Patent Publication Number: US-6220378-B1

Title: Drive mechanism for a track type work machine having enhanced durability

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to drive mechanism for a track type work machine and more specifically to drive mechanism for a track type work machine which has enhanced durability. 
     BACKGROUND OF THE INVENTION 
     Work machines which utilize a track type drive mechanism have several advantages over wheel type work machines. For instance, the tracks give the work machine excellent traction by spreading the weight of the machine over a large area. Thus, the track type work machine has enhanced off road capabilities. 
     A typical undercarriage of a track type drive mechanism consists of a track having of a number of shoes linked together, a sprocket which drives the track, and a number of rollers which distribute the weight of the work machine along the track. 
     The shoes include a drive lug, and a number of links. The drive lugs engage the drive sprocket in order to advance the track. The links pivotably couple each shoe to the adjacent shoes so as to allow the track to advance in along an oval path. 
     Certain track configurations support the weight of the work machine on the number of links. In particular, the number of rollers are rotatably supported by the links during advancement of the track. A disadvantage to such a configuration is that the stress of advancing the track around the work machine and the stress of supporting the weight of the work machine are combined which may decrease the useful life of the number of links. 
     A further disadvantage to such a configuration is that the various wear surfaces of each of the links cannot be replaced independently of each other. In some abrasive environments, such as environments which include sand, the upper surface of the links which is in contact with the rollers are subjected wear at a greater rate than the surfaces of the links which pivotably couple the links together. Thus, it is likely that wear on the upper surface would require the entire link to be replaced even though the surfaces that pivotably couple the links together still possess significant useful life. 
     What is needed therefore is an apparatus and method for driving a track type work machine which overcomes one or more of the above-mentioned drawbacks. 
     DISCLOSURE OF THE INVENTION 
     In accordance with a first embodiment of the present invention, there is provided a drive mechanism for a work machine. The drive mechanism includes a sprocket drive assembly which includes a hub, and a plurality of engagement members each being supported by the hub. The drive mechanism further includes a track assembly having a plurality of shoes which are secured to one another to form a track and a plurality of drive lugs which are mounted to the plurality of shoes. The drive lugs engage the plurality of engagement members when the hub is rotated. The track assembly further has a support roller mechanically coupled to a chassis of the work machine. Each of the drive lugs includes a rail segment on which the support roller is supported during advancement of the track. 
     In accordance with a second embodiment of the present invention, there is provided a drive mechanism for a work machine. The drive mechanism includes a sprocket drive assembly which includes a hub, and a plurality of engagement members each being supported by the hub. The drive mechanism further includes a track assembly having a plurality of shoes which are secured to one another to form a track, and a plurality of drive lugs which are mounted to the plurality of shoes. The drive lugs engage the plurality of engagement members when the hub is rotated. Each of the drive lugs includes a left lateral rail segment and a right lateral rail segment. The track assembly further includes a support roller mechanically coupled to a chassis of the work machine. The support roller includes a left wheel and a right wheel. The left wheel is supported by the left lateral rail segment during advancement of the track. The right wheel is supported by the right lateral rail segment during advancement of the track. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a tracked type work machine which incorporates the features of the present invention therein; 
     FIG. 2 is a perspective view of a drive sprocket, a roller assembly, and a portion of the track assembly of the work machine of FIG. 1; 
     FIG. 3 is a cross sectional view of the drive sprocket, the roller assembly, and the portion of the track shown in FIG. 2 as view along the line  2 — 2  of FIG. 2 in the direction of the arrows; 
     FIG. 4 is a perspective view of a shoe assembly of the track assembly of FIG. 2; 
     FIG. 5 is an exploded view of the shoe assembly of FIG. 4; 
     FIG. 6 is a front elevational view of a roller assembly positioned on the track assembly of FIG. 2 as viewed along the line  2 — 2  in the direction of the arrows; 
     FIG. 7 is a perspective view of a roller supported by a shoe assembly of the track assembly shown in FIG. 2; 
     FIG. 8 is a fragmentary exploded view of the drive sprocket of FIG. 2; and 
     FIG. 9 is a front elevational view of a roller assembly positioned on the track assembly of FIG.  1 . 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     While the invention is susceptible to various modifications and alternative forms, a specific embodiment thereof has been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     Referring now to FIG. 1, there is shown a tracked type work machine  10  which incorporates the features of the present invention therein. The work machine  10  includes a chassis  12 , an engine assembly  14 , a work implement  16  and a track drive mechanism  18 . It should be noted that the work machine  10  further includes a second track drive mechanism (not shown) substantially identical to the mechanical configuration of the track drive mechanism  18  shown in FIG.  1 . As a result, only the track drive mechanism  18  is described in detail. 
