Driving force transmission apparatus

In a driving force transmission apparatus, an oil inlet portion for introducing hydraulic fluid from one of spaces to the other one of the spaces is formed, and an apparatus case has an oil outlet portion for delivering the hydraulic fluid from the other one of the spaces to an outside of the other one of the spaces with an oil delivery capacity lower than an oil introducing capacity of the oil inlet portion. A pressure regulation valve for bringing the oil inlet portion into an open state upon reception of hydraulic pressure higher than or equal to a predetermined pressure from hydraulic fluid is arranged at the oil inlet portion.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2012-098753 filed on Apr. 24, 2012 including the specification, drawings and abstract, is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a driving force transmission apparatus that transmits driving force from an input shaft to an output shaft in, for example, an automobile.

2. Description of the Related Art

There is a conventional driving force transmission apparatus that is mounted in, for example, a four-wheel-drive vehicle and in which a pair of rotary members are coupled to each other by a clutch such that torque is transmittable therebetween (see, for example, Japanese Patent Application Publication No. 2008-121796 (JP 2008-121796 A)).

The driving force transmission apparatus is formed of a first rotary member, a second rotary member, a friction clutch and a pressing force application mechanism. The first rotary member rotates together with an input shaft. The second rotary member is rotatable about the axis of the first rotary member. The clutch couples the second rotary member and the first rotary member to each other such that torque is transmittable therebetween. The pressing force application mechanism applies pressing force, which is used as clutch actuating force, to the clutch.

The first rotary member is formed of a closed-end cylindrical housing that is open toward one side. The first rotary member is coupled to the input shaft. The first rotary member rotates upon reception of the driving force from a driving source, such as a vehicle engine, via the input shaft.

The second rotary member is arranged so as to be rotatable relative to the first rotary member, and is coupled to an output shaft. The second rotary member is accommodated in an apparatus case together with the first rotary member.

The clutch includes inner clutch plates and outer clutch plates, and is arranged between the first rotary member and the second rotary member. The clutch couples the first rotary member and the second rotary member to each other such that torque is transmittable therebetween, by frictionally engaging the inner clutch plates and the outer clutch plates with each other.

The pressing force application mechanism includes a piston and a pressing force application member. The piston is movable back and forth inside the apparatus case (cylinder). The pressing force application member applies the moving force of the piston as pressing force, to the clutch. The pressing force application mechanism is arranged on the input side of the clutch.

With the above-described configuration, when driving force from the engine is input into the first rotary member via the input shaft, the first rotary member rotates about its axis. At this time, if hydraulic fluid is supplied into the cylinder, pressure due to the hydraulic fluid acts on the piston.

Therefore, the pressing force application mechanism is actuated, the piston moves toward the clutch accordingly, and the moving force is applied from the pressing force application member to the clutch as pressing force.

Then, the inner clutch plates and the outer clutch plates of the clutch approach each other and then frictionally engage with each other, and the first rotary member and the second rotary member are coupled to each other due to the frictional engagement such that torque is transmittable therebetween. Thus, the driving force from the engine is transmitted from the input shaft to the output shaft via the driving force transmission apparatus.

With the driving force transmission apparatus described in JP 2008-121796 A, it is desirable to reduce the amount of lubricating oil (to lower the oil level) in the housing in order to reduce adverse influence due to so-called drag torque that is generated on the basis of the viscosity of the lubricating oil while the four-wheel-drive vehicle travels in a two-wheel-drive mode.

On the other hand, if the amount of lubricating oil in the housing is excessively small while the four-wheel-drive vehicle travels in a four-wheel-drive mode, there is a possibility that the clutch plates may be damaged due to heat of the clutch plates, which is generated through frictional engagement between the inner clutch plates and the outer clutch plates of the clutch, and the durability of the clutch may decrease.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a driving force transmission apparatus configured to keep the oil level of lubricating oil in an apparatus case at a desired oil level to reduce drag torque and to suppress a decrease in the durability of a clutch.

An aspect of the invention relates to a driving force transmission apparatus, including: a first rotary member that is rotated by a driving source of a vehicle; a second rotary member that is arranged on a rotation axis of the first rotary member so as to be rotatable relative to the first rotary member; a clutch that is interposed between the second rotary member and the first rotary member, and that couples the first rotary member and the second rotary member to each other such that the first rotary member and the second rotary member are disengageable from each other; a pressing force application mechanism that includes a piston that applies pressing force to the clutch through movement due to supplied hydraulic fluid; and an apparatus case that has two spaces that are adjacent to each other via the piston of the pressing force application mechanism. An oil inlet portion for introducing the hydraulic fluid from the oil supply-side space among the two spaces to the clutch-side space among the two spaces is formed. The apparatus case has an oil outlet portion for delivering the hydraulic fluid from the clutch-side space to an outside of the clutch-side space with an oil delivery capacity lower than an oil introducing capacity of the oil inlet portion. A pressure regulation valve that brings the oil inlet portion into an open state upon reception of a hydraulic pressure higher than or equal to a predetermined pressure from the hydraulic oil is arranged at the oil inlet portion.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1schematically shows a four-wheel-drive vehicle200. As shown inFIG. 1, the four-wheel-drive vehicle200includes a driving force transmission system201, an engine202, a transmission203, front wheels204R,204L that serve as main drive wheels, and rear wheels205R,205L that serve as auxiliary drive wheels.

