Abstract:
A disconnect mechanism for a drive unit (e.g., a wheel drive unit or an integrated drive unit) including a wheel carrying hub rotatably attached to a frame and a reciprocal input shaft operable to selectively connect a power source (e.g., a hydraulic motor) to a transmission connected to the hub. The disconnect mechanism is connected to the input shaft and is adapted to be selectively hand actuated. The disconnect mechanism includes a hand rotatable knob rotatably connected to a cover removably attached to and enclosing an end of the hub. The knob includes an annular helical surface (i.e., ramp surface) operable to axially displace the input shaft from a first position in which the input shaft is drivingly connected to the transmission and, thus, the hub to a second position in which the input shaft is disconnected from the transmission. A disconnect pin is positioned intermediate the disconnect knob and the input shaft and rides on the ramp surface of the disconnect knob. A resilient member biases the input shaft into continuous contact (via the disconnect pin) with the ramp surface of the disconnect knob.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention.  
           [0002]    The present invention relates to a drive unit (e.g., an integrated drive unit, or a wheel drive unit), and, more specifically, to an improved drive unit including an improved disconnect device for selectively disengaging the input shaft from the gear reduction train to allow the gear reduction train to freewheel.  
           [0003]    2. Description of the Related Art.  
           [0004]    Wheel drive units include a single housing having a transmission connectable to a power source mounted exterior of the housing. In one known wheel drive unit, a sleeve is utilized to link a power source to an input shaft selectively drivingly engaged with the transmission. In some configurations, the wheel drive unit is configured to accommodate axial displacement of the input shaft to allow the input shaft to be disengaged from the transmission so that the wheel drive unit will freewheel.  
           [0005]    Integrated drive units include a single housing containing both a power input device and a transmission. The power input device can be, e.g., a hydraulic motor. In one known integrated drive unit, the hydraulic motor is linked to the transmission of the integrated drive unit via an output shaft (drivingly engaged with the motor) coupled to an input shaft (selectively drivingly engaged with the transmission). In some configurations, the integrated drive unit is configured to accommodate axial displacement of the input shaft to allow the input shaft to be disengaged from the transmission so that the integrated drive unit will freewheel. For the purposes of this document, “drive unit” generically refers to either a wheel drive unit or an integrated drive unit.  
           [0006]    One known drive unit utilizes an externally positioned cover or “hat” to position the input shaft in either an engaged or a disengaged position. The hat includes a protrusion having a hollow interior sized to accommodate a plunger pin which abuts the input shaft. When the input shaft is engaged with the transmission, the hat is positioned with its protrusion facing outwardly from the integrated drive unit, with the plunger pin positioned in the hollow interior of the hat-shaped disconnect cover. The disconnect device is typically secured to the drive unit via a screw or other conventional fastener. To disengage the input shaft from the transmission, the fastener is removed so that the hat may be reversed (i.e., rotated 180°) such that the protrusion extends toward the integrated drive unit and axially displaces the plunger pin and, consequently, the input shaft, thereby disengaging the input shaft from the transmission. After repositioning the hat, the fastener is replaced so that the drive unit is maintained in its freewheel configuration. This disconnect mechanism requires the use of tools for placing the drive unit in its freewheel configuration. Utilizing a hat mechanism of this type requires the location of the appropriate tool to remove the fastener holding the hat in place and is relatively tedious to perform.  
           [0007]    Prior art disconnect mechanisms typically extend past the hub of the drive unit and are, therefore, particularly susceptible to damage and inadvertent contact by external influences, such as rocks, mud and vegetation, for example. These influences can bind the disconnect mechanism and create an impediment to proper disconnect operation. Mechanisms of this type utilize axially exerted force (with respect to the input shaft) to effect disconnection and can be difficult to operate.  
           [0008]    What is needed in the art is a disconnect mechanism for use with a drive unit which disconnect mechanism is quick and easy to utilize without requiring the use of tools.  
           [0009]    What is further needed in the art is a disconnect mechanism for use with a drive unit which provides an operator with tactile indication of engagement and disengagement of the transmission.  
           [0010]    What is additionally needed in the art is a disconnect mechanism for use with a drive unit that is protected from external influences such as, e.g., rocks, mud, and vegetation.  