     The engine assembly  14  is mounted to the chassis  12 . The engine assembly  14  includes an engine (not shown) and a transmission (not shown). The engine generates mechanical energy which is transferred to the transmission which outputs the mechanical energy to a drive shaft  20  as shown in FIG.  2 . The transmission allows an operator to selectively change the gear ratios between the engine and the drive shaft  20 . The change of ratios allows the engine assembly  14  to provide a range of torque outputs to the track drive mechanism  18  for various operating conditions. In particular, high torque may be required at low speeds whereas low torque may be required at high speeds. 
     Referring now to FIG. 2, the track drive mechanism  18  includes a drive sprocket  22 . The drive sprocket  22  includes a hub  24  which is mechanically coupled to the drive shaft  20 , such that rotation of the drive shaft  20  in the general direction of arrow  26  causes the hub  24  to rotate in the general direction of arrow  26 , whereas rotation of the drive shaft  20  in the general direction of arrow  28  causes the hub  24  to rotate in the general direction of arrow  28 . The outer surface of the hub  24  includes twelve mounting tabs  30  defined thereon. Each of the mounting tabs  30  projects radially outward from the center of the hub  24 . 
     Referring now to FIG. 8, the drive sprocket  22  further includes a first side support  32  secured to the hub  24 . In particular, the first side plate  32  is secured to each of the tabs  30  by a fastener, such as a bolt  34 . More specifically, the bolt  34  is inserted through an aperture in the first side support  32  and received into a threaded aperture  35  defined in the right side of each of the tabs  30 . It should be appreciated that the first side support plate  32  is embodied as a two of separate components as shown in FIG. 2 in order to facilitate maintenance of the drive sprocket  22 . In particular, the first side support plate  32  is composed of two semi circular portions  32 ′ so as to allow one semi circular portion  32 ′ of the first support plate  32  to be removed, while the other semicircular portion  32 ′ remains secured to the hub  24 . 
     The drive sprocket  22  further includes a second side support  36  secured to the hub  24 . In a manner similar to the first side plate  32 , the second side plate  36  is secured to each of the tabs  30  by a fastener, such as a bolt  37 . More specifically, the bolt  37  is inserted through an aperture in the second side support  36  and received into a threaded aperture  38  shown in FIG. 3 defined in the left side of each of the tabs  30 . It should be appreciated that the second side support plate  36  is embodied as a two of separate components in order to facilitate maintenance of the drive sprocket  22 . In particular, the second side support plate  36  is composed of two semi circular portions  36 ′ shown in FIG. 3 so as to allow one semi circular portion  36 ′ of the second support plate  36  to be removed, while the other semicircular portion  36 ′ remains secured to the hub  24 . 
     The first side support plate  32  includes twelve projections, each of which has a pin aperture  40  defined therethrough. Similarly, each of the second side support plates  36  has twelve projections, each of which has a pin apertures  41  defined therethrough. 
     A bushing or engagement member  44  is configured to span the distance between the first engagement plate  32  and the second engagement plate  36  between the aperture  40  of the first engagement plate  32  and the aperture  41  of the second engagement plate  36 . In particular, the engagement member  44  is rotatably secured to both the first side support plate  32  and the second side support plate  36 . More specifically, each engagement member  44  has a pin opening  46  defined therethrough (see FIG.  8 ). A pin  42  is received through the aperture  41  in the second side support  36 , through the pin opening  46  of the engagement member  44 , and through the aperture  40  of the first side support plate  32 . Furthermore, a pin fastener  43  secures the pin  42  to the first side support  32  and the second side support  36  such that the pin  42  is prevented from moving in the general direction of arrows  48  and  50 . It should be appreciated that each of the twelve engagement members  44  are free to rotate relative to the respective pin  42  in the general direction of arrows  26  and  28 . 
     Referring now to FIG. 1, the work machine  10  further includes a roller frame  52  welded to the chassis  12 . Five roller assemblies  54  are attached to the underside of the roller frame  52 . 
