Patent Publication Number: US-2011053726-A1

Title: Gear Reduction and Coupling Assembly

Description:
FIELD OF THE INVENTION 
     The present invention relates to a gear reduction and coupling assembly. It will be described hereinafter more particularly in terms of its application within an overall drive configuration for a battle tank, bulldozer or other skid steered tracked or wheeled vehicle of the kind described in WO-02/083483 or WO-2006/021745, although it may be found more generally useful where a planetary gear reduction stage needs to transmit high torque to a shaft or other member and the arrangement needs to accommodate a certain angular misalignment between the two. 
     BACKGROUND OF THE INVENTION 
     Any practical transmission mounting arrangement and vehicle structure will have certain inaccuracies, and corresponding angular misalignments between nominally coaxially coupled transmission components must therefore be accommodated. One way of achieving this in the case of splined couplings is to crown the splines on one of the components. 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 so that when meshed with a set of parallel splines on the other component the assembly can rotate together with some angular misalignment without jamming. A disadvantageous consequence of such crowning, however, is that there is only a small contact area (near point contact) between each internal and external mating spline tooth, which reduces the maximum torque which can be transmitted through the coupling as compared to parallel-to-parallel spline sets which can spread the load over a larger contact area for the same coupling diameter. One way in which to carry higher torque through a crowned spline coupling, however, is to increase its diameter. Increasing diameter reduces the total shear load carried by the teeth and also increases the circumference of the coupling, thus accommodating more teeth and/or larger teeth. Torque capacity therefore nominally increases in proportion to diameter squared. 
     SUMMARY OF THE INVENTION 
     With the foregoing in mind, in one aspect the present invention resides in a gear reduction and coupling assembly comprising: a planetary gear mechanism having an input through the sun gear and output through the planet carrier; and a coupling member coupled to said carrier through a crowned spline coupling of a first diameter; said coupling member also being adapted to be coupled to a subsequent transmission member through a splined coupling of a second diameter; said first diameter being substantially greater than said second diameter. 
     In this way angular misalignment between the subsequent transmission member (typically a shaft) and the planet carrier of the gear reduction assembly can be accommodated by virtue of the crowned splines in the larger diameter coupling between the coupling member and the planet carrier. Conventional parallel-to-parallel spline sets can therefore be used in the smaller diameter coupling between the coupling member and the subsequent member, and a similar torque transmitting capacity achieved for both of the couplings with the coupling member. 
     In principle the splines which are crowned in the coupling between the coupling member and the planet carrier can be either the external spline set or the internal spline set, and either component can be configured to carry the external set or the internal set. In a preferred embodiment, however, the crowned splines are an external set provided on the planet carrier which mate with internal parallel splines on the coupling member. 
     Similarly either of the coupling member or subsequent transmission member can be configured to carry the external or internal spline set in the smaller diameter coupling, although when for example it is desired to minimise the diameter of a shaft which constitutes the subsequent member then that component will carry the external splines. 
     The face width of the splines in the coupling between the planet carrier and coupling member is typically substantially less than the face width of the splines between the coupling member and subsequent transmission member. 
     The coupling member is preferably axially located with respect to the planet carrier though a spherical joint held in the carrier—or by any other form of joint which can both axially locate the coupling member and permit its articulation relative to the planet carrier occasioned by the misalignment between the subsequent transmission member and planet carrier—and through flexible means permitting limited axial displacement between the coupling member and planet carrier. 
     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 a respective gear reduction and coupling assembly according to the first aspect of the invention within each said transmission; the sun gear of each said assembly being arranged to be driven from the respective said propulsion motor; and the coupling member in each said assembly being coupled to a respective transmission member leading towards the respective said drive member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other 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 practical implementation of a gear reduction and coupling assembly according to the invention as embodied in the drive configuration of  FIG. 1 ; and 
         FIG. 4  illustrates the typical form of a crowned spline. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  illustrates diagrammatically one form of vehicular drive configuration with which gear reduction and coupling 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   e.    
     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 respective 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 a preferred embodiment the shafts  18   a , 18   b  need to be of relatively small diameter to pass through the brakes  5   a , 5   b  and into the final drives  6   a ,  6   b —where they are preferably coupled in accordance with our copending United Kingdom patent application no 0915306.5. There is preferably also provision for withdrawing these coupling shafts from the central assembly after assembly so that the central assembly can be lifted out of the vehicle if required, for example for maintenance purposes. 
