Patent Publication Number: US-8523738-B2

Title: Method of shifting a tandem drive axle having an inter-axle differential

Description:
CLAIM OF PRIORITY 
     The present application claims priority to and incorporates by reference U.S. Provisional Application No. 61/435,007 filed Jan. 21, 2011, entitled “Two Speed Tandem Drive Axle Having an Inter-Axle Differential.” 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a vehicle drive train and to a method of shifting a tandem drive axle for the vehicle drive train having an inter-axle differential from a first operating state to a second operating state. 
     BACKGROUND OF THE INVENTION 
     Vehicles incorporating tandem drive axles benefit in many ways over vehicles having a single driven axle. Inter-axle differentials in such vehicles may be configured to distribute torque proportionately or disproportionately between the axles. Additionally, shift mechanisms may be provided to such vehicles to permit the disengagement of one of the driven axles or to transition from single axle operation to tandem axle operation, among other benefits. However, such versatility typically requires the incorporation of additional drive train components into the vehicle at added expense and weight. Such added weight results in a decreased fuel efficiency of the vehicle. 
     Components of the tandem drive axle may also be selected based on a gear reduction ratio present in an axle. Axle ratios may be of a two-speed configuration to permit the vehicle to operate in a low speed and high torque manner or in a high speed and low torque manner. It is preferred to drive both axles when the low speed and high torque manner of operation is desired (to distribute the higher torque amongst a greater number of wheels) and it is advantageous to operate a single axle when the high speed and low torque manner of operation is desired (to decrease windage and frictional losses when torque distribution is of lower concern). However, incorporation of both the two-speed configuration, an axle disconnect function, and the inter axle differential may be prohibitive with respect to cost and weight. Such added weight, windage losses, and frictional losses result in a decreased fuel efficiency of the vehicle. 
     Additionally, when components are provided to permit the disengagement of one of the driven axles or to transition from the vehicle from single axle operation to tandem axle operation, a complexity of the tandem drive axle is increased. Typically, such operations may require manual engagement by an operator when specific conditions are present or may require the operator to stop the vehicle to engage specific components. Such features, while providing additional functionality to the vehicle, may not be properly implemented by the operator or may not be used at all by the operator. When such features are not properly implemented, a shift from one operating state to another may result in damage to the tandem drive axle or a rough shift, either of which typically result in a dissatisfaction of the operator. 
     It would be advantageous to develop a method of shifting a tandem drive axle system from a low speed and high torque tandem axle manner of operation to a high speed and low torque single axle manner of operation that reduces windage and frictional losses and facilitates improved shifting from one operating state to another without excessively increasing a cost and a weight of the tandem drive axle. 
     SUMMARY OF THE INVENTION 
     Presently provided by the invention, a method of shifting a tandem drive axle system from a low speed and high torque tandem axle manner of operation to a high speed and low torque single axle manner of operation that reduces windage and frictional losses and facilitates improved shifting from one operating state to another without excessively increasing a cost and a weight of the tandem drive axle, has surprisingly been discovered. 
     In one embodiment, the present invention is directed to a method of shifting a power distribution unit for a vehicle from a first operating state to a second operating state. The method includes the steps of drivingly engaging a first axle assembly with a first output of the power distribution unit and drivingly engaging a second axle assembly with a second output of the power distribution unit. Next, the method includes the step of drivingly engaging an input of the power distribution unit with an output of a power source, the power distribution unit including an inter-axle differential, the first output, the second output, and a first clutching device having a first position and a second position, the first clutching device in the first position locking the inter-axle differential, engaging the first output with the input of the power distribution unit, and disengaging the second output from the inter-axle differential, the first clutching device in the second position unlocking the inter-axle differential and engaging the first output and the second output with the inter-axle differential. Next, the method includes the steps of placing the first clutching device in one of the first position and the second position and applying a rotational force to the input of the power distribution unit. Next, the method includes the steps of adjusting the rotational force transferred to the power distribution unit to facilitate moving the first clutching device and moving the first clutching device from one of the first position and the second position to a third position, the first clutching device in the third position neither locking the inter-axle differential nor engaging the second output with the inter-axle differential. Lastly, the method includes the steps of adjusting a rotational speed of the input of the power distribution unit to facilitate moving the first clutching device from the third position, moving the first clutching device from the third position to one of the first and second positions, and adjusting the rotational force transferred to the power distribution unit. 
     Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a schematic view of a tandem drive axle system including a power distribution unit according to an embodiment of the present invention; 
         FIG. 2  is a chart illustrating a first example of shifting the power distribution unit from a first operating state to a second operating state; 
         FIG. 3  is a chart illustrating a second example of shifting the power distribution unit from the first operating state to the second operating state; 
         FIG. 4  is a chart illustrating a third example of shifting the power distribution unit from the first operating state to the second operating state; 
         FIG. 5  is a chart illustrating a first example of shifting the power distribution unit from the second operating state to the first operating state; and 
         FIG. 6  is a schematic view of a tandem drive axle system including a power distribution unit according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
       FIG. 1  illustrates a drive axle system  10  for a vehicle having a power source  11 . The drive axle system  10  preferably includes a power distribution unit  12 , a first axle assembly  14 , and a second axle assembly  16 . The drive axle system  10  is drivingly engaged with a power source  11 . As shown, the drive axle system  10  includes the three assemblies  12 ,  14 ,  16 , but it is understood the drive axle system  10  may include fewer or more assemblies or components. 
     The power source  11  is drivingly engaged with an input shaft  18  of the power distribution unit  12 , and applies power thereto. The power source  11  is, for example, an internal combustion engine; however, it is understood that the power source  11  may include an electric motor or another source of rotational output. It is understood that the power source  11  may be a hybrid power source including both an internal combustion engine and an electric motor. Further, it is understood that the power source  11  may include a transmission (not shown) as known in the art. Further, it is understood that the power source  11  may include a clutch (not shown) as known in the art, for one of reducing and interrupting a rotational force transferred to the power distribution unit  12 . 
     The power distribution unit  12  includes an input shaft  18 , an inter-axle differential  19 , a first output gear  20 , a plurality of driving pinions  21 , a transfer shaft  22 , a second output gear  24 , and a clutch  28 . Preferably, the components  18 ,  19 ,  20 ,  21 ,  22 ,  24 ,  28  are formed from a hardened steel, however the components  18 ,  19 ,  20 ,  21 ,  22 ,  24 ,  28  may be formed from any other rigid material. As shown, power distribution unit  12  includes the seven components  18 ,  19 ,  20 ,  21 ,  22 ,  24 ,  28  disposed in a housing  30  but it is understood the power distribution unit  12  may include fewer or more components. 
     The input shaft  18  is at least partially disposed in the housing  30 . Preferably, the input shaft  18  is an elongate member, however the input shaft  18  may be any other shape. Bearings  32  disposed between the input shaft  18  and the housing  30  permit the input shaft  18  to rotate about an axis of the input shaft  18 . The input shaft  18  has a first end portion  33 , a middle portion  34 , and a second end portion  35 . 
     The middle portion  34  has a diameter greater than a diameter of the first end portion  33 . The middle portion  34  is a substantially disc shaped body drivingly coupled to the input shaft  18 . Alternately, the middle portion  34  may be integrally formed with the input shaft  18 . 
     The second end portion  35  is a substantially hollow body having a diameter greater than a diameter of the first end portion  33  and the middle portion  34 . The second end portion  35  is drivingly coupled to the middle portion  34 . Alternately, the second end portion  35  may be integrally formed with the input shaft  18  and the middle portion  34 . The second end portion  35  has a pinion carrier  36 , a first set of clutch teeth  37 , and an engagement portion  38  formed thereon. 
     The pinion carrier  36  is a substantially disc shaped body drivingly coupled to the second end portion  35  of the input shaft  18 . The pinion carrier  36  includes a plurality of pinion supports  39  protruding from a first side of the pinion carrier  36  into the second end portion  35  of the input shaft  18 . The engagement portion  38  is formed on a second side of the pinion carrier  36 . As is known in the art, the pinion carrier  36  is also known as a planet carrier. 
     The engagement portion  38  is a conical surface oblique to the input shaft  18 , however, the engagement portion  38  may have any other shape. The first set of clutch teeth  37  are formed on the pinion carrier  36  radially inward from the engagement portion  38 . 
     The inter-axle differential  19  includes the pinion carrier  36 , the plurality of driving pinions  21 , the first output gear  20 , and the transfer shaft  22 . The inter-axle differential  19  is a planetary differential as known in the art; however, it is understood that the inter-axle differential  19  may be a bevel gear differential or any other type of differential. 
     The plurality of driving pinions  21  are rotatably disposed on the pinion supports  39  of the pinion carrier  36 . Each of the driving pinions  21  have gear teeth formed on an outer surface thereof. As is known in the art, each of the driving pinions  21  is also known as a planet gear. Preferably, bearings are disposed between each of the driving pinions  21  and the pinion supports  39 , however, the driving pinions  21  may be directly mounted on the pinion supports  39 . 
     The first output gear  20  is a gear concentrically disposed within the second end portion  35  of the input shaft  18 . The first output gear  20  is a substantially cup shaped body having an inner surface having gear teeth  40  formed on. As is known in the art, the first output gear  20  is known as a ring gear. The gear teeth  40  are engaged with the gear teeth formed on the outer surface of each of the driving pinions  21 . 
