CA1255121A - Five gear torque transfer assembly - Google Patents

Abstract

VEHICLE GEAR ASSEMBLY FOR TORQUE TRANSFER TO
TWO OR FOUR-WHEELS
ABSTRACT
A gear assembly for selectively shifting the driving torque in an engine driven vehicle to either a 2 or 4-wheel high mode while in motion, or from a neutral position to a 4-wheel low mode and return.
The assembly has five main gears including an input set of two and an output set of three constant mesh gears rotatably mounted in fixed positions and a synchronizer in between the two sets in selective driving association with the output gear set. Selective transfer of torque from the input drive to the assembly to either or both the front and rear output shafts is effected by operation of a single shift rod which in turn moves a single fork both for activation of the synchronizer and for shifting the assembly into its 2 and 4-wheel high, neutral and 4-wheel low drive positions through interconnecting gear clutch collars. A poppet member arrangement in the form of a poppet ball or slug provides an interlocking connection between clutch collars for their positive positioning in the drive positions manually selected by way of the shift rod.

Description

~3S 1 BACKGROUND OF T~IE INVENTION

This lnvention is a ~ear system which enables transfer of engine supplied drive power -to and between two and four-wheel high speed drive modes while the vehicle is in motion as well as allowing independent shi~ting of power to a ~-wheel low speed drive mode from a stopped neutral position.
Four-wheel drive vehicles are highly desirable for off-road travel over rou~h terrain because o~ their greater reliability in providing traction in such conditions, but on the usual hard improved surface roads they can be more economically operated as a two-wheel conventional drive vehicle. The capability of shifting of a four-wheel drive vehicle to two-wheel driving and return therefore is highly desirable dependent on the conditions of traction confronted.
Gear transfer arrangements of the prior art have enabled conversion of vehicles -to two-wheel or four wheel d-ive modes, but in many arrangements the shifting between such modes has not been possible while the vehicle is in ?n motion. In one such arrangemen-t the position of the locking drive hub oE the usual non-driven wheels must be manually locked or unlocked to correspond to the four or two-wheel drive shift control positions desired. Thus conversion from two-wheel to four-wheel drive and return has been cumbersome, ~5 time consuming and subject to error in combinations of settings required to obtain the wheel driving relationships desired.

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In another -type of prior ar-t year shif-t sys-tem, a belt drive arrangemen-t is relied upon in which a chain bel-t is utilized to supply power to the gear sys-tem.
The efficiency of this system suffers from greater power consumption in that all gears of the assembly opera-te all the time with corresponding constant splashing -through oil without relief in shifting from one drive mocle to another. In addition, the chain belt drive calls for a greater amount of space in comparison to a corresponding gear assembly.
Other all gear systems exlst which include constant mesh gear arrangements but lac]c the simple reliable fixed relation of gears of the present invention in addition to lacking means for synchronizing gear speeds for a smooth shift between two and four-wheel drive modes while the vehicle is in motion and a single fork means effecting shifts between operating modes.
In view of the foregoing, it is an object of the present invention to provide an improved torque transfer system in the form o, a gear assembly unit incorporating a synchronizer for ma-tching gear speeds to permlt shifting of torque from a two to a four-wheel drive mode and return while the vehicle is in mo-tion and in which only a single actuating fork is required to change operating modes.
Another object of the invention is to provide an improved rugged and long life torque -transfer unit in which reliability is promotecl by having all the gears rotatably in an axially fixed position in constant mesh ..
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relationshlp and in which torque transfers through -the system can be effected by the movement of a single shiEt fork.
SUM~ARY OF TEE INVE~TION
The invention is an improved vehicle torque transfer gear mechanism in which the gear assembly includes an input set of two constantly meshed gears in side by side spaced relation with an output set of -three constantly meshed gears and a gear speed synchronizer be-tween the two gear sets associated with the output set to match speeds of the two gear sets to enable a smooth shifting of torque paths between two and four-wheel high drive modes while the vehicle is in motion. Clutching collars all movable by a single shift fork effect -the changes in drive modes selected by way oE a shift control rod.
~he ~our-wheel low speed drive mode is set by shifting from a stop/neutral position. A novel concentric arrangement of clutching collars with interlocking slug means in the form of poppet balls or palls provide a space saving means for shifting torque paths. In addition the annular synchronizer in applicant's arrangement is positively piloted by support about opposite peripheral edges thereby assuring more square and positive alignmen-t and suppor-t of the synchronizer a~ainst wobble ~or reliable operation.
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~'', A fea-ture oE the inven-tion is that power -transfer is effected through an all year assembly in which only two gears o~erate in -the two-wheel drive mode while all five gears are called into operation only in the lesser used four-wheel drive mode -thus promoting high efficiency in over all operation of the power transfer unit.
Still another feature of the invention is tha-t only a single fork is required for shift of torque -transfer collars and for ac-tivation of the synchronlzer to effect shifts of torque paths through the gear assembl~.

