Patent Publication Number: US-2016236732-A1

Title: Track-module apparatus with load-independent load distribution

Description:
FIELD OF THE INVENTION 
     The invention relates generally to the field of vehicle track-module systems of the type typically for use in place of vehicle wheels and, more particularly, to track modules having leading and trailing wheels and at least one load-supporting bogie wheel between the leading and trailing wheels, all of which are engaged by an endless track extending around the wheels to drive a vehicle along the ground. 
     BACKGROUND OF THE INVENTION 
     Agricultural vehicles such as tractors, combines and the like are commonly used in agricultural fields for a variety of jobs, and construction vehicles and other large work vehicles are used for many different jobs on a variety of ground surfaces. Typically, these vehicles have large wheels with tires on which the vehicles are supported on the ground. However, for improved traction, vehicle track-module systems (or “track modules” or “track-module apparatus”) are used in place of wheels with tires, and such track-module systems provide a much larger ground-surface engagement area supporting vehicle weight and tends to prevent vehicles from becoming bogged down in mud or other soft ground surfaces. 
     Among the challenges encountered in the use of vehicle track-module apparatus is the need to distribute the load supported by the track module among the various wheels. These loads are both static and dynamic and may change during operation of the vehicle. Loads change as the vehicle encounters uneven ground, as the vehicle turns and as the slope of the ground being traversed changes. Ideally, all wheels remain in contact with the ground through the endless belt and share a portion of the load at all times. 
     One track-module unit which is intended to distribute load relatively evenly is disclosed in U.S. Pat. No. 7,628,235 (Satzler et al.) owned by CLAAS Industrietechnik GmbH of Paderhorn, Germany. A vehicle track roller unit is disclosed which has at least one pivotable subframe and at least one further pivotable subframe, and each of the subframes rotatably accommodates at least one land wheel. At least one subframe is pivotably mounted on the vehicle, and the at least one further subframe is pivotably mounted on the at least one pivotable subframe. 
     Another vehicle track-module unit is disclosed in United States Published Patent Application No. 2013/0154345 (Schultz et al.) owned by CLAAS Selbstfahrende Erntemaschinen GmbH of Harsewinkel, Germany. A vehicle track unit is disclosed which has a plurality of supporting rollers arranged one behind the other in the direction of travel of the vehicle and around which a belt is wrapped. The rollers are adjusted by way of at least one actuator between a first configuration, in which all supporting rollers are loaded, and a second configuration, in which at least one outer roller of the supporting rollers is relieved. An energy source delivers drive energy required to adjust the configuration. An energy accumulator is charged by the drive energy source and connected to the actuator in order to provide the actuator with the drive energy required to adjust the configuration. 
     CLAAS also has its Lexion Terra Trac product line which includes configurations which are intended to address some of these challenges. However, none of these prior art systems includes all of the elements of the present invention and meets the needs as outlined above. 
     OBJECTS OF THE INVENTION 
     It is an object of this invention to provide an improved vehicle track-module apparatus which has high load-supporting capability while maintaining lower contact forces on the ground by providing lower loading per axle from more even load distribution. 
     Another object of this inventive vehicle track-module apparatus is to minimize the unsprung mass of the track-module apparatus. 
     Another object of the inventive vehicle track-module apparatus is to provide track-module apparatus which shares load changes between axles. 
     A further object of the inventive vehicle track-module apparatus is to provide track-module apparatus in which each axle is able to move vertically in an independent manner. 
     Yet another object of the inventive vehicle track-module apparatus is to provide track-module apparatus in which the load distribution on the wheels is independent of vertical load. 
     Still another object of the inventive vehicle track-module apparatus is to provide track-module apparatus which has independent roll-mode movement for all bogie axes. 
     An additional object of this invention is to provide improved vehicle track-module apparatus which includes an articulating bogie assembly and which includes independent accommodation of bogie roll motion. 
     Yet another object of the present invention is to provide track-module apparatus which reduces wear on the flexible track. 
     These and other objects of the invention will be apparent from the following descriptions and from the drawings. 
     BRIEF SUMMARY OF THE INVENTION 
     This invention is an improvement in vehicle track-module apparatus which includes a module frame, a drive wheel rotatable with respect to the frame, leading and trailing ground-engaging wheels, at least one ground-engaging bogie wheel, and an endless track extending around the wheels and driven by its engagement with the drive wheel. 
     The inventive improvement comprises: (1) a leading suspension arm rotatably attached to the frame at a leading arm axis and extending forwardly to a leading-arm distal end at which a leading-wheel assembly is rotatably attached and rearwardly to a rearward suspension end; (2) a trailing suspension arm rotatably attached to the frame at a trailing arm axis and extending rearwardly to a trailing-arm distal end at which a trailing-wheel assembly is rotatably attached and forwardly to a forward suspension end; (3) a bogie assembly having a bogie mount and the at least one leading bogie wheel rotatable with respect thereto; and (4) leading and trailing suspension elements rotatably attached to and extending from the rearward and forward suspension ends, respectively, to rearward and forward rotatable bogie-mount connections, respectively. 