     The first output gear  20  includes an output shaft  41  drivingly coupled thereto. Alternately, the first output gear  20  may be integrally formed with the output shaft  41 . The first output gear  20  is drivingly engaged with the first axle assembly  14  through the output shaft  41 . The output shaft  41  is collinear with the input shaft  18 . Bearings  32  disposed between the output shaft  41  and the housing  30  support the output shaft  41  and permit the output shaft  41  to rotate about an axis of the output shaft  41 . 
     A bevel gear pinion  42  is drivingly coupled to the output shaft  41  opposite the first output gear  20 . Alternately, the bevel gear pinion  42  may be integrally formed with the output shaft  41 . As is known in the art, the bevel gear pinion  42  has gear teeth formed on an outer surface thereof. The bevel gear pinion  42  may be one of a hypoid gear, a spiral bevel gear, a straight bevel gear, or any other gear known to those skilled in the art. 
     The transfer shaft  22  is a hollow shaft rotatably disposed in the housing  30  and having an axis of rotation concurrent with the axis of rotation of the input shaft  18 . Preferably, the transfer shaft  22  is a hollow elongate cylindrical member, however the transfer shaft  22  may be any other shape. Bearings may be disposed between the transfer shaft  22  and pinion carrier  36  to permit the transfer shaft  22  to rotate about an axis of the transfer shaft  22 . The transfer shaft  22  has a first end portion  43  having a first set of clutch teeth  44  formed on an outer surface thereof, and a second end portion  45 , having a second set of gear teeth  46  formed on an outer surface thereof. 
     The first end portion  43  and the second end portion  45  are integrally formed with the transfer shaft  22 . The first set of clutch teeth  44  and the second set of gear teeth  46  are formed in the transfer shaft  22 . Alternately, the first end portion  43  and the second end portion  45  may be formed separate from and drivingly coupled to the transfer shaft  22 . As is known in the art, the second end portion  45  having the gear teeth  46  is known as a sun gear. The second set of gear teeth  46  are engaged with the plurality of driving pinions  21  and the first set of clutch teeth  44  are disposed adjacent the first set of clutch teeth  37  of the pinion carrier  36 . The first portion  43  of the transfer shaft  22  may be selectively engaged with the second output gear  24  or the pinion carrier  36   
     The second output gear  24  is a gear concentrically disposed about a portion of the transfer shaft  22 . The second output gear  24  has a central perforation having a diameter greater than a diameter of the transfer shaft  22 . The second output gear  24  is a substantially disc shaped body having a first end portion  47 , a second end portion  48  defining an outer diameter of the second output gear  24 , and an engagement portion  49 . Bearings  32  disposed between the second output gear  24  and the housing  30  permit the second output gear  24  to rotate about an axis of the second output gear  24 . The axis of the second output gear  24  is concurrent with the axis of the input shaft  18 . A first set of clutch teeth  50  are formed on the first end portion  47  adjacent the first set of clutch teeth  44  of the transfer shaft  22 . A second set of gear teeth  51  are formed on the second end portion  48 . The second output gear  24  is drivingly engaged with the second axle assembly  16 . 
     The engagement portion  49  is formed in the second output gear  24  intermediate the first end portion  47  and the second end portion  48 . As shown, the engagement portion  49  is a conical surface oblique to the input shaft  18 ; however, the engagement portion  49  may have any other shape. 
     The clutch  28  is a shift collar concentrically disposed about the transfer shaft  22 . The clutch  28  includes a set of inner clutch collar teeth  52  formed on an inner surface thereof, a first synchronizer  53 , and a second synchronizer  54 . The set of inner clutch collar teeth  52  are engaged with the first set of clutch teeth  44  of the transfer shaft  22 . The clutch  28  can be slidably moved along the axis of the input shaft  18  as directed automatically by a controller  55  while maintaining engagement of the inner clutch collar teeth  52  and the first set of clutch teeth  44 . A shift fork  56  disposed in an annular recess formed in the clutch  28  moves the clutch  28  along the axis of the input shaft  18  into a first position, a second position, or a third position. A first actuator  57 , which is drivingly engaged with the shift fork  56 , is engaged to position the shift fork  56  as directed by the controller  55 . Consequently, the shift fork  56  positions the clutch  28  into the first position, the second position, or the third position. In the first position, the inner clutch collar teeth  52  of the clutch  28  are drivingly engaged with the first set of clutch teeth  44  of the transfer shaft  22  and the first set of clutch teeth  37  of the pinion carrier  36 . In the second position, inner clutch collar teeth  52  of clutch  28  are drivingly engaged with the first set of clutch teeth  44  of the transfer shaft  22  and the first set of clutch teeth  50  of the second output gear  24 . In the third position, the inner clutch collar teeth  52  of the clutch  28  are only drivingly engaged with the first set of clutch teeth  44  of the transfer shaft  22 . It is understood the clutch  28 , the clutch teeth  37 ,  44 ,  50 ,  52 , the synchronizers  53 ,  54 , and the engagement portions  38 ,  49  may be substituted with any clutching device that permits selective engagement of a driving and a driven part. 
     The first synchronizer  53  is an annular body coupled to the clutch  28  adjacent the engagement portion  38  of the pinion carrier  36 . The first synchronizer  53  has a first conical engagement surface  58 . Alternately, the first synchronizer  53  may have an engagement surface having any other shape. When the clutch  28  is moved from the third position towards the first position, the first conical engagement surface  58  contacts the engagement portion  38  of the pinion carrier  36 , causing the clutch  28  to act upon the pinion carrier  36 . When the clutch  28  is moved further towards the first set of clutch teeth  37  of the input shaft  18 , the clutch continues to act upon the pinion carrier  36  as the inner clutch collar teeth  52  become drivingly engaged with the first set of clutch teeth  44  of the transfer shaft  22  and the first set of clutch teeth  37  of the pinion carrier  36 . 
     The second synchronizer  54  is an annular body coupled to the clutch  28  adjacent the first end portion  47  of the second output gear  24 . The second synchronizer  54  has a second conical engagement surface  59 . Alternately, the second synchronizer  54  may have an engagement surface having any other shape. When the clutch  28  is moved from the third position into the second position, the second conical engagement surface  59  contacts the engagement portion  49  of the second output gear  24 , causing the clutch  28  to act upon the second output gear  24 . When the clutch  28  is moved further towards the first set of clutch teeth  50  of the second output gear  24 , the clutch  28  continues to act upon the second output gear  24  as the inner clutch collar teeth  52  become drivingly engaged with the first set of clutch teeth  44  of the transfer shaft  22  and the first set of clutch teeth  50  of the second output gear  24 . 
     The first axle assembly  14  includes the bevel gear pinion  42 , a first driving gear  60 , a first wheel differential  61 , and a first pair of output axle shafts  62 . Preferably, the components  42 ,  60 ,  61 ,  62  are formed from a hardened steel, however the components  42 ,  60 ,  61 ,  62  may be formed from any other rigid material. As shown, the first axle assembly  14  includes the four components  42 ,  60 ,  61 ,  62  disposed in a first axle housing  63  but it is understood the first axle assembly  14  may include fewer or more components. 
     The first driving gear  60  is coupled to a housing of the first wheel differential  61  by a plurality of fasteners or a weld and is rotatable about an axis of the first pair of output axle shafts  62  within the first axle housing  63 . Alternately, the first driving gear  60  may be integrally formed with the first wheel differential  61 . As is known in the art, the first driving gear  60  has gear teeth formed on an outer surface thereof. The first driving gear  60  may be one of a hypoid gear, a spiral bevel gear, a straight bevel gear, or any other gear known to those skilled in the art. The first driving gear  60  is drivingly engaged with the bevel gear pinion  42  and has a first gear ratio. As a non-limiting example, the first gear ratio may be a 2.26:1 ratio, but it is understood that other ratios may be used. The output shaft  41  is drivingly engaged with the first driving gear  60  of the first axle assembly  14  through a single gear mesh. 
     The first wheel differential  61  is a bevel gear style differential as is known in the art having a plurality of driving pinions and a pair of side gears drivingly engaged with the first pair of output axle shafts  62 . The first wheel differential  61  is rotatably disposed within the first axle housing  63  about the axis of the first pair of output axle shafts  62 . Alternately, other styles of differentials may be used in place of the first wheel differential  61 . 
     The first pair of output axle shafts  62  are elongate cylindrical members having a common axis rotatably mounted within the first axle housing  63 . Bearings  32  disposed between the first pair of output axle shafts  62  and the first axle housing  63  permit the first pair of output axle shafts  62  to rotate therein. The side gears of the first wheel differential  61  are disposed on first ends of each of the first output axle shafts  62  and wheels (not shown) are disposed on second ends of each of the first output axle shafts  62 . 
     The second axle assembly  16  includes an inter-axle shaft  64 , a second driving gear  65 , a second wheel differential  66 , a second pair of output axle shafts  67 , and an axle clutch  68 . Preferably, the components  64 ,  65 ,  66 ,  67 ,  68  are formed from a hardened steel, however the components  64 ,  65 ,  66 ,  67 ,  68  may be formed from any other rigid material. As shown, the second axle assembly  16  includes the five components  64 ,  65 ,  66 ,  67 ,  68  disposed in a second axle housing  69  but it is understood the second axle assembly  16  may include fewer or more components. 
     The inter-axle shaft  64  comprises at least one elongate cylindrical member drivingly engaged with the second output gear  24  through a driven gear  70  coupled to the inter-axle shaft  64 . As illustrated, the inter-axle shaft  64  comprises a plurality of elongate cylindrical members connected by joints. Bearings  32  disposed between the inter-axle shaft  64  and the housing  30  permit the inter-axle shaft  64  to rotate therein. 
