Patent Publication Number: US-2006012247-A1

Title: Drive sprocket for a tracked vehicle

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      The present application is a continuation of U.S. application Ser. No. 10/636,917 filed on Aug. 8, 2003. Through the &#39;917 application, the present application claims priority to US Provisional Application No.: 60/402,088 filed on Aug. 9, 2002. The disclosure of both are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to drive sprockets for tracked vehicles, and relates more specifically to the tooth design for such drive sprockets having improved traction with reduced noise generation.  
      2. Description of Related Art  
      Tracked vehicles such as snowmobiles and snow groomers drivingly engage the ground through one or more endless tracks. Endless tracks conventionally include an outer side with a pattern of projecting lugs or ribs that are designed to engage the snow or other ground surface, apply traction, and propel the vehicle. Conventional endless tracks also include an inner side that engages one or more drive sprockets, which, in turn, are operatively connected to a propulsion system of the vehicle.  
      The drive sprockets and the inner side of the endless track typically include mating teeth that provide traction between the drive sprockets and the endless track. Conventional drive sprockets use either external or internal teeth.  
       FIGS. 9 and 10  illustrate a drive sprocket  1010  that includes a plurality of radially-extending sprocket teeth  1020  projecting outwardly from an outer circumferential side of the sprocket  1010 . An endless track  1030  includes a plurality of longitudinally spaced holes that define a plurality of track teeth  1040 . Reinforcing metal bars (not shown) extend laterally across the endless track  1030  through the track teeth  1040  to reinforce the track teeth  1040  and the track  1030 . The track teeth  1040  mesh with the sprocket teeth  1020  to provide traction between the drive sprocket(s)  1010  and the track(s)  1030 . In the typical example where the drive sprocket  1010  is used, two such sprockets  1010  usually are provided to engage each endless track  1030 .  
      Each track tooth  1040  is surrounded by a metal alignment cleat  1050  that meshes with the sprocket  1010  to keep the endless track  1030  laterally aligned with the drive sprockets  1010 . As the drive sprockets  1010  rotate to propel the vehicle, the cleats  1050  rattle against the valleys formed between adjacent sprocket teeth  1020 . This metal-to-metal contact creates noise, especially when the vehicle travels quickly.  
       FIGS. 11-13  illustrate an additional conventional drive sprocket  1060  that includes a plurality of axially-extending sprocket teeth  1070 . An endless track  1080  includes a plurality of longitudinally-spaced track teeth  1090  projecting inwardly from an inner side of the endless track  1080 . The sprocket teeth  1070  engage the track teeth  1090  to provide traction between the sprocket  1060  and the endless track  1080 . Alignment cleats  1050  are laterally offset from the track teeth  1090  on the endless track  1080  and the sprocket teeth  1070 . Consequently, the cleats  1050  do not typically rattle against the sprockets  1060  as much as in the previous example, and noise is reduced as compared to sprockets  1010  that rely on radially-extending teeth  1020 .  
      While the use of axially-extending teeth  1070  instead of radially-extending teeth  1020  reduces noise, the axially-extending teeth  1070  are not as effective at generating traction with the track  1080  as the radially-extending teeth  1020 . The reduction in traction may be caused, in part, by the fact that the internal track teeth  1090  are typically not reinforced and therefore deform under high loads. Accordingly, a greater number of sprockets  1060  must be used to generate sufficient traction with the endless track  1080 . As illustrated in  FIGS. 8 and 9 , four internally toothed sprockets  1060  (two inner sprockets  1060  with two sets of sprocket teeth  1070  and two outer sprockets  1060  with one set of sprocket teeth  1070 ) are typically used.  
      As a result of this, a need has developed for a sprocket construction that provides the superior traction of the drive sprocket  1010  while also minimizing the generation of noise as does the drive sprocket  1030 .  
