U.S. patent application number 09/725937 was filed with the patent office on 2001-06-07 for power transmission system for four-wheel drive vehicles.
This patent application is currently assigned to Honda Giken Kogyo Kabushiki Kaisha. Invention is credited to Arai, Kentaro, Kunii, Rikiya.
Application Number | 20010002629 09/725937 |
Document ID | / |
Family ID | 18375517 |
Filed Date | 2001-06-07 |
United States Patent
Application |
20010002629 |
Kind Code |
A1 |
Arai, Kentaro ; et
al. |
June 7, 2001 |
Power transmission system for four-wheel drive vehicles
Abstract
In a power transmission system for a four-wheel drive vehicle, a
multiple disk clutch for distributing a proportion of the torque of
the front wheels to the rear wheels is provided between a driving
shaft which is connected to the front wheels which are main driven
wheels and a driven shaft which is connected to the rear wheels
which are auxiliary driven wheels. Between the driving shaft and
the driven shaft a torque cam mechanism, a hydraulic pump and a
bidirectional clutch mechanism are provided, in that order. When
the rotational rate of the front wheels exceeds the rotational rate
of the rear wheels, the bidirectional clutch mechanism is engaged,
a first rotor of the hydraulic pump which is operatively connected
to the front wheels and a second rotor which is operatively
connected to the rear wheels rotate relative to each other, and a
rotational load is generated by a hydraulic circuit. A first cam
element and a second cam element of the torque cam mechanism rotate
relative to each other as a result of the above-mentioned
rotational load so as to generate a thrust force, and this thrust
force causes engagement of the multiple disk clutch so as to put
the vehicle into a four-wheel drive mode. It is thus possible to
decrease the torque transmission capacity of the bidirectional
clutch mechanism so reducing its size and cost.
Inventors: |
Arai, Kentaro; (Wako-shi,
JP) ; Kunii, Rikiya; (Wako-shi, JP) |
Correspondence
Address: |
ARMSTRONG,WESTERMAN, HATTORI,
MCLELAND & NAUGHTON, LLP
1725 K STREET, NW, SUITE 1000
WASHINGTON
DC
20006
US
|
Assignee: |
Honda Giken Kogyo Kabushiki
Kaisha
Tokyo
JP
|
Family ID: |
18375517 |
Appl. No.: |
09/725937 |
Filed: |
November 30, 2000 |
Current U.S.
Class: |
180/233 ;
192/103R; 192/35 |
Current CPC
Class: |
B60K 17/34 20130101;
B60K 23/08 20130101 |
Class at
Publication: |
180/233 ; 192/35;
192/103.00R |
International
Class: |
F16D 043/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 3, 1999 |
JP |
11-345278 |
Claims
1. A power transmission system for four-wheel drive vehicles for
distributing a proportion of the torque of the main driven wheels
which are directly driven by an engine to auxiliary driven wheels
via a driving shaft, a multiple disk clutch and a driven shaft,
comprising: a torque cam mechanism comprising a first cam element
and a second cam element which are capable of rotating relative to
each other, the torque cam mechanism generating a thrust force for
engaging the multiple disk clutch by the relative rotation of the
two cam elements; a bidirectional clutch mechanism comprising a
first clutch element and a second clutch element which are capable
of rotating relative to each other, the bidirectional clutch
mechanism engaging the two clutch elements with each other
regardless of the rotational direction of the first clutch element
when the rotational rate of the first clutch element exceeds the
rotational rate of the second clutch element; and a load generating
means which comprises a first rotor and a second rotor which are
capable of rotating relative to each other, the load generating
means generating a rotational load by the relative rotation of the
two rotors, wherein the driving shaft is connected to the first cam
element of the torque cam mechanism, the second cam element of the
torque cam mechanism is connected to the first rotor of the load
generating means, the second rotor of the load generating means is
connected to the first clutch element of the bidirectional clutch
mechanism, and the second clutch element of the bidirectional
clutch mechanism is connected to the driven shaft.
2. A power transmission system for four-wheel drive vehicles
according to claim 1, wherein said load generating means comprises
a hydraulic pump.
3. A power transmission system for four-wheel drive vehicles
according to claim 1, wherein said load generating means comprises
a power generator.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a power transmission system
for four-wheel drive vehicles which distributes a proportion of the
torque of the main driven wheels which are directly driven by an
engine to auxiliary driven wheels via a multiple disk clutch.