         SUMMARY OF THE INVENTION  
         [0011]    The foregoing shortcomings of the prior art are addressed and overcome by the present invention. The present invention provides a rotating disconnect knob having an annular helical surface (i.e., ramp surface) for placing the input shaft of a drive unit in either an engaged or a disengaged position with respect to the transmission. The disconnect knob of the current invention is manually actuatable and provides a quick and easy mechanism for placing a drive unit in a freewheel configuration.  
           [0012]    The rotating disconnect knob of the present invention is applicable to an integrated drive unit including a fixed spindle and a rotatable hub connected to a wheel. The integrated drive unit includes a drive mechanism (e.g., hydraulic motor) for selectively driving the hub, and an output/input shaft combination for transmitting power output from the drive mechanism to power input to the transmission. The output/input shaft combination comprises an output shaft rotatably fixed to an input shaft such that relative axial movement may be effected. The input shaft is selectively axially displaceable from a first position in which the input shaft is drivingly connected to the hub (via the transmission) to a second position in which the input shaft is disconnected from the transmission, and, thus, the hub. The annular helical, or ramped surface of the disconnect knob is operable to axially displace the input shaft between the first and the second positions. In one exemplary embodiment, the rotating disconnect knob of the present invention contacts the input shaft via a plunger pin. In this embodiment, the disconnect knob includes an engage detent and a disengage detent for signaling achievement of the first and second positions of the disconnect knob, respectively. The disengage knob is positioned within a counterbore formed in the hub, and is thereby generally protected from external influences such as, e.g., rocks, mud, and vegetation.  
           [0013]    The rotating disconnect knob of the present invention is equally applicable to a wheel drive unit including a fixed spindle and a rotatable hub connected to a wheel. The wheel drive unit generally includes a sleeve for selectively connecting an input shaft to a power source mounted outside the wheel drive unit housing. The input shaft is axially displaceable relative to the sleeve from a first position in which the input shaft is drivingly connected to the hub (via the transmission) to a second position in which the input shaft is disconnected from the transmission, and, thus, the hub. As in the case of an integrated drive unit, the annular helical, or ramped surface of the disconnect knob is operable to axially displace the input shaft between the first and the second positions.  
           [0014]    The invention, in one form thereof, comprises a drive unit including a fixed spindle connectable to a vehicle and a rotatable hub connected to a wheel. In this form of the current invention, the drive unit comprises a gear set operable to be selectively driven by a power source, with the hub selectively operably coupled to the power source through the gear set; an input shaft for selectively connecting the power source to the gear set, with the input shaft being selectively axially displaceable from an engaged position in which the input shaft is drivingly engaged with the gear set to a disengaged position in which the input shaft is disengaged from the gear set. An actuatable ramped surface is connected to the input shaft so that actuation of the ramped surface operates to axially displace the input shaft between the engaged and disengaged positions.  
           [0015]    The invention, in another form thereof, comprises a method of placing a drive unit in a freewheel position, wherein the drive unit includes a fixed spindle and a rotatable hub connected to a wheel; the drive unit further includes a gear set selectively driven by a power source, with the hub selectively operably coupled to the power source through the gear set; an input shaft for selectively connecting the power source to the gear set; wherein the input shaft is selectively axially displaceable from an engaged position in which the input shaft is drivingly engaged with the gear set to a disengaged position in which the input shaft is disengaged from the gear set; and an actuatable ramped surface connected to the input shaft, whereby actuation of the ramped surface operates to axially displace the input shaft between the engaged and the disengaged positions; said method comprising the step of: actuating the ramped surface.  
           [0016]    An advantage of the present invention is the ability to place a drive unit in a freewheel configuration without the use of tools.  