      Others have attempted to solve these problems.  FIGS. 14-16  illustrate an additional conventional drive sprocket  1110  that includes a plurality of axially-extending sprocket teeth  1120 . The sprocket teeth  1120  engage the track teeth  1090  of the endless track  1080 , as shown, for example, in  FIGS. 12 and 13 . The drive sprocket  1110  further includes a plurality of radially-extending sprocket teeth  1130  projecting outwardly from the an outer circumferential side of the sprocket  1110 . The radially-extending sprocket teeth  1   130  mesh with track teeth  1040 , as shown, for example in  FIGS. 9 and 10  to provide traction between the drive sprocket  1110  and the endless track. The radially-extending sprocket teeth  1130  extend directly from the outer circumference of the sprocket  1110  and have a width substantially the same as the sprocket teeth  1120 . The drive sprocket  1110  exhibits many of the drawbacks identified above.  
     SUMMARY OF THE INVENTION  
      Accordingly, one aspect of embodiments of the present invention provides a drive sprocket that generates more traction than conventional drive sprockets.  
      An additional aspect of embodiments of the present invention provides an improved drive sprocket that reduces noise.  
      A further aspect of embodiments of the present invention provides an improved drive sprocket that includes at least two sets of teeth.  
      A further aspect of embodiments of the present invention provides a drive sprocket with both radially-extending and axially-extending teeth.  
      A further aspect of embodiments of the present invention provides a sprocket for a vehicle having an endless track and a power plant. The sprocket includes a sprocket wheel that is engagable with the power plant of the vehicle. The sprocket wheel has a rotational axis, a perimetrical surface, and first and second axial surfaces. The sprocket also includes a first plurality of teeth extending radially outwardly from the perimetrical surface in spaced-apart relation, and at least a second plurality of teeth extending axially outwardly from the first axial surface in spaced-apart relation. A third plurality of teeth can extend axially outwardly from the second axial surface in spaced-apart relation.  
      The sprocket wheel, the first plurality of teeth, and the second plurality of teeth may be integrally formed. The sprocket may also include a third plurality of teeth extending axially outwardly from the second axial surface in spaced-apart relation. Each of the first plurality of teeth, each of the second plurality of teeth, and each of the third plurality of teeth may be radially aligned.  
      The perimetrical surface defines valleys between adjacent ones of the first plurality of teeth. The perimetrical surface may be generally cylindrically shaped such that each valley comprises an arc-shaped surface.  
      Each of the second plurality of teeth define an outward surface that may be disposed radially farther from the rotational axis than adjacent valleys.  
      Each of the second plurality of teeth may have two notches therein. Each of the second plurality of teeth has a base portion and a tip portion. The notches may be formed in the base portion.  
      In a circumferential direction, each of the second plurality of teeth may be wider than each of the first plurality of teeth.  
      Embodiments of the present invention are also directed toward a vehicle that includes a frame, a power plant supported by the frame, and a sprocket operatively connected to the power plant. The sprocket is one of the previously described sprockets.  
      The endless track may include a belt having an outer side and an inner side. The belt has a plurality of holes therethrough. The portions of the belt between the holes define a first plurality of track teeth. The endless track also includes a plurality of lugs projecting from the outer side, and a second plurality of track teeth projecting from the inner side. The second plurality of track teeth engage the second plurality of sprocket teeth.  
      Each of the first plurality of track teeth may longitudinally register with each of the second plurality of track teeth, and each of the first plurality of sprocket teeth may be radially aligned with each of the second plurality of sprocket teeth.  
      The endless track may further include a third plurality of track teeth projecting from the inner side. The third plurality of track teeth engages the third plurality of sprocket teeth. Each of the second and third pluralities of track teeth may be disposed laterally adjacent to each of the first plurality of teeth, on opposite sides thereof. Each of the first plurality of sprocket teeth, each of the second plurality of sprocket teeth, and each of third plurality of sprocket teeth may be radially aligned.  