[0003] 2. Description of the Related Art
[0004] Such a power transmission system for four-wheel drive
vehicles is disclosed in FIG. 10 of Japanese Patent Application
Laid-open No. 9-202152. In this system, a driving shaft which
rotates in operative connection with front wheels which are the
main driven wheels and a driven shaft which rotates in operative
connection with rear wheels which are the auxiliary driven wheels
are connected to each other via a multiple disk clutch, and a
bidirectional clutch mechanism is provided on the aforementioned
driven shaft. The bidirectional clutch mechanism has the function
of enhancing the ground covering properties of the vehicle by being
engaged when the front wheels slip, which causes the rotational
rate of the front wheels to exceed the rotational rate of the rear
wheels, thereby distributing the torque of the front wheels to the
rear wheels when the vehicle is travelling either forward or
backward, and the function of avoiding influencing the operation of
the ABS (anti-lock braking system) by cancelling the engagement
when the front wheels are locked, which causes the rotational rate
of the front wheels to become lower than the rotational rate of the
rear wheels, so as to prevent the torque of the front wheels from
being distributed to the rear wheels.
[0005] In the above-mentioned conventional system, because the
bidirectional clutch mechanism is provided on the driven shaft
which transmits the torque of the front wheels to the rear wheels,
the aforementioned torque is transmitted directly via the
bidirectional clutch mechanism. Therefore, it is necessary to use a
large and expensive bidirectional clutch mechanism having a large
torque transmission capacity, which is the main cause of the
increase in size and cost of the power transmission system.
SUMMARY OF THE INVENTION
[0006] The present invention has been conceived in view of the
above-mentioned circumstances, and it is an object of the present
invention to reduce the torque transmission capacity of a
bidirectional clutch mechanism which is used in a power
transmission system for four-wheel drive vehicles and to reduce the
size and cost of the bidirectional clutch mechanism.
[0007] In accordance with a first aspect of the present invention,
in order to achieve the above-mentioned object, a power
transmission system for four-wheel drive vehicles is proposed for
distributing a proportion of the torque of the main driven wheels
which are directly driven by an engine to auxiliary driven wheels
via a driving shaft, a multiple disk clutch and a driven shaft,
comprising a torque cam mechanism which comprises a first cam
element and a second cam element which can rotate relative to each
other and which generates a thrust force for engaging the multiple
disk clutch by the relative rotation of the two cam elements, a
bidirectional clutch mechanism which comprises a first clutch
element and a second clutch element which can rotate relative to
each other and which engages the two clutch elements with each
other regardless of the rotational direction of the first clutch
element when the rotational rate of the first clutch element
exceeds the rotational rate of the second clutch element and a load
generating means comprising a first rotor and a second rotor which
can rotate relative to each other which generates a rotational load
by the relative rotation of the two rotors, wherein the driving
shaft is connected to the first cam element of the torque cam
mechanism, the second cam element of the torque cam mechanism is
connected to the first rotor of the load generating means, the
second rotor of the load generating means is connected to the first
clutch element of the bidirectional clutch mechanism, and the
second clutch element of the bidirectional clutch mechanism is
connected to the driven shaft.
[0008] In accordance with a second aspect of the present invention,
in addition to the above-mentioned first aspect, a power
transmission system for four-wheel drive vehicles is proposed in
which the above-mentioned load generating means is a hydraulic
pump.
[0009] In accordance with a third aspect of the present invention,
in addition to the above-mentioned first aspect, a power
transmission system for four-wheel drive vehicles is proposed in
which the above-mentioned load generating means is a power
generator.
[0010] In accordance with the above-mentioned arrangements, the
bidirectional clutch mechanism is in a disengaged state when the
vehicle is travelling forward at a constant speed, where the
rotational rate of the main driven wheels coincides with the
rotational rate of the auxiliary driven wheels and when the vehicle
is braking when travelling forward where the rotational rate of the
main driven wheels is less than the rotational rate of the
auxiliary driven wheels. As a result, the second rotor of the load
generating means rotates under no load by being dragged by the
first rotor, the torque cam mechanism does not transmit any torque
and no thrust force is thus generated, the multiple disk clutch is
disengaged, and the vehicle is maintained in a two-wheel drive
state.