           [0017]    Another advantage of the present invention is the provision of a manually operable mechanism for placing a drive unit in a freewheel condition, which manually operable mechanism includes tactile feedback indicative of the freewheel condition. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:  
         [0019]    [0019]FIG. 1A is a partial sectional view of an integrated drive unit including a rotating disconnect knob in accordance with the present invention;  
         [0020]    [0020]FIG. 1B is a partial sectional view of a wheel drive unit including a rotating disconnect knob in accordance with the present invention;  
         [0021]    [0021]FIG. 2 is a plan view of the disconnect mechanism taken along line  2 - 2  of FIG. 1;  
         [0022]    [0022]FIG. 3 is a partial sectional view of the integrated drive unit of FIG. 1;  
         [0023]    [0023]FIG. 4 is a partial sectional view of the integrated drive unit of FIG. 1 illustrating the integrated drive unit in a disengaged (i.e., freewheel position);  
         [0024]    [0024]FIG. 5 is a partial sectional view of the integrated drive unit of FIG. 1 illustrating the embodiment of the disconnect input shaft engaged with the transmission;  
         [0025]    [0025]FIG. 6 is a perspective view of a first embodiment of the rotating disconnect knob of the present invention;  
         [0026]    [0026]FIG. 7 is a plan view thereof;  
         [0027]    [0027]FIG. 8 is a plan view of a second embodiment of the rotating disconnect knob of the present invention;  
         [0028]    [0028]FIG. 9 is a plan view of a third embodiment of the rotating disconnect knob of the present invention; and  
         [0029]    [0029]FIG. 10 is a plan view of a fourth embodiment of the disconnect knob of the present invention. 
     
    
       [0030]    Corresponding reference characters indicate corresponding parts throughout the several views. Although the drawings represent embodiments of the present invention, the drawings are not necessarily to scale and certain features may be exaggerated to better illustrate and explain the present invention. The exemplifications set out herein illustrate exemplary embodiments of the present invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0031]    Referring now to the drawings and particularly to FIG. 1A, an exemplary embodiment of an integrated drive unit in accordance with the present invention is illustrated. Integrated drive unit  10  includes transmission  12  and hydraulic motor  14 . Hydraulic motor  14  is selectively connectable, in driving relationship, to transmission  12  via motor output shaft  16  and input shaft  18 . Input shaft  18  is selectively disengageable from transmission  12  via disconnect mechanism  11  as will be further described hereinbelow.  
         [0032]    Referring now to FIG. 1B, an exemplary embodiment of a wheel drive unit in accordance with the present invention is illustrated. Wheel drive unit  13  includes transmission  12 , input shaft  18 , and sleeve  17 . Sleeve  17  is operable to selectively connect transmission  12  (via input shaft  18 ) to a power source. Sleeve  17  is splined to input shaft  18 , and, in use, will be rotationally fixed to and driven by a power source. Input shaft  18  is selectively disengageable from transmission  12  via disconnect mechanism  11  as will be further described hereinbelow. The remainder of this detailed description will refer to integrated drive unit  10  illustrated in FIG. 1A, however, the disconnect mechanism of the present invention is equally applicable to wheel drive unit  13  illustrated in FIG. 1B. Wheel drive unit  13  generally differs from integrated drive unit  10  only in that wheel drive unit  13  does not house a power source (e.g., a hydraulic motor). With this in mind, the corresponding parts of wheel drive unit  13  and integrated drive unit  10  are indicated with consistent reference numerals and the operation of wheel drive  13  is not separately described for the sake of brevity.  
         [0033]    Referring again to FIG. 1A, hydraulic motor  14  is in fluid communication with a hydraulic pump (not shown) installed in a vehicle (not shown) onto which the integrated drive unit, or a plurality of integrated drive units are installed. Motor pistons  20  are hydraulically actuated by the hydraulic pump and function in conjunction with swash plate  22  to provide rotary motion to hydraulic motor barrel  24  as is known in the art. Hydraulic motor barrel  24  is splined to motor output shaft  16 . Motor output shaft  16  is further splined to input shaft  18 , and input shaft  18  is selectively splined to primary sun gear  26 . In this way, rotary motion of hydraulic motor barrel  24  can be transmitted into rotary motion of primary sun gear  26 .  