      When the endless track does not deform, the first plurality of track teeth preferably do not engage the perimetrical surface. Similarly, when the endless track does not deform, the first plurality of track teeth preferably do not engage the first plurality of sprocket teeth. The second plurality of track teeth may include a flexible, resilient material. When the second plurality of track teeth deform under a load exerted thereon by the second plurality of sprocket teeth, the first plurality of sprocket teeth may engage the first plurality of track teeth to supplement traction between the sprocket and the endless track.  
      The endless track may further include a plurality of cleats. Each cleat includes a base portion secured to one of the first plurality of track teeth and at least one cleat portion projecting from the base portion away from the inner side. When the second plurality of teeth do not deform, the base portions of the cleats do not contact the perimetrical surface. When the second plurality of track teeth deform under a load exerted thereon by the second plurality of sprocket teeth, the first plurality of sprocket teeth engage the cleats to enhance traction between the sprocket and the endless track.  
      The perimetrical surface may define a sprocket valley between adjacent teeth of the first plurality of sprocket teeth. The inner side of the belt may define a track valley between adjacent teeth of the second plurality of track teeth. Each of the second plurality of sprocket teeth defines an outward surface, and the outward surfaces of the second plurality of sprocket teeth engage the track valleys as the sprocket rotates such that when the endless track does not deform, the first plurality of track teeth do not contact the sprocket valleys.  
      The endless track may further include a plurality of alignment cleats. When the endless track does not deform, the base portions of the cleats preferably do not contact the sprocket valleys.  
      The endless track may be a resilient, flexible material. The first and second pluralities of track and sprocket teeth may be sized and spaced such that the first plurality of sprocket teeth drivingly engage the first plurality of track teeth only when a portion of the endless track deforms longitudinally as the second plurality of sprocket teeth apply a load thereto.  
      When the endless track is laterally aligned with the sprocket, the cleat portions of the cleats preferably do not touch the sprocket as the sprocket rotates. As the sprocket rotates, the cleat portions preferably extend into the notches of adjacent ones of the second plurality of sprocket teeth.  
      The first plurality of track teeth may engage the first plurality of sprocket teeth as the sprocket rotates.  
      Embodiments of the present invention are also directed toward a sprocket for a vehicle having an endless track and a power plant. The sprocket includes a sprocket wheel engagable with the power plant of the vehicle. The sprocket wheel has a rotational axis. The sprocket also includes at least first and second pluralities of sprocket teeth projecting outwardly from the sprocket wheel.  
      Embodiments of the present invention are also directed toward a vehicle that includes a frame, a power plant supported by the frame, and at least one sprocket operatively connected to the power plant. Each of the at least one sprockets includes a sprocket wheel rotationally supported by the frame and operatively connected to the power plant. Each sprocket also includes first and second laterally adjacent pluralities of sprocket teeth projecting outwardly from the sprocket wheel. The vehicle further includes an endless track supported by the frame. The endless track passes around the at least one sprocket and has first, second, and third pluralities of laterally adjacent track teeth that engage the first, second, and third pluralities of sprocket teeth, respectively. The at least one sprocket may consist of two sprockets. A portion of the track may be longitudinally, resiliently deformable and the first plurality of sprocket teeth may only engage the third plurality of track teeth when the track longitudinally deforms  
      Additional and/or alternative objects, features, aspects, and advantages of the present invention will become apparent from the following description, the accompanying drawings, and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a better understanding of the present invention as well as other objects and further features thereof, reference is made to the following description which is to be used in conjunction with the accompanying drawings, where:  
       FIG. 1  is a side view of a snowmobile according to an embodiment of the present invention;  
