Four-wheel vehicle drive system

A four-wheel vehicle drive system, comprising an engine having an output shaft in a lateral direction of the vehicle, a power transmission gear unit including transmission input and output shafts having axes of rotation parallel with the engine output shaft, a final reduction gear rotatable about an axis parallel with the transmission input and output shafts, the transmission output shaft being in driving engagement with the final reduction gear, a gear housing rotatable with the final reduction gear, a main transaxle casing having enclosed therein the transmission gear unit, final reduction gear and gear housing, an auxiliary transaxle casing secured to the main transaxle casing, a first wheel drive gear unit to split driving power from the final reduction gear into two components and including a differential gear assembly enclosed in the auxiliary transaxle casing and operative to transmit one of the power components to a pair of road wheels, a second wheel drive gear unit enclosed in the auxiliary transaxle casing and operative to transmit therethrough the other of the two driving power components in a fore-and-aft direction of the vehicle to drive another pair of road wheels, and low-and-high speed shifting means including a shift gear assembly enclosed within the gear housing and operative to transfer driving power from the final reduction gear to the differential gear assembly selectively with two different gear ratios.

FIELD OF THE INVENTION 
The present invention relates to a four-wheel drive system for an 
automotive vehicle having at least two pairs of road wheels consisting of 
a pair of front road wheels and a pair of rear road wheels and, more 
particularly, to a transaxle mechanism for use in a four-wheel drive 
system for such a wheeled vehicle. 
DESCRIPTION OF THE PRIOR ART 
In an automotive vehicle equipped with a four-wheel drive system, it is 
desired to provide not only clutch and transmission gear units but 
low-and-high speed shifting means adapted to transfer the power output of 
the transmission gear unit selectively with two different input/output 
gear ratios therethrough so as to enable the drive system to drive the 
front and rear road wheels with torques adequate for various operational 
and road conditions. Whereas, a four-wheel drive system is known which is 
of the type using a power plant positioned to have an axis of rotation in 
a lateral direction of the vehicle body. Typical examples of such a 
four-wheel drive system are disclosed in, for example, Japanese 
Provisional Publications of Patent No. 55-11948 and No. 55-17727. The 
prior-art four-wheel drive system therein shown is, however, not provided 
with the low-and-high speed shifting means of the above described nature. 
If a transaxle mechanism originally designed to form part of a two-wheel 
drive system is to be modified to construct a transaxle mechanism for a 
four-wheel drive system having low-and-high speed shifting means, it is 
required to have the low-and-high speed shifting means positioned between 
the laterally positioned power plant and a front-wheel differential gear 
assembly forming part of the transaxle mechanism. For this purpose, 
drastic modification is required of the construction and arrangement of 
the transaxle mechanism for the two-wheel drive system. 
In an effort to provide enhanced compatibility between a transaxle 
mechanism for a two-wheel drive system and that for a four-wheel drive 
system, a transaxle mechanism for use with a laterally positioned internal 
combustion engine has been proposed which features low-and-high speed 
shifting means provided between the clutch and transmission gear units of 
the transaxle mechanism. An example of a four-wheel drive system using 
such a transaxle mechanism is taught in Japanese Provisional Publication 
of Utility Model No. 55-170129. Considerable design modifications and 
production costs therefor are, however, still required for re-constructing 
a transaxle mechanism for a two-wheel drive system into a transaxle 
mechanism for a four-wheel drive system. 