[0011] Because the bidirectional clutch mechanism is in an engaged
state when the vehicle starts to travel forward and when the
vehicle accelerates in the forward direction where the rotational
rate of the main driven wheels exceeds the rotational rate of the
auxiliary driven wheels, the first clutch element of the
bidirectional clutch mechanism brakes the second rotor of the load
generating means so causing rotation relative to the first rotor.
As a result, the load generating means generates a load, the torque
cam mechanism transmits the torque so as to generate a thrust
force, the multiple disk clutch is therefore engaged and the
vehicle switches over to a four-wheel drive state.
[0012] When the vehicle is travelling backward the direction in
which each of the elements of the power transmission system rotates
is opposite to the rotational direction when the vehicle is
travelling forward, and since the bidirectional clutch mechanism
engages the first clutch element with the second clutch element
regardless of the rotational direction of the first clutch element
when the rotational rate of the first clutch element exceeds the
rotational rate of the second clutch element, the bidirectional
clutch mechanism is disengaged when the vehicle is travelling
backward at a constant speed and when the vehicle is being braked
backward in the same manner as when it is travelling forward so as
to maintain the vehicle in a two-wheel drive state, and the
bidirectional clutch mechanism is engaged so as to switch the
vehicle over to a four-wheel drive state when the vehicle starts to
travel backward and when the vehicle accelerates backward.
[0013] Torque transmitted from the main driven wheels to the
auxiliary driven wheels is not directly applied to the
bidirectional clutch mechanism; only a small torque which is
transmitted by the torque cam mechanism is applied to the
bidirectional clutch mechanism, and it is therefore possible to
decrease the torque transmission capacity of the bidirectional
clutch mechanism, thereby reducing the size and the cost
thereof.
[0014] With regard to the load generating means, a hydraulic pump
or a power generator can be used.
[0015] The above-mentioned objects, other objects, characteristics
and advantages of the present invention will become apparent from
an explanation of preferable embodiments which will be described in
detail below by reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 to FIG. 6 illustrate a first embodiment of the
present invention.
[0017] FIG. 1 is a diagram showing the arrangement of the entire
power transmission line of a four-wheel drive vehicle.
[0018] FIG. 2 is a diagram showing the structure of a power
transmission system.
[0019] FIG. 3A and FIG. 3B are enlarged cross sectional views at
line 3-3 in FIG. 2.
[0020] FIG. 4 is an enlarged cross sectional view at line 4-4 in
FIG. 2.
[0021] FIG. 5A, FIG. 5B and FIG. 5C are diagrams for explaining the
action of a bidirectional clutch mechanism.
[0022] FIG. 6 is a schematic view showing the power transmission
path.
[0023] FIG. 7 is a diagram showing the structure of a power
transmission system of a second embodiment of the present
invention.
[0024] FIG. 8 is a diagram showing the structure of a power
transmission system of a third embodiment of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] Practical features of the present invention are explained
below by reference to embodiments of the present invention shown in
the attached drawings.
[0026] FIG. 1 to FIG. 6 illustrate the first embodiment of the
present invention.
[0027] As shown in FIG. 1, the output from an engine E mounted in
the front part of a four-wheel drive vehicle is input into a
differential gear 2 at the front via a transmission 1, the output
from the differential gear 2 is transmitted to right and left front
wheels Wf, Wf, which are main driven wheels, via drive shafts 3, 3.
Furthermore, the output from the engine E which has been input into
the differential gear 2 is input to a power transmission system T
which is described hereinafter, via a bevel gear 4 and a driving
shaft 5, the output from the power transmission system T is
transmitted to a differential gear 8 at the rear via a driven shaft
6 and a bevel gear 7, and furthermore the output from the
differential gear 8 is transmitted to right and left rear wheels
Wr, Wr, which are auxiliary driven wheels, via drive shafts 9,
9.
[0028] As shown in FIG. 2, the power transmission system T which is
placed between the driving shaft 5 which rotates in operative
connection with the rotation of the front wheels Wf, Wf and the
driven shaft 6 which rotates in operative connection with the
rotation of the rear wheels Wr, Wr comprises a multiple disk clutch
11, a torque cam mechanism 12, a hydraulic pump 13 and a
bidirectional clutch mechanism 14 which are placed in that order
from the driving shaft 5 side to the driven shaft 6 side.