       FIG. 2  is a perspective view of a drive sprocket of the snowmobile illustrated in  FIG. 1 ;  
       FIG. 3  is a front cross-sectional view of the drive axle and drive sprockets, taken along the line  3 - 3  in  FIG. 1 ;  
       FIGS. 4 and 5  are side views of the drive sprocket and track of the snowmobile illustrated in  FIG. 1 ;  
       FIG. 6  is a partial perspective view of the track of the snowmobile illustrated in  FIG. 1 ;  
       FIG. 7  is a partial top view of the circumferential edge of the drive sprocket according to another embodiment of the present invention;  
       FIG. 8  is a partial top view of the circumferential edge of the drive sprocket according to yet another embodiment of the present invention;  
       FIGS. 9 and 10  are side views of one conventional drive sprocket and track;  
       FIG. 11  is a perspective view of a second conventional drive sprocket;  
       FIG. 12  is a front view of a plurality of conventional drive sprockets, like the sprocket illustrated in  FIG. 11 , incorporated into the propulsion system of a snowmobile;  
       FIG. 13  is a partial exploded view of the propulsion system illustrated in  FIG. 12 ;  
       FIG. 14  is a partial perspective view of another conventional drive sprocket;  
       FIG. 15  is an enlarged perspective view of the sprocket teeth of the drive sprocket of  FIG. 14 ; and  
       FIG. 16  is a partial top view of the circumferential edge of the drive sprocket of  FIG. 14  illustrating the axially-extending sprocket teeth and the radially-extending sprocket teeth. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION  
      As illustrated in  FIG. 1 , a snowmobile  10  according to an embodiment of the present invention includes a frame  15  that supports a pair of selectively steerable skis  20 . An endless track  30  is supported by the frame  15  through a slide rail suspension system  40 . The frame  15  also supports a straddle-type seat  50 .  
      The snowmobile  10  includes a propulsion unit  60  (shown in phantom), such as an internal combustion engine, that is operatively connected to the endless track  30  via a drive axle  70 . A continuously variable transmission (not shown) connects the propulsion unit  60  to the drive axle  70 . Two drive sprockets  80 , as shown in  FIG. 3  are mounted to the drive axle  70  for common rotational movement about a drive sprocket axis  85 . As illustrated in  FIGS. 2-6 , each sprocket  80  includes three sets of sprocket teeth  90 ,  100 ,  110  that engage three corresponding sets of track teeth  120 ,  130 ,  140  in the endless track  30  to provide traction between the sprockets  80  and the endless track  30 .  
      The drive sprocket  80  comprises a generally disc-shaped sprocket wheel  150  that has an outer perimetrical surface  160  and two opposing axial surfaces  170 ,  180 . The sprocket wheel  150  has a central bore  190  through which the drive axle  70  extends. The bore  190  and the drive axle  70  have mating hexagonal cross-sections that rotationally secure the sprocket  80  to the drive axle  70 . Alternative methods of rotationally securing the sprocket  80  to the drive axle  70  may also be used (e.g., a key and keyway, square cross-sections, radial pins, etc.).  
      The sprocket teeth  100  comprise circumferentially-spaced teeth that project radially outwardly from the perimetrical surface  160 . The sprocket teeth  90 ,  110  comprise circumferentially-spaced teeth that project axially outwardly from the axial surfaces  170 ,  180 , respectively. Because the sprocket teeth  110  are mirror images of the sprocket teeth  90 , only the sprocket teeth  90  will be discussed in detail below. It is to be understood that the description of the sprocket teeth  90  applies equally to the sprocket teeth  110 .  
      The sprocket wheel  150  and the sprocket teeth  90 ,  100 ,  110  are integrally formed, for example by integral metal casting. To reduce the weight of the snowmobile  10 , the sprocket  80  preferably comprises a strong, light material such as aluminum or plastic. Also, while the sprocket  80  is preferably made of plastic, it is contemplated that the sprocket  80  may be made of any other material including steel or a composite material including aluminum. In addition, the sprocket  80  could be made from a composite including carbon fibers. In other words, the exact composition of the sprocket  80  may be selected from a wide variety of substances without departing from the scope of the present invention. In addition, the sprocket teeth  90 ,  100 ,  110  may be formed separately from the sprocket wheel  150  and subsequently rigidly fastened (via glue, rivets, welds, bolts, etc.) to the sprocket wheel  150 .  
