Shock control arrangement in hydraulic control system of automatic power transmission

What is claimed is:

1. In an automatic power transmission having an automatic forward drive range condition, a reverse drive gear condition and first, second and third gear ratios in the automatic forward drive range condition and including a transmission mechanism having incorporated therein a fluid operated first frictional unit contributive to the selection of the reverse drive gear condition and the shifting from the second gear ratio to the third gear ratio in the automatic forward drive range, a fluid operated second frictional unit contributive to the shifting from the first gear ratio to the second gear ratio in the automatic forward drive range condition and a fluid operated third frictional unit contributive to the selection of the automatic forward drive range condition, and a hydraulic control system including control fluid pressure generating means for producing a control fluid pressure throughout the automatic forward drive range and reverse drive range conditions, a shock control arrangement for reducing shocks to be produced during selection of any of the automatic forward drive range and reverse drive gear conditions or upshifting between any two of the first, second and third gear ratios in the automatic forward drive range condition, comprising

a pressure accumulator unit operatively intervening between the control fluid generating means and each of the first, second and third frictional units and including a fluid operated piston having a first pressure acting area to be acted upon by a fluid pressure and urge the piston to move in a first direction when the first frictional unit is to be actuated, a second pressure acting area to be acted upon by a fluid pressure and urge the piston to move in a second direction opposite to the first direction when the second frictional unit is to be actuated and a third pressure acting area to be acted upon by a fluid pressure and urge the piston to move in the first direction when the third frictional unit is to be actuated, and biasing means for constantly urging the piston to move in the second direction;

passageway means for discharging the fluid pressure to act on the second pressure acting area;

valve means operatively intervening between the second frictional unit and the control pressure generating means and responsive to the fluid pressure to actuate the first frictional unit and act on the first pressure acting area, said valve means including a valve element which is movable into and out of a limit position providing communication between the second frictional unit and the control pressure generating means and closing said passageway means and which is responsive to the fluid pressure to actuate the first frictional unit for being urged to move away from said limit position thereof, and biasing means for constantly urging the valve element toward the limit position thereof.

2. A shock control arrangement as set forth in claim 1, in which said hydraulic control system further includes first circuit means for providing communication between said control pressure generating means and said first frictional unit, second circuit means for providing communication between said control pressure generating means and said second frictional unit and third circuit means for providing communication between said control pressure generating means and said third frictional unit, said shock control arrangement further comprising a first fluid passageway constantly communicating with said first circuit means and open to said first pressure acting area, a second fluid passageway constantly communicating with said second frictional unit and open to said second pressure acting area, the second fluid passageway forming part of said passageway means communicable with said control pressure generating means across said valve means and through said second circuit means, a third fluid passageway constantly communicating with said third circuit means and open to said third pressure acting area, and a fourth passageway providing constant communication between said first conduit means and said valve means.

3. A shock control arrangement as set forth in claim 2, in which said valve element has a pressure acting area and in which said fourth passageway is constantly open to said pressure acting area of the valve element for urging the valve element away from said limit position thereof when a fluid pressure is developed in the fourth passageway.

4. A shock control arrangement as set forth in claim 2, in which said valve means comprises, in addition to said valve element as a first valve element, a second valve element arranged in series with said first valve element and having a pressure acting area, the second valve element being movable into and out of a limit position and engageable at one end with said first valve element when moved out of the limit position thereof, the biasing means of said valve means being operative to urge both of the first and second valve elements toward the respective limit positions thereof.

5. A shock control arrangement as set forth in claim 4, in which said first valve element has a first pressure acting area to be acted upon by fluid pressure in said second fluid passageway and urge the first valve element to move away from the limit position thereof when a fluid pressure is developed in the second fluid passageway, and a second pressure acting area to be acted upon by fluid pressure in said second circuit means and urge the first valve element to move toward the limit position thereof when a fluid pressure is developed in the second circuit means.

6. A shock control arrangement as set forth in claim 5, in which said valve means comprises a fluid inlet port constantly communicating with said second circuit means, a fluid outlet port constantly communicating with said second passageway and said second frictional unit and a drain port and in which said first valve element comprises first and second lands forming said first and second pressure actings areas, respectively, of the first valve element and arranged to allow said fluid outlet port selectively to the fluid inlet port or said drain port depending upon the position of said first valve element and to provide communication between the fluid inlet and outlet ports when the first valve element is in said limit position thereof, said first and second lands being further arranged so that one of the lands is on the point of one of the fluid inlet and drain ports when the other of the lands is on the point of opening up the other of the ports.

