Drive unit and electrically assisted vehicle

A drive unit includes an electric motor including an output shaft including an output gear; a pedal crank shaft extending through a housing, rotatably supported by the housing, and including a driven gear; and a transmission to transmit a torque of the output gear of the electric motor to the driven gear. The transmission includes a decelerator rotatably supported by the housing, and an idle gear rotatably supported by the housing. The decelerator increases the torque of the output shaft of the electric motor, and the increased torque is transmitted to the driven gear via the idle gear.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to Japanese Patent Application No. 2020-081834 filed on May 7, 2020. The entire contents of this application are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a drive unit attachable to a vehicle frame of an electrically assisted vehicle, and also to an electrically assisted vehicle including such a drive unit.

2. Description of the Related Art

An example of vehicle movable in accordance with power generated by a power source is an electrically assisted bicycle by which power provided by a rider pedaling the bicycle is assisted by an electric motor (see, for example, Japanese Laid-Open Patent Publication No. 2014-196080). An electrically assisted bicycle causes the electric motor to generate drive power in accordance with human power applied by the rider to pedals. Thus, the electrically assisted bicycle may alleviate the load imposed on the rider while, for example, running on a slope or running with a cargo.

The electrically assisted bicycle includes a drive unit including an electric motor and the like. Known drive units include a drive unit of a type located in a hub of a rear wheel and a drive unit of a type attached to a bottom end of a vehicle frame (in the vicinity of a bottom bracket). Recently, the latter type of drive unit is becoming mainstream.

The electrically assisted bicycle disclosed in Japanese Laid-Open Patent Publication No. 2014-196080 includes a drive unit attached to a bottom end of a vehicle frame. The drive unit includes a housing, an electric motor, a pedal crank shaft and the like. The electric motor is accommodated in the housing, and generates drive power that assists the pedal effort of the rider.

The pedal crank shaft is located to pass through the housing in a left-right direction of the vehicle. Pedals are attached to the pedal crank shaft via arms. The rotation of the pedal crank shaft is transmitted to the rear wheel via a drive sprocket, a chain, a driven sprocket and the like.

In a drive unit of an electrically assisted bicycle, many components other than a battery, namely, an electric motor, a decelerator, a pedal crank shaft, a controller and the like are accommodated in one case in a concentrated manner. Some electrically assisted bicycles adopt a mounting system called a “center mounting system”, by which such a drive unit is attached to a position corresponding to the position of a pedal crank shaft of a general bicycle.

This mounting system allows heavy mechanical and electronic parts to be located in a center bottom portion of the vehicle in a concentrated manner, and thus has an advantage of improving the weight balance of the vehicle. This mounting system allows many components to be accommodated in one case in a concentrated manner, and thus also has an advantage of allowing the drive unit to be mounted on many types of bicycle frames with a minimum change at low cost.

Meanwhile, in the case where the drive unit is located at a position in an electrically assisted bicycle corresponding to the position of the pedal crank shaft in a general bicycle, there occurs a problem that the rear center length (length between the pedal crank shaft and the axle of the rear wheel) is made longer than that of the general bicycle. In general, in the case where the rear center length is made longer, the agility in handling is decreased.

For a type of vehicle running on a non-asphalt-paved road, such as a mountain bike or the like, it is important that an appropriate minimum ground clearance (distance from the ground to the drive unit) is provided. In the case where the minimum ground clearance is short, the location where the vehicle can run is limited, which decreases the running performance.

In the case where the drive unit has a large size, the parts around the drive unit, for example, a rear suspension, a battery and the like are restricted in the positional arrangement thereof.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide drive units each having a decreased size, and electrically assisted vehicles including such drive units.

A drive unit according to a preferred embodiment of the present invention is usable in an electrically assisted vehicle. The drive unit includes an electric motor including an output shaft including an output gear; a housing accommodating a portion of, or an entirety of, the electric motor; a pedal crank shaft extending through the housing, rotatably supported by the housing, and including a driven gear; a transmission to transmit a torque of the output gear of the electric motor to the driven gear; and a resultant force output shaft rotatable coaxially with the pedal crank shaft to combine a pedal effort and an assist power of the electric motor. The transmission includes a decelerator rotatably supported by the housing in the housing, and an idle gear rotatably supported by the housing in the housing. The decelerator increases the torque of the output shaft of the electric motor, and the increased torque is transmitted to the driven gear via the idle gear.

In order to shorten the rear center length of the electrically assisted vehicle, it is conceivable to shorten the diameter of the driven gear provided on the pedal crank shaft. If a “two-axial decelerator” is located between the driven gear on the pedal crank shaft and the electric motor, it is difficult to shorten the width of the drive unit in the axial direction. The decelerator includes the rotation shaft on which two gears having different diameters from each other and including different numbers of teeth from each other are located thereon in the axial direction. The two-axial decelerator includes two such rotation shafts, and the two rotation shafts are shifted from each other in the axial direction. Therefore, the size of the two-axial decelerator in the axial direction is increased.

According to a preferred embodiment of the present invention, the idle gear is used so that the diameter of the driven gear may be decreased, and thus the rear center length of the electrically assisted vehicle may be shortened with no increase in the width (size in the axial direction) of the drive unit. The use of the idle gear may also improve the degree of freedom of positional arrangement of the electric motor, the decelerator and the driven gear, and thus may further decrease the size of the drive unit.

In a preferred embodiment of the present invention, the electric motor may be supported by the housing such that the output shaft rotates about a first central axis. The decelerator may be supported by the housing in the housing so as to be rotatable about a second central axis, and may include a first transmission gear, a second transmission gear including teeth of a smaller number than that of the first transmission gear, and a transmission shaft to transmit a rotation of the first transmission gear to the second transmission gear. The idle gear may be supported by the housing in the housing so as to be rotatable about a third central axis. The pedal crank shaft may extend through the housing along a fourth central axis, and may be supported by the housing so as to be rotatable about the fourth central axis.

The torque increased by the two transmission gears having different numbers of teeth from each other may be transmitted to the driven gear via the idle gear.

In a preferred embodiment of the present invention, the idle gear may be engaged with each of the second transmission gear of the decelerator and the driven gear.

The idle gear is engaged with each of the second transmission gear and the driven gear so that the torque may be transmitted from the decelerator to the driven gear via the single idle gear. The presence of such an idle gear may easily decrease the size of the driven gear.

In a preferred embodiment of the present invention, the first transmission gear of the decelerator may be engaged with the output gear of the electric motor.

Since the single decelerator and the single idle gear transmit the torque of the electric motor to the driven gear, the size of the drive unit may be decreased with no use of any extra gear.

In a preferred embodiment of the present invention, a distance from a plane including the first central axis and the fourth central axis to the third central axis may be longer than a distance from the plane to the second central axis, and the idle gear may have a diameter less than any of a diameter of the driven gear and a diameter of the first transmission gear.

The idle gear does not decelerate or accelerate, and thus merely needs to have such a size as to achieve power transmission between the decelerator and the driven gear. Therefore, the idle gear may have a diameter less than each of the diameter of the first transmission gear of the decelerator and the diameter of the driven gear on the pedal crank shaft. The distance from the plane including the first central axis and the fourth central axis to the third central axis is longer than the distance from the plane to the second central axis so that the idle gear having a relatively small diameter may be located outward of the first transmission gear. In other words, the first transmission gear having a relatively long diameter may be located at a more inward position in the housing, and thus may be prevented from protruding outward. The housing may have an outer contour along the idle gear having a relatively small diameter, not along the first transmission gear having a relatively long diameter. Therefore, the size of the drive unit may be decreased.