     A bevel gear pinion  71  is drivingly coupled to the inter-axle shaft  64  opposite the driven gear  70 . As is known in the art, the bevel gear pinion  71  has gear teeth formed on an outer surface thereof. The bevel gear pinion  71  may be one of a hypoid gear, a spiral bevel gear, a straight bevel gear, or any other gear known to those skilled in the art. 
     The second driving gear  65  is a ring style bevel gear as is known in the art having a set of gear teeth engaged with the gear teeth formed on the bevel gear pinion  71 . The second driving gear  65  is coupled to a housing of the second wheel differential  66  by a plurality of fasteners or a weld and is rotatable about an axis of the second pair of output axle shafts  67  within the second axle housing  69 . Alternately, the second driving gear  65  may be integrally formed with the second wheel differential  66 . The second driving gear  65  is drivingly engaged with the bevel gear pinion  71  and has a second gear ratio. As a non-limiting example, the second gear ratio may be a 4.88:1 ratio, which is a lower gear ratio than the first gear ratio, but it is understood that other ratios or a ratio equal to the first gear ratio may be used. 
     The second wheel differential  66  is a bevel gear style differential as is known in the art having a plurality of driving pinions and a pair of side gears drivingly engaged with the second pair of output axle shafts  67 . The second wheel differential  66  is rotatably disposed within the second axle housing  69  about the axis of the second pair of output axle shafts  67 . Alternately, other styles of differentials may be used in place of the second wheel differential  66 . 
     The second pair of output axle shafts  67  are elongate cylindrical members having a common axis rotatably mounted within the second axle housing  69 . Bearings  32  disposed between the pair of second output axle shafts  67  and the second axle housing  69  permit the second pair of output axle shafts  67  to rotate therein. The side gears of the second wheel differential  66  are disposed on first ends of each of the second output axle shafts  67  and wheels (not shown) are disposed on second ends of each of the second output axle shafts  67 . 
     The axle clutch  68  is a dog style clutch that divides one of the second output axle shafts  67  into first and second portions. Alternately, the axle clutch  68  may be a component of the second wheel differential  66  which engages a side gear of the second wheel differential  66  and one of the second output axle shafts  67  or any other clutching device as known in the art. The axle clutch  68  may also be a plate style clutch or any other style of clutch. The axle clutch  68  has a plurality of teeth formed thereon for selectively engaging corresponding teeth formed on the first portion and the second portion of the second output axle shafts  67 . The axle clutch  68  is urged into an engaged position or a disengaged position by a shift fork  73 . A second actuator  74 , which is drivingly engaged with the shift fork  73 , is engaged to position the shift fork  73 , and thus the axle clutch  68 , as directed by the controller  55 . When the axle clutch  68  is in the engaged position, the first portion of one of the second output axle shafts  67  is drivingly engaged with the second portion of one of the second output axle shafts  67 . 
     The controller  55  is in communication with the power source  11 , the first actuator  57 , the second actuator  74 , and at least one sensor  75 . Preferably, the controller  55  is in electrical communication with the power source  11 , the first actuator  57 , the second actuator  74 , and the at least one sensor  75 . Alternately, the controller  55  may be in communication with the power source  11 , the first actuator  57 , the second actuator  74 , and the at least one sensor  75  using pneumatics, hydraulics, or a wireless communication medium. 
     The controller  55  is configured to accept an input containing information regarding at least one of an operating condition of the power source  11 , a temperature of the second axle assembly  16 , a speed of a portion of the transfer shaft  22 , a speed of the second output gear  24 , a speed of a portion of the second axle assembly  16 , an amount of the rotational force transferred to the power distribution unit  12 , a position of the clutch  28 , and a position of the axle clutch  68 . The controller  55  uses the input to adjust the at least one of the operating condition of the power source  11 , the position of the clutch  28 , the position of the axle clutch  68 , and a duration between successive positions of the clutch  28 . The controller  55  performs the adjustment to the operating condition of the power source  11 , the position of the clutch  28 , the position of the axle clutch  68 , and the duration between successive positions of the clutch  28  based on at least one of the operating condition of the power source  11 , the temperature of the second axle assembly  16 , the speed of the second output gear  24 , the speed of a portion of the second axle assembly  16 , the amount of the rotational force transferred to the power distribution unit  12 , the position of the clutch  28 , and the position of the axle clutch  68 . The controller  55  references at least one of a series of instructions and conditions, an operator input, at least one data table, and at least one algorithm to determine the adjustment made to the operating condition of the power source  11 , the position of the clutch  28 , the position of the axle clutch  68 , and the duration between successive positions of the clutch  28 . 
     The at least one sensor  75  may be disposed within the housing  30 , the first axle housing  63 , and the second axle housing  69 . Further, it is understood that the at least one sensor  75  may be disposed on an outer surface of one of the housings  30 ,  63 ,  69  or mounted elsewhere on the vehicle. The at least one sensor  75  is configured as known in the art to monitor at least one of the operating condition of the power source  11 , the temperature of the second axle assembly  16 , the speed of a portion of the transfer shaft  22 , the speed of the second output gear  24 , the speed of a portion of the second axle assembly  16 , the amount of a rotational force transferred to the power distribution unit  12 , the position of the clutch  28 , and the position of the axle clutch  68 . The operating condition of the power source  11  may be at least one of an indication that the power source  11  is operating, a rotational speed of the power source  11 , a state of a transmission forming a portion of the power source  11 , and a speed of the vehicle. 
     In use, a method for use with the drive axle system  10  facilitates shifting the power distribution unit  12  from a first operating state to a second operating state. 
     When the power distribution unit  12  is placed in the first operating state, only the first axle assembly  14  is driven in a high speed and low torque manner of operation. The first operating state is employed when the vehicle reaches a “cruising” speed, which typically requires a reduced amount of torque to maintain the “cruising” speed. In the first operating state, the clutch  28  is placed in a first position. In the first position, the inter-axle differential  19  is locked and the first output gear  20  is drivingly engaged with the input shaft  18  through the inter-axle differential  19  in the locked condition. When the inter-axle differential  19  is locked, the pinion carrier  36 , the plurality of driving pinions  21 , the first output gear  20 , and the transfer shaft  22  rotate concurrently because inner clutch collar teeth  52  of the clutch  28  are drivingly engaged with the first set of clutch teeth  37  of the pinion carrier  36  and the first set of clutch teeth  44  of the transfer shaft  22 . Further, in the first position, the second output gear  24  is disengaged from the clutch  28  and the transfer shaft  22 , and thus the inter-axle differential  19  and the input shaft  18 . When the power distribution unit  12  is placed in the first operating state, the axle clutch  68  may be disengaged, permitting the second output gear  24 , the driven gear  70 , the inter-axle shaft  64 , the second driving gear  65 , and the second wheel differential  66  to coast to an idle condition. 
     When the power distribution unit  12  is placed in the second operating state, the first axle assembly  14  and the second axle assembly  16  are simultaneously driven in a low speed and high torque manner of operation. The second operating state is employed when the vehicle is operated at lower speeds or when the vehicle is accelerating. When the vehicle is operated at lower speeds or when the vehicle is accelerating, an increased amount of torque is typically required. In the second operating state, the clutch  28  is placed in a second position. In the second position, the inter-axle differential  19  is unlocked and the output shaft  41  of the first output gear  20  and the second output gear  24  are drivingly engaged with the input shaft  18  through the inter-axle differential  19 . The pinion carrier  36  simultaneously drives the first output gear  20  and the transfer shaft  22  through the plurality of driving pinions  21 . When the inter-axle differential  19  is unlocked, the pinion carrier  36 , the plurality of driving pinions  21 , the first output gear  20 , and the transfer shaft  22  are free to rotate with respect to one another. Further, in the second position, the first set of clutch teeth  50  of the second output gear  24  and the first set of clutch teeth  44  of the transfer shaft  22  are engaged with the inner clutch collar teeth  52  of the clutch  28 . When the power distribution unit  12  is placed in the second operating state, the axle clutch  68  is engaged, permitting the second output gear  24  to drive the second pair of output axle shafts  67  through the driven gear  70 , the inter-axle shaft  64 , the second driving gear  65 , and the second wheel differential  66 . 
       FIGS. 2-4  are three charts illustrating three non-limiting examples of shifting the power distribution unit  12  from a first operating state to a second operating state. 
     The shifting procedure as illustrated in  FIG. 2  is employed by the controller  55  when the temperature of the second axle assembly  16  is above the predetermined value prior to initiation of the shifting procedure. Further, it is understood that the example illustrated in  FIG. 2  may be selected by the controller  55  on the basis that the shifting procedure illustrated in  FIG. 2  is advantageous when the temperature of the second axle assembly  16  is within a predetermined temperature range. A horizontal axis shown in  FIG. 2  indicates a duration of time from a first chronological reference point, A, to a fourth chronological reference point, D. Chronological reference points B and C respectively occur between points A and D. 
     A vertical axis shown in  FIG. 2  indicates a rotational speed of the first output gear  20 , the transfer shaft  22 , the second output gear  24 , and the power source  11 . The vertical axis begins at a rotational speed of zero and increases as the vertical axis extends away from the horizontal axis. A rotational speed of the power source  11  depicted in  FIG. 2  is merely for purposes of example, and the shifting procedure is not limited to the depicted speeds. 
     Point A indicates a starting time of the shifting procedure. At point A, the power distribution unit  12  is in the first operating state. In the first operating state, the clutch  28  is in the first position. When directed by the controller  55  or by an operator of the vehicle, the shifting procedure is initiated by verifying disengagement of the axle clutch  68  and by adjusting the rotational force transferred to the power distribution unit  12 . 
