Abstract:
A tandem axle system for a vehicle has a forward drive assembly selectively connected to a rear drive assembly. The forward drive assembly provides constant drive to a forward axle. The rear drive assembly is selectively connected to the forward drive assembly via at least one clutch. Engagement or disengagement of the at least one clutch provides drive to a rear axle or allows the rear axle to idle, respectively.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to tandem axle systems for vehicles. More specifically, the present invention relates to tandem axle systems for vehicles having a first driven axle and a second, selectively driven axle.  
       BACKGROUND OF THE INVENTION  
       [0002]     Those skilled in the art know that traditional tandem axle drivelines of trucks comprise 6×4 drivelines (i.e., 2 wheels on the steer axle and 4 driving wheels on tandem axles behind the steer axle) or 6×2 drivelines (i.e., 2 wheels on the steer axle and 4 wheels on the tandem axles behind the steer axle where only two wheels are on a drive axle).  
         [0003]     The 6×2 drivelines are often undesirable since they lack the needed tractive effort under poor traction conditions.  
         [0004]     The 6×4 drivelines can be undesirable since they can be inefficient, costly and heavy. They are inefficient due to the windage loss of rotating gears in oil. The cost and weight associated with 6×4 drivelines can be, at least partially, attributable to the inter-axle differentials and helical drop gears as well as wheel differentials.  
         [0005]     The 6×4 drivelines are also undesirable since, under most driving traction coefficient conditions, two drive axles are not required to develop the necessary tractive effort for a truck, such as a Class 8 truck.  
         [0006]     Various patents depict and describe attempts by others to disengage one of the two drive axles of a 6×4 tandem when it is not needed. For example, U.S. Pat. No. 5,711,389 teaches that the forward axle of the tandem can be selectively engaged or disengaged by a disk-type friction clutch or clutch pack. The rear axle of the tandem is always driven by a through shaft.  
         [0007]     U.S. Pat. No. 1,927,276 teaches a clutch member designed to engage and disengage a through shaft. The through shaft drives an inter-axle shaft connected to a rear axle. A drop gear set is used to provide drive to the forward axle.  
         [0008]     U.S. Pat. No. 2,064,262 describes a forward drive axle and a rear drive axle that are substantially similar. Both drive axles have a selectively operable clutch comprising a slidable collar. A collar is mounted on splines of the output shaft of the forward drive axle and can slide onto splines on a through shaft. The through shaft provides drive to the rear drive axle. Joining the output shaft of the forward drive axle with the through shaft via the collar connects the forward drive axle and the rear drive axle. Sliding the collar either off of the splines of the through shaft or the output shaft disconnects the forward drive axle and the rear drive axle.  
         [0009]     U.S. Pat. No. 4,046,210 teaches a clutch mechanism located in the forward drive axle designed to selectively connect and disconnect the forward drive axle with the rear drive axle. The clutch mechanism comprises a clutch gear mounted on splines of a through shaft. The clutch gear is selectively moveable on the splines into and out of engagement with a clutch gear on the input shaft. A drop gear set, located forward of the clutch mechanism, provides drive from the input shaft to the forward drive axle.  
         [0010]     In light of the disadvantages of the representative prior art discussed above, it would be desirable for a tandem axle drive system to eliminate the interaxle differential, the rear axle wheel differential and the drop gear set in the forward axle yet which allows the forward axle to selectively provide drive to the rear axle and allows the rear axle of the tandem to idle when not driven.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention is a tandem axle system for a vehicle comprising a forward drive assembly having a forward input shaft and a forward pinion gear axially aligned with and connected to the forward input shaft. The forward pinion gear directly drives a forward ring gear via a hypoid engagement. A rear drive assembly has a rear pinion gear where the rear pinion gear is connected to a rear ring gear. The rear drive assembly is selectively connected to the forward drive assembly.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description when considered in the light of the accompanying drawings in which:  
         [0013]      FIG. 1  is a top, schematic view of a vehicle of the present invention;  
         [0014]      FIG. 2  is a partial, schematic side view of a forward drive assembly;  
         [0015]      FIG. 3  is a partial, schematic side view of another forward drive assembly of the present invention;  
         [0016]      FIG. 4  is a partial, schematic side view of another forward drive assembly of the present invention;  
         [0017]      FIG. 5  is a partial, schematic top view of a rear drive assembly of the present invention; and  
         [0018]      FIG. 6  is a partial, schematic top view of another rear drive assembly of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     It is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions, directions or other physical characteristics relating to the embodiments disclosed are not to be considered as limiting, unless the claims expressly state otherwise.  