[0029] The multiple disk clutch 11 governs the transmission and
blocking of torque between the driving shaft 5 and the driven shaft
6 and is formed by alternately superimposing a plurality of
frictional engagement members 16 . . . supported on a clutch outer
15 which rotates together with the driving shaft 5 and a plurality
of frictional engagement members 18 . . . supported on a clutch
inner 17 which rotates together with the driven shaft 5, and the
two frictional engagement members 16 . . . , 18 . . . come into
close contact with each other by receiving a thrust force from the
torque cam mechanism 12, which is described hereinafter so as to
engage the driving shaft 5 with the driven shaft 6. In the state in
which the multiple disk clutch 11 is engaged, torque is transmitted
from the front wheels Wf, Wf to the rear wheels Wr, Wr, and in the
state in which the engagement of the multiple disk clutch 11 is
released, the transmission of torque from the front wheels Wf, Wf
to the rear wheels Wr, Wr is blocked.
[0030] As is clear by referring to FIG. 3A and FIG. 3B together
with FIG. 2, the torque cam mechanism 12 comprises a first cam
element 19 which is connected by splines to the clutch outer 15 and
a second cam element 21 which is connected to the forward end of a
sleeve 20 coaxially fitted around an outer circumference of the
driven shaft 6, and a plurality of balls 22 . . . are supported
between the plurality of triangular cam channels 19a . . . , 21a .
. . which are formed on the surfaces of the first cam element 19
and the second cam element 21, respectively, that face each
other.
[0031] The hydraulic pump 13 which forms a load generating means of
the present invention comprises, for example, a known vane pump; a
pump rotor which forms a first rotor 23 of the load generating
means is connected to the rear end of the aforementioned sleeve 20
and a cam ring which forms a second rotor 24 of the load generating
means is connected to a first clutch element 29 of the
bidirectional clutch mechanism 14 which is described hereinafter.
The hydraulic pump 13 comprises a first port 13a and a second port
13b; when the first rotor 23 and the second rotor 24 rotate
relative to each other in one direction the hydraulic oil which is
taken in through the first port 13a discharges into the second port
13b, and when the first rotor 23 and the second rotor 24 rotate
relative to each other in the other direction the hydraulic oil
which is taken in through the second port 13b discharges into the
first port 13a.
[0032] A hydraulic circuit 25 which is connected to the hydraulic
pump 13 is formed by connecting in parallel an orifice 26 which is
placed between the first port 13a and the second port 13b, a relief
valve 27 which opens when the oil pressure of the first port 13a
exceeds the oil pressure of the second port 13b by a predetermined
value, and a relief valve 28 which opens when the oil pressure of
the second port 13b exceeds the oil pressure of the first port 13a
by a predetermined value.
[0033] As is clear by referring to FIG. 4 together with FIG. 2, the
bidirectional clutch mechanism 14 comprises a ring-shaped first
clutch element 29 which is positioned outermost in the radial
direction and is connected to the second rotor 24 of the hydraulic
pump 13, a second clutch element 30 which is positioned coaxially
inside the first clutch element 29 and is connected to the outer
circumference of the driven shaft 6, an annular retainer 31 which
is placed in a rotatable manner between the first and second clutch
elements 29, 30, and a plurality of sprags 32 . . . which are
supported so as to fit inside a plurality of pockets 31a . . .
formed in the retainer 31 at predetermined intervals and a
plurality of indentations 29a . . . formed on the inner
circumference of the first clutch element 29 at these predetermined
intervals. The indentations 29a . . . are formed on the inner
circumference of the first clutch element 29, which is the outer of
the two clutch elements, and a circular surface 30a is formed on
the outer circumference of the second clutch element 30, which is
the inner of the two clutch elements. The sprags 32 . . . are
therefore retained by being surrounded by the indentations 29a . .
. of the first clutch element 29, the circular surface 30a of the
second clutch element 30 and the pockets 31a . . . of the retainer
31.
[0034] Shoes 31c . . . provided at the tip end of arms 31b . . .
which extend from the retainer 31 are frictionally engaged in a
slidable manner with the inner surface of a casing 33 of the power
transmission system T. A pin 34 which protrudes from the retainer
31 in the radial direction engages with a notch 29b formed on the
inner circumference of the first clutch element 29 so as to limit
the angular range over which the retainer 31 is capable of rotating
relative to the first clutch element 29. Furthermore, the retainer
31 and the sprags 32 . . . are urged towards the neutral position
shown in FIG. 5A by means of springs 35 . . . , 35 . . . which are
provided at both edges of the pockets 31a . . . of the retainer
31.
