Power transfer mechanism for four-wheel drive

A power transfer mechanism comprises a planetary gear assembly incorporating double planet gears which are in mesh with each other and journalled on a carrier. Input member is drivingly connected to the ring gear and first output member is drivingly connected to carrier. The mechanism further includes means shiftable to one of a plurality of operating positions engaging sun gear with a second output member.

BACKGROUND OF THE INVENTION 
The present invention relates to a power transfer mechanism for a 
four-wheel drive motor vehicle. 
A power transfer mechanism for a four-wheel drive automotive vehicle is 
known. The known mechanism includes a sub-transmission which may be 
shifted to establish a high-range mode or a low-range mode. The mechanism 
also includes a center differential which absorbs a difference in rotation 
between two output members for front axle and rear axle of an associated 
four-wheel drive vehicle. The mechanism, therefore, requires a large 
number of component parts, resulting in a construction which is bulky, 
heavy and expensive to manufacture. 
In order to alleviate the above deficiencies, there has been proposed a 
power transfer mechanism incorporating a single planetary gear assembly as 
disclosed in U.S. Pat. No. 4,074,591. The planetary gear assembly includes 
a shifting mechanism. The shifting mechanism may be shifted to establish a 
high-range mode wherein the carrier acts as an input member and the sun 
and ring gears as output members to establish torque proportioning 
differential action. The mechanism may be shifted to establish a low-range 
mode wherein the sun gear acts as an input member, the ring gear as a 
reaction member, and the carrier as an output member to provide locked-up 
reduction drive. The use of a single planetary gear assembly provides a 
construction which is less bulky, compact, and less heavy. The shifting 
mechanism includes a pair of shifting sleeves, each supporting a floating 
collar. In the high-range mode, one sleeve engages the input with the 
carrier, and its associated floating collar engages the sun gear with one 
output member. The other sleeve engages the ring gear with the other 
output member, and its associated floating collar is disengaged. In the 
low-range mode, the one sleeve engages the input with the sun gear, and 
its associated floating collar engages the carrier with one output member. 
The other sleeve engages the carrier with the other output member, and its 
associated floating collar grounds the ring gear to the housing. The 
shifting mechanism is mounted in the housing and disposed radially 
outwardly of the input and output members and radially inwardly of the sun 
gear. As a result, this disposition of the shifting mechanism is a 
stumbling block in reducing the over all dimension of the entire mechanism 
and causes a complicated structure. The fact that the radial dimension is 
relatively large poses a problem in its installation in an associated 
four-wheel drive vehicle because the vehicle floor has to be elevated so 
as to secure the road clearance high enough for the associated four-wheel 
drive to take full advantage of the power transfer mechanism, thus 
sacrificing the vehicle's cabin space, or the road clearance has to be 
sacrificed so as to maintain the vehicle floor as low as possible and to 
secure sufficiently large vehicle's cabin, thus preventing the associated 
four-wheel drive vehicle from exibiting good running performance in 
passing through the rough terrain. Besides, in the high-range mode, since 
the input member is engaged with the carrier, and the ring gear is engaged 
with the output members for transferring torque to the rear axle of an 
associated four-wheel drive vehicle and the sun gear is engaged with the 
other output member for transferring torque to the front axle of the 
four-wheel drive vehicle, torque transferred to the rear axle is always 
larger than torque transferred to the front axle by a considerable amount 
owing to considerable difference in length between the arms with which the 
ring and sun gears turn the associated output members. This causes an 
associated four-wheel drive to show oversteering characteristic when 
making a turn. Lastly, since the input is engaged with the sun gear (in 
the low-range mode) or the carrier (in the high-range mode), the reduction 
ratio having a sufficiently small value cannot be obtained. 
There is a need to provide a power transfer mechanism which establishes at 
least approximately even distribution in torque between two output members 
when the mechanism is shifted to the torque proportioning differential 
drive range. 
An object of the present invention is to provide a power transfer mechanism 
which meets the above mentioned need. 