         [0034]    Primary sun gear  26  is in toothed engagement with primary planet gear  28 . While only one primary planet gear  28  is illustrated in FIG. 1, multiple primary planet gears (e.g., three or four) can be utilized. Primary planet gear  28  is rotatably supported by primary planet carrier  30 , with bearing  32  interposed therebetween. Retaining washer  34  is affixed to primary planet carrier  30  and resists axial movement of primary planet gear  28 . Ring gear  36  is press fit to hub  40  and is in toothed engagement with primary planet gear  28 . Rotation of primary sun gear  26  causes rotational movement of primary planet gear  28  about ring gear  36 , and consequently, rotational movement of primary planet carrier  30 . Primary planet carrier  30  is in splined engagement with secondary sun gear  38 . Secondary sun gear  38  includes a clearance hole through which input shaft  18  passes. Secondary sun gear  38  is in toothed engagement with secondary planet gears  42 . Again, while only one secondary planet gear  42  is illustrated in FIG. 1, multiple secondary planet gears (e.g., 3 or 4) can be utilized.  
         [0035]    Secondary planet gears  42  are rotatably supported by secondary planet carrier  44 , with bearings  46  interposed therebetween. Separation washer  48  is placed between axially aligned sets of bearings  46 , while retaining washer  50  is affixed to secondary planet carrier  44  and resists axial movement of each secondary planet gear  42 . Secondary planet carrier  44  is splined to spindle  52  at location  54 , and spindle  52  is affixed to frame  56  of the vehicle (not shown) via bolts  58 . Secondary planet carrier  44  is stationary relative to the vehicle frame and therefore rotational movement of secondary planet gears  42  causes rotational movement of ring gear  36  and consequently hub  40  to which ring gear  36  is affixed. Hub  40  is affixed to wheel  27  via bolts  62  and nuts  64 , and therefore, rotational movement of ring gear  36  is translated into rotational movement of wheel  27 . Integrated drive unit  10  includes a brake mechanism depicted generally at  60 . The structure of an applicable brake mechanism is disclosed in U.S. Provisional Patent Application No. 60/192,909, filed Mar. 29, 2000, assigned to the assignee of the present invention, the disclosure of which is herein explicitly incorporated by reference.  
         [0036]    Referring to FIGS. 1A, 2, and  3 , integrated drive unit  10  includes disconnect mechanism  11  recessed within hub  40 . Hub  40  includes end  66  having counterbore  68  with retaining ring groove  70  formed therein to receive a conventional coil-type retaining ring  72 . Retaining ring  72  secures cover  74  within counterbore  68  of hub  40 . Cover  74  includes outer surface  76 , inner surface  78  and peripheral surface  79 . Cover  74  is substantially U-shaped in cross section, having a cupped portion facing inwardly toward input shaft  18  within hub  40 . Peripheral surface  79  of cover  74  includes seal groove  80  in which seal  82  is positioned to seal cover  74  to hub  40 . Seal  82  is, e.g., an elastomeric ring. Cover  74  includes drain hole  84  with oil plug  86  positioned therein. Hub  40  includes inner hub cavity  83  which is at least partially filled with lubricant, e.g., oil, to lubricate transmission  12 . Lubricant may be drained from, or added to inner hub cavity  83  via drain hole  84  in cover  74 .  
         [0037]    As illustrated, e.g., in FIGS. 2 and 3, cover  74  includes eccentrically positioned recess  88  sized to accommodate rotatable disengage knob  90 . Referring now to FIG. 3, knob  90  is rotatably attached to cover  74  by fastener  94 . Fastener  94  traverses aperture  91  in knob  90  and is threadably engaged with cover  74  such that knob  90  is rotatably attached to cover  74 . Cover  74  includes centrally located through hole  100  which reciprocally supports disconnect pin  98  extending therethrough. Embedded washer  96  is fixed to inner surface  78  of cover  74  to protect cover  74  from damage due to adjacently positioned sun gear  26  and to discourage axial displacement of sun gear  26 . Knob  90  is fitted with O-rings  92 ,  149  to prevent lubrication from escaping inner hub cavity  83  through aperture  100  in cover  74 .  