The driving force transmission system201is arranged on a driving force transmission path that extends from the transmission203to the rear wheels205R,205L in the four-wheel-drive vehicle200, together with a front differential206and a rear differential207. The driving force transmission system201is mounted on a vehicle body (not shown) of the four-wheel-drive vehicle200.

The driving force transmission system201includes a driving force transmission apparatus1, a propeller shaft2and a driving force interrupting device3. The driving force transmission system201is configured to be able to shift the four-wheel-drive vehicle200from a four-wheel-drive mode to a two-wheel-drive mode or from the two-wheel-drive mode to the four-wheel-drive mode. The details of the driving force transmission apparatus1will be described later.

The front differential206includes side gears209R,209L, a pair of pinion gears210, a gear support member211and a front differential case212. The front differential206is arranged between the transmission203and the driving force interrupting device3. The side gear209L is coupled to a front wheel axle shaft208L, and the side gear209R is coupled to a front wheel axle shaft208R. The pinion gears210are arranged such that the gear axes thereof are perpendicular to the gear axes of the side gears209R,209L, and are in mesh with the side gears209R,209L. The gear support member211supports the pinion gears210such that the pinion gears210are rotatable. The front differential case212accommodates the gear support member211, the pinion gears210and the side gears209R,209L.

The rear differential207includes side gears214R,214L, a pair of pinion gears215, a gear support member216and a rear differential case217. The rear differential207is arranged between the propeller shaft2and the driving force transmission apparatus1. The side gear214L is coupled to a rear wheel axle shaft213L, and the side gear214R is coupled to a rear wheel axle shaft213R. The pinion gears215are arranged such that the gear axes thereof are perpendicular to the gear axes of the side gears214R,214L, and are in mesh with the side gears214R,214L. The gear support member216supports the pinion gears215such that the pinion gears215are rotatable. The rear differential case217accommodates the gear support member216, the pinion gears215and the side gears214R,214L.

The engine202drives the front wheels204R,204L by outputting driving force to the front wheel axle shafts208R,208L via the transmission203and the front differential206.

The engine202drives the rear wheel205L by outputting driving force to the rear wheel axle shaft213L via the transmission203, the driving force interrupting device3, the propeller shaft2and the rear differential207. The engine202drives the rear wheel205R by outputting driving force to the rear wheel axle shaft213R via the transmission203, the driving force interrupting device3, the propeller shaft2, the rear differential207and the driving force transmission apparatus1.

The propeller shaft2is arranged between the driving force transmission apparatus1and the driving force interrupting device3. The propeller shaft2receives the driving force of the engine202from the front differential case212, and then transmits the driving force from the front wheels204R,204L side toward the rear wheels205R,205L.

A front wheel gear mechanism6is arranged at the front wheel-side end portion of the propeller shaft2. The front wheel gear mechanism6is formed of a drive pinion60and a ring gear61that are in mesh with each other. A rear wheel gear mechanism7is arranged at the rear wheel-side end portion of the propeller shaft2. The rear wheel gear mechanism7is formed of a drive pinion70and a ring gear71that are in mesh with each other.

The driving force interrupting device3is formed of a dog clutch that includes a first spline tooth portion30, a second spline tooth portion31and a sleeve32. The first spline tooth portion30is non-rotatable with respect to the front differential case212. The second spline tooth portion31is non-rotatable with respect to the ring gear61. The dog clutch has the sleeve32that is able to be spline-fitted to the first spline tooth portion30and the second spline tooth portion31. The driving force interrupting device3is arranged at the front wheels204R,204L side of the four-wheel-drive vehicle200. The driving force interrupting device3is connected to a vehicle electronic control unit (ECU) (not shown) via an actuator (not shown). The driving force interrupting device3couples the propeller shaft2and the front differential case212to each other such that the propeller shaft2and the front differential case212are disengageable from each other.

FIG. 2shows the driving force transmission apparatus. As shown inFIG. 2, the driving force transmission apparatus1includes an apparatus case4, a clutch (multiple disk clutch)8, a housing12, an inner shaft13and a pressing force application mechanism15. The driving force transmission apparatus1is arranged at the rear wheel205R (shown inFIG. 1) side of the four-wheel-drive vehicle200(shown inFIG. 1).

The driving force transmission apparatus1couples the propeller shaft2(shown inFIG. 1) and the rear wheel axle shaft213R (shown inFIG. 1) to each other such that the propeller shaft2and the rear wheel axle shaft213R are disengageable from each other. That is, the rear wheel axle shaft213R and the propeller shaft2are coupled to each other via the driving force transmission apparatus1. The rear wheel axle shaft213L (shown inFIG. 1) and the propeller shaft2are coupled to each other without providing the driving force transmission apparatus1therebetween.