     Referring now to FIG. 7, each roller assembly  54  includes a right roller mount  56  and a left roller mount  58 . The right roller mount  56  has two fastener apertures  61  defined therein which allows a fastener (not shown) to secure the right roller mount  56  to the underside of the roller frame  52 . Similarly, the left roller mount  58  has two fastener apertures  61  defined therein which allows a fastener (not shown) to secure the left roller mount  58  to the underside of the roller frame  52 . Moreover, the right roller mount  56  includes a right axle aperture  60  defined in the lower portion of the right roller mount  56  whereas the left roller mount  58  includes a left axle aperture  59  defined in the lower portion of the left roller mount  56 . 
     Referring now to FIGS. 6 and 7, the roller assembly  54  further includes a roller  62  and an axle  64 . The roller  62  has an axle aperture  66  shown in FIG. 6 defined therein. In particular, the axle  64  is received through the axle aperture  66 . More specifically, a set of bearings (not shown) are interposed between the axle  64  and the axle aperture  66  in order to reduce frictional losses between the axle  64  and the roller  62 . Furthermore, the roller  62  is free to rotate relative to the axle  64  in the general directions of arrows  70  and  72 . 
     A right end of the roller axle  64  is then secured to the right axle aperture  60  defined in the lower portion of the right roller mount  56  whereas a left end of the roller axle  64  is secured to the left axle aperture  59  defined in the lower portion of the left roller mount  58 . Thus, the roller assembly  54  is secured to the roller frame  52 , and the roller  62  is free to rotate relative to the roller frame  52  in the general direction of arrows  70  and  72 . It should be appreciated that work machine  10  includes five roller assemblies secured to the roller frame  52  each of which are substantially identical in configuration. 
     Referring now to FIGS. 4 and 5, the tracked drive mechanism  18  includes thirty seven shoe assemblies  80 . Each shoe assembly  80  includes a shoe  81 , a right link  82  and a left link  92 . The shoe  81  has a tread  83  defined therein. The tread  83  projects downwardly from the main body of the shoe  81 . 
     The right link  82  includes a right link strap  86 , a right pin receptacle  88 , and a right link pin  90 . The right link  82  is secured to the shoe  81 . In particular, the right link strap  86  of the right link  82  is secured to the shoe  81  by a pair of fasteners such as bolts  89  received through a pair of apertures  87  and secured to a pair of nuts  85 . 
     In a similar manner, the left link  92  includes a left link strap  96 , a left pin receptacle  98 , and a left link pin  100 . The left link  92  is secured to the shoe  81 . In particular, the left link strap  96  of the left link  92  is secured to the shoe  81  by a pair of fasteners such as bolts  98  received through a pair of apertures  87  and secured to a pair of nuts  85 . 
     In order to form a track assembly  99 , thirty seven shoe assemblies  80  are linked together as shown in FIG.  1 . In particular, the right link pin  90  of a first shoe assembly  80  is configured to be received by the right pin receptacle  88  of an adjacent shoe assembly  80 ′ as shown in FIG.  7 . It should be appreciated that the right link pin  90  allows the first shoe assembly  80  to rotate relative to the second shoe assembly  80 ′ at the right link pin  90  in the general direction of arrows  102  and  104 . Furthermore, the right pin receptacle  88  of the first shoe assembly  80  prevents movement of the second shoe assembly  80 ′ in the general direction of arrows  108 , while permitting movement of the second shoe assembly  80 ′ with respect to the first shoe assembly  80  in the general direction of arrow  106 . 
     Similarly, the left link pin  100  of a first shoe assembly  80  is configured to be received by the left pin receptacle  98  of an adjacent shoe assembly  80 ′ as shown in FIG.  7 . It should be appreciated that the left link pin  100  allows the first shoe assembly  80  to rotate relative to the second shoe assembly  80 ′ in the general direction of arrows  102  and  104 . Furthermore, the right pin receptacle  98  of the first shoe assembly  80  prevents movement of the second shoe assembly  80 ′ in the general direction of arrows  106 , while permitting movement of the second shoe assembly  80 ′ with respect to the first shoe assembly  80  in the general direction of arrow  108 . 