SUMMARY OF THE INVENTION 
According to the present invention, a power transfer mechanism incorporates 
a planetary gear assembly which includes a ring gear drivingly connected 
to an input member, a carrier drivingly connected to one of output 
members, a sun gear, a plurality of first planet gears journalled on the 
carrier and being in mesh with the ring gear, a plurality of second planet 
gears journalled on the carrier, each of the second planet gears being in 
mesh with an associated one of the first planet gears and being in mesh 
with the sun gear. The planetary gear assembly including means for 
shifting the power transfer mechanism to a plurality of operating modes. 
The shifting means may be shifted to one of the plurality of operating 
positions engaging the sun gear with the other output member.. 
The planetary gear assembly may include a rotary element in mesh with one 
of the first and second planet gears. The shifting means may be shifted to 
locked-up reduction ratio drive position wherein the rotary element is 
grounded and the both output members are engaged with each other. The 
shifting means may be shifted to another drive position engaging the sun 
gear with the rotary element, thus establishing a torque path to one 
output member only.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the accompanying drawings, the preferred embodiments according 
to the present invention are described. Referring particularly to FIG. 1, 
there is shown schematically a housing 10 of a transfer case for use in an 
associated four-wheel drive automotive vehicle. An arm 12 is rigidly 
secured to housing 10 and thus forms a portion thereof. Arm 12 defines a 
brake element which may be teeth (not shown). 
An input member 14 which may be the output shaft of a manual or automatic 
transmission incorporated in an associated automotive vehicle extends and 
is rotatable about the common axis about which a first output member 16 is 
rotatable. First output member 16 may be an output shaft which transfers 
torque to the rear axle of an associated four-wheel drive vehicle. First 
output member 16 includes a radially outwardly extending portion 18 which 
defines on its outer periphery a clutch element which may be teeth (not 
shown). A second output member 20 is a sprocket which transfers torque 
through a chain 22 to the front axle of an associated four-wheel drive 
vehicle. Output member 20 defines a sleeve portion 24 rotatably mounted on 
output member 16, first and second radially outwardly extending portions 
26, 28 extending from the both axial ends of the sleeve portion 24, 
respectively. First and second radially outwardly extending portions 26, 
28 define on their outer peripheries splines 26a, 28a, respectively. 
A planetary gear assembly 30 is mounted within housing 10. Assembly 30 
includes a ring gear 32. Via a radially extending portion 34, ring gear 32 
is drivingly connected to the input shaft 14. A plurality of first planet 
gears 36 are journalled on a carrier 38 and in mesh with ring gear 32. A 
plurality of second planet gears 40 are journalled on carrier 38 and each 
of second planet gears 40 is in mesh with one of first planet gears 36. 
Second planet gears 40 are in mesh with a sun gear 42. Carrier 38 is 
drivingly connected to first output shaft 16. Sun gear 42 includes a 
sleeve portion 44 extending axially from the sun gear 42 and rotatably 
mounted on first output shaft 16, and a radially outwardly extending 
portion 46 extending from the sleeve portion 44 and defining on its outer 
periphery a clutch element which may be teeth (not shown). A rotary 
element 48 in the form of a second ring gear is in mesh with second planet 
gears 40. Rotary element 48 includes a floating collar portion 50 mounted 
around the sleeve portion 44 of sun gear 42, and a radially outwardly 
extending portion 52 which defines on its outer periphery a clutch element 
which may be spline 52a. 
Planetary gear assembly 30 incorporates a shifting mechanism 54 which 
includes a first clutch sleeve or shift member 56 which is engaged with 
spline 52a of the radially outwardly extending portion 52 of rotary 
element 48. Thus, clutch sleeve 56 is axially shiftable relative to but 
rotatable with rotary element 48. A shift fork 58 engages in a groove 60 
defined by clutch sleeve 56. 
Shifting mechanism 54 also includes a second clutch sleeve or shift member 
62 which is engaged with spline 26a of first radially outwardly extending 
portion 26 of second output member 20. Thus, clutch sleeve 62 is axially 
shiftable relative to but rotatable with second output member 20. A shift 
fork 64 engages in a groove 66 defined by the clutch sleeve 62. 