     The term “axis” as used herein pertains to a pivot joint which includes the necessary components such as bearings and other structure to permit rotation about such axis. A more complete description of the use of this and related terms is presented later in this document. 
     The term “suspension elements” as used herein refers to components in the suspension system which provide spring force and/or damping in the system. 
     The term “ground-engaging” as used herein with respect to a wheel means that the wheel bears on the ground through the endless track that engages the wheel under normal operating conditions. 
     The term “bogie wheel(s)” as used herein refers to one or more wheels providing support for a vehicle in a middle ground-engaging region of a track module, with other ground-engaging support being provided rearward and/or forward of the bogie wheels(s). 
     The term “therebetween” when referring to the position of ground-engaging bogie wheels means that the bogie wheels are positioned behind the leading ground-engaging wheel(s) and ahead of the trailing ground-engaging wheel(s) along the direction of travel. The term “idler” as used herein refers to wheel which is not a driven wheel but turn only by virtue of its engagement with the endless track. 
     The term “load- and ground-responsive” as used herein with respect to suspension joints means that the above-ground positions of such joints are variable, including with respect to the frame, and depend on the contour of the ground under the track and on the total loading on the track module, however caused. 
     In some preferred embodiments of the improved vehicle track-module apparatus, the leading-arm and trailing-arm axes coincide at a suspension-arm axis. In some such embodiments, the suspension-arm axis is at a level lower than the drive wheel axis, and in some of these embodiments, the suspension-arm axis is rearward of drive wheel axis. 
     In highly-preferred embodiments, (a) the bogie assembly further includes a bogie-assembly arm rotatably attached (i) at a bogie-assembly arm distal end to the leading suspension arm at a first bogie-assembly axis between the leading-arm distal end and the suspension-arm axis and (ii) at a bogie-assembly arm proximal end to the bogie mount at a second bogie-assembly axis, (b) the leading suspension element is rotatably attached (i) to the rearward suspension end at a leading suspension-element pivot and (ii) to the rearward bogie-mount connection at a first bogie-assembly pivot, and (c) the trailing suspension element is rotatably attached (i) to the forward suspension end at a trailing suspension-element pivot and (ii) to the forward bogie-mount connection at a second bogie-assembly pivot. 
     In some of these highly-preferred embodiments, the at least one bogie wheel includes at least one leading bogie wheel and at least one trailing bogie wheel, and the bogie mount includes (a) a bogie-mount forward portion having the at least one leading bogie wheel attached thereto at a leading bogie axis, the bogie-mount forward portion including the forward bogie-mount connection, and (b) a bogie-mount rearward portion having the at least one trailing bogie wheel attached thereto at a trailing-bogie axis, the bogie-mount rearward portion including the rearward bogie-mount connection. Some of these embodiments may also include at least two leading bogie wheels and at least two trailing bogie wheels, and in these embodiments, the leading bogie axis rotates on a leading bogie roll axis perpendicular thereto and the trailing bogie axis rotates on a trailing bogie roll axis perpendicular thereto. 
     In some of the preferred embodiments, the bogie-mount forward and rearward portions are rotatably attached at a third bogie-assembly axis, and in some of these embodiments, the bogie mount further includes a bogie-mount middle portion having at least one middle bogie wheel attached thereto at a middle bogie axis. 
     In other highly-preferred embodiments, the inventive vehicle track-module apparatus includes at least two leading bogie wheels, at least two trailing bogie wheels, and at least two middle bogie wheels and (a) the leading bogie axis rotates on a leading bogie roll axis perpendicular thereto, (b) the middle bogie axis rotates on a middle bogie roll axis perpendicular thereto, and (c) the trailing bogie axis rotates on a trailing bogie roll axis perpendicular thereto. In some of these embodiments, the bogie-mount forward and middle portions are rotatably attached at a third bogie-assembly axis. 
     In certain highly-preferred embodiments, the vehicle track-module apparatus further includes a tensioning element having first and second ends, the first end rotatably attached to the leading suspension arm at a proximal tensioning pivot between the leading-arm distal end and the suspension-arm axis. In such embodiment, the leading-wheel assembly includes (a) at least one leading wheel rotatable about a leading-wheel axis and (b) a wheel linkage at the leading-wheel axis. The distal end of the leading suspension arm is rotatably attached to the wheel linkage at a wheel offset axis offset from the leading-wheel axis and the second end of the tensioning element is rotatably attached to the wheel linkage at a distal tensioning pivot offset from the leading-wheel axis. Also, the tensioning axes is angularly displaced therearound such that the wheel linkage is a class  2  lever with the wheel offset axis being the fulcrum thereof. In some of these embodiments, the trailing-wheel assembly includes a trailing-wheel axis at the trailing-arm distal end and at least one trailing wheel rotatable about the trailing-wheel axis. 
