Patent Publication Number: US-2011048814-A1

Title: Final Drive Assembly

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to final drive assemblies and more particularly (though not exclusively) to final drives for tracked or wheeled skid steered vehicles. 
     SUMMARY OF THE INVENTION 
     The present invention as is described hereinafter in terms of its application within an overall drive configuration for a battle tank, bulldozer or other skid steered vehicle of the kind described in WO-02/083483 or WO-2006/021745, although it may be found more generally useful for vehicular applications. 
     The invention resides in various aspects of the assembly of which a preferred embodiment is described hereinafter and of which specific objects include a compact assembly with minimal intrusion within the vehicle hull, with provision to accommodate some misalignment with, and disengagement from, the inboard transmission. In accordance with one such aspect a final drive assembly comprises a planetary gear reduction mechanism of which the input is through the sun gear and the output is through the planet carrier, the sun gear turning with a first, hollow shaft which is driven through a second shaft extending coaxially within the first shaft, and the first and second shafts being coupled through a coupling member splined to each said shaft. 
     Preferably the coupling member and first shaft are coupled through a crowned spline coupling having a diameter substantially greater that the splined coupling between the coupling member and second shaft. 
     A mechanism may be provided for displacing the second shaft axially with respect to the first shaft while the splined couplings of the coupling member to each said shaft are maintained. 
     The first shaft is preferably rotationally supported by bearings located outboard of the sun gear in the sense of the intended mounting of the assembly to a vehicle, and such bearings may act between the first shaft and a third shaft which is arranged to be driven by the planet carrier. 
     The assembly of planet carrier and planet gears preferably floats between the sun gear and annulus of the planetary gear reduction mechanism and the planet carrier is preferably single sided and supports the planet gears in cantilever fashion. Its output is preferably through a crowned spline coupling. 
     The assembly may be combined with a track drive sprocket for a tracked vehicle arranged to be driven through said assembly, and/or with a brake arranged to act on the first shaft. 
     In another aspect the invention resides in a drive configuration for a skid steered vehicle comprising: a pair of propulsion motors coupled through respective transmissions to drive a respective drive member (such as a track drive sprocket in the case of a tracked vehicle or a wheel hub in the case of a wheeled vehicle) at a respective side of the vehicle; at least one steer motor coupled to a differential gear mechanism coupled between said propulsion motors to selectively impose a speed difference between said drive members; and each transmission comprising a respective final drive assembly or combination as defined above associated with the respective drive member. 
    
    
     
       DESCRIPTION OF THE SEVERAL FIGURES 
       These and other aspects and features of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a diagrammatic illustration of a drive configuration for a skid steered vehicle in which the invention may be embodied; 
         FIG. 2  illustrates schematically a mechanism for the controlled differential of the configuration of  FIG. 1 ; 
         FIG. 3  is an axial cross-section through a preferred form of a final drive assembly according to the invention for incorporation in the drive configuration of  FIG. 1 ; and 
         FIG. 4  illustrates the typical form of a crowned spline. 
     
    
    
     DESCRIPTION OF THE INVENTION, 
       FIG. 1  illustrates diagrammatically one form of vehicular drive configuration with which final drive assemblies in accordance with the present invention may be found particularly useful, being a track drive arrangement for a skid steered vehicle according to WO-02/083483 or WO-2006/021745. In this Figure a transverse drive arrangement comprises two electric propulsion motors  1   a  and  1   b  with associated gear change units  2   a  and  2   b  turning drive shafts  3   a  and  3   b  respectively. Outbound of these units the transmission includes in each case a gear reduction stage  4   a,    4   b,  a brake  5   a,    5   b  and a final drive gear reduction  6   a,    6   b,  leading to respective track drive sprockets  7   a  and  7   b  at opposite sides of the vehicle. Inboard the motors  1   a,    1   b  are coupled through the shafts  3   a,    3   b  to opposite sides of a controlled differential device  8  having an input from one or more electric steer motors  9 . 
