Abstract:
A drive unit disconnect mechanism is operable to transmit or disconnect power from a vehicle power source to a driven unit, and can be disengaged by multiple methods depending on how much force is required to separate the internal components of the mechanism. In a first, relatively quicker method, a handle is pulled axially away from the hub to withdraw and internal gear from splined engagement with a corresponding driven gear. In a second, relatively slower method, the handle is rotated to threadably withdraw the entire disconnect mechanism from the hub, which in turn withdraws the internal gear from splined engagement with the driven gear. The quicker method is desirable in most instances, but the slower method allows disconnection of the driven unit from the vehicle power source when gear pressure prevents or impedes the quicker method.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 61/370,474 filed Aug. 4, 2010 and entitled QUICK DISCONNECT FOR A DRIVE UNIT, the entire disclosure of which is hereby expressly incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to vehicle power transmission units, and, more particularly, to disconnect devices for selectively disengaging the vehicle transmission unit from driven vehicle wheels. 
     2. Description of the Related Art 
     Large industrial machinery, such as earth moving equipment and other construction vehicles, may use individual power transmission units mounted at the hub of each driven wheel to accomplish sufficient gear reduction for the heavy loads, large wheels, and low speeds frequently employed by such vehicles. These individual power transmission units are sometimes referred to as “wheel drives” and may house a transmission connectable to a power source mounted exterior of the wheel drive. For example, a wheel drive unit may operate to link a power source output shaft to a driven unit (such as a wheel), such that the driven unit is selectively drivingly engaged with the primary vehicle transmission. 
     In some configurations, a wheel drive unit may be configured to accommodate axial displacement of a coupling shaft or sleeve, which selectively disengages the driven unit from the powered transmission output shaft, which in turn disengages the wheel from the primary vehicle power source and allows the vehicle to “free wheel”. Generally speaking, these selectively engageable drive units utilize axially moveable structures contained within the hub of the drive unit, with the structures manually accessible from the outside of the vehicle for engagement or disengagement. When an operator wishes to disengage or engage an individual wheel from the vehicle&#39;s primary power source via a drive unit, the operator manipulates the axially engageable structure to toggle an internal gearing mechanism between engaged and disengaged configurations. 
     One known disconnect system which operates to disconnect a driven shaft from individual wheels is disclosed in U.S. Pat. No. 5,597,058 to Ewer. The &#39;058 patent discloses a hub lock for a vehicle, moveable between a manually engaged position and a disengaged position. For manual engagement and disengagement, a dial is turned to cause a nut to move inwardly or outwardly on threads formed on the dial. In the engaged position, the nut is moved outwardly to compress a first spring, which urges a clutch ring into engagement with a drive gear and thereby interlocks the drive axle with the hub lock housing. When the nut is moved inwardly, a second spring is compressed on the opposite side of the clutch ring to urge the clutch ring out of engagement with the drive gear. 
     Integrated drive units are similar to regular wheel drive units, but further include an integrated power input device, such as a hydraulic motor. For example, a hydraulic motor may be linked to the integrated drive unit via an output shaft (driven by the motor) coupled to an input shaft selectively engaged with the wheel-driving output of the drive unit. Like a non-integrated wheel drive unit, integrated drive units may be configured to accommodate axial displacement of a coupling shaft to allow the input shaft to be disengaged from the wheel so that the wheel can rotate independently of the vehicle&#39;s primary transmission, i.e., “free wheel”. 
     One example of a disconnect mechanism used with an integrated drive unit is disclosed in U.S. Pat. No. 4,588,322 to Shoemaker et al. The &#39;322 patent discloses a disconnect mechanism with a disconnect shaft that moves into and out of driving engagement with a coupling sleeve. A spring resiliently urges the disconnect shaft toward an engaged position, forming a splined engagement between the shaft and the sleeve. This splined engagement transfers driving force from a hydraulic motor to the disconnect shaft via the sleeve, the disconnect shaft drives a spindle, and the spindle drives a wheel hub. To disengage the disconnect shaft from the sleeve, a handle is pulled directly outwardly from a slot formed in the spindle, and the handle is then rotated out of alignment with the slot to maintain the disengaged position of the disconnect shaft against the bias of the compressed spring. When so disengaged, the spindle and disconnect shaft rotate freely without resistance from the motor. Pulling the handle is the only disclosed method of disengaging the disconnect shaft from the coupling sleeve; no alternative methods of disengagement are shown or described. 
     Another disconnect system for use with an integrated drive unit is disclosed in U.S. Pat. No. 5,261,801 to Stone. The &#39;801 patent discloses an engagement/disengagement mechanism in which a handle is actuated to disengage a hydraulic motor from a driven mechanism. A block or clip is inserted between the handle and the housing of the driven mechanism to maintain disengagement. To reengage the hydraulic motor with the driven mechanism, the block is removed and the handle is pushed and rotated until splines connected to the handle line up with coacting splines on the driven mechanism. A spring may be provided to urge the splines into engagement. 
     Disconnect mechanisms for wheel-mounted drive units are particularly useful for certain applications, such as towing of industrial machinery. When such machinery is in use under its own power, a drive unit serves to couple each wheel to the wheel&#39;s individual motor (in the case of integrated drive units) or to the primary vehicle power source (in the case of non-integrated drive units). However, when the machinery is not in use, it may be desirable to tow the machinery to another location. To avoid towing against the resistance of the motor(s) or vehicle transmission, a disconnect mechanism may be used to disengage each wheel from its respective drive unit so that the wheels can “freewheel” during the towing procedure. 