     In some highly-preferred embodiments, the rotatable attachments of the leading and trailing suspension elements at the rearward and forward suspension ends, respectively, are configured to permit rotation having at least two degrees-of-freedom and the rearward and forward rotatable bogie-mount connections are configured to permit rotation having at least two degrees-of-freedom. 
     In certain highly-preferred embodiments, the rotatable attachments of the first and second ends of the tensioning element are configured to permit rotation having at least two degrees-of-freedom. 
     In certain highly-preferred embodiments, the leading and trailing suspension elements include gas-filled components to provide spring force. In some of these embodiments, the leading and trailing suspension elements further include hydraulic piston and cylinder components. Also, in some of these embodiments, the leading and trailing suspension elements are on a common hydraulic circuit. And some of these embodiments further include an external accumulator hydraulically connected to the common hydraulic circuit. 
     In certain embodiments of the inventive vehicle track-module apparatus, the leading and trailing wheels have equal diameters, and in some embodiments, the bogie wheels have equal diameters. 
     In another aspect of the invention, the leading wheel(s) are leading idler wheel(s), the trailing wheel(s) are trailing idler wheel(s), the drive wheel is positioned above and between the idler wheels, the leading-wheel assembly is a leading-idler assembly, and the trailing-wheel assembly is a trailing-idler assembly. 
     Some embodiments of the inventive track-module apparatus are configured such that the drive wheel is driven by a driving mechanism configured to be powered by the vehicle, and in some of these embodiments, the driving mechanism is a gearbox attachable to a driven axle of a vehicle. 
     Some embodiments of the inventive vehicle track-module apparatus include a tensioning element having first and second ends with the first end rotatably attached to the trailing suspension arm at a proximal tensioning pivot between the trailing-arm distal end and the trailing arm axis. In these embodiments, the trailing-wheel assembly includes (a) at least one trailing wheel rotatable about a trailing-wheel axis and (b) and a wheel linkage at the trailing-wheel axis. The distal end of the trailing suspension arm is rotatably attached to the wheel linkage at a wheel offset axis offset from the trailing-wheel axis; the second end of the tensioning element is rotatably attached to the wheel linkage at a distal tensioning pivot which is offset from the trailing-wheel axis; and the distal tensioning pivot and the wheel offset axis are angularly displaced around the trailing-wheel axis such that wheel linkage is a class  2  lever with the wheel offset axis being the fulcrum thereof. In some of these embodiments, the leading-wheel assembly includes a leading-wheel axis at the leading-arm distal end and at least one leading wheel rotatable about the leading-wheel axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective drawing of an embodiment of the vehicle track-module apparatus of this invention. 
         FIG. 1B  is a perspective drawing of the embodiment of  FIG. 1A  as viewed from the side opposite that shown in  FIG. 1A . 
         FIG. 2  is a perspective drawing of portions of the embodiment of the vehicle track-module apparatus of  FIGS. 1A and 1B , illustrating the suspension linkage components without the drive wheel, endless polymeric track, wheels, frame and suspension elements. 
         FIG. 3  is an exploded perspective drawing of the embodiment of  FIGS. 1A and 1B . 
         FIG. 4  is a side-elevation drawing of the embodiment of  FIGS. 1A and 1B  with the near set of idler and bogie wheels removed to show the linkages. 
         FIG. 5  is side-elevation drawing of portions of the embodiment of the vehicle track-module apparatus of  FIGS. 1A and 1B , showing the suspension linkage components without the drive wheel and the endless track and wheels. 
         FIG. 6A  is a perspective drawing of the bogie mount portions of the bogie assembly of the vehicle track-module apparatus of  FIGS. 1A and 1B . 
         FIG. 6B  is a perspective drawing of the bogie mount of  FIG. 6A  showing the bogie-mount forward portion rotated with respect to the bogie-mount rearward portion around the third bogie-assembly axis. 
         FIGS. 7A-7F  are side-elevation drawings of the embodiment of  FIGS. 1A and 1B  illustrating the movement of the inventive vehicle track-module apparatus as it traverses over a small bump along its path of travel.  FIG. 7A  shows the track-module apparatus just prior to encountering the bump. 
         FIG. 7B  shows the track-module apparatus with its leading idler wheels over the bump. 
         FIG. 7C  shows the track-module apparatus with its leading bogie wheels over the bump. 
         FIG. 7D  shows the track-module apparatus with its middle bogie wheels over the bump. 
         FIG. 7E  shows the track-module apparatus with its trailing bogie wheels over the bump. 