Thus, when the rear wheel axle shaft213R is coupled to the propeller shaft2by the driving force transmission apparatus1, the rear wheel axle shaft213L and the propeller shaft2are coupled to each other via the gear mechanism7and the rear differential207(both are shown inFIG. 1) such that torque is transmittable therebetween, and the rear wheel axle shaft213R and the propeller shaft2are coupled to each other via the gear mechanism7and the rear differential207such that torque is transmittable therebetween. On the other hand, when the rear wheel axle shaft213R is disconnected from the propeller shaft2by the driving force transmission apparatus1, the left rear wheel axle shaft213L remains coupled to the propeller shaft2via the gear mechanism7and the rear differential207, whereas the right rear wheel axle shaft213R is disconnected from the propeller shaft2.

The apparatus case4is formed of two cases40,41that are arranged next to each other along a rotation axis O. The apparatus case4is fitted to the vehicle body (not shown) of the four-wheel-drive vehicle200(shown inFIG. 1). Two spaces42a,42bare formed in the apparatus case4. The spaces42a,42bare located next to each other via a piston150(described later) of the pressing force application mechanism15. The space42afunctions as a fluid supply-side space (cylinder), and the space42bfunctions as a clutch-side space. Oil inlet portions47and an oil outlet portion48are arranged in the apparatus case4. The oil inlet portions47are used to introduce hydraulic fluid (lubricating oil) from the space42ainto the space42b. The oil outlet portion48is used to deliver the lubricating oil from the space42bto the outside. The details of the oil inlet portions47will be described later (described in association with the pressing force application mechanism15).

The oil outlet portion48is an oil flow passage formed of a through-hole (round hole) that is communicated (connected) with an oil tank43and the space42b. The oil outlet portion48is formed in a body portion400(described later) of the case40. Through the oil outlet portion48, the lubricating oil flows from the space42bto the oil tank43.

The case40has the body portion400and a bottom portion401. The entirety of the case40is formed of a closed-end cylindrical member that is open toward the case41(right side inFIG. 2).

The body portion400is open toward both sides in the rotation axis O. The entirety of the body portion400is formed of an open-end cylindrical member that accommodates part of the housing12. A piston 150-side open end face of the body portion400is formed of a spring receiving face400athat receives the spring force of a return spring49. The oil tank43is formed on the radially outer side of the body portion400. The lubricating oil is stored in a space between the outer periphery of the body portion400and the inner periphery of the case41. A seal member50is attached to the piston 150-side open end portion of the body portion400. The seal member50is interposed between the outer periphery of the body portion400and the inner periphery of the case41, and seals the oil tank43.

The bottom portion401faces the piston150via the clutch8, and the like, and the entirety of the bottom portion401is formed of an annular member that closes an opening portion of the body portion400, which is on the side opposite to a piston 150-side opening portion. The bottom portion401forms the oil tank43together with the body portion400and the case41. The bottom portion401has an insertion hole401athrough which the housing12is passed along the rotation axis O. A cylindrical portion401bis formed at the outer opening periphery of the insertion hole401a. The cylindrical portion401bprotrudes toward the rear differential207(shown inFIG. 1), and has a central axis that coincides with the rotation axis O. A seal mechanism51is attached to the cylindrical portion401b. The seal mechanism51is interposed between the inner periphery of the cylindrical portion401band the outer periphery of the inner shaft13. In addition, the bottom portion401has a first oil flow hole401cand a second oil flow hole401d. The first oil flow hole401ccommunicates with the space42b, and is used to determine the upper limit height of the oil level (oil height) of lubricating oil in the space42b. The second oil flow hole401dcommunicates with the oil tank43. The first oil flow hole401cand the second oil flow hole401dare connected to each other by a pipe52.

The case41has an insertion hole41athrough which the inner shaft13(described later) is passed along the rotation axis O, and the entirety of the case41is formed of a closed-end cylindrical member that is open toward the case40(left side inFIG. 2). A cylindrical portion410ais formed at the inner opening periphery of the insertion hole41a. The cylindrical portion410aprotrudes toward the clutch8, and has a central axis that coincides with the rotation axis O. A cylindrical portion411ais formed at the outer opening periphery of the insertion hole41a. The cylindrical portion411aprotrudes in a direction opposite to the direction in which the cylindrical portion410aprotrudes, and has a central axis that coincides with the rotation axis O.

An annular inner flange412ais formed at a clutch 8-side end portion of the cylindrical portion410a. The inner flange412aprotrudes from the inner periphery of the cylindrical portion410a. A needle roller bearing53is fitted to the cylindrical portion410a. The needle roller bearing53is interposed between the inner periphery of the cylindrical portion410aand the outer periphery of the inner shaft13, and arranged on the opposite side of the inner flange412afrom the clutch8.

