Transfer case for part time front wheel drive in a four wheel drive motor vehicle

The present invention provided a power transfer system for a four-wheel drive vehicle. The transfer system includes a part-time transfer case having an input member rotatably driven by the vehicle's engine, first and second output members operable to rotatively drive respective front and rear axle assemblies, and an intermediate shaft through which power is selectively transferred from the input member to one or both of the first and second output members. In operation, driven rotation of the intermediate shaft causes power to be constantly delivered to the first output member and the front axle assembly for establishing a two-wheel drive mode of operation. Moreover, selective actuation of a mode shift arrangement functions to interconnect the second output member to the intermediate shaft for also delivering power to the rear axle assembly for establishing a four-wheel drive mode of operation.

BACKGROUND OF THE INVENTION 
The present invention relates generally to four-wheel drive vehicles and, 
more particularly, to a part-time transfer case for use in front wheel 
drive vehicular applications. 
With the advent of increased consumer popularity in four-wheel drive 
passenger cars and sport/utility vehicles, part-time transfer cases are 
more frequently being incorporated in vehicular driveline applications. As 
is known, conventional part-time transfer cases are used in four-wheel 
drive vehicles to normally deliver power to the rear wheels of the vehicle 
for establishing a two-wheel drive operating mode. However, such part-time 
transfer cases are also operable for selectively delivering power to both 
the front and rear wheels for establishing a four-wheel drive operating 
mode. Since the weight distribution for most four-wheel drive vehicles is 
biased toward the front wheels (i.e., due to the weight of the engine and 
transmission), during two-wheel drive operation the greatest potential for 
wheel slip due to lost traction is associated with the driven rear wheels. 
This being the case, the need exists to provide a part-time transfer case 
which normally delivers power to the front wheels of the vehicle in the 
two-wheel drive mode and yet is operable for selectively delivering power 
to both the front and rear wheels in the four-wheel drive mode. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to overcome the 
disadvantages associated with conventional four-wheel drive vehicles by 
providing a power transfer system including a part-time transfer case 
which can be selectively shifted between a two-wheel drive mode for 
normally delivering power to the front wheels, and a four-wheel drive mode 
for selectively delivering power to all four wheels. 
As a related object, the part-time transfer case of the present invention 
can be readily substituted for conventional transfer cases in a manner 
permitting utilization of standardized high-volume driveline components. 
Otherwise stated, it is an object of the present invention to provide a 
part-time transfer case that is normally operative in a front-wheel drive 
mode and yet which is adapted to cooperate with original equipment 
driveline components from conventional four-wheel drive vehicles. 
It is yet another object of the present invention to provide the part-time 
transfer case with a mode shift arrangement that is operable for 
permitting the vehicle operator to selectively transfer power to the rear 
wheels in addition to the front wheel for establishing the four-wheel 
drive mode of operation. 
In accordance with the above and other objects, the present invention is 
preferably embodied by a part-time transfer case having an input member 
rotatably driven by the vehicle's engine, first and second output members 
operable to rotatively drive respective front and rear axle assemblies, 
and an intermediate shaft through which power is selectively transferred 
from the input member to one or both of the first and second output 
members. In operation, driven rotation of the intermediate shaft causes 
power to be constantly delivered to the first output member and the front 
axle assembly for establishing the two-wheel drive mode. Moreover, 
selective actuation of the mode shift arrangement functions to 
interconnect the second output member to the intermediate shaft for also 
delivering power to the rear axle assembly, thereby establishing the 
four-wheel drive mode.

DETAILED DESCRIPTION OF THE INVENTION 
In general, the present invention is directed to a part-time transfer case 
that is adapted for normally delivering drive torque to the front wheels 
of the four-wheel drive motor vehicle for establishing a two-wheel drive 
mode. The transfer case includes means for permitting the vehicle operator 
to selectively transfer drive torque to the rear wheels in addition to the 
front wheels for establishing a four-wheel drive mode. Such a four-wheel 
drive arrangement is advantageous in vehicular applications where the 
vehicle's weight distribution is biased over or toward the front wheels so 
as to deliver drive torque to the wheels having the greatest tractive 
potential during two-wheel drive operation. As will be appreciated, the 
part-time transfer case of the present invention can be originally 
installed or easily retro-fitted into conventional four-wheel drive 
vehicles without necessitating the use of new or modified driveline 
components. 
