Automatic transmission

An automatic transmission includes a piston movable in an axial direction, a plurality of friction plates disposed on a side of a first surface of the piston, a fastening hydraulic chamber applying a hydraulic pressure to a second surface of the piston to move the piston to a fastening position where the friction plates are pressed to be fastened to each other, a release hydraulic chamber applying a hydraulic pressure to the first surface of the piston to move the piston to a release position where the friction plates are released, and a hydraulic pressure control valve that supplies and discharges the hydraulic pressure to and from each of the fastening hydraulic chamber and the release hydraulic chamber. An area of the second surface of the piston for receiving the hydraulic pressure is set to be larger than an area of the first surface for receiving the hydraulic pressure.

TECHNICAL FIELD

The present invention relates to an automatic transmission that includes a friction fastening element including a plurality of friction plates and a piston.

BACKGROUND ART

An automatic transmission incorporated in a vehicle such as an automobile includes a planetary gear set and a plurality of friction fastening elements such as a multi-plate clutch and a multi-plate brake. As the friction fastening elements are selectively fastened according to the operation state of an engine, it is possible to automatically shift to a predetermined gear position. The friction fastening element includes a plurality of friction plates disposed with a clearance therebetween and a piston pressing the friction plates. The piston is moved between a fastening position where the friction plates are pressed to be fastened to each other and a release position where pressing of the friction plates is released and thus the friction plates are in a released state.

Patent Literature 1 discloses an automatic transmission in which a fastening hydraulic chamber is disposed on one side in an axial direction of a piston and a centrifugal hydraulic chamber (release hydraulic chamber) is disposed on the other side, and a through-hole through which the fastening hydraulic chamber communicates with the centrifugal hydraulic chamber is formed in the piston. A predetermined hydraulic pressure is supplied from different oil passages to the fastening hydraulic chamber and the centrifugal hydraulic chamber. A pressure regulator valve is assembled in the oil passage for the centrifugal hydraulic chamber.

It is necessary in the automatic transmission to shorten a fastening control time while reducing a fastening shock applied to the friction fastening elements when being switched from a released state to a fastened state. To shorten the fastening control time and reduce the fastening shock at the same time, it is necessary to cause a large flow rate of oil to flow at a precharge hydraulic pressure or the like for the purpose of shortening the fastening control time, and at the same time, to reduce the flow rate (reduce the hydraulic pressure) of oil immediately before a stroke is completed for the purpose of reducing the fastening shock. In this case, precise flow rate control must be executed, so that hydraulic pressure control tends to be complicated. Consequently, it takes a long time to execute fastening control for fastening the friction fastening elements, thus reducing response of the friction fastening elements.

CITATION LIST

Patent Literature

Patent Literature 1: JP 10-131984 A

SUMMARY OF INVENTION

An object of the present invention is to provide an automatic transmission including a friction fastening element that can reduce a fastening shock without requiring complicated hydraulic pressure control and can shorten a fastening control time.

An automatic transmission according to an aspect of the present invention includes a piston that has a first surface and a second surface opposing to each other in an axial direction and that is movable in the axial direction, a plurality of friction plates disposed on a side of the first surface of the piston, a fastening hydraulic chamber that applies a hydraulic pressure to the second surface of the piston to move the piston to a fastening position where the friction plates are pressed to be fastened to each other, a release hydraulic chamber that applies a hydraulic pressure to the first surface of the piston to move the piston to a release position where the friction plates are released, a hydraulic pressure control valve that has an output port of the hydraulic pressure, and supplies and discharges the hydraulic pressure to and from each of the fastening hydraulic chamber and the release hydraulic chamber, a first oil passage that allows the output port of the hydraulic pressure control valve to communicate with the fastening hydraulic chamber, and a second oil passage that allows the output port to communicate with the release hydraulic chamber. An area of the second surface of the piston for receiving the hydraulic pressure is set to be larger than an area of the first surface for receiving the hydraulic pressure.

DESCRIPTION OF EMBODIMENTS

[Overall Configuration of Automatic Transmission]

FIG. 1is a conceptual diagram of a configuration of an automatic transmission1for an automobile (a vehicle) according to an embodiment of the present invention. The automatic transmission1includes a transmission case2, an input shaft3that is disposed within the transmission case2and extends from an engine side, an output gear4, four planetary gear sets functioning as transmission mechanisms (a first planetary gear set11, a second planetary gear set12, a third planetary gear set13, and a fourth planetary gear set14), two brakes (a first brake21and a second brake22), and three clutches (a first clutch31, a second clutch32, and a third clutch33).

Power generated in an engine is input to the input shaft3. The output gear4outputs drive force having a predetermined transmission ratio determined by the transmission mechanism. The present embodiment exemplifies a so-called torque-converter-less automatic transmission in which power from an engine is input to an input unit not via a torque converter (a hydraulic transmission device).

The transmission case2includes an outer peripheral wall2a, a first middle wall2bat an end part of the outer peripheral wall2aon an engine side, a second middle wall2con the side, of the first middle wall2b, opposite to the engine side, a third middle wall2din an axially middle part of the outer peripheral wall2a, a side wall2eat an end part of the outer peripheral wall2aon the side opposite to the engine side, a boss part2fextending from a center part of the side wall2etoward the engine side, and a cylinder part2gextending from an inner peripheral side end part of the second middle wall2ctoward the side opposite to the engine side.

The four planetary gear sets11to14are disposed in the order of, from the engine side, the first planetary gear set11, the second planetary gear set12on an inner peripheral side and the third planetary gear set13on an outer peripheral side that are disposed to radially overlap to each other, and the fourth planetary gear set14. The first planetary gear set11includes a carrier11c, a pinion (not shown) supported by the carrier11c, a sun gear11s, and a ring gear11r. The first planetary gear set11is of a single pinion type in which the pinion directly meshes with the sun gear11sand the ring gear11r. The second planetary gear set12, the third planetary gear set13, and the fourth planetary gear set14are also of a single pinion type, and include carriers12c,13c, and14c, pinions (not shown), sun gears12s,13s, and14s, and ring gears12r,13r, and14r, respectively.

The ring gear12rof the second planetary gear set12and the sun gear13sof the third planetary gear set13that are disposed to radially overlap to each other are integrated by welding, shrink fitting, or the like. That is, the ring gear12rand the sun gear13sare always coupled to each other, thus forming an integral rotating element15. The sun gear11sof the first planetary gear set11is always coupled to the sun gear12sof the second planetary gear set12. The ring gear11rof the first planetary gear set11is always coupled to the carrier14cof the fourth planetary gear set14. The carrier11cof the first planetary gear set11is always coupled to the carrier13cof the third planetary gear set13. The input shaft3is always coupled to the carrier12cof the second planetary gear set12. The output gear4is always coupled to the carrier11cof the first planetary gear set11and the carrier13cof the third planetary gear set13. The output gear4is rotatably supported via a bearing41by the cylinder part2gof the transmission case2.