         FIG. 7F  shows the track-module apparatus with its trailing idler wheels over the bump. 
         FIG. 8A  is a side-elevation drawing of the embodiment of  FIGS. 1A and 1B  illustrating the movement of the inventive vehicle track-module apparatus as it traverses an uphill path. 
         FIG. 8B  is a side-elevation drawing of the embodiment of  FIGS. 1A and 1B  illustrating the movement of the inventive vehicle track-module apparatus as it traverses a downhill path. 
         FIG. 9  is a schematic drawing of the leading and trailing suspension elements in a hydraulic circuit. 
         FIG. 10  is a schematic diagram of the embodiment of  FIGS. 1A and 1B  illustrating the supported load FL and the five resulting wheel loads F 1  through F 5 . 
         FIG. 11  is a side-elevation drawing (similar to  FIG. 4 ) of a first alternative embodiment of the vehicle track-module apparatus of this invention. Such embodiment is similar to the embodiment of  FIG. 4  but includes only leading and trailing bogie wheels with corresponding modifications to the components used in the suspension system. 
         FIG. 12  is a side-elevation drawing (similar to  FIG. 4 ) of a second alternative embodiment of the vehicle track-module apparatus of this invention. Such embodiment is similar to the embodiment of  FIG. 11  but does not include a tensioning element and the leading-idler assembly includes only the leading idler wheel. 
         FIG. 13  is a side-elevation drawing (similar to  FIG. 4 ) of a third alternative embodiment of the vehicle track-module apparatus of this invention. Such embodiment is similar to the embodiment of  FIG. 4  but does not include a tensioning element and the leading-idler assembly includes only the leading idler wheel. 
         FIG. 14  is a side-elevation drawing (similar to  FIG. 4 ) of a fourth alternative embodiment of the vehicle track-module apparatus of this invention. Such embodiment is similar to the embodiment of  FIG. 13  but does not include the third bogie-assembly axis. 
         FIG. 15A  is side-elevation drawing of portions of the embodiment of the vehicle track-module apparatus of  FIG. 1  to illustrate the detail of an end of the tensioning element. 
         FIG. 15B  is a sectional view of  FIG. 15A . 
         FIG. 15C  is an enlargement of a portion of  FIG. 15B  particularly showing an end of the tensioning element. 
         FIG. 16  is a table of reference numbers for the components and other things illustrated in  FIGS. 1A-15C and 17A-18  and for the forces represented in the drawings. 
         FIG. 17A  is a table of dimensions for an exemplary track-module apparatus. 
         FIG. 17B  is a set of five tables illustrating five different sets of loads on the exemplary apparatus of  FIG. 17A  and the five resulting load distributions. 
         FIG. 18  is a schematic diagram of an embodiment of the inventive track-module apparatus in which a tensioning element is mounted with respect to the trailing idler wheel. 
     
    
    
     DETAILED DESCRIPTIONS OF PREFERRED EMBODIMENTS 
       FIG. 1A  is a perspective drawing of an embodiment  10  of the vehicle track-module apparatus of this invention. (As referred to herein, an embodiment of a track-module apparatus and the track-module apparatus itself may be referred to using the same reference number. Thus, for example, “embodiment  10 ” and “track-module apparatus  10 ” refer to the same apparatus.) Embodiment  10  includes a module frame  12 , a drive wheel  14  which is rotatable with respect to frame  12 , the drive wheel having a drive wheel axis  16 , ground-engaging leading wheels  18  and ground-engaging trailing wheels  20  and ground-engaging bogie wheels  56 ,  60  and  64 , and an endless track  22  which extends around wheels  14 ,  18 ,  20 ,  56 ,  60  and  64  and is driven by its engagement with drive wheel  14 . 
       FIG. 1B  is a perspective drawing of track-module apparatus  10  of  FIG. 1A  as viewed from the side opposite that shown in  FIG. 1A . Referring to  FIG. 1B , track-module apparatus  10  includes a vehicle connection  11  at which track-module apparatus  10  is attached to a vehicle and a drive gearbox  14   g  which receives power from the vehicle through a drive power input shaft  14   p.    
     In embodiment  10 , leading wheels  18  are leading idler wheels  18 , and trailing wheels  20  are trailing idler wheels  20 . In some embodiments of the inventive track-module apparatus, it is contemplated that a leading or trailing wheel may also function as the drive wheel. Also in embodiment  10 , endless track  22  is an endless polymeric track  22 . It is contemplated that within the scope of the present invention, endless track  22  may be constructed of a wide variety of materials and structures including metallic components such as are presently known in some tracked vehicles. The specific properties and materials of the endless track are not central to the inventive concepts of the present invention. 