     As discussed above with respect to &#39;058, &#39;322 and &#39;801 patents, substantial design efforts have focused on providing hub-based connection/disconnection mechanisms. These known disconnect mechanisms purport to provide convenience to the user, but do so at a cost in terms of flexibility and robustness. 
     For example, industrial machinery utilizing drive units may be subjected to frequent engagement and disengagement in harsh and unpredictable service environments, such as in off-road in varied terrain, in heavy-duty applications, and/or in inclement weather conditions. On hilly terrain, a vehicle may be parked on a grade and oriented up or down the slope. When so parked, the vehicle will typically be left in gear so that the transmission resists potential rolling of the vehicle up or down the slope. However, this resistance places the transmission components under pressure, including the components of a quick-disconnect mechanism. 
     Spring pressure may be insufficient to overcome the substantial frictional forces that arise between components of a wheel drive when under pressure, effectively rendering a mechanism that relies on such disengagement mechanisms unable to effect the desired disconnection of the wheel drive. Similarly, disengaging a disconnect mechanism by a user-exerted pull-out force may also be difficult or impossible when the mechanism is under pressure, particularly where the user himself may be subject to adverse conditions (i.e., rain, mud, cold, etc). 
     Therefore, what is needed is a hub based engagement/disengagement drive unit mechanism that is robust, intuitive, manipulable without any special tools, and easy to operate under a wide variety of operating conditions, while also being actuatable when the associated wheel drive is under pressure. 
     SUMMARY 
     The present disclosure provides a drive unit disconnect mechanism operable to transmit or disconnect power from a vehicle power source to a driven unit, in which the mechanism can be disengaged by multiple methods depending on how much force is required to separate the internal components of the mechanism. In a first, relatively quicker method, a handle is pulled axially away from the hub to withdraw and internal gear from splined engagement with a corresponding driven gear. In a second, relatively slower method, the handle is rotated to threadably withdraw the entire disconnect mechanism from the hub, which in turn withdraws the internal gear from splined engagement with the driven gear. The quicker method is desirable in most instances, but the slower method allows disconnection of the driven unit from the vehicle power source when gear pressure prevents or impedes the quicker method. 
     The mechanism includes an outer shaft threadably engaged to a hub of a drive unit and an inner shaft axially moveable within the outer shaft. Optionally, the inner shaft may be spring biased toward an engaged position. To effect quick disengagement, a handle coupled to the inner shaft is pulled outwardly, axially displacing the inner shaft and rapidly withdrawing a coupling gear from engagement with a driven gear. The handle may then be rotated to lock the disconnect mechanism in this disengaged configuration. 
     In addition to axial displacement of the handle and inner disconnect shaft to quickly engage or disengage the disconnect mechanism, the handle may be used to rotate the outer shaft to at least partially threadably disengage the outer shaft from the hub, which in turn places the disconnect mechanism in a “screw-disengaged” configuration. As the outer shaft is threadably withdrawn from the hub, the inner shaft and coupling gear are axially displaced together with the outer shaft. As these components move axially outward along the thread axis, the coupling gear is slowly withdrawn from engagement with the driven gear. Advantageously, this threaded disengagement may be effected even when the drive unit and disconnect unit are under pressure, such as when the associated vehicle is parked on a hill. If such pressure prevents the coupling gear from being “quick-disconnected” by axially displacing the handle and inner shaft, the handle can instead be rotated to affect the slower “screw-disengagement” of the mechanism. 
     In one form thereof, the present invention provides a disconnect mechanism having an engaged configuration and a disengaged configuration, the mechanism comprising: a hub attachable to a driven unit, the hub having an inward side and an opposing, user-accessible outward side, the hub having a threaded bore extending from the inward side to the outward side; an outer shaft having a bore extending axially therethrough, the outer shaft defining a threaded engagement with the bore of the hub to axially move the outer shaft between an outer-shaft seated position in which the outer shaft is relatively inwardly disposed with respect to the hub, and an outer-shaft withdrawn position in which the outer shaft is relatively outwardly disposed with respect to the hub; an inner shaft received within the outer shaft, the inner shaft axially movable with respect to the outer shaft between an inner-shaft seated position in which the inner shaft is relatively inwardly disposed with respect to the outer shaft, and an inner-shaft withdrawn position in which the inner shaft is relatively outwardly disposed with respect to the outer shaft; and a coupling gear axially fixed to the inner shaft, the coupling gear defining: an engaged position corresponding to the engaged configuration of the disconnect mechanism, the coupling gear in the engaged position when the outer shaft is in the outer-shaft seated position and the inner shaft is in the inner-shaft seated position, and a disengaged position corresponding to the disengaged configuration of the disconnect mechanism, the coupling gear in the disengaged position when the outer shaft is in the outer-shaft withdrawn position or the inner shaft is in the inner-shaft withdrawn position. 
     In another form thereof, the present invention provides a disconnect mechanism for selectively disconnecting a hub from a power source, the mechanism comprising: a hub attachable to a driven unit, the hub having an inward side and an opposing, user-accessible outward side, the hub having a threaded bore extending from the inward side to the outward side; a coupling gear axially movable with respect to the hub between an engaged position in which the coupling gear extends relatively further outwardly and a disengaged position in which the coupling gear extends relatively further inwardly; means for axially toggling the coupling gear between the engaged position and the disengaged position by direct axial displacement, wherein the coupling gear is axially displaced with no mechanical advantage; and means for threadably toggling the coupling gear between the engaged position and the disengaged position by threaded rotational displacement, wherein the coupling gear is axially displaced with a mechanical advantage. 