A first rotating member34is coupled to the sun gear14sof the fourth planetary gear set14. The first rotating member34extends toward the side opposite to the engine side. Similarly, a second rotating member35is coupled to the ring gear13rof the third planetary gear set13. A third rotating member36is coupled to the integral rotating element15. These rotating members34and35also extend toward the side opposite to the engine side. A fourth rotating member37is coupled via the input shaft3to the carrier12cof the second planetary gear set12.

The first brake21is disposed on the first middle wall2bof the transmission case2. The first brake21includes a cylinder211, a piston212fitted into the cylinder211, and an operation hydraulic chamber213defined by the cylinder211and the piston212. As a predetermined fastening hydraulic pressure is supplied to the operation hydraulic chamber213in the first brake21, a friction plate is fastened to fix the sun gear11sof the first planetary gear set11and the sun gear12sof the second planetary gear set12to the transmission case2.

The second brake22is disposed on the third middle wall2d. The second brake22includes a cylinder23, a piston24fitted into the cylinder23, and a fastening hydraulic chamber26defined by the cylinder23and the piston24. As a predetermined fastening hydraulic pressure is supplied to the fastening hydraulic chamber26in the second brake22, a friction plate is fastened to fix the ring gear14rof the fourth planetary gear set14to the transmission case2. The present embodiment shows an example of applying a friction fastening element including characteristics of the present invention to the second brake22. The second brake22is described below in detail with reference toFIG. 3and the following drawings.

The first to third clutches31to33are disposed at an end part within the transmission case2on the side opposite to the engine side. The first to third clutches31to33are disposed to radially overlap to each other in such a manner that at the axially same position, the second clutch32is placed on an inner peripheral side of the first clutch31and the third clutch33is placed on an inner peripheral side of the second clutch32.

The first clutch31connects and disconnects the sun gear14sof the fourth planetary gear set14to and from the ring gear13rof the third planetary gear set13. That is to say, the first clutch31switches a connection state of the first rotating member34coupled to the sun gear14sand the second rotating member35coupled to the ring gear13r.

The second clutch32connects and disconnects the sun gear14sof the fourth planetary gear set14to and from the integral rotating element15(that is, the ring gear12rof the second planetary gear set12and the sun gear13sof the third planetary gear set13). That is to say, the second clutch32switches a connection state of the first rotating member34coupled to the sun gear14sand the third rotating member36coupled to the integral rotating element15.

The third clutch33connects and disconnects the sun gear14sof the fourth planetary gear set14to and from the input shaft3and the carrier12cof the second planetary gear set12. That is to say, the third clutch33switches a connection state of the first rotating member34coupled to the sun gear14sand the fourth rotating member37coupled via the input shaft3to the carrier12c.

The connection state of the first rotating member34to the second rotating member35is switched by the first clutch31. The connection state of the first rotating member34to the third rotating member36is switched by the second clutch32. The connection state of the first rotating member34to the fourth rotating member37is switched by the third clutch33. That is, the first rotating member34is one rotating member of two rotating members whose connection state is switched by the clutches31to33, which is common to the clutches. For this reason, a common rotating member30including a wall part perpendicular to a shaft center is disposed near the side wall2eof the transmission case2on the side of the first to third clutches31to33opposite to the engine side. The first rotating member34is coupled to the common rotating member30.

The common rotating member30is shared by the first to third clutches31to33. Cylinders, pistons, operation hydraulic chambers, operation hydraulic pressure passages, centrifugal balance hydraulic chambers, centrifugal balance chamber components, and the like included in the clutches31to33are supported by the common rotating member30.FIG. 1schematically shows pistons31p,32p, and33pof the first clutch31, the second clutch32, and the third clutch33, respectively. A common member38that holds friction plates of the second clutch32and the third clutch33is attached to these clutches.

As described above, the automatic transmission1according to the present embodiment includes a transmission mechanism that includes the first to fourth planetary gear sets11to14, and the first and second brakes21and22and the first to third clutches31to33that function as five friction fastening elements. In addition, the transmission mechanism changes the transmission ratio of the input shaft3and the output gear4.FIG. 2is a fastening table of the five friction fastening elements included in the automatic transmission1. As shown in the fastening table ofFIG. 2, three friction fastening elements of the five friction fastening elements are selectively fastened (the symbol ∘ is given to the three friction fastening elements), it is possible to shift to first to eighth forward gears and a back gear. Referring toFIG. 2, CL1, CL2, and CL3denote the first clutch31, the second clutch32, and the third clutch33, respectively. BR1and BR2denote the first brake21and the second brake22, respectively.

FIG. 3shows a schematic cross-section of a configuration of a friction fastening element of the automatic transmission1according to the embodiment of the present invention and a block configuration of a hydraulic mechanism of the friction fastening element. An example of applying the friction fastening element to the second brake22is shown herein. InFIG. 3(andFIGS. 4 to 8below), the axial direction of the input shaft3is an X direction and the radial direction of the automatic transmission1is a Y direction. Regarding the X direction, for convenience, the left side on the drawing is denoted by −X and the right side on the drawing in the X direction is denoted by +X.

The second brake22is, as described above, a friction fastening element that is disposed in the cylinder23formed by the third middle wall2d. The second brake22includes the piston24, a sealing ring25, a fastening hydraulic chamber26, a release hydraulic chamber27, a return spring28, and a friction plate unit5(a plurality of friction plates). A hydraulic mechanism80is provided for the second brake22. The hydraulic mechanism80includes an oil pump81, a hydraulic circuit82including a reducing valve6and a linear solenoid valve7(hydraulic pressure control valve), and a hydraulic pressure control unit83that controls the oil pump81and the hydraulic circuit82.

The third middle wall2dis constituted by a first wall part201extending radially inward from the outer peripheral wall2aof the transmission case2and a second wall part202extending axially (in the −X direction) from a radially inward edge of the first wall part201. The outer peripheral wall2aand the second wall part202radially oppose to each other with a predetermined distance therebetween. The space formed by the outer peripheral wall2a, the first wall part201, and the second wall part202constitutes the space for the above-described cylinder23in the second brake22. The first wall part201is provided with a first supply port203for supplying a hydraulic pressure to the fastening hydraulic chamber26. The second wall part202is provided with a second supply port204for supplying a hydraulic pressure to the release hydraulic chamber27.

The piston24includes a first surface24A and a second surface24B axially opposing to each other, and is capable of axially moving in the space between the outer peripheral wall2aand the second wall part202(in the cylinder23). The first surface24A faces the release hydraulic chamber27and the second surface24B faces the fastening hydraulic chamber26. The piston24is moved between a release position where the friction plate unit5is released (for example, a position shown inFIG. 4) and a fastening position where pressing force is applied to the friction plate unit5to fasten the friction plate unit5(a position shown inFIG. 8).

The piston24includes a pressing piece241adjacent to the outer peripheral wall2aand a pressure-receiving piece242that is slidably in contact with inner peripheral surfaces of the outer peripheral wall2aand the second wall part202. A through-hole243axially passing through the pressure-receiving piece242is formed in the pressure-receiving piece242. A sealing member245is fitted into inner and outer peripheral surfaces of the pressure-receiving piece242.