     The step of adjusting the rotational force transferred to the power distribution unit  12  may be performed by one of adjusting an operating condition of the power source  11  and at least partially disengaging a clutch (not shown) forming a portion of the power source  11 . When the step of adjusting the rotational force transferred to the power distribution unit  12  is performed by adjusting the operating condition of the power source  11 , the operating condition of the power source  11  may be adjusted by one of increasing or decreasing a fuel supplied to the power source  11 . When the rotational force is a positive rotational force (meaning the power source  11  is applying a rotational force to the power distribution unit  12 ) the fuel supplied to the power source  11  is decreased to reduce the rotational force. When the rotational force is a negative rotational force (meaning the power distribution unit  12  is applying a rotational force to the power source  11 ) the fuel supplied to the power source  11  is increased to increase the rotational force. When the step of one of reducing and interrupting the rotational force transferred to the power distribution unit  12  is performed by at least partially disengaging a clutch or other device (neither are shown) associated with the power source  11 , an amount of engagement of the clutch or other device (neither are shown) associated with the power source  11  is decreased to reduce the rotational force. Adjusting the rotational force transferred to the power distribution unit  12  as mentioned hereinabove is performed until the rotational force transferred to the power distribution unit  12  is about equal to an amount of rotational force applied by the power distribution unit  12  to the power source  11 . 
     When the rotational force transferred to the power distribution unit  12  is about equal to an amount of rotational force applied by the power distribution unit  12 , the controller  55  engages the first actuator  57  to move the clutch  28  from the first position to the third position. Point B of  FIG. 2  indicates a time in the shifting procedure when the clutch  28  is placed in the third position. As mentioned hereinabove, when the clutch  28  is placed in the third position, the inter-axle differential  19  is unlocked and the second output gear  24  is disengaged from the transfer shaft  22 , and thus the inter-axle differential  19 . Once the clutch  28  is placed in the third position, the controller  55  further engages the first actuator  57  to move the clutch  28  from the third position to the second position while simultaneously adjusting the rotational speed of the power source  11 . 
     As shown in  FIG. 2 , the duration of time between points B and C represents a duration of time where the clutch  28 , which is drivingly engaged with the transfer shaft  22 , is acting upon the second output gear  24  but before the inner clutch collar teeth  52  engage the first set of clutch teeth  50  of the second output gear  24 . When the clutch  28  acts upon the second output gear  24 , a rotational force is applied to the second output gear  24 . The rotational force applied to the second output gear  24  causes the second output gear  24 , the driven gear  70 , the inter-axle shaft  64 , the second driving gear  65 , and the second wheel differential  66  to adjust the rotational speed from the idle condition to one of a predetermined speed and a target speed. 
     The predetermined speed is obtained by the controller  55  by referencing information stored in the at least one data table. The target speed is calculated by the controller  55  using the at least one algorithm and a speed of the vehicle. To facilitate adjusting the rotational speed to one of the predetermined speed and the target speed, the rotational speed of the power source  11  is adjusted. As a non-limiting example, the rotational speed of the power source  11  may be adjusted by one of increasing or decreasing a fuel supplied to the power source  11 . A shown in  FIG. 2 , the rotational speed of the power source  11  is increased between points B and C. A near constant rotational speed of the first pair of output axle shafts  62  (as the vehicle coasts during the shifting procedure) backdrives the first output gear  20 . The first output gear  20 , backdriven at the near constant rotational speed, permits increases in the rotational speed of the power source  11  to be directly reflected in the rotational speed of the transfer shaft  22  through the inter-axle differential  19 . The concurrent rotation of the pinion carrier  36  (as driven by the input shaft  18 ) and the first output gear  20  drives the transfer shaft  22  through the plurality of driving pinions  21  to adjust the rotational speed of the second output gear  24 , the driven gear  70 , the inter-axle shaft  64 , the second driving gear  65 , and the second wheel differential  66  through the clutch  28 . 
     A shown in  FIG. 2 , the one of the predetermined speed and the target speed are obtained when the rotational speed of the transfer shaft  22  and the second output gear  24  are about equal. Further, the one of the predetermined speed and the target speed permits a meshing engagement between the second output gear  24  and the transfer shaft  22  with the clutch  28 . 
     When the one of the predetermined speed and the target speed are obtained, the controller  55  further engages the first actuator  57  to move the clutch  28  from the third position to the second position. Point C of  FIG. 2  indicates a time in the shifting procedure when the clutch  28  is placed in the second position. As mentioned hereinabove, when the clutch  28  is placed in the second position, the inter-axle differential  19  is unlocked and the second output gear  24  is engaged with the transfer shaft  22 , and thus the inter-axle differential  19 . 
     When the one of the predetermined speed and the target speed are obtained, the controller  55  commands the second actuator  74  to move the axle clutch  68  to an engaged position. Point C of  FIG. 2  indicates the time of the shifting procedure when the controller  55  commands the second actuator  74  to move the axle clutch  68  to an engaged position. Point D of  FIG. 2  indicates the time of the shifting procedure when the axle clutch  68  is engaged. As shown in  FIG. 2 , the duration of time between points C and D represents a duration of time after the controller  55  commands the second actuator  74  to engage the axle clutch  68  but before the axle clutch  68  is engaged. The axle clutch  68  may not immediately engage due to a misalignment between the first portion and the second portion of one of the second pair of output axle shafts, a slipping condition of one of the wheels (not shown) coupled to the second pair of output axle shafts  67 , or due to both conditions. As shown in  FIG. 2 , once the one of the predetermined speed and the target speed are obtained, a substantially constant rotational speed of the power source  11  is maintained by the controller  55 . When the axle clutch  68  is engaged, the second pair of output axle shafts  67  is drivingly engaged with the second output gear  24  through the driven gear  70 , the inter-axle shaft  64 , the second driving gear  65 , and the second wheel differential  66 . 
     Following point D, the step of one of increasing and resuming the rotational force transferred to the power distribution unit  12  may be performed by one of adjusting an operating condition of the power source  11  and by engaging the clutch (not shown) forming a portion of the power source  11 . When the step of increasing the rotational force transferred to the power distribution unit  12  is performed by adjusting the operating condition of the power source  11 , the operating condition of the power source  11  may be adjusted by increasing the fuel supplied to the power source  11 . When the step of increasing and resuming the rotational force transferred to the power distribution unit  12  is performed by engaging the clutch (not shown), the amount of engagement of the clutch (not shown) associated with the power source  11  is increased to increase the rotational force. Adjusting the rotational force transferred to the power distribution unit  12  as mentioned hereinabove completes the shifting procedure as illustrated in  FIG. 2 , and the controller  55  returns control of one of the operating conditions of the power source  11  and the clutch (not shown) to the operator. 
     Once the drive axle system  10  is placed in the second operating state, the rotational force applied to the power distribution unit  12  by the power source  11  is distributed between the first output gear  20  and the second output gear  24  through the inter-axle differential  19 . A rotational difference of the first output gear  20  and the second output gear  24  caused by a difference between the first gear ratio and the second gear ratio is accommodated by the inter-axle differential  19 . Because the inter-axle differential  19  accommodates the rotational difference between the first gear ratio and the second gear ratio, a cumulative gear ratio is provided. The cumulative gear ratio is intermediate the first gear ratio and the second gear ratio. 
     The shifting procedure as illustrated in  FIG. 3  is employed by the controller  55  when the temperature of the second axle assembly  16  is below the predetermined value prior to initiation of the shifting procedure. Further, it is understood that the example illustrated in  FIG. 3  may be selected by the controller  55  on the basis that the shifting procedure illustrated in  FIG. 3  is advantageous when the temperature of the second axle assembly  16  is within a predetermined temperature range. The shifting procedure illustrated in  FIG. 3  may be selected when the temperature of the second axle assembly  16  is low enough to substantially reduce an effectiveness of the synchronizer  54  of the clutch  28  in acting upon the second output gear  24  due to an increase in viscosity of the lubricant disposed in the second axle assembly  16 . A horizontal axis shown in  FIG. 3  indicates a duration of time from a first chronological reference point, A, to a fifth chronological reference point, E. Chronological reference points B, C, and D respectively occur between points A and E. 
     A vertical axis shown in  FIG. 3  indicates the rotational speed of the first output gear  20 , the transfer shaft  22 , the second output gear  24 , and the power source  11 . The vertical axis begins at a rotational speed of zero and increases as the vertical axis extends away from the horizontal axis. The rotational speed of the power source  11  depicted in  FIG. 3  is merely for purposes of example, and the shifting procedure is not limited to the depicted speeds. 
     Point A indicates a starting time of the shifting procedure. At point A, the power distribution unit  12  is in the first operating state. In the first operating state, the clutch  28  is in the first position. When directed by the controller  55  or by an operator of the vehicle, the shifting procedure is initiated by verifying disengagement of the axle clutch  68  and by adjusting the rotational force transferred to the power distribution unit  12 . 