In a preferred embodiment of the present invention, a distance from the plane including the first central axis and the fourth central axis to a farthest point of the idle gear may be longer than each of a distance from the plane to a farthest point of the output gear, a distance from the plane to a farthest point of the first transmission gear, and a distance from the plane to a farthest point of the driven gear, and the idle gear may have a diameter less than any of a diameter of the driven gear and a diameter of the first transmission gear.

The idle gear does not decelerate or accelerate, and thus merely needs to have such a size as to achieve power transmission between the decelerator and the driven gear. Therefore, the idle gear may have a diameter less than each of the diameter of the first transmission gear of the decelerator and the diameter of the driven gear on the pedal crank shaft. The distance from the plane including the first central axis and the fourth central axis to the farthest point of the idle gear is longer than each of the distance from the plane to the farthest point of the output gear, the distance from the plane to the farthest point of the first transmission gear, and the distance from the plane to the farthest point of the driven gear. With this structure, the idle gear having a relatively small diameter may be located outward of the first transmission gear. In other words, the first transmission gear having a relatively long diameter may be located at a more inward position in the housing, and thus may be prevented from protruding outward. The housing may have an outer contour along the idle gear having a relatively small diameter, not along the first transmission gear having a relatively long diameter. Therefore, the size of the drive unit may be decreased.

In a preferred embodiment of the present invention, as seen in an axial direction parallel to the first central axis, a triangle having three axes among the first through fourth central axes as apexes may have the remaining one axis among the first through fourth central axes located therein.

The gear rotating about the remaining one axis may be located at a more inward position in the housing, and thus the gear may be prevented from protruding outward. With this structure, the size of the drive unit may be decreased.

In a preferred embodiment of the present invention, as seen in an axial direction parallel to the first central axis, the second central axis may be located in a triangle having the first central axis, the third central axis, and the fourth central axis as apexes.

The idle gear does not decelerate or accelerate, and thus merely needs to have such a size as to achieve power transmission between the decelerator and the driven gear. Therefore, the idle gear may have a diameter less than each of the diameter of the first transmission gear of the decelerator and the diameter of the driven gear on the pedal crank shaft. The second central axis is located in a triangle having the first central axis, the third central axis, and the fourth central axis as the apexes. With this structure, the first transmission gear having a relatively long diameter may be located at a more inward position in the housing, and thus may be prevented from protruding outward. The housing may have an outer contour along the idle gear having a relatively small diameter, not along the first transmission gear having a relatively long diameter. Therefore, the size of the drive unit may be decreased.

In a preferred embodiment of the present invention, as seen in an axial direction parallel to the first central axis, at least a portion of the driven gear may overlap the first transmission gear.

The driven gear and the first transmission gear are located such that at least a portion of the driven gear and the first transmission gear overlap each other, and consequently, the second central axis and the fourth central axis may be closer to each other. With this structure, the first central axis and the fourth central axis may be closer to each other. The distance between the first central axis and the fourth central axis may be shortened, and thus the size of the drive unit in the front-rear direction may be decreased. The size of the drive unit in the front-rear direction is decreased so that the space for the battery unit to be located in the down tube is provided.

In a vehicle in which the battery unit is located in the down tube, the battery unit is located in a space between the pedal crank shaft and the front wheel in the front-rear direction of the vehicle (located in a front center portion). An attempt to increase the capacitance of the battery results in an increase in the size of the battery unit, and the front center length needs to be longer accordingly. However, in the case where the front center length is longer, there occurs a problem that the operability of the vehicle is decreased.

According to a preferred embodiment of the present invention, the size of the drive unit in the front-rear direction is decreased so that the degree of freedom of positional arrangement of the battery unit may be increased. The battery unit having a large capacitance may easily be located without increasing the front center length.

In a preferred embodiment of the present invention, as seen in an axial direction parallel to the first central axis, at least a portion of a region where the driven gear and the idle gear are engaged with each other may overlap the first transmission gear.

The first transmission gear, the idle gear, and the driven gear are located such that at least a portion of the region where the driven gear and the idle gear are engaged with each other overlaps the first transmission gear. Consequently, the second central axis and the fourth central axis may be closer to each other. With this structure, the first central axis and the fourth central axis may be closer to each other. The distance between the first central axis and the fourth central axis may be shortened, and thus the size of the drive unit in the front-rear direction may be decreased.

In a preferred embodiment of the present invention, as seen in an axial direction parallel to the first central axis, the third central axis may overlap the first transmission gear.

The idle gear and the decelerator are located such that the third central axis and the first transmission gear overlap each other, and consequently, the second central axis and the third central axis may be closer to each other. With this structure, the first central axis and the fourth central axis may be closer to each other. The distance between the first central axis and the fourth central axis may be shortened, and thus the size of the drive unit in the front-rear direction may be decreased.

In a preferred embodiment of the present invention, the pedal crank shaft may be provided with a one-way clutch.

The pedal crank shaft is provided with the one-way clutch so that a structure is provided in which a forward rotation of the pedal crank shaft is transmitted to the drive sprocket but a reverse rotation of the pedal crank shaft is not transmitted to the drive sprocket.

An electrically assisted vehicle according to a preferred embodiment of the present invention includes the above-described drive unit.

The electrically assisted vehicle includes the drive unit in which the driven gear on the pedal crank shaft has a small diameter so that the rear center length of the electrically assisted vehicle may be shortened.

The drive unit according to a preferred embodiment of the present invention uses the idle gear so that the diameter of the driven gear may be decreased, and thus the rear center length of the electrically assisted vehicle may be shortened with no increase in the width (size in the axial direction) of the drive unit. The use of the idle gear may improve the degree of freedom of positional arrangement of the electric motor, the decelerator, and the driven gear, and thus may further decrease the size of the drive unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, drive units and electrically assisted vehicles including the drive units according to preferred embodiments of the present invention will be described with reference to the drawings. In the description of the preferred embodiments, like components will bear like reference signs, and overlapping descriptions will be omitted. In the preferred embodiments of the present invention, “front”, “rear”, “left”, “right”, “up” and “down” respectively refer to “front”, “rear”, “left”, “right”, “up” and “down” based on a state where a rider is sitting on a saddle (seat) of the electrically assisted vehicle while facing a handle. The following preferred embodiments are merely illustrative, and the present invention is not limited to the following preferred embodiments in any way.

Electrically Assisted Bicycle

With reference toFIG.1, an electrically assisted bicycle10as an example of electrically assisted vehicle according to a preferred embodiment of the present invention will be described.FIG.1is a right side view generally showing a structure of the electrically assisted vehicle10.

The electrically assisted bicycle10includes a vehicle frame12, a front wheel14F, a rear wheel14R, a handle16and a saddle18. The electrically assisted bicycle10further includes a drive unit20and a battery unit26.

The vehicle frame12includes a head tube121, a top tube122, a down tube123, a seat tube124, and a bracket125.

The head tube121is located in a front portion of the vehicle frame12, and extends in an up-down direction. A stem27is rotatably inserted into the head tube121. A handle16is secured to a top end of the stem27. A front fork28is secured to a bottom end of the stem27. The front wheel14F is rotatably attached to a bottom end of the front fork28. More specifically, the front wheel14F is supported by the vehicle frame12via the stem27and the front fork28.

The top tube122is located to the rear of the head tube121, and extends in a front-rear direction. A front end of the top tube122is connected with the head tube121. A rear end of the top tube122is connected with the seat tube124.

