Transfer case with integrated viscous coupling

A transfer case for a four-wheel drive vehicle adapted to provide two- to four-wheel drive on-demand in the high range or four-wheel drive in the low range. The transfer case includes an input shaft coupled to the sun gear of a planetary gear set, a main drive shaft, a first output and a second output. Provided in fixed association with the main drive shaft is a viscous fluid coupling chamber, and associated with the chamber is a drive sprocket. A range clutch sleeve and locking clutch assembly is slidably disposed on the main drive shaft between the planetary gear set and the rotary member for selectively providing two- to four-wheel on-demand high range power, neutral, and four-wheel drive low range power. Inner and outer relatively rotational drum housings surround the aft end of the main drive shaft for defining an annular viscous fluid coupling chamber therebetween. The inner drum is splined to the main drive shaft for rotation with that shaft while the outer drum is fixedly connected with the rotary member for providing torque to the second output.

BACKGROUND OF THE INVENTION 
This invention relates to transfer cases for four-wheel drive systems, and 
more particularly to a transfer case incorporating a viscous coupling and 
a gear reduction planetary assembly. 
Four-wheel drive systems have been in existence for many decades. The 
output of the engine has been conventionally split between the front 
wheels and the rear wheels by a transfer case. One type of system used for 
many years provided undifferentiated power to all wheels of the vehicle. 
This gave the vehicle good mobility under adverse surface conditions. 
However, this type of drive was not particularly useful for normal highway 
driving in that, because all four wheels operated at the same speed, the 
vehicle could not be turned without sliding or scuffing one or more tires. 
This drawback resulted in the development of systems which included a 
conventional two-wheel drive mechanism. For highway use the two-wheel 
drive was utilized and this caused the vehicle to assume the 
characteristics of most other two-wheel drive vehicles. 
It is desirable in a four-wheel drive vehicle to obtain the benefits of 
differentiation provided in conventional two-wheel drive vehicles while 
also obtaining the benefits of a conventional four-wheel drive, when 
needed. In these circumstances, transfer cases were developed that were 
manually actuated to lock out a differential unit when the differentiation 
was not desired. However, the procedures of manually locking out a 
differential unit are not practical in that the user must stop the vehicle 
prior to engaging the all-wheel drive system. Accordingly, efforts have 
been made to replace the manual lock-out type operation by an automatic 
locking apparatus. One type of automatically operated apparatus developed 
utilizes electronic sensing apparatus which senses a difference in 
rotation speeds of two rotatable speeds of two rotatable parts of the 
differential. When the difference in speeds exceeds a predetermined value, 
a solenoid is actuated to operate a mechanical clutching mechanism which 
couples the two rotatable parts together and renders the differential 
ineffective. 
Another type of automatic apparatus includes a viscous coupling having a 
plurality of plates connected to one rotatable member and interleaved with 
a plurality of plates connected to another rotatable member. To this end, 
viscous couplings have been used in numerous power transmission 
applications such as four-wheel drive transfer cases, differentials and 
limited-slip intra-axle and inter-axle devices. In such couplings, viscous 
fluid substantially fills the housing containing the interleaved plates. 
As the speed difference between two rotating parts increases, the viscous 
fluid is sheared by the interleaved elements, which results in a tendency 
to interlock the rotatable parts. Examples of transfer cases which 
incorporate a viscous coupling therein are disclosed in U.S. Pat. No. 
4,031,780 to Dolan et al., U.S. Pat. No. 5,046,998 to Frost, and in U.S. 
Pat. No. 5,078,660 to Williams et al., all commonly assigned to the 
assignee of the instant application. 
In the patent to Dolan et al., a transfer case for a four-wheel drive train 
is disclosed that includes a differential incorporating both a viscous 
coupling and a viscous fluid operated mechanical clutch which may be 
operated to inhibit and prevent differentiation. 