The pressing piece241projects from the pressure-receiving piece242toward the −X side, and includes a distal end surface24C applying pressing force to the friction plate unit5on a distal end side (the −X side) in its movement direction. The pressure-receiving piece242is a partition wall that defines the fastening hydraulic chamber26and the release hydraulic chamber27. In the present embodiment, the fastening hydraulic chamber26may communicate with the release hydraulic chamber27via the through-hole243. The sealing member245allows the axial movement of the piston24and at the same time, seals the gap between the inner peripheral surface of the pressure-receiving piece242and the inner peripheral surface of the second wall part202and the gap between the outer peripheral surface of the pressure-receiving piece242and the inner peripheral surface of the outer peripheral wall2a.

The through-hole243is a cylindrical hole with different diameters in the axial direction, and includes a large-diameter part w with a relatively large diameter, a small-diameter part n with a relatively small diameter, and a middle part m between the large-diameter part w and the small-diameter part n. The large-diameter part w is disposed near the second surface24B of the pressure-receiving piece242, that is, on a side of the fastening hydraulic chamber26. The small-diameter part n is disposed near the first surface24A, that is, on a side of the release hydraulic chamber27. The middle part m is a tapered part that gradually reduces its inner diameter from the large-diameter part w toward the small-diameter part n.

A pressure ball244(regulation part) is disposed in the through-hole243for the purpose of regulating an oil flow from the fastening hydraulic chamber26to the release hydraulic chamber27. The pressure ball244has an outer diameter that is smaller than an inner diameter of the large-diameter part w and is larger than an inner diameter of the small-diameter part n. If the hydraulic pressure of the release hydraulic chamber27is higher than the hydraulic pressure of the fastening hydraulic chamber26, the pressure ball244floats in the large-diameter part w and does not regulate the oil flow from the fastening hydraulic chamber26to the release hydraulic chamber27. On the other hand, if the hydraulic pressure of the fastening hydraulic chamber26is higher than the hydraulic pressure of the release hydraulic chamber27, the pressure ball244is locked to the middle part m to close the through-hole243.

The sealing ring25is a ring-shaped plate member that is disposed on a side of the first surface24A of the piston24to face the pressure-receiving piece242. The sealing ring25is disposed between the pressing piece241of the piston24and the second wall part202and defines the release hydraulic chamber27with the pressing piece241and the second wall part202. A sealing member251is attached to inner and outer peripheral surfaces of the sealing ring25. The sealing member251seals the gap between an outer peripheral edge of the sealing ring25and the inner peripheral surface of the pressing piece241and the gap between the inner peripheral surface of the sealing ring25and the inner peripheral surface of the second wall part202.

The fastening hydraulic chamber26is the space to which a hydraulic pressure for moving the piston24in a direction toward the fastening position (in the −X direction) is supplied. The fastening hydraulic chamber26is the space defined by the first wall part201, the second wall part202, the outer peripheral wall2a, and the second surface24B of the piston24. That is, the fastening hydraulic chamber26applies the hydraulic pressure to the second surface24B to move the piston24to the fastening position where the friction plate unit5is pressed to be fastened (friction plates are fastened).

The release hydraulic chamber27is the space to which a hydraulic pressure for moving the piston24in a direction toward the release position (in the +X direction) is supplied. The release hydraulic chamber27is the space defined by the second wall part202, the pressing piece241of the piston24, a surface of the sealing ring25on the +X side, and the first surface24A of the piston24. That is, the release hydraulic chamber27applies the hydraulic pressure to the first surface24A to move the piston24to the release position where the friction plate unit5is released. The return spring28that biases the piston24in the +X direction is disposed in the release hydraulic chamber27. When no hydraulic pressure is applied to the fastening hydraulic chamber26, the return spring28moves (returns) the piston24in the +X direction.

The hydraulic-pressure-receiving area of the second surface24B of the piston24is set to be larger than the hydraulic-pressure-receiving area of the first surface24A of the piston24. Referring toFIG. 3, the region on the first surface24A to which a hydraulic pressure from the release hydraulic chamber27is applied, that is, the pressure-receiving area of the first surface24A is schematically shown as “region A”. The region on the second surface24B to which a hydraulic pressure from the fastening hydraulic chamber26is applied, that is, the pressure-receiving area of the second surface24B is schematically shown as “region B”. In the present embodiment, the relationship between these pressure-receiving areas is represented as region B>region A.

There is a difference in the pressure-receiving area between the region A and the region B, and thus the piston24can be moved based on the difference in the pressure-receiving area. That is, when the same hydraulic pressure is simultaneously supplied to the fastening hydraulic chamber26and the release hydraulic chamber27, the first surface24A and the second surface24B receive this hydraulic pressure. In this case, the pressure-receiving area of the second surface24B is larger than the pressure-receiving area of the first surface24A, and thus pressing force in the −X direction acts on the piston24according to the difference in the pressure-receiving area. The through-hole243is formed in the piston24. Consequently, when the pressing force acts in the −X direction, oil in the release hydraulic chamber27flows via the through-hole243into the fastening hydraulic chamber26. The piston24is thus moved in the −X direction. That is, the pressing force in the +X direction, which is received by the first surface24A, and the pressing force in the −X direction, which is received by the second surface24B, are offset, and thus the piston24is moved in the −X direction based on the pressing force corresponding to the difference in the pressure-receiving area.

The friction plate unit5includes a plurality of friction plates disposed with a clearance therebetween, and is disposed on the side of the first surface24A of the piston24. Specifically, the friction plate unit5is configured such that a plurality of drive plates51and a plurality of driven plates52are alternately arranged with a predetermined clearance C therebetween. A facing is applied on both surfaces of the drive plate51. The drive plates51are spline-coupled to a first spline part53. The driven plates52are spline-coupled to a second spline part54. The first spline part53corresponds to the outer peripheral part of the ring gear14rof the fourth planetary gear set14shown inFIG. 1. The second spline part54is provided in a part of the outer peripheral wall2aof the transmission case2.

The distal end surface24C of the piston24abuts against the driven plate52placed closest to the +X side and applies pressing force to the friction plate unit5. A retaining plate55is disposed adjacent to the drive plate51placed closest to the −X side. The retaining plate55regulates the movement of the drive plates51and the driven plates52in the −X direction.

The hydraulic mechanism80supplies a predetermined hydraulic pressure to the friction fastening elements included in the automatic transmission1and discharges the predetermined hydraulic pressure. The oil pump81of the hydraulic mechanism80is driven by the engine to cause oil to flow into required portions and generates a predetermined hydraulic pressure. The hydraulic circuit82selectively supplies a hydraulic pressure to the first brake21, the second brake22, and the first to third clutches31to33that function as the friction fastening elements to shift to the transmission gears shown inFIG. 2.FIG. 3shows only the reducing valve6and the linear solenoid valve7for supplying and discharging a hydraulic pressure to and from the second brake22.