         [0038]    Rotation of knob  90  causes axial displacement of disconnect pin  98  to disengage or engage moveable input shaft  18  from sun gear  26 , as further described hereinbelow. Referring to FIG. 3, input shaft  18  includes external splines  102  operable to engage splines  104  of sun gear  26 . Input shaft  18  traverses clearance hole  110  in secondary sun gear  38  and is axially displaceable relative thereto. Disconnect pin  98  includes a pair of opposing rounded ends  114 ,  115 . End  114  abuts piloting counterbore  112  in input shaft  18 , while end  115  abuts knob  90 .  
         [0039]    As illustrated in FIG. 2, knob  90  includes outer portion  116  including base  122  (FIG. 3) connected by outer wall  120  to top portion  118  having a periphery formed by alternating convex portions  130  and concave portions  132 . As illustrated in FIG. 2, outer wall  120  follows the periphery of top portion  118  to form a plurality of protrusions corresponding to convex portions  130  (FIG. 6). As illustrated in FIG. 2, knob  90  includes a first raised triangular portion embossed with the letter “E” to form engage indicator  124 . Similarly, knob  90  includes second raised triangular portion embossed with the letter “D” to form indicator  126 . Cover  74  includes a raised triangular portion forming indicator  128  which aligns with raised portions  124 ,  126 , respectively, when the integrated drive unit is in the engaged or disengaged position. As described above, outer wall  120  includes alternating convex portions  130  and concave portions  132 . This construction of outer wall  120  provides an operator with a utilitarian gripping structure to facilitate hand rotation of knob  90 . Specifically, the protrusions in outer wall  120  corresponding to convex portions  130  provide structure against which a tangential force can be applied to rotate knob  90 . In one exemplary embodiment, knob  90  has width “W” (FIG. 2) measuring 3¼ inches.  
         [0040]    Referring to FIGS. 6 and 7, knob  90  includes inner portion  133 , with ramp  146  extending from planar floor  135  of knob  90 . Ramp  146  engages disconnect pin  98  and is operable to effect axial displacement of disconnect pin  98  as will be further described herein below. O-ring groove  136  is provided at the periphery of base  122 . Circumferentially arranged ribs  138  are positioned within O-ring groove  136  and are spaced about the periphery of base  122 . Ribs  138  provide discrete “pinch” points along the circumference of O-ring  92  (FIG. 3) and thereby function to retain O-ring  92  in O-ring groove  136  and to prevent rotation of O-ring  92  relative to knob  90 . Boss  140  is centrally located within inner portion  133  and includes elongate aperture  91  sized to accommodate fastener  94  (FIG. 3) for rotatably supporting knob  90  as described above. Referring to FIG. 3, counterbore  144  in knob  90  accommodates head  142  of fastener  94 . Positioned intermediate aperture  91  and counterbore  144  is intermediate counterbore  180  having O-ring  149  positioned therein (FIG. 5). Washer  143  may be provided between head  142  of fastener  94  and intermediate counterbore  180  to retain O-ring  149  within intermediate counterbore  180 . O-rings  92 ,  149  prevent oil in hub cavity  83  from escaping through aperture  100  in cover  74 . In another embodiment (illustrated in FIG. 5), O-ring  149  is positioned within groove  145  formed in elongate aperture  91  of knob  90 .  
         [0041]    As illustrated in FIG. 6, inner portion  133  of knob  90  includes helical annular ramp  146 . Annular ramp  146  includes ramped surface  147  and inner and outer wall surfaces  148  and  150 , respectively. Ribs  152  are attached to inner wall surface  148  and extend inwardly therefrom to centrally located boss  140 . Advantageously, ribs  152  increase the structural integrity of knob  90  and decrease the likelihood that knob  90  will lift away from surface  153  (FIG. 3) of cover  74  in response to rotation of knob  90  and the consequent axial force applied to disconnect pin  98 . Therefore, ribs  152  decrease the likelihood of a fluid leak along O-ring  92  when knob  90  is actuated.  
         [0042]    As illustrated in FIG. 6, ramp  146  includes pinnacle  154  and base  156  (FIGS.  6 - 7 ). Pinnacle  154  and base  156  are separated by approximately 180° on annular ramp  146 . Referring to FIG. 3, ramped surface  147  of annular ramp  146  provides a bearing surface for end  115  of disconnect pin  98 . Ramped surface  147  acts against disconnect pin  98  to axially displace input shaft  18  as is further described hereinbelow. Input shaft  18  and ramped surface  147  remain in contact with disconnect pin  98  due to the biasing force of spring  157  (FIG. 1).  