7. A hydraulic control system for an automatic power transmission having a first frictional unit and a second frictional unit, the control system comprising:

a servo motor for the first frictional unit;

a servo motor for the second frictional unit including an apply-side chamber, a release-side chamber and a pressure responsive piston which takes a brake apply position when said apply-side chamber is pressurized with said release-side chamber being drained and takes a brake release position when said release-side chamber is pressurized;

a source of fluid pressure adapted to be used for operating the first and second frictional units;

a single pressure accumulator unit including a bore, a fluid operated piston movably disposed in said bore to form within said bore a first chamber and a second chamber, said fluid operated piston having a small surface area exposed to said first chamber and a large surface area exposed to said second chamber and movable in one direction to increase the volume of said first chamber and to contract the volume of said second chamber and movable in the opposite direction to increase the volume of said second chamber and to contract the volume of said first chamber, said pressure accumulator unit including biasing means disposed in said second chamber for biasing said piston in a direction to maximize the volume of said second chamber;

a first passageway communicating with said servo motor for the first frictional unit, said release-side chamber of said servo motor for the second frictional unit and said first chamber of said pressure accumulator unit;

a second passageway communicating with said apply-side chamber of said servo motor for the second frictional unit;

a valve including a pressure control chamber communicating with said second chamber of said pressure accumulator unit, an inlet port communicating with said second passageway and a drain port;

said valve including a valve element movable to a predetermined position wherein said inlet port is uncovered to open into said pressure control chamber and said drain port is covered so that the pressure in said second passageway is modulated to a level lower than the maximum pressure which prevails therein;

said valve including first means for urging said valve element, in one direction, toward said predetermined position, and second means for urging said valve element, in an opposite direction to said one direction, in response to pressure within said first passageway.

8. A hydraulic control system as claimed in claim 7, in which said valve includes third means for urging said valve element in said opposite direction in response to pressure within said control chamber.

9. A hydraulic control system as claimed in claim 8, in which said second urging means of said valve includes a differential area piston having a large surface area exposed to the pressure within said first passageway and a small surface area exposed to the pressure within said pressure control chamber of said valve.

10. A hydraulic control system as claimed in claim 9, in which said first urging means of said valve includes a chamber, other than said control chamber, communicating with said inlet port and a spring disposed therein.

11. A hydraulic control system as claimed in claim 7, further comprising:

a third frictional unit having a servo-motor;

means defining a third chamber in said pressure accumulator intermediate of said first and second chamber;

a third passageway providing fluid communication between said servo-motor of said third frictional unit and said third chamber;

the arrangement of the foregoing being such that upon introduction of pressurized hydraulic fluid into said third passageway, said piston of said accumulator unit is moved against the bias of said biasing means in said second chamber.

Description Submit all comments and votes

FIELD OF THE INVENTION

The present invention relates to an automotive automatic power transmission having a transmission mechanism to be operated by a hydraulic control system and, more particularly, to a shock control arrangement to be incorporated into the hydraulic control system of an automatic power transmission for reducing mechanical shocks to be produced during shifting to predetermined gear positions and between predetermined automatic forward drive gear ratios.

BACKGROUND OF THE INVENTION

In an automatic power transmission for an automotive vehicle, the gear ratios in the forward and reverse drive ranges of the transmission are selected by selectively actuating fluid operated frictional units including, for example, a high-and-reverse clutch, a forward drive clutch, a brake band and a low-and-reverse brake which are all operated by fluid pressure delivered from a hydraulic control system. In a known power transmission using these frictional units, the forward drive clutch in particular is maintained coupled throughout the conditions in which an automatic forward drive range is established in the transmission. When forward drive clutch alone is permitted to be operative, the first or "low" gear ratio in the automatic forward drive range is established in the transmission mechanism. If the brake band is applied with the forward drive clutch maintained in the coupled condition, then a shift is made in the transmission mechanism from the first gear ratio to the second gear ratio in the automatic forward drive range. If, furthermore, the brake band is released and the high-and-reverse clutch in turn is made operative with the forward drive clutch held coupled, there is produced in the transmission mechanism an upshift from the second gear ratio to the third or "high" gear ratio in the automatic forward drive range. When, on the other hand, both of the high-and-reverse clutch and the low-and-reverse brake are made operative, a reverse drive gear position is selected in the transmission mechanism.