The present invention contemplates elimination of these drawbacks of known 
four-wheel drive systems of the described characters. It is, accordingly, 
a prime object of the present invention to provide a four-wheel drive 
system including a transaxle mechanism which has low-and-high speed 
shifting means incorporated therein and which can be constructed by 
slightly modifying a transaxle mechanism originally designed for use in a 
two-wheel drive system of an automotive vehicle. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a four-wheel 
drive system for a vehicle with first and second pairs of road wheels, 
comprising a power unit having an output shaft rotatable about an axis in 
a lateral direction of the vehicle; a power transmission gear unit 
including transmission input and output shafts each having an axis of 
rotation parallel with an extension of the axis of rotation of the output 
shaft of the power unit; a final reduction gear rotatable about an axis 
parallel with the respective axes of rotation of the transmission input 
and output shafts, the transmission output shaft being held in driving 
engagement with the final reduction gear; a gear housing rotatable with 
the final reduction gear about the axis of rotation of the final reduction 
gear; a main transaxle gear casing having enclosed therein the 
transmission gear unit, the final reduction gear and the gear housing; an 
auxiliary transaxle gear casing secured to the main transaxle gear casing; 
a first wheel drive gear unit comprising power splitting gear means 
operative to split driving power from the final reduction gear into two 
power components and a differential gear assembly operative to transmit 
one of the two power components to the first pair of road wheels, the 
power splitting means and the differential gear assembly being enclosed 
within the auxiliary transaxle gear casing; a second wheel drive gear unit 
enclosed within the auxiliary transaxle gear casing and operative to 
transmit therethrough the other of the two driving power components in a 
fore-and-aft direction of the vehicle, the power splitting gear means 
operatively intervening between the final reduction gear and the second 
wheel drive gear unit; and low-and-high speed shifting means operative to 
transfer driving power from the final reduction gear to the differential 
gear assembly selectively with two different gear ratios, the low-and-high 
speed shifting means including a low-and-high speed shift gear assembly 
enclosed within the gear housing. In the transaxle mechanism thus 
constructed and arranged, the differential gear assembly of the first 
wheel drive gear unit preferably comprises a pair of rotatable output 
members which have respective axes of rotation substantially aligned with 
each other and through which one of the aforesaid two power components is 
to be transmitted to the first pair of road wheels, the low-and-high speed 
shift gear assembly comprising power input and output members rotatable 
about a common axis substantially aligned with the axis of rotation of the 
output members of the differential gear assembly, the power input member 
of the low-and-high speed shift gear assembly being rotatable with the 
final reduction gear. More specifically, the differential gear assembly of 
the first wheel drive gear unit may comprise a rotatable input member and 
a pair of rotatable output members which have respective axes of rotation 
substantially aligned with each other and through which one of the 
aforesaid two power components is to be transmitted to the first pair of 
road wheels, the low-and-high speed shifting means comprising a constant 
power input member rotatable with the final reduction gear, a constant 
power output member rotatable with the input member of the differential 
gear assembly and a lockable output member engageable with the auxiliary 
transaxle gear casing, the input and output members of the low-and-high 
speed shift gear assembly being rotatable about a common axis 
substantially aligned with the axis of rotation of the output members of 
the differential gear assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawings, a four-wheel drive system according to the 
present invention comprises a transaxle mechanism in combination with a 
power unit typically constituted by an internal combustion engine which is 
schematically indicated at 1 in FIG. 1. The internal combustion engine 1 
has a power output shaft la which is constituted by, for example, the 
crankshaft of an ordinary internal combustion engine for automotive use. 
The transaxle mechanism of the four-wheel drive system proposed by the 
present invention is to be used in a vehicle having the engine positioned 
laterally of the vehicle body. The engine 1 is thus installed on the body 
structure (not shown) of a wheeled vehicle in such manner that the output 
shaft la extends in a lateral direction of the vehicle body. The transaxle 
mechanism of the system embodying the present invention is further assumed 
as being incorporated in an automotive vehicle of the front-engine design 
and, thus, the engine 1 is positioned in a front portion of the vehicle 
body. As is further shown in FIG. 1 of the drawings, the transaxle 
mechanism for use in such a four-wheel drive system includes a clutch unit 
2, and a power transmission gear unit 3. In the embodiment of the present 
invention as herein shown, it is assumed by way of example that the clutch 
unit 2 is of the mechanical friction-disc type and that the power 
transmission gear unit 3 is of the manually operated synchromesh type. The 
transaxle mechanism of the system embodying the present invention 
comprises, in addition to the transmission gear unit 3, a front-wheel 
drive gear unit 4 and a rear-wheel drive gear unit 5 which are enclosed 
together with the transmission gear unit 3 within a transaxle casing 
structure fixedly mounted on the vehicle body. 