In the patent to Frost, a transfer case for a four-wheel drive vehicle is 
disclosed that provides a drive range planetary gear set that is 
selectively positionable between low and high speed output positions and a 
neutral position. A dual planetary inter-axle differential gear set is 
axially spaced from the range planetary gear set. The latter gear set may 
be positioned to provide either a two-wheel drive mode to a first output 
or a full-time four-wheel drive mode with differential action between the 
first output and a second output. A viscous fluid clutch is connected 
between the first and second outputs for modifying the torque division 
between these outputs. 
Finally, in the patent to Williams et al., a transfer case for full-time 
four-wheel drive vehicles is disclosed having limited slip between the 
front and rear drive lines. Like the transfer case of the Frost patent, 
the transfer case of the Williams et al. patent also includes a drive 
range planetary gear set that is selectively positionable between low and 
high drive ranges and a neutral position. A viscous coupling chamber is 
provided in conjunction with a dual planetary differential gear set for 
providing full-time four-wheel drive differentiation with limited slip 
between first and second outputs. 
The present invention relates to an transfer case which includes both 
mechanical and viscous coupling features. The present invention provides a 
transfer case having operating characteristics and advantages different 
from those of the prior art. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a transfer case for 
on-demand four-wheel drive operation for a four-wheel drive vehicle having 
an improved coupling arrangement. Two-wheel high range operation to 
four-wheel high range operation is made possible by incorporation of a 
viscous coupling uniquely arranged within the transfer case in a compact 
and simplified manner. 
Still another object of this invention is to provide a transfer case of the 
class described which utilizes a characteristic of viscous fluid of a 
viscous coupling to direct torque to the front wheel set. 
It is another object of the present invention to provide an improved 
transfer case as set forth above wherein the viscous coupling is arranged 
in association with a planetary gear set. 
A further object of this invention is to provide a transfer case of the 
type described that may selectively lock out the viscous coupling to 
provide a positive low drive to the front and rear sets of wheels. 
A further object of this invention is to provide a transfer case of the 
class described which is economical in construction and efficient in 
operation. 
The present invention achieves these objectives in an improved viscous 
coupling for use in an on-demand four-wheel drive transfer case which 
functions to automatically provide all-wheel drive when conditions require 
all-wheel traction. The viscous coupling is operably installed between an 
input member and its associated planetary gear set and first and second 
output members of the transfer case and is constructed in a compact 
arrangement. 
The planetary gear set includes a sun gear that is integrally fixed to the 
output end of the input shaft. An axially movable range clutch sleeve 
surrounds a main drive shaft and is splined thereto. The sleeve is movable 
between the three operating positions of the transfer case of the present 
invention. 
In its high range on-demand drive position (two-wheel high range to 
four-wheel high range), the range clutch sleeve is positioned between 
internal splines of the sun gear and the main shaft, thus interlocking the 
two. Rotational movement is delivered to the rear wheels from the first 
output member and to the inner drum and the inner plates of the viscous 
coupling assembly associated with the inner drum. A plurality of outer 
plates are fitted between the inner plates and are fixedly attached to an 
outer drum, together with front and rear cover plates. These components 
form a rotatable housing that may be rotated on the inner drum. The inner 
drum, the outer drum, and the front and rear cover plates together 
comprise a sealed viscous coupling assembly. The assembly is mostly filled 
with a viscous fluid. 
The front cover plate of the rotatable housing is fixedly attached to a 
drive sprocket. The drive sprocket has a chain drivingly connected to the 
front axle via the second output member and a front axle drive shaft. 
With power delivered to the inner drum from the vehicle engine, and with 
the vehicle being driven under normal operating conditions such as on a 
highway, only a small amount of torque is transmitted through the viscous 
coupling assembly to the chain drive sprocket because the rotational 
difference between the inner and outer plates is very small. However, on 
uneven surfaces that cause the rotation of the inner plates (connected 
indirectly to the rear wheels) to rotate at a portionally greater rate 
than the outer plates (connected indirectly to the front wheels), a 
relatively large torque is delivered to the front wheels due to the 
viscous shear resistance of the viscous fluid between the interleaved 
clutch plates. 