BRIEE DESCRIPTION OF T~IE DRAWI~GS
Figure 1 is a partia.l.ly schematic ~lan view of components of a four-wheel drive system for an automotlve vehicle including a gear transfer unit of the present invention;
Figure 2 is an enlarged and elevational view of the rear output end of the case for the four-wheel gear transfer unit of Figure 1;
Figure 3 is a cross-sectional view of the transfer case of Figure 2 as taken on line 3-3 showing -the five gear assembly with the svnchronizer located between the input and output gear sets;
Figure ~ is a cross-sectional view of the transfer case shown in Figure 2 showing the gear assembly as vi.ewed on line 4-~ illustrating the shift rod and shift ~ork for engagement wi-th clutch collars to e.ffect chan~es in drive . : :

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modes of -the vehicle;
Figure 5 is an enlarged somewhat schematic view of a portion of the gear assembly oE Figure 3 showing the clutch collar position and the synchronizer setting to provide a two-wheel high speed drive mode for the vehicle;
Figure 6 is a view of part of the gear assembly of Figure 3 showin~ the shift collar and synchronizer settings for a four-wheel high drive mode of the vehicle;
Figure 7 is a view of part of the gear assembly o~ Figure 3 showing the shift collar and sychronizer settings for the neutral mode of the vehicle; and Figure 8 is a view of part of the gear assembly of Figure 3 showing the shift clutch collar and synchronizer lS settings for a four-wheel low drive mode of the vehicle.

BEST MODE OF CARRYING OUT I~VE~TION
Referring to the drawings in grea-ter detail, Figure 1 shows the case 10 for the torque transfer mechanism of the present invention positioned in a four-wheel vehicle behind the engine 11 and its transmission 12 which has a tail shaft (not shown) to which the unit 10 is connected for receipt of engine driving power. Power is transmitted from the unit 10 to the rear wheels 1~ through a rear differential 13 and power supplied to the frcnt wheels 16 is supplied through a front wheel differential 15.

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Figure 2 shows the rear output end oE the gear case 10 with its rear drive shaE-t por-tion 21 includiny the rear output shaft 30, closed front drive shaft portion 22 and the closed intermediate shaf-t por-tion 23.
Figure 3 is a cross-sectional view of the case 10 as taken on line 3-3 of Figure 2 showing the input drive shaft ~0 and the associated gear assembly for dri~7ing the rear wheel and front wheel output shafts 30 and 32, respectively. The input dri~7e shaft 20 is a~ially aligned with -the rear output shaft 30, each shaf-t being rotatable independent of the other, but interassociated by providing the input shaft 20 with a reduced diameter end portion extendin~ coaxially into a hollowed end portion of the rear output shaft 30. A small gap 36 between the mated coaxial ends of the shafts provides a space for a needle bearing 37 t~ provide mu-tual support for the two shafts 20 and 30.
Two sets of constant mesh gears make up the torque transfer gear assembly, namely an input gear se-t and an output gear set. The input gear set includes an input gear 40 in constant mesh with a larger diameter low speed gear 45. The input gear is fixed on the input shaft 20 and can be formed integrally with the inpu-t shaft as shown to provide a solid constantly aligned combination. The low ~5 speed gear 45 is rotatably mounted on roller bearings on an intermediate shaft 47.
The output gear set includes a rear drive gear ' ~ . .