Each of the frictional units thus operative is actuated to couple or apply by a control fluid pressure, unusally called line pressure, developed by a pressure regulator valve incorporated in the hydraulic control system. If the line pressure is supplied at an uncontrolled rate to any one or two of these fluid operated frictional units during shifting from the neutral gear position to the automatic forward drive range position or the reverse drive gear position, mechanical shocks tend to be created in the transmission mechanism and will give the occupants of the vehicle an unpleasant sensation.

The brake band to produce the second forward drive gear ratio is automatically operated to apply or release by means of a fluid operated band servo unit depending upon the vehicle speed under automatic forward drive range condition. If the line pressure is supplied to the band servo unit at an uncontrolled rate during shifting from the first gear ratio to the second gear ratio in the automatic forward drive range, mechanical shocks are also produced in the transmission mechanism.

In order to alleviate these shocks to be produced during selection of the automatic forward drive range position or the reverse drive gear position or upshifting between the gear ratios in automatic forward drive range, there is proposed and put to practical use a hydraulic transmission control system having incorporated therein a plurality of pressure accumulator units each of which is arranged to lessen the shocks to be produced during each of the shifts from the neutral gear position to the automatic forward drive gear position and the reverse drive gear position and the upshifts between the gear ratios in the automatic forward drive range. Installation of such a plurality of pressure accumulator units results in an enlarged and intricate construction of the control valve assembly incorporating the hydraulic transmission control system and further in an increased production cost of the transmission system as a whole.

The present invention contemplates elimination or reduction of the mechanical shocks usually concommitant with the selection of the automatic forward drive range and reverse drive gear position and the upshifting between the gear ratios in the automatic forward drive range by means of a shock control arrangement using a single pressure accumulator unit so as to provide compact and small sized construction of the transmission control valve assembly without having recourse to increasing the production cost of the transmission system having the shock control arrangement.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided in an automatic power transmission having an automatic forward drive range condition, a reverse drive gear condition and predetermined first, second and third gear ratios in the automatic forward drive range condition and including a transmission mechanism having incorporated therein a fluid operated first frictional unit contributive to the selection of the reverse drive gear condition and the shifting from the second gear ratio to the third gear ratio in the automatic forward drive range, a fluid operated second frictional unit contributive to the shifting from the first gear ratio to the second gear ratio in the automatic forward drive range condition and a fluid operated third frictional unit contributive to the selection of the automatic forward drive range condition, and a hydraulic control system including control fluid pressure generating means for producing a control fluid pressure throughout the automatic forward drive range and reverse drive range conditions, a shock control arrangement for reducing shocks to be produced during selection of any of the automatic forward drive range and reverse drive gear conditions or upshifting between any two of the first, second and third gear ratios in the automatic forward drive range condition, comprising a pressure accumulator unit operatively intervening between the control fluid generating means and each of the first, second and third frictional units and including a fluid operated piston having a first pressure acting area to be acted upon by a fluid pressure and urge the piston to move in a first direction when the first frictional unit is to be actuated, a second pressure acting area to be acted upon by a fluid pressure and urge the piston to move in a second direction opposite to the first direction when the second frictional unit is to be actuated and a third pressure acting area to be acted upon by a fluid pressure and the piston to move in the first direction when the third frictional unit is to be actuated, and biasing means for constantly urging the piston to move in the second direction; passageway means for discharging the fluid pressure to act on the second pressure acting area; and valve means operatively intervening between the second frictional unit and the control pressure generating means and responsive to the fluid pressure to actuate the first frictional unit and act on the first pressure acting area, the valve means including a valve element which is movable into and out of a limit position providing communication between the second frictional unit and the control pressure generating means and closing the above mentioned passageway means and which responsive to the fluid pressure to actuate the first frictional unit for being urged to move away from the limit position, and biasing means for constantly urging the valve element toward the limit position thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic view showing a transmission mechanism which may from part of an automatic power transmission including a hydraulic control system into which a shock control arrangement provided by the present invention is to be incorporated;

FIGS. 2A and 2B are a schematic view showing a hydraulic control system including a preferred embodiment of a shock control arrangement according to the present invention; and

FIG. 3 is a schematic view showing, partly in section, another preferred embodiment of a shock control arrangement according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Power Transmission Mechanism--General Construction