Upon the range clutch sleeve being shifted rearwardly from its high range 
position to its neutral position, the sleeve becomes disengaged from the 
planetary gear set, thus no power at all is delivered to the main drive 
shaft. 
The low range four-wheel drive mode is engaged when the range clutch sleeve 
is shifted rearwardly to its aft-most position, pushing with it an 
axially-movable locking clutch. The sleeve interlocks the planet pinion 
carrier and the main drive shaft, thus reducing rotational speed of the 
main drive compared with that of the input shaft. The locking clutch is 
also splined to the main drive shaft and includes external splines that 
are engaged with internal splines defined in a counterbore of the 
differential gear. This arrangement "locks out" the viscous coupling 
assembly by providing a direct, locked connection between the planet 
pinion carrier and the differential gear, in turn directly driving the 
front wheels. 
Other objects and advantages will be made apparent as the description 
progresses.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In general, the present invention discloses a novel viscous coupling 
drivingly attached to a gear reduction planetary assembly. Referring to 
the drawings, there is shown in FIG. 1 a portion of an exemplary transfer 
case 10 for use in four-wheel drive motor vehicles according to the 
present invention incorporating a helical planetary gear reduction 
assembly 40. Vehicle transfer case 10 and helical planetary gear reduction 
assembly 40 are thoroughly disclosed in U.S. Pat. No. 4,677,873 to 
Eastman, commonly assigned to the assignee of the instant application, the 
disclosure of which is incorporated by reference herein. In general, the 
transfer case 10 is adapted for conventional interconnection to a drive 
train (engine and transmission) for driving front and rear ground wheels 
supported on front and rear axles, respectively. 
The transfer case 10 includes a housing assembly 12 formed by front and 
rear housing sections 14 and 16, respectively, which are suitably 
connected by a plurality of threaded bolts, only one of which is shown at 
18. The front housing section 14 is adapted to receive a vehicle 
transmission output shaft 20 for rotation about a longitudinal axis 22. An 
input stub shaft 24, aligned on longitudinal axis 22, has its internal 
splines 26 engaged with external splines 28 of transmission output shaft 
20. The stub shaft 24 is shown rotatably supported in a hub portion 30 of 
the front housing section 14 by a ball bearing assembly 32 and sealingly 
enclosed by a collar member 34 secured by bolts 36. 
With reference to FIG. 2, the input stub shaft 24 has an input sun gear 38 
operatively associated with a helical planetary gear set reduction 
assembly 40. More specifically, the sun gear 38 is formed integrally with 
the inner end of the stub shaft 24. A helical planetary gear set reduction 
assembly 40 is a speed reduction apparatus operable for preferably 
defining high, low and neutral range positions as will be described 
hereinafter. It will be appreciated that the planetary gear set reduction 
assembly 40 is merely exemplary of a suitable two-speed reduction 
apparatus for use in the on-demand transfer case 10 of the present 
invention. 
As noted, the sun gear 38 is formed integrally with the inner end of stub 
shaft 24. Helical teeth 42 of the sun gear 38 are meshed with teeth 44 of 
a plurality of planet pinion gears, one of which is shown at 46. Each 
planet pinion gear 46 is rotatably journalled on a pin 48 supported in a 
planetary gear set carrier 50. The planetary gear set carrier 50 includes 
fore and aft carrier ring members 52 and 54, suitably interconnected as by 
machine bolts (not shown). It will be noted that the aft carrier member 54 
is formed with a central bore 56 having internal splines 58 concentrically 
arranged about the longitudinal axis 22. The planet pinion gears 46 mesh 
with a helical annulus gear 60 suitably mounted in a splined, press-fit 
manner to an inner annulus surface 62 formed in the front housing section 
14. The annulus gear 60 is also retained against rearward movement by a 
snap ring 64 received in an internal annular notch 66. 
With reference to FIGS. 2 and 3, a first or main drive shaft 68, aligned 
concentrically with longitudinal axis 22, has a pilot end portion 70 
journally supported in an input shaft axial counterbore 72 by roller 
bearings 74 and an output end portion 76 journally supported in a hub 
portion 78 of the rear housing section 16 by a ball bearing assembly 80. 