The linear solenoid valve7is a hydraulic pressure control valve that supplies and discharges a hydraulic pressure to and from the fastening hydraulic chamber26and the release hydraulic chamber27. The linear solenoid valve7includes an input port71to which oil is introduced from the oil pump81, an output port72that outputs the oil (the hydraulic pressure), a drain port73that discharges the oil, and a spool (not shown) operated by a coil being energized. When the hydraulic pressure is supplied to the fastening hydraulic chamber26and the release hydraulic chamber27, the spool is operated to allow the input port71to communicate with the output port72. When the oil is discharged, the output port72communicates with the drain port73. As the amount of energization to the coil in the linear solenoid valve7is controlled, the amount of oil discharged from the output port72is controlled.

The hydraulic circuit82includes a first oil passage74for allowing the linear solenoid valve7to communicate with the fastening hydraulic chamber26and a second oil passage75for allowing the linear solenoid valve7to communicate with the release hydraulic chamber27. Specifically, an upstream end of the first oil passage74is connected to the output port72whereas a downstream end thereof is connected to the first supply port203communicating with the fastening hydraulic chamber26. An upstream end of the second oil passage75is connected to the output port72whereas a downstream end thereof is connected to the second supply port204communicating with the release hydraulic chamber27. That is, the first oil passage74and the second oil passage75do not receive oil supplied from different hydraulic pressure supply passages. Instead, the first oil passage74and the second oil passage75receive oil from the output port72of the linear solenoid valve7that is common to the first oil passage74and the second oil passage75.

The second oil passage75is divided into an upstream oil passage751and a downstream oil passage752with the below-described reducing valve6interposed therebetween. When the friction plate unit5is shifted from the released state to the fastened state, a hydraulic pressure is simultaneously supplied from the output port72of the linear solenoid valve7through the first oil passage74and the second oil passage75to the fastening hydraulic chamber26and the release hydraulic chamber27.

The reducing valve6is attached to the second oil passage75and regulates the hydraulic pressure of the release hydraulic chamber27so as not to exceed a predetermined hydraulic pressure. The reducing valve6includes a plurality of ports a, b, c, d, e, and f, and a spool61that switches the ports. The ports a and b communicate with a spring chamber in which a return spring62biasing the spool61in the +X direction is accommodated. The port c is an input port c, and the port d is an output port d. A downstream end of the upstream oil passage751of the second oil passage75is connected to the input port c. An upstream end of the downstream oil passage752is connected to the output port d, and thus the output port d is connected to the second supply port204.

The port e is a drain port e, and the port f is a feedback port f. When the biasing force of the return spring62is larger than the hydraulic pressure input to the feedback port f, the input port c communicates with the output port d. The upstream oil passage751thus communicates with the downstream oil passage752, so that the hydraulic pressure can be supplied to the release hydraulic chamber27. Meanwhile, when the hydraulic pressure exceeding the biasing force of the return spring62is input to the feedback port f, the spool61is moved in the −X direction by the hydraulic pressure and the output port d communicates with the drain port e. The hydraulic pressure can thus be discharged from the release hydraulic chamber27. That is, if the hydraulic pressure of the release hydraulic chamber27is increased, the hydraulic pressure supplied from the feedback port f to the reducing valve6is also increased. The spool61is then operated to allow the output port d to communicate with the drain port e, thus reducing the hydraulic pressure of the release hydraulic chamber27. When the hydraulic pressure is reduced and the biasing force of the return spring62becomes larger, the spool61returns to allow the input port c to communicate with the output port d, and thus the hydraulic pressure can be supplied to the release hydraulic chamber27.

The hydraulic pressure control unit83controls an operation of a solenoid of the linear solenoid valve7, thus controlling the hydraulic pressure supplied to the fastening hydraulic chamber26and the release hydraulic chamber27. In addition, the hydraulic pressure control unit83controls solenoid valves of other friction fastening elements and the like, and also controls the hydraulic pressure supplied to the first brake21and the first to third clutches31to33.

Next, an operation of the second brake22shown inFIG. 3is described with reference to the schematic cross-sectional views shown inFIGS. 4 to 8.FIG. 4shows a wait state where no hydraulic pressure is supplied via the linear solenoid valve7to the fastening hydraulic chamber26and the release hydraulic chamber27. The piston24is pressed in the +X direction by the biasing force of the return spring28without being affected by a hydraulic pressure, thus being located at the release position. The distal end surface24C of the piston24is spaced apart from the friction plate unit5by a predetermined distance, and the drive plates51and the driven plates52in the friction plate unit5are released. As the piston24is moved in the +X direction, the capacity of the fastening hydraulic chamber26is minimized and on the other hand, the capacity of the release hydraulic chamber27is maximized.

FIG. 5shows a state, after the wait state ofFIG. 4, where oil starts to flow into the fastening hydraulic chamber26and the release hydraulic chamber27. The hydraulic pressure control unit83allows the input port71of the linear solenoid valve7to communicate with the output port72thereof, thus executing control such that the oil discharged from the oil pump81flows in the first oil passage74and the second oil passage75. In the reducing valve6, the input port c communicates with the output port d by the biasing force of the return spring62. The oil starts to flow from the output port72of the linear solenoid valve7, which is common to the first oil passage74and the second oil passage75, through the first oil passage74to the fastening hydraulic chamber26and at the same time, to flow from the output port72through the upstream oil passage751, the reducing valve6, and the downstream oil passage752of the second oil passage75to the release hydraulic chamber27. In such a state, the pressing force by a hydraulic pressure does not, of course, act on the piston24. The piston24is thus completely moved in the +X direction by the biasing force of the return spring28.

FIG. 6shows a state where after oil starts to flow as shown inFIG. 5, the fastening hydraulic chamber26and the release hydraulic chamber27are filled with the oil and thus the piston24is moved in the −X direction. When the same hydraulic pressure is applied to the fastening hydraulic chamber26and the release hydraulic chamber27, the piston24is moved based on a difference in the pressure-receiving area between the first surface24A and the second surface24B. As described above, the pressure-receiving area of the second surface24B is larger than the pressure-receiving area of the first surface24A, and thus pressing force D1in the −X direction acts on the piston24based on the difference in the pressure-receiving area. That is, pressing force D1=hydraulic pressure×(area of region B−area of region A). The pressing force D1moves the piston24in the −X direction. The pressing force D1needs to exceed the biasing force of the return spring28in the +X direction. For this reason, the difference in the pressure-receiving area is set in view of the biasing force of the return spring28.

When the piston24is moved in the −X direction, the hydraulic pressure of the release hydraulic chamber27is increased. In addition, the release hydraulic chamber27has large capacity when the piston24is moved in the +X direction. Consequently, a large amount of oil is present in the release hydraulic chamber27. As shown by an arrow D11inFIG. 6, the oil in the release hydraulic chamber27flows via the through-hole243into the fastening hydraulic chamber26. The oil may flow reversely in the second oil passage75, as shown by an arrow D12, depending on the hydraulic pressure of the release hydraulic chamber27.