     Bogie wheels  56  are leading bogie wheels, bogie wheels  60  are trailing bogie wheels, and bogie wheels  64  are middle bogie wheels. Bogie wheels  56 ,  60  and  64  are part of a bogie assembly  46 . Embodiment  10  also includes a leading suspension arm  24 , a trailing suspension arm  34 , a leading suspension element  68 , a trailing suspension element  70 , and a tensioning element  106 . 
     The direction of forward travel of the track module of embodiment  10  (and other similar embodiments presented herein) is defined by leading idler wheels  18  being ahead of trailing idler wheels  20 .  FIG. 4  includes an arrow  122  indicating the direction of travel applicable to all embodiments as defined by the leading and trailing components of the embodiments. 
       FIGS. 2 through 6B  illustrate track-module apparatus  10  and various partial assemblies thereof in several views in order to show more clearly the various aspects of inventive track-module apparatus  10 .  FIG. 2  is a perspective drawing of portions of track-module apparatus  10 , illustrating the several suspension linkage components without drive wheel  14 , track  22 , leading idler wheels  18 , trailing idler wheels  20 , leading bogie wheels  56 , trailing bogie wheels  60 , middle bogie wheels  64 , frame  12 , leading suspension element  68 , trailing suspension element  70 , and tensioning element  106 .  FIG. 3  is an exploded perspective drawing of track-module apparatus  10 . 
       FIG. 4  is a side-elevation drawing of track-module apparatus  10  with the near (in the drawing) set of idler wheels  18  and  20  and bogie wheels  56 ,  60  and  64  removed to show the various elements of embodiment  10  more clearly. 
       FIG. 5  is side-elevation drawing of portions of vehicle track-module apparatus  10 . Similar to  FIG. 2 ,  FIG. 5  illustrates various components of apparatus  10  with some components not shown to increase the visibility of other components. 
       FIGS. 6A and 6B  are perspective drawings of the bogie mount portions of bogie assembly  46  of vehicle track-module apparatus  10  without bogie wheels  56 ,  60  and  64 .  FIG. 6A  illustrates bogie mount  48  oriented as if apparatus  10  were on a flat portion of the ground.  FIG. 6B  illustrates bogie mount  48  as if apparatus  10  were on uneven ground to illustrate some of the degrees-of-freedom available in the configuration of bogie mount  48  of bogie assembly  46 . Further description is presented below in this document. 
     The following description of track-module apparatus  10  (and later of other embodiments) refers to  FIGS. 1A-6B  together. Note that in all of the drawings, a “+” symbol is used to indicate an axis of rotation. In general, as used herein, the term “axis” as stated above pertains to a pivot joint which includes the necessary bearing structure and other components to permit rotation about such axis. As an example, drive wheel axis  16  about which drive wheel  14  rotates is indicated in  FIG. 4  by a “+” symbol. Portions of axle and bearing structure (not shown) which are needed for drive wheel  14  to rotate around axis  16  are assumed to be part of embodiment  10 . 
     In six instances within apparatus  10 , the “+” symbol indicates a pivot joint which may provide more than one degree-of-freedom of relative motion. This is indicated by (a) the name including the word “pivot” rather than “axis” and (b) the relevant reference number ending with the letter “p”. These instances are  82   p ,  84   p ,  86   p ,  88   p ,  112   p  and  116   p . As described later in this document, such higher number of degrees-of-freedom of relative motion may be provided by the use of spherical bearings. It should be understood that it is intended that in some embodiments, such “pivots” may also simply be axes configured for single degree-of-freedom rotation. The use of the term “pivot” is not intended to limit the scope of the present invention to multiple degrees-of-freedom motion at such locations within embodiments having such pivots. Acceptable components necessary to provide the required number of degrees-of-freedom are well-known to those skilled in the mechanical arts. 
     Leading suspension arm  24  is rotatably attached to frame  12  at a leading arm axis  26  and extends forward to a leading-arm distal end  28  at which a leading-wheel assembly  30  is rotatably attached. In apparatus  10 , leading-wheel assembly  30  is also called leading-idler assembly  30  since in apparatus  10 , leading wheel  18  is leading idler wheel  18 . Leading suspension arm  24  extends rearwardly to a rearward suspension end  32 . In similar fashion, trailing suspension arm  34  is rotatably attached to frame  12  at a trailing arm axis  36  and extends rearward to a trailing-arm distal end  38  at which a trailing-wheel assembly  42  is attached. In apparatus  10 , trailing-wheel assembly  42  is also called trailing-idler assembly  42  since in apparatus  10 , trailing wheel  20  is trailing idler wheel  20 . 