[0035] FIG. 6 is a schematic diagram of the power transmission path
of the power transmission system T, which will assist in the
understanding of the structure thereof. As shown in the figure, the
front wheels Wf, Wf, the driving shaft 5, the first cam element 19
and the second cam element 21 of the torque cam mechanism 12, the
first rotor 23 and the second rotor 24 of the hydraulic pump 13,
the first clutch element 29 and the second clutch element 30 of the
bidirectional clutch mechanism 14, the driven shaft 6 and the rear
wheels Wr, Wr are connected in series. Thick solid lines linking
each of the components with each other denote direct connections
which do not allow relative rotation and fine double lines, a, b,
and c denote connections which do allow relative rotation.
[0036] An action of the embodiment of the present invention
comprising the aforementioned arrangement is explained by reference
mainly to FIG. 6.
[0037] (1) When travelling forward at a constant speed
[0038] When a vehicle is travelling forward at a constant speed in
which the front wheels Wf, Wf and the rear wheels Wr, Wr rotate at
the same speed, the engagement of the multiple disk clutch 11 is
released, and the distribution of torque from the front wheels Wf,
Wf to the rear wheels Wr, Wr is blocked so as to put the vehicle in
a two-wheel drive state. The action when travelling forward at a
constant speed is explained below.
[0039] Rotation of the front wheels Wf, Wf, which are driven by the
engine E, is transmitted to the torque cam mechanism 12 through the
driving shaft 5. Because the torque cam mechanism 12 has the
structure in which balls 22 . . . are held between the cam channels
19a . . . of the first cam element 19 and the cam channels 21a . .
. of the second cam element 21, rotation of the first cam element
19 is transmitted to the second cam element 21 via the balls 22 . .
. . At this stage, since a load is not being applied to the second
cam element 21 as described hereinafter, the torque cam mechanism
12 does not transmit substantial torque, the first cam element 19
and the second cam element 21 do not rotate relative to each other
(see FIG. 3A), and the torque cam mechanism 12 does not generate a
thrust force for engaging the multiple disk clutch 11.
[0040] When the rotation is transmitted to the first rotor 23 of
the hydraulic pump 13 which is connected to the second cam element
21 of the torque cam mechanism 12, since a load is not being
applied to the second rotor 24 as described hereinafter, the second
rotor 24 is dragged by the rotation of the first rotor 23 and
rotates at the same speed as that of the first rotor 23, and the
hydraulic pump 13 idles with no load, neither taking in nor
discharging any hydraulic oil.
[0041] The first clutch element 29 of the bidirectional clutch
mechanism 14 rotates by being connected to the second rotor 24 of
the hydraulic pump 13, the second clutch element 30 rotates by
being connected to the rear wheels Wr, Wr via the driven shaft 6,
and at this stage since the rotational rate of the front wheels Wf,
Wf coincides with the rotational rate of the rear wheels Wr, Wr,
the first and second clutch elements 29, 30 of the bidirectional
clutch mechanism 14 rotate in the same direction at the same speed,
thereby, bringing about a slip state in which no torque is
transmitted.
[0042] That is to say, as shown in FIG. 5B if the second clutch
element 30 of the bidirectional clutch mechanism 14, which rotates
in operative connection with the rotation of the rear wheels Wr,
Wr, rotates in the forward direction shown by the arrow Nr, the
retainer 31 which is dragged by the second clutch element 30 also
rotates in the forward direction, but because the retainer 31 is
retarded by the shoes 31c . . . (see FIG. 2) which frictionally
engage with the casing 33, the rotation of the retainer 31 is
retarded by a predetermined angle relative to the first clutch
element 29, and the pin 34 stops at a position in which it is in
contact with one edge of the notch 29b of the first clutch element
29 (FIG. 5B). In this state torque is transmitted from the first
clutch element 29 to the second clutch element 30 only when the
rotational rate Nf of the first clutch element 29 in the forward
direction exceeds the rotational rate Nr of the second clutch
element 30 in the forward direction, and no torque is transmitted
from the first clutch element 29 to the second clutch element 30
when the rotational rate Nf of the first clutch element 29 in the
forward direction coincides with or becomes less than the
rotational rate Nr of the second clutch element 30 in the forward
direction.