Description will be hereinafter made regarding the general construction and arrangement of a representative example of an automatic power transmission mechanism to which a hydraulic control system embodying the present invention is to be applied. The transmission mechanism forms part of the power train of an automotive vehicle equipped with a power plant such as an internal combustion engine 10 having a crankshaft 12 as the power output delivering member as partially and schematically illustrated in FIG. 1 of the drawings and is operatively connected to the crankshaft 12 of the engine 10 through a hydrodynamic torque converter 14. The torque converter 14 is herein assumed to be of the three member design by way of example and is thus shown comprising a driving member or pump impeller 16, a driven member or turbine runner 18, and a reaction member or stator 20 as is well known in the art. The pump impeller 16 is connected by a converter cover 22' and a converter driving plate 24 to the crankshaft 12 of the engine 10 and is rotatable with the engine crankshaft 12 about an axis which is aligned with the axis of rotation of the crankshaft 12. The turbine runner 18 is mounted on a turbine support disc 26 which is keyed or splined to a transmission input shaft 28 having a center axis which is also aligned with the axis of rotation of the engine crankshaft 12. The stator 20 serving as the reaction member of the torque converter 14 is positioned between the pump impeller 16 and the turbine runner 18 thus arranged and is mounted on a stator support hollow shaft 30 through a torque converter one-way clutch assembly 32. The stator support hollow shaft 30 has the transmission input shaft 28 axially passed therethrough in substantially coaxial relationship and is fixedly connected to or forms part of a stationary wall structure 34. The stator 20 is permitted to rotate about the center axis of the transmission input shaft 28 in the same direction as the direction of rotation the pump impeller 16 of the torque converter 14 and accordingly as the direction of rotation of the engine crankshaft 12. Though not shown, each of the pump impeller 16, turbine runner 18 and stator 20 of the torque converter 14 has a number of vanes arranged and inclined in symmetry about the center axis of the transmission input shaft 28. Behind the torque converter 14 thus constructed and arranged is positioned a transmission oil pump assembly 36 including, though not shown, an oil pump body bolted or otherwise secured to the above mentioned stationary wall structure 34 and a drive gear keyed or splined to an oil pump support sleeve 38 coaxially surrounding and rotatable on the outer peripheral surface of the stator support hollow shaft 30 and welded or otherwise securely connected to the pump impeller 16 of the torque converter 14.

When the engine 10 is in operation, the driving power produced by the engine is delivered from the crankshaft 12 of the engine 10 to the pump impeller 16 of the torque converter 14 through the converter driving plate 24 and the converter cover 22 and is transmitted from the pump impeller 16 to the transmission input shaft 28 through the turbine runner 18 of the torque converter 14 with a torque multiplied by means of the stator 20 at a ratio which is variable with the ratio between the revolution speed of the engine crankshaft 12 driving the pump impeller 16 and the revolution speed of the transmission input shaft 28 driven by the turbine runner 18 of the torque converter 14, as is well known in the art. The pump impeller 16 of the torque converter 14 drives not only the turbine runner 18 of the torque converter but the transmission oil pump assembly 36 through the pump support sleeve 38 so that the oil pump assembly 36 delivers oil under pressure which is also variable with the revolution speed of the crankshaft 12 of the engine 10.

The power transmission mechanism herein shown is assumed to be of the three forward speed and one reverse speed type by way of example and comprises first and second or high-and-reverse and forward drive clutches 40 and 42 which are positioned in series at the rear of the transmission oil pump assembly 36. The high-and-reverse clutch 40 compreses a plurality of clutch discs 40a keyed or splined at their inner peripheral edges to a clutch hub 44 and clutch plates 40b keyed or splined at their outer peripheral edges to a front clutch drum 46 which is in part positioned between the clutches 40 and 42 as shown. Likewise, the forward drive clutch 42 comprises a plurality of clutch discs 42a keyed or splined at their inner peripheral edges to a clutch hub 48 and clutch plates 42b keyed or splined at their outer peripheral edges to a rear clutch drum 50. The clutch hub 44 for the high-and-reverse clutch 40 and the rear clutch drum 50 for the forward drive clutch 42 are integral with each other and are rotatable with the transmission input shaft 28 with the rear clutch drum 50 keyed or splined to a rear end portion of the transmission input shaft 28 which axially projects from the stator support hollow shaft 30 as shown. The clutch discs 40a of the high-and-reverse clutch 40 and the clutch plates 42b of the forward drive clutch 42 thus serve as driving friction elements and, accordingly, the clutch plates 40b of the high-and-reverse clutch 40 and the clutch discs 42a of the forward drive clutch 42 serve as driven friction elements in the clutches 40 and 42, respectively. Though not shown in the drawings, each of the clutches 40 and 42 has incorporated therein a return spring urging the clutch discs and plates of the clutch to be disengaged from one another and a clutch piston which is adapted to bring the clutch discs and plates into engagement with one another when moved by a fluid pressure developed in a fluid chamber which is formed between the piston and the clutch drum 46, as is well known in the art.