      As illustrated in  FIGS. 4 and 5 , the sprocket teeth  90 ,  100 ,  110  are preferably radially aligned such that each sprocket tooth  90  is disposed at the same circumferential position as a corresponding one of each of the sets of sprocket teeth  100 ,  110 . However, the sets of sprocket teeth  90 ,  100 ,  110  may alternatively be out of phase with each other without departing from the scope of the present invention.  
      As illustrated in  FIGS. 2, 4  and  5 , the perimetrical surface  160  defines sprocket valleys  200  between adjacent sprocket teeth  100 . The perimetrical surface  160  is generally cylindrically shaped such that the sprocket valleys  200  each have an arc-shaped surface. Each of the sprocket teeth  90  have radially outward surfaces  90   a  that extend radially outwardly farther from the rotational axis  85  than the adjacent sprocket valleys  200 .  
      Each sprocket tooth  90  includes a base portion  90   c  connecting the sprocket wheel  150  to the tip portion  90   b . Notches  210  are formed on opposite circumferential sides of the base portion  90   c  of each sprocket tooth  90 . The notches  210  delimit the transition point between the base portion  90   c  and the tip portion  90   b.    
      As illustrated in  FIGS. 1 and 6 , the endless track  30  comprises an endless flexible belt  220  with an inner side  220   a  and an outer side  220   b . The endless track  30  includes a plurality of lugs (or ribs)  230  that project from the outer side  220   b  to give the endless track  30  traction against the snow as the endless track  30  propels the snowmobile  10 .  
      As illustrated in  FIG. 6 , the track teeth  130  are defined by a plurality of longitudinally spaced holes  240  through the endless belt  220 . As illustrated in  FIGS. 4 and 5 , inner surfaces  130   a  of the track teeth  130  are defined by the inner side  220   a  of the belt  220 .  
      The endless track  30  also comprises a plurality of alignment cleats  250  that are mounted onto the track teeth  130 . As illustrated in  FIGS. 4 and 5 , each cleat  250  includes a base portion  250   a  and a cleat portion  250   b . The base portion  250   a  has a generally C-shaped cross-section that wraps around the inner surface  130   a  of one of the track teeth  130 . The cleat portion  250   b  projects inwardly away from the inner side  220   a  of the belt  220 . The alignment cleats  250  preferably comprise a strong, light, stamped sheet of metal such as steel. As would be appreciated by those skilled in the art, however, any other suitable material (e.g., aluminum, etc.) may be used. Moreover, the cleats  250  need not be stamped from a metal sheet but may be cast or molded into the appropriate configuration.  
      Two sets of the track teeth  120 , 140  project inwardly from the inner side  220   a  of the endless belt  220 . The track teeth  120  are longitudinally spaced from each other around the inner side  220   a . The track teeth  140  are also longitudinally spaced from each other around the inner side  220   a . Track valleys  260  are defined by the inner side  220   a  of the endless belt  220  between longitudinally adjacent pairs of the track teeth  120 ,  140 . The track teeth  120 ,  140  are positioned laterally adjacent to the track teeth  130  but are disposed on opposite lateral sides of the track teeth  130 . Each of the track teeth  130  longitudinally registers with one tooth from each of the sets of track teeth  120 ,  140  (i.e., teeth from each set of track teeth  120 ,  130 ,  140  are longitudinally aligned) in the preferred example. Of course, if desired, the track teeth  120 ,  130 ,  140  need not be longitudinally aligned. Offset track teeth  120 ,  130 ,  140  are also considered to fall within the scope of the present invention.  
      The endless track  30  comprises a strong, flexible material such as rubber reinforced with fabric and metal. The endless belt  220  and track teeth  120 ,  130 ,  140  are integrally formed with each other. Alternatively, any one or more of the track teeth  120 ,  130 ,  140  may be formed separately from the remaining components of the endless track  30  and subsequently attached to the endless track  30  (via glue, bolts, rivets, etc.).  