50 in cons-tant mesh rela-tion with a smaller intermedia-te gear 55 which in -turn is in constan-t mesh with the front output gear 57 which is of the same .size as the rear drive gear 50. The rear drive gear 50 is suppor-ted ro-tationally on the rear output shaft 30 while the i.ntermediate gear 55 is rotatably mounted on roller bearings on the intermediate shaft 47 and the front drive gear is fixed in splined relation on a ball bea.ring supported front output shaft 32.
The drive power paths through the gear assembly are changed by laterally shifting of collars on and between splines 48 and 68 at the confronting exterior ends of the input and rear output shafts 20 and 30, respectively, splines 51 of a hub of the rea,r drive gear 50 and confronting hubs of the in-ter~edia-te gear 55 and low speed gear ~5 on the intermediate shaft 47.

TWO--WEIEEL HIGH MODE

Figures 5 and 6 show a coaxial collar assembly in which in-ternal splines 61 of an annular outer collar 60 make slideable driving engagemen-t with external teeth 67 of a smaller annular interior collar 65. The interior collar 65 has internal splines 66 which in the high speed two-wheel drive mode of the assembly bridges the gap bet~7een t,he confronting ends of the input and rear output shafts 20 and 30 and engages matching peripheral splines 48 and 68 about the ends of shafts 20 and 30, respectively. A series of ': :

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elongated cylindrical locking slugs 69 with hemi-spherical ends, herein alternatively termed palls 69, are each snugly fit wi-thin one of a series of apertures in a thicker e~tra strength marginal edge 70 provided on the inner collar 65. The collar edge 70 as shown extends over -the end portions of splines 68 on shaft 30 where the ends of the palls 69 make engagement wi-th ma-tching locking recesses 71 in the splines 68. The palls 69 are sufficiently long to be backed by the splines of the outer collar 60 thereby causing the palls to positively lock the collar 65 in its bridging relation in the two wheel drive mode until the collar 60 is slid laterally as described hereinafter, where an opposite series of pall matching recesses 72 in the interior splines 61 of -the outer collar 60 receive the opposite ends of the palls to release -the palls and the collar 65 from their locked -two-wheel position.
A pin type synchronizer 25 is mated in selective driving relation with the rear output drive gear 50 and has its locking ring 27 joined integrally with the outer circumferential periphery of the collar 60 so tha-t upon movement of collar 60, the synchronizer is activated to establish a driving relation with the rear drive gear 50.
The synchronizer includes a cone shaped clutching ring 29 which selectively makes a locked driving communicatlon or a disengaged non-driving relation with a finished internal rim 35 of the rear drive gear 50. The synchronizer in the two-wheel high mode represented in Figure 5 is illustrated ,: ~"` '.

in its disengaged rela-tion. A set oE synchronize:r blocking pins 26 e~-tend between -the clutching ring 29 and an opposite retaining ring 31 and in doing so, pass through separate retaining apertures in the locking ring 27. The blocking pins 26 each have a circumferential groove 28 which when the synchronizer is inactive, is located in the region of the aperture in the locking ring 27 through which i-t passes, thus resulting in the pins 26 being held in a released condition within the locking ring 27. The retaining .-ing 31 in the present invention is piloted by a residence backing groove 33 in a stationary portion of the case for the transfer mechanism 10, which as shown is in the form of a projecting portion 34. Thus the synchronizer can be stably retained against possible wobble within the assembly and maintained in dimensionally square association with the rear drive gear 50.
When the synchronizer locking ring 27 is moved laterally for selection of a next drive mode, which from the two-wheel drivejarrangement represented in Figures 3 and 5 would be the four-wheel high drive mode of Figure 6, the ring 27 passes over the shoulders of the blocking pin grooves 28 and causes the pins to take on positions which results in the cone shaped clutching ring 29 engaging the rear drive gear rim surface 35 in driving relation.