As described above, the fastening hydraulic chamber26receives the oil from the release hydraulic chamber27and thus it requires only a small amount of oil to be supplied through the first oil passage74to the fastening hydraulic chamber26. That is, it is only required that a flow rate of oil for generating the pressing force D1based on the difference in the pressure-receiving area is supplied via the linear solenoid valve7. The hydraulic pressure response in moving the piston24in the −X direction is thus improved. In response to the movement of the piston24, the distal end surface24C approaches the friction plate unit5and the return spring28is gradually compressed.

FIG. 7shows a state where a stroke of the piston24is completed. In such a state, the piston24is moved in the −X direction and thus the distal end surface24C reaches a position (fastening position) of abutting against the friction plate unit5(the driven plate52). Even in this state, as described inFIG. 6, only the pressing force D1based on the difference in the pressure-receiving area acts on the second surface24B, and oil flows as shown by the arrows D11and D12.

When the distal end surface24C abuts against the friction plate unit5and the piston24presses the friction plate unit5, a clearance between the drive plate51and the driven plate52is reduced and friction fastening force is generated therebetween. At this time, only the pressing force D1still contributes to pressing. In initial fastening, the drive plate51and the driven plate52are fastened at a low fastening pressure. This contributes to a reduction in fastening shock in the friction plate unit5.

FIG. 8shows a state where the friction plate unit5is fastened at a predetermined fastening pressure. In such a state, the hydraulic pressure control unit83controls the linear solenoid valve7to supply a predetermined fastening hydraulic pressure (a line pressure) from the output port72. The fastening hydraulic pressure is thus applied through the first oil passage74and the second oil passage75to the fastening hydraulic chamber26and the release hydraulic chamber27, respectively. However, the reducing valve6is operated to regulate the hydraulic pressure of the release hydraulic chamber27not so as to exceed a predetermined pressure (a predetermined hydraulic pressure lower than the fastening hydraulic pressure). That is, when the hydraulic pressure of the release hydraulic chamber27is increased and the hydraulic pressure exceeding the biasing force of the return spring62is input to the feedback port f of the reducing valve6, the spool61is moved in the −X direction by the hydraulic pressure and the output port d communicates with the drain port e. The hydraulic pressure of the release hydraulic chamber27is thus kept at a constant pressure. Consequently, only the hydraulic pressure of the fastening hydraulic chamber26is increased.

As the hydraulic pressure of the fastening hydraulic chamber26is increased, the pressure ball244is moved in the −X direction to close the through-hole243. The movement of oil from the fastening hydraulic chamber26to the release hydraulic chamber27is regulated. Large pressing force D2in the −X direction thus acts on the piston24based on the fastening hydraulic pressure. Pressing force D2=fastening hydraulic pressure×area of region B. That is, the pressing force D2corresponding to the fastening hydraulic pressure is applied to the overall second surface24B and thus the piston24is pressed in the −X direction. The pressing force D2is applied via the distal end surface24C to the friction plate unit5. The friction plate unit5is thus fastened at a predetermined brake fastening pressure.

[Change in Fastening Pressure Before and after Pressure Regulation]

FIG. 9is a graph showing a relationship between a hydraulic pressure applied to the fastening hydraulic chamber26and the release hydraulic chamber27and a change in the brake fastening pressure of the second brake22. In the brake fastening pressure on the vertical axis, a level L1indicates a level at which the reducing valve6starts to be operated and a level L2indicates a level at which a predetermined fastening hydraulic pressure is applied. Further referring toFIG. 9, a solid line C1indicates a fastening pressure change characteristic according to the present embodiment and a dot line C2indicates a fastening pressure change characteristic according to a comparative example.

At a time point T1, no oil is filled in the fastening hydraulic chamber26and the release hydraulic chamber27. The time point T1corresponds to the wait state described with reference toFIG. 4or the state ofFIG. 5where oil starts to flow in the fastening hydraulic chamber26and the release hydraulic chamber27. As shown inFIGS. 4 and 5, the piston24is pressed in the +X direction by the biasing force of the return spring28, thus being located at the release position.

The time point T2is a timing when the oil is filled in the fastening hydraulic chamber26and the release hydraulic chamber27and the piston24starts to be moved in the −X direction based on a difference in the pressure-receiving area between the first surface24A and the second surface24B. A time point T3is a timing when the piston24is completely moved in the −X direction, the distal end surface24C abuts against the friction plate unit5, and the clearance C between the plates51and52is reduced (a so-called zero-touch state).FIG. 6shows the state of the piston24between the time point T2and the time point T3. During the period between the time point T2and the time point T3, the piston24does not abut against the friction plate unit5. A brake fastening pressure is thus not generated.

After the time point T3, the brake fastening pressure is generated. In the present embodiment, however, in initial fastening after the time point T3, the brake fastening pressure is not increased sharply. This is because, as shown inFIG. 7, after a stroke of the piston24is completed, the piston24is still moved, for a while, based on the difference in the pressure-receiving area between the first surface24A and the second surface24B and thus only small pressing force D1is applied to the friction plate unit5.

A time point T4is a timing when the hydraulic pressure of the fastening hydraulic chamber26and the release hydraulic chamber27is higher than a predetermined pressure and the reducing valve6starts to be operated (brake fastening pressure=L1). As described with reference toFIG. 8, when the reducing valve6is operated, the hydraulic pressure of the release hydraulic chamber27is regulated to a constant pressure and thus large pressing force D2in the −X direction acts on the piston24. Consequently, the friction plate unit5also receives large pressing force and thus the brake fastening pressure is increased greatly.

As described above, during the period from the time point T3when the friction plate unit5is fastened to the time point T4when the reducing valve6is operated, the piston24is moved based on the difference in the pressure-receiving area between the first surface24A and the second surface24B. After the time point T4, the piston24is moved by the increased pressure of the fastening hydraulic chamber26. As shown by the solid line C1, the brake fastening pressure is increased moderately from the time point T3to the time point T4, and is then increased sharply after the time point T4. Thus, a fastening shock in initial fastening of the friction plate unit5is reduced. Meanwhile, if it depends only on the increased pressure of the fastening hydraulic chamber26in the initial fastening, as shown by the dot line C2, the brake fastening pressure is increased sharply after the time point T3. Thus, a relatively large fastening shock may be generated.

The friction fastening element or the automatic transmission1according to the present embodiment as described above achieves the following operations and effects. The automatic transmission1includes the linear solenoid valve7shared by the fastening hydraulic chamber26and the release hydraulic chamber27. Specifically, the automatic transmission1includes the first oil passage74for allowing the output port72of the linear solenoid valve7to communicate with the fastening hydraulic chamber26and the second oil passage75for allowing the output port72to communicate with the release hydraulic chamber27. When the friction plate unit5is shifted from the released state to the fastened state, a hydraulic pressure is simultaneously supplied from the output port72through the first oil passage74and the second oil passage75to the fastening hydraulic chamber26and the release hydraulic chamber27.