Forward movement of the ball bearing assembly 80 is prevented by an 
annular wall 82 defined in the hub portion 78. Aftward movement of the 
assembly 80 is checked by inner and outer bearing retainer rings 84 and 
86, respectively. A rear oil seal 88 sealingly encloses the bearing 
assembly 80 and provides a seal about the output end portion 76 of the 
drive shaft 68 to prevent leakage of lubricating oil. 
The main drive shaft 68 is concentrically surrounded by a range clutch 
sleeve 90 axially slidable thereon by means of collar internal splines 92 
engaged with external splines 94 formed on the drive shaft 68. The range 
clutch sleeve 90 is formed with external splines 96 and, in its "X" 
position illustrated in FIG. 2, is shown slidably engaged with sun gear 
internal splines 98 concentrically arranged about the longitudinal axis 22 
of the main drive shaft 68 and provided in an axial counterbore in the 
right or aft end of the input stub shaft 24. Torque or power flow is 
transferred directly from the input stub shaft 24 through engagement of 
the splines 96 and 98 in conjunction with the engagement of the collar 
internal splines 92 and the output shaft external splines 94, thereby 
establishing the direct high range position. 
With reference to FIG. 3, the aft carrier internal splines 58 are placed in 
sliding meshed engagement with the external splines 96 upon the range 
clutch sleeve 90 being slid rearwardly to position "Z" for defining the 
"low" drive range of the transfer case 10. It will be noted that when the 
range clutch sleeve 90 is moved rearwardly a predetermined distance from 
its high range position ("X" of FIG. 2), its external splines 96 are 
disengaged both from the sun gear internal splines 98 and the aft carrier 
internal splines 58 for defining a "neutral" drive range position 
indicated by position "Y". With the transfer case 10 shifted in neutral, 
rotation of the input stub shaft 24 drives only the planet pinion gears 46 
and the fore and aft carrier ring members 52 and 54, respectively, around 
the annulus gear 60. Thus, in the neutral position no torque is 
transmitted to the drive shaft 68, and thus no power is transmitted to the 
vehicle's wheels (not shown). 
As will be appreciated, the range clutch sleeve 90 is selectively shiftable 
through coaxial movement of a range fork 100. The fork 100 is axially 
movable on a shift rail 102. The rail 102 is fixed between a front support 
104 defined in the front housing section 14 and a rear support 106 defined 
in the rear housing section 16. Axial movement of the fork 100 is 
controlled by a sector assembly 108 comprising a sector plate 110 
connected to the fork 100 by a range pin 112. The sector assembly 108 
further includes an operating lever 114 operably connected to the plate 
110 by a sector shaft 116. It is to be understood that the range fork 100 
can be selectively shifted by the vehicle operator through the sector 
assembly 108 between the two-wheel to four-wheel on-demand mode and 
full-time four-wheel mode either manually (i.e., via a shift lever) or 
electrically (i.e., via a motor driven system). 
The main drive shaft 68 is also concentrically surrounded by a locking 
clutch 118 axially slidable thereon by means of hub internal splines 120 
engaged with the external splines 94 formed on the drive shaft 68. The 
locking clutch 118 is formed with external clutch splines 122. The 
external clutch splines 122 are placed in sliding meshed engagement with 
internal splines 124 defined in a counterbore 126 of a differential gear 
128. The differential gear 128 concentrically surrounds the main drive 
shaft 68. While engagement of the differential gear 128 is discussed 
below, when not engaged, the differential gear 128 is allowed to freely 
rotate on the shaft 68, rotatably supported by a plurality of needle 
bearings 130. A biasing element 132 encourages the locking clutch 118 away 
from the differential gear 128 and assures that the range clutch sleeve 90 
remains in its "X" position (or high position) when not selectively 
engaged in either its "Y" position (or neutral position; see FIG. 3) or 
its "Z" position (or low position; also of FIG. 3). The biasing element 
132 is preferably a coil spring that concentrically surrounds the drive 
shaft 68 and is positioned between the locking clutch 118 and a locking 
clutch washer 134. The washer 134 is biased against the differential gear 
128. A snap ring 136 limits aftward movement of the differential gear 128. 
A drive sprocket 138 is fixedly splined to the differential gear 128 in a 
conventional manner for rotation therewith. 