     In yet another form thereof, the present invention provides a method of disengaging a driven unit from a vehicle power source, the method comprising: receiving a hub; receiving a driven gear; receiving a disconnect mechanism operable to selectively engage the hub with the driven gear via a coupling gear, the disconnect mechanism defining: an engaged configuration in which the hub is operably engaged with the driven gear; a handle-disengaged configuration in which the hub is disengaged from the driven gear by direct axial displacement of the coupling gear; and a screw-disengaged configuration in which the hub is disengaged from the driven gear by threaded disengagement of the disconnect mechanism from the hub; and with the disconnect mechanism in the engaged configuration, assessing whether the coupling gear and driven gear are under sufficient pressure to prevent disengagement of the hub from the driven gear by placing the disconnect mechanism into the handle-disengaged configuration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features and advantages of the present disclosure, 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: 
         FIG. 1A  is a perspective view of a drive unit disconnect mechanism connected to a drive unit assembly, with the disconnect mechanism shown in an engaged position; 
         FIG. 1B  is a perspective, section view of the disconnect mechanism and drive unit of  FIG. 1A ; 
         FIG. 1C  is an elevation, section view of the disconnect mechanism and drive unit of  FIG. 1A ; 
         FIG. 2A  is a perspective view of the disconnect mechanism and drive unit of  FIG. 1A , with the disconnect mechanism shown in a “handle-disengaged” configuration; 
         FIG. 2B  is a perspective, section view of the disconnect mechanism and drive unit of  FIG. 2A ; 
         FIG. 2C  is an elevation, section view of the disconnect mechanism and drive unit of  FIG. 2A ; 
         FIG. 3A  is a perspective view of the disconnect mechanism and drive unit of  FIG. 1A , with the disconnect mechanism shown in a “screw-disengaged” configuration; 
         FIG. 3B  is a perspective, section view of the disconnect mechanism and drive unit of  FIG. 3A ; 
         FIG. 3C  is an elevation, section view of the disconnect mechanism and drive unit of  FIG. 3A ; 
         FIG. 4  is an elevation, section, partially-exploded view of the disconnect mechanism and drive unit of  FIG. 1A ; 
         FIG. 5  is an elevation, section view of the disconnect mechanism and drive unit of  FIG. 1A , with the disconnect mechanism shown removed from the drive unit; 
         FIG. 6A  is a schematic illustration of a method of disengaging a gear by directly axially displacing the gear; and 
         FIG. 6B  is a schematic illustration of a method of disengaging a gear by rotating a shaft, such that which threaded interaction between the shaft and a surrounding structure causes axial displacement of the gear. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates an exemplary embodiment of the invention, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION 
     The present disclosure provides a drive unit disconnect mechanism  10  configurable between an engaged position ( FIGS. 1A-1C ), a “handle-disengaged” position ( FIG. 2A-2C ), and a “screw-disengaged” position ( FIGS. 3A-3C ). In the engaged configuration, coupling gear  12  of mechanism  10  rotationally fixes output hub  14  to planetary gear transmission assembly  101  of drive unit  100  ( FIGS. 1B and 1C , described in detail below). 
     In the handle-disengaged position, handle  16  is used to draw inner disconnect shaft  18  out of outer disconnect shaft  20 , so that coupling gear  12  is drawn into output hub  14  and fully disengages from planetary transmission assembly  101  of drive unit  100  ( FIGS. 2B and 2C ). As best shown in  FIG. 2A , visual indication of the handle-disengaged configuration is provided by inner disconnect shaft  18  protruding from outer disconnect shaft  20  and handle  16  being out of alignment with slots  56  formed in outer shaft  20 . 
     In the screw-disengaged configuration, outer disconnect shaft  20  is at least partially threadably disengaged from output hub  14 , with inner disconnect shaft  18  and handle  16  axially displacing together with outer disconnect shaft  20 . When outer disconnect shaft  20  is sufficiently axially displaced, coupling gear  12  disengages from planetary transmission assembly  101  of drive unit  100  in a similar manner as in the handle-disengaged configuration described above. 
     As schematically illustrated in  FIG. 6A , disconnect mechanism  10  may be quickly and easily reconfigured from the engaged configuration to the handle-disengaged configuration by simply pulling and subsequently rotating handle  16 , provided the operator can exert the required force F 1  to overcome any friction between coupling gear  12  and the mating structures (described below) of planetary transmission assembly  101 . On the other hand, when coupling gear  12  is under a high-pressure engagement with such mating structures of planetary transmission assembly  101 , force F 1  may not be sufficient to overcome the resulting high friction applied to coupling gear  12 . Advantageously, as schematically illustrated in  FIG. 6B  and described in detail below, the mechanical advantage afforded by the screw-disengagement method allows coupling gear  12  to be disengaged using force F 1 ′, which has about the same magnitude as force F 1  but exerts a much larger disengaging force F 2  on coupling gear  12 . 
     Also advantageously, as clearly shown in  FIGS. 1A ,  2 A and  3 A, disconnect mechanism  10  provides clear visual indication of its various configurations. Thus, a brief visual inspection of disconnect mechanism  10  is sufficient to determine whether output hub  14  (and any wheel connected thereto) is operably connected to drive unit  100  (in the engaged configuration) or will “freewheel” with respect to drive unit  100  (in either the screw-disengaged or handle-disengaged configurations). 