The mechanism for effecting a change of drive mode by way of a single manually operated control rod (not shown) is illustrated in Figure 4 which is a cross-sectional view of the torque transfer unit as taken on line 4-~of ~igure 2. A shi:E-t rod 80 connected to and moved by the manually opera-ted control rod has a shift fork 90 mounted thereon in close fit sliding relation by way of a fork hub 93 which is backed by a retaining ring 98 fixed on shaft 80. An upper arm 91 of the shift fork makes enqagement with the rotatable synchronizer locking ring 27 which in turn is directly connected -to the outer shift collar 60 so that upon lateral movement of the fork 90 both the locking ring and the collar 60 are correspondingly laterally moved. An oppositely extending lower arm 92 of the fork similarly engages a low speed rotatable collar 52 to permit la-teral movement thereof bv way of an exterior detent 54 pro~ecting therefrom. The collar 52 has internal splines 53 which communicate in sliding relation with mating splines 46 and 56 on the hubs of gears 45 and 55, respectively, both rotatably mounted in side-by-side relation on t~e intermediate shart 47.
The hub 93 of fork 90 has a hollow cup-like opening 95 for accommodati.on of a biasing spring 94 surrounding the shaft 80 and extending outwardly from t.he hollow hub portion 95 for spring pressure relation against an annular spring retainer washer 96 mounted on the shaft 80 and backed by a retaining ring 97 Eixed on the shaft. The pressure of the spring 94 against the washer 96 bi.ases the fork hub 93 toward and against another retaining ring 98 also fixed on the shaft 80.

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ll The shaft ~0 has a series of side-by-sicle circum-ferential mode position grooves 8~, 85, 86 and 87 for positioning of the shift rod 80 in semi-locked pressure relation for the two-wheel high, four-wheel high, neutral and four-wheel low speed drive modes, respectively. An e~tra groove 88 is provided for association with switching means for activation or signalling of auxiliary components if desired. At least one poppet ball 81 makes pressure engagement with the matching rounded grooves each in turn under the biased influence of a spring 82 which communicates with the ball within a residence opening 83 for the spring and ball in -the case for unit. When the shaft 80 is moved longitudinally the pressure of the poppet ball 81 communicating with each respective groove is overcome to effect a shift in mode. When the shift rod 80 is moved to a position corresponding to the four-wheel high drive where the poppet ball pressure is exerted in groove 84, the pressure of the spring 82 is overcome and the ball 81 passes over -the ridge of groove 84 and drops into groove 85 to lock the fork 90 in this position.
Figures 5 to ~ illustrate in sequence the power paths throu~h the effective components of the gear assembly for -the two-wheel high, -the four-wheel high, neutral and four-wheel low modes, respectively. The synchroni~er 25 in the -two wheel high mode, as described above, is in active condition represented in Fi~ure 5 while the input drive shaft 20 is connected directly to the rear output shaft 30 by the inner collar 65, the interior splines 66 of which ~ 3~ ~

establish a locked interenyayiny drlviny relation between splines ~8 of the input shaft and s~lines 68 o~ -the ou-tput sha-ft 30.

FOUR--WHEEL HIGEI MODE
Upon shift of the rod 80 from its -two-wheel high drive mode to the four-wheel high mode represented in Figure 6, the poppe-t ball 81 is biased in-to the groove 85 to hold the shift rod 80 in position~ The synchronizer locking ring 27 and the integrally associated external collar 60 of the coaxial collar assembly are thereby moved laterally toward the rear drive gear 50. In sequence, the synchronizer 25 is first activated by movement of its locking ring 27 over a portion of each of the blocking pins 26 from the grooves 28 which brings the clutch ring 29 into frictional driving engagement with the rim 35 of the rear drive gear 50. The gear 50 is thereupon gradually brought up to the rotational speed of the interconnected input and rear output shafts 20 and 30, respectively, whereupon the collar 60 and its splines 61 can mate and slide into positive driving relation with the splines 51 of gear 50. The rear drive shaft 30 and the constant mesh output gear set including rear drive gear 50, intermediate gear 55, the front drive gear 57 and the front drive shaft on which it is fixed are thus all driven in unison in the four-wheel high speed mode by the input drive shaft 20.

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NEUTRAL MOD~

When the shi-Et rod 80 is moved to the neutral mode represented in Figure 7, the fork 90 moves the synchronizer locking ring 27 and collar 60 still another 5 step closer to -the output gear 50 over its splines 51.
This movement causes a radial inward thicker marginal pro~ecting portion 73 of the collar 60 to engage -the ma-tching radial outward projection 74 at the marginal edge 70 of collar 65 and to draw the collar with it away from engagement with splines 48 of the input shaft 200 As may be seen in Figure 6, the repositioning of the collar 65 from its locked conditlon of the two-wheel high mode releases the locking pall 69 because o:E the p.resence of a pall recess 72 in splines 61 which allows the locking pall 69 to be accepted in recess 72 from the locking recess 71. This in turn interlocks the exterior and interior collars 60 and 65 in an eY~tended relation as illustrated in Figure 7 with the collar 65 withdrawn from its driven communication with the splines 48 of the input shaft 20. The output gear set is thus placed in an idle neutral condition with neither ~he rear or Eront output shafts 30 and 32 connected for receipt of power.