A viscous coupling assembly, generally indicated as 140, includes a front 
cover plate 142 that is fixedly coupled to the differential gear 128. 
Fixed coupling is provided by internal splines 144 defined in the 
counterbore 146. The internal splines 144 are in constant meshed 
engagement with external splines 148 formed on the aft end of the 
differential gear 128. The coupling assembly 140 further includes a rear 
cover plate 150 that defines an annular ring. An inner drum 152 is shown 
to concentrically surround the drive shaft 68 and is adapted to be fixedly 
engaged with the drive shaft 68 so as to rotate therewith. More 
specifically, the inner drum 152 is coupled for rotation with the main 
drive shaft 68 through a series of splines 154 interiorly formed thereon. 
Forward movement of the inner drum 152 along the shaft is prohibited by a 
radial annual shoulder 156 defined on the drive shaft 68, while aftward 
movement is checked by an inner retaining ring 158. 
A rotatable drum housing assembly 160 encircles the inner drum 152 and 
generally includes a cylindrical outer drum 162 which is fixedly connected 
to the front cover plate 142 and the rear cover plate 150. So constructed, 
the drum housing assembly 160 and the inner drum 152 are capable of 
rotating relative to one another. 
The drum housing assembly 160 encloses the inner drum 152 with the inner 
surfaces of outer drum 162 and front and rear cover plates 142 and 150, 
respectively, defining an internal chamber 164. 
The chamber 164 is hermetically sealed around the inner drum 152 by fore 
and aft seals 166 and fore and aft back-up rings 168. Disposed within the 
chamber 164 are two sets of interleaved viscous coupling plates, 
cumulatively designated as 170, which substantially fill the chamber 164. 
One set of plates, hereinafter referred to as inner plates 172, are 
mounted for rotation with the inner drum 152 while the second set of 
plates, hereinafter referred to as outer plates 174, are mounted for 
rotation with the outer drum 162. 
In general, the viscous coupling plates 170 are formed from relatively thin 
plate stock and are generally ring shaped. As shown in FIG. 4, each of the 
inner plates 172 includes splines 176 formed along its inner circumference 
which are configured to meshingly engage axial splines 178 formed on the 
exterior surface of the inner drum 152. The inner plates 172 are 
positioned in a spaced relationship and are so maintained by spacer rings 
180. 
The outer plates 174 are mounted to the outer drum 162 via external splines 
182 formed around the outer circumference of outer plates 174. In 
particular, the splines 182 meshingly engage axial splines 184 formed 
interiorly of the outer drum 162. 
In the exemplary viscous coupling apparatus shown, the spacer rings 180 are 
not used with the outer plates 174 and thus allow axial movement of the 
outer plates 174 between the adjacent inner plates 172 along the splines 
184. However, in an alternative embodiment, the outer plates 174 may be 
axially spaced by the spacer rings 186 and fixed relative to the outer 
drum 162 while the inner plates 172 are axially movable therebetween along 
the inner drum 152. 
The chamber 164 is substantially filled, typically ninety percent or 
greater, with a viscous fluid such as silicone oil, the remaining volume 
of chamber 164 being filled with air or some other inert gas. To 
facilitate filling of the chamber 164, the rear cover plate 150 is 
provided with a fill port 188 and a fill plug 190. An O-ring seal 192 is 
provided to prevent the escape of fluid between the wall of the fill port 
188 and the fill plug 190. 
During operation of the viscous coupling assembly 140 (this presumes that 
the range clutch sleeve 90 is in its "X" or on-demand high range 
position), the main drive shaft 68 will be driven by the vehicle's source 
of power of transmission via the helical planetary gear set reduction 
assembly 40 to cause the inner drum 152 and its associated inner plates 
172 to rotate. 