In such a configuration, the hydraulic-pressure-receiving area of the second surface24B of the piston24is set to be larger than the hydraulic-pressure-receiving area of the first surface24A of the piston24. For this reason, the hydraulic pressure applied on the first surface24A from the side of the release hydraulic chamber27and the hydraulic pressure applied on the second surface24B from the side of the fastening hydraulic chamber26are offset. The piston24is then moved in a fastening direction by the pressing force D1based on a difference in the pressure-receiving area, the difference being generated because the pressure-receiving area of the second surface24B is larger than the pressure-receiving area of the first surface24A. When the friction plate unit5is shifted from the release position to the fastening position, the piston24is moved by weak pressing force D1corresponding to the difference in the pressure-receiving area. It is thus possible to reduce a fastening shock of the friction plate unit5. In addition, no complicated hydraulic pressure control is necessary to reduce the fastening shock. That is, it is possible to avoid control of reducing the flow rate of oil immediately before a stroke of a piston is completed. It is thus possible to reduce a fastening control time.

As the piston24includes the through-hole243for allowing the fastening hydraulic chamber26to communicate with the release hydraulic chamber27, oil flows via the through-hole243into the fastening hydraulic chamber26if the pressure of the release hydraulic chamber27is increased. Consequently, when the piston24is moved in the fastening direction, the fastening hydraulic chamber26can receive the oil supplied from the release hydraulic chamber27. It thus requires only a small amount of oil to be supplied through the first oil passage74to the fastening hydraulic chamber26. That is, it is only required that an amount of oil for generating the pressing force D1based on the difference in the pressure-receiving area is supplied via the linear solenoid valve7to the fastening hydraulic chamber26.

The piston24can be moved with a small amount of oil, and thus it is possible to improve response when the friction plate unit5is fastened. This is effective in a case of increasing the clearance C between the drive plate51and the driven plate52for the purpose of reducing so-called drag resistance of the friction plate unit5. That is, if the increased clearance C requires an increased amount of movement of the piston24needed for friction fastening, it requires only a relatively small amount of oil to be flown from the first oil passage74into the fastening hydraulic chamber26. The response of friction fastening is not degraded. As a result, it is possible to reduce the drag resistance and improve the response of friction fastening.

The pressure ball244, which regulates an oil flow from the fastening hydraulic chamber26to the release hydraulic chamber27, is disposed in the through-hole243. The pressure ball244closes the through-hole243if necessary to prevent a reverse oil flow. It is thus possible to separate the fastening hydraulic chamber26from the release hydraulic chamber27in the standpoint of a pressure, and to apply large pressing force D2to the piston24toward the fastening direction.

The second oil passage75includes the reducing valve6that prevents the hydraulic pressure of the release hydraulic chamber27from exceeding a predetermined hydraulic pressure. As the reducing valve6regulates the hydraulic pressure of the release hydraulic chamber27, the piston24can be smoothly moved in the fastening direction. Specifically, after the piston24abuts against the friction plate unit5and the clearance C between the plates51and52is reduced, a predetermined fastening hydraulic pressure is supplied through the first oil passage74to the fastening hydraulic chamber26. Meanwhile, the hydraulic pressure of the release hydraulic chamber27is regulated by the reducing valve6. As a result, the piston24can be quickly moved to the fastening position.

In addition, the automatic transmission1includes the linear solenoid valve7as a hydraulic pressure control valve for the fastening hydraulic chamber26and the release hydraulic chamber27. It is thus possible to regulate the amount of oil supplied based on the amount of energization to a solenoid coil included in the linear solenoid valve7, thus executing hydraulic pressure control with high precision.

First Modified Embodiment

The embodiment described above shows the example of applying a friction fastening element including characteristics of the present invention to the second brake22. The present invention is applicable not only to a brake for an automatic transmission but also to a clutch for an automatic transmission.FIG. 10schematically shows an example of applying the present invention to the first clutch31, which is one of the friction fastening elements included in the automatic transmission1.

The first clutch31includes a drum91, a piston92, a sealing ring93, a fastening hydraulic chamber94, and a release hydraulic chamber95. The first modified embodiment is similar to the above embodiment in that the first clutch31fastens and releases the friction plate unit5, and the reducing valve6and the linear solenoid valve7are used as a hydraulic pressure mechanism for the first clutch31.

The drum91is rotatably supported about an axis of a center shaft of the automatic transmission1by the transmission case2. The drum91includes a disk part910extending in the Y direction, an outer cylinder part911having a larger diameter than the disk part910and extending from a radially outward edge of the disk part910in the −X direction, and an inner cylinder part912coaxially disposed inside the outer cylinder part911. The inner cylinder part912has a first supply port913and a second supply port914for supplying a hydraulic pressure.

The piston92corresponds to the piston31pshown inFIG. 1, and includes a pressure-receiving part921, a small cylinder part922, and a large cylinder part923. The pressure-receiving part921includes a first surface92A and a second surface92B that receive a hydraulic pressure. The first surface92A is placed on a side of the friction plate unit5and the second surface92B is placed on the opposite side to the first surface92A. In addition, the pressure-receiving part921includes a through-hole924axially passing through the pressure-receiving part921, and a pressure ball925is disposed in the through-hole924. An inner cylinder part926projects from a radially inward edge of the pressure-receiving part921in the −X direction. A third supply port927communicating with the second supply port914is formed in the inner cylinder part926. An −X side edge of the large cylinder part923presses the friction plate unit5. A sealing ring93is disposed between the piston92and the friction plate unit5and closes an opening between the large cylinder part923and the inner cylinder part926.

The fastening hydraulic chamber94(the operation hydraulic chamber described above) is a space between the pressure-receiving part921of the piston92(on the side of the second surface92B) and the disk part910of the drum91, and receives a hydraulic pressure from the first oil passage74via the first supply port913. The release hydraulic chamber95(the centrifugal balance hydraulic chamber described above) is a space defined by the pressure-receiving part921of the piston92(on the side of the first surface92A), the small cylinder part922, the large cylinder part923, and the sealing ring93, and receives a hydraulic pressure from the second oil passage75via the second supply port914and the third supply port927. A return spring96that biases the piston92in the +X direction is disposed in the release hydraulic chamber95. When the friction plate unit5is shifted from the released state to the fastened state, a hydraulic pressure is simultaneously supplied from the output port72of the linear solenoid valve7through the first oil passage74and the second oil passage75to the fastening hydraulic chamber94and the release hydraulic chamber95.

The first surface92A of the piston92receives a hydraulic pressure from the release hydraulic chamber95and the second surface92B receives a hydraulic pressure from the fastening hydraulic chamber94. In this case, the hydraulic-pressure-receiving area of the second surface92B is set to be larger than the hydraulic-pressure-receiving area of the first surface92A. The piston92includes the small cylinder part922and the large cylinder part923that are successively connected in the −X direction in the pressure-receiving part921. The release hydraulic chamber95thus includes a small capacity part95A on the +X side (within the small cylinder part922) and a large capacity part95B on the −X side (within the large cylinder part923). In a clutch, the release hydraulic chamber95is required to have a function of canceling out a centrifugal hydraulic pressure of the fastening hydraulic chamber94.