A seal mechanism54is attached to the cylindrical portion411a. The seal mechanism54is interposed between the inner periphery of the cylindrical portion411aand the outer periphery of the inner shaft13. A piping portion413ais provided on the outer periphery of the cylindrical portion411a. The piping portion413acommunicates with the space42a, and communicates with the oil tank43via an oil flow passage417. The piping portion413ahas a sensor fitting hole414athat communicates with the inside of the piping portion413a. A pressure sensor55is fitted in the sensor fitting hole414a.

A pump418is arranged on the oil flow passage417. The pump418is used to deliver lubricating oil from the oil tank43to the space42avia the piping portion413aand to discharge the lubricating oil from the space42ato the oil tank43. The pump418is, for example, a trochoid pump, and is connected to a driving source419. When the pump418is driven in the forward direction by the driving source419, the lubricating oil is delivered from the oil tank43to the space42avia the oil flow passage417, the piping portion413a, and the like. When the pump418is driven in the reverse direction by the driving source419, the lubricating oil is discharged from the space42ato the oil tank43via the piping portion413a, the oil flow passage417, and the like.

The clutch8is formed of a friction clutch that includes a plurality of inner clutch plates80and a plurality of outer clutch plates81. The clutch8is arranged between the housing12that may function as a first rotary member and the inner shaft13that may function as a second rotary member.

Any adjacent inner and outer clutch plates among the inner clutch plates80and the outer clutch plates81of the clutch8may be frictionally engaged with each other. Also, the frictional engagement between the inner clutch plates80and the outer clutch plates81of the clutch8may be cancelled. In this way, the clutch8couples the housing12and the inner shaft13to each other such that the housing12and the inner shaft13are disengageable from each other.

The inner clutch plates80and the outer clutch plates81are alternately arranged along the rotation axis O, and each are entirely formed of an annular friction plate. A clearance C between any adjacent two clutch plates among the inner clutch plates80and the outer clutch plates81is set to such a value that the clutch plates do not frictionally engage with each other due to drag torque based on the viscosity of lubricating oil when the four-wheel-drive vehicle200(shown inFIG. 1) travels in the two-wheel-drive mode.

Each of the inner clutch plates80has a straight spline fitting portion80aat its inner peripheral portion. The straight spline fitting portions80aof the inner clutch plates80are fitted to a straight spline fitting portion130aof the cylindrical portion13a(inner shaft13). Thus, the inner clutch plates80are connected to the inner shaft13so as to be non-rotatable but movable relative to the inner shaft13.

A plurality of oil holes80bis formed in each of the inner clutch plates80. The oil holes80bare arranged in the circumferential direction of the inner clutch plates80, and are open toward both sides in the rotation axis O.

Each of the outer clutch plates81has a straight spline fitting portion81aat its outer peripheral portion. The outer clutch plates81are coupled to the housing12by fitting the straight spline fitting portions81ato a straight spline fitting portion120a(described later) of the cylindrical portion12a(housing12). Thus, the outer clutch plates81are connected to the housing12so as to be non-rotatable but movable relative to the housing12.

Among the outer clutch plates81, the outer clutch plate closest to the piston150functions as an input portion of the clutch8. When the outer clutch plate closest to the piston150receives a first pressing force P1that is directed from the piston150of the pressing force application mechanism15toward the clutch8via a pressing member11, the clearance C between any adjacent two of the inner clutch plates80and the outer clutch plates81is reduced to, for example, 0 (C=0) due to the movement in the pressing direction. When the outer clutch plate closest to the piston150among the outer clutch plates81receives a second pressing force P2(P2≧P1) that is directed from the piston150of the pressing force application mechanism15toward the clutch8via the pressing member11after application of the first pressing force P1, any adjacent two of the inner clutch plates80and the outer clutch plates81frictionally engage with each other due to the movement in the pressing direction.

The pressing member11has a straight spline fitting portion11athat is exposed to the space42bof the apparatus case4, and the entirety of the pressing member11is formed of an annular member. The pressing member11is coupled to the housing12by fitting the straight spline fitting portion11ato the straight spline fitting portion120aof the cylindrical portion12a, on the clutch8side of the pressing force application mechanism15. Thus, the pressing member11is connected to the housing12so as to be non-rotatable but movable relative to the housing12. The pressing member11is rotatably supported by the piston150of the pressing force application mechanism15via a needle roller bearing44.

FIG. 3shows a state where the housing is splined-fitted to the outer clutch plates. As shown inFIG. 2, the housing12has the cylindrical portion12aand a shaft portion12b, and is arranged on the axis (rotation axis O) of the rear wheel axle shaft213R (shown inFIG. 1). The housing12is rotatably supported by the inner periphery of the case40, which defines the insertion hole401a, via a ball bearing20. The bottom portion of the cylindrical portion12aof the housing12is rotatably supported by the bottom portion401of the case40via a needle roller bearing21.