The power transmission gear unit 3 is enclosed within a main transaxle gear 
casing 6 forming part of the above mentioned transaxle casing structure 
and comprises input and output shafts 7 and 8. The input shaft 7 has 
opposite end portions respectively journaled in bearings 9 and 9' received 
in the transaxle gear casing 6 and extends in alignment with the axis of 
rotation of the engine output shaft 1a. The transmission output shaft 8 
likewise has opposite end portions respectively journaled in bearings 10 
and 10' received in the transaxle gear casing 6 and extends in parallel 
with the transmission input shaft 7. The transmission input shaft 7 is 
selectively coupled to and uncoupled from the engine output shaft 1a 
through the clutch unit 2. The transmission gear unit 3 is assumed to be 
of the four-forward-speed and one-reverse-speed type and thus comprises 
five input gears rotatable with the transmission input shaft 7 and 
consisting of first-speed to fourth-speed forward drive gears 11 to 14, 
and a reverse drive gear 15. On the other hand, the transmission output 
shaft 8 has mounted thereon four driven gears rotatable independently of 
one another on the shaft 8 and consisting of first-speed to fourth-speed 
driven gears 16 to 19. The gears 16 to 19 are held in mesh with the drive 
gears 11 to 14, respectively, on the transmission input shaft 7. The 
transmission gear unit 3 further comprises a reverse idler shaft 20 having 
an idler gear 21 rotatable and axially slidable thereon and movable on the 
shaft 20 into and out of an axial position held in mesh with the reverse 
drive gear 15 on the transmission input shaft 7 as indicated by broken 
lines in FIG. 1. The reverse idler shaft 20 also extends in parallel with 
the transmission input shaft 7 and has opposite end portions secured to 
the main transaxle gear casing 6. 
The transmission gear unit 3 further comprises first-second and 
third-fourth speed synchronizer clutch assemblies 22 and 23, each of which 
is rotatable with the transmission output shaft 8. The first-second speed 
synchronizer clutch assembly 22 is provided between the first-speed and 
second-speed driven gears 16 and 17 and is selectively engageable with 
these gears 16 and 17. Likewise, the third-fourth speed synchronizer 
clutch assembly 23 is provided between the third-speed and fourth-speed 
driven gears 18 and 19 and is selectively engageable with the gears 18 and 
19. The transmission output shaft 8 has fixedly mounted thereon a 
transmission output gear 24 which is thus rotatable with the transmission 
output shaft 8. The transmission output gear 24 is held in mesh with an 
annular final reduction gear 25 also enclosed within the main transaxle 
gear casing 6 and rotatable with respect to the gear casing 6 about an 
axis parallel with the axis of rotation of the transmission output shaft 
8. 
As will be better seen from FIG. 2 of the drawings, the main transaxle gear 
casing 6 has further enclosed therein a low-and-high speed shift gear 
assembly 26 as well as the above described clutch unit 2, transmission 
gear unit 3 and the final reduction gear 25 and is securely connected to 
an auxiliary transaxle gear casing 27. The low-and-high speed shift gear 
assembly 26 forms part of low-and-high speed shifting means in the 
transaxle mechanism of the four-wheel drive system according to the 
present invention. In the shown embodiment of the present invention, the 
low-and-high speed shift gear assembly 26 is operative to selectively 
produce two input/output gear ratios and is constituted by a planetary 
gear assembly which comprises a hollow gear housing 28 which is integral 
with the above mentioned final reduction gear 25 and which has opposite 
axial boss portions respectively journaled in bearings 29 and 29' received 
in the auxiliary transaxle gear casing 27. The gear housing 28 is thus 
rotatable with the final reduction gear 25 about the axis of rotation of 
the gear 25 with respect to the auxiliary transaxle gear casing 27. The 
planetary gear assembly further comprises an externally toothed sun gear 
30, an internally toothed ring gear 31 and a plurality of externally 
toothed planet pinions 32. The sun gear 30 is constituted by an axial 
portion of a sleeve 33 axially passing through the gear housing 28 and 
having a center axis coincident with the axis of rotation of the housing 
28. The sleeve 33 is rotatable with respect to the transaxle gear casing 
27 and the gear housing 28 about the axis of rotation of the housing 28 
and has an externally serrated axial portion 34 axially projecting 
outwardly from the gear housing 28 in a direction opposite to the final 
reduction gear 25 as shown. The ring gear 31 is constituted by an annular 
inner wall portion of the gear housing 28 and coaxially encircles the sun 
gear 30. The planet pinions 32 intervene between the sun and ring gears 30 
and 31 and are held in mesh with these gears 30 and 31. The planet pinions 
32 are connected together by an internally serrated pinion carrier 35 
rotatable with respect to the auxiliary transaxle gear casing 27 and the 
gear housing 28 about an axis aligned with the common axis of rotation of 
the sun and ring gears 30 and 31. Each of the planet pinions 32 is, thus, 
rotatable not only about the center axis thereof with respect to the 
pinion carrier 35 but, together with the pinion carrier 35, about the 
common axis of rotation of the sun gear 30 and ring gear 31 with respect 
to the auxiliary transaxle gear casing 27 and the gear housing 28. 