Generally, the drum housing assembly 160 and its associated outer plates 
174 move as a unit and will be rotating under substantially similar 
conditions (that is, the vehicle is traveling on dry pavement with the 
drum housing assembly 160 rotating with the front wheels while the inner 
drum 152 is rotating with the rear wheels). Where the conditions involve a 
slight differential in rotational speeds between the inner drum 152 and 
the drum housing assembly 160, the fluid will permit viscous shearing and 
accommodate the rotational difference by allowing slip. However, as the 
rotational speed differential and viscous shearing rate increase, the 
apparent viscosity of the fluid will decrease which results in a softening 
is more than offset by the increase in shear torque generated by the 
increase in speed and, as the viscous shearing rate increase, the viscous 
coupling assembly 140 becomes increasingly rigid thereby transmitting an 
increased amount of torque. 
If a substantially continuous speed differential is maintained over a 
period of time, the temperature within the chamber 164 will begin to 
increase causing the viscous fluid to expand. Since the fluid will 
naturally expand at a rate greater than that of the chamber 164, the 
internal pressure of the chamber 164 will rise. During the rise in chamber 
pressure, the gas or air contained within the chamber 164 dissolves into 
the silicone oil. This change in the distribution of air acts to modify 
its flow patterns and will allow for the development of pressure 
differentials. 
In response to the pressure differentials, each of the axially movable 
plates (that is, the outer plates 174 in the illustrated embodiment) will 
axially shift and establish frictional contact with an adjacent 
non-movable plate (that is, the inner plate 172). With the establishment 
of frictional contact between the inner and outer plates 172 and 174, 
respectively, a sudden and sharp increase in transmitted torque occurs 
without a corresponding increase in different rotational speed. This is 
referred to as the "humping" phenomenon or torque progression, and is well 
illustrated by line A of FIG. 5 in which a diagram illustrating torque 
(expressed in newton-meters [Nm] on the Y-axis) versus revolutions per 
minute (on the X-axis) as shown. 
As may be seen on the graph, the torque increases according to a 
predictable slope relative to the engine speed until frictional contact 
between the inner and outer plates 172 and 174, respectively, is 
established, at which time a dramatic increase in torque occurs. (Line A 
represents experimental data derived from a working embodiment of the 
present invention in which the viscous coupling assembly 140 contained 
twenty-four pairs of plates with 60,000 cSt silicone in which the chamber 
164 was ninety percent filled. Conversely, line B was derived from data 
developed from a working embodiment incorporating twenty-seven pairs of 
plates with 10,000 cSt silicone and a chamber 164 being eighty-eight 
percent filled. As may be clearly seen, no torque progression occurred in 
the latter embodiment.) 
As is known, and with reference back to FIG. 4, the axial movement of the 
outer plates 174 relative to the outer drum 162 is promoted by radially 
extending slits 194 and other openings 196 being provided in the plate 172 
to facilitate the development of the pressure differentials. 
When frictional contact is developed between the inner plates 172 and the 
outer plates 174, the front cover plate 142 rotates in the direction of 
the main drive shaft 68 because the differential gear 128 is in constant 
mesh with the front cover plate 142. The differential gear 128 has 
external splines 198 which engage the internal splines 200 of the drive 
sprocket 138. Thus, the differential gear 128 carries the drive sprocket 
138 for rotation therewith. A chain 202 driven by the drive sprocket 138, 
in turn, rotates a driven sprocket 204 which drives a front or second 
drive shaft 206. The drive sprocket 138 is fixed on the differential gear 
128 between a snap ring 208 and a radial annual shoulder 210 formed on the 
differential gear 128. 
The second drive shaft 206 is supported and retained in the transfer case 
10 by a front bearing assembly 212 supported in the front housing section 
14 and a rear bearing assembly 214 located in the rear housing section 16. 
A second drive shaft yoke 216 is secured to the forward end of the second 
drive shaft 206 with a yoke nut 218 with the second drive shaft yoke 216 
being sealed by an oil seal 220. The second drive shaft yoke 216 extends 
forwardly and is adapted for connecting to a drive shaft to drive the 
front axle of a vehicle. 