     It should be appreciated that the right pin receptacle  88  of the first shoe assembly  80  prevents the movement of the right link  82  of the second shoe assembly  80 ′ in the general direction of arrow  106  whereas the left pin receptacle  98  of the first shoe assembly  80  prevents the movement of the left link  92  of the second shoe assembly  80 ′ in the general direction of arrow  108 . Thus, when the right link  82  of the first shoe assembly  80  is coupled to the right link  82  of an adjacent second link  80 ′ and the left link  92  of the first shoe assembly  80  is coupled to the left link  92  of the adjacent second shoe assembly  80 ′, the first shoe assembly  80  is coupled to the second shoe assembly  80 ′ such that the second shoe assembly  80 ′ can only rotate relative to the link pins  90 ,  100  in the general directions of arrows  102  and  104  and is restricted from moving in either the general direction of arrow  106  or the general direction of arrow  108 . It should further be appreciated that the right link pin  90  and the left link pin  100  share a common pivot axis  101 . The first shoe assembly  80  rotates about the pivot axis  101  as the first shoe assembly  80  moves relative to the second shoe assembly  80 ′. 
     Referring now to FIG. 1, the track drive assembly  18  further includes a carrier roller  110  and an idler roller  112 . The carrier roller  110  is rotatably coupled to the chassis  12  of the work machine  10  such that the carrier roller  110  is free to rotate relative to the work machine  10  in the general direction of arrows  114  and  116 . The idler roller  112  is rotatably secured to the roller frame  52  such the idler roller  112  is free to rotate relative to the roller frame  52  in the general direction of arrows  114  and  116 . It should be appreciated that the drive sprocket  22 , the carrier roller  110 , the idler roller  112 , and the five rollers assemblies  54  define a generally oval path around the roller frame  52 . 
     It should be appreciated that thirty seven shoe assemblies  80  are required to form a track assembly  99 . It should further be appreciated that the last shoe assembly is linked to the first shoe assembly in order to form the generally oval shape track assembly  99 . It should still further be appreciated that the track assembly  99  extends around the oval path defined by drive sprocket  22 , the carrier roller  110 , the idler roller  112 , and the five rollers assemblies  54 . 
     Referring now to FIGS. 4,  5 , and  7 , the shoe assembly  80  further includes a drive lug  118  secured to the shoe  81 . In particular, a pair of bolts  91  are inserted through a pair of apertures  93  in the shoe  81  and rotatably received through a threaded aperture (not shown) on the underside of the drive lug  118  as shown in FIG.  5 . 
     The drive lug  118  includes a central drive segment  122  which has a first grasping notch  124  and a second grasping notch  126  defined therein. It should be appreciated that the first grasping notch  124  of a shoe assembly  80  lies proximate to the second grasping notch  126  of an adjacent shoe assembly  80 ′ as shown in FIG.  2 . It should further be appreciated that the pivot axis  101  lies between the first gasping notch  124  of a first shoe assembly  80  and the second grasping notch  126  of an adjacent second shoe assembly  80 ′ such that when the second shoe assembly  80 ′ is rotated relative to the first shoe assembly  80  about the pivot axis  101  in the general direction of arrow  104 , the upper portion of the second grasping notch  126  of the second shoe assembly  80 ′ is moved toward the upper portion of the first grasping notch  124  of the first shoe assembly  80 . Similarly, when second shoe assembly  80 ′ is rotated relative to the first shoe assembly  80  about the pivot axis  101  in the general direction of arrow  102 , the upper portion of the second grasping notch  126  of the second shoe assembly  80 ′ is moved away from the upper portion of the first grasping notch  124  of the first shoe assembly  80 . 
     Referring now to FIG. 3, as the drive sprocket is rotated in the general direction of arrow  28 , the lower portion of the track assembly  99  engages the engagement members  44  of the drive sprocket  22  as the lower portion of the track is advanced in the general direction of arrow  128 . In particular, pairs of shoe assemblies  80  cooperate to engage a respective engagement member  44  of the drive sprocket  22 . Similarly, the upper portion of the track assembly  99  disengages an engagement member  44  of the drive sprocket  22  as the upper portion of the track assembly  99  advances in the general direction of arrow  129 . 
     Shoe assemblies  80 A through  80 J show the progressive stages of the track assembly  99  engaging and disengaging a respective engagement member  44  of the drive sprocket  22 . Shoe assembly  80 A and shoe assembly  80 B are not engaged with the drive sprocket  22 . However, the shoe assembly  80 A and the shoe assembly  80 B are being positioned to engage the engagement member  44 A. In particular, as the engagement member  44 A is moving in the general direction of arrow  28 , the engagement member is being positioned between the first grasping notch  124  of the shoe assembly  80 A and the second grasping notch  126  of the shoe assembly  80 B. 