         [0020]     Referring now to  FIG. 1 , a vehicle  10  having an engine  12  drivingly connected to a change speed transmission  14  is depicted. A shaft  16  is connected to the output portion of the transmission  14 , such as by a yoke  18  as known to those skilled in the art, and is drivingly connected to an input, such as by a yoke  20  also as known to those skilled in the art, of a forward drive axle housing  22  of tandem axles  24 .  
         [0021]     As described in more detail below, drive is transmitted from the yoke  20  to a first forward drive axle  26  and a second forward drive axle  28  located within the forward drive axle housing  22 . The first forward drive axle  26  and the second forward drive axle  28  are half shafts as known to those skilled in the art. The first forward drive axle  26  provides drive to at least one wheel  32  and associated tire (not shown) and the second forward drive axle provides drive to at least one wheel  32 A and associated tire (not shown), as known to those skilled in the art.  
         [0022]     A through shaft, numbered generically with reference number  34 , extends through the forward drive axle housing  22  and is drivingly connected to an inter-axle driveline  36 . The inter-axle driveline  36  connects the forward drive axles  26 ,  28  with a first rear drive axle  38  and a second rear drive axle  40 . More specifically, the inter-axle driveline  36  transmits drive to an input, such as a yoke  42 , as known to those skilled in the art, for the rear drive axles  38 ,  40 . The rear drive axles  38 ,  40  are located within a rear drive axle housing  44 . The drive axles  38 ,  40  are half shafts as known to those skilled in the art. The first rear drive axle  38  provides drive to at least one wheel  46  and associated tire (not shown) and the second rear drive axle  40  provides drive to at least one wheel  48  and associated tire (not shown), as known to those skilled in the art.  
         [0023]     Turning now to  FIG. 2 , one embodiment of selected components of the forward drive axle housing  22  is depicted. One half of the yoke  20  connecting the forward drive axles  26 ,  28  with the shaft  16  is depicted. The yoke  20  is mounted on an input shaft  50 . The input shaft  50  is mounted for rotation within the forward drive axle housing  22  on at least two bearings. A bearing cage  52 , as known to those skilled in the art, may be secured to the housing  22 . The bearing cage  52  supports one of the bearings called an inner pinion bearing  54 . An outer pinion bearing  56  comprises the second bearing.  
         [0024]     A seal  58 , as known by those skilled in the art, is located about the input shaft  50  to prevent dirt and debris from entering the forward drive axle housing  22  and from lubricant (not shown) located within the forward drive axle housing  22  from escaping.  
         [0025]     The input shaft  50  is integrally formed with a pinion gear  60 , thus axially aligning the pinion gear  60  with the input shaft  50 . Those skilled in the art will appreciate that a non-integrally formed input shaft and pinion gear are also within the scope of the present invention.  
         [0026]     The pinion gear  60  is part of a hypoid gear set  62  also comprising a ring gear  64 . The ring gear  64  is drivingly connected to a differential case (not shown), as known by those skilled in the art. The differential case is connected to a differential spider (not shown) having at least one shaft with pinions located on the ends of the shaft. The pin ions are connected to side gears (not shown), which are connected to the first and second forward drive axles  26 ,  28 . Thus, drive is provided to the first and second forward drive axles  26 ,  28  as described above and as known by those skilled in the art.  