     In embodiment  10 , trailing-idler assembly  42  primarily comprises trailing idler wheels  20  which are rotatably attached at a trailing-idler axis  118 . Trailing suspension arm  34  extends forwardly to a forward suspension end  40 . In embodiment  10 , leading arm axis and trailing arm axis  36  are coincident and together form suspension-arm axis  44 . Such coincidence is not intended to be limiting; other configurations of the inventive track-module apparatus in which leading arm axis  26  and trailing arm axis  36  are not coincident are contemplated within the scope of this invention. 
     Suspension-arm axis  44  of embodiment  10  is shown as being rearward of and below drive wheel axis  16  as defined by direction-of-travel arrow  122  in  FIG. 4 . Such relative positioning with respect to drive wheel axis  16  is not intended to be limiting; other relative positions of leading arm axis  26  and trailing arm axis  36  are contemplated within the scope of this invention. 
     Bogie assembly  46  includes two leading bogie wheels  56 , two middle bogie wheels  64 , and two trailing bogie wheels  60 . Bogie assembly  46  also includes a bogie mount  48  which includes bogie-mount forward portion  50 , a bogie-mount middle portion  54 , a bogie-mount rearward portion  52 , and a bogie-mount arm  102 . Leading bogie wheels  56  are rotatable with respect to bogie-mount forward portion  50  around a leading bogie axis  58 . In addition, leading bogie axis  58  rotates through a limited range of angles about a leading bogie roll axis  96  which is perpendicular to leading bogie axis  58 . 
     In a similar fashion, such relative rotational movement is also provided for middle bogie wheels  64  and trailing bogie wheels  60 . Middle bogie wheels  64  are rotatable with respect to bogie-mount middle portion  54  around a middle bogie axis  66 . Middle bogie axis  66  rotates through a limited range of angles about a middle bogie roll axis  100  which is perpendicular to middle bogie axis  66 . Trailing bogie wheels  60  are rotatable with respect to bogie-mount rearward portion  52  around a trailing bogie axis  62 . Trailing bogie axis  62  rotates through a limited range of angles about a trailing bogie roll axis  98  which is perpendicular to trailing bogie axis  62 . 
     Bogie mount  48  also includes bearings  96   b ,  100   b  and  98   b , configured as follows: (1) bearing  96   b  at leading bogie roll axis  96 ; (2) bearing  100   b  at middle bogie roll axis  100 ; and (3) bearing  98   b  at trailing bogie roll axis  98 . Bogie assembly  46  also includes a leading bogie axle assembly  96   a  to which leading bogie wheels  56  are rotatably attached, a middle bogie axle assembly  100   a  to which middle bogie wheels  64  are rotatably attached, and a trailing bogie axle assembly  98   a  to which trailing bogie wheels  60  are rotatably attached. Bearings  96   b ,  100   b  and  98   b  are configured to permit bogie axle assemblies  96   a ,  100   a  and  98   a , respectively, to rotate on such bearings around leading bogie roll axis  96 , middle bogie roll axis  100  and trailing bogie roll axis  98 , respectively. Leading bogie roll axis  96  and trailing bogie roll axis  98  are indicated at respective ends of bogie mount  48  in  FIGS. 6A and 6B . Also in  FIGS. 6A and 6B , middle bogie roll axis  100  is indicated by dotted lines at middle bogie roll axis bearing  100   b  but should be understood to be located internally in the center of bearing  100   b , parallel to such dotted lines and not on the surface of bearing  100   b.    
     Bogie mount  48  of bogie assembly  46  is rotatably attached at a first bogie-assembly axis  78  to leading suspension arm  24  at a location along arm  24  between leading arm axis  26  and leading-arm distal end  28  by a bogie-assembly arm  72  at a bogie-assembly arm distal end  74 . Bogie-assembly arm  72  also includes a bogie-assembly arm proximal end  76  which is rotatably attached to a bogie-mount arm  102  of bogie mount  48  at a second-bogie-assembly axis  80 . 
     Bogie mount  48  of bogie assembly  46  is also attached to leading suspension arm  24  and trailing suspension arm  34  by suspension elements  68  and  70 . Leading suspension element  68  is rotatably attached to rearward suspension end  32  of leading suspension arm  24  at a leading suspension-element pivot  82   p  and is rotatably attached to bogie-mount rearward portion  52  at a first bogie-assembly pivot  84   p  at a rearward bogie-mount connection  92 . Trailing suspension element  70  is rotatably attached to forward suspension end  40  of trailing suspension arm  34  at trailing suspension-element pivot  86   p  and is rotatably attached to bogie-mount forward portion  50  at a second bogie-assembly pivot  88   p  at a forward bogie-mount connection  94 . 
     Within bogie mount  48  of bogie assembly  46  in track-module apparatus  10 , bogie-mount forward portion  50  and bogie-mount middle portion  54  are rotatably attached at a third bogie-assembly axis  90 . 