The down tube123is located to the rear of the head tube121, and extends in the front-rear direction. The down tube123is located below the top tube122. A front end of the down tube123is connected with the head tube121. In the example shown inFIG.1, a front portion of the down tube123is also connected with a front end portion of the top tube122. A rear end of the down tube123is connected with the bracket125.

The battery unit26is attached to the down tube123. In the example shown inFIG.1, the battery unit26is attached to the inside of the down tube123. The battery unit26supplies electric power to the drive unit20. The battery unit26includes a battery and a control circuit. The battery is a rechargeable battery rechargeable and dischargeable. The control circuit controls the charge and discharge of the battery, and also monitors the output current, the remaining battery level and the like of the battery.

The seat tube124is located to the rear of the top tube122and the down tube123, and extends in the up-down direction. A bottom end of the seat tube124is connected with the bracket125. More specifically, the seat tube124extends upward from the bracket125.

In the example shown inFIG.1, the seat tube124is bent at a middle position in the up-down direction. As a result, a bottom portion of the seat tube124extends in the up-down direction, whereas a top portion of the seat tube124extends in a direction inclined with respect to the up-down direction.

A seat post29is inserted into the seat tube124. The saddle18is attached to a top end of the seat post29.

The bracket125is located at a bottom end of the vehicle frame12. The bracket125supports the drive unit20. The drive unit20attached to the vehicle frame12generates drive power to be transmitted to a wheel (in this example, the rear wheel14R). The details of the drive unit20will be described below.

The vehicle frame12further includes a swing arm30, a pair of connection arms303and a suspension304. The swing arm30includes a pair of chainstays301and a pair of seatstays302.

The pair of chainstays301each extend in the front-rear direction. The pair of chainstays301are located side by side in the left-right direction. The rear wheel14R is located between the pair of chainstays301. The pair of chainstays301are located bilaterally symmetrically. Therefore,FIG.1shows only the right chainstay301.

A front end portion of each of the chainstays301is attached to the bracket125. More specifically, each chainstay301extends rearward from the bracket125. Each chainstay301is swingable, about an axis line extending in the left-right direction, with respect to the bracket125.

An axle141of the rear wheel14R is non-rotatably attached to a rear end portion of each chainstay301. More specifically, the rear wheel14R is supported by the pair of chainstays301so as to be rotatable about the axle141. In other words, the rear wheel14R is supported by the vehicle frame12. A multi-stage driven sprocket32is secured to the rear wheel14R.

The pair of seatstays302each extend in the front-rear direction. The pair of seatstays302are located side by side in the left-right direction. The rear wheel14R is located between the pair of seatstays302. The pair of seatstays302are located bilaterally symmetrically. Therefore,FIG.1shows only the right seatstay302.

A rear end portion of the left seatstay302is connected with the rear end portion of the left chainstay301. A rear end portion of the right seatstay302is connected with the rear end portion of the right chainstay301.

The pair of connection arms303each extend in the front-rear direction. The pair of connection arms303are located side by side in the left-right direction. The seat tube124is located between the pair of connection arms303. The pair of connection arms303are located bilaterally symmetrically. Therefore,FIG.1shows only the right connection arm303.

Each of the connection arms303is attached to the seat tube124. Each connection arm303is swingable, about an axis line extending in the left-right direction, with respect to the seat tube124.

As seen in a side view of the vehicle, a front end of each connection arm303is located to the front of the seat tube124. As seen in a side view of the vehicle, a rear end of each connection arm303is located to the rear of the seat tube124.

A rear end portion of the right connection arm303is attached to a front end portion of the right seatstay302. The right connection arm303is swingable, about an axis line extending in the left-right direction, with respect to the right seatstay302.

A rear end portion of the left connection arm303is attached to a front end portion of the left seatstay302. The left connection arm303is swingable, about an axis line extending in the left-right direction, with respect to the left seatstay302.

The suspension304is located to the front of the seat tube124and to the rear of the down tube123. A top end portion of the suspension304is attached to the pair of connection arms303. The suspension304is swingable, about an axis line extending in the left-right direction, with respect to the pair of connection arms303. A bottom end portion of the suspension304is attached to the bracket125. The suspension304is swingable, about an axis line extending in the left-right direction, with respect to the bracket125. The position at which the suspension304is attached to the bracket125is to the front of the position at which the seat tube124is attached to the bracket125.

A drive sprocket34is attached to the drive unit20via a support member33. A chain is wound along the drive sprocket34and the driven sprocket32.

Drive Unit

With reference toFIG.2, an example of the structure of the drive unit20will be described.FIG.2is a cross-sectional view showing an example of the internal structure of the drive unit20.

As shown inFIG.2, the drive unit20includes a housing21, a pedal crank shaft22, a rotation shaft23, a transmission mechanism40and an electric motor25.

First, a structure of the housing21according to this preferred embodiment will be described.

The housing21is secured to the bracket125(FIG.1) via a plurality of tightening tools. The housing21includes a first case211, a second case212, and a cover213. The first case211, the second case212and the cover213are each made of a metal material (e.g., an aluminum alloy).

The first case211fits to the second case212from the left in the left-right direction. The first case211and the second case212are secured to each other via a plurality of tightening tools. As a result, a space214is formed between the first case211and the second case212.

The cover213fits to the first case211from the left in the left-right direction. The cover213and the first case211are secured to each other via a plurality of tightening tools. As a result, a space215enclosed by the cover213is formed to the left of the first case211. The motor25is accommodated in the space215.

Now, a structure of the pedal crank shaft22according to this preferred embodiment will be described.

The pedal crank shaft22is located to extend through the housing21in the left-right direction of the vehicle, and is rotatably supported by the housing21. A central axis line CL4of the pedal crank shaft22extends in the left-right direction. As seen in an axial direction of the pedal crank shaft22(in a thrust direction), the central axis line CL4is a rotation center axis RC4(fourth central axis) of the pedal crank shaft22. The pedal crank shaft22is rotatable, about the central axis line CL4, with respect to the housing21.

The pedal crank shaft22extends through the housing21along the fourth central axis RC4, and is supported by the housing21so as to be rotatable about the fourth central axis RC4. In the housing21, the pedal crank shaft22is rotatably supported by a pair of bearings38L and38R. The bearing38L is located on the left side in the axial direction, and is secured to the first case211. The bearing38R is located on the right side in the axial direction, and is secured to the second case212.

The pedal crank shaft22extends through the rotation shaft23. The rotation shaft23is accommodated in the housing21. The details of the rotation shaft23will be described below. A pair of, namely, left and right, crank arms35(seeFIG.1) are attached to the pedal crank shaft22. A pedal37(seeFIG.1) is attached to each of the crank arms35.

Now, a structure of the electric motor25and the transmission mechanism40according to this preferred embodiment will be described.

The electric motor25is accommodated in the housing21, and is secured to the housing21. The electric motor25generates drive power that assists the running of the electrically assisted bicycle10. The electric motor25includes a stator251and a rotor252.

The stator251includes a plurality of bobbins2512, around each of which a coil2511is wound. An iron core2513is inserted into each of the bobbins2512. The stator251is located in the space215. In this state, the stator251is secured to the first case211.

A support member253is attached to the stator251. The support member253is made of a resin material. A plurality of busbars (not shown) are embedded in the support member253. The busbars are each connected with the coil2511corresponding thereto. Transmission of electric power to the busbars is controlled so that a magnetic force is generated in the stator251.