     As the drive sprocket  22  advances in the general direction of arrow  28 , a pair of shoe assemblies is moved into a position shown by shoe assembly  80 B and  80 C. In this position, the rotation of the drive sprocket  22  places the engagement member  44 B between the shoe assembly  80 B and the shoe assembly  80 C. In particular, the engagement member  44 B is positioned between the first grasping notch  124  of the shoe assembly  80 B and the second grasping notch  126  of the shoe assembly  80 C. It should be appreciated that the engagement member  44 B is positioned between the shoe assembly  80 B and the shoe assembly  80 C. However, the engagement member  44 B is not engaged to the track assembly  99 . 
     As the drive sprocket  22  advances in the general direction of arrow  28 , a pair of shoe assemblies are moved into a position shown by shoe assembly  80 C and  80 D. As the shoe assembly  80 D moves around the drive sprocket  22 , the shoe assembly  80 D rotates relative to the shoe assembly  80 C in the general direction of arrow  104  about the pivot axis  101  defined by the right link pin  90  and the left link pin  100  as shown in FIG.  2 . As discussed above, when the shoe assembly  80 D rotates relative to an adjacent shoe assembly  80 C, the upper portion of the second grasping notch  126  of the shoe assembly  80 D is moved toward the upper portion of the first grasping notch  126  of the shoe assembly  80 C. 
     As the second grasping notch  124  of the shoe assembly  80 D moves toward the grasping notch  126  of the shoe assembly  80 C, the track assembly  99  engages the engagement member  44 C of the drive sprocket  22 . In particular, the first grasping notch  124  of the shoe assembly  80 C applies a force against the engagement member  44 C in the general direction of arrow  104  whereas the second grasping notch  126  of the shoe assembly  80 D applies a force against the engagement member  44 C in the general direction of arrow  102 . Thus, the engagement member  44 C is engaged to the track assembly  99  by the first grasping notch  124  of the shoe assembly  80 C and second grasping notch  126  of the shoe assembly  80 D. 
     It should be appreciated that as the drive sprocket  22  advances in the general direction of arrow  28 , a pair of shoe assemblies remains engaged to the sprocket  22  until the pair of shoe assemblies are moved into a position shown by shoe assembly  80 H and shoe assembly  80 I. It should still further be appreciated that the engagement of the track assembly  99  to a rotating engagement member  44  allows the engagement member  44  to remain engaged to the track assembly  99  even though the engagement member  44  may rotate relative to the drive sprocket  22  thus preventing the sliding contact between the engagement member  44  and the track assembly  99 . Eliminating the sliding contact reduces the wear caused by the first grasping notch  124  and second grasping notch  126  of the drive lugs  118  of the track assembly  99  engaging and disengaging the engagement members  44  of the drive sprocket  22 . 
     As the pair of shoe assemblies are moved into a position shown by shoe assembly  80 I and  80 J, the track assembly  99  disengages from the drive sprocket  22 . In particular, as the drive sprocket  22  is rotated in the general direction of arrow  28 , the engagement member  44 E is moved away from the first grasping notch  124  of the shoe assembly  80 I and away from the second grasping notch  126  of the shoe assembly  80 J in the general direction of arrow  28 . In addition, as the track assembly  99  advances in the general direction of arrow  129 , the shoe assembly  80 J rotates relative the shoe assembly  80 I such that the upper portion of the first grasping notch  124  of the shoe assembly  80 I is moved away from the second grasping notch  126  of the shoe assembly  80 J. Thus, the force of the first notch  124  of the shoe assembly  80 I and the force of the second notch  126  of the shoe assembly  80 J are removed from the engagement member  44 E allowing the engagement member  44 E to disengage from the track assembly  99 . 
     It should be appreciated that the track assembly  99  advances in a clockwise manner in the direction of arrows  128 ,  28 , and  129  respectively due to the movement of the drive shaft  20  shown in FIG. 2, which rotates the drive sprocket  22  in the general direction of arrow  28 . As the drive sprocket  22  is rotated in the general direction of arrow  28 , the engagement members  44  move the track assembly  99  in the general direction of arrow  28 . In particular, the shoe assemblies  80 C,  80 D,  80 E,  80 F,  80 G,  80 H, and  80 I are fully or partially engaged to a respective pair of engagement members  44  of the drive sprocket  22  such that rotation of the drive sprocket in the general direction of arrow  28  moves the corresponding engaged shoe assembly  80  of the track assembly  99  in the general direction of arrow  28 . In addition, rotation of the drive sprocket in the general direction of arrow  26  moves the corresponding engaged shoe assembly  80  of track assembly  99  in the general direction of arrow  26 . 