     1. Disconnect Mechanism Construction and Assembly 
     As best seen in  FIGS. 4 and 5 , drive unit disconnect mechanism  10  includes coupling gear  12 , output hub  14 , handle  16 , inner disconnect shaft  18 , outer disconnect shaft  20 , spring  22  and various o-rings and retaining rings used in assembly of mechanism  10  (as described below). Coupling gear  12  is axially received at gear end  24  of inner shaft  18 , with inner shoulder  26  of coupling gear  12  abutting outer shoulder  28  of inner shaft  18 . Coupling gear retaining ring  30  is then fixed to inner shaft  18  ( FIG. 4 ), such that coupling gear  12  is captured between shoulder  28  and retaining ring  30  and thereby axially fixed to gear end  24  of inner shaft  18 . Spring  22  is received upon inner shaft  18  and seated against inner shoulder  26  of coupling gear  12 . Inner shaft  18  is then placed into bore  32  of outer disconnect shaft  20  ( FIG. 5 ), such that the other end of spring  22  contacts end surface  34  of outer shaft  20 . With spring  22  captured between shoulder  26  of coupling gear  12  and end surface  34  of outer shaft  20 , any further axial movement of inner shaft  18  into bore  32  of outer shaft  20  compresses spring  22  and biases coupling gear  12  away from outer shaft  20 , as discussed in detail below. 
     Outer shaft  20  is installed to hub  14  either before or after inner shaft  18  is received within bore  32  of outer shaft  20 . Outer shaft  20  includes threaded portion  58 , which engages inner threads  60  formed in bore  48  of hub  14 . When outer shaft  20  is fully seated and threadably engaged within bore  48 , stepped portion  53  is fully received in a correspondingly large-diameter portion of bore  48  and shoulder  50  of outer shaft  20  seats against the corresponding shoulder  51  in output hub  14  ( FIGS. 4 and 5 ). As described in detail below, interaction between shoulders  50 ,  51  limits further inward axial movement of outer shaft  20  with respect to hub  14 , but allows outer shaft  20  to be freely threadably disengaged from the user-accessible side of hub  14 . 
     With outer shaft  20  coupled to hub  14  and inner shaft  18  fully received within bore  32  of outer shaft  20 , handle  16  is passed through transverse bore  36  formed in handle end  38  of inner shaft  18 . Outer shaft  20  includes slot  56 , which is most clearly shown in  FIG. 5  as part of a secondary elevation view of outer shaft  20  appearing beneath the primary elevation view thereof. As illustrated in  FIG. 5 , the second elevation view is rotated  90  degrees about the axis of outer shaft  20 . Handle  16  is received in slot  56  upon assembly, and can be toggled between a fully seated position ( FIGS. 1A-1C ) and a withdrawn position ( FIGS. 2A-2C ). In an exemplary embodiment, spring  22  is slightly compressed when handle  16  is fully seated at the bottom of slot  56 , thereby maintaining a spring bias against coupling gear  12 . Handle  16  and coupling gear  12  cooperate to define the limits of axial travel of inner shaft  18  with respect to outer shaft  20 , as described in detail below. 
     O-ring  40  may be installed within groove  42  formed in bore  32  of outer shaft  20  to provide a seal between bore  32  and the external environment. Similarly, o-ring  44  may be installed into groove  46  ( FIG. 5 ) formed in bore  48  of output hub  14  prior to outer disconnect shaft  20  being received therein, thereby sealing bore  48  from the external environment. 
     Referring to  FIG. 4 , outer splines  52  formed on coupling gear  12  engage inner splines  54  formed in a portion of bore  48  of output hub  14  when shafts  18 ,  20  are assembled to hub  14 . Splines  52 ,  54  enmesh to rotationally fix coupling gear  12  to output hub  14 . As described in detail below, the axial translation of coupling gear  12  into and out of bore  48  of output hub  14  operates to engage and disengage output hub  14  from the planetary transmission assembly  101  contained within drive unit  100 . 
     With drive unit disconnect mechanism  10  assembled, mechanism  10  may be mated to a drive unit, such as drive unit  100 . Although drive unit  100  is an exemplary drive unit adapted for use with disconnect mechanism  10 , it is contemplated that disconnect mechanism  10  may be mated to any number of different drive units or other power transmission units within the scope of the present disclosure. 
     In an exemplary embodiment, disconnect mechanism  10  is coupled to a wheel-mounted transmission, such as planetary transmission assembly  101 , via drive unit  100 . Drive unit  100  is in turn connected to a single, central power source which supplies the motive force for multiple wheels of a vehicle. Planetary transmission assembly  101  amplifies the torque available to the adjacent driven wheel, but also amplifies any residual torque applied to the internal gears of transmission assembly  101  and disconnect mechanism  10  when the vehicle is parked. As described below, disconnect mechanism  10  is particularly well-suited to wheel-mounted, high-reduction transmission applications because the screw-disengagement method of disengagement accommodates substantial residual torque. 
     As best seen in  FIG. 4 , drive unit  100  includes drive unit hub  102  including flange  104 . Flange  104  may be used to mount drive unit  100  to another structure, i.e., a vehicle frame, using mounting bolts  105 . Planetary transmission assembly  101  is bolted to drive unit hub  102  via ring gear  106 , which includes internal ring gear splines  107  adapted to engage planet gears  110 ,  112  (as described below). Also attached to ring gear  106 , and mounted generally opposite drive unit hub  102 , is cover  108 . For purposes of the present disclosure, cover  108  is considered to be at an “input side” of drive unit  100 , in that a powered input shaft (not shown) enters through cover  108  from a primary vehicle power source and/or primary vehicle transmission. Conversely, output hub  14  is considered to be mounted at an “output side” of drive unit  100 , in that power output is provided to a driven unit, i.e., a vehicle wheel, via disconnect mechanism  10  mounted within hub  102 . 