     Turning to  FIG. 3 , this illustrates a practical implementation of the gear reduction stage  4   a , and its relationship to the drive shaft  3   a  and to the coupling shaft  18   a  (itself not shown in  FIG. 3 ) which leads to the brake  5   a  and final drive  6   a  in the left hand transmission as viewed in  FIG. 1 . The corresponding components in the right hand transmission are identical in mirror image. 
     The illustrated gear reduction stage comprises a planetary gear set of which the sun gear  19  is formed on the end of the drive shaft  3   a . The latter is itself driven by the rotor of the propulsion motor  1   a  through the gear change unit  2   a  which is preferably in accordance with WO-05/054712. The sun gear  19  meshes with a set of helically cut planet gears  20  borne on respective pins  21  in a planet carrier  22  and the planet gears also mesh with a fixed annulus or ring gear  23  formed on the inside surface of the casing  24  of this part of the transmission. As the shaft  3   a  and sun gear  19  rotate the planet carrier  22  is also caused to turn, but at a slower rate, by virtue of the revolution of the planet gears  20  around the ring gear  23 , and it provides the output of the gear reduction stage. The assembly of the planet carrier  22  and planet gears  20  “floats” inside the ring gear  23  and the drive shaft  3   a  “floats” with it. In this respect a central spigot  25  from the planet carrier is received in the end of the drive shaft  3   a  and borne rotationally relative to that shaft by a spherical roller bearing  26  and a needle roller bearing  27 , the bearing  26  also serving to locate the carrier  22  axially with respect to the shaft  3   a . In this respect the bearing  26  is itself axially located between a shoulder  28  on the shaft  3   a  and a snap ring  29 , and is also clamped between the spigot  25  and a threaded retainer  30  by means of a bolt  31 . Although not readily apparent in  FIG. 3  there is a small radial clearance between the retainer  30  and the internal surface of the shaft  3   a.    
     The rotation of the planet carrier  22  is transmitted to the subsequent coupling shaft  18   a  through a coupling member  32  having a set of internal parallel splines  33  to mate with external parallel splines on that shaft. In this respect it is anticipated that the coupling shaft may be angularly misaligned from the planet carrier  22  by up to 1°, and by virtue of the shaft&#39;s parallel-to-parallel splined connection to the coupling member  32  that member may be equally misaligned with respect to the planet carrier. To accommodate such misalignment the flange of the carrier  22  is formed on its outside perimeter with a set of external crowned splines  34 . The typical form of such a spline is shown in  FIG. 4 , the flanks being curved to present a convex profile in the lengthwise direction. These splines mate with a set of internal parallel splines provided on the coupling member  32  at a substantially greater radius than the splines  33 . It will be observed that the face width of the splines in the coupling between the planet carrier  22  and the coupling member  32 , i.e. the dimension w in  FIG. 4 , is substantially less than the face width of the splines (e.g.  33 ) in the coupling between the coupling shaft and the coupling member  32 , this being necessary due to the crowned form of the splines  34  and the need to prevent the splines jamming as the misaligned components rotate together. Despite the crowning of the splines  34 , however, and the consequently small contact area between each mating spline tooth in the coupling between the planet carrier  22  and the coupling member  32 , the torque transmitting capacity of that coupling can match that of the coupling between the parallel-to-parallel splines of the coupling shaft and the member  32  by virtue of its substantially greater diameter. 
     The coupling member  32  is located axially in the assembly by a ball joint (spherical joint)  35  held in the planet carrier spigot  25  by the bolt  31 , and from which a pair of discs  36  radiate and are fixed to the member  32  by a series of screws such as indicated at  37 . The ball joint  35  permits the articulation of the coupling member  32  relative to the planet carrier  22  occasioned by the anticipated degree of misalignment of the coupling shaft. The discs  36  are also of such flexibility as to permit a small degree of axial movement of the coupling member  32  relative to the planet carrier  22 . This is a protection mechanism for the bearing  26  in the event that a high axial load is applied through the coupling shaft to the member  32 , for example due to flexure of the vehicle chassis, and particularly under high torque operation when the friction due to loading on the splines between the coupling shaft  18   a  and member  32  will preclude any axial play between those components. 
     An oil seal assembly  38  around the hub of the coupling member  32  is also mounted in the casing  24  such as to accommodate the articulation of the coupling member.