      While only one lateral side of the endless track  30  has been described in detail, it is to be understood that, as is shown in  FIGS. 3 and 6 , additional, laterally-offset sets of track teeth mirror the track teeth  120 ,  130 ,  140 . The additional track teeth engage the second sprocket  60 .  
      Hereinafter, the engagement between the sprocket  80  and the endless track  30  is described with specific reference to  FIGS. 4 and 5 . The sprockets  380  and  480  described below engage the endless track  30  in a similar manner.  
      During low-load operation of the snowmobile  10  (e.g., during low acceleration, constant low speed use, coasting, etc.), successive sprocket teeth  90 ,  110  engage successive track teeth  120 ,  140 , respectively. During this low-load engagement, the traction provided between the sprocket teeth  90 ,  110  on the two laterally spaced sprockets  80  and track teeth  120 ,  140  is sufficient to prevent the endless track  30  from slipping relative to the sprockets  80 .  
      The sprocket  80  and track  30  reduce noise by reducing or eliminating rattling contact between the alignment cleats  250  and the sprocket  80 . As the sprocket  80  rotates, the outer surfaces  90   a  of the sprocket teeth  90  register with and contact the track valleys  260 . Similarly, the inner surfaces  130   a  of the track teeth  130  and their surrounding cleat bases  250   a  register with the sprocket valleys  200 . However, because the sprocket valleys  200  are disposed radially inwardly on the sprocket  80  relative to the outer surfaces  90   a  and because the cleat base portions  250   a  are disposed at generally the same level (in a direction perpendicular to the inner side  220   a  of the endless track  220 ) as the track valleys  260 , the cleat base portions  250   a  remain slightly spaced from the sprocket valleys  200 . Consequently, the metal cleats  250  do not rattle against the sprocket  80  to generate noise.  
      In the illustrated embodiment, while sprocket tooth and valley height differences are used to prevent the cleats  250  from rattling against the sprockets  80 , various other dimensions may alternatively be altered to achieve the same result without departing from the scope of the present invention. For example, the track valleys  260  could be built up slightly to space the cleats farther away from the sprocket valleys. Alternatively, the cleats could be disposed in depressions formed on the inner side  220   a  of the endless belt  220  to create a height gap between the cleats  250  and the track valleys  260 .  
      As the sprocket  80  rotates, the cleat portions  250   b  of the cleats  250  extend into the notches  210  formed in adjacent ones of the sprocket teeth  120 . If the cleat portions  250   b  are longitudinally narrow enough, the cleat portions  250   b  may simply extend into an open area formed between the notches  210  of adjacent sprocket teeth  120 . Because the circumferential width of the space formed between adjacent notches  210  is larger than the longitudinal width of the cleat portions  250   b , the cleat portions  250   b  do not typically contact the sprocket teeth  90 , even when the track teeth  120 ,  140  deform slightly in its longitudinal direction. However, if the endless track  30  and sprocket  80  begin to misalign, the cleat portions  250   b  contact the sprocket  80  to urge the track  30  back into alignment with the sprocket  80 . Accordingly, unless the cleats  250  are realigning the endless track  30  and sprocket  80 , cleat-to-sprocket rattling is reduced or eliminated altogether.  
      During low-load operation, the endless track  30  does not significantly longitudinally deform and the track and sprocket teeth  90 ,  110 ,  120 ,  140  provide sufficient traction between the sprockets  80  and the endless track  30 . Accordingly, the sprocket  80  and track  30  are designed so that the sprocket teeth  100  (and the cleat base portions  250   a  that surround the sprocket teeth  100 ) do not contact or engage the track teeth  130 . In a circumferential direction of the sprocket  80 , the sprocket teeth  100  are slightly narrower than the sprocket teeth  90 ,  110 . However, the engaging faces of the track teeth  120 ,  130 ,  140  are longitudinally aligned. Consequently, when the sprocket teeth  90 ,  110  engage the track teeth  120 ,  140 , a slight circumferential gap is formed between the mating faces of the sprocket teeth  100  and the track teeth  130  (and the cleat base portions  250   a ). The cleat base portions  250   a  do not, therefore, contact or rattle against the sprocket teeth  100 .  