The rotor252is located inward of the stator251. A central axis line CL1of the rotor252is parallel or substantially parallel to the central axis line CL4of the pedal crank shaft22. More specifically, the rotor252is parallel or substantially parallel to the central axis line CL4of the pedal crank shaft22. As seen in the axial direction of the pedal crank shaft22, the central axis line CL1is a rotation center axis RC1(first central axis) of the rotor252.

The rotor252includes a rotor main body2521and an output shaft2522. An outer circumferential surface of the rotor main body2521is magnetized with N poles and S poles alternately in a circumferential direction.

The output shaft2522extends through the rotor main body2521. The output shaft2522is secured to the rotor main body2521. More specifically, the output shaft2522is rotatable together with the rotor main body2521.

In the housing21, the output shaft2522is supported by the housing21so as to be rotatable about the first central axis RC1. The output shaft2522is supported by two bearings42L and42R so as to be rotatable, about the central axis line CL1, with respect to the housing21. The bearing42L is secured to the cover213. The bearing42R is located to the right of the rotor main body2521, and is secured to the first case211. The output shaft2522extends through the first case211. A portion of the output shaft2522that is located in the space214includes an output gear252A thereon. The output gear252A is, for example, a helical gear.

The transmission mechanism40is accommodated in the housing21. Specifically, the transmission mechanism40is located in the space214. The transmission mechanism40includes a decelerator24, an idle gear41, and a rotation shaft43. The transmission mechanism40transmits a torque of the output gear252A of the electric motor25to a driven gear233described below.

The decelerator24is rotatably supported by the housing21, and increases the torque of the output gear252A of the electric motor25. The decelerator24includes a first transmission gear241, a second transmission gear242, and a transmission shaft243. A central axis line CL2of the transmission shaft243is parallel or substantially parallel to the central axis line CL4of the pedal crank shaft22. More specifically, the transmission shaft243extends parallel or substantially parallel to the central axis line CL4of the pedal crank shaft22. As seen in an axial direction of the transmission shaft243, namely, in the axial direction of the pedal crank shaft22, the central axis line CL2is a rotation center axis RC2(second central axis) of the transmission shaft243. In the housing21, the decelerator24is supported by the housing21so as to be rotatable about the second central axis RC2.

The first transmission gear241is located on a right portion of the transmission shaft243in the axial direction. A left portion of the transmission shaft243is rotatably supported by a bearing44L. The first transmission gear241located on the right portion of the transmission shaft243is rotatably supported by a bearing44R. The transmission shaft243and the first transmission gear241are supported by the two bearings44L and44R so as to be rotatable about the central axis line CL2. The bearing44L is secured to the first case211. The bearing44R is secured to the second case212.

The first transmission gear241is meshed with the output gear252A of the electric motor25. With this structure, the drive power generated by the electric motor25is transmitted to the first transmission gear241from the output gear252A.

A one-way clutch244is located between the first transmission gear241and the transmission shaft243. The one-way clutch244couples the transmission shaft243and the first transmission gear241to each other. The one-way clutch244regulates the rotation of the first transmission gear241with respect to the transmission shaft243to one direction. A rotation force of the output gear252A acting in such a direction as to rotate the rear wheel14R (FIG.1) of the electrically assisted bicycle10forward is transmitted to the transmission shaft243via the first transmission gear241, whereas a rotation force of the output gear252A acting in such a direction as to rotate the rear wheel14R rearward is not transmitted to the transmission shaft243. The one-way clutch244also prevents a forward rotation force of the pedal crank shaft22generated by human power of the rider from being transmitted to the electric motor25.

The first transmission gear241has a diameter longer than that of the output gear252A of the electric motor25, and includes teeth of a larger number than that of the output gear252A. More specifically, the first transmission gear241is decelerated more than the output gear252A.

The second transmission gear242is made of a metal material (e.g., iron). The second transmission gear242is located on the transmission shaft243. The second transmission gear242is located at a position different from that of the first transmission gear241in the axial direction of the transmission shaft243. The second transmission gear242has a diameter less than that of the first transmission gear241, and includes teeth of a smaller number than that of the first transmission gear241. The transmission shaft243and the second transmission gear242are integral in this preferred embodiment, but are not limited to this. The second transmission gear242may be secured to the transmission shaft243by serration coupling (or by press-fit). The second transmission gear242is rotatable together with the transmission shaft243. The transmission shaft243transmits the rotation of the first transmission gear241to the second transmission gear242.

The idle gear41is made of a metal material (e.g., iron). The idle gear41is located on the rotation shaft43. The idle gear41is secured to the rotation shaft43by, for example, a tightening tool, but is not limited to this. The idle gear41may be secured to the rotation shaft43by serration coupling (or by press-fit). The idle gear41and the rotation shaft43may be integral. The idle gear41is rotatable together with the rotation shaft43.

A central axis line CL3of the rotation shaft43is parallel or substantially parallel to the central axis line CL4of the pedal crank shaft22. More specifically, the rotation shaft43extends parallel or substantially parallel to the central axis line CL4of the pedal crank shaft22. As seen in an axial direction of the rotation shaft43, namely, in the axial direction of the pedal crank shaft22, the central axis line CL3is a rotation center axis RC3(third central axis) of the rotation shaft43. In the housing21, the idle gear41secured by the rotation shaft43is supported by the housing21so as to be rotatable about the third central axis RC3.

The rotation shaft43is supported by two bearings46L and46R so as to be rotatable about the central axis line CL3. The bearings46L and46R are secured to the first case211. The idle gear41is located closer to the bearing46R than to the bearing46L in the axial direction of the rotation shaft43. The idle gear41is meshed with the second transmission gear242of the decelerator24. With this structure, the output torque of the electric motor25increased by the decelerator24is transmitted to the idle gear41.

Now, a structure of the vicinity of the pedal crank shaft22will be described.

The rotation shaft23is coaxial with the pedal crank shaft22, and is rotatable together with the pedal crank shaft22. The rotation shaft23includes a coupling shaft231and a one-way clutch50.

The coupling shaft231has a cylindrical shape. The pedal crank shaft22is inserted into the coupling shaft231. The coupling shaft231is coaxial with the pedal crank shaft22.

A left end portion of the coupling shaft231is coupled with the pedal crank shaft22by serration coupling or the like. As a result, regardless of whether the pedal crank shaft22is rotated forward or rearward, the coupling shaft231is rotated together with the pedal crank shaft22.

A torque detection device232is located around the coupling shaft231. The torque detection device232is supported by the coupling shaft231, and is not rotatable with respect to the first case211. The torque detection device232detects a torque generated in the coupling shaft231when the driver steps on the pedals. The torque detection device232is, for example, a magnetostrictive torque sensor. The torque detection device232outputs a signal in accordance with the detected torque to a controller mounted on a substrate48. The controller refers to the torque detected by the torque detection device232to learn the state of the pedaling performed by the driver and control the electric motor25.

The one-way clutch50is located to the right of the torque detection device232in the axial direction of the pedal crank shaft22. The one-way clutch50is located on the pedal crank shaft22via the coupling shaft231. The one-way clutch50is coaxial with the pedal crank shaft22. The one-way clutch50includes an inner member51and an outer member52.

The inner member51of the one-way clutch50has a cylindrical shape. A right portion of the coupling shaft231is inserted into the inner member51. The inner member51is coaxial with the coupling shaft231. In this state, the right portion of the coupling shaft231is coupled with the inner member51by serration coupling or the like. As a result, regardless of whether the coupling shaft231is rotated forward or rearward, the inner member51is rotated together with the coupling shaft231. More specifically, regardless of whether the pedal crank shaft22is rotated forward or rearward, the inner member51is rotated together with the pedal crank shaft22. The coupling shaft231and the inner member51act as a crank rotation input shaft that is rotatable integrally with the pedal crank shaft22.