     Embodiment  10  includes a tensioning element  106  which provides attachment between leading suspension arm  24  and leading-idler assembly  30 . Leading-idler assembly  30  includes leading idler wheels  18  and a leading-idler axis  104  about which leading idler wheels  18  rotate. Leading-idler assembly  30  also includes a wheel linkage  120  at leading-idler axis  104 ; in apparatus  10 , wheel linkage  120  is idler linkage  120 . Leading-arm distal end  28  is rotatably attached to idler linkage  120  at an idler offset axis  114  which is offset from leading-idler axis  104 . 
     A tensioning-element first end  108  of tensioning element  106  is rotatably attached to leading suspension arm  24  at a proximal tensioning pivot  112   p  at forward suspension end  40  between leading-arm distal end  28  and suspension-arm axis  44 . A tensioning-element second end  110  is rotatably attached to leading-idler assembly  30  at a distal tensioning pivot  116   p  offset from leading-idler axis  104 . Idler offset axis  114  is parallel to leading-idler axis  104  and angularly displaced therearound such that idler linkage  120  is a class  2  lever with idler offset axis  114  being the fulcrum thereof. Tension forces on track  22  are provided through idler wheels  18  by tensioning element  106  through the class  2  lever action of idler linkage  120  acted on by tensioning element  106 . 
     Suspension elements  68  and  70  and tensioning element  106  may provide both spring and damping forces. In some embodiments, such elements may be gas-filled and include a liquid-filled cavity to provide both types of forces for the suspension system. Such elements are well-known to those skilled in the art of vehicle suspension. Further description of suspension elements  68  and  70  is provided in the description of  FIG. 9 . 
       FIGS. 7A through 8B  illustrate the kinematics of track-module apparatus  10  under various operating conditions. Each such drawing is a side-elevation illustration of apparatus  10  under representative conditions to show the relative movement of the components of apparatus  10  under such conditions.  FIGS. 7A-7F  illustrate the movement of vehicle track-module apparatus  10  as it traverses over a small bump  126  on the ground  124  along its path of travel.  FIG. 7A  shows track-module apparatus  10  just prior to encountering bump  126 .  FIG. 7B  shows apparatus  10  with its leading idler wheels  18  over bump  126 .  FIG. 7C  shows apparatus  10  with leading bogie wheels  18  over bump  126 .  FIG. 7D  shows apparatus  10  with middle bogie wheels  64  over bump  126 .  FIG. 7E  shows apparatus  10  with trailing bogie wheels  60  over bump  126 .  FIG. 7F  shows apparatus  10  with trailing idler wheels  20  over bump  126 . 
       FIG. 8A  is a side-elevation drawing of track-module apparatus  10  illustrating apparatus  10  as it traverses an uphill portion  128  of ground  124 . Similarly,  FIG. 8B  is a side-elevation drawing of apparatus  10  illustrating apparatus  10  as it traverses a downhill portion  130  of ground  124 . Each of the drawings of  FIGS. 7A through 8B  illustrate idler wheels  18  and  20  and bogie wheels  56 ,  60  and  64  all in contact with ground  124  in order to support some portion of the loads on apparatus  10 . 
       FIG. 9  is a schematic drawing of leading suspension element  68  and trailing suspension element  70  in a hydraulic circuit  134 . Suspension elements  68  and  70  each include cylinders hydraulic  136  containing hydraulic fluid  144  and gas-filled cylinders  138  containing gas  146  separated by pistons  140 . Hydraulic cylinders  136  and gas-filled cylinders  138  are movably sealed for relative movement by seals  142 , and gas-filled cylinders  138  and pistons  140  are movably sealed for relative movement by another set of seals  142  such that the volumes of hydraulic fluid  144  and gas  146  may both change under loads which are applied across suspension elements  68  and  70 . In such components, gas  146  is typically nitrogen but other gases may be used. 
     Hydraulic cylinders  136  are interconnected by a hydraulic conduit  148  placing suspension elements  68  and  70  in a common hydraulic circuit such that the pressures in suspension elements  68  and  70  are equal. Gas  146  in gas-filled cylinders  138  enables suspension elements  68  and  70  to provide spring forces to the suspension system of apparatus  10  while hydraulic fluid  144  flowing through hydraulic conduit  148  enables suspension elements  68  and  70  to provide damping forces to the suspension system of apparatus  10 . 
     Hydraulic circuit  134  also includes an external accumulator  150  connected to hydraulic conduit  148  by an accumulator conduit  156 . Accumulator  150  includes both hydraulic fluid  144  and gas  146  in sealed separation from one another by an accumulator piston  152  movably sealed within accumulator  150  by accumulator seal  154 . Gas  146  within accumulator  150  provides additional spring force to the suspension system of apparatus  10  while hydraulic fluid  144  flowing through accumulator conduit  156  and hydraulic conduit  148  provides additional damping force to suspension system of apparatus  10 . 