With particular reference to FIG. 1, a drivetrain 10 for a four-wheel drive 
vehicle is shown which incorporates the novel principles of the present 
invention. The motor vehicle drivetrain 10 has a pair of front wheels 12 
and a pair of rear wheels 14 both drivable from a source of power, such as 
an engine 16, through a transmission 18 which may be of either the manual 
or automatic type. In the particular embodiment shown, drivetrain 10 is 
arranged as a front wheel drive system which incorporates a part-time 
transfer case 20 operable to receive drive torque from engine 16 and 
transmission 18 for normally driving front wheels 12 in a two-wheel drive 
mode of operation. In addition, part-time transfer case 20 is adapted to 
permit a vehicle operator to also transfer drive torque to rear wheels 14 
for defining a four-wheel drive mode of operation. Front wheels 12 and 
rear wheels 14 are connected at opposite ends of front and rear axle 
assemblies 24 and 26, respectively. As is known in the art, a front 
differential 28 is interconnected between front axle assembly 24 and one 
end of a front drive shaft 30. The opposite end of front drive shaft 30 is 
interconnected to a first output shaft 32 of transfer case 20. As such, 
front wheels 12 are rotatably driven by front drive shaft 30 when 
part-time transfer case 20 is operating in either of the two-wheel drive 
mode or the four-wheel drive mode. Similarly, rear axle assembly 26 
includes a rear differential 34 that is coupled in driven relationship to 
one end of a rear drive shaft 36, the opposite end of which is coupled to 
a second output shaft 38 of part-time transfer case 20 for driving rear 
wheels 14 when part-time transfer case 20 is operating in the four-wheel 
drive mode. 
With particular reference now to FIGS. 2, a transmission output shaft 40 is 
shown which couples transmission 18 to transfer case 20 for supplying 
power (i.e., drive torque) thereto. Part-time transfer case 20 is shown to 
include a housing 42 formed by front and back half sections 44 and 46, 
respectively, that are suitably interconnected by a plurality of bolts 48. 
Front half section 44 receives transmission output shaft 40 within an 
internally splined input shaft 50. Input shaft 50 is rotatably mounted in 
a collar portion 52 by bearing assembly 54 with collar portion 52 being 
secured by bolts 56 to housing front half section 44. A speed reduction 
unit, such as planetary gear set 58, is provided for establishing a direct 
drive ("high"), reduced ratio ("low") and a "neutral" speed ranges as will 
be described further hereinafter. Planetary gear set 58 includes a sun 
gear 60 that is integrally formed on input shaft 50 and shown meshed with 
a plurality of planet gears 62. Each planet gear 62 is rotatably 
journalled on a pin 64 supported in a planetary carrier 66. Planetary 
carrier 66 includes fore and aft ring members 68 and 70, respectively. In 
addition, planet gears 62 also mesh with an annulus gear 72 that is 
fixedly secured to the inner surface of housing front section 44. Thus, 
rotation of input shaft 50 causes carrier 66 to rotate at a reduced speed 
ratio relative thereto. As will be appreciated, planetary gear set 58 is 
merely exemplary of one type of speed reduction unit which is well suited 
for use in part-time transfer case 20. 
With continued reference to FIG. 2, an intermediate shaft 74 is aligned on 
the longitudinal axis of input shaft 50 and has a first end portion 76 
supported by bearing assembly 78 in an axial bore 80 formed in input shaft 
50. Likewise, a second end portion 82 of intermediate shaft 74 is 
supported by bearing assembly 84 in an axial bore 86 formed in second 
output shaft 38. Moreover, a range collar 88 is supported for rotation 
with, and axial sliding movement on, intermediate shaft 74 by means of 
collar internal splines 90 engaged with external splines 92 formed on 
intermediate shaft 74. Range collar 88 also includes external clutch teeth 
94 shown slidably engaged with internal clutch teeth 96 formed in an axial 
counterbore of input shaft 50. In the position shown, power (i.e., drive 
torque) is transferred from input shaft 50 to intermediate shaft 74 via 
range collar 88, thereby establishing a direct drive or "high" range 
position indicated by the construction line "H". When range collar 88 is 
slid rearwardly to establish a reduced ratio or "low" range position, as 
indicated by construction line "L", its external clutch teeth 94 engage 
internal clutch teeth 98 formed on aft carrier ring 70. It will be noted 
that when range collar 88 is moved rearwardly a predetermined distance 
from its high range "H" position, its external clutch teeth 94 are 
disengaged from sun gear internal clutch teeth 96 and aft carrier ring 
internal clutch teeth 98 for defining a "neutral" drive range position 
indicated by construction line "N". With part-time transfer case 20 
shifted into the "neutral" position, input shaft 50 is disconnected from 
range collar 88 such that no torque is transmitted to intermediate shaft 
74, whereby no power is transmitted to either of the vehicle's front 
wheels 12 or rear wheels 14. Preferably, range collar 88 is selectively 
shiftable via movement of a range fork, partially shown at 99, that is 
coupled to a conventional shift fork assembly (not shown) in a manner 
known in the art. It is to be understood that the conventional shift fork 
assembly can be selectively shifted by the vehicle operator between the 
various speed ranges either manually (i.e. via a shift lever) or 
electrically (i.e., via a motor driven system). 