     The step of adjusting the rotational force transferred to the power distribution unit  12  may be performed by one of adjusting an operating condition of the power source  11  and at least partially disengaging a clutch (not shown) forming a portion of the power source  11 . When the step of adjusting the rotational force transferred to the power distribution unit  12  is performed by adjusting the operating condition of the power source  11 , the operating condition of the power source  11  may be adjusted by one of increasing or decreasing a fuel supplied to the power source  11 . When the rotational force is a positive rotational force (meaning the power source  11  is applying a rotational force to the power distribution unit  12 ) the fuel supplied to the power source  11  is decreased to reduce the rotational force. When the rotational force is a negative rotational force (meaning the power distribution unit  12  is applying a rotational force to the power source  11 ) the fuel supplied to the power source  11  is increased to increase the rotational force. When the step of one of reducing and interrupting the rotational force transferred to the power distribution unit  12  is performed by at least partially disengaging a clutch or other device (neither are shown) associated with the power source  11 , an amount of engagement of the clutch or other device (neither are shown) associated with the power source  11  is decreased to reduce the rotational force. Adjusting the rotational force transferred to the power distribution unit  12  as mentioned hereinabove is performed until the rotational force transferred to the power distribution unit  12  is about equal to an amount of rotational force applied by the power distribution unit  12  to the power source  11 . 
     When the rotational force transferred to the power distribution unit  12  is about equal to an amount of rotational force applied by the power distribution unit  12 , the controller  55  engages the first actuator  57  to move the clutch  28  from the first position to the third position. Point B of  FIG. 3  indicates a time in the shifting procedure when the clutch  28  is placed in the third position. As mentioned hereinabove, when the clutch  28  is placed in the third position, the inter-axle differential  19  is unlocked and the second output gear  24  is disengaged from the transfer shaft  22 , and thus the inter-axle differential  19 . 
     Once the clutch  28  is placed in the third position, the controller  55  decreases a rotational speed of the power source  11 . As a non-limiting example, the rotational speed of the power source  11  may be decreased by decreasing a fuel supplied to the power source  11 . A shown in  FIG. 3 , the rotational speed of the power source  11  is decreased between points B and C. A near constant rotational speed of the first pair of output axle shafts  62  (as the vehicle coasts during the shifting procedure) backdrives the first output gear  20 . The first output gear  20 , backdriven at the near constant rotational speed, permits decreases in the rotational speed of the power source  11  to be directly reflected in the rotational speed of the transfer shaft  22  through the inter-axle differential  19 . The concurrent rotation of the pinion carrier  36  (as driven by the input shaft  18 ) and the first output gear  20  retards the transfer shaft  22  through the plurality of driving pinions  21  to adjust the rotational speed of the transfer shaft  22 . The rotational speed of the transfer shaft  22  is adjusted to facilitate a smooth engagement of the clutch  28  with the second output gear  24  when the clutch  28  is moved from the third position to the second position. As a non-limiting example, the rotational speed of the power source  11  may be decreased between points B and C to decrease the rotational speed of the transfer shaft  22  to about zero. Point C of  FIG. 3  indicates a time in the shifting procedure when the clutch  28  is placed in the second position. 
     When the rotational speed of the transfer shaft  22  is adjusted to facilitate a smooth engagement of the clutch  28  with the second output gear  24 , the controller  55  further engages the first actuator  57  to move the clutch  28  from the third position to the second position while simultaneously adjusting a rotational speed of the power source  11 . 
     As shown in  FIG. 3 , the duration of time between points C and D represents a duration of time where the inner clutch collar teeth  52  are drivingly engaged with the clutch teeth  44  of the transfer shaft  22  and the clutch teeth  50  of the second output gear  24  but before the controller  55  commands the second actuator  74  to move the axle clutch  68  to an engaged position. 
     When the clutch  28  is engaged with the second output gear  24 , the second output gear  24  is drivingly engaged with the transfer shaft  22 . When the second output gear  24  is drivingly engaged with the transfer shaft  22 , the rotational speed of the second output gear  24 , the driven gear  70 , the inter-axle shaft  64 , the second driving gear  65 , and the second wheel differential  66  may be adjusted to one of a predetermined speed and a target speed. 
     Further, when the second output gear  24  is drivingly engaged with the transfer shaft  22 , the second driving gear  65  and the second wheel differential  66  imparts energy to the lubricant disposed within the second axle assembly  16 . The duration of time between points C and D may be determined by the controller based on the temperature of the second axle assembly  16  as indicated by the sensor  75 . The controller  55  may increase or decrease the duration of time between points C and D until the temperature of the second axle assembly  16  is above the predetermined value. As a non-limiting example, the predetermined value may be about 20° Fahrenheit. 
     The predetermined speed is obtained by the controller  55  by referencing information stored in the at least one data table. The target speed is calculated by the controller  55  using the at least one algorithm and a speed of the vehicle. To facilitate adjusting the rotational speed to one of the predetermined speed and the target speed, the rotational speed of the power source  11  is adjusted. As a non-limiting example, the rotational speed of the power source  11  may be adjusted by one of increasing or decreasing a fuel supplied to the power source  11 . A shown in  FIG. 3 , the rotational speed of the power source  11  is increased between points C and D. A near constant rotational speed of the first pair of output axle shafts  62  (as the vehicle coasts during the shifting procedure) backdrives the first output gear  20 . The first output gear  20 , backdriven at the near constant rotational speed, permits increases in the rotational speed of the power source  11  to be directly reflected in the rotational speed of the transfer shaft  22  through the inter-axle differential  19 . The concurrent rotation of the pinion carrier  36  (as driven by the input shaft  18 ) and the first output gear  20  drives the transfer shaft  22  through the plurality of driving pinions  21  to adjust the rotational speed of the second output gear  24 , the driven gear  70 , the inter-axle shaft  64 , the second driving gear  65 , and the second wheel differential  66  through the clutch  28  placed in the second position. 
     A shown in  FIG. 3 , the one of the predetermined speed and the target speed are obtained when the rotational speed of the second output gear  24 , the driven gear  70 , the inter-axle shaft  64 , the second driving gear  65 , and the second wheel differential  66  permits a meshing engagement between the first portion of one of the second pair of output axle shafts  67  and the second portion of one the second pair of output axle shafts  67  with the axle clutch  68 . 
     When the one of the predetermined speed and the target speed are obtained, the controller  55  commands the second actuator  74  to move the axle clutch  68  to the engaged position. Point D of  FIG. 3  indicates the time of the shifting procedure when the controller  55  commands the second actuator  74  to move the axle clutch  68  to the engaged position. Point E of  FIG. 3  indicates the time of the shifting procedure when the axle clutch  68  is engaged. As shown in  FIG. 3 , the duration of time between points D and E represents a duration of time after the controller  55  commands the second actuator  74  to engage the axle clutch  68  but before the axle clutch  68  is engaged. The axle clutch  68  may not immediately engage due to a misalignment between the first portion and the second portion of one of the second pair of output axle shafts, a slipping condition of one of the wheels (not shown) coupled to the second pair of output axle shafts  67 , or due to both conditions. As shown in  FIG. 3 , once the one of the predetermined speed and the target speed are obtained, a substantially constant rotational speed of the power source  11  is maintained by the controller  55 . When the axle clutch  68  is engaged, the second pair of output axle shafts  67  is drivingly engaged with the second output gear  24  through the driven gear  70 , the inter-axle shaft  64 , the second driving gear  65 , and the second wheel differential  66 . 
     Following point E, the step of one of increasing and resuming the rotational force transferred to the power distribution unit  12  may be performed by one of adjusting an operating condition of the power source  11  and by engaging the clutch (not shown) forming a portion of the power source  11 . When the step of increasing the rotational force transferred to the power distribution unit  12  is performed by adjusting the operating condition of the power source  11 , the operating condition of the power source  11  may be adjusted by increasing the fuel supplied to the power source  11 . When the step of increasing and resuming the rotational force transferred to the power distribution unit  12  is performed by engaging the clutch (not shown), the amount of engagement of the clutch (not shown) associated with the power source  11  is increased to increase the rotational force. Adjusting the rotational force transferred to the power distribution unit  12  as mentioned hereinabove completes the shifting procedure as illustrated in  FIG. 3 , and the controller  55  returns control of one of the operating conditions of the power source  11  and the clutch (not shown) to the operator. 
     Once the drive axle system  10  is placed in the second operating state, the rotational force applied to the power distribution unit  12  by the power source  11  is distributed between the first output gear  20  and the second output gear  24  through the inter-axle differential  19 . A rotational difference of the first output gear  20  and the second output gear  24  caused by a difference between the first gear ratio and the second gear ratio is accommodated by the inter-axle differential  19 . Because the inter-axle differential  19  accommodates the rotational difference between the first gear ratio and the second gear ratio, a cumulative gear ratio is provided. The cumulative gear ratio is intermediate the first gear ratio and the second gear ratio. 
     The shifting procedure as illustrated in  FIG. 4  is employed by the controller  55  when the temperature of the second axle assembly  16  is below the predetermined value prior to initiation of the shifting procedure. Further, it is understood that the example illustrated in  FIG. 4  may be selected by the controller  55  on the basis that the shifting procedure illustrated in  FIG. 4  is advantageous when the temperature of the second axle assembly  16  is within a predetermined temperature range. The shifting procedure illustrated in  FIG. 3  may be selected when the temperature of the second axle assembly  16  is low enough to reduce an effectiveness of the synchronizer  54  of the clutch  28  in acting upon the second output gear  24  due to an increase in viscosity of the lubricant disposed in the second axle assembly  16 . A horizontal axis shown in  FIG. 4  indicates a duration of time from a first chronological reference point, A, to a fifth chronological reference point, E. Chronological reference points B, C, and D respectively occur between points A and E. 
     A vertical axis shown in  FIG. 4  indicates a rotational speed of the first output gear  20 , the transfer shaft  22 , the second output gear  24 , and the power source  11 . The vertical axis begins at a rotational speed of zero and increases as the vertical axis extends away from the horizontal axis. The rotational speed of the power source  11  depicted in  FIG. 4  is merely for purposes of example, and the shifting procedure is not limited to the depicted speeds. 