An operation of the first clutch31configured as described above is similar to the operation of the second brake22described in the above embodiment. When a hydraulic pressure is supplied to the fastening hydraulic chamber94and the release hydraulic chamber95, the piston92is moved in the −X direction (fastening direction) by relatively small pressing force based on a difference in the pressure-receiving area between the first surface92A and the second surface92B. In initial fastening, the piston92continues to be moved for a certain period of time, based on the difference in the pressure-receiving area. When the reducing valve6starts to be operated, the second surface92B receives large pressing force.

In the case of the first clutch31, the vertical axis shown inFIG. 9indicates a transmission torque of the clutch. During the period from the time point T3in a zero-touch state to the time point T4when the reducing valve6starts to be operated, the friction plate unit5is fastened with a small torque. It is thus possible to reduce a fastening shock. At the time point T4, the pressure of the fastening hydraulic chamber94is increased and thus large pressing force is applied to the piston92, so that the transmission torque is also increased.

Second Modified Embodiment

The embodiment described above shows the example in which the hydraulic circuit82includes the reducing valve6that prevents the hydraulic pressure of the release hydraulic chamber27from exceeding a predetermined hydraulic pressure. A second modified embodiment shows an example in which the hydraulic circuit82further includes a predetermined-hydraulic-pressure change unit capable of changing the predetermined hydraulic pressure of the reducing valve6.FIG. 11shows a schematic cross-section of a configuration of the second brake22according to the second modified embodiment and a block configuration of a hydraulic mechanism of the second brake22. The second brake22according to the second modified embodiment is different from the second brake22shown inFIG. 3in that a hydraulic circuit82A includes a predetermined-hydraulic-pressure control valve76(predetermined-hydraulic-pressure change unit) that changes conditions for the reducing valve6to regulate the pressure of the release hydraulic chamber27. Other portions are the same as in the embodiment described above, and thus descriptions thereof are appropriately omitted.

As described above, the reducing valve6includes a plurality of ports a, b, c, d, e, and f, and the spool61(valve body) that switches the ports. The input port c (first port) communicates with the output port72of the linear solenoid valve7, and the output port d (second port) communicates with the release hydraulic chamber27. The drain port e (third port) discharges the hydraulic pressure of the release hydraulic chamber27via the output port d. An output hydraulic pressure supplied from the output port d to the release hydraulic chamber27is applied to the feedback port f (fourth port). In addition, the feedback port f is used for moving the spool61in the −X direction (first direction) for the purpose of reducing the output hydraulic pressure.

The port a and the port b (fifth port) communicate with a spring chamber having the return spring62accommodated therein. The feedback port f is disposed at an end part of the spool61on the +X side, and the ports a and b opposing the feedback port f are disposed at an end part of the spool61on the −X side. In the present modified embodiment, the port a is closed, and a regulation hydraulic pressure for changing the operation hydraulic pressure of the reducing valve6is applied to the port b. That is, it is possible to change the predetermined hydraulic pressure at which the reducing valve6starts the pressure regulation operation, and thus it is possible to apply a regulation hydraulic pressure via the predetermined-hydraulic-pressure control valve76to the port b. When the regulation hydraulic pressure is applied via the port b to the spring chamber, force for moving the spool61in the +X direction (second direction) acts on the spool61, against the output hydraulic pressure applied to the feedback port f.

The predetermined-hydraulic-pressure control valve76is a linear solenoid valve, and includes an input port761in which oil is introduced from the oil pump81, an output port762(change output port) that outputs a hydraulic pressure, a drain port763that discharges oil, and a spool (not shown) that is operated by energizing a coil. An oil passage764allows the output port762to communicate with the port b of the reducing valve6. The predetermined-hydraulic-pressure control valve76is a normally closed type solenoid valve in which the output port762is always closed, that is, when the coil is not energized, the spool closes the output port762.

When a regulation hydraulic pressure is applied from the predetermined-hydraulic-pressure control valve76to the spring chamber of the reducing valve6, the input port761communicates with the output port762by an operation of the spool performed by energizing the coil. As the amount of energization to the coil is controlled, the regulation hydraulic pressure output from the output port762can be controlled. When the regulation hydraulic pressure is discharged, the output port762communicates with the drain port763.

In the reducing valve6, when the biasing force of the return spring62is larger than the output hydraulic pressure input from the output port d to the feedback port f, the input port c communicates with the output port d and the output hydraulic pressure is supplied to the release hydraulic chamber27. Meanwhile, when the output hydraulic pressure exceeding the biasing force of the return spring62is input to the feedback port f, the spool61is moved in the −X direction by the hydraulic pressure and the output port d communicates with the drain port e. The operation when no regulation hydraulic pressure is supplied from the predetermined-hydraulic-pressure control valve76to the port b has been described above, and this operation is similar to that of the above embodiment.

Meanwhile, when the regulation hydraulic pressure is supplied to the port b, the force for moving the spool61in the +X direction is increased. That is, the operation hydraulic pressure of the reducing valve6is changed. In this case, unless an output hydraulic pressure higher than the output hydraulic pressure before the regulation hydraulic pressure is supplied is input to the feedback port f, the spool61is not moved in the −X direction. If the operation of supplying a regulation hydraulic pressure is performed, for example, after the clearance C between the plates51and52is reduced (FIG. 7), it is possible to reduce a speed at which the piston24is moved from a zero clearance position to a fastening position.

The operation hydraulic pressure of the reducing valve6may be changed depending only on a regulation hydraulic pressure supplied from the port b to the hydraulic chamber corresponding to the spring chamber, without using the return spring62in the reducing valve6. In this case, it is possible to extend the range of accelerating or decelerating the movement speed of the piston24.

According to the second modified embodiment, the regulation hydraulic pressure is supplied from the port b, and thus conditions for the reducing valve6to regulate the pressure of the release hydraulic chamber27can be changed. It is thus possible to control the operation of moving the piston24to the fastening position. The reducing valve6includes the port b to which a regulation hydraulic pressure is applied, for moving the spool61in the +X direction. The port b opposes the feedback port f for moving the spool61in the −X direction. As the regulation hydraulic pressure supplied from the port b is changed, conditions for the reducing valve6to regulate the pressure of the release hydraulic chamber27can be changed easily.

The predetermined-hydraulic-pressure control valve76is a normally closed type linear solenoid valve. For this reason, the regulation hydraulic pressure is not always applied to the port b. The regulation hydraulic pressure is applied to the port b only when necessary. It is thus possible to achieve a fail-safe function that allows the reducing valve6to be operated normally even if the predetermined-hydraulic-pressure control valve76fails, and thus it is possible to perform a friction fastening operation.

As described above, the automatic transmission1according to the present embodiments moves the piston24,92based on a difference in the pressure-receiving area between the first surface24A,92A of the piston24,92and the second surface24B,92B of the piston24,92. It is thus possible to reduce a fastening shock without requiring complicated hydraulic pressure control, and to shorten a fastening control time.