The outer member52of the one-way clutch50has a cylindrical shape. The pedal crank shaft22is inserted into the outer member52. A slide bearing49is located between the outer member52and the pedal crank shaft22. With this structure, the outer member52is rotatable coaxially with the pedal crank shaft22.

A latchet mechanism as a one-way clutch mechanism is located between the outer member52and the inner member51. With this structure, a forward rotation force of the inner member51is transmitted to the outer member52, whereas a rearward rotation force of the inner member51is not transmitted to the outer member52. A forward rotation force of the outer member52generated by the rotation of the electric motor25is not transmitted to the inner member51.

The outer member52is supported by the bearing38R so as to be rotatable, about the central axis line CL4of the pedal crank shaft22, with respect to the housing21. The outer member52extends through the second case212. The drive sprocket34is attached to a portion of the outer member25that is outward of (to the right of) the housing21.

The outer member52includes the driven gear233. The driven gear233is located on the pedal crank shaft22via the one-way clutch50and the coupling shaft231. The driven gear233is meshed with the idle gear41. The driven gear233has a diameter longer than that of each of the second transmission gear242and the idle gear41, and includes teeth of a larger number than that of each of the second transmission gear242and the idle gear41. More specifically, the driven gear233is rotated at a rotation rate lower than the rotation rate of each of the second transmission gear242and the idle gear41. The idle gear41is meshed with each of the second transmission gear242and the driven gear233so that the output torque of the electric motor25increased by the decelerator24may be transmitted to the driven gear233via the single idle gear41.

The outer member52transmits resultant force of the human power (pedal effort) transmitted to the coupling shaft231and assist drive power of the electric motor25to the drive sprocket34. The outer member52defines a resultant force output shaft235, which combines the human power that is input via the one-way clutch50and the assist drive power that is input via the driven gear233and outputs the resultant force. The resultant force output shaft235rotates coaxially with the pedal crank shaft22. The resultant force output shaft235is included in the rotation shaft23.

Idle Gear

Now, the drive unit20including the idle gear41according to this preferred embodiment will be described in more detail.

As described above, in the case where the drive unit is located at a position in the electrically assisted bicycle corresponding to the position of the pedal crank shaft of a general bicycle, there occurs a problem that the rear center length is longer than in the general bicycle.

In order to shorten the rear center length of the electrically assisted vehicle, it is conceivable to shorten the diameter of the driven gear provided on the pedal crank shaft. If a “two-axial decelerator” is located between the driven gear and the electric motor, it is difficult to shorten the width of the drive unit in the axial direction. Two gears having different diameters from each other and including different numbers of teeth from each other are located in the axial direction on the rotation shaft of the decelerator. Therefore, the size of the rotation shaft in the axial direction tends to be increased. The two-axial decelerator includes two such rotation shafts, and the two rotation shafts are shifted from each other in the axial direction. Therefore, the size of the two-axial decelerator in the axial direction is increased.

In this preferred embodiment, the idle gear41is provided on a power transmission path between the decelerator24and the driven gear233so that the diameter of the driven gear233may be decreased. Even in the case where the diameter of the driven gear233is decreased, the output torque of the electric motor25may be transmitted to the driven gear233via the idle gear41. In this preferred embodiment, such use of the idle gear41allows the diameter of the driven gear233to be decreased, thus to shorten the rear center length of the electrically assisted bicycle10, with no increase in the width (size in the axial direction) of the drive unit20. The use of the idle gear41may also improve the degree of freedom of the positional arrangement of the electric motor25, the decelerator24and the driven gear233, and thus may further decrease the size of the drive unit20.

Hereinafter, the positional arrangement of the components of the drive unit20that makes it possible to decrease the size of the drive unit20will be described.

FIG.3throughFIG.6each show a positional arrangement of the components of the drive unit20as seen in the axial direction. The axial direction is parallel or substantially parallel to the first through fourth central axes RC1through RC4.

As described above, the output shaft2522and the output gear252A of the electric motor25rotate about the first central axis RC1. The first transmission gear241, the second transmission gear242and the transmission shaft243of the decelerator24rotate about the second central axis RC2. The idle gear41and the rotation shaft43rotate about the third central axis RC3. The pedal crank shaft22and the driven gear233rotate about the fourth central axis RC4. The diameter of the idle gear41is less than any of the diameter of the driven gear233and the diameter of the first transmission gear241.

With reference toFIG.3, a plane including the first central axis RC1and the fourth central axis RC4is labeled as a plane61. InFIG.3, a line (dashed line) along which the plane61and the sheet of the figure cross each other represents the position of the plane61. This dashed line is a perpendicular line crossing each of the first central axis RC1and the fourth central axis RC4at the right angle, and corresponds to a perpendicular line connecting the first central axis RC1and the fourth central axis RC4to each other. This is also applicable toFIG.4.

The distance from the plane61to the third central axis RC3is labeled as d3. The distance from the plane61to the second central axis RC2is labeled as d2. Distance d3is longer than distance d2. More specifically, the idle gear41is located outward of the decelerator24in the housing21.

The idle gear41does not decelerate or accelerate, and thus merely needs to have such a size as to achieve power transmission between the decelerator24and the driven gear233. Therefore, the idle gear41may have a diameter less than each of the diameter of the first transmission gear241of the decelerator24and the diameter of the driven gear233. The idle gear41and the decelerator24are located such that distance d3is longer than distance d2, and consequently, the idle gear41having a relatively short diameter may be located outward of the first transmission gear241. In other words, the first transmission gear241having a relatively long diameter may be located at a more inward position in the housing21, and thus may be prevented from protruding outward. The housing21may have an outer contour along the idle gear41having a relatively short diameter, and not along the first transmission gear241having a relatively long diameter. Therefore, the size of the drive unit20may be decreased. The size of the drive unit20in the up-down direction may be decreased, and thus the minimum ground clearance of the electrically assisted bicycle10may be longer.

As shown inFIG.3, the idle gear41is located below the drive unit20so that the degree of freedom of positional arrangement of the suspension304(FIG.1) may be increased. Substantially the same effect is provided even in the case where the idle gear41is located above the drive unit20. However, in the vehicle shown inFIG.1in which the suspension304is located above the drive unit20, if the idle gear41is located above the drive unit20, the position of the suspension304is influenced. The idle gear41is located below the drive unit20so that the degree of freedom of positional arrangement of the suspension304may be increased.

With reference toFIG.4, the positional relationship of the components of the drive unit20will be described further.

The distance from the plane61to a farthest point of the idle gear41is labeled as L3. The farthest point of the idle gear41is the position, on a tip circle connecting tips of the teeth of the idle gear41, that is farthest from the plane61. The distance from the plane61to a farthest point of the output gear252A is labeled as L1. The distance from the plane61to a farthest point of the first transmission gear241is labeled as L2. The distance from the plane61to a farthest point of the driven gear233is labeled as L4. The farthest point of the output gear252A is the position, on a tip circle connecting tips of the teeth of the output gear252A, that is farthest from the plane61. The farthest point of the first transmission gear241is the position, on a tip circle connecting tips of the teeth of the first transmission gear241, that is farthest from the plane61. The farthest point of the driven gear233is the position, on a tip circle connecting tips of the teeth of the driven gear233, that is farthest from the plane61.

Distance L3is longer than any of distances L1, L2and L4. Especially, distance L3is longer than distance L2so that the idle gear41having a relatively short diameter may be located outward of the first transmission gear241. More specifically, the first transmission gear241having a relatively long diameter may be located at a more inward position in the housing21, and thus may be prevented from protruding outward. With this structure, the size of the drive unit20may be decreased.