     Contained within (and including) stationary ring gear  106  is a planetary transmission assembly  101 . Referring still to  FIG. 4 , planetary transmission assembly  101  includes stationary ring gear  106 , a plurality of output planet gears  110  coupled to output planet gear carrier  114 , a plurality of input planet gears  112  coupled to input planet gear carrier  116 , output sun gear  118  and powered input sun gear  120 . Input sun gear  120  is rotated by a powered input shaft (not shown) which may be powered, for example, by a vehicle engine via a primary vehicle transmission. Input planet gears  112 , which are in splined engagement with both input sun gear  120  and stationary splines  107  of ring gear  106 , are in turn rotated about the axis of input sun gear  120 , and also about the axes of respective input-side coupling shafts  122 . Input planet gear carrier  116 , which is also rotatably coupled to each of shafts  122 , rotates about the axis of input sun gear  120  together with input planet gears  112 . Input planet gear carrier  116  is fixedly coupled with output sun gear  118 , which therefore rotates at the same rotational speed as input planet gear carrier  116 . 
     In similar fashion to input sun gear  120  and input planet gears  112 , the rotation of output sun gear  118  drives rotation of output planet gears  110  (which are in splined engagement with sun gear  118  and splines  107  of ring gear  106 ) about an axis of output sun gear  118  and about respective axes of output-side coupling shafts  124 . Output planet gear carrier  114 , which is coupled to output-side coupling shafts  124 , rotates about the axis of output sun gear  118  together with output planet gears  110 . Owing to the various gear reductions created by planetary transmission assembly, output planet gear carrier  114  rotates much more slowly than input sun gear  120  (and the motor shaft which drives input sun gear  120 ). 
     Drive unit hub  102  includes bore  128  sized to receive disconnect mechanism  10 . More particularly, bearings  130  disposed in bore  128  have inside diameters sized to correspond with respective outside diameters of output hub  14 . Thus, when output hub  14  is received within bore  128  (as shown in  FIGS. 1C ,  2 C and  3 C), bearings  130  allow disconnect mechanism  10  to rotate freely with respect to hub  102 . Output hub retaining ring  132  retains output hub  14  (and, concomitantly, disconnect mechanism  10 ) within bore  128  of hub  102 . Bulb seal  134  may be provided to seal bore  128  from the outside environment. 
     With disconnect mechanism  10  assembled and coupled to drive unit  100 , drive unit  100  may be installed to a vehicle frame, for example, with a driven input shaft connected to drive unit  100  via input sun gear  120 , and a driven unit such as a vehicle wheel connected to output hub  14  via wheel bolts  136 . As described in detail below, output planet gear carrier  114  serves as the “engagement point” for operably coupling disconnect mechanism  10  with the input shaft (not shown) via drive unit  100 . More particularly, output planet gear carrier  114  includes inner splines  126  adapted to mate with outer splines  52  of coupling gear  12 , such that output planet gear carrier  114  selectively drives output hub  14  (and any wheel or other driven unit attached thereto) depending on whether disconnect mechanism  10  is in an engaged or disengaged configuration. 
     While the planetary transmission assembly  101  shown and described herein is used for the illustrative embodiment of the present disclosure, it is also within the scope of the present disclosure to use any gear, power transmission unit or transmission assembly in conjunction with a disconnect mechanism made in accordance with the present disclosure. For example, any power transmission unit adapted to mate with coupling gear  12  may be used with disconnect mechanism  10 . Another exemplary drive unit is disclosed in U.S. Pat. No. 6,607,049, entitled QUICK DISCONNECT FOR AN INTEGRATED DRIVE UNIT, filed Mar. 6, 2001 and commonly assigned with the present application, the entire disclosure of which is hereby incorporated by reference herein. 
     2. Disconnect Mechanism Function 
     Referring now to  FIGS. 1A-1C , drive unit disconnect mechanism  10  is shown in an engaged configuration. In this configuration, handle  16  is fully seated in slot  56  formed in the end of outer shaft  20  ( FIG. 1A ). Threads  58  of outer shaft  20  are also fully threadably engaged with threads  60  of output hub  14 , such that stepped portion  53  of outer shaft  20  is fully received within bore  48 , and shoulder  50  of outer shaft  20  is seated against shoulder  51  of output hub  14 . In this engaged configuration, coupling gear  12  protrudes from bore  48  of output hub  14  ( FIGS. 1C and 4 ). When disconnect mechanism  10  is assembled to drive unit  100 , as discussed above, the protrusion of coupling gear  12  from bore  48  results in engagement of outer splines  52  of coupling gear  12  with inner splines  126  of output planet gear carrier  114  ( FIG. 1C ). Thus, outer splines  52  of coupling gear  12  engage both output planet gear carrier  114  and inner splines  54  of hub  14 , which rotationally fixes hub  14  and output planet gear carrier  114  to one another. When so rotationally fixed, power to input sun gear  120  is transmitted through drive unit  100  and coupling gear  12  to hub  14 , with an associated gear reduction between sun gear  120  and hub  14 . 