The embodiments described above exemplify a planetary gear automatic transmission. The present invention is applicable to other automatic transmissions such as a continuously variable transmission (CVT) and a dual clutch transmission (DCT).

Finally, characteristic configurations disclosed in the embodiments described above, and operations and effects based on such configurations are described.

An automatic transmission according to an aspect of the present invention includes a piston that has a first surface and a second surface opposing to each other in an axial direction and that is movable in the axial direction, a plurality of friction plates disposed on a side of the first surface of the piston, a fastening hydraulic chamber that applies a hydraulic pressure to the second surface of the piston to move the piston to a fastening position where the friction plates are pressed to be fastened to each other, a release hydraulic chamber that applies a hydraulic pressure to the first surface of the piston to move the piston to a release position where the friction plates are released, a hydraulic pressure control valve that has an output port of the hydraulic pressure, and supplies and discharges the hydraulic pressure to and from each of the fastening hydraulic chamber and the release hydraulic chamber, a first oil passage that allows the output port of the hydraulic pressure control valve to communicate with the fastening hydraulic chamber, and a second oil passage that allows the output port to communicate with the release hydraulic chamber. An area of the second surface of the piston for receiving the hydraulic pressure is set to be larger than an area of the first surface for receiving the hydraulic pressure.

According to the automatic transmission, a hydraulic pressure is supplied from the output port of the hydraulic pressure control valve through the first oil passage and the second oil passage to the fastening hydraulic chamber and the release hydraulic chamber. There is a difference in the pressure-receiving area between the first surface and the second surface of the piston. Accordingly, the hydraulic pressure applied on the first surface from the side of the release hydraulic chamber and the hydraulic pressure applied on the second surface from the side of the fastening hydraulic chamber are offset. The piston can be moved in a fastening direction by pressing force based on a difference in the pressure-receiving area, the difference being generated because the pressure-receiving area of the second surface is larger than the pressure-receiving area of the first surface. When the friction plates are shifted from the released state to the fastened state, the piston is moved by the pressing force corresponding to the difference in the pressure-receiving area. It is thus possible to reduce a fastening shock without requiring complicated hydraulic pressure control. In addition, it is possible to avoid control of reducing the flow rate of oil immediately before the stroke of the piston is completed for the purpose of reducing the fastening shock. It is thus possible to shorten a fastening control time.

In the automatic transmission described above, the piston preferably includes a through-hole that allows the fastening hydraulic chamber to communicate with the release hydraulic chamber.

According to the automatic transmission, because the piston includes the through-hole, oil flows via the through-hole into the fastening hydraulic chamber when the pressure of the release hydraulic chamber is increased. When the piston is moved in the fastening direction, the fastening hydraulic chamber can receive the oil supplied from the release hydraulic chamber. It thus requires only a small amount of oil to be supplied through the first oil passage to the fastening hydraulic chamber. It is thus possible to improve response when the friction plates are fastened to each other. Even when a clearance between the friction plates is increased for reducing so-called drag resistance, that is, even when the amount of movement of the piston required for friction fastening is increased, the amount of oil to be flown through the first oil passage into the fastening hydraulic chamber can be relatively small. It is thus possible to reduce the drag resistance and improve the response of friction fastening.

In the automatic transmission described above, a regulation part that regulates an oil flow from the fastening hydraulic chamber to the release hydraulic chamber is preferably provided in the through-hole.

According to the automatic transmission, the through-hole is closed when necessary to prevent a reverse oil flow, and a hydraulic pressure can be effectively received by the second surface.

In the automatic transmission described above, the second oil passage preferably includes a reducing valve that prevents the hydraulic pressure of the release hydraulic chamber from exceeding a predetermined hydraulic pressure.

According to the automatic transmission, by regulating the hydraulic pressure of the release hydraulic chamber using the reducing valve, it is possible to smoothly move the piston in the fastening direction. For example, after the piston abuts against the friction plates and the clearance between the friction plates is reduced, a predetermined fastening hydraulic pressure is supplied through the first oil passage to the fastening hydraulic chamber, while the hydraulic pressure of the release hydraulic chamber is regulated by the reducing valve. As a result, the piston can be quickly moved to the fastening position. Further, as the hydraulic pressure of the release hydraulic chamber is lower than that of the fastening hydraulic chamber, it is possible to ensure fastening force in friction fastening (force in the fastening direction of the piston).

Preferably, the automatic transmission described above further includes a predetermined-hydraulic-pressure change unit that changes the predetermined hydraulic pressure.

By changing the predetermined hydraulic pressure, it is possible to change conditions for the reducing valve to regulate the pressure of the release hydraulic chamber. As the conditions for regulating the pressure of the release hydraulic pressure are controlled, for example, the operation of moving the piston to the fastening position can be controlled.

In the automatic transmission described above, the reducing valve preferably includes a plurality of ports and a valve body that switches the plurality of ports. The plurality of ports preferably include a first port that communicates with the output port of the hydraulic pressure control valve, a second port that communicates with the release hydraulic chamber, a third port that discharges a hydraulic pressure of the release hydraulic chamber via the second port, a fourth port to which an output hydraulic pressure from the second port is applied and that moves the valve body in a first direction so as to reduce the output hydraulic pressure, and a fifth port that moves the valve body in a second direction opposite to the first direction. A regulation hydraulic pressure is preferably applied to the fifth port, and the predetermined-hydraulic-pressure change unit preferably changes the regulation hydraulic pressure so as to change the predetermined hydraulic pressure.

According to the automatic transmission, the reducing valve includes the fifth port functioning as a feedback port of the output hydraulic pressure. The fifth port is a port to which a regulation hydraulic pressure is applied, for moving the valve body in the second direction opposite to the first direction. The fifth port opposes the fourth port for moving the valve body in the first direction. Consequently, by changing the regulation hydraulic pressure, the conditions for the reducing valve to regulate the pressure of the release hydraulic chamber can be changed easily.

In this case, the predetermined-hydraulic-pressure change unit is preferably a predetermined-hydraulic-pressure control valve that includes a change output port communicating with the fifth port and controls the regulation hydraulic pressure, and the predetermined-hydraulic-pressure control valve is preferably a normally closed type solenoid valve in which the change output port is always closed.

According to the automatic transmission, the regulation hydraulic pressure is not always applied to the fifth port. The regulation hydraulic pressure is applied to the fifth port only when necessary. It is thus possible to allow the reducing valve to be operated normally even if the predetermined-hydraulic-pressure control valve fails, and thus it is possible to perform a friction fastening operation.

In the automatic transmission described above, the hydraulic pressure control valve is preferably constituted by a linear solenoid valve. It is thus possible to regulate the amount of oil supplied based on the amount of energization to a solenoid coil, thus executing hydraulic pressure control with high precision.

According to the present invention described above, the piston is moved based on a difference in the pressure-receiving area between the first surface and the second surface of the piston. It is thus possible to provide an automatic transmission including a friction fastening element that can reduce a fastening shock without requiring complicated hydraulic pressure control and can shorten a fastening control time.