[0043] As hereinbefore described, when the vehicle is travelling
forward at a constant speed in which the rotational rates Nf, Nr of
the first clutch element 29 and the second clutch element 30
coincide with each other, the bidirectional clutch mechanism 14 is
not engaged, and because the first clutch element 29 can rotate
with no load the second rotor 24 of the hydraulic pump 13 which is
connected to the first clutch element 29 can rotate without any
load. Therefore, torque transmission between the first cam element
19 and the second cam element 21 of the torque cam mechanism 12
does not occur, the phases of the first and second cam elements 19,
21 are maintained in the state shown in FIG. 3A, and the torque cam
mechanism 12 does not generate a thrust force for engaging the
multiple disk clutch 11.
[0044] (2) When starting to travel forward or accelerating
forward
[0045] When the front wheels Wf, Wf slip as a result of a rapid
start or rapid acceleration on a road surface having a low
coefficient of friction, the rotational rate of the front wheels
Wf, Wf exceeds the rotational rate of the rear wheels Wr, Wr, the
multiple disk clutch 11 is engaged, torque is distributed from the
front wheels Wf, Wf to the rear wheels Wr, Wr and the vehicle is
put into a four-wheel drive state. The action when starting to
travel forward or accelerating forward is explained below.
[0046] At the above-mentioned time when the vehicle is travelling
forward at a constant speed, the rotational rates Nf, Nr of the
first clutch element 29 and the second clutch element 30 of the
bidirectional clutch mechanism 14 become identical, but when the
front wheels Wf, Wf slip the rotational rate Nf of the first clutch
element 29 of the bidirectional clutch mechanism 14, which is
operatively connected to the rotation of the front wheels Wf, Wf
exceeds the rotational rate Nr of the second clutch element 30,
which is operatively connected to the rotation of the rear wheels
Wr, Wr. When the rotational rate Nf of the first clutch element 29
in the forward direction exceeds the rotational rate Nr of the
second clutch element 30 in the forward direction in FIG. 5B, the
bidirectional clutch mechanism 14 is engaged and the first clutch
element 29 and the second clutch element 30 are joined
together.
[0047] At this stage the rotational rate Nr of the second clutch
element 30, which is directly connected to the rear wheels Wr, Wr
via the driven shaft 6 is unchanged, but the rotational rate Nf of
the first clutch element 29, which is connected to the front wheels
Wf, Wf via the hydraulic pump 13 and the torque cam mechanism 12 is
decreased to the same level as the rotational rate Nr of the second
clutch element 30 as a result of the load which is applied by the
second clutch element 30. When the rotation of the first clutch
element 29 of the bidirectional clutch mechanism 14 is thus braked,
since the rotation of the second rotor 24 of the hydraulic pump 13
which is connected to the first clutch element 29 is also braked,
the first rotor 23 and the second rotor 24 rotate relative to each
other so as to discharge hydraulic oil from the first port 13a, and
this hydraulic oil returns to the second port 13b passing through
the orifice 26 so as to generate a rotational load in the hydraulic
pump 13. In addition, when the discharge pressure of the hydraulic
pump 13 reaches an upper limit the one relief valve 27 opens so as
to restrain the rotational load applied to the hydraulic pump 13 to
an upper limit.
[0048] When the rotational load so generated in the hydraulic pump
13 brakes the rotation of the first rotor 23, a difference in
rotation is caused between the second cam element 21 of the torque
cam mechanism 12, which rotates by being connected to the first
rotor 23 and the first cam element 19 of the torque cam mechanism
12, which rotates by being connected to the front wheels Wf, Wf.
The phases of the cam channel 19a of the first cam element 19 and
the cam channel 21a of the second cam element 21 are displaced so
as to generate a thrust force (FIG. 3B), and this thrust force
makes the frictional engagement members 16 . . . , 18 . . . of the
multiple disk clutch 11 come into close contact with each other so
as to engage them. As a result, the torque of the front wheels Wf,
Wf is distributed to the rear wheels Wr, Wr via the driving shaft
5, the multiple disk clutch 11 and the driven shaft 6 and the
vehicle is put into a four-wheel drive state.