Shifting mechanism 54 also includes a third clutch sleeve or shift member 
68 which is engaged with spline 28a of second radially outwardly extending 
portion 28 of second output member 20. Thus, clutch sleeve 68 is axially 
shiftable relative to but rotatable with second output member 20. A shift 
fork 70 engages in a groove 72 defined by clutch sleeve 68. 
HIGH-RANGE FOUR-WHEEL DRIVE MODE 
As shown in FIG. 2, shifting mechanism 54 which may be shifted via shift 
forks 58, 64, 70 by a shift lever (not shown) is in the high-range 
four-wheel drive position establishing torque proportioning four-wheel 
drive. Teeth of second clutch sleeve 62 are in mesh with teeth of radially 
outwardly extending portion 46 of sun gear 42. As a result, sun gear 42 is 
engaged with second output member 20 through second clutch sleeve 62. The 
other two clutch sleeves 56, 68 are disengaged. 
Torque is transferred from input member 14 to ring gear 32. Second ring 
gear 48 is free to rotate. First and second planet gears 36, 40 are free 
to planatate, and torque is transferred from carrier 38 to first output 
member 16 and from sun gear 42 to second output member or sprocket 20 in 
accordance with their associated rear and front axles. Difference in 
rotation between front and rear axles which may occur when an associated 
four-wheel vehicle makes a turn causes rotation of first and second planet 
gears 36, 40. Thus, planetary gear assembly 30 acts as a center 
differential to absorb difference in rotation. 
The length of arm during transferring torque from carrier 38 to first 
output member 16 and the length of arm during transferring torque from 
carrier 38 to sun gear 42, viz., during transferring torque from carrier 
38 to second output member 20, are approximately 1:1. As a result, torque 
transferred to the front axle and that to the rear axle are approximately 
1:1, avoiding oversteering owing to excessively uneven distribution of 
torque in favour of the rear axle, thus enhancing controllability and 
stability of the associated four-wheel drive vehicle. 
LOCK-UP HIGH-RANGE FOUR-WHEEL DRIVE MODE 
In FIG. 3, shifting mechanism 54 has been shifted to the lock-up high-range 
four-wheel drive position establishing locked-up high-range four-wheel 
drive. Teeth of second clutch sleeve 62 remains in mesh with teeth of 
radially outwardly extending portion 46 of sun gear 42, and teeth of third 
clutch sleeve 68 are now in mesh with teeth of radially outwardly 
extending portion 18 of first output member 16. As a result, second output 
member 20 is engaged with first output member 16 through third clutch 
sleeve 68, causing sun gear 42 and carrier 38 to rotate as a unit. 
Planetary gear assembly 30 does not act as a center differential, 
preventing slip between first and second output members 16, 20, thereby 
making it easy for an associated four-wheel drive vehicle to pass through 
rough terrain. 
LOCK-UP LOW RANGE FOUR-WHEEL DRIVE MODE 
In FIG. 4, shifting mechanism 54 has been shifted to the lock-up low-range 
four-wheel drive position establishing locked-up four-wheel reduction 
ratio drive. Teeth of third clutch sleeve 68 is in mesh with teeth of 
radially outwardly extending portion 18 of first output member 16. As a 
result, second output member 20 is engaged with first output member 16 
through third clutch sleeve 68. Teeth of second clutch sleeve 62 is out of 
mesh with teeth of radially outwardly extending portion 46 of sun gear 42. 
Teeth of first clutch sleeve 56 is in mesh with teeth of arm 12. As a 
result, the second ring gear 48 is grounded or anchored to housing 10. 
Torque is transferred from input member 14 to ring gear 32. Second ring 
gear 48 acts as a reaction member. First and second planet gears 36, 40 
planatate and reduction ratio torque is directed to carrier 38. As both 
first and second output members 16, 20 are engaged with carrier 38, power 
transfer mechanism in low-range four-wheel drive mode provides locked-up 
reduction ratio torque to both first and second output members 16, 20, 
thus enabling the associated four-wheel drive vehicle to perform 
four-wheel drive with sufficiently high power. 