With reference toFIG.5, the positional relationship of the components of the drive unit20will be described still further.

In the drive unit20as seen in the axial direction, a triangle having three axes, among the first through fourth central axes RC1through RC4, as apexes is set. The remaining one axis among the first through fourth central axes RC1through RC4is located inside the triangle.FIG.5shows, as an example, a triangle63having the central axes RC1, RC3and RC4as the apexes as seen in the axial direction. In the example shown inFIG.5, the second central axis RC2of the first transmission gear241is located inside the triangle63having the central axes RC1, RC3and RC4as the apexes. The first transmission gear241having a relatively long diameter is located at a more inward position in the housing21so that the first transmission gear241may be prevented from protruding outward. With this structure, the size of the drive unit20may be decreased.

Now, the positional relationship between the first transmission gear241of the decelerator24and the driven gear233will be described.

With reference toFIG.3, in this preferred embodiment, in the drive unit20as seen in the axial direction, the first transmission gear241and the driven gear233are located such that at least a portion of the driven gear233overlaps the first transmission gear241.

The driven gear233and the first transmission gear241are located such that at least a portion of the driven gear233and the first transmission gear241overlap each other, and consequently, the second central axis RC2and the fourth central axis RC4may be closer to each other. With this structure, the first central axis RC1and the fourth central axis RC4may be closer to each other. The distance between the first central axis RC1and the fourth central axis RC4may be shortened, and thus the size of the drive unit20in the front-rear direction may be decreased.

The size of the drive unit20in the front-rear direction is decreased so that a space for the battery unit26(FIG.1) to be located in the down tube123(FIG.1) is provided. In a vehicle in which the battery unit is located in the down tube, the battery unit is located in a space between the pedal crank shaft and the front wheel in the front-rear direction of the vehicle (located in a front center portion). An attempt to increase the capacitance of the battery results in an increase in the size of the battery unit, and the front center length needs to be longer accordingly. However, in the case where the front center length is longer, there occurs a problem that the operability of the vehicle is decreased.

According to this preferred embodiment, the size of the drive unit20in the front-rear direction is decreased so that the degree of freedom of positional arrangement of the battery unit26may be increased. The battery unit26having a large capacitance may easily be located without increasing the front center length.

Now, with reference toFIG.6, the positional relationship among the first transmission gear241of the decelerator24, the idle gear41and the driven gear233will be described.

In the drive unit20as seen in the axial direction, the driven gear233and the idle gear41are engaged with each other in a region65(black portion inFIG.6). In this preferred embodiment, the first transmission gear241, the idle gear41and the driven gear233are located such that at least a portion of the region65, where the driven gear233and the idle gear41are engaged with each other, overlaps the first transmission gear241.

The first transmission gear241, the idle gear41and the driven gear233are located such that at least a portion of the region65, where the driven gear233and the idle gear41are engaged with each other, overlaps the first transmission gear241. Consequently, the second central axis RC2and the fourth central axis RC4may be closer to each other. With this structure, the first central axis RC1and the fourth central axis RC4may be closer to each other. The distance between the first central axis RC1and the fourth central axis RC4may be shortened, and thus the size of the drive unit20in the front-rear direction may be decreased.

Now, with respect toFIG.6, the positional relationship between the third central axis RC3and the first transmission gear241of the decelerator24will be described. The idle gear41rotates about the third central axis RC3.

In the drive unit20as seen in the axial direction, the third central axis RC3overlaps the first transmission gear241. The idle gear41and the decelerator24are located such that the third central axis RC3and the first transmission gear241overlap each other, and consequently, the second central axis RC2and the third central axis RC3may be closer to each other. With this structure, the first central axis RC1and the fourth central axis RC4may be closer to each other. The distance between the first central axis RC1and the fourth central axis RC4may be shortened, and thus the size of the drive unit20in the front-rear direction may be decreased.

Support Structure of the Driven Gear

Now, a structure that supports the driven gear233will be described.FIG.7is a cross-sectional view showing an example of structure of the driven gear233and the vicinity thereof in the drive unit20.

As described above, the driven gear233is provided in the outer member52of the one-way clutch50. The drive sprocket34is attached to a portion of the outer member52that is outward of (to the right of) the housing21.

While the electrically assisted bicycle10is running with both of the outer member52and the drive sprocket34rotating, a load is applied from the drive sprocket34to the outer member52. Because of the load from the drive sprocket34, precession is caused in the outer member52including the driven gear233. In the case where the precession is large, there occurs a problem that the meshing precision of the driven gear233and the idle gear41is decreased.

In this preferred embodiment, two bearings381R and382R are provided as the bearing38R supporting the outer member52. The bearings381R and382R are each a rolling-element bearing including an inner race, an outer race and a rolling element. The outer race of each of the bearings381R and382R is press-fit into the second case212of the housing21, and the outer member52is clearance-fit into the inner race thereof.

In the left-right direction of the drive unit20, the bearing382R is located at an outer position whereas the bearing381R is located at an inner position. The bearings381R and382R may be in contact with each other, or may be located separately from each other.

The outer member52is supported by the two bearings381R and382R so that the rotation shake of the outer member52may be decreased, and thus the precession of the outer member52may be decreased. The decrease in the precession may improve the meshing precision of the driven gear233and the idle gear41. With this structure, the noise generated when the driven gear233and the idle gear41are meshed with each other may be decreased, and abrasion of the rotation shaft may be decreased. In addition, the sealability of the bearings381R and382R supporting the outer member52may be improved.

Now, the positional relationship among the drive sprocket34, the driven gear233and the bearings381R and382R will be described.

In the axial direction of the pedal crank shaft22, the distance to the most inward position of the bearing381R (left end portion of the bearing381R) from a position341, in the outer member52, to which the drive sprocket34is attached is labeled as a. In the axial direction of the pedal crank shaft22, the distance from the attachment position341to the center of a portion where the driven gear233and the idle gear41are meshed with each other is labeled as b. The relationship between distance a and distance b may be represented by expression 1 below.
a/b≥0.5  expression 1

The bearing381R is located at an inner position in the left-right direction of the drive unit20. As the position of the bearing381R is farther from the position341at which the drive sprocket34is attached, the precession of the outer member52may be decreased more. The bearing381R is located such that the left end portion thereof is located inward of the middle position of distance b in the left-right direction of the drive unit20, and consequently, the precession of the outer member52may be decreased more.

The bearing381R is located far from the position341at which the drive sprocket34is attached, whereas the bearing382R is located as close as possible to the attachment position341, so that the precession of the outer member52may be decreased more. For example, the bearing382R is located such that a left end portion thereof is located outward of the middle position of distance b in the left-right direction of the drive unit20. Consequently, the precession of the outer member52may be decreased more.

The bearings381R and382R may have the same size as, or different sizes from, each other.

A multi-row bearing may be used instead of the bearings381R and382R. Examples of the usable multi-row bearing include a multi-row angular bearing, a multi-row deep groove bearing and the like, but are not limited any of these. A needle bearing may be used instead of the bearings381R and382R.

Heat Dissipation Structure

Now, a structure that dissipates heat generated in the coils2511of the electric motor25to the outside will be described.FIG.8is a cross-sectional view showing a structure of the electric motor25and the vicinity thereof in the drive unit20. In the example shown inFIG.8, a heat dissipation agent2515(hatched portion inFIG.8) is located between the stator251of the electric motor25and the cover213of the housing21. The heat dissipation agent2515is provided between the coils2511wound around the bobbins2512of the stator251and the cover213so that the heat generated in the coils2511may be dissipated outside efficiently.