[0049] Thus, when the front wheels Wf, Wf slip a proportion of the
torque of the above-mentioned front wheels Wf, Wf is distributed to
the rear wheels Wr, Wr so putting the vehicle in a four-wheel drive
state, and the ground covering properties of the vehicle can be
improved. Moreover, the level of torque distributed to the rear
wheels Wr, Wr can be increased according to the increase in the
difference between the rotational rate of the front wheels Wf, Wf
and that of the rear wheels Wr, Wr, that is to say, according to
the increase in the degree of slip of the front wheels Wf, Wf. The
torque transmission from the front wheels Wf, Wf to the rear wheels
Wr, Wr is carried out by the multiple disk clutch 11, only a small
amount of the torque which is applied between the first and second
cam elements 19, 21 of the torque cam mechanism 12 is transmitted
to the bidirectional clutch mechanism 14 and, therefore, not only
can the size and weight be reduced by using the bidirectional
clutch mechanism 14 having a small torque transmission capacity,
but the durability can also be enhanced.
[0050] (3) When braking while travelling forward
[0051] When a vehicle travelling forward on a road surface having a
low coefficient of friction is braked rapidly, because the braking
force applied to the front wheels Wf, Wf is set so as to be larger
than the braking force applied to the rear wheels Wr, Wr, there are
cases in which the front wheels lock first and the rotational rate
of the rear wheels Wr, Wr exceeds the rotational rate of the front
wheels Wf, Wf. If the multiple disk clutch 11 is engaged and the
vehicle is put into a four-wheel drive state in such cases, because
there is a possibility that the operation of the ABS (anti-lock
braking system) might be affected so degrading the braking
performance, it is necessary to maintain the vehicle in a two-wheel
drive state when braking while travelling forward. The action when
braking while travelling forward is explained below.
[0052] At the aforementioned time when travelling forward at a
constant speed, the rotational rates Nf, Nr of the first clutch
element 29 and the second clutch element 30 of the bidirectional
clutch mechanism 14 are identical, but if the front wheels Wf, Wf
are locked, the rotational rate Nf of the first clutch element 29
of the bidirectional clutch mechanism 14, which is operatively
connected to the rotational rate of the front wheels Wf, Wf,
becomes less than the rotational rate Nr of the second clutch
element 30, which is operatively connected to the rotation of the
rear wheels Wr, Wr. When the rotational rate Nf of the first clutch
element 29 in the forward direction becomes less than the
rotational rate Nr of the second clutch element 30 in the forward
direction in FIG. 5B, the bidirectional clutch mechanism 14 is
disengaged and the first clutch element 29 and the second clutch
element 30 are separated from each other.
[0053] That is to say, because the first clutch element 29 of the
bidirectional clutch mechanism 14 can rotate at a rotational rate
less than that of the second clutch element 30 without receiving
any load from the second clutch element 30, rotation of the second
rotor 24 of the hydraulic pump 13, which is connected to the second
clutch element 30, is not restrained, and the first rotor 23 and
the second rotor 24 of the hydraulic pump 13 therefore rotate at
the same speed in a state in which no load is being applied. As a
result, the first cam element 19 and the second cam element 21 of
the torque cam mechanism 12 rotate in the same phase without
transmitting any torque, and because no thrust force for engaging
the multiple disk clutch 11 is generated the vehicle is maintained
in a two-wheel drive state.
[0054] (4) When travelling backward
[0055] When a vehicle is travelling backward it can switch between
a two-wheel drive state and a four-wheel drive state in the same
manner as in the above-mentioned case when it is travelling
forward. In detail, the two-wheel drive state is maintained when
travelling backward at a constant speed or in the case where the
front wheels Wf, Wf are locked when braking while the vehicle is
travelling backward, and it is switched over to the four-wheel
drive state in the case where the front wheels Wf, Wf slip when
starting to travel backward or when travelling backward with rapid
acceleration. The action when travelling backward is explained
below.
[0056] When a vehicle is travelling backward, since the rotational
direction of all the elements in FIG. 6 are reversed, the second
clutch element 30 of the bidirectional clutch mechanism 14 rotates
in the direction shown by the arrow Nr in FIG. 5C. As a result, the
retainer 31 which is dragged by the rotation of the second clutch
element 30 in the reverse direction rotates in the reverse
direction, but because the retainer 31 is braked by the shoes 31c .
. . which frictionally engage with the casing 33 (FIG. 2), its
rotation is retarded relative to the first clutch element 29 by a
predetermined angle in the rotational direction so as to be in the
state shown in FIG. 5C. In this state it is only when the
rotational rate Nf of the first clutch element 29 in the reverse
direction exceeds the rotational rate Nr of the second clutch
element 30 in the reverse direction that a torque is transmitted
from the first clutch element 29 to the second clutch element 30,
and when the rotational rate Nf of the first clutch element 29 in
the reverse direction coincides with or becomes less than the
rotational rate Nr of the second clutch element 30 in the reverse
direction, no torque is transmitted from the first clutch element
29 to the second clutch element 30.