The front-wheel drive gear unit 4 of the four-wheel drive system embodying 
the present invention comprises a pair of front-wheel side gear shafts 36 
and 36' having respective axes of rotation which are aligned with the axis 
of rotation of the final reduction gear 25 and which are thus parallel 
with the input and output shafts 7 and 8 of the transmission gear unit 
(FIG. 1). One front-wheel side gear shaft 36 has an inner axial portion 
rotatably received in a hollow shaft 37 axially extending in part through 
the sleeve 33 and having an axially outer end portion splined to the 
pinion carrier 35 of the planetary gear assembly. The side gear shafts 36 
and 36' have serrated inner end portions and axially extend from a 
frontwheel differential gear assembly 38 in opposite directions laterally 
of the vehicle body as will be seen from FIG. 1. The front-wheel 
differential gear assembly 38 is also enclosed within the auxiliary 
transaxle gear casing 27 and comprises a gear housing 39 which is 
rotatable about an axis aligned with the axes of rotation of the side gear 
shafts 36 and 36'. The gear housing 39 has opposite axial boss portions 
respectively journaled in bearings 40 and 40' received in the auxiliary 
transaxle gear casing 27 and has carried therein a pair of differential 
bevel pinions 41 which are rotatably mounted on a common pinion cross 
shaft 42 secured to the gear housing 39 and extending at right angles to 
the axis of rotation of the gear housing 39. The individual bevel pinions 
41 are, thus, rotatable not only together with the gear housing 39 and 
cross shaft 42 about the axis of rotation of the gear housing 39 but also 
independently of one another about the center axis of the cross shaft 42, 
viz., about an axis perpendicular to the axis of rotation of the gear 
housing 39. The differential bevel pinions 41 intervene between and are 
held in mesh with a pair of differential side bevel gears 43 and 43' which 
are also carried in the differential gear housing 39 and which are 
rotatable about the axis of rotation of the gear housing 39. The bevel 
gears 43 and 43' constitute power output members of the differential gear 
assembly 38 and have respective axes of rotation aligned with the common 
axis of rotation of the sun gear 30, ring gear 31 and pinion carrier 35 of 
the planetary gear assembly. The side bevel gears 43 and 43' are fixedly 
connected to or splined to the serrated inner end portions of the side 
gear shafts 36 and 36', respectively, extending in a lateral direction of 
the vehicle body. The front-wheel side gear shafts 36 and 36' form part of 
front axle assemblies and are operatively connected at their axially outer 
ends to front wheel drive shafts 44 and 44' through suitable coupling 
means such as constant-velocity or universal coupling units 45 and 45', 
respectively. The front wheel drive shafts 44 and 44' also extend in a 
lateral direction of the vehicle body and are in turn connected at their 
outer axial ends to the front wheel axles for front road wheels 46 and 46' 
via suitable coupling means such as constant-velocity or universal units 
47 and 47', respectively, as shown in FIG. 1. The gear housing 39 of the 
differential gear assembly 38 has one of its axial boss portions formed 
with an externally serrated, hollow axial extension 39a splined to an 
internally serrated axially inner end portion of the hollow shaft 37 as 
shown in FIG. 2. The hollow axial extension 39a of the gear housing 39 is 
also externally serrated and is splined to a second clutch gear 51 which 
will become apparent as the description proceeds. 