To provide means for transferring power from intermediate shaft 74 to first 
output shaft 32, a chain carrier 100 is fixedly retained on intermediate 
shaft 74. Chain carrier 100 includes a drive sprocket 102 drivingly 
engaging a chain, shown in dashed lines at 104, which, in turn, is coupled 
to a lower driven sprocket 106. In addition, driven sprocket 106 is 
coupled to, or an integral portion of, first output shaft 32 of part-time 
transfer case 20. Thus, torque is transferred to front wheels 12 when 
range collar 88 is in either of its high "H" range or low "L" range 
positions. First output shaft 32 is shown mounted for rotation within 
front housing section 44 by ball bearing assembly 108 and within rear 
housing section 46 by roller bearing assembly 110. Furthermore, transfer 
case first output shaft 32 is adapted to be operably connected by a first 
coupling 112 to the motor vehicle's front drive shaft 30. Similarly, 
second output shaft 38 is suitably connected by a second coupling (not 
shown) or splines 114 to the vehicle's rear drive shaft 36. Second output 
shaft 38 is shown to have one end supported for rotation within web 
portion 116 by a suitable bearing assembly 117. The remainder of second 
output shaft 38 extends through a tubular housing extension 118 and is 
supported for rotation therein via bearing assemblies 120 and 122. 
According to the teaching of the present invention, means are provided for 
mechanically shifting part-time transfer case 20 between the normal 
two-wheel drive mode and the four-wheel drive mode. According to the 
embodiment shown, the means to shift transfer case 20 between its 
two-wheel drive mode and four-wheel drive mode includes a "mode" shift 
arrangement 124 having a hub member 126 that is fixed for rotation with 
intermediate shaft 74. In addition, a mode sleeve 128 is formed with 
internal spline teeth 130 which are in constant axial sliding engagement 
with external spline teeth 132 formed on hub member 126. A mode fork, 
partially shown at 134, is coupled to mode sleeve 128 for permitting the 
vehicle operator to axially shift mode sleeve 128 via selective actuation 
of the shift fork assembly. Mode sleeve 128 is shown disengaged from 
external spline teeth 136 formed on second output shaft 38, as is 
indicated by the construction line "X", for causing intermediate shaft 74 
to only transfer drive torque to first output shaft 32, thereby 
establishing the two-wheel drive mode of operation. When it is desired to 
shift part-time transfer case 20 into the four-wheel drive mode, mode 
sleeve 128 is selectively shifted from the two-wheel drive mode "X" 
position to the four-wheel drive mode position, indicated by the 
construction "Y", by sliding internal spline teeth 130 into driving 
engagement with external spline teeth 136 on second output shaft 38. In 
this "lock-up" condition, drive torque is delivered from intermediate 
shaft 74 to second output shaft 38 as well as to front output shaft 32. 
Mode shift arrangement 124 is shown as a non-synchronized arrangement that 
is capable of being shifted "on-the-fly" between the two-wheel drive and 
four-wheel drive mode positions. More specifically, with rear axle 
assembly 26 being of the solid axle type, non-driven rotation of rear 
wheels 14 cause concomitant rotation of rear drive shaft 36 and, in turn, 
second output shaft 38. As such, when the vehicle is moving in a 
substantially straight ahead direction, second output shaft 38 is 
rotatably driven (via rear axle assembly 26, etc.) with virtually no speed 
differential with respect to intermediate shaft 74. Therefore, selective 
actuation of the shift fork assembly causes corresponding axial movement 
of mode sleeve 128 such that transfer case 20 can be shifted "on-the-fly" 
from its front-wheel, two-wheel drive mode to its four-wheel drive mode 
without the need of a conventional speed synchronization device. However, 
it is to be understood that transfer case 20 is readily adapted for use 
with virtually any type of synchronized mode shift system when rear axle 
assembly 26 is of the split-shaft type such as that shown and described in 
U.S. Pat. No. 4,381,828 issued May 3, 1983 to Lunn. In such a case, a 
speed synchronizer apparatus would be operably installed between hub 
member 126 and the front end of second output shaft 38 to cause speed 
synchronization between driven intermediate shaft 74 and non-driven second 
output shaft 38 prior to permitting mode sleeve 128 to be moved into 
"lock-up" engagement with external spline teeth 136 on second output shaft 
38. 
FIG. 3 presents the preferred shift pattern associated with the shift fork 
assembly for shifting transfer case 20 between its various drive modes and 
speed ranges. As is conventional, it is generally preferred that movement 
of mode sleeve 128 and range sleeve 88 be coordinated to maintain direct 
drive (i.e., high-range) operation during shifting between the two-wheel 
drive and four-wheel drive modes. One particular shift mechanism that is 
suited for use with the present invention is disclosed in U.S. Pat. No. 
5,076,112 to Williams and which is commonly owned with the present case, 
the entire disclosure of which is expressly incorporated by reference 
herein. 
The foregoing discussion disclose and describes merely an exemplary 
embodiment of the present invention. One skilled in the art will readily 
recognize from such discussion, and from the accompanying drawings and 
claims, that various changes, modifications and variations can be made 
therein without departing from the true spirit and fair scope of the 
invention as defined in the following claims.