     Point A indicates a starting time of the shifting procedure. At point A, the power distribution unit  12  is in the first operating state. In the first operating state, the clutch  28  is in the first position. When directed by the controller  55  or by an operator of the vehicle, the shifting procedure is initiated by verifying disengagement of the axle clutch  68  and by adjusting the rotational force transferred to the power distribution unit  12 . 
     The step of adjusting the rotational force transferred to the power distribution unit  12  may be performed by one of adjusting an operating condition of the power source  11  and at least partially disengaging a clutch (not shown) forming a portion of the power source  11 . When the step of adjusting the rotational force transferred to the power distribution unit  12  is performed by adjusting the operating condition of the power source  11 , the operating condition of the power source  11  may be adjusted by one of increasing or decreasing a fuel supplied to the power source  11 . When the rotational force is a positive rotational force (meaning the power source  11  is applying a rotational force to the power distribution unit  12 ) the fuel supplied to the power source  11  is decreased to reduce the rotational force. When the rotational force is a negative rotational force (meaning the power distribution unit  12  is applying a rotational force to the power source  11 ) the fuel supplied to the power source  11  is increased to increase the rotational force. When the step of one of reducing and interrupting the rotational force transferred to the power distribution unit  12  is performed by at least partially disengaging a clutch or other device (neither are shown) associated with the power source  11 , an amount of engagement of the clutch or other device (neither are shown) associated with the power source  11  is decreased to reduce the rotational force. Adjusting the rotational force transferred to the power distribution unit  12  as mentioned hereinabove is performed until the rotational force transferred to the power distribution unit  12  is about equal to an amount of rotational force applied by the power distribution unit  12  to the power source  11 . 
     When the rotational force transferred to the power distribution unit  12  is about equal to an amount of rotational force applied by the power distribution unit  12 , the controller  55  engages the first actuator  57  to move the clutch  28  from the first position to the third position. Point B of  FIG. 4  indicates a time in the shifting procedure when the clutch  28  is placed in the third position. As mentioned hereinabove, when the clutch  28  is placed in the third position, the inter-axle differential  19  is unlocked and the second output gear  24  is disengaged from the transfer shaft  22 , and thus the inter-axle differential  19 . 
     Once the clutch  28  is placed in the third position, the controller  55  further engages the first actuator  57  to move the clutch  28  from the third position to the second position while simultaneously adjusting a rotational speed of the power source  11 . As shown in  FIG. 4 , the rotational speed of the power source  11  is decreased between points B and C as the second synchronizer  54  of the clutch  28  acts upon the second output gear  24  but before the clutch  28  is placed in the second position. 
     As a non-limiting example, the rotational speed of the power source  11  may be decreased by decreasing a fuel supplied to the power source  11 . A shown in  FIG. 4 , the rotational speed of the power source  11  is decreased between points B and C. A near constant rotational speed of the first pair of output axle shafts  62  (as the vehicle coasts during the shifting procedure) backdrives the first output gear  20 . The first output gear  20 , backdriven at the near constant rotational speed, permits decreases in the rotational speed of the power source  11  to be directly reflected in the rotational speed of the transfer shaft  22  through the inter-axle differential  19 . The concurrent rotation of the pinion carrier  36  (as driven by the input shaft  18 ) and the first output gear  20  retards the transfer shaft  22  through the plurality of driving pinions  21  to adjust the rotational speed of the transfer shaft  22 . The rotational speed of the transfer shaft  22  is adjusted to facilitate a smooth engagement of the clutch  28  with the second output gear  24  when the clutch  28  is moved from the third position to the second position. Point C indicates a time in the shifting procedure when the clutch  28  is placed in the second position. 
     When one of the rotational speed of the power source  11  and the rotational speed of the transfer shaft  22  are adjusted to facilitate a smooth engagement of the clutch  28  with the second output gear  24 , the controller  55  further engages the first actuator  57  to engage the clutch  28  with the second output gear  24  then increasing a rotational speed of the power source  11 . Between points C and D, when the clutch  28  is placed in the second position, the rotational speed of the power source  11  is increased. 
     As shown in  FIG. 4 , the duration of time between points C and D represents a duration of time where the inner clutch collar teeth  52  are drivingly engaged with the clutch teeth  44  of the transfer shaft  22  and the clutch teeth  50  of the second output gear  24  but before the controller commands the second actuator  74  to move the axle clutch  68  to an engaged position. 
     When the clutch  28  is engaged with the second output gear  24 , the second output gear  24  is drivingly engaged with the transfer shaft  22 . When the second output gear  24  is drivingly engaged with the transfer shaft  22 , the rotational speed of the second output gear  24 , the driven gear  70 , the inter-axle shaft  64 , the second driving gear  65 , and the second wheel differential  66  may be adjusted to one of a predetermined speed and a target speed. 
     Further, when the second output gear  24  is drivingly engaged with the transfer shaft  22 , the second driving gear  65  and the second wheel differential  66  imparts energy to the lubricant disposed within the second axle assembly  16 . The duration of time between points C and D may be determined by the controller based on the temperature of the second axle assembly  16  as indicated by the sensor  75 . The controller  55  may increase or decrease the duration of time between points C and D until the temperature of the second axle assembly  16  is above the predetermined value. As a non-limiting example, the predetermined value may be about 20° Fahrenheit. 
     The predetermined speed is obtained by the controller  55  by referencing information stored in the at least one data table. The target speed is calculated by the controller  55  using the at least one algorithm and a speed of the vehicle. To facilitate adjusting the rotational speed to one of the predetermined speed and the target speed, the rotational speed of the power source  11  is adjusted. The rotational speed of the power source  11  is adjusted by adjusting the rotational speed of the power source  11 . As a non-limiting example, the rotational speed of the power source  11  may be adjusted by increasing a fuel supplied to the power source  11 . A shown in  FIG. 4 , the rotational speed of the power source  11  is increased between points C and D. A near constant rotational speed of the first pair of output axle shafts  62  (as the vehicle coasts during the shifting procedure) backdrives the first output gear  20 . The first output gear  20 , backdriven at the near constant rotational speed, permits increases in the rotational speed of the power source  11  to be directly reflected in the rotational speed of the transfer shaft  22  through the inter-axle differential  19 . The concurrent rotation of the pinion carrier  36  (as driven by the input shaft  18 ) and the first output gear  20  drives the transfer shaft  22  through the plurality of driving pinions  21  to adjust the rotational speed of the second output gear  24 , the driven gear  70 , the inter-axle shaft  64 , the second driving gear  65 , and the second wheel differential  66  through the clutch  28  placed in the second position. 
     A shown in  FIG. 4 , the one of the predetermined speed and the target speed are obtained when the rotational speed of the second output gear  24 , the driven gear  70 , the inter-axle shaft  64 , the second driving gear  65 , and the second wheel differential  66  permits a meshing engagement between the first portion of one of the second pair of output axle shafts  67  and the second portion of one the second pair of output axle shafts  67  with the axle clutch  68 . 
     When the one of the predetermined speed and the target speed are obtained, the controller  55  commands the second actuator  74  to move the axle clutch  68  to the engaged position. Point D of  FIG. 4  indicates the time in the shifting procedure when the controller  55  commands the second actuator  74  to move the axle clutch  68  to the engaged position. Point E of  FIG. 4  indicates the time of the shifting procedure when the axle clutch  68  is engaged. As shown in  FIG. 4 , the duration of time between points D and E represents a duration of time after the controller  55  commands the second actuator  74  to engage the axle clutch  68  but before the axle clutch  68  is engaged. The axle clutch  68  may not immediately engage due to a misalignment between the first portion and the second portion of one of the second pair of output axle shafts, a slipping condition of one of the wheels (not shown) coupled to the second pair of output axle shafts  67 , or due to both conditions. As shown in  FIG. 4 , once the one of the predetermined speed and the target speed are obtained, a substantially constant rotational speed of the power source  11  is maintained by the controller  55 . When the axle clutch  68  is engaged, the second pair of output axle shafts  67  is drivingly engaged with the second output gear  24  through the driven gear  70 , the inter-axle shaft  64 , the second driving gear  65 , and the second wheel differential  66 . 
     Following point E, the step of one of increasing and resuming the rotational force transferred to the power distribution unit  12  may be performed by one of adjusting an operating condition of the power source  11  and by engaging the clutch (not shown) forming a portion of the power source  11 . When the step of increasing the rotational force transferred to the power distribution unit  12  is performed by adjusting the operating condition of the power source  11 , the operating condition of the power source  11  may be adjusted by increasing the fuel supplied to the power source  11 . When the step of increasing and resuming the rotational force transferred to the power distribution unit  12  is performed by engaging the clutch (not shown), the amount of engagement of the clutch (not shown) associated with the power source  11  is increased to increase the rotational force. Adjusting the rotational force transferred to the power distribution unit  12  as mentioned hereinabove completes the shifting procedure as illustrated in  FIG. 4 , and the controller  55  returns control of one of the operating conditions of the power source  11  and the clutch (not shown) to the operator. 
     Once the drive axle system  10  is placed in the second operating state, the rotational force applied to the power distribution unit  12  by the power source  11  is distributed between the first output gear  20  and the second output gear  24  through the inter-axle differential  19 . A rotational difference of the first output gear  20  and the second output gear  24  caused by a difference between the first gear ratio and the second gear ratio is accommodated by the inter-axle differential  19 . Because the inter-axle differential  19  accommodates the rotational difference between the first gear ratio and the second gear ratio, a cumulative gear ratio is provided. The cumulative gear ratio is intermediate the first gear ratio and the second gear ratio. 
