at an opening end on the side of the second face54jopposed to the differential gear casing61of the differential gear system6. The pocket54mtemporality holds the lubricating oil supplied to the pinion gear supporting portion54a, thereby enabling proper lubrication of the bearing56.

By the way, some of the lubricating oil supplied from the space portion S to the bearing65flows to the outer periphery of the differential gear casing61by passing through the bearing65, and receives a centrifugal force caused by the rotation of the differential gear casing61and moves radially outward along the input plate61b. The input plate61bof the embodiment includes a third guide portion61ethat guides the lubricating oil moving radially outward along the input plate61bto the two upper pinion gear supporting portions54a. The third guide portion61eis an edge of an annular recess formed on a surface of the input plate61bopposed to the pinion gear53, and the lubricating oil moving radially outward along the input plate61bis guided to the pinion gear supporting portion54aby the edge formed in a position opposed to the pinion gear supporting portion54ain the radial direction. Some of the lubricating oil splashed by the second gear52also flows in the same manner.

[Dimensional Relation Between Second Gear and Pinion Holder]

A dimensional relation between the second gear52and the pinion holder54will be described below with reference toFIG. 7.

As illustrated inFIG. 7, an inner diameter R1of the second gear52(gear portion52a) in the meshing portion M with the pinion gear53(small diameter gear53b) is set to be equal to or smaller than an outer diameter R2of the pinion holder54(pinion gear supporting portion54a). The inner diameter R1of the second gear52(gear portion52a) is a distance from an axial center O of the pinion shaft53cto the tip (edge) of the gear, and the outer diameter R2of the pinion holder54(pinion gear supporting portion54a) is a distance from the axial center O of the pinion shaft53cto the outer peripheral surface of the pinion gear supporting portion54a. Thus, a deceleration ratio of the second gear52is set to be large at the meshing portion M. In addition, this can reduce the size in the radial direction around the pinion gear53while improving the degree of freedom in layout around the pinion holder54. For example, it is possible to make the outer diameter of the second-gear small diameter portion52dof the second gear52small while making it easy to enlarge the bearing56and secure a lubrication path around the pinion holder54.

In the connecting portion52bbetween the second gear52and the differential gear casing61, an inner diameter R3of the second gear52is set to be larger than the outer diameter R2of the pinion holder54(pinion gear supporting portion54a). The inner diameter R3of the second gear52is a distance from the axial center O of the pinion shaft53cto the tip of the spline tooth. Thus, the pinion holder54can be assembled after the second gear52is assembled, and the transmission5and the differential gear system6can be assembled according to assembling procedures to be described below.

[Assembling Procedure of Transmission and Differential Gear System]

As illustrated inFIGS. 8A to 8D, in assembling the transmission5and the differential gear system6in the manufacturing process of the power system1, after the first gear51is assembled, three pinion gears53are first assembled as illustrated inFIG. 8A. Thus, one end of the pinion shaft53cof the pinion gear53is rotatably supported by the partition wall43through the bearing55, and the large diameter gear53aof the pinion gear53meshes with the first gear51.

Next, as illustrated inFIG. 8B, the second gear52is assembled. Thus, the gear portion52aof the second gear52meshes with the small diameter gear53bof the pinion gear53.

Next, as illustrated inFIG. 8C, the pinion holder54is assembled. Since the pinion holder54has the outer diameter R2smaller than the inner diameter R3of the second gear52in the connecting portion52bbetween the second gear52and the differential gear casing61, it can be assembled even after the second gear52is assembled. Then, the pinion holder54rotatably supports the other end of the pinion shaft53cthrough the bearing56, and is fixed to the partition wall43with the bolt57.

Next, as illustrated inFIG. 8D, the differential gear system6is assembled. Thus, one end of the differential gear casing61of the differential gear system6is rotatably supported by the pinion holder54through the bearing65, and the input plate61bof the differential gear casing61is mechanically connected to the connecting portion52bof the second gear52. Reference numeral58indicates a clip (retaining ring) for restricting movement of the input plate61bin the axial direction.

As described above, according to the embodiment, since the pinion holder54has the pinion gear supporting portion54awhich is disposed on the side of the differential gear casing61of the differential gear system6to support the pinion gear53, relative to the meshing portion M between the second gear52mechanically connected to the differential gear casing61of the differential gear system6and the pinion gear53, the pinion gear53can be appropriately supported. This enables appropriate power transmission from the pinion gear53to the second gear52.

In addition, since the inner diameter R1of the second gear52in the meshing portion M with the pinion gear53is equal to or smaller than outer diameter R2of the pinion holder54, the deceleration ratio of the second gear52can be increased. Further, this can reduce the size in the radial direction around the pinion gear53while improving the degree of freedom in layout around the pinion holder54.

In the embodiment, since the pinion holder54for supporting the other end of the pinion shaft53cis fixed to the partition wall43for supporting one end of the pinion shaft53c, the partition wall43has both the support function of the pinion shaft53cand the support function of the pinion holder54, so that the components can be used in common and thus the number of components can be reduced.

In the embodiment, since the pinion gear53has the large diameter gear53aand the small diameter gear53band the small diameter gear53bmeshes with the second gear52, enlargement in dimension in the radial direction can be prevented.

In the embodiment, since the inner diameter R3of the second gear52is larger than the outer diameter R2of the pinion holder54in the connecting portion52bbetween the second gear52and the differential gear casing61, the pinion holder54can be assembled after the second gear52is assembled.

In the embodiment, the second gear52includes the second-gear large diameter portion52cconstituting the connecting portion52bwith the differential gear casing61, and the second-gear small diameter portion52dconstituting the meshing portion M with the pinion gear53, the outer diameter of the second-gear small diameter portion52dis smaller than that of the second-gear large diameter portion52c, and thus it is possible to prevent enlargement in dimension in the radial direction around the pinion gear53. Further, the deceleration ratio of the second gear52can be increased at the meshing portion M.

In the embodiment, since the pinion gear53, the second gear52, the pinion holder54, and the differential gear system6are assembled in this order after the first gear51is assembled, it is possible to improve the degree of freedom in layout around the pinion holder54while making it easy to enlarge the bearing56and secure a lubrication path.

It is noted that the invention is not limited to the above-described embodiment, but can be appropriately modified and improved, for example.

For example, the pinion holder54can select any shape as long as the inner diameter R1of the second gear52at the meshing portion M is equal to or smaller than the outer diameter R2of the pinion holder54(pinion gear supporting portion54a).

In addition, the number of pinion gears53is not limited to three, but may be one, two, or four or more.

The power system1may adopt a forced lubrication system using an oil pump instead of or together with the splashing type lubrication system.