     Referring now to FIGS. 4 through 6, the drive lug  118  further includes a right rail  130  and a left rail  132 . The right rail  130  and the left rail  132  are steel or composite surfaces that are highly resistant to wear. The roller  62  further includes a right wheel  134  and a left wheel  136  as shown in FIG.  6 . The right wheel  134  and the left wheel  136  are spaced apart such that the right wheel  134  and the left wheel  136  straddle the central drive segment  122  of the drive lug  118 . 
     By straddling the central drive segment  122 , the roller  62  is aligned with the central drive segment  122  ensuring that the track assembly  99  is properly aligned with the rollers  62  of the roller assemblies  54 . It should be appreciated that each of the five rollers  62  are positioned such that the rollers  62  align the track assembly  99  with the engagement members  44  of the drive sprocket  22 . Thus, alignment of the track assembly  99  with the rollers  62  aligns the track assembly  99  with the engagement members  44  of the drive sprocket  22 . It should further be appreciated that large size of the drive portion  122  of the drive lugs  124  makes it less likely that the drive portion  122  will jump out of alignment with the right wheel  134  and the left wheel  136  as the track assembly  99  is advanced. 
     The right wheel  134  of the roller  62  is supported by the right rail  130 . Similarly, the left wheel  136  is supported by the left rail  132 . It should be appreciated that the rollers  62  support the entire weight of the work machine  10 . It should be appreciated that the weight of the work machine supported on the right rail  130  and the left rail  132  is further supported and dispersed by the shoe  81 . Dispersing the weight of the work machine  10  is an advantage of the tracked type work machine  10 . The low ground pressure resulting from dispersing the weight over the shoes  81  allows the tracked type work machine  10  to operate in conditions where a wheel type work machine would be unsuitable. It should further be appreciated that the tread  83  of each shoe  81  provides traction as the track assembly  99  is moved in the general direction of arrows  128  or  129 . 
     In the present invention, the right link  82  is spaced apart from the right rail  130  whereas the left link  92  is spaced apart from the left rail  132 . Many track assembly configurations for tracked type work machines combine the functions of the right link  82  with the right rail  130  and combine functions of the left link  92  with the left rail  132 . 
     Separating the rails  130 ,  132  from the links  82 ,  92  allows the stress of moving the rails to be primarily carried by the links  82 ,  92  whereas the stress of supporting the weight of the work machine  10  is primarily carried by the rails  130 ,  132 . Thus, the links  82 ,  92  can be replaced independently of the rails  130  and  132  on the same shoe assembly  80 . 
     Referring now to FIG. 9, the idler roller  112  further includes a right rim  140 , a left rim  142 , and a central portion  144 . The idler roller  112  is in contact with the shoe assemblies  80  as the track assembly  99  is advanced. In particular, the central portion  144  of the idler roller  112  is in contact with an upper surface  123  of the central drive segment  122  of the drive lug  118 . As shown in FIG. 1, the track assembly movably contacts approximately half or one hundred eighty degrees of the outer circumference of the idler roller  112  at any given point in time. Thus, most of the wear on the shoe assemblies  80  caused by advancing the track assembly  99  around the idler roller  112  will occur on surface  123 . 
     It should be appreciated that the components of the shoe assembly  80  that are most likely to need replacement due to wear are all included in the drive lug  118 . In particular, the first grasping notch  124  and the second grasping notch  126  are subject to wear from engaging and disengaging engagement members  44  of the drive sprocket  22  as the track assembly  99  is advanced. In addition, the rails  130 ,  132  are subject to wear as the rollers  62  move across the rails  130 ,  132 . Moreover, the upper surface  123  of the drive lug  118  is subject to wear as the track assembly  99  is advanced around the idler roller  112 . Thus, replacement of the drive lug  118 , replaces the five components of the shoe assembly  80  that are most subject to wear: the first grasping notch  124 , the second grasping notch  126 , the right rail  130 , the left rail  132 , and the upper surface  123 . 