The heat dissipation agent2515may be made of any material having a high heat conductivity. Examples of the material usable for the heat dissipation agent2515include highly heat-conductive epoxy and unsaturated polyester resins each having an alumina filler incorporated thereto, but are not limited to any of these.

It is possible to dissipate the heat even in the case where the entire circumference of each of the coils2511wound around the bobbins2512is covered with the heat dissipation agent2515. However, in the case where the heat dissipation agent2515is provided to fill the space so as to cover the entire circumference of each of the coils2511, there occurs a problem that the weight of the drive unit20is increased. In the case where the entire circumference of each of the coils2511is covered with the heat dissipation agent2515, a mold to be filled with the heat dissipation agent2515is needed, and thus there occurs a problem that the production cost and the number of production steps are increased.

In this preferred embodiment, the heat dissipation agent251is provided only between the stator251and the cover213so that the heat generated in the coils2511may be dissipated to the outside efficiently while reducing or minimizing the increase in the weight.

With reference toFIG.8, the cover213in this preferred embodiment includes an inner wall2131. The cover213has a shape with which a left portion of each of the bobbins2512, around each of which the coil2511is wound, is covered with the inner wall2131and an outer circumferential wall2132of the cover213. During the production of the drive unit20, the heat dissipation agent2515is applied between the inner wall2131and the wall2132of the cover213. The cover213having the heat dissipation agent2515applied thereto is attached to the first case211of the housing21, and thus the structure including the heat dissipation agent2515between the coils2511and the cover213may easily be realized. A mold to be filled with the heat dissipation agent2515is not needed, and thus the production cost and the number of production steps may be decreased.

The cover213includes the inner wall2131, so that while the heat dissipation agent2515is applied to the cover213, the heat dissipation agent2515may be prevented from flowing to an area where the heat dissipation agent2515is not necessary. The left portion of each of the bobbins2512, around each of which the coil2511is wound, is covered with the inner wall2131and the wall2132of the cover213, so that the heat dissipation agent2515may be prevented from flowing toward the rotor252.

Preferred embodiments of the present invention have been described above. The present invention is not limited to the above-described preferred embodiments. For example, in the above-described preferred embodiments, the electrically assisted bicycle including the suspension is described as an example. Preferred embodiments of the present invention are preferably applicable also to an electrically assisted bicycle with no suspension.

In the above-described preferred embodiments, the drive unit20(FIG.2) includes four shafts, namely, the output shaft2522, the transmission shaft243, the rotation shaft43and the pedal crank shaft22. The number of the shafts is not limited to four. Preferred embodiments of the present invention are applicable to a drive unit including five or more shafts. For example, preferred embodiments of the present invention are applicable to a drive unit in which a gear is provided between the output gear252A of the electric motor25and the first transmission gear241of the decelerator24and a torque is transmitted from the output gear252A to the first transmission gear241via the above-mentioned gear.

In the above-described preferred embodiments, the entirety of the electric motor25is accommodated in the housing21. The structure of the housing21is not limited to this. Only a portion of the electric motor25may be accommodated in the housing21. For example, a left portion of the first case211may have an opening through which the electric motor25may extend, and the electric motor25may be attached such that a portion thereof is located in the housing21through the opening. In this case, the opening may be provided with a dust-proof and waterproof cover.

The cover213(FIG.2) may be a portion of the housing21, and may be included in the housing21. The cover213may have such a shape as to cover a side surface of the electric motor25, and the electric motor25may be supported by the cover213. A form in which the electric motor25is supported by the cover213is encompassed in the form in which the electric motor25is supported by the housing21.

In the above-described preferred embodiments, the electrically assisted bicycle with two wheels is described as an example of the electrically assisted vehicle10. The present invention is not limited to this. For example, the electrically assisted vehicle10may be an electrically assisted vehicle with three or more wheels.

In the above-described preferred embodiments, the drive wheel to which the human power generated by the rider stepping on the pedals and the assist power generated by the motor are transmitted is the rear wheel. The present invention is not limited to this. The human power and the assist power may be transmitted to the front wheel, or both of the front wheel and the rear wheel, in accordance with the form of the electrically assisted bicycle.

In the above-described preferred embodiments, the vehicle is the electrically assisted bicycle, but alternatively, may be a vehicle other than the electrically assisted bicycle. Preferred embodiments of the present invention are preferably applicable to any vehicle in which the drive unit is required to have a decreased size.

Illustrative preferred embodiments of the present invention have been described above.

A drive unit20according to a preferred embodiment of the present invention is usable in an electrically assisted vehicle10. The drive unit includes an electric motor25including an output shaft2522including an output gear252A; a housing21accommodating a portion of, or the entirety of, the electric motor25; a pedal crank shaft22extending through the housing21, rotatably supported by the housing21, and provided with a driven gear233; a transmission mechanism40to transmit a torque of the output gear252A of the electric motor25to the driven gear233; and a resultant force output shaft235rotatable coaxially with the pedal crank shaft22, the resultant force output shaft235combining a pedal effort and an assist power of the electric motor25. The transmission mechanism40includes a decelerator24rotatably supported by the housing21in the housing21, and an idle gear41rotatably supported by the housing21in the housing21. The decelerator24increases the torque of the output shaft252A of the electric motor25, and the increased torque is transmitted to the driven gear233via the idle gear41.

In order to shorten the rear center length of the electrically assisted vehicle10, it is conceivable to shorten the diameter of the driven gear233provided on the pedal crank shaft22. If a “two-axial decelerator” is located between the driven gear233on the pedal crank shaft22and the electric motor25, it is difficult to shorten the width of the drive unit20in the axial direction. Two gears having different diameters from each other and including different numbers of teeth from each other are located, in the axial direction, on the rotation shaft of the decelerator. Therefore, the size of the rotation shaft in the axial direction tends to be increased. The two-axial decelerator includes two such rotation shafts, and the two rotation shafts are located as shifted from each other in the axial direction. Therefore, the size of the two-axial decelerator in the axial direction is increased.

According to a preferred embodiment of the present invention, the idle gear41is used so that the diameter of the driven gear233may be decreased, and thus the rear center length of the electrically assisted bicycle10may be shortened, with no increase in the width (size in the axial direction) of the drive unit20. The use of the idle gear41may also improve the degree of freedom of positional arrangement of the electric motor25, the decelerator24and the driven gear233, and thus may further decrease the size of the drive unit20.

In a preferred embodiment of the present invention, the electric motor25may be supported by the housing21such that the output shaft2522rotates about a first central axis RC1. The decelerator24may be supported by the housing21in the housing21so as to be rotatable about a second central axis RC2, and may include a first transmission gear241, a second transmission gear242including teeth of a smaller number than that of the first transmission gear241, and a transmission shaft243to transmit a rotation of the first transmission gear241to the second transmission gear242. The idle gear41may be supported by the housing21in the housing21so as to be rotatable about a third central axis RC3. The pedal crank shaft22may extend through the housing21along a fourth central axis RC4, and may be supported by the housing21so as to be rotatable about the fourth central axis RC4.

The torque increased by the two transmission gears having different numbers of teeth from each other may be transmitted to the driven gear233via the idle gear41.

In a preferred embodiment of the present invention, the idle gear41may be engaged with each of the second transmission gear242of the decelerator24and the driven gear233.

The idle gear41is engaged with each of the second transmission gear242and the driven gear233so that the torque may be transmitted from the decelerator24to the driven gear233via the single idle gear41. The presence of such an idle gear41may easily decrease the size of the driven gear233.