FOUR-W~IEEL LOW MODE
Upon movement of the shift rod 80 ~rom the neutral position to the four-wheel low drive mode represente~

1'1 in Figure 8, -the interlocked extended collars 60 and 65 are advanced still fur-ther toward the rear outpu-t gear 50 while maintaining their connected relation with the splines 61 and 68 and correspondingly maintaining an interlocked relation of the output gear 50 with the rear output shaft 30. At the same time the low speed collar 52 is moved into position to bridge splines 46 and 56 of the low speed gear 45 and intermedia-te gear 55, respectively, thereby resulting in these two gears rotating in unison. The torque of the input shaft 20 thus is transmitted Erom the inpu-t gear 40 to the low speed and intermediate gears 45 and 55, respectively, from which it is transmitted to both the rear and front output gears 50 and 57, respectively, and correspondingly the rear and front output shafts 30 and 32 to which they are connected. ~hus the four wheels of the vehicle are supplied with torque power at the same rotational speed since both the rear drive gear 50 and front drive gear 57 are of the same size and are both driven by the smaller intermediate gear 55 which in turn is driven by the larger low speed gear 45, consequently providing a higher torque and lower speed power to the output shaft in comparison to the power supplied to the input shaft 20.

SHIFT UNDER WINDUP STRESS

As the shift rod 80 is moved into position from the two-wheel high mode illustrated in Figure 3~4/ and 5 to the four-wheel high, neutral and four-wheel low modes . . .
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of Figures 6, 7,and 8, respectively, the retaining ring 98 pushes the hub 93 of the fork 90 laterally ~o activate the synchronizer and position clutch collars for the corresponding operating modes without significant restraint upon movement of the shift rod 80. In reverse sequence, however, the assembly at times has transient internal stresses of drive wheel windup developed therein, such as by reason of vehicle turns which result in greater travel of the front wheels relative to the rear wheels, which cause a restraint in movement of collars for shifts from four-wheel drive modes. More specifically, upon motion of the shif-t rod 80 from the four-wheel high mode of Figure 6 to the two-wheel high mode of Figure 5, stresses can arise between splines 51 and 61 of the rear output gear 50 and the collar 60, respectively, such that the collar 60 is restrained from lateral movement to effect its disengagement from gear 50 for shift into the two-wheel high mode illustrated in Figure 5. The shift rod 80 in being moved under such assembly stressed conditi.ons slides within the fork hub 93 càusing the retaining washer 96 to compress spring 95 within the hollow 94 of the fork hub 93 and correspondingly exert pressure on the hub 93 -to promote the disengagement between collar 60 and gear 50 upon reduc-tion of the stresses therebetween with continuing travel of vehicle. Thus the actual shift of the fork 90 under the biasing ac-tion of the spring 95 from -the four wheel high position to the two-wheel high position may be delayed slightly until windup s-tresses which may be present in the assembly are reduced sufficien-tly that the spring 95 can move the fork arm 90 laterally a distance limited bv the retaining ring 98 and corresponding to the new mode.
Similarly when the rod 80 in being shifted from its four-wheel low drive position to the neutral position, like stresses between gear splines 61 and collar splines 51 and between collar splines 66 and splines 68 at the rear output shaft 30 can restrain and delay movement of the extended interlocked collars 60 and 65 from the position of the four-wheel drive illustrated in Figure 8 to the position illustrated in Figure 7 where the splines 53 of the low speed collar 52 are disengaged from the intermediate gear splines 56. The spring 95 is compressed and is effective during such a transfer to promote the spline disengagement upon reduction of stress between the collar splines 61 and the gear splines 51. It will be recognized in view of the foregoing that the spring 95 is selected to exert sufficient force to move the fork 90 and 2n collars of the assembly upon reduction of transfer restraining stresses, b-ut no-t so large as to exert a back pressure against the spring retainer 96 to overcome the locking action imparted on the shift rod 80 by the spring 82 on the poppet ball 81 in the shift rod groove of the selected operating mode.