The cylindrical portion12ais open toward the space42bof the apparatus case4, and is arranged at one-side end portion (right-side end portion inFIG. 2) of the housing12. The internal space of the cylindrical portion12afunctions as a clutch accommodating space. The straight spline fitting portion120ais formed in the inner periphery of the cylindrical portion12a. The straight spline fitting portion120ais fitted to the straight spline fitting portions81aof the outer clutch plates81and the straight spline fitting portion11aof the pressing member11. As shown inFIG. 3, the cylindrical portion12ahas through-holes122a. Each through-hole122ais located between any adjacent two spline teeth among spline teeth121aof the straight spline fitting portion120a, and has an axis L that is perpendicular to the axis (rotation axis O) of the housing12.

The shaft portion12bis arranged at the other-side end portion (left-side end portion inFIG. 2) of the housing12. The shaft portion12bis coupled to the side gear214R through spline-fitting by inserting its distal end portion into the opening portion of the side gear214R. Thus, the shaft portion12bis connected to the side gear214R so as to be non-rotatable but movable relative to the side gear214R. The shaft portion12bhas a recessed hole120bthat communicates with the inside of the cylindrical portion12a.

The inner shaft13has cylindrical portions13ato13c(13bis a shaft portion) and step faces13d,13e. The inner shaft13is arranged on the rotation axis O of the housing12, and the entirety of the inner shaft13is formed of a closed-end cylindrical member that is open toward one side (right side inFIG. 2) in the axial direction. The outside diameters of the cylindrical portions13ato13care set to sizes different from one another. The outside diameter of the cylindrical portion13ais set to the maximum size (maximum outside diameter). The outside diameter of the shaft portion13bis set to the minimum size (minimum outside diameter). The outside diameter of the cylindrical portion13cis set to a size (intermediate outside diameter) in between the outside diameter of the cylindrical portion13aand the outside diameter of the shaft portion13b. The distal end portion of the rear wheel axle shaft213R (shown inFIG. 1) is inserted and accommodated in the opening portion of the inner shaft13. The rear wheel axle shaft213R is coupled to the inner shaft13through spline-fitting so as to be non-rotatable but movable relative to the inner shaft13.

The cylindrical portion13ahaving the maximum outside diameter is located at the axially center portion of the inner shaft13, at a position between the shaft portion13bhaving the minimum outside diameter and the cylindrical portion13chaving the intermediate outside diameter. The cylindrical portion13ahaving the maximum outside diameter is rotatably supported by the bottom portion of the cylindrical portion12aof the housing12via a spacer23and a needle roller bearing24arranged on the step face13d, and is rotatably supported by the flange end face of the inner flange412aof the case41via a spacer25and a needle roller bearing26arranged on the step face13e. The straight spline fitting portion130ais formed in the outer periphery of the cylindrical portion13ahaving the maximum outside diameter. The straight spline fitting portion130ais exposed to the inside of the cylindrical portion12aof the housing12, and is fitted to the straight spline fitting portions80aof the inner clutch plates80of the clutch8.

The shaft portion13bhaving the minimum outside diameter is arranged at one side (left side inFIG. 2) of the inner shaft13, and is rotatably supported in the recessed hole120bof the housing12via a needle roller bearing27.

The cylindrical portion13chaving the intermediate outside diameter is arranged at the other side (right side inFIG. 2) of the inner shaft13. The cylindrical portion13chaving the intermediate outside diameter is rotatably supported by the inner periphery of the cylindrical portion410aof the case41via the needle roller bearing53.

The step face13dis located between the cylindrical portions13a,13b. The step face13eis located between the cylindrical portions13a,13c.

As shown inFIG. 2, the pressing force application mechanism15includes the piston150and pressure regulation valves151, and is accommodated in the apparatus case4(case41). In the pressing force application mechanism15, the piston150moves toward the clutch8upon reception of the hydraulic pressure of lubricating oil on the space42aside, and applies the pressing force (the first pressing force P1and the second pressing force P2) to the clutch8.

The piston150has annular portions150ato150c. The piston150is interposed between the piston 150-side end face of the pressing member11and the bottom face of the case41so as to move back and forth. The entirety of the piston150is formed of an annular member through which the cylindrical portion410ais passed. The piston150applies, to the clutch8, the first pressing force P1for reducing the clearance between adjacent clutch plates prior to an opening operation of the pressure regulation valve151and the second pressing force P2for frictionally engaging the clutch plates with each other. The first pressing force P1and the second pressing force P2each are larger than the spring force a of the return spring49(a<P1≦P2).

The outside diameters of the annular portions150ato150care set to sizes different from one another. The outside diameter of the annular portion150ais set to the maximum outside diameter. The outside diameter of the annular portion150bis set to the minimum outside diameter. The outside diameter of the annular portion150cis set to the intermediate outside diameter in between the outside diameter of the annular portion150aand the outside diameter of the annular portion150b.

The annular portion150ahaving the maximum outside diameter is arranged at axial one side (right side inFIG. 2) of the piston150. A seal member57and a seal member58are attached to the annular portion150ahaving the maximum outside diameter. The seal member57is interposed between the outer periphery of the annular portion150aand the inner periphery of the case41. The seal member58is interposed between the inner periphery of the annular portion150aand the outer periphery of the cylindrical portion410a. The clutch 8-side end face of the annular portion150ahaving the maximum outside diameter is formed of a spring receiving face150dthat faces the spring receiving face400aof the case40via the return spring49.