         [0027]     As shown in  FIG. 2 , the pinion gear  60  has a recess  66  for receiving a portion  68  of a through shaft  70 . The through shaft  70  may be secured within the recess  66  of the pinion gear  60  with mechanical fasteners, welding or the like. It is preferred, however, that a first set of splines  72  in an interior portion  74  of the recess  66  mate with a second set of splines  76  on the through shaft  70 . Regardless of the method used to join the through shaft  70  and the pinion gear  60 , it is preferable that the pinion gear  60  provides rotational drive to the through shaft  70 .  
         [0028]     Based on  FIG. 2 , it can be appreciated that the input shaft  50 , the pinion gear  60  and the through shaft  70  share the same axis of rotation  78 .  
         [0029]     The through shaft  70  extends across the forward drive axle housing  22 . Preferably, the through shaft  70  extends under the second forward drive axle  28 , as shown in  FIG. 2 .  
         [0030]     Those skilled in the art will appreciate that the differential spider, pinions and side gears discussed above may have to be offset to one side of the forward drive axle housing  22  to provide sufficient clearance for the through shaft  70 . The through shaft  70  is supported for rotation with respect to the forward drive axle housing  22  on at least one rear bearing. Preferably, two rear bearings  80  support through shaft  70 . A seal  82  is located outwardly from the bearings  80  to prevent lubricant (not shown) from escaping from the forward drive axle housing  22  and to prevent dirt and debris from entering the forward drive axle housing  22 .  
         [0031]     A yoke  84  is preferably attached to a portion  86  of the through shaft  70 , as known by those skilled in the art. The yoke  84  is connected to the inter-axle driveline  36  described above and depicted in  FIG. 1 .  
         [0032]     Referring now to  FIG. 3 , another embodiment of selected components of the forward drive axle housing  22  is depicted. Reference numbers used in  FIG. 3  for like components discussed above and depicted in  FIG. 2  will be used for the depicted present embodiment. One half of the yoke  20  connecting the forward drive axles  26 ,  28  with the shaft  16  is depicted. The yoke  20  is mounted on an input shaft  88 . The input shaft  88  is mounted for rotation within the forward drive axle housing  22  on at least two bearings. A bearing cage  52 , as known to those skilled in the art, may be secured to the housing  22 . The bearing cage  52  supports one of the bearings called an inner pinion bearing  54 . An outer pinion bearing  56  comprises the other bearing.  
         [0033]     A seal  58 , as known by those skilled in the art, is located about the input shaft  88  to prevent dirt and debris from entering the forward drive axle housing  22  and to prevent lubricant (not shown) located within the forward drive axle housing  22  from escaping.  
         [0034]     The input shaft  88  is integrally formed with a pinion gear  90 , thus axially aligning the pinion gear  90  with the input shaft  88 . Those skilled in the art will appreciate that a non-integrally formed input shaft  88  and pinion gear  90  are also within the scope of the present invention.  
         [0035]     The pinion gear  90  is part of a hypoid gear set  92  also comprising a ring gear  94 . The ring gear  94  is drivingly connected to a differential case (not shown), as known by those skilled in the art. The differential case is connected to a differential spider (not shown) having at least one shaft with pinions located on the ends of the shaft. The pinions are connected to side gears (not shown), which are connected to the first and second forward drive axles  26 ,  28 . Thus, drive is provided to the first and second forward drive axles  26 ,  28  as described above and as known by those skilled in the art.  
         [0036]     As shown in  FIG. 3 , the pinion gear  90  and a through shaft  96  are integrally formed. It can be appreciated, based on  FIG. 3 , that the input shaft  88 , the pinion gear  90  and the through shaft  96  preferably share the same axis of rotation  98 . The input shaft  88  drives the pinion gear  90  and the through shaft  96 .  
         [0037]     The through shaft  96  extends across the forward drive axle housing  22 . Preferably, the through shaft  96  extends under the second forward drive axle  28 , as shown in  FIG. 3 . Those skilled in the art will appreciate that the differential spider, pinions and side gears discussed above may have to be offset to one side of the forward drive axle housing  22  to provide sufficient clearance for the through shaft  96 .  
         [0038]     The through shaft  96  has a male portion  100  designed to fit within a female portion  102  of a shaft  104 . Preferably, the male portion  100  is free to rotate within the female portion  102 .  