     Suspension elements  68  and  70  and tensioning element  106  may provide suspension forces which are variable. For example, the damping forces may depend on the direction of the movement (extension or contraction) of the element in order to provide a specific desired suspension performance. 
     The operation of the components of hydraulic circuit  134  are well-known to those skilled in mechanical systems.  FIG. 9  is intended only to be schematic. For example, the functions of accumulator piston  152  and accumulator seal  154  may be provided by a membrane, a bladder or other similar component. In similar fashion, the components of suspension elements  68  and  70  may also be different from those described above while providing similar operation of suspension elements  68  and  70 . 
       FIG. 10  is a schematic diagram of the embodiment of  FIGS. 1A and 1B , illustrating a supported load FL and a set of five resulting wheel loads F 1  through F 5 . The load on leading wheels  18  is referred to as F 1 ; the load on leading bogie wheels  56  is referred to as F 2 ; the load on middle bogie wheels  64  is referred to as F 3 ; the load on trailing bogie wheels  60  is referred to as F 4 ; and the load on trailing wheels  20  is referred to as F 5 . Since all of supported load FL acts on suspension-arm axis  44 ,  FIG. 10  shows FL at such location in the schematic diagram of  FIG. 10 . 
     The load FL supported by track-module apparatus  10  may have both vertical and horizontal components depending on the specific operational situation. These include at least the following: (a) the portion of the vehicle weight supported by apparatus  10 ; (b) pulling forces when the vehicle is pulling a load; and (c) braking forces which in an emergency braking situation may be quite high. Also, of course, each of the resulting forces F 1  through F 5  may also have both vertical and horizontal components, and all of these forces vary with the slope of the ground being traversed. 
       FIGS. 11 through 14  are side-elevation drawings (similar to  FIG. 4 ) illustrating several alternative embodiments  10   a  through  10   d , respectively, of the vehicle track-module apparatus of this invention. In each of  FIGS. 11-14 , the same reference numbers are used for components similar to those of track-module apparatus  10 . 
       FIG. 11  is a side-elevation drawing (similar to  FIG. 4 ) of a first alternative embodiment  10   a  of the vehicle track-module apparatus of this invention. Embodiment  10   a  is similar to embodiment  10  except that middle bogie wheels  64  have been eliminated with corresponding changes in other components to accommodate such modification. A track module similar to first alternative embodiment  10   a  may be used to reduce complexity and cost when compared to embodiment  10  and/or may be used when the distance between the leading and trailing wheels needs to be shorter than is provided by embodiment  10 . 
       FIG. 12  is a side-elevation drawing (similar to  FIG. 4 ) of a second alternative embodiment  10   b  of the vehicle track-module apparatus of this invention. Embodiment  10   b  is similar to first alternative embodiment  10   a  except that tensioning element  106  has been eliminated and leading-idler assembly  30  primarily includes only leading idler wheels  18 . A track module similar to second alternative embodiment  10   b  may be used to reduce complexity and cost when compared to embodiment  10 . Similar to first embodiment  10   a , second embodiment  10   b  may also provide a shorter distance between the leading and trailing wheels if such a configuration is desirable. 
       FIG. 13  is a side-elevation drawing (similar to  FIG. 4 ) of a third alternative embodiment  10   c  of the vehicle track-module apparatus of this invention. Embodiment  10   c  is similar to embodiment  10  except that tensioning element  106  has been eliminated and leading-idler assembly  30  primarily includes only leading idler wheels  18 . A track module similar to third alternative embodiment  10   c  may be used to reduce complexity and cost when compared to embodiment  10 . 
       FIG. 14  is a side-elevation drawing (similar to  FIG. 4 ) of a fourth alternative embodiment  10   d  of the vehicle track-module apparatus of this invention. Embodiment  10   d  is similar to third alternative embodiment  10   c  except that third bogie-assembly axis  90  has been eliminated with corresponding changes in other components to accommodate such modification. A track module similar to fourth alternative embodiment  10   d  may be used to reduce complexity and cost when compared to embodiment  10 . In the case of fourth embodiment  10   d , a degree-of-freedom within bogie assembly  46  has been removed; under certain operational conditions such as travel mainly on generally even terrain, this reduction in compliance may be acceptable. 
       FIG. 15A  is side-elevation drawing of portions of vehicle track-module apparatus  10  to illustrate the detail of tensioning-element first end  108  of tensioning element  106 .  FIG. 15B  is a sectional view (section A-A) as indicated in  FIG. 15A . Section A-A passes through proximal tensioning pivot  112   p  at the rotatable attachment between tensioning element  106  and leading suspension arm  24 . 
       FIG. 15C  is a further enlargement of a portion of  FIG. 15B  to show even more detail of proximal tensioning pivot  112   p . As described above, certain pivot joints within apparatus  10  involve structures which provide more