The annular portion150bhaving the minimum outside diameter is passed through the pressing member11and faces the clutch8(the piston 150-side end face of the inner clutch plate80), and is arranged at an axial other side (left side inFIG. 2) of the piston150.

The annular portion150chaving the intermediate outside diameter faces the piston 150-side end face of the pressing member11via the needle roller bearing44, and is arranged at the axial center portion of the piston150. The outer periphery of the annular portion150chaving the intermediate outside diameter is tapered such that the outside diameter gradually decreases from the annular portion150ahaving the maximum outside diameter toward the annular portion150bhaving the minimum outside diameter.

The piston150has a plurality of (for example, six in the present embodiment) oil inlet portions47that is arranged at equal intervals around the rotation axis O.

Each of the oil inlet portions47has two holes47a,47b(large hole47a, small hole47b) having hole diameters different from each other and an annular seat face47clocated between both holes47a,47b. Each of the oil inlet portions47is an oil flow passage formed of a through-hole (round hole) that extends from the annular portion150ahaving the maximum outside diameter to the annular portion150bhaving the minimum outside diameter. The oil introducing capacity of the oil inlet portions47is larger than the oil delivery capacity of the oil outlet portion48. That is, an introducing rate Q1of the lubricating oil that is introduced from the space42ato the space42bvia the oil inlet portions47is set to a flow rate higher than a delivery rate Q2(Q2<Q1) of the lubricating oil that is delivered from the space42bto the outside (oil tank43) via the oil outlet portion48. The flow rate difference (Q1−Q2) is created by, for example, setting the opening area of the oil inlet portions47to an area smaller than the opening area of the oil outlet portion48.

Therefore, while the four-wheel-drive vehicle200(shown inFIG. 1) travels in the four-wheel-drive mode, the lubricating oil supplied to the space42aafter opening the oil inlet portions47flows through the oil inlet portions47into the space42b, and then part of the lubricating oil flows from the space42bthrough the oil outlet portion48to the outside. However, a predetermined amount of lubricating oil is ensured in the space42b. While the four-wheel-drive vehicle200travels in the two-wheel-drive mode, the lubricating oil flows from the space42bthrough the oil outlet portion48to the outside (oil tank43). Therefore, it is possible to avoid the situation where the amount of lubricating oil in the space42b, that is, the clutch-side space, is excessively small while the four-wheel-drive vehicle200travels in the four-wheel-drive mode, and it is possible to avoid the situation where the amount of lubricating oil in the clutch-side space becomes an amount, at which drag torque is generated, while the four-wheel-drive vehicle200travels in the two-wheel-drive mode.

The pressure regulation valve151is arranged in each of the oil inlet portions47. The pressure regulation valve151opens or closes upon reception of hydraulic pressure of the lubricating oil that is supplied to the space42aof the apparatus case4.

Each pressure regulation valve151is arranged in the large hole47aof a corresponding one of the oil inlet portions47. Each pressure regulation valve151enters an open state upon reception of a hydraulic pressure higher than or equal to a predetermined pressure P3(P3>P2). That is, in an open state of the pressure regulation valves151, the oil inlet portions47are open. When a hydraulic pressure lower than the predetermined pressure is applied, the pressure regulation valves151do not enter an open state and are kept in a closed state. That is, in a closed state of the pressure regulation valves151, the oil inlet portions47are closed. Therefore, when the piston150receives a hydraulic pressure corresponding to the first pressing force P1from lubricating oil, the piston150moves toward the clutch8while not opening the oil inlet portions47, and applies the first pressing force P1to the clutch8together with the pressing member11. After that, when the piston150receives a hydraulic pressure corresponding to the second pressing force P2from lubricating oil, the piston150moves toward the clutch8while not opening the oil inlet portions47, and applies the second pressing force P2to the clutch8together with the pressing member11. Each pressure regulation valve151is, for example, a relief valve that includes a valve element152and a spring153. The valve element152opens and closes the oil inlet portion47, and is able to be seated on the seat face47c. The spring153applies, to the valve element152, spring force in a direction in which the oil inlet portion47is closed.

Next, the operation of the driving force transmission apparatus according to the present embodiment will be described with reference toFIG. 1andFIG. 2.

InFIG. 1, when the four-wheel-drive vehicle200is in the two-wheel-drive mode, the rotational driving force of the engine202is transmitted to the front differential206via the transmission203. The rotational driving force of the engine202is transmitted from the front differential206to the front wheels204R,204L via the front wheel axle shafts208R,208L, and the front wheels204R,204L are rotated.

In this case, inFIG. 2(upper half), the pump418is in a non-driven state. Therefore, the pressing force application mechanism15is not actuated.