         [0043]    Referring to FIG. 1A, retaining ring  163  is engaged in groove  165  formed in input shaft  18 . Washer  167  abuts retaining ring  163  and provides support for one end of compression spring  157 . The opposing end of spring  157  abuts motor output shaft  16  (in the wheel drive unit embodiment illustrated in FIG. 1B, the opposing end of spring  157  abuts a washer placed about, but not secured to input shaft  18 ). Spring  157  biases input shaft  18  toward cover  74 . If an operator rotates knob  90  toward the disengaged position, the biasing force of the spring must be overcome to inwardly displace input shaft  18 . Similarly, if an operator rotates knob  90  toward the engaged position, the biasing force of spring operates to axially displace input shaft  18  into the engaged position illustrated, e.g., in FIG. 3.  
         [0044]    [0044]FIG. 4 illustrates knob  90  rotated into the disengaged position corresponding to disengage indicator  126  (FIG. 2) being aligned with indicator  128  on cover  74  (corresponding to an 180° rotation of knob  90  from the position illustrated in FIG. 2). The disengage position corresponds to disconnect pin  98  abutting pinnacle  154  of ramped surface  147 . Detent  158  at pinnacle  154  provides a mechanism for retaining disconnect pin  98  in the disengaged position, as is further discussed hereinbelow. Similarly, FIG. 3 illustrates disconnect knob  90  positioned in the engaged position (also illustrated in FIG. 2) with disconnect pin  98  being positioned at base  156  of ramped surface  147 . Detent  160  is positioned at base  156  of ramped surface  147  and functions to retain disconnect pin  98  in the engaged position.  
         [0045]    Axial displacement of disconnect pin  98  is effected by rotation of knob  90 . Rotation of knob  90  causes disconnect pin  98  to ride on ramped surface  147  and thereby be axially displaced into one of its engaged and disengaged positions. As described above, spring  157  biases input shaft  18  into abutting relationship with disconnect pin  98  and thereby facilitates displacement of input shaft  18  from its disengaged position to its engaged position. Detents  158 ,  160  are opposite indicators  126 ,  124  of knob  90 , respectively, whereby engaged indicator  124  is aligned with indicator  128  on cover  74  when disconnect pin is positioned within detent  160  (at base  156  of ramped surface  147 ). Similarly, disengage indicator  126  is aligned with indicator  128  on cover  74  when disconnect pin  98  is positioned within detent  158  (at pinnacle  154  of ramped surface  147 ).  
         [0046]    In one exemplary embodiment, knob  90  is manufactured from a fiber-reinforced Nylon copolymer such as, e.g., Grivory GV-6H manufactured by, e.g., EMS-American Grilon Inc., Sumter, S.C., 29151. Grivory GV-6H has a Shore D hardness of 91. Generally, knob  90  will be constructed from a rigid and durable material having a generally high resistance to lubricants. Advantageously, knob  90  may be formed from an injection moldable material to facilitate manufacture thereof. Cover  74  may also be formed from Grivory GV-6H, or another suitable fiber-reinforced Nylon copolymer.  
         [0047]    Referring to FIGS.  6 - 7 , pinnacle  154  of ramped surface  147  includes detent  158 . Detent  158  is operable to “lock” disconnect pin  98  in the disengaged position as briefly described supra. Specifically, end  115  (FIG. 3) of disconnect pin  98  is retained within detent  158  to retain the disengaged position as illustrated in FIG. 4. In operation, detent  158  provides an operator with tactile indication of disengagement. Similarly, base  156  of ramped surface  147  includes detent  160 . Detent  160  is operable to “lock” disconnect pin  98  in the engaged position. In the engaged position, end  115  (FIG. 3) of disconnect pin  98  is retained within detent  160  to retain disconnect knob  90  in the engaged position. Similar to detent  158 , detent  160  provides an operator with tactile indication of engagement.  