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Although the drawings and description of -the invention are herein presented in relation oE -the preferred form of the invention, it will be recognized tha-t variations are possible within the broad scope of the invention.
For example, the synchronizer in the described embodiment is associated with the rear output drive gear, but instead can be included in the assembly in association wi-th the intermediate gear or with the front output drive gear if the latter is made free to rotate on its supporting shaft. Still further, the pilot support for the synchronizer can be any of a number of housing supports built into the casing for the assembly.
In view of the foregoing it will be understood that many variations of the arrangement of my invention can be provided within the broad scope of the principles embodied therein. Thus while a particular preferred embodiment of my invention has been shown and described, it is intended by the appended claims to cover all such modifications which fall within the true spirit and scope of the invention.

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Claims (18)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A gear assembly for selectively shifting a power driven vehicle to a two-wheel or a four-wheel drive mode comprising a case for said gear assembly, an input shaft for supply driving power to said assembly, a first output shaft for driving a first pair of wheels and a second output shaft for driving a second pair of wheels, a gear set comprising three constant mesh gears including a first output gear for free rotation on said first output shaft, an intermediate gear in meshed relation with said first output gear and an intermediate shaft on which said intermediate gear is rotatably mounted and a second output gear for said second wheels fixed to said second output shaft and having a meshed driven relation with said intermediate gear, means for selectively connecting said input shaft in direct driving relation with said first output shaft, to drive said first pair of wheels, a synchronizer mounted for direct rotation concentrically with said first output shaft and for frictional engagement with the input shaft side of said freely rotatable first output gear, means for selectively activating said synchronizer to establish such frictional engagement of the synchronizer with said first output gear to effect matching of rotational speeds between said first output gear and first output shaft, means for positive connection of said first output gear and first output shaft for rotation in unison upon such matching of speeds and shift control means for making the selective connections and for activation of said synchronizer in said assembly corresponding to desired operating modes whereby the driving power of said input shaft can be selectively supplied to said first pair of wheels alone or to both said pairs of wheels.

2. A vehicle gear assembly as set forth in claim 1 wherein said shift control means comprises a single shift fork for making the selective connections.

3. A vehicle gear assembly as set forth in claim 1 wherein said means for connecting said input shaft directly with said first output shaft comprises a first annular collar having internal splines, said input shaft and first output shaft being axially aligned and having matching external splines with which the internal splines of said collar are slideably mateable whereby said collar can be selectively moved onto one of said shafts and alternately in bridging connecting relation between said shafts.

4. A vehicle gear assembly as set forth in claim 3 wherein said means for connecting said input shaft with said first output shaft includes a second annular collar concentric with and external of said first collar, said first collar having a locking member within an aperture therein for locking said first collar to said output shaft in its bridging relation between said shafts, said second collar being laterally moveable over said first collar to hold said locking member in position to effect the locked relation between said second collar having an interior recess for receipt of said locking member to release said locked relation between shafts when said collar is moved to a position with said recess over said locking member.

5. A vehicle gear assembly as set forth in claim 4 in which said selective activating means for said synchronizer comprises a ring member joined to and laterally moveable with said second collar.

6. A vehicle gear assembly as set forth in claim 5 in which said means for positive connection of said first output gear and first output shaft comprises splines on the interior of said second collar and matching splines on the hub of said first output gear engageable by said interior splines of said second collar upon lateral movement thereof whereby the positive connection between said first output gear and first output shaft can he established for supply of power to both pairs of wheels of the vehicles.

7. A gear assembly as set forth in claim 1 including a second gear set comprising two constant mesh gears including an input gear fixed on and rotated in unison with said input shaft and a low speed gear in constant mesh with and larger in diameter than said input gear, said low speed gear being mounted in freely rotatable relation on said intermediate shaft, means for selectively connecting said low speed gear for rotation in unison with said intermediate gear of said first gear set whereby drive power supplied from said first gear set to said second gear set for said two output shafts is supplied at a lower rotational speed than the rotational speed of said input gear.