     The mechanism of one suitable form of differential  8  is illustrated schematically in  FIG. 2 . It comprises an opposed pair of planetary gear sets each comprising a sun gear  10   a,    10   b,  planet gears  11   a,    11   b  and an annulus or ring gear  12 ,  12   b,  with the planet carriers  13   a,    13   b  of each set interconnected by a cross shaft  14  passing through the sun gears. The annuli  12   a,    12   b  are coupled to the respective adjacent drive shafts  3   a,    3   b  and the sun gears  10   a,    10   b  are fast with respective input gears  15   a,    15   b  which can be driven when required in this case by a coupled pair of steer motors  9   a,    9   b.  The steer motors are in this respect each coupled to a shaft  16  carrying a pinion  17   a  meshing with gear  15   a,  and a pinion  17   b  meshing through an idler gear  17   c  with gear  15   b,  so that the direction of rotation of the gear  15   b  in response to rotation of the shaft  16  is reversed as compared to the direction of rotation of the gear  15 a. 
     During straight running of the vehicle the steer motors  9   a,    9   b  are energised to hold the shaft  16  stationary, so the input gears  15   a,    15   b  and sun gears  10   a,    10   b  are likewise held stationary. Energising the propulsion motors  1   a,    1   b  to drive the sprockets  7   a,    7   b  in this condition also rotates the annuli  12   a,    12   b  to cause the planet gears  11   a,    11   b  to revolve about the sun gears  10   a,    10   b.  Due to their connection by the shaft  14  the two planet carriers  13   a,    13   b  must rotate at the same speed, also equalising the speeds of the two annuli  12   a,    12   b  and the two connected shafts  3   a,    3   b  and related transmission trains in this condition. The actual power distribution between the two transmissions will be determined by the torque required to drive the respective sprockets  7   a,    7   b  with torque being transferred through the controlled differential  8  from one side to the other as required e.g. in respect to changing ground conditions. 
     To turn the vehicle in one sense while being propelled by the motors  1   a,    1   b  as above the steer motors  9   a,    9   b  are energised to rotate the shaft  16  in a corresponding sense, thus causing the input gears  15   a,    15   b  and their respective sun gears  10   a,    10   b  to rotate in mutually opposite senses. The effect, since the two planet carriers  13   a,    13   b  must always turn together, is to increase the rate of rotation of the individual planet gears  11   a,  or  11   b  in that set for which the sun gear  10   a  or  10   b  is turning in the opposite sense to the respective annulus  12   a  or  12   b,  and to decrease the rate of rotation of the individual planet gears  11   a  or  11   b  in that set for which the sun gear  10   a  or  10   b  is turning in the same sense as the respective annulus  12   a  or  12   b.  This in turn causes the annuli  12   a,    12   b  and respective connected transmissions to the sprockets  7   a,    7   b  to run at different speeds thus turning the vehicle in the required sense, while power from the slower running transmission is mechanically regenerated to the faster running transmission through the controlled differential  8 . To turn the vehicle in the opposite sense the steer motors  9   a,    9   b  are energised to rotate the shaft  16  in the opposite sense and so forth, and it will be appreciated that for a given forward speed of the vehicle the turning radius in either sense will depend on the speed at which the steer motors are operated—the faster the steer motors the tighter the turn. In the limit, with zero forward speed the vehicle can be made to perform a neutral turn—i.e. “turning on the spot”—by driving the two transmissions in opposite directions through the differential  8 . 
     In a functionally equivalent arrangement one of the sun gears  10   a  or  10   b  can be permanently locked in place and a single gear train used from the shaft  16  to turn the other sun gear as required. 
     In practice the propulsion motors  1   a,    1   b , gear change units  2   a,    2   b,  gear reduction stages  4   a ,  4   b,  controlled differential  8  and steer motor(s)  9  of  FIG. 1  are integrated together in a first major assembly A 1  centrally of the vehicle while the brakes  5   a,    5   b,  final drives  6   a,    6   b  and track drive sprockets  7   a,    7   b  are integrated together in separate assemblies A 2  and A 3  to each side of the vehicle linked to the central assembly through respective coupling shafts  18   a,  and  18   b.  In an alternative embodiment a brake acting on the transmission could instead be incorporated in the assembly A 1 . 
     Turning to  FIG. 3  this illustrates schematically the main components of the final drive  6   a  and track drive sprocket  7   a  in the left hand assembly A 2  as viewed in  FIG. 1 , the corresponding components in the right hand assembly A 3  being identical in mirror image. 