The power transmission mechanism shown in FIG. 1 further comprises first and second planatary gear assemblies 52 and 54 which are arranged in series at the rear of the forward drive clutch 42. The first planatary gear assembly 52 comprises an externally toothed sun gear 52a and an internally toothed ring gear 52b which have a common axis of rotation aligned with the center axis of the transmission input shaft 28. The clutch hub 48 for the forward drive clutch 42 has a rear extension or flange 48a to which the ring gear 52b of the first planetary gear assembly 52 is keyed or splined as diagrammatically illustrated in the drawing. The first planatary gear assembly 52 further comprises at least two planet pinions 52c each of which is in mesh with the sun and ring gears 52a and 52b and which is rotatable about an axis around the common axis of rotation of the sun and ring gears 52a and 52b. The planet pinions 52c of the first planatary gear assembly 52 are jointly connected to a pinion carrier 56. The second planatary gear assembly 54 is constructed similarly to the first planatary gear assembly 52 and thus comprises an externally toothed sun gear 54a and an internally toothed ring gear 54b which have a common axis of rotation aligned with the center axis of the transmission input shaft 28. The sun gears 52a and 54a of the first and second planatary gear assemblies 52 and 54, respectively, are jointly splined or otherwise fastened to a connecting shell 58 enclosing the forward drive clutch 42 and the first planatary gear assembly 52 therein and integral with or securely connected to the front clutch drum 46 for the high-and-reverse clutch 40. The second planetary gear assembly 54 further comprises at least two planet pinions 54c each of which is in mesh with the sun and ring gears 54a and 54b and which is rotatable about an axis around the common axis of rotation of the sun and ring gears 54a and 54b. The planet pinions 54c of the second planetary gear assembly 54 are jointly connected to a pinion carrier 60 which is keyed or splined at its outer peripheral edge to a connecting drum 62 enclosing the second planetary gear assembly 54 therein. The connecting drum 62 has a rear axial extension extending rearwardly away from the second planetary gear assembly 54 as shown. The respective sun gears 52a and 54a of the first and second planetary gear assemblies 52 and 54 are formed with axial bores through which a transmission output shaft 64 having a center axis aligned with the center axis of the transmission input shaft 28 is passed through and axially extends rearwardly away from the second planetary gear assembly 54. The transmission output shaft 64 is connected to the pinion carrier 56 of the first planetary gear assembly 52 direction at its foremost end portion and further to the ring gear 53b of the second planetary gear assembly 54 through a generally disc shaped connecting member 66 which is keyed or splined at its inner peripheral edge to an intermediate axial portion of the transmission output shaft 64 and at its outer peripheral edge to the ring gear 54b of the second planetary gear assembly 54. The clutches 40 and 42, the planetary gear assemblies 52 and 54 and the connecting members between the clutches and planetary gear assemblies are enclosed within a transmission case (not shown). The previously mentioned stationary wall structure 34 integral with or securely connected to the stator support hollow shaft 30 may be constituted by a front end portion of the transmission case.

Within a rear end portion of the transmission case is positioned a low-and-reverse brake 68. The low-and-reverse brake 68 is herein assumed to be of the multiple disc type by way of example and is, thus, shown composed of a plurality of brake discs 68a keyed or splined at their inner peripheral edges to the rear axial extension of the connecting drum 62 engaging the pinion carrier 60 of the second planetary gear assembly 54, and a plurality of brake plates 68b which are keyed or splined at their outer peripheral edges to a stationary wall structure 34'. The stationary wall structure 34' may be constituted by a rear end portion of the transmission case. Though not shown in the drawings, the low-and-reverse brake 68 has further incorporated therein a return spring urging the brake discs and plates 68a and 68b of the brake unit to be disengaged from one another and a brake piston which is adapted to bring the brake discs and plates 68a and 68b into engagement with one another when the piston is moved by a fluid pressure developed in a fluid chamber which is formed between the piston and the above mentioned stationary wall structure 34', as is well known in the art. It is apparent that the low-and-reverse brake 68 of the multiple disc type as above described may be replaced with a brake unit of the cone type which is well known in the art.