With reference to the three shift positions designated in FIG. 6, operation 
of the transfer case 10 will now be described in fuller detail. Operating 
state No. 1 is the "2WH TO 4WH" on-demand drive mode wherein the range 
clutch sleeve 90 is located in the leftward "X" position with the locking 
clutch 118 biased to the leftward "A" position. As noted, torque or power 
flow is transferred from the sun gear 38 to the main drive shaft 68 via 
the range clutch sleeve 90. This high range torque drive is then 
transferred to the inner drum 152 of the viscous coupling assembly 140 and 
directly to the rear wheels (not shown). When the rotational speed of the 
inner plates 172 (splined to the inner drum 152) is more or less the same 
as the rotational speed of the outer plates 174 (splined to the outer drum 
162) as would be the case on smooth, even surfaces, no shearing of the 
viscous fluid takes place, thus no torque is transferred to the front 
wheels, thus the vehicle remains in its conventional two-wheel drive mode. 
However, in the event the vehicle overpasses uneven terrain or a loose 
surface, the difference between the rotational speeds of the inner 
(driving) and outer (driven) plates becomes increasingly acute, and the 
viscous fluid is sheared by the interleaved plates which results in their 
tendency to become interlocked. As a result, the front cover plate 70 of 
the drum housing assembly 160 turns the drive sprocket 138 through the 
interconnection of the splines 144 and 148. Rotation of the drive sprocket 
138 drives the chain 202 which, in turn, drives the vehicle's front wheels 
via the second drive shaft 206. Thus, in the No. 1 operating state shown 
in FIG. 6, the sun gear internal splines 98 of the sun gear 38 provides 
torque to the rear wheels while the viscous coupling assembly 140 allows 
for torque to be directed to the front wheels as needed for traction. 
Upon transfer case 10 being placed in its No. 2 "NEUTRAL" operating state 
(with the range clutch sleeve 90 moved rightwardly to the "Y" position and 
the locking clutch 118 still in its unengaged position), the sun gear 
internal splines 98 are disengaged from the range clutch external splines 
96 whereby no input power is transferred from the input stub shaft 24 to 
the clutch sleeve 90 and thus no output torque is delivered to either 
first or second drive shafts 68 and 206, respectively. In the neutral 
state, the viscous coupling assembly 140 permits front and rear axle 
differentiation when the vehicle is being towed. 
Operating state No. 3 is the "4WL LOCK" drive mode wherein the range clutch 
sleeve 90 is shifted rightwardly from its high-range "X" position past its 
neutral position "Y" to its low-range position "Z" and the locking clutch 
118 is forcibly shifted rightwardly to its viscous coupling assembly 140 
"lock-out" position "B" against the biasing force of the biasing element 
132. With the locking clutch 118 in its "B" position, its internal splines 
120 engage the splines 92 of the main drive shaft 68 and its external 
clutch splines 122 engage the internal splines 124 of the differential 
gear 128 thereby "locking out" the viscous coupling assembly 140. Thus, 
power flow from the input stub shaft 24 passes through the planetary gear 
set carrier 50 at a reduced speed and into the range clutch sleeve 90 for 
delivery without differentiation to both the rear wheels via the main 
drive shaft 68 and the front wheels via the second drive shaft 206. The 
second drive shaft 206 is positively rotated by the drive chain 202 at the 
same speed as the main drive shaft 68, and there is no inter-axle 
differentiation. Furthermore, because the viscous coupling assembly 140 is 
locked out, there is no viscous coupling involvement in the low-range. 
While the specific embodiment of the invention has been shown and described 
in detail to illustrate the principles of the present invention, it will 
be understood that the invention may be embodied otherwise without 
departing from such principles. For example, one skilled in the art will 
readily recognize from such discussion and from the accompanying drawings 
and claims as various changes, modifications and variations can be made 
therein without departing from the spirit and scope of the invention as 
defined in the following claims.