     The track drive sprocket  7   a  is attached by bolts (not shown) through a flange  19  to a hollow output shaft  20  which is formed in two parts  20 A and  20 B splined together at  20 C and locked by a ring  20 D. The shaft  20  is rotationally supported by a pair of bearings  21  and  22  in a fixed casing  23  which has a mounting flange  24  by which it is attached by bolts (not shown) to a side plate  25  of the vehicle hull. The bearing  21  is a spherical roller bearing which axially locates the shaft  20  and is positioned at approximately the centre line of the track to minimise any moment loads on the shaft  20  and inner bearing  22 , (the flange  19  of the track drive sprocket itself being offset from the centre line). The bearing  22  is a needle roller bearing and supports any moment loads which are seen at the sprocket  7   a;  it can also permit some axial movement to accommodate differential thermal expansion and cumulative tolerances. Within the output shaft  20  a hollow input shaft  26  is rotationally supported by a pair of bearings  27  and  28 , the shaft  26  being formed in two parts  26 A and  26 B attached together at  26 C by bolts (not shown). Bearing  27  is a spherical roller bearing carried from the output shaft  20  which axially locates the shaft  26  and bearing  28  is a needle roller bearing between the shafts  20  and  26  which can permit some axial movement to accommodate differential thermal expansion and cumulative tolerances. The bearings  27  and  28  are well spaced to provide good support to the sun gear  38  and components of the brake assembly  5   a  which are located inboard of the bearings. 
     The input shaft  26  is arranged to be driven by the coupling shaft  18   a  from the central assembly Al of the drive arrangement, the shaft  18   a  extending with clearance through the brake assembly  5   a  and coaxially within the shaft  26 . In this respect the shaft  18   a  is splined at its inboard end as at  29  to the output component (not shown) of the gear reduction stage  4   a.  This output component may be for example the respective gear reduction stage planet carrier in the case of an assembly of the kind shown in  FIG. 3  of WO-2006/021745 or the coupling member in the case of an assembly of the kind shown in  FIG. 3  of copending U.S. patent application Ser. No. 12/821402. At its outboard end the shaft  18   a  is splined as at  30  to a coupling member  31 , through parallel external and internal spline sets on these components. The coupling member  31  is also splined as at  32  to the input shaft  26 . The external splines on this coupling member, which mate with (parallel) internal splines on the input shaft, are crowned splines, that is to say the flanks of the teeth are modified as compared to the usual parallel form and are convex in the lengthwise direction. The typical form of a crowned spline is shown in  FIG. 4 . By virtue of such crowning some angular misalignment between the shafts  18   a  and  26 , typically up to 1°, can be accommodated without the splines jamming as the assembly rotates. This coupling can therefore compensate for some inaccuracies in the mounting of the central assembly and/or final drive assembly in the vehicle. The bolted joint  26 C of input shaft  25  outboard of the splined coupling  32  allows assembly of the components and cutting of the internal spline teeth on that shaft. 
     It is observed that the crowned spline coupling  32  between the coupling member  31  and input shaft  26  is of a larger diameter than the parallel-to-parallel splined coupling  30  between the coupling shaft  18   a  and coupling member  31 . It can therefore accommodate more and/or larger teeth than the inner coupling. The couplings  30  and  32  can therefore be of similar torque transmitting capacity despite the reduced contact area between individual internal and external spline teeth in the coupling  32  due to the crowning. 