In a preferred embodiment of the present invention, the first transmission gear241of the decelerator24may be engaged with the output gear252A of the electric motor25.

Since the single decelerator24and the single idle gear41transmit the torque of the electric motor25to the driven gear233, the size of the drive unit20may be decreased with no use of any extra gear.

In a preferred embodiment of the present invention, distance d3from a plane61including the first central axis RC1and the fourth central axis RC4to the third central axis RC3may be longer than distance d2from the plane61to the second central axis RC2, and the idle gear41may have a diameter less than any of a diameter of the driven gear233and a diameter of the first transmission gear241.

The idle gear41does not decelerate or accelerate, and thus merely needs to have such a size as to achieve power transmission between the decelerator24and the driven gear233. Therefore, the idle gear41may have a diameter less than each of the diameter of the first transmission gear241of the decelerator24and the diameter of the driven gear233on the pedal crank shaft22. Distance d3from the plane61including the first central axis RC1and the fourth central axis RC4to the third central axis RC3is longer than distance d2from the plane61to the second central axis RC2so that the idle gear41having a relatively short diameter may be located outward of the first transmission gear241. In other words, the first transmission gear241having a relatively long diameter may be located at a more inward position in the housing21, and thus may be prevented from protruding outward. The housing21may have an outer contour along the idle gear41having a relatively short diameter, not along the first transmission gear241having a relatively long diameter. Therefore, the size of the drive unit20may be decreased.

In a preferred embodiment of the present invention, distance L3from the plane61including the first central axis RC1and the fourth central axis RC4to a farthest point of the idle gear41may be longer than each of distance L1from the plane61to a farthest point of the output gear252A, distance L2from the plane61to a farthest point of the first transmission gear241, and distance L4from the plane61to a farthest point of the driven gear233, and the idle gear41may have a diameter less than any of a diameter of the driven gear233and a diameter of the first transmission gear241.

The idle gear41does not decelerate or accelerate, and thus merely needs to have such a size as to achieve power transmission between the decelerator24and the driven gear233. Therefore, the idle gear41may have a diameter less than each of the diameter of the first transmission gear241of the decelerator24and the diameter of the driven gear233on the pedal crank shaft22. Distance L3from the plane61including the first central axis RC1and the fourth central axis RC4to the farthest point of the idle gear41is longer than each of distance L1from the plane61to the farthest point of the output gear252A, distance L2from the plane61to the farthest point of the first transmission gear241, and distance L4from the plane61to the farthest point of the driven gear233. With this structure, the idle gear41having a relatively short diameter may be located outward of the first transmission gear241. In other words, the first transmission gear241having a relatively long diameter may be located at a more inward position in the housing21, and thus may be prevented from protruding outward. The housing21may have an outer contour along the idle gear41having a relatively short diameter, and not along the first transmission gear241having a relatively long diameter. Therefore, the size of the drive unit20may be decreased.

In a preferred embodiment of the present invention, as seen in an axial direction parallel to the first central axis RC1, a triangle having three axes among the first through fourth central axes RC1through RC4as apexes may have the remaining one axis located therein.

The gear rotating about the remaining one axis may be located at a more inward position in the housing21, and thus the gear may be prevented from protruding outward. With this structure, the size of the drive unit20may be decreased.

In a preferred embodiment of the present invention, as seen in an axial direction parallel to the first central axis RC1, the second central axis RC2may be located inside a triangle having the first central axis RC1, the third central axis RC3and the fourth central axis RC4as apexes.

The idle gear41does not decelerate or accelerate, and thus merely needs to have such a size as to achieve power transmission between the decelerator24and the driven gear233. Therefore, the idle gear41may have a diameter less than each of the diameter of the first transmission gear241of the decelerator24and the diameter of the driven gear233on the pedal crank shaft22. The second central axis RC2is located inside a triangle having the first central axis RC1, the third central axis RC3and the fourth central axis RC4as the apexes. With this structure, the first transmission gear241having a relatively long diameter may be located at a more inward position in the housing21, and thus may be prevented from protruding outward. The housing21may have an outer contour along the idle gear41having a relatively short diameter, and not along the first transmission gear241having a relatively long diameter. Therefore, the size of the drive unit20may be decreased.

In a preferred embodiment of the present invention, as seen in an axial direction parallel to the first central axis RC1, at least a portion of the driven gear233may overlap the first transmission gear241.

The driven gear233and the first transmission gear241are located such that at least a portion of the driven gear233and the first transmission gear241overlap each other, and consequently, the second central axis RC2and the fourth central axis RC4may be closer to each other. With this structure, the first central axis RC1and the fourth central axis RC4may be closer to each other. The distance between the first central axis RC1and the fourth central axis RC4may be shortened, and thus the size of the drive unit20in the front-rear direction may be decreased. The size of the drive unit20in the front-rear direction is decreased so that the space for the battery unit26to be located in the down tube123is provided.

In a vehicle in which the battery unit is located in the down tube, the battery unit is located in a space between the pedal crank shaft and the front wheel in the front-rear direction of the vehicle (located in a front center portion). An attempt to increase the capacitance of the battery results in an increase in the size of the battery unit, and the front center length needs to be longer accordingly. However, in the case where the front center length is longer, there occurs a problem that the operability of the vehicle is decreased.

According to a preferred embodiment of the present invention, the size of the drive unit20in the front-rear direction is decreased so that the degree of freedom of positional arrangement of the battery unit26may be increased. The battery unit26having a large capacitance may easily be located without increasing the front center length.

In a preferred embodiment of the present invention, as seen in an axial direction parallel to the first central axis RC1, at least a portion of a region65where the driven gear233and the idle gear41are engaged with each other may overlap the first transmission gear241.

The first transmission gear241, the idle gear41and the driven gear233are located such that at least a portion of the region65, where the driven gear233and the idle gear41are engaged with each other, overlaps the first transmission gear241. Consequently, the second central axis RC2and the fourth central axis RC4may be closer to each other. With this structure, the first central axis RC1and the fourth central axis RC4may be closer to each other. The distance between the first central axis RC1and the fourth central axis RC4may be shortened, and thus the size of the drive unit20in the front-rear direction may be decreased.

In a preferred embodiment of the present invention, as seen in an axial direction parallel to the first central axis RC1, the third central axis RC3may overlap the first transmission gear241.

The idle gear41and the decelerator24are located such that the third central axis RC3and the first transmission gear241overlap each other, and consequently, the second central axis RC2and the third central axis RC3may be closer to each other. With this structure, the first central axis RC1and the fourth central axis RC4may be closer to each other. The distance between the first central axis RC1and the fourth central axis RC4may be shortened, and thus the size of the drive unit20in the front-rear direction may be decreased.

In a preferred embodiment of the present invention, the pedal crank shaft22may be provided with a one-way clutch50.

The pedal crank shaft22is provided with the one-way clutch50so that a structure in which a forward rotation of the pedal crank shaft22is transmitted to the drive sprocket34but a reverse rotation of the pedal crank shaft22is not transmitted to the drive sprocket34may be provided.

An electrically assisted vehicle10according to a preferred embodiment of the present invention includes the above-described drive unit20.

The electrically assisted vehicle10includes the drive unit20in which the driven gear233provided on the pedal crank shaft22has a short diameter so that the rear center length of the electrically assisted vehicle10may be shortened.

Preferred embodiments of the present invention are especially useful in the field of electrically assisted vehicles and drive units mountable on the electrically assisted vehicles.