The low-and-high speed shifting means of the drive system embodying the 
present invention comprises, in addition to the above described 
low-and-high speed shift gear assembly 26, a low-and-high speed shift 
control assembly 48 enclosed within the transaxle gear casing 27 and 
adapted to have the low-and-high speed shift gear assembly 26 conditioned 
to selectively produce the two input/output gear ratios. The low-and-high 
speed shift control assembly 48 comprises a driving member 49 having an 
externally serrated annular portion 49a and a tubular extension 49b. The 
serrated annular portion 49a coaxially surrounds the axial portion of the 
hollow shaft 37 and is located axially adjacent the externally serrated 
axial extension 39a of the differential gear housing 39. The tubular 
extension 49b extends from the annular portion 49a in coaxial relationship 
to the hollow shaft 37 and has an internally serrated end portion splined 
to the previously mentioned externally serrated peripheral portion 34 of 
the sleeve 33. The driving member 49 is thus rotatable with the final 
reduction gear 25 about the axis of rotation of the side gear shaft 36 and 
has its tubular extension 49b rotatably received in inner wall portions 
of the auxiliary transaxle gear casing 27. The low-and-high speed shift 
control assembly 48 further comprises first and second clutch gears 50 and 
51 disposed also in coaxial relationship to the hollow shaft 37 and in 
such a manner that the annular portion 49a of the driving member 49 
axially intervenes therebetween. The second clutch gear 51 is internally 
serrated and is splined to the previously mentioned externally serrated 
axial extension 39a of the differential gear housing 39. Furthermore, the 
second clutch gear 51 has an externally serrated annular portion 51a 
axially adjacent one end face of the serrated annular portion 49a of the 
driving member 49. On the other hand, the first clutch gear 50 is securely 
connected to the auxiliary transaxle gear casing 27 and has an externally 
serrated annular portion 50a axially adjacent the other end face of the 
serrated annular portion 49a of the driving member 49. The externally 
serrated annular portion 49a of the driving member 49 is splined to an 
internally serrated, annular coupling sleeve 52. The coupling sleeve 52 is 
axially movable on the externally serrated annular portion 49a of the 
driving member 49 selectively into engagement with the externally serrated 
annular portion 50a of the first clutch gear 50 or the externally serrated 
annular portion 51a of the second clutch gear 51. The coupling sleeve 52 
is formed with an external circumferential groove having fitted therein a 
clutch actuating fork 53. Though not shown in the drawings, the clutch 
actuating fork 53 is connected through a suitable mechanical linkage to 
manually or otherwise operated low-and-high speed shift control means so 
that the coupling sleeve 52 is axially moved selectively into engagement 
with the first or second clutch gear 50 or 51. The sun gear 30 of the 
planetary gear assembly constituting the previously described low-and-high 
speed shift gear assembly 26 is thus connected to and rotatable with the 
driving member 49 of the above described low-and-high speed shift control 
assembly 48 through the sleeve 33, while the pinion carrier 35 of the 
planetary gear assembly is connected to and rotatable with the second 
clutch gear 51 of the low-and-high speed shift control assembly 48 through 
the hollow shaft 37 and the gear housing 39 of the front-wheel 
differential gear assembly 38. 
The front-wheel drive gear unit 4 further comprises power splitting gear 
means enclosed within the auxiliary transaxle gear casing 27 and operative 
to split driving power from the low-and-high speed shift control assembly 
48 into two driving power components one of which is to be transmitted to 
the front-wheel differential gear assembly 38 and the other of which is to 
be transmitted to the rear-wheel drive gear unit 5. In the shown 
embodiment of a four-wheel drive system according to the present 
invention, such power splitting gear means comprises a first power 
transfer gear 54 which is constituted by a portion of or otherwise 
coaxially rotatable with the differential gear housing 39, and a second 
power transfer gear 55 which is coaxially rotatable on a power transfer 
gear shaft 56 and which is held in mesh with the first power transfer gear 
54. The power transfer gear shaft 56 is rotatable about an axis parallel 
with the side gear shaft 36 and has an axial end portion journaled in a 
bearing 57 received in the transaxle gear casing 27 and an externally 
serrated opposite axial end portion 56a. 
On the other hand, the rear-wheel drive gear unit 5 is also enclosed within 
the auxiliary transaxle gear casing 27 and comprises a 
two-wheel/four-wheel drive shift gear assembly 58 adapted to selectively 
establish or cut off driving connection from the first power transfer gear 
54 to the rear-wheel driveline. In the system embodying the present 
invention, such a two-wheel/four-wheel drive shifting gear assembly 58 
comprises at least three clutch members consisting of a first clutch 
member held in driving connection to the rear wheel driveline and 
rotatable with, for example, the power transfer shaft 56, a second clutch 
member rotatable with the second power transfer gear 55, and a third 
clutch member rotatable with the first clutch member and selectively 
movable into and out of engagement with the first and second clutch 
members. In the embodiment herein shown, the first clutch member is 
constituted by an externally serrated annular clutch member 59 mounted on 
or splined to an axial end portion of the power transfer shaft 56 and thus 
coaxially rotatable with the shaft 56. On the other hand, the second 
clutch member is constituted by a clutch gear 60 coaxially rotatable on 
the power transfer shaft 56 and integral or otherwise rotatable with the 
second power transfer gear 55. The clutch gear 60 has an externally 
serrated annular portion located axially adjacent the clutch member 59. 