     Provision is also made for withdrawing the coupling shaft  18   a  from the central assembly A 1  after assembly so that the central assembly can be lifted out of the vehicle if required, for example for maintenance purposes. To this end the withdrawal mechanism comprises a screw  33  mounted to the shaft  26  and threaded through a nut  34  captive in the shaft  18   a.  The screw  33  is accessible, for winding using a standard socket wench or the like, after removal of a protective cap  35 , to draw the shaft  18   a  to the left as viewed in  FIG. 3 . This mechanism normally rotates as a whole with the shafts  18   a  and  26  so eliminates the need for a thrust bearing. The external splines on the shaft  18   a  which mate with the coupling member  31  at  30  are extended in the lengthwise direction as seen in the Figure so that the shaft can be withdrawn from the assembly A 1  without disengaging from its coupling with the coupling member. A compression spring  36  is trapped between an abutment in the end of the shaft  26  and a tube  37  which abuts the nut  24 , and urges the shaft  18   a  to the right (as viewed) to the extent permitted by the engagement of the nut  34  with the screw  33 . Winding the screw to withdraw the shaft  18   a  compresses the spring  36  further. If, upon winding the screw in the opposite direction to re-engage the shaft  18   a  with the assembly A 1 , the splines on the relevant output component from that assembly are not correctly aligned with the splines  29  on the shaft  18   a,  the shaft will be blocked from rightward movement and winding the screw  33  will instead cause the latter to protrude to the left from the end of the shaft  26 . As soon as the output component is turned by the transmission to align its splines with the splines  29 , however, the shaft  18   a  and screw  33  will shift to the right under the action of the spring  33  and the shaft will be re-engaged with the assembly A 1 . 
     The sun gear  38  of a planetary gear reduction mechanism is formed on the input shaft  26  and the annulus or ring gear  39  of this mechanism is formed on the interior of the fixed casing  23 . Between these gears revolve a set of planet gears  40 , borne on pins  41  with taper roller bearings  42  in a planet carrier  43 . This planetary gear mechanism is the largest diameter component of the final drive assembly and is mounted approximately on the same plane as the vehicle hull side plate  25  so that it does not have to be accommodated within the envelope of the track drive sprocket  7   a.  All of the other final drive components, including the bearings  21 ,  22 ,  27  and  28 , are located outboard of the planetary mechanism and do not occupy space within the vehicle hull. 
     The assembly of planet carrier  43  and planet gears  40  “floats” between the sun gear  38  and annulus  39  to maximise load sharing between the planet gears. Being of relatively large diameter the gears in the planetary mechanism are of relatively narrow face width, as the larger the diameter the less tolerant is this type of mechanism of any misalignments which might otherwise cause the gears to fail prematurely. The planet carrier  43  is single sided, meaning that the pins  41  are cantilevered and there is no additional structure linking two sides of the carrier, so more space is available for planet gears. Typically there may be up to six planet gears  40  to maximise load carrying capacity. The illustrated carrier  43  is a relatively heavy and stiff component suited to high load applications. Alternatively, for lower load applications the planet carrier may be of a light weight design which allows some flexibility and enhances load sharing between all planet gears, in which case self aligning planet gear bearings would be used in place of the taper bearing set  42 . In any event, the single sided carrier arrangement is suited to the large diameter narrow face width planetary gear set since this allows for relatively stiff, short length and large diameter cantilevered planet pins  41 . A more conventional double sided planet carrier could, however, be used in other embodiments. 
     The output from the planetary gear mechanism is through the carrier  43 , which necessarily rotates at a reduced rate as compared to the coupling and input shafts  18   a  and  26 . It is splined as at  44  to the output shaft  20  and consequently drives the track drive sprocket  7   a.  An internal parallel spline set is cut on the carrier  43  and a crowned external spline set is cut on the output shaft  20 , thus to accommodate the “float” of the carrier to maximise load sharing between the planet gears. 
     In an alternative embodiment the planet carrier could be fast with the output shaft and the sun gear allowed to float. This may be preferred when a brake is not associated with the final drive assembly. 
     In the illustrated embodiment, however, the brake  5   a  (not shown in  FIG. 3 ) is mounted from the rear of the casing  23  and acts on an inboard extension  26 D of the input shaft  26 , consequently braking the track drive sprocket  7   a  via the planetary gear mechanism and output shaft  20 . It is preferably a self-energising, liquid-cooled brake according to WO-2007/107771 and WO-2008/047072. It enables the vehicle to remain braked notwithstanding any withdrawal of the shaft  18   a  from the central assembly A 1  as described above. 
     While in the crowned splined couplings  32  and  44  between the coupling member  31  and input shaft  26  and between the planet carrier  43  and output shaft  20  the crowned splines have been indicated as the external spline set in each case, it could alternatively be the internal spline set that is crowned in either case, or indeed both spline sets could be crowned in either coupling. Equally at least the coupling  44  between the planet carrier  43  and output shaft  20  could be modified so that the external spline set is provided on the planet carrier and the internal spline set is provided on the output shaft instead of vice versa.