Therefore, the second pressing force P2that is the clutch actuating force is not applied from the piston150of the pressing force application mechanism15to the clutch8via the pressing member11, the inner clutch plates80and the outer clutch plates81of the clutch8do not frictionally engage with each other, and the rotational driving force of the engine202is not transmitted from the housing12to the inner shaft13. In this case, in the apparatus case4, the lubricating oil in the space42b, that is, the clutch-side space, is delivered to the oil tank43via the oil outlet portion48. Therefore, occurrence of drag torque based on the viscosity of lubricating oil between the inner clutch plates80and outer clutch plates81of the clutch8is suppressed.

On the other hand, in order to shift the four-wheel-drive vehicle200from the two-wheel-drive mode into the four-wheel-drive mode, the propeller shaft2is coupled to the rear wheel axle shaft213L by the driving force transmission apparatus1such that torque is transmittable therebetween, and then the front differential case212is coupled to the propeller shaft2by the driving force interrupting device3such that torque is transmittable therebetween.

When the propeller shaft2is coupled to the rear wheel axle shaft213R, the pump418is driven by the driving source419in the forward direction to supply the lubricating oil in the oil tank43to the space42aof the apparatus case4. In this case, when the pump418is driven, the lubricating oil in the oil tank43flows through the oil flow passage417into the piping portion413a, and then flows through the piping portion413ainto the space42a.

Therefore, the piston150receives hydraulic pressure from the lubricating oil in an initial state shown inFIG. 2(upper half), and the pressing force application mechanism15is actuated. In this case, when the pressing force application mechanism15is actuated, the piston150moves toward the clutch8in the space42aand the space42bin a state where the oil inlet portions47are closed, and applies the first pressing force P1to the pressing member11.

Accordingly, the pressing member11moves together with the piston150toward the clutch8in the space42bagainst the spring force of the return spring49, and moves the entirety of the clutch8to bring the clutch8into contact with the bottom portion of the housing12.

Thus, the clearance C between any adjacent two clutch plates among the inner clutch plates80and the outer clutch plates81becomes, for example, 0 (C=0).

Next, the pump418is driven by the driving source419in the forward direction, and the lubricating oil in the oil tank43is supplied to the space42a. In this case, when the lubricating oil is supplied to the space42a, the piston150receives a hydraulic pressure higher than the pressure corresponding to the first pressing force P1from lubricating oil, moves in the space42aand the space42bagainst the spring force of the return spring49while not opening the oil inlet portions47, and applies the second pressing force P2to the pressing member11.

Accordingly, when the pressing member11receives the second pressing force P2, the pressing member11moves together with the piston150in the space42btoward the clutch8against the spring force of the return spring49to cause the entirety of the clutch8to be further pressed against the bottom portion of the housing12and moved. At this time, when the piston150receives a hydraulic pressure higher than the second pressing force P2from the lubricating oil, the pressure regulation valves151are brought into an open state, and the lubricating oil is introduced from the space42ato the space42bvia the oil inlet portions47. In this case, the lubricating oil in the space42bis delivered to the oil tank43via the oil outlet portion48. However, because the introducing rate Q1of lubricating oil is higher than the delivery rate Q2of lubricating oil, it is possible to avoid the situation where the amount of lubricating oil in the space42bis excessively small.

Thus, any adjacent two clutch plates among the inner clutch plates80and the outer clutch plates81frictionally engage with each other, and the housing12and the inner shaft13are coupled to each other via the clutch8.

Therefore, the rotational driving force of the engine202is transmitted from the housing12to the inner shaft13, and further transmitted from the inner shaft13to the rear wheel205R via the rear wheel axle shaft213R. Thus, the rear wheel205R is rotated.

The propeller shaft2and the rear wheel axle shaft213L are coupled to each other without providing the driving force transmission apparatus1therebetween. Therefore, the rotational driving force of the engine202is transmitted from the propeller shaft2to the rear wheel205L via the rear wheel axle shaft213L while the front differential case212is coupled to the propeller shaft2, and the rear wheel205L is rotated.

In order to disconnect the propeller shaft2and the rear wheel axle shaft213R from each other, the pump418is driven by the driving source419in the reverse direction, and the lubricating oil in the space42ais returned to the oil tank43. In this case, when the pump418is driven, the lubricating oil in the space42aflows out to the piping portion413a, and flows out from the piping portion413athrough the oil flow passage417to the oil tank43. In addition, the lubricating oil in the space42bflows through the oil outlet portion48to the oil tank43.

According to the above-described first embodiment, the following advantageous effects are obtained.

(1) It is possible to keep the oil level of the lubricating oil in the apparatus case4(space42b) at a desired oil level when the four-wheel-drive vehicle200travels in the four-wheel-drive mode and in the two-wheel-drive mode. Therefore, it is possible to reduce drag torque, and it is possible to suppress a decrease in the durability of the clutch8. In this case, connection of the housing12with the inner shaft13is achieved by applying the first pressing force P1and the second pressing force P2to the clutch8. Therefore, it is possible to increase the clearance between the clutch plates of the clutch8, and it is possible to further improve the effect of reducing drag torque.

(2) The piston150is returned to the initial position by the return spring49. Therefore, it is possible to drain the lubricating oil to the outside of the apparatus case4with the use of the piston150, and it is possible to smoothly return the lubricating oil to the oil tank43.