The two-wheel/four-wheel drive shifting gear assembly 58 further comprises 
an internally serrated, annular coupling sleeve 61 which is splined to the 
clutch member 59. The coupling sleeve 61 constitutes the above mentioned 
third clutch member. The coupling sleeve 61 is formed with an external 
circumferential groove having fitted therein a clutch actuating fork 62. 
Though not shown in the drawings, the clutch actuating fork 62 is 
connected through a suitable mechanical linkage to manually or otherwise 
operated two-wheel/four-wheel drive shift control means so that the 
coupling sleeve 61 is axially moved selectively into engagement with the 
clutch gear 60. 
The rear-wheel drive gear unit 5 further comprises a right-angle power 
transfer gear assembly 63 operatively connected through a rear wheel 
driveline to the wheel axles for rear road wheels. The right-angle power 
transfer gear assembly 63 consists of a driving bevel gear 64 and a driven 
bevel gear 65. The driving bevel gear 64 has a boss portion journaled in a 
bearing 66 received in the auxiliary transaxle gear casing 27 and is 
splined to the serrated axial end portion 56a of the power transfer shaft 
56 and is thus coaxially rotatable with the power transfer shaft 56. The 
driven bevel gear 65 is held in mesh with the driving bevel gear 64 and is 
rotatable about an axis perpendicular to the axis of rotation of the power 
transfer shaft 56. The driven bevel gear 65 has a rearward axial extension 
65a journaled in bearings 67 and 67' received in the auxiliary transaxle 
gear casing 27. The axial extension 65a of the driven bevel gear 65 
projects rearwardly from the gear casing 27 through an opening formed 
therein as shown in FIG. 2 and is connected through a suitable joint unit 
such as a constant-velocity or universal coupling unit 68 to a propeller 
shaft 69 extending rearwardly from the gear 65 in a fore-and-aft direction 
of the vehicle body as shown in FIG. 1. The propeller shaft 69 forms part 
of the rear wheel driveline and is connected at its rear end to a 
rear-wheel final reduction and differential gear assembly 70 through a 
constant-velocity or universal coupling unit 71. The rear-wheel final 
reduction and differential gear assembly 70 has a power input member 
connected to the propeller shaft 69 through the coupling unit 71 and a 
pair of power output members connected to side gear shafts 72 and 72', 
respectively, and is adapted to produce between the input member and each 
of the output members a gear ration equal to that achieved in the 
differential gear assembly 38 of the front-wheel drive gear unit 4. The 
side gear shafts 72 and 72' axially extend in a lateral direction of the 
vehicle body from the gear assembly 70 similarly to the front-wheel side 
gear shafts 36 and 36'. These side gear shafts 72 and 72' are connected at 
their axially outer ends to rear wheel drive shafts 73 and 73' through 
constant-velocity or universal coupling units 74 and 74', respectively. 
The rear-wheel drive shafts 73 and 73' also extend in a lateral direction 
of the vehicle body and are connected at their outer axial ends to the 
rear wheel axles for front road wheels 75 and 75' through 
constant-velocity or universal coupling units 76 and 76', respectively. 
Description will now be made regarding the operation of the four-wheel 
drive system including the transaxle mechanism constructed and arranged as 
hereinbefore described. 
When the engine 1 is in operation and the clutch unit 2 is in a coupled 
condition, the driving power delivered from the output shaft 1a of the 
engine 1 is transmitted through the clutch unit 2 to the input shaft 7 of 
the power transmission gear unit 3. If, under these conditions, one of the 
driven gears 16 and 19 on the transmission output shaft 8 is coupled to 
the shaft 8 through the associated synchronizer clutch assembly 22 or 23 
or the reverse idler gear 21 is held in mesh with the reverse drive gear 
15 and the synchronizer clutch assembly 22, the driving power carried to 
the transmission input shaft 7 is transmitted to the transmission output 
shaft 8 through the selected pair of gears on the shafts 7 and 8 or 
through the gears 15 and 21. The transmission output shaft 8 is, as a 
result, driven for rotation at a speed proportioned in the selected ratio 
to the rotational speed of the transmission input shaft 7. The rotation of 
the transmission output shaft 8 is transmitted via the transmission output 
gear 24 on the shaft 8 to the final reduction gear 25 which is held in 
constant mesh with the transmission output gear 24. The rotation of the 
final reduction gear 25 is carried through the gear housing 28 to the ring 
gear 31 of the planetary gear assembly constituting the low-and-high speed 
shift gear assembly 26 and drives the ring gear 31 for rotation about the 
center axis of the front-wheel side gear shaft 36. The ring gear 31 in 
turn drives the planet pinions 32 for rotation about the respective center 
axes thereof and thereby further drives the pinion carrier 35 and 
accordingly the hollow shaft 37 for rotation about the common axis of 
rotation of the sun and ring gears 30 and 31. The rotation of the hollow 
shaft 37 is transmitted to the gear housing 39 of the differential gear 
assembly 38 and causes the gear housing 39 to rotate about the aligned 
axes of rotation of the side gear shafts 36 and 36'. 