         [0048]    A number of alternative embodiments of disconnect knob  90  will now be described. Common elements of the various embodiments of the disconnect knob will be denoted with reference numerals having a letter appended thereto, and many of these common elements will not be specifically referred to for the sake of brevity. FIG. 8 illustrates knob  90   a  in accordance with an alternative embodiment of the present invention. Rather than a single ramp ( 146 ) as illustrated in FIGS.  6 - 7 , knob  90   a  includes a pair of ramps  164 ,  166 , having ramped surfaces  169 ,  171 , respectively. Ramps  164 ,  166  are substantially concentric and are spaced whereby end  115  (FIG. 3) of disconnect pin  98  contacts both ramped surfaces  169 ,  171 . Both ramps  164  and  166  extend from floor  135   a  of base  122  (FIG. 2). Ribs  152   a  are attached to inner wall surface  168  of ramp  164  and extend inwardly to centrally located boss  140   a . It is contemplated that additional ribs may extend between ramps  164 ,  166 . Detent  158   a  of knob  90   a  differs from detent  158  of knob  90  (FIGS.  6 - 7 ) in that it is formed in both ramps  164 ,  166 .  
         [0049]    A third embodiment of a knob  90   b  according to the present invention is illustrated in FIG. 9 and differs from the first embodiment knob  90  (FIGS.  6 - 7 ) in that ramp  146   b  intersects column portion  172 . Detent  158   b  is formed in column portion  172  and overlays end  115  of disconnect pin  98  (in the disengaged position) such that the force generated by contact between end  115  of disconnect pin  98  and detent  158   b  is substantially uniformly transferred therebetween. Ramp  146   b  includes ramped surface  147   b  to provide a bearing surface for end  115  of disconnect pin  98 . Ramped surface  147   b  is operable to displace input shaft  18  as described hereinabove with respect to knob  90  illustrated, e.g., in FIG. 6.  
         [0050]    Knob  90   c  according to yet another embodiment of the present invention is illustrated in FIG. 10 and includes chord  182  connected to annular ramp  146   c . Annular ramp  146   c  includes ramped surface  147   c  operable to displace disconnect pin  98  as described above. Detent  158   c  is formed in both ramped surface  147   c  and chord  182  to facilitate retention of disconnect pin  98  therein.  
         [0051]    [0051]FIG. 5 illustrates an alternative embodiment drive unit according to the present invention. Certain elements are indicated by primed reference numerals to differentiate between the corresponding elements of the embodiments illustrated in FIGS. 1A and 1B, and many of these common elements will not be specifically referred to for the sake of brevity. Input shaft  18 ′ and sun gear  26 ′ of drive unit  15  have a longer axial length of engagement relative to input shaft  18  and sun gear  26  of integrated drive unit  10  or wheel drive unit  13  (see, e.g., FIG. 3). With this in mind, the disconnect mechanism of drive unit  15  must be operable to axially displace input shaft  18 ′ a greater distance relative to the displacement of input shaft  18  to effect disengagement of input shaft  18 ′. Therefore, disconnect knob  90 ′ includes ramp  146 ′ having pinnacle  154 ′ displaced a greater distance from top portion  118 ′ (of knob  90 ′) relative to the displacement of pinnacle  154  from top portion  118  of disconnect knob  90  (See e.g., FIG. 4). This additional length of pinnacle  154 ′ allows for greater axial displacement of disconnect pin  98 ′ and, consequently, greater axial displacement of input shaft  18 ′. As illustrated in FIG. 5, pinnacle  154 ′ extends past base  122 ′ of disconnect knob  90 ′. With this in mind, cover  74 ′ includes annular channel  176  to accommodate a portion of ramp  146 ′ (including pinnacle  154 ′). Detents  158 ′,  160 ′ function to retain disconnect pin  98  in the engaged, and disengaged positions, respectively.  
         [0052]    Notably, the outer dimensions of the disconnect mechanisms in accordance with the present invention are such that the disconnect knob is encircled by the portion of hub  40  extending from cover  74 . This arrangement advantageously allows hub  40  to provide protection to knob  90  so that the recessed knob is less likely to be struck or otherwise disrupted by external influences, such as, e.g., airborne rocks or dirt.  
         [0053]    While this invention has been described as having exemplary designs, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.