As both first and second planet gears 36, 40 rotate during their planet 
movement, the reduction ratio may be increased to a value near 2. 
TWO-WHEEL DRIVE MODE 
In FIG. 5, shifting mechanism 54 has been shifted to the two-wheel drive 
position establishing two-wheel drive. Teeth of first clutch sleeve 56 is 
out of mesh with teeth of arm 12 rigidly secured to housing 10 and is now 
in mesh with teeth of radially outwardly extending portion 46 of sun gear 
42. As a result, sun gear 42 is engaged through first clutch sleeve 56 
with second ring gear 48. Teeth of second clutch sleeve 62 is out of mesh 
with teeth of radially outwardly extending portion 46 of sun gear 42. As a 
result, sun gear 42 is disengaged from second output member 20. Teeth of 
third clutch sleeve 68 is out of mesh with teeth of radially outwardly 
extending portion 18 of first output member 16. As a result, second output 
member 20 is disengaged from first output member 16. 
Torque is transferred from input member 14 to ring gear 32, and torque is 
directed through first planet gears 36 and carrier 38 to first output 
member 16, thus transferring torque only to first output member 16 that 
transfers torque to the rear axle of the associated four-wheel drive 
vehicle. 
From the previous description of the embodiment, it will be understood that 
since input torque is transferred to the ring gear and torque is 
transferred from carrier to the first output member and from the sun gear 
to the second output member when the shifting mechanism is in the 
four-wheel drive position, the ratio of torque transferred to the first 
output member to that to the second output member has been made almost 1:1 
or it is made 1:1. 
It will also be understood that the transfer mechanism provides the 
two-wheel drive. 
The shifting mechanism comprises three clutch sleeves 56, 62, 68 and these 
sleeves are axially offset from the planetary gear assembly, thus 
decreasing the overall radial dimension of the power transfer mechanism. 
Although in the previously described embodiment, the shifting mechanism 
using the clutch sleeves is described, the shifting mechanism may take 
other form, such as one using friction clutch and/or brake means. 
Although, the previously described embodiment uses second ring gear 48 as 
the rotary element which is grounded to housing (see FIG. 4) or engaged 
with sun gear 42 (see FIG. 5), a second sun gear in mesh with first planet 
gears may be used instead of second ring gear 48 as such rotary element. 
This will be more specifically described referring to FIG. 6 which shows a 
second embodiment of a power transfer mechanism. 
As will be readily understood to those skilled in the art, the second 
embodiment is substantially the same as the forst embodiment shown in FIG. 
1 except that a planetary gear assembly 30A incorporates a rotary element 
in the form of a second sun gear 80 which is in mesh with first planet 
gears 36 instead of second ring gear 48 and that a shifting mechanism 54A 
includes first and second clutch sleeves or shift members 82, 84 which are 
sligtly different from their counterparts 56, 62 shown in FIG. 1 in their 
mounting manner. 
Referring to FIG. 6, first clutch sleeve 82 is engaged with spline 12a 
formed on an arm 12A extending from a housing 10. Thus, first clutch 
sleeve 82 is axially shiftable relative to spline 12a formed on arm 12A. 
Clutch sleeve 82 is shiftable rightwardly as viewed in FIG. 6 to a 
position engaging with teeth (not shown) formed on a radially outwardly 
extending portion 52A of second sun gear 80. A shift fork 86 engages in a 
groove 88 defined by clutch sleeve 82. 
Second clutch sleeve 84 is engaged with spline 90 formed on a radially 
outwardly extending portion 46A of sun gear 42. Thus, second clutch sleeve 
84 is axially shiftable relative to spline 90 of radially outwardly 
extending portion 46A of sun gear 42. Second clutch sleeve 84 is shiftable 
rightwardly as viewed in FIG. 6 to a position engaging with teeth formed 
on a radially outwardly extending portion 26A of a second output member 
20A. Second clutch sleeve 84 is shiftable leftwardly as viewed in FIG. 6 
to a position engaging with teeth formed on a radially outwardly extending 
portion 52A of second sun gear 80. A shift fork 92 engages in a groove 94 
defined by clutch sleeve 84. 