      As the load exerted on the endless track  30  by the sprocket  80  increases, the sprocket and track teeth  90 ,  110 ,  120 ,  140  become less capable of handling the increased tractional load between the sprocket  80  and the endless track  30 . Simultaneously, the endless track  30  deforms longitudinally. The longitudinal deformation of the endless track  30  closes the gap between the cleat base portions  250   a  (and the track teeth  130 ) and the sprocket teeth  100 . The sprocket teeth  100 , therefore, engage the cleat base portions  250   a  and track teeth  130  to provide additional sprocket-to-track traction.  
      While engagement between the sprocket teeth  100  and the track teeth  130  causes the sprocket  80  to rattle against the cleat base portions  250   a , the high-load situations when the additional traction is required typically occur when the snowmobile  10  is traveling slowly but accelerating quickly. The slower rotational speed of the sprocket  80  in this situation minimizes the increased noise caused by cleat  250  rattling.  
      While in the illustrated embodiment, the sprocket teeth  100  selectively engage the track teeth  130  during high loads by reducing the circumferential length of each sprocket tooth  100 , the same effect may be obtained by modifying various other components of the endless track  30  and/or sprocket  80 . For example, instead of circumferentialiy narrowing the sprocket teeth  100 , the holes  240  in the endless track  30  could be longitudinally widened slightly. Alternatively, the sprocket teeth  100  could be circumferentially shifted slightly relative to the sprocket teeth  90 ,  110  or the track teeth  130  could be longitudinally shifted slightly relative to the track teeth  120 ,  140 . Consequently, the sprocket teeth  100  would be slightly out of phase (in a trailing direction) with the track teeth  130 .  
      The sprocket  80  is designed to be used with a conventional track like the track  1030  illustrated in  FIGS. 9 and 10 . The track  1030  includes all of the functional components of the endless track  30 . However, if, as is discussed above, one of the track components is modified to achieve one of the objectives of the present invention, a specifically designed track would replace the conventional track  1030 .  
      Furthermore, while in the illustrated embodiment, the sprocket teeth  100  selectively engage the track teeth  130  only during high loads, such selective engagement is not required to practice the present invention. For example, the sprocket and track teeth  90 ,  100 ,  110 ,  120 ,  130 ,  140  may be positioned and sized such that all three sets of sprocket teeth  90 ,  100 ,  110  simultaneously, continuously engage all three sets of track teeth  120 ,  130 ,  140 , respectively, to continuously provide increased traction between the sprocket  80  and the endless track  30 .  
      As illustrated in  FIG. 3 , because the sprocket and track teeth  90 ,  100 ,  110 ,  120 ,  130 ,  140  provide substantial traction between each sprocket  80  and the endless track  30 , only two sprockets  80  are required. This reduces the weight of the moving parts of the snowmobile  10  as compared to conventional four sprocket arrangements (see, e.g.,  FIG. 12 ). While two sprockets  80  are used in the illustrated embodiment, greater or fewer sprockets  80  may alternatively be used. The precise number of sprockets  80  that should be used to drive an endless track will be dictated by the tractional requirements of the specific tracked vehicle. Furthermore, a sprocket  80  may be used in conjunction with one or more conventional sprockets such as the previously described sprockets  1010 ,  1060  (see  FIGS. 9, 10  and  11 ).  