If, in this instance, the annular coupling sleeve 52 of the low-and-high 
speed shift control assembly 48 is held in the axial position engaging the 
annular portion 50a of the first clutch gear 50 integral with the 
auxiliary transaxle gear casing 27, the driving member 49 is locked to the 
gear casing 27 by the coupling sleeve 52. Under these conditions, the sun 
gear 30 of the planetary gear assembly constituting the low-and-high speed 
shift gear assembly 26 is held stationary with respect to the transaxle 
gear casing 27 so that the pinion carrier 35 is driven to rotate about the 
common center axis of sun and ring gears 30 and 31 at a speed lower than 
the speed of rotation of the ring gear 31. The rotation of the ring gear 
31 is transmitted through the hollow shaft 37 to the gear housing 39 of 
the differential gear assembly 38. The differential gear housing 39 is 
accordingly driven for rotation about an axis at right angles to the cross 
shaft 42 at a speed lower than the speed of rotation of the final 
reduction gear 25. 
If, on the other hand, the annular coupling sleeve 52 of the low-and-high 
speed shift control assembly 48 is held in the axial position engaging the 
serrated annular portion 51a of the second clutch gear 51 integral with 
the differential gear housing 39, the driving member 49 rotatable with the 
sun gear 30 is driven for rotation with the second clutch gear 51 and 
accordingly with the hollow shaft 37 rotating with the pinion carrier 35 
of the shift gear assembly 26. Under these conditions, the sun gear 30, 
ring gear 31 and pinion carrier 35 are caused to turn as a single unit 
about the common axis of rotation of the sun and ring gears 30 and 31 by 
the final reduction gear 25. Accordingly, the driving power transmitted 
from the final reduction gear 25 to the low-and-high speed shift gear 
assembly 26 is transmitted to the gear housing 39 of the differential gear 
assembly 38 through the sleeve 33, driving member 49, coupling sleeve 52 
and second clutch gear 51 as well as the hollow shaft 37. The gear housing 
39 of the differential gear assembly 38 is thus driven for rotation at a 
speed equal to the speed of rotation of the final reduction gear 25 about 
the axis at right angles to the cross shaft 42. Driving power is in these 
manners transmitted from the final reduction gear 25 to the gear housing 
39 of the differential gear assembly 38 selectively at two different 
speeds depending upon the axial positions of the coupling sleeve 52 on the 
annular portion 49a of the driving member 49. As will have been understood 
from the foregoing description, the planetary gear assembly constituting 
the low-and-high speed shift gear assembly 26 in the system embodying the 
present invention consists essentially of a constant power input member 
constituted by the ring gear 31, a constant power output member 
constituted by the pinion carrier 35 and a lockable power output member 
constituted by the sun gear 30. If desired, however, such an arrangement 
of the planetary gear assembly may be modified as long as the planetary 
gear assembly comprises a constant power input member constituted by one 
of the sun gear 30, ring gear 31 and pinion carrier 35, a constant power 
output member constituted by one of the remaining two of the sun gear 30, 
ring gear 31 and pinion carrier 35, and a lockable power output member 
constituted by the remaining one of the sun gear 30, ring gear 31 and 
pinion carrier 35. 
The driving power transmitted from the final reduction gear 25 to the gear 
housing 39 of the front-wheel differential gear assembly 38 is also 
carried not only to the bevel pinions 41 of the differential gear assembly 
38 through the pinion cross shaft 42 but to the first power transfer gear 
54 integral with the differential gear housing 39. In the differential 
gear assembly 38, the gear housing 39 drives the differential bevel 
pinions 41 for rotation with the housing 39 about the center axis of the 
pinion cross shaft 42. The differential bevel pinions 41 in turn drive the 
differential side bevel gears 43 and 43' for rotation with respect to the 
gear housing 39 about an axis at right angles to the center axis of the 
pinion cross shaft 42. Thus, the driving power transmitted to the 
differential gear assembly 38 is further split into two output components, 
which are transmitted to the side gear shafts 36 and 36' and further 
through these shafts 36 and 36', coupling units 45 and 45', front-wheel 
drive shafts 44 and 44' and coupling units 47 and 47' to the wheel axles 
of the front road wheels 46 and 46', respectively (FIG. 1). 