     Drive unit disconnect mechanism  10  may be moved to one of two disengaged configurations, in which output hub  14  rotates independently of output planet gear carrier  114 . In the first disengaged configuration, referred to herein as the “handle-disengaged” configuration and schematically illustrated in  FIG. 6A , handle  16  of disconnect mechanism  10  is pulled out of slots  56  to axially displace inner shaft  18  and coupling gear  12  by distance D 1 , thereby disconnecting coupling gear  12  from output planet gear carrier  114 . In the second configuration, referred to herein as the “screw-disengaged” configuration and illustrated schematically in  FIG. 6B , handle  16  is left seated in slots  56  and instead used to rotate outer shaft  20  to at least partially threadably disengage outer shaft  20  from hub  14  by distance D 1 , thereby axially displacing coupling gear  12  out of engagement with output planet gear carrier  114 . 
     The handle-disengaged configuration of disconnect mechanism  10  is illustrated in  FIGS. 2A-2C . To reconfigure disconnect mechanism  10  from the engaged configuration to the handle disengaged configuration, handle  16  is grasped and pulled out of slot  56  ( FIG. 2A ) in outer shaft  20 . Referring to  FIG. 2C , pulling handle  16  in this way, with sufficient force to counteract the biasing force of spring  22 , axially slides inner disconnect shaft  18  with respect to outer disconnect shaft  20  against the biasing force of spring  22 . Inner disconnect shaft  18  draws coupling gear  12  into bore  48  of output hub  14  ( FIG. 2C ), disengaging coupling gear  12  from splined engagement with output planet gear carrier  114 . When handle  16  is clear of slots  56  ( FIG. 2A ), handle  16  may be rotated out of alignment with slot  56 , then aligned with and seated in detents  66  ( FIGS. 3A and 3C ) to retain inner disconnect shaft  18  in the axially displaced, outer position. 
     As best seen in  FIG. 2C , when handle  16  is pulled out of slot  56  to axially displace inner shaft  18 , coupling gear  12  becomes fully disengaged from output planet gear carrier  114 . Disconnect mechanism  10  is therefore in the “handle-disengaged” configuration, in which output hub  14  is freely rotatable independent of the planetary transmission assembly  101 , (i.e., in a “free wheel” configuration). Moreover, output hub  14  is independent of any driving influence from the vehicle power source and/or primary vehicle transmission when disconnect mechanism  10  is disengaged. 
     Between the engaged and handle disengaged configurations, disconnect mechanism  10  may be placed in a “waiting-to-engage” configuration. The waiting-to-engage configuration occurs when handle  16  is realigned with slots  56  of outer shaft  20  and released, thereby freeing spring  22  to bias coupling gear  12  towards engagement with output planet gear carrier  114 , but outer splines  52  of coupling gear  12  are not properly aligned with inner splines  126  of output planet gear carrier  114 . With splines  52 ,  126  misaligned, coupling gear  12  will not engage and disconnect mechanism will instead enter the “waiting-to-engage” configuration. In this configuration, handle  16  remains aligned with, and partially captured within slots  56 , while spring  22  continues to urge coupling gear  12  toward engagement with output planet gear carrier  114 . As soon as either output hub  14  or output planet gear carrier  114  begins to rotate, outer splines  52  will align with inner splines  126  and spring  22  will push coupling gear  12  into engagement with output planet gear carrier  114 . Thus, placing disconnect mechanism  10  into the waiting-to-engage configuration causes disconnect mechanism  10  to “automatically” move from the waiting-to-engage configuration to the engaged configuration as soon as the alignment of the internal gears makes such engagement possible. 
     The second or “screw-disengaged” configuration is illustrated in  FIGS. 3A-3C . In this configuration, handle  16  is left engaged within slot  56  of outer shaft  20 . Rather than axially displacing inner shaft  18  with respect to outer shaft  20 , as described above with respect to the handle-disengagement method, the screw-disengagement method axially displaces both inner and outer shafts  18 ,  20 . Handle  16  is used to rotate outer disconnect shaft  20  with respect to output hub  14 , which remains stationary (such as by being coupled with a vehicle wheel at rest). As outer threads  58  of outer disconnect shaft  20  threadably disengage from inner threads  60  of output hub  14 , outer shaft  20  withdraws from hub  14 . Further, where screw-disengagement disconnect mechanism  10  begins from a fully engaged position ( FIG. 1C ), interaction between shoulder  50  of outer shaft  20  and shoulder  51  of hub  14  will only permit rotation (and axial displacement) in one direction, i.e., outwardly from hub  14  and toward the screw-disengaged configuration. 
     When outer shaft  20  has been sufficiently threadably disengaged from output hub  14 , as shown in  FIGS. 3B and 3C , coupling gear  12  is retracted into bore  48  ( FIG. 2C ) of hub  14 . As described above with respect to the handle-disengagement method, such retraction of coupling gear  12  rotatably decouples output hub  14  from output planet gear carrier  114 , allowing output hub  14  to “free wheel” with respect to the planetary transmission assembly  101 . As described in detail below, the screw-disengagement method offers the benefit of a mechanical advantage compared to the handle-disengagement method; the screw-disengagement therefore offers a greater force for withdrawing coupling gear  12  from engagement with output planet gear carrier  114 , for any given operator-produced exertion force against handle  16 . This greater withdrawal force is beneficial in effecting disengagement when splines  52 ,  126  are under pressure or otherwise tightly engaged with one another. 
     As best seen by comparison of  FIGS. 3C and 4 , the total axial travel of coupling gear  12  towards a “disengaged” position is limited by impingement of inner face  62  of coupling gear  12  upon shoulder  64  formed within bore  48  of output hub  14 . Conversely, axial displacement of coupling gear  12  in the other direction (i.e., towards an “engaged” position) is limited by impingement of handle  16  upon the ends of slots  56  formed in outer shaft  20 , and/or by impingement of shoulder  50  formed by stepped portion  53  of outer shaft  20  upon the corresponding shoulder  51  formed within bore  48  of output hub  14 . Thus, coupling gear  12  is only permitted to move within a controlled axial range when assembled to disconnect mechanism  10  as described above. 