         [0039]     It is also preferred that a clutch  106  selectively joins the through shaft  96  with the shaft  104 . In a preferred embodiment depicted in  FIG. 3 , the clutch  106  comprises a clutch gear  108  located on a portion  110  of the through shaft  96  for rotation therewith and a complementary clutch gear  112  located on the shaft  104 .  
         [0040]     A portion of the complementary clutch gear  112  is connected to a shift fork  114 . The shift fork  114  may be hydraulically driven, or pneumatically driven. It is also within the scope of the present invention for the shift fork  114  to be manually or electrically driven. The shift fork  114  is axially movable by a control means  116 , thus bringing clutch gears  108 ,  112  into and out of engagement with one another. It can be appreciated that when clutch gears  108 ,  112  are brought into mesh with one another via the shift fork  114  and the control means  116 , the shaft  104  rotates with the through shaft  96 . When the control means  116  and shift fork  114  disengage the clutch gears  108 ,  112 , the through shaft  96  no longer provides rotational drive to the shaft  104 .  
         [0041]     The shaft  104  is supported for rotation with respect to the forward drive axle housing  22  on at least one rear bearing. Preferably, two rear bearings  118  support the shaft  104 . A seal  120  is located outwardly from the bearings  118  to prevent lubricant from escaping from the forward drive axle housing  22  and to prevent dirt and debris from entering the forward drive axle housing  22 .  
         [0042]     A yoke  84  is preferably attached to an end portion  124  of the shaft  104 , as known by those skilled in the art. The yoke  84  is connected to the inter-axle driveline  36  described above and depicted in  FIG. 1 .  
         [0043]     Referring now to  FIG. 4 , another embodiment of selected components of the forward drive axle housing  22  is depicted. Again, like reference numbers have been used for this embodiment for like structures discussed above. One half of the yoke  20  connecting the forward drive axles  26 ,  28  with the shaft  16  is depicted. The yoke  20  is mounted on an input shaft  126 . The input shaft  126  is mounted for rotation within the forward drive axle housing  22  on at least one bearing  128 .  
         [0044]     A seal  58 , as known by those skilled in the art, is located about the input shaft  126  to prevent dirt and debris from entering the forward drive axle housing  22  and to prevent lubricant (not shown) located within the forward drive axle housing  22  from escaping.  
         [0045]     The input shaft  126  has an outwardly, radially extending flange  130 . The flange  130  is secured to a cup  132  preferably with one or more mechanical fasteners  134 . The flange  130  and the cup  132  may also be welded together or integrally formed together. The flange  130  therefore provides rotational drive to the cup  132 . The cup  132  is drivingly connected to a pinion gear  136 .  
         [0046]     The pinion gear  136  is mounted for rotation within the housing  22  on an inner pinion gear bearing  138  and an outer pinion gear bearing  140 . The outer pinion gear bearing  140  may be supported within the housing  22  with an integrally formed outer pinion gear bearing support  142 , as shown in  FIG. 4 . The inner pinion gear bearing  138  may be supported within the housing  22  with a bolt-on cage  144 , as also shown in  FIG. 4 . Those skilled in the art will appreciate other bearing support mechanisms and systems are within the scope of the present invention.  
         [0047]     The input shaft  126  is preferably axially aligned with the pinion gear  136 .  
         [0048]     The pinion gear  136  is part of a hypoid gear set  146  also comprising a ring gear  148 . The ring gear  148  is drivingly connected to a differential case (not shown), as known by those skilled in the art. The differential case is connected to a spider having at least one shaft with pinions located on the ends of the shaft. The pinions are connected to side gears (not shown), which are connected to the first and second forward drive axles  26 ,  28 . Thus, drive is provided to the first and second forward drive axles  26 ,  28  as described above and as known by those skilled in the art.  
         [0049]     At least one tab  150  extends radially outward through a slot  152  in the cup  132 . A plurality of tabs or a circumferential ring may also be used. The tab  150  is preferably mounted on a clutch gear  154 . The clutch gear  154  is slidable in the axial direction on a plurality of splines  156  on the pinion gear  136 .  