On the other hand, the driving power transmitted to the first power 
transfer gear 54 of the power splitting means is carried to the second 
power transfer gear 55 on the power transfer gear shaft 56. If, in this 
instance, the coupling sleeve 61 of the two-wheel/four-wheel drive 
shifting gear assembly 58 is held in the axial position engaging the 
serrated annular portion of the clutch gear 60, the driving power imparted 
from the first power transfer gear 54 to the second power transfer gear 55 
is transmitted through the coupling sleeve 61 and the annular clutch 
member 59 to the power transfer gear shaft 56 and drives the shaft 56 for 
rotation about the center axis thereof. The power transfer gear shaft 56 
thus drives the driving bevel gear 64 of the right-angle power transfer 
gear assembly 63 for rotation with the power transfer gear shaft 56, and 
the driving bevel gear 64 in turn drives the driven bevel gear 65 for 
rotation about the axis thereof in a fore-and-aft direction of the vehicle 
body. The driving power transmitted to the driving member 49 is, thus, 
carried not only to the front road wheels 46 and 46' as above described 
but also to the rear-wheel final reduction and differential gear assembly 
70 (FIG. 1) via the coupling unit 68, propeller shaft 69 and coupling unit 
71. The rear-wheel final reduction and differential gear assembly 70 
splits the input driving power into two driving power components 
respectively driving the rear-wheel side gear shafts 72 and 72'. The 
driving power components are further transmitted via the coupling units 74 
and 74', rear-wheel drive shafts 73 and 73' and coupling units 76 and 76' 
to the wheel axles of the rear road wheels 75 and 75', respectively. The 
front road wheels 46 and 46' and the rear road wheels 75 and 75' are thus 
driven for rotation so that the vehicle operates in a four-wheel driven 
mode. If, however, the coupling sleeve 61 of the two-wheel/four-wheel 
drive shifting gear assembly 58 is held in the axial position disengaged 
from the serrated annular portion of the clutch gear 60, the clutch member 
59 and accordingly the power transfer gear shaft 56 are isolated from the 
driving power transmitted to the second power transfer gear 55 and are 
allowed to idle together with the driving bevel gear 64 on the shaft 56. 
In this instance, only the front road wheels 46 and 46' are driven for 
rotation so that the vehicle operates in a two-wheel driven mode. 
While the power unit in the four-wheel drive system embodying the present 
invention has been assumed as being positioned in a front portion of the 
vehicle body, the power unit of a transaxle mechanism of a four-wheel 
drive system according to the present invention may be installed in a 
lengthwise middle or rear portion of the vehicle body. 
While, furthermore, the system embodying the present invention has been 
described as using the power transmission system of the manually-operated 
type, it will be apparent that a four-wheel drive system according to the 
present invention may be of the type which uses a power transmission 
system of the automatically-operated type. 
One of the outstanding advantages of the four-wheel drive system proposed 
by the present invention as thus far described is that the low-and-high 
speed shift gear assembly 26 forming part of the low-and-high speed 
shifting means is incorporated into the gear housing 28 which is integral 
with the final reduction gear 25 and within which a differential gear 
assembly is to be installed in the case of a two-wheel driven vehicle. By 
virtue of such an arrangement, those members, units and assemblies 
required in a four-wheel driven vehicle such as, for example, the 
low-and-high speed shift control assembly 48, front-wheel differential 
gear assembly and the power splitting gears 54 and 55 can be accommodated 
within an additional casing constituted by the gear casing 27. In 
accordance with the present invention, the transaxle mechanism including 
low-and-high speed shifting means can thus be realized simply by modifying 
a transaxle mechanism for a two-wheel vehicle drive system in such a 
manner that the gear housing having the differential gear assembly for the 
two-wheel drive system is adapted to have accommodated therein the 
low-and-high speed shift gear assembly 26 and that the auxiliary transaxle 
gear casing 27 is attached additionally to the main transaxle gear casing 
6. This will contribute to significant reduction of the production cost of 
a transaxle mechanism for use in a four-wheel drive system having a 
low-and-high speed shift feature as well as a two-wheel/four-wheel shift 
feature.