[0057] As hereinbefore described, because the bidirectional clutch
mechanism 14 is not engaged when the vehicle is travelling backward
at a constant speed where the rotational rates Nf, Nr of the first
clutch element 29 and the second clutch element 30 coincide with
each other, and when the vehicle is braked while travelling
backward where the rotational rate Nf of the first clutch element
29 becomes less than the rotational rate Nr of the second clutch
element 30 and the first clutch element 29 can rotate without a
load, the second rotor 24 of the hydraulic pump 13, which is
connected to the first clutch element 29, can rotate without a
load. Therefore, no torque is transmitted between the first cam
element 19 and the second cam element 21 of the torque cam
mechanism 12, and the multiple disk clutch 11 is disengaged so as
to maintain a two-wheel drive state.
[0058] When the front wheels Wf, Wf slip when the vehicle is
starting to travel backward and when the vehicle is rapidly
accelerating while travelling backward thereby causing the
rotational rate Nf of the first clutch element 29 to exceed the
rotational rate Nr of the second clutch element 30, the
bidirectional clutch mechanism 14 is engaged and the first clutch
element 29 is braked by receiving a load from the second clutch
element 30. As a result, the first rotor 23 and the second rotor 24
of the hydraulic pump 13 rotate relative to each other so as to
generate a rotational load, torque is transmitted between the first
cam element 19 and the second cam element 21 of the torque cam
mechanism 12 so generating a thrust force, and the multiple disk
clutch 11 is engaged by this thrust force and the vehicle is put
into a four-wheel drive state.
[0059] In addition, the rotational direction of the hydraulic pump
13 when the vehicle is starting to travel backward or is rapidly
accelerating while travelling backward is the reverse of that when
the vehicle is starting to travel forward or is rapidly
accelerating forward; the first port 13a becomes an induction port
and the second port 13b becomes a discharge port. Therefore, the
upper limit for the oil pressure is restrained by the other relief
valve 28.
[0060] The second embodiment of the present invention is explained
by reference to FIG. 7.
[0061] The second embodiment differs from the aforementioned first
embodiment in terms of the layout of the bidirectional clutch
mechanism 14. That is to say, in the first embodiment the
bidirectional clutch mechanism 14 is placed coaxially on the driven
shaft 6, but in the second embodiment the bidirectional clutch
mechanism 14 is placed at a position away from the driven shaft 6.
A gear 41 provided on the first clutch element 29 of the
bidirectional clutch mechanism 14 meshes with a gear 42 provided on
the second rotor 24 of the hydraulic pump 13, and a gear 43
provided on the second clutch element 30 of the bidirectional
clutch mechanism 14 meshes with a gear 44 provided on the driven
shaft 6. In this case, the gear ratio of the two gears 41, 42 on
the first clutch element 29 side coincides with the gear ratio of
the two gears 43, 44 on the second clutch element 30 side.
[0062] The third embodiment of the present invention is explained
by reference to FIG. 8.
[0063] In the third embodiment a power generator 45 is used as the
load generating means instead of the hydraulic pump 13 of the first
embodiment. The power generator 45 comprises a first rotor 46 which
forms a power generator rotor on the inner side thereof and a
second rotor 47 which forms a stator on the outer side thereof; the
first rotor 46 is connected to the second cam element 21 of the
torque cam mechanism 12 via the sleeve 20, and the second rotor 47
is connected to the first clutch element 29 of the bidirectional
clutch mechanism 14. Both ends of the coil of the second rotor 47
are connected to a controller 48. When the first rotor 46 and the
second rotor 47 of the power generator 45 rotate relative to each
other, since the load so generated acts so as to suppress the
rotation of the first rotor 46, it can exhibit the same function as
that of the hydraulic pump 13 of the first embodiment.
[0064] Thus, the same operational effects as those obtained by the
first embodiment can be achieved by the second and third
embodiments.
[0065] The embodiments of the present invention are explained in
detail above, but the present invention can be modified in a
variety of ways without departing from the spirit and scope of the
invention.
[0066] For example, the structure of the bidirectional clutch
mechanism 14 is not limited to that described in the embodiments,
and rollers may be used instead of the sprags 32.
* * * * *