     Upon reengagement of disconnect mechanism  10  from the screw-disengaged configuration, disconnect mechanism  10  may move into an engaged configuration or a waiting-to-engage configuration. As described above with respect to the handle-disengaged configuration, splines  52 ,  126  of coupling gear  12  and output planet gear carrier  114 , may not align as outer shaft  20  is rotated to fully threadably engage threads  58  of outer shaft  20  with threads  60  of hub  14 . If this is the case, handle  16  will outwardly advance in slots  56  as outer shaft  20  is rotated back towards a fully engaged position, with mechanism  10  entering the waiting-to-engage configuration when outer shoulder  50  of outer shaft  20  seats against shoulder  51  of hub  14  ( FIGS. 4 and 5 ). Handle  16 , inner shaft  18 , and coupling gear  12  will “snap” into the engaged configuration as soon as splines  52  of coupling gear  12  align with splines  126  of output planet gear carrier  114 , as described above. 
     3. Disconnect Mechanism Features and Benefits 
     Advantageously, disconnect mechanism  10  is particularly well-suited for use with planetary transmission assembly  101  and other gear-reduction transmission systems because the screw-disengagement method (described in detail above) offers a mechanical advantage that facilitates disconnection of gears under high pressure. 
     For example, a vehicle parked up- or down-hill with the transmission engaged will “come to rest” against the resistive force of the (unpowered) transmission. Stated another way, the tendency of the vehicle to roll downhill is counteracted by tension or pressure in the parts of the transmission, which in turn are created by the inertial forces of a shut-off vehicle motor. 
     In the context of a quick-disconnect system, this pressure is also transmitted to the internal gears of the quick disconnect which couples the wheel of the vehicle to the motor and primary transmission. If the vehicle also includes a hub-based wheel drive unit  100  including additional gear reduction for each vehicle wheel, such as via planetary transmission assembly  101  described above, even greater pressure may be exerted between the internal gears of the disconnect mechanism. Particularly for the heavy construction vehicles often used with high-reduction wheel drives, the pressure on the internal gears of a quick disconnect can become substantial on even a modest grade. 
     In the case of the handle-disengagement method, force F 1  exerted on handle  16  is equal to the force exerted on coupling gear  12 , as illustrated in  FIG. 6A . Where such pressure exists between coupling gear  12  of disconnect mechanism  10  and output planet gear carrier  114 , i.e., where the associated vehicle is parked on a sloped surface and resting against the transmission gears, exerted force F 1  on coupling gear  12  by pulling handle  16  directly away from hub  14  (illustrated schematically in  FIG. 6A ) may not be sufficient to dislodge coupling gear  12 . Exertion of a larger force, such as force F 2  ( FIG. 6B ) may not within the physical ability of the operator. 
     Where the operator assesses that force F 1  generated by pulling directly on handle  16  will not be sufficient to dislodge coupling gear  16 , the mechanical advantage of offered by the screw-disengagement method can be used to overcome such pressure. Like the handle-disengagement method, the screw-disengagement method is simple operation that requires no tools, as described above. 
     Referring now to the illustrative embodiment of  FIG. 6B , the screw-disengagement method is accomplished by applying force F 1 ′ to each side of handle  16 . For purposes of the present discussion, force F 1 ′ is taken to be approximately equal to force F 1  used for the handle-disengagement method ( FIG. 6A ), though either of forces F 1 , F 1 ′ may be any force within the normal range of forces exerted by a human hand. Exerting force F 1 ′ on handle  16  results in a much smaller axial displacement of inner shaft  18  for a given amount of movement applied to handle  16 . Thus, the screw-disengagement utilizes much more motion of handle  16  to accomplish a given axial displacement of coupling gear  12 , which results in mechanical advantage as described below. 
     The equation
 
[Work]=[Force]*[Distance]
 
can be rearranged as
 
[Force]=[Work]/[Distance],
 
which stands for the proposition that spreading a given amount of work over a greater distance lowers the amount of force needed to accomplish that work. For purposes of the present discussion, applicants assume for mathematical simplicity that coupling gear  12  is axially displaced by a distance of 1-inch against the forces resisting such axial displacement (which forces are mostly comprised of friction between coupling gear  12  and output planet gear carrier  114 ). Assuming a given amount of frictional resistance, the amount of work required to move coupling gear  12  1-inch is the same regardless of whether the handle-disengagement or screw-disengagement method is used.
 
     In the case of quick disconnect mechanism  10 , the mechanical work required to displace coupling gear  12  is accomplished by user-exerted work on handle  16 . Performing this user-exerted work via the handle-disengagement method requires that the mechanical work of dislodging coupling gear  12  be accomplished over the 1-inch travel of handle  16 ; performing the user-exerted work on handle  16  by the screw-disengagement method accomplishes the same mechanical work on coupling gear  12  over a much larger distance, giving rise to a mechanical advantage. 
     One full rotation of handle  16  axially displaces coupling gear  12  by the distance between an adjacent pair of threads  58 ,  60  (i.e., the “pitch” of threads  58 ,  60 ). In an exemplary embodiment, threads  58 ,  60  are male and female 1¼-7 UNC threads, respectively, meaning the mating threaded portions of outer shaft  20  and hub  14  are each 1¼-inches in diameter and have 7 threads per inch of axial travel. Thus, one full rotation of handle  16  axially displaces outer shaft  20  (and coupling gear  12 ) by 1/7-inch, and the user-exerted work required to axially displace coupling gear  12  by 1-inch is spread over 7 full rotations of handle  16 . 