      Variations of the drive sprocket  80  are illustrated in  FIGS. 7 and 8 . Like the sprocket  80 , each sprocket  380  shown in  FIG. 7  includes three sets of sprocket teeth  390 ,  400 ,  410  that engage three corresponding sets of track teeth  120 ,  130 ,  140  in the endless track  30  to provide traction between the sprockets  80  and the endless track  30 . The sprocket  380  has a similar construction to the drive sprocket  80 . For the sake of brevity, the common components including the endless track  30  will not be described in further detail. Reference is made to the description above.  
      The sprocket teeth  400  comprise circumferentially-spaced teeth that project radially outwardly from the perimetrical surface  160 . The sprocket teeth  390 ,  410  comprise circumferentially-spaced teeth that project axially outwardly from the axial surfaces  170 ,  180 , respectively. The sprocket teeth  390  and  410  are substantially mirror images. The drive sprocket  480  is illustrated in  FIG. 8 , the sprocket  480  differs from sprocket  380  in that one set of the axially-extending teeth is missing.  
      Like the sprocket teeth  90 ,  100  and  110 , the sprocket teeth  390 ,  400 ,  410  are preferably radially aligned such that each sprocket tooth  390  is disposed at the same circumferential position as a corresponding one of each of the sets of sprocket teeth  400 ,  410 , as shown in  FIG. 7 . In the embodiment of  FIG. 8 , the sprocket teeth  390  are disposed at the same circumferential position as the sprocket teeth  500 .  
      The perimetrical surface  160  defines sprocket valleys  200  between adjacent sprocket teeth  400  or  500 . Like the sprocket teeth  90 , each of the sprocket teeth  390  has radially outward surfaces  390   a  that extend radially outwardly farther from the rotational axis  85  than the adjacent sprocket valleys  200 . Each sprocket tooth  390  includes a base portion  390   c  connecting the sprocket wheel  150  to the tip portion  390   b . Notches  391  are formed on opposite circumferential sides of the base portion  390   c  of each sprocket tooth  390 . The notches  391  delimit the transition point between the base portion  390   c  and the tip portion  390   b . Each tip portion  390   b  includes a notch  392  formed therein. The notches  392  provide a valuable reduction in weight without sacrificing performance.  
      The sprocket teeth  400  in  FIG. 7  and sprocket teeth  500  in  FIG. 8  have similar construction. The size of the teeth  500  are narrower when compared to the width of the teeth  400  due to the reduced thickness of the sprocket  480 . In the embodiments illustrated in  FIGS. 7 and 8 , the teeth  400  and teeth  500  have a width that is narrower than the teeth  390  and/or teeth  410 . With such an arrangement, the teeth  400  and the teeth  500  can be formed on top of the sprocket teeth  390  and  410  rather than on the circumferential side of the sprocket like the prior art. With such a construction, the perimetrical surface  160  of the sprockets  380  and  480  does not contact the endless track  30 , which further reduces the generation of noise. The sprockets  380  and  480  engage the endless track  30  in the same manner as described above in connection with sprocket  80 . As shown in  FIGS. 7 and 8 , the sides of the teeth  500  may have a concave curvature, which further reduces the weight of the sprocket.  
      While the present invention has been described and illustrated as being embodied in a snowmobile  10 , the present invention is not limited to snowmobiles. Rather, the present invention is considered applicable to the propulsion of endless tracks used on all types of tracked vehicles, including snow groomers, plows and muskeets.  
      Furthermore, Additional teeth may be provided on the sprocket  80 ,  380 ,  480  and/or the endless track  30  that do not correspond to any of the sprocket or track teeth  90 ,  100 ,  110 ,  120 ,  130 ,  140 . In other words, not every tooth on the sprocket  80  and track  30  needs to be one of the teeth  90 ,  100 ,  110 ,  120 ,  130 ,  140 .  
      The foregoing illustrated embodiments are provided to illustrate the structural and functional principles of the present invention and are not intended to be limiting. To the contrary, the principles of the present invention are intended to encompass any and all changes, alterations and/or substitutions within the spirit and scope of the following claims.