         [0050]     Another clutch gear  158  is mounted on a through shaft  160  so that clutch gear  158  is adjacent clutch gear  154 . The tab  150  is connected to a shift fork  162 . The shift fork  162  may be pneumatically driven, hydraulically driven, electrically driven or manually driven. Regardless of the means to drive the shift fork  162 , it is connected to at least one control means  164 .  
         [0051]     The control means  164  axially moves the shift fork  162  which in turn moves the clutch gear  154  into or out of engagement with clutch gear  158 . Engagement of the clutch gears  154 ,  158  causes the through shaft  160  to rotate with the input shaft  126 . The pinion gear  136  is concentrically located about the through shaft  160  and the pinion gear  136  and through shaft  160  are free to rotate with respect to one another.  
         [0052]     It can be appreciated, based on  FIG. 4 , that the input shaft  126 , the through shaft  160  and the pinion gear  136  share the same axis of rotation  166  and are axially aligned with one another. Those skilled in the art will appreciate that the spider pinions and side gears discussed above may have to be offset to one side of the forward drive axle housing  22  to provide sufficient clearance for the through shaft  160 .  
         [0053]     The through shaft  160  extends across the forward drive axle housing  22 . Preferably, the through shaft  160  extends under the second forward drive axle  28 , as shown in  FIG. 4 .  
         [0054]     The through shaft  160  is supported for rotation with respect to the forward drive axle housing  22  on at least one rear bearing. Preferably, two rear bearings  168  support the through shaft  160 . A seal  170  is located outwardly from the bearings  168  to prevent lubricant from escaping from the forward drive axle housing  22  and to prevent dirt and debris from entering the forward drive axle housing  22 .  
         [0055]     A yoke  84  is preferably attached to an end portion  174  of the through shaft  160 , as known by those skilled in the art. The yoke  84  is connected to the inter-axle driveline  36  described above and depicted in  FIG. 1 .  
         [0056]     Those skilled in the art will note that, in the preferred embodiments of the forward drive axle housing  22  described above, helical drop gear components are not required. Also, only one differential is required due to the elimination of the power divider differential and elimination of the rear axle wheel differential, for the complete tandem axle set.  
         [0057]     Referring now to  FIG. 5 , selected components within a rear drive axle housing  176  are depicted. An input shaft  178  having a yoke  42  for connection with the inter-axle drive line  36  is provided. The input shaft  178  is mounted for rotation within the rear drive axle housing  176  with at least one bearing  180 . A seal  182  is preferably located about the input shaft  178  to prevent dirt and debris from entering into the rear drive axle housing  176  and to prevent lubrication (not shown) from escaping from the housing  176 .  
         [0058]     The input shaft  178  is connected to a rear pinion gear  184 . Preferably, the input shaft  178  is located within the rear pinion gear  184 , so that the rear pinion gear  184  is concentric with a portion of the input shaft  178 . Of course, this arrangement puts the rear pinion gear  184  on the same rotational axis  185  as the input shaft  178 . The input shaft  178  may rotate with the rear pinion gear  184  or separate therefrom.  
         [0059]     A clutch gear  186  is mounted for rotation with the input shaft  178 . Preferably, the clutch gear  186  is mounted on the input shaft  178  via splines  188  that allow the clutch gear  186  to move axially with respect to the input shaft  178 . The clutch gear  186  is connected to a shift fork  190 . The shift fork  190  is connected to a pneumatic, hydraulic, electric, and/or manual control means  192 .  
         [0060]     A complementary clutch gear  194  is located on the rear pinion gear  184  adjacent to clutch gear  186 . Those skilled in the art will appreciate that the control means  192  can axially move the shift fork  190 , thus axially moving clutch gear  186  into and out of engagement with clutch gear  194 . Engagement of the clutch gears  186 , 194  results in rotational drive being supplied from the input shaft  178  to the rear pinion gear  184 .  
         [0061]     The rear pinion gear  184  is mounted for rotation within the housing  122  on at least one bearing. Preferably, the rear pinion gear  184  is mounted for rotation within the housing  122  on an inner pinion bearing  196  and an outer pinion bearing  198 . The inner pinion bearing  196  may be mounted on bearing cage  200 , as shown in  FIG. 5 .  