     In this exemplary embodiment, handle  16  is about 2¼ inches long (i.e., sized to be easily grasped by the hand of an operator), so total movement of an end of handle  16  during one full rotation is equal to pi*2.25, which is slightly more than 7 inches. Total movement of handle  16  in the screw disengagement method is therefore [7 handle rotations]*[−7 inches per handle rotation]=˜49 inches. This total movement by the screw-disengagement method compares to only 1-inch for the direct handle-disengagement method, meaning the amount of force required to do the work of disengaging coupling gear  12  by the screw-disengagement method is 1/49 th  the force required by the handle-disengagement method. Stated another way, screw-disengagement of the exemplary disconnect mechanism  10  can dislodge a coupling gear  12  under 49 times more pressure with output planet gear carrier  114  than can be done with the handle disengagement method, for a given user-exerted force on handle  16 . 
     As detailed above, the screw-disengagement method advantageously allows coupling gear  12  to be disengaged from planet gear carrier  114  from a high-pressure, high friction engagement. On the other hand, where the pressure between coupling gear  12  and output planet gear carrier  114  is relatively small (i.e., when the associated vehicle is parked on level ground), disconnect mechanism  10  offers the handle-disengagement method for a much faster, tool-free and simple disengagement method. 
     Regardless of the disengagement method, an operator can engage or disengage disconnect mechanism  10  even in adverse environmental conditions. For example, because fine motor tasks are not required with the present tool-less design (i.e., aligning a pin with a hole, aligning a tool with a part, etc), disconnect mechanism  10  can be manipulated by an operator wearing gloves or mittens, even in cold, wet or muddy conditions. Similarly, since no external parts are needed to use disconnect mechanism  10 , no parts needed for engagement/disengagement of same can be lost or misplaced. 
     Also advantageously, each of the engaged, handle disengaged, screw-disengaged, and waiting-to-engage configurations of disconnect mechanism  10  are visually distinct configurations, thereby enabling an observer to readily ascertain the configuration of disconnect mechanism  10  (i.e., handle-disengaged, screw-disengaged, engaged, or waiting-to-engage) with only a moment of visual or tactile inspection. 
     In the engaged configuration, stepped portion  53  of outer shaft  20  is fully received within bore  48  of output hub  14 , and handle  16  is fully seated within slots  56  of outer shaft  20 . Further, inner disconnect shaft  18  appears inset within bore  32  of outer disconnect shaft  20  from the operator-accessible side of mechanism  10 , such that a portion of the inner wall defined by bore  32  is visible. 
     In the handle-disengaged configuration, stepped portion  53  of outer shaft  20  remains fully seated within bore  48  of output hub  14 , but handle  16  is fully removed from slots  56  and is rotated out of alignment therewith. Handle  16  may be engaged with detents  66 . Further, a portion of inner disconnect shaft  18  protrudes out of bore  32  of outer disconnect shaft  20 , exposing part of the outer arcuate face of inner shaft  18 . 
     In the screw-disengaged configuration, handle  16  is received within slots  56  and inner disconnect shaft  18  is inset within bore  32  of outer disconnect shaft  20 , similar to the engaged configuration discussed above. Unlike the engaged configuration, however, stepped portion  53  of outer shaft  20  protrudes substantially outwardly from output hub  14 , and the end of outer shaft  20  protrudes farther outwardly from hub  14  than when in the engaged configuration. Mechanism  10  is in a fully screw-disengaged configuration when handle  16  can no longer be rotated to further extract shaft  20  from hub  14  (owing to the impingement of coupling gear  12  upon shoulder  64  formed in hub  14 , as described above). 
     Finally, in the waiting-to-engage configuration, stepped portion  53  of outer shaft  20  is fully seated against output hub  14 , similar to the engaged and handle-disengaged positions described above. Handle  16  is aligned with, but only partially received within slots  56 . Handle  16  is not fully received within slots  56 , and inner shaft  18  protrudes slightly from bore  32  of outer shaft  20 , exposing a portion of the outer arcuate face of shaft  18 . 
     Thus, each configuration of disconnect mechanism  10  is unique and easily distinguishable from the other configurations. Notably, the “telltale” visual cues corresponding to the handle and screw-disengaged configurations cannot coexist because of the axial displacement limitations imposed by coupling gear  12  and output hub  14 . More particularly, inner face  62  of coupling gear  12  contacts shoulder  64  within bore  48  of output hub  14  when disconnect mechanism  10  is in the screw-disengaged configuration. Therefore, handle  16  cannot be extracted from slots  56 . Similarly, stepped portion  53  of outer shaft  20  cannot be significantly displaced away from its seated position within bore  48  of output hub  14  when handle  16  is removed from slots  56 , again because further axial displacement is prevented by the impingement upon inner face  62  upon shoulder  64 . 
     Also advantageously, disconnect mechanism  10  automatically reorients itself from the waiting-to-engage configuration to the engaged configuration under the biasing force of spring  22 . Thus, disconnect mechanism  10  may simply be placed in the waiting-to-engage configuration, with the operator assured that slight vehicle movement will subsequently place disconnect mechanism  10  in the engaged configuration. As noted above, the operator can visually verify that such engagement has occurred with a brief glance after such slight vehicle motion has occurred. 
     While this disclosure has been described as having an exemplary design, 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.