       FIG. 5  is a chart illustrating an example of shifting the power distribution unit  12  from a second operating state to a first operating state.  FIG. 5  illustrates a non-limiting example of shifting the power distribution unit  12  from the second operating state to the first operating state. 
     A horizontal axis shown in  FIG. 5  indicates a duration of time from a first chronological reference point, A, to a third chronological reference point, C. Chronological reference point B occurs between points A and C. 
     A vertical axis shown in  FIG. 5  indicates a rotational speed of the first output gear  20 , the transfer shaft  22 , the second output gear  24 , and the power source  11 . The vertical axis begins at a rotational speed of zero and increases as the vertical axis extends away from the horizontal axis. The rotational speed of the power source  11  depicted in  FIG. 5  is merely for purposes of example, and the shifting procedure is not limited to the depicted speeds. 
     Point A indicates a starting time of the shifting procedure. At point A, the power distribution unit  12  is in the second operating state. In the second operating state, the clutch  28  is in the second position. When directed by the controller  55  or by an operator of the vehicle, the shifting procedure is initiated by disengaging the axle clutch  68  and by adjusting the rotational force transferred to the power distribution unit  12 . 
     Point A of  FIG. 5  indicates the time of the shifting procedure when the controller  55  commands the second actuator  74  to move the axle clutch  68  to a disengaged position. Point B of  FIG. 5  indicates the time of the shifting procedure when the axle clutch  68  is disengaged. As shown in  FIG. 5 , the duration of time between points A and B represents a duration of time after the controller  55  commands the second actuator  74  to disengage the axle clutch  68  but before the axle clutch  68  is disengaged. The axle clutch  68  may not immediately disengage due to a slipping condition of one of the wheels (not shown) coupled to the second pair of output axle shafts  67 . As shown in  FIG. 5 , to facilitate disengagement of the axle clutch  68 , a substantially constant rotational speed of the power source  11  is maintained by the controller  55 . When the axle clutch  68  is disengaged, the second pair of output axle shafts  67  is drivingly disengaged from the second output gear  24 . 
     The step of adjusting the rotational force transferred to the power distribution unit  12  may be performed by one of adjusting an operating condition of the power source  11  and at least partially disengaging a clutch (not shown) forming a portion of the power source  11 . When the step of adjusting the rotational force transferred to the power distribution unit  12  is performed by adjusting the operating condition of the power source  11 , the operating condition of the power source  11  may be adjusted by one of increasing or decreasing a fuel supplied to the power source  11 . When the rotational force is a positive rotational force (meaning the power source  11  is applying a rotational force to the power distribution unit  12 ) the fuel supplied to the power source  11  is decreased to reduce the rotational force. When the rotational force is a negative rotational force (meaning the power distribution unit  12  is applying a rotational force to the power source  11 ) the fuel supplied to the power source  11  is increased to increase the rotational force. When the step of one of reducing and interrupting the rotational force transferred to the power distribution unit  12  is performed by at least partially disengaging a clutch or other device (neither are shown) associated with the power source  11 , an amount of engagement of the clutch or other device (neither are shown) associated with the power source  11  is decreased to reduce the rotational force. Adjusting the rotational force transferred to the power distribution unit  12  as mentioned hereinabove is performed until the rotational force transferred to the power distribution unit  12  is about equal to an amount of rotational force applied by the power distribution unit  12  to the power source  11 . 
     When the rotational force transferred to the power distribution unit  12  is about equal to an amount of rotational force applied by the power distribution unit  12 , the controller  55  engages the first actuator  57  to move the clutch  28  from the second position to the third position. Point B of  FIG. 5  indicates a time in the shifting procedure when the clutch  28  is placed in the third position. As mentioned hereinabove, when the clutch  28  is placed in the third position, the inter-axle differential  19  is unlocked and the second output gear  24  is disengaged from the transfer shaft  22 , and thus the inter-axle differential  19 . As shown in  FIG. 5 , between points B and C, when the clutch  28  is moved to the third position and the axle clutch  68  is disengaged, the second output gear  24 , the driven gear  70 , the inter-axle shaft  64 , the bevel gear pinion  72 , the second driving gear  65 , and the second wheel differential  66  coast to the idle condition. 
     Once the clutch  28  is placed in the third position, the controller  55  further engages the first actuator  57  to move the clutch  28  from the third position to the first position while simultaneously adjusting a rotational speed of the power source  11 . As shown in  FIG. 5 , the rotational speed of the power source  11  is decreased as the first synchronizer  53  of the clutch  28  acts upon the pinion carrier  36  but before the clutch  28  is placed in the first position. 
     As a non-limiting example, the rotational speed of the power source  11  may be decreased by decreasing a fuel supplied to the power source  11 . A shown in  FIG. 5 , the rotational speed of the power source  11  is decreased between points B and C. A near constant rotational speed of the first pair of output axle shafts  62  (as the vehicle coasts during the shifting procedure) backdrives the first output gear  20 . The first output gear  20 , backdriven at the near constant rotational speed, permits decreases in the rotational speed of the power source  11  to be directly reflected in the rotational speed of the transfer shaft  22  through the inter-axle differential  19 . The concurrent rotation of the pinion carrier  36  (as driven by the input shaft  18 ) and the first output gear  20  retards the transfer shaft  22  through the plurality of driving pinions  21  to adjust the rotational speed of the transfer shaft  22 . The rotational speed of the transfer shaft  22  is adjusted to facilitate a smooth engagement of the clutch  28  with the first output gear  20  when the clutch  28  is moved from the third position to the first position. 
     When one of the rotational speed of the power source  11  and the rotational speed of the transfer shaft  22  are adjusted to facilitate a smooth engagement of the clutch  28  with the first output gear  20 , the controller  55  further engages the first actuator  57  to move the clutch  28  from the third position to the first position. Point C of  FIG. 5  indicates a time in the shifting procedure when the clutch  28  is placed in the first position. As mentioned hereinabove, when the clutch  28  is placed in the first position, the inter-axle differential  19  is locked and the first output gear  20  is engaged with the input shaft  18  through the inter-axle differential  19  in the locked condition. 
     Following point C, the step of one of increasing and resuming the rotational force transferred to the power distribution unit  12  may be performed by one of adjusting an operating condition of the power source  11  and by engaging the clutch (not shown) forming a portion of the power source  11 . When the step of increasing the rotational force transferred to the power distribution unit  12  is performed by adjusting the operating condition of the power source  11 , the operating condition of the power source  11  may be adjusted by increasing the fuel supplied to the power source  11 . When the step of increasing and resuming the rotational force transferred to the power distribution unit  12  is performed by engaging the clutch (not shown), the amount of engagement of the clutch (not shown) associated with the power source  11  is increased to increase the rotational force. Adjusting the rotational force transferred to the power distribution unit  12  as mentioned hereinabove completes the shifting procedure as illustrated in  FIG. 5 , and the controller  55  returns control of one of the operating conditions of the power source  11  and the clutch (not shown) to the operator. 
       FIG. 6  depicts yet another embodiment of the present invention. The embodiment shown in  FIG. 6  is similar to the embodiment shown in  FIG. 1 . Similar features of the embodiment shown in  FIG. 6  are numbered similarly in series, with the exception of the features described below. 
       FIG. 6  illustrates a drive axle system  10 ′ for a vehicle having a power source  11 ′. The drive axle system  10 ′ preferably includes a power distribution unit  612 , a first axle assembly  14 ′, and a second axle assembly  16 ′. The drive axle system  10 ′ is drivingly engaged with a power source  11 ′. As shown, the drive axle system  10 ′ includes the three assemblies  612 ,  14 ′,  16 ′, but it is understood the drive axle system  10 ′ may include fewer or more assemblies or components. 
     The power distribution unit  612  includes an input shaft  618 , an inter-axle differential  619 , a first output gear  620 , a plurality of driving pinions  621 , a transfer shaft  622 , a second output gear  624 , and a clutch  628 . As shown, power distribution unit  612  includes the seven components  618 ,  619 ,  620 ,  621 ,  622 ,  624 ,  628  disposed in a housing  30 ′ but it is understood the power distribution unit  612  may include fewer or more components. 
     The tandem drive axle system  10 ′ includes the input shaft  618  at least partially disposed in the housing  30 ′. Preferably, the input shaft  618  is an elongate cylindrical member, however the input shaft  618  may be any other shape. Bearings  32 ′ disposed between the input shaft  618 ′ and the housing  30 ′ and the input shaft  618  and the transfer shaft  622  permit the input shaft  618  to rotate about an axis of the input shaft  618 . The input shaft  618  has a first end portion  633 , having a first set of clutch teeth  637  formed thereon, a middle portion  634 , and a second end portion  635 , having a pinion carrier  636  disposed thereon. 
     The first end portion  633  has a diameter greater than a diameter of the middle portion  634 . The first end portion  633  is a substantially disc shaped body drivingly coupled to the input shaft  618 . Alternately, the first end portion  633  may be integrally formed with the input shaft  618 . The first end portion  633  includes an engagement portion  638  formed therein adjacent an outer peripheral edge thereof. As shown, the engagement portion  638  is a conical surface oblique to the input shaft  618 , however, the engagement portion  638  may have any other shape. The first set of clutch teeth  637  are formed on the first end portion  633  intermediate the input shaft  618  and the engagement portion  638 . 