         [0062]     The rear pinion gear  184  is connected to a rear ring gear  202 . The rotational drive provided by the rear pinion gear  184 , when it is drivingly connected to the input shaft  178 , drives the rear ring gear  202 .  
         [0063]     As shown in  FIG. 5 , the rear ring gear  202  rotates about the first rear axle drive shaft  38  and the second rear axle drive shaft  40 . Preferably, the rear ring gear  202  has a first set of clutch teeth  204  and a second set of clutch teeth  206 .  
         [0064]     The first set of clutch teeth  204  on the rear ring gear  202  are adjacent a clutch gear  208  mounted for rotation on the first rear axle drive shaft  38 . Preferably, the clutch gear  208  is mounted on the first rear axle drive shaft  38  on a plurality of splines  210  that facilitate inward and outward movement of the clutch gear  208  with respect to the first rear axle drive shaft  38 . The clutch gear  208  is connected to a shift fork  212 . The shift fork  212  is controlled by a control means  214  that may be pneumatically, hydraulically, electrically or manually driven.  
         [0065]     The control means  214  can move the shift fork  212  inwardly and outwardly thus moving the clutch gear  208  into and out of engagement with the first set of clutch teeth  204 . Engagement of the first set of clutch teeth  204  with the clutch gear  208  provides rotational drive to the first rear axle drive shaft  38 .  
         [0066]     The second set of clutch teeth  206  on the rear ring gear  202  are adjacent a clutch gear  216  mounted for rotation on the second rear axle drive shaft  40 . Preferably, the clutch gear  216  is mounted on the second rear axle drive shaft  40  on a plurality of splines  218  that facilitate inward and outward movement of the clutch gear  216  with respect to the second rear axle drive shaft  40 . The clutch gear  216  is connected to a shift fork  220 . The shift fork  220  is controlled by a control means  222  that may be pneumatically, hydraulically, electrically or manually driven.  
         [0067]     The control means  222  can move the shift fork  220  inwardly and outwardly thus moving the clutch gear  216  into and out of engagement with the second set of clutch teeth  206 . Engagement of the second set of clutch teeth  206  with the clutch gear  216  provides rotational drive to the second rear axle drive shaft  40 . Preferably, the shift forks  212 ,  220  operate simultaneously with one another to provide identical rotational drive to the first rear axle drive shaft  38  and the second rear axle drive shaft  40 .  
         [0068]     It is within the scope of the present invention for the shift fork  190  to disengage clutch gear  186  with clutch gear  194  to prevent the pinion gear  184  from rotating with the input shaft  178 . It is also within the scope of the present invention for the shift forks  212 ,  220  to disengage the first set of clutch teeth  204  with clutch gear  208  and to disengage the second set of clutch teeth  206  with clutch gear  216  to prevent the ring gear  202  from rotating.  
         [0069]     Referring now to  FIG. 6 , selected components within yet another embodiment of a rear drive axle housing  176 ′ are depicted. Certain components in  FIG. 6  that are similar to components depicted in  FIG. 5  are described above and designated with prime. Components which may be identical between FIGS.  5  and  FIG. 6  use the same reference numbers without a prime designation.  
         [0070]     An input shaft  178 ′ having a yoke  42 ′ for connection with the inter-axle drive line  36  is provided. The input shaft  178 ′ is mounted for rotation within the rear drive axle housing  176 ′ with at least one bearing  180 ′. A seal  182 ′ is preferably located about the input shaft  178 ′ to prevent dirt and debris from entering into the rear drive axle housing  176 ′ and to prevent lubrication (not shown) from escaping from the housing  176 ′.  
         [0071]     The input shaft  178 ′ is connected to a rear pinion gear  184 ′. Preferably, the input shaft  178 ′ is located within the rear pinion gear  184 ′, so that the rear pinion gear  184 ′ is concentric with a portion of the input shaft  178 ′. Of course, this arrangement puts the rear pinion gear  184 ′ on the same rotational axis  185 ′ as the input shaft  178 ′.  