(3) After the lubricating oil is supplied from the oil tank43to the space42aby driving the pump418, a hydraulic circuit that introduces the lubricating oil from the space42ato the space42bvia the oil inlet portions47and then delivers the lubricating oil to the oil tank43via the oil outlet portion48is formed, and the lubricating oil smoothly flows in the hydraulic circuit.

(4) The upper limit height of the oil level of the lubricating oil in the space42bis determined by the first oil flow hole401c. Therefore, the oil level of the lubricating oil in the space42bdoes not become an oil level higher than the upper limit height. Therefore, it is possible to avoid the situation where an excessive amount of lubricating oil is stored in the space42b.

(5) The oil inlet portions47are formed in the piston15. Therefore, the oil inlet portions47are arranged inside the apparatus case4, and, as a result, it is possible to reduce the size of the apparatus as a whole as compared to the case where the oil inlet portions47are arranged outside of the apparatus case4.

Next, a driving force transmission apparatus100according to a second embodiment of the invention will be described with reference toFIG. 4.FIG. 4shows the driving force transmission apparatus. InFIG. 4, the components having the same or equivalent functions to those inFIG. 2are denoted by the same reference numerals as those inFIG. 2, and the detailed description is omitted.

As shown inFIG. 4, the driving force transmission apparatus100according to the second embodiment of the invention differs from the driving force transmission apparatus1according to the first embodiment in that, instead of the oil inlet portions47arranged inside the apparatus case4, an oil inlet portion101is arranged outside of the apparatus case4(outside of the oil supply-side space42aand the clutch-side space42b).

Therefore, the oil inlet portion101has an oil flow passage101a. One end portion of the oil flow passage101ais connected to the space42avia a pipe102, and the other end portion of the oil flow passage101ais connected to the space42bvia a pipe103. The oil introducing capacity of the oil inlet portion101is higher than the oil delivery capacity of the oil outlet portion48as in the case of the first embodiment.

The pressure regulation valve151is arranged in the oil inlet portion101. The pressure regulation valve151opens or closes upon reception of hydraulic pressure of the lubricating oil that is supplied to the space42aof the apparatus case4.

In the thus configured driving force transmission apparatus100, while the four-wheel-drive vehicle200(shown inFIG. 1) travels in the four-wheel-drive mode, the lubricating oil supplied to the space42aafter opening the oil inlet portion101flows through the oil inlet portion101into the space42b, and then part of the lubricating oil flows from the space42bthrough the oil outlet portion48to the outside. However, a predetermined amount of lubricating oil is kept in the space42b. While the four-wheel-drive vehicle200travels in the two-wheel-drive mode, the lubricating oil in the space42bflows through the oil outlet portion48to the outside (oil tank43). Therefore, it is possible to avoid the situation where the amount of lubricating oil in the space42b, that is, the clutch-side space is excessively small while the four-wheel-drive vehicle200travels in the four-wheel-drive mode, and it is possible to avoid the situation where the amount of lubricating oil in the clutch-side space becomes an amount, at which drag torque is generated, while the four-wheel-drive vehicle200travels in the two-wheel-drive mode.

According to the above-described second embodiment, the following advantageous effects are obtained in addition to the advantageous effects (1) to (4) of the first embodiment.

The oil inlet portion101is arranged outside of the apparatus case4. Therefore, it is not necessary to employ the configuration employed in the first embodiment (configuration that the oil inlet portions are provided in the piston150). Thus, the shape of the piston150is not complex, and it is possible to easily form the piston150.

The driving force transmission apparatus according to the invention is described on the basis of the above embodiments. However, the invention is not limited to the above embodiments. It is possible to implement the invention in various other embodiments without departing from the scope of the invention. For example, the following modifications may be made.

(1) In the above-described embodiments, the description is made on the case where the first pressing force P1for reducing the clearance C between any adjacent inner clutch plate80and outer clutch plate81to, for example, 0 (C=0) is applied to the piston150by the pressing force application mechanism15. However, the invention is not limited to this configuration. The first pressing force P1for reducing the clearance between any adjacent clutch plates of the clutch8as compared to that in the initial state may be applied to the piston150by actuating the pressing force application mechanism15. That is, in short, the invention just needs to be configured such that the pressing force application mechanism applies, to the piston, the first pressing force for reducing the clutch plate clearance of the clutch.

(2) In the above-described embodiments, the description is made on the case where the housing12is coupled to the input shaft side and the inner shaft13is coupled to the output shaft side. However, the invention is not limited to this configuration. Similar advantageous effects are obtained when the housing is coupled to the output shaft side and the inner shaft is coupled to the input shaft side.

According to the invention, it is possible to keep the oil level of lubricating oil in the apparatus case at a desired oil level height during application of pressing force. Therefore, it is possible to suppress a decrease in the durability of the clutch. Oil is drained to the tank through the oil outlet portion of the apparatus case when pressing force is not required, and oil in the clutch space decreases. Therefore, it is possible to reduce drag torque.