     The pinion carrier  636  is a substantially disc shaped body having a plurality of pinion supports  639  protruding therefrom adjacent a peripheral edge of the pinion carrier  636 , however, the pinion carrier  636  may be any other rounded shape and may have a plurality of recesses or perforations formed therein. As is known in the art, the pinion carrier  636  is also known as a planet carrier. 
     The plurality of driving pinions  621  are rotatably coupled to the pinion supports  639 . Each of the driving pinions  621  have gear teeth formed on an outer surface thereof. As is known in the art, each of the driving pinions  621  is also known as a planet gear. Preferably, bearings are disposed between each of the driving pinions  621  and the pinion supports  639 , however, the driving pinions  621  may be directly mounted on the pinion supports  639 . 
     The transfer shaft  622  is a hollow shaft concentrically disposed about the input shaft  618 . Preferably, the transfer shaft  622  is a hollow elongate cylindrical member, however the transfer shaft  622  may be any other shape. Bearings  32 ′ disposed between the transfer shaft  622  and the housing  30 ′ and the input shaft  618  and the transfer shaft  622  permit the transfer shaft  622  to rotate about an axis of the transfer shaft  622 . The axis of the transfer shaft  622  is concurrent with the axis of the input shaft  618 . The transfer shaft  622  has a first end portion  643 , having a first set of clutch teeth  644  formed on an outer surface thereof, and a second end portion  645 , having a second set of gear teeth  646  formed on an outer surface thereof. 
     The first end portion  643  and the second end portion  645  are substantially disc shaped bodies having an outer diameter greater than a diameter of the transfer shaft  622 . The first end portion  643  and the second end portion  645  are drivingly coupled to the transfer shaft  622 . Alternately, the first end portion  643  and the second end portion  645  may be integrally formed with the transfer shaft  622  and may have a diameter substantially equal to the transfer shaft  622 . Similarly, the first set of clutch teeth  644  and the second set of clutch teeth  646  may be formed directly in the transfer shaft  622 . As is known in the art, the second end portion  645  having the clutch teeth  646  is known as a sun gear. The second set of clutch teeth  646  are engaged with the plurality of driving pinions  621  and the first set of clutch teeth  644  are disposed adjacent the first set of clutch teeth  637  of the input shaft  618 . 
     The second output gear  624  is a gear concentrically disposed about the input shaft  618  and the transfer shaft  622 . The second output gear  624  has a central perforation having a diameter greater than a diameter of the transfer shaft  622 . The second output gear  624  is a substantially disc shaped body having a first end portion  647 , a second end portion  648  defining an outer diameter of the second output gear  624 , and an engagement portion  649 . Bearings (not shown) disposed between the transfer shaft  622  and the second output gear  624  permit the second output gear  624  to rotate about an axis of the second output gear  624 . The axis of the second output gear  624  is concurrent with the axis of the input shaft  618 . A first set of clutch teeth  650  are formed on the first end portion  647  adjacent the first set of clutch teeth  644  of the transfer shaft  622 . A second set of gear teeth  651  are formed on the second end portion  648 . 
     The engagement portion  649  is formed in the second output gear  624  intermediate the first end portion  647  and the second end portion  648 . As shown, the engagement portion  649  is a conical surface oblique to the input shaft  618 ; however, the engagement portion  649  may have any other shape. 
     The clutch  628  is a shift collar concentrically disposed about the input shaft  618  and the transfer shaft  622 . The clutch  628  includes a set of inner clutch collar teeth  652  formed on an inner surface thereof, a first synchronizer  653 , and a second synchronizer  654 . The set of inner clutch collar teeth  652  are engaged with the first set of clutch teeth  644  of the transfer shaft  622 . The clutch  628  can be slidably moved along the axis of the input shaft  618  as directed automatically by the controller  55 ′ while maintaining engagement of the inner clutch collar teeth  652  and the first set of clutch teeth  644 . A shift fork  56 ′ disposed in an annular recess formed in the clutch  628  moves the clutch  628  along the axis of the input shaft  618  into a first position, a second position, or a third position. The first actuator  57 ′, which is drivingly engaged with the shift fork  56 ′, is engaged to position the shift fork  56 ′ as directed manually by the controller  55 ′. Consequently, the shift fork  56 ′ positions the clutch  628  into the first position, the second position, or the third position. In the first position, the clutch  628  is drivingly engaged with the first set of clutch teeth  644  of the transfer shaft  622  and the first set of clutch teeth  637  of the input shaft  618 . In the second position, the clutch  628  is drivingly engaged with the first set of clutch teeth  644  of the transfer shaft  622  and the first set of clutch teeth  650  of the second output gear  624 . In the third position, the inner clutch collar teeth  652  of the clutch  628  are only drivingly engaged with the first set of clutch teeth  644  of the transfer shaft  622 . It is understood the clutch  628 , the clutch teeth  637 ,  644 ,  650 ,  652 , the synchronizers  653 ,  654 , and the engagement portions  638 ,  649  may be substituted with any clutching device that permits selective engagement of a driving and a driven part. 
     The first synchronizer  653  is an annular body coupled to the clutch  628  adjacent the first end portion  633  of the input shaft  618 . The first synchronizer  653  has a first conical engagement surface  658 . Alternately, the first synchronizer  653  may have an engagement surface having any other shape. When the clutch  628  is moved from the third position towards the first position, the first conical engagement surface  658  contacts the engagement portion  638  of the first end portion  633  of the input shaft  618 , causing the clutch  628  to act upon the input shaft  618 . When the clutch  628  is moved towards the first set of clutch teeth  637  of the input shaft  618 , the clutch  628  continues to act upon the input shaft  618  as the inner clutch collar teeth  652  become drivingly engaged with the first set of clutch teeth  644  of the transfer shaft  622  and the first set of clutch teeth  637  of the input shaft  618 . 
     The second synchronizer  654  is an annular body coupled to the clutch  628  adjacent the first end portion  647  of the second output gear  624 . The second synchronizer  654  has a second conical engagement surface  659 . Alternately, the second synchronizer  654  may have an engagement surface having any other shape. When the clutch  628  is moved from the third position into the second position, the second conical engagement surface  659  contacts the engagement portion  649  of the first end portion  647  of the second output gear  624 . When the clutch  628  is moved further towards the first set of clutch teeth  650  of the second output gear  624 , the clutch  628  continues to act upon the second output gear  624  as the inner clutch collar teeth  652  become drivingly engaged with the first set of clutch teeth  644  of the transfer shaft  622  and the first set of clutch teeth  650  of the second output gear  24 . 
     The first output gear  620  is a gear concentrically disposed about the input shaft  618  and the pinion carrier  636 . The first output gear  620  has a central recess having a diameter greater than an outer diameter of the pinion carrier  636 . The first output gear  620  is a substantially cup shaped body having an inner surface having gear teeth  640  formed on. As is known in the art, the first output gear  620  is known as a ring gear. The gear teeth  640  are engaged with the gear teeth formed on the outer surface of each of the driving pinions  621 . 
     The first output gear  620  includes an output shaft  641  drivingly coupled thereto. Alternately, the first output gear  620  may be integrally formed with the output shaft  641 . The output shaft  641  is collinear with the input shaft  618 . Bearings  32 ′ disposed between the output shaft  641  and the housing  30 ′ support the first output gear  620  and permit the output shaft  641  to rotate about an axis of the output shaft  641 . 
     An axle clutch  668  is a shift collar having a conical engagement surface that divides one of the second output axle shafts  67 ′ into first and second portions. Alternately, the axle clutch  668  may be a plate style clutch or any other style of friction clutch. The axle clutch  668  has a plurality of teeth formed thereon for selectively engaging corresponding teeth formed on the first portion and the second portion of the second output axle shafts  67 ′. The axle clutch  668  is urged into an engaged position or a disengaged position by a shift fork  73 ′. A second actuator  74 ′, which is drivingly engaged with the shift fork  73 ′, is engaged to position the shift fork  73 ′, and thus the axle clutch  668 , as directed by the controller  55 ′. When the axle clutch  668  is in the engaged position, the first portion of one of the second output axle shafts  67 ′ is drivingly engaged with the second portion of one of the second output axle shafts  67 ′. 
     The axle clutch  668  may be selectively engaged to impart energy to a lubricant disposed within the second axle housing  69 ′. Preferably, when the axle clutch  668  is used to impart energy to the lubricant disposed within the second axle housing  69 ′, the axle clutch  668  is a clutch capable of acting upon on a connecting component in a variable manner, such as a shift collar having a conical engagement surface. When the axle clutch  668  is used to impart energy to the lubricant disposed within the second axle housing  69 ′, the controller  55 ′ one of engages and partially engages the axle clutch  668  until a temperature of the second axle assembly  16 ′ is above a predetermined value. As a non-limiting example, the predetermined value may be about 20° Fahrenheit. 
     The axle clutch  668  and the clutch  628  may be simultaneously used to impart energy to the lubricant disposed within the second axle housing  69 ′. When the axle clutch  668  and the clutch  628  are simultaneously used to impart energy to the lubricant disposed within the second axle housing, the axle clutch  668  cooperates with the clutch  628  to adjust the rotational speed of the second output gear  624 , the driven gear  70 ′, the inter-axle shaft  64 ′, the second driving gear  65 ′, and the second wheel differential  66 ′ to one of the predetermined speed and a target speed. 
     In use, the method for use of the drive axle system  10 ′ facilitates shifting from the first operating state to the second operating state. Similarly, the shifting procedures described above for use with the drive axle system  10 , may be used with the drive axle system  10 ′, accommodating for the differences of the embodiment shown in  FIG. 6  as described hereinabove. 
     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.