         [0072]     The rear pinion gear  184 ′ is mounted for rotation within the housing  176 ′ on at least one bearing  180 ′. Preferably, the rear pinion gear  184 ′ is mounted for rotation within the housing  122 ′ on an inner pinion bearing  196 ′ and an outer pinion bearing  180 ′. The inner pinion bearing  196 ′ may be mounted on bearing cage  200 ′ as shown in  FIG. 6 .  
         [0073]     The rear pinion gear  184 ′ is connected to a rear ring gear  202 ′. The rotational drive provided by the rear pinion gear  184 ′, when it is drivingly connected to the input shaft  178 ′, drives the rear ring gear  202 ′.  
         [0074]     As shown in  FIG. 6 , the rear ring gear  202 ′ rotates about the first rear axle drive shaft  38  and the second rear axle drive shaft  40 . Preferably, the rear ring gear  202 ′ has a first set of teeth  204 ′ and a second set of teeth  206 ′.  
         [0075]     The first set of teeth  204 ′ on the rear ring gear  202 ′ are adjacent a clutch gear  208 ′ mounted for rotation on the first rear axle drive shaft  38 . Preferably, the clutch gear  208 ′ is mounted on the first rear axle drive shaft  38  on a plurality of splines  210 ′ that facilitate inward and outward movement of the clutch gear  208 ′ with respect to the first rear axle drive shaft  38 . The clutch gear  208 ′ is connected to a shift fork  212 ′. The shift fork  212 ′ is controlled by a control means  214 ′ that may be pneumatically, hydraulically, electrically or manually driven.  
         [0076]     The control means  214 ′ can move the shift fork  212 ′ inwardly and outwardly thus moving the clutch gear  208 ′ into and out of engagement with the first set of teeth  204 ′. Engagement of the first set of teeth  204 ′ with the clutch gear  208 ′ provides rotational drive to the first rear axle drive shaft  38 .  
         [0077]     The second set of teeth  206 ′ on the rear ring gear  202 ′ are adjacent a clutch gear  216 ′ mounted for rotation on the second rear axle drive shaft  40 . Preferably, the clutch gear  216 ′ is mounted on the second rear axle drive shaft  40  on a plurality of splines  218 ′ that facilitate inward and outward movement of the clutch gear  216 ′ with respect to the second rear axle drive shaft  40 . The clutch gear  216 ′ is connected to a shift fork  220 ′. The shift fork  220 ′ is controlled by a control means  222 ′ that may be pneumatically, hydraulically, electrically or manually driven.  
         [0078]     The control means  222 ′ can move the shift fork  220 ′ inwardly and outwardly thus moving the clutch gear  216 ′ into and out of engagement with the second set of teeth  206 ′. Engagement of the second set of teeth  206 ′ with the clutch gear  216 ′ provides rotational drive to the second rear axle drive shaft  40 . Preferably, the shift forks  212 ′,  220 ′ operate simultaneously with one another to provide identical rotational drive to the first rear axle drive shaft  38  and the second rear axle drive shaft  40 .  
         [0079]     It is also within -the scope of the present invention for the shift forks  212 ′,  220 ′ to disengage the first set of teeth  204 ′ with clutch gear  208 ′ and to disengage the second set of teeth  206 ′ with clutch gear  216 ′ to prevent the ring gear  202 ′ from rotating.  
         [0080]     Based upon the above-described preferred embodiment of the present invention, those skilled in the art will appreciate that a rear axle differential is not required. The rear ring gear  202 ,  202 ′, thus provides selective, direct drive to the first rear axle drive shaft  38  and the second rear axle drive shaft  40 .  
         [0081]     It must also be appreciated that the assembly described above and depicted in  FIG. 2  can be mated to the assembly described above and depicted in  FIG. 5  via the inter axle driveline  36 . Further, it can be appreciated that the assemblies described above and depicted in FIGS.  3  or  4  can be mated to the assembly described above and depicted in  FIG. 6  via the inter axle driveline  36 .  
         [0082]     In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiments. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.