The low-and-reverse brake 68 is paralleled in effect by a transmission one-way clutch 70 which is positioned within the rear axial extension of the above mentioned connecting drum 68. The transmission one-way clutch 70 is assumed to be of the sprag type by way of example and is, thus, shown comprising a stationary inner race member 70a, a rotatable outer race member 70b and a series of spring loaded sprag segments 70c disposed between the inner and outer race members 70a and 70b. The stationary inner race member 70a is centrally bored to have the transmission output shaft 64 axially passed therethrough and is bolted or otherwise securely fastened to the stationary wall structure 34' which may form part of the transmission case. On the other hand, the rotatable outer race member 70b is keyed or splined along its outer periphery to the rear axial extension of the connecting drum 62 carrying the brake discs 68a of the low-and-reverse brake 68. The sprag segments 70c provided between the inner and outer race members 70a and 70b are arranged in such a manner that the sprag segments 70c are caused to stick to the inner and outer race members 70a and 70b and thereby lock up the rotatable outer race member 70b to the stationary inner race member 70a when the outer race member 70b is urged to turn about the center axis of the transmission output shaft 64 in a direction opposite to the direction of rotation of the crankshaft 12 of the engine 10, viz., to the direction of rotation of the transmission output shaft 64 to produce a forward drive mode of an automotive vehicle. The direction of rotation of any member rotatable about an axis coincident or parallel with the center axis of the transmission output shaft 64 will be herein referred to as forward direction if the direction of rotation of the member is identical with the direction of rotation of the transmission output shaft 64 to produce a forward drive condition in a vehicle and as reverse direction if the direction of rotation of the member is identical with the direction of rotation of the transmission output shaft 64 to produce a rearward drive condition of the vehicle. Thus, the above described transmission one-way clutch 70 is adapted to allow the connecting drum 62 and accordingly the pinion carrier 60 of the second planetary gear assembly 54 to turn in the forward direction about the center axis of the transmission output shaft 64 but prohibit the connecting drum 60 and the pinion carrier 60 from being rotated in the reverse direction about the center axis of the transmission output shaft 64. The forward direction herein referred to is identical with the direction of rotation of the crankshaft 12 of the engine 10 and accordingly with the direction of rotation of the transmission input shaft 28. It is apparent that the transmission one-way clutch 70 of the sprag type as above described may be replaced with a one-way clutch of the well known cam and roller type if desired.

The power transmission mechanism shown in FIG. 1 further comprises a brake band 72 wrapped around the outer peripheral surface of an axial portion of the connecting shell 58 integral with or securely fastened to the clutch drum 46 for the high-and-reverse clutch 40. The brake band 72 is anchored at one end to the transmission casing and is at the other end connected to or engaged by a fluid operated band servo unit 74 which is illustrated at the top of FIG. 2A. Referring to FIG. 2A, the band servo unit 74 has a housing formed with brake-apply and brake-release fluid chambers 76 and 76' which are separated by a servo piston 78 connected by a piston rod 80 to the brake band 72. The servo piston 78 is axialy moved in a direction to cause the brake band 72 to be contracted and tightened upon the outer peripheral surface of the connecting shell 58 when there is a fluid pressure developed in the brake-apply fluid chamber 76 in the absence of a fluid pressure in the brake-release fluid chamber 76'. The servo piston 78 is biased to axially move in a direction to contract the brake-apply fluid chamber, via., cause the brake band 72 to be disengaged from the connecting shell 58 by means of a return spring 82 incorporated into the servo unit 74. Furthermore, the piston 78 and the housing of the servo unit 74 are designed so that the piston 78 has a differential pressure acting area effective to move the piston in the particular direction when the piston is subjected to fluid pressures on both sides thereof. When a fluid pressure is built up in the brake-release fluid chamber 76', the servo piston 78 is axially moved in a direction to cause the brake band 72 to expand and disengage from the connecting shell 58 regardless of the presence or absence of a fluid pressure in the brake-apply fluid chamber 76 of the servo unit 74.

Turning back to FIG. 1, the output shaft 64 of the power transmission mechanism thus constructed and arranged projects rearwardly from the transmission case and has mounted thereon a transmission governor assembly 84