HIGH-RANGE FOUR-WHEEL DRIVE MODE 
Shifting mechanism 54A may be shifted to the high-range four-wheel drive 
position establishing torque proportioning four-wheel drive. Teeth of 
second clutch sleeve 84 are in mesh with teeth of radially outwardly 
extending portion 26A of second output member 20A. As a result, sun gear 
42 is engaged with second output member 20A through second clutch sleeve 
84. The other two clutch sleeves 82, 68 are disengaged. 
Torque is transferred from input member 14 to ring gear 32. First and 
second planet gears 36, 40 are free to planatate, and torque is 
transferred from carrier 38 to first output member 16 and from sun gear 42 
to second output member or sprocket 20A in accordance with their 
associated rear and front axles. Difference in rotation between front and 
rear axles which may occur when an associated four-wheel vehicle makes a 
turn causes rotation of first and second planet gears 36, 40. Thus, 
planetary gear assembly 30 acts as a center differential to absorb 
difference in rotation. 
LOCK-UP HIGH-RANGE FOUR-WHEEL DRIVE MODE 
Shifting mechanism 54A has been shifted to the lock-up high-range 
four-wheel drive position establishing locked-up high-range four-wheel 
drive. Teeth of second clutch sleeve 84 remains in mesh with teeth of 
radially outwardly extending portion 26A of second output member 20A, and 
teeth of third clutch sleeve 68 are now in mesh with teeth of radially 
outwardly extending portion 18 of first output member 16. As a result, 
second output member 20A is engaged with first output member 16 through 
third clutch sleeve 68, causing sun gear 42 and carrier 38 to rotate as a 
unit. Planetary gear assembly 30A does not act as a center differential, 
preventing slip between first and second output members 16, 20A. 
LOCK-UP LOW-RANGE FOUR-WHEEL DRIVE MODE 
Shifting mechanism 54A has been shifted to the lock-up low-range four-wheel 
drive position establishing locked-up four-wheel reduction ratio drive. 
Teeth of third clutch sleeve 68 is in mesh with teeth of radially 
outwardly extending portion 18 of first output member 16. As a result, 
second output member 20A is engaged with first output member 16 through 
third clutch sleeve 68. Teeth of second clutch sleeve 84 is out of mesh 
with teeth of radially outwardly extending portion 26A of second output 
member 20A. Teeth of first clutch sleeve 82 is in mesh with teeth of 
radially outwardly extending portion 52A of second sun gear 80. As a 
result, the second sun gear 80 is grounded or anchored to housing 10. 
Torque is transferred from input member 14 to ring gear 32. Second sun gear 
80 acts as a reaction member. First and second planet gears 36, 40 
planatate and reduction ratio torque is directed to carrier 38. As both 
first and second output members 16, 20A are engaged with carrier 38, power 
transfer mechanism in low-range four-wheel drive mode provides locked-up 
reduction ratio torque to both first and second output members 16, 20A. 
TWO-WHEEL DRIVE MODE 
Shifting mechanism 54A has been shifted to the two-wheel drive position 
establishing two-wheel drive. Teeth of first clutch sleeve 82 is out of 
mesh with teeth of radially outwardly extending portion 52A of second sun 
gear 80. Teeth of second clutch sleeve 84 is now in mesh with teeth of 
radially outwardly extending portion 46A of second sun gear 80. As a 
result, sun gear 42 is engaged through second clutch sleeve 84 with second 
sun gear 80. Teeth of second clutch sleeve 84 is out of mesh with teeth of 
radially outwardly extending portion 26A of second output member 20A. As a 
result, sun gear 42 is disengaged from second output member 20A. Teeth of 
third clutch sleeve 68 is out of mesh with teeth of radially outwardly 
extending portion 18 of first output member 16. As a result, second output 
member 20A is disengaged from first output member 16. 
Torque is transferred from input member 14 to ring gear 32, and torque is 
directed through first planet gears 36 and carrier 38 to first output 
member 16, thus transferring torque only to the first output member 16.