8. A gear assembly as set forth in claim 7 wherein said intermediate gear is smaller in diameter than said low speed gear whereby the rotational speed of said output shafts is further lowered in comparison to the rotational speed of said input shaft.

9. A gear assembly as set forth in claim 8 wherein said synchronizer is disposed within the assembly between said two gear sets.

10. The gear assembly of claim 9 wherein said synchronizer in operation is supported on one side in engaging relation with said first output gear, and an internal portion of said casing is provided to pilot the opposite side of said synchronizer whereby tendencies toward wobble which might otherwise arise are minimized.

11. A gear assembly as set forth in claim 9 wherein said shift fork control means comprises a shift rod indexed for the different operating modes of the assembly and a single operating fork extending in the region between said gear sets.

12. A gear assembly as set forth in claim 11 wherein the shift fork is mounted on said shift rod.

13. A gear assembly as set forth in claim 11 wherein the shift rod is longitudinally movable relative to said fork and said fork is spring biased in one direction to allow indexing movement of said rod opposite to said biasing direction regardless of operating stresses of the gear assembly sufficient to prevent corresponding motion of said fork, said spring being of size to provide a biasing force adequate to move said fork to the mode position set by said shift rod upon sufficient reduction in such operating stresses of the gear assembly.

14. The gear assembly of claim 13 wherein a series of side by side grooves are provided on said shift rod, each said groove corresponding to a different position of said rod for a separate operating mode of the assembly and a spring biased locking member located to effect at least a semi-locked engagement with each groove of said shift rod corresponding to a selected mode position, the spring biasing action of said locking member being sufficient to maintain said rod in selected positions against opposing force exerted on said shift rod by the biasing spring action on said fork.

15. A gear assembly for selectively shifting a power driven vehicle to a two-wheel or a four-wheel drive mode comprising a casing for said gear assembly, an input shaft for supply of driving power to said assembly, a first output shaft for driving a first pair of wheels and a second output shaft for driving a second pair of wheels, a gear set comprising three constant mesh gears including a first output gear mounted on said first output shaft, an intermediate gear in meshed relation with said first output gear and an intermediate shaft on which said intermediate gear is rotatably mounted and a second output gear for said second wheels mounted on said second output shaft and having a meshed driven relation with said intermediate gear, a synchronizer mounted for coaxial rotation with one of said output shafts and for selective frictional driving connection with the input shaft side of the one output gear mounted thereon, said output gear on said one output shaft being rotatable relative to said one output shaft, the output gear on the other of said output shafts being fixed on said other of said output shafts, means for activating said synchronizer to frictionally engage said one output gear to effect matching of rotational speeds between said one output gear and said one output shaft on which it is mounted and means for establishing a positive connection between said output gear and said one shaft on which it is mounted whereby said output gear and one output shaft are driven in unison, and shift control means for selectively connecting said input shaft to said one output shaft and through said synchronizer to said one output gear of said set and consequently to all three meshed gears whereby power can be supplied selectively to two or four-wheels of the vehicle.

16. The gear assembly as set forth in claim 15 wherein said synchronizer is mounted for rotation with said first output gear and including a second gear set comprising two constant mesh gears one of which is an input gear fixed on and rotated with said input shaft and the other being a low speed gear in constant mesh with and larger in diameter than said input gear, said low speed gear being freely rotatable on said intermediate shaft, means for selectively connecting said low speed gear for rotation in unison with the intermediate gear of said three gear set whereby power is supplied to said three gear set at a lower rotational speed than the rotational speed of said input shaft, said synchronizer being supported in said assembly between said gear sets, a single shift fork between said gear sets for activating said synchronizer and for changing connections between said gears and shafts and a manual shift control rod for changing position of said fork to effect changes in connections through said assembly corresponding to operating modes of said assembly selected by the positioning of said control rod.

17. The gear assembly as set forth in claim 16 wherein said connection means for positively connecting said output gear and said one shaft on which is is mounted and means for connecting said input shaft to said one output shaft comprise coaxial collars movable laterally with respect to each other in extended interlinked association.

18. The gear assembly as set forth in claim 17 wherein said means for connecting said low speed gear for rotation with the intermediate gear is movable in alignment with the external collar of said coaxial collars.