     Operation of the work machine  10  in different environments causes the drive lug  118  to wear at different rates as compared to the links  82 ,  92 . Combining multiple wear surfaces into the drive lug  118  allows the material composition of the wear surfaces to be changed without changing the links  82 ,  92 . The track assembly  99  can be configured for the various environments by altering the material composition of a single component (i.e. the drive lugs  118 ). In a high wear environment, the track assembly  99  can be configured with drive lugs made of a more wear resistant material to increase the life of the drive lugs  118 . In a low wear environment, the track assembly  99  can be configured with drive lugs made of a less costly less wear resistant material. 
     A further advantage of the present invention is that the track drive mechanism  18  can be configured for use in an abrasive environment, such an environment that includes sand, which increases the rate the rails  130 ,  132  wear. An advantage to flat rails  130 ,  132  which are not combined with the respective links  82 ,  92  is that a resilient material, such as rubber, can be applied to the upper surface of the rails  130 ,  132  in order to reduce the wear on the rails  130 ,  132 . In addition, the resilient material may also be applied to the outer surfaces of the wheels  134 ,  136  which contact the rails  130 ,  132  to further reduce wear of the rails  130 ,  132  when operating in an abrasive environment. Moreover, the resilient material has the added advantage of reducing noise generated during advancement of the track assembly  99 . 
     INDUSTRIAL APPLICABILITY 
     In operation, the track assembly  99  is formed by coupling the right links  82  and the left links  92  of the thirty seven shoe assemblies  80 . The track assembly  99  is mounted on the work machine  10  in a manner that follows the generally oval path defined by the drive sprocket  22 , the carrier roller  110 , the idler roller  112 , and the five rollers assemblies  54  as shown in FIG.  1 . The central drive segment  122  of the drive lugs  118  of each of the shoe assemblies  80  cooperate with the first wheel  134  and the second wheel  136  of the roller assemblies  54  to align the track assembly  99  with the rollers  62  as shown in FIG.  6 . 
     In addition, the right rail  130  and the left rail  132  of the drive lug  118  support the right wheel  134  and the left wheel  136  respectively of the roller assemblies  54  which in turn support the entire weight of the work machine  10 . The rails  130 ,  132  transfer this weight to the shoes  81  which in turn distribute the weight of the work machine  10  over a large area. The right rail  130 , left rail  132 , right wheel  134 , and left wheel  136  can alternately have a resilient material, such as rubber, applied thereto in order to reduce wear on the right rail  130  and left rail  132  when operating in an abrasive environment. 
     In order to drive the track assembly  99 , the engine assembly  14  provides power to rotate a drive shaft  20  which rotates the drive sprocket  22  in the general direction of arrows  26  or  28 . The power is transferred from the drive sprocket  22  to the track assembly  99  by a rotating engagement member  44  attached to the drive sprocket  22 . In particular, as the track assembly  99  is advanced around the sprocket assembly  22 , the first grasping notch  124  of a first shoe assembly  80  cooperates with the second grasping notch  126  of an adjacent shoe assembly  80  to engage the rotatable engagement member  44  of the drive sprocket  22 . Since the engagement member  44  is free to rotate in the general direction of arrows  26  and  28  relative to the drive sprocket  22 , the amount of friction and wear between the drive sprocket  22  and the track assembly  99  is reduced. 
     As the lower portion track assembly  99  moves in the general direction of arrows  128  or  129 , the treads  83  of the each of the shoe assemblies  80  engages the ground in order to move the tracked type work machine  10 . In particular, the track type work machine  10  is moved to position the work implement  16  as required for a work operation. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description is to be considered as exemplary and not restrictive in character, it being understood that only the preferred embodiment has being shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. 
     For example, although the drive lugs  118  are configured as a single component, and have significant advantages thereby in the present invention, the drive lugs  118  could be embodied with the rails  130 ,  132  as separate components from the drive segment  122 . Such a configuration would allow the rails  130 ,  132  to replaced independently of the drive segment  122 . 
     Moreover, although the idler roller  112  is configured to contact the drive lugs  118  on the upper surface  123 , and has significant advantages thereby in the present invention, the right rim  140  and the left rim  142  could be configured to contact the right rail  130  and the left rail  132  respectively. Such a configuration would place the wear caused by advancing the track assembly  99  around the idler roller  112  on the rails  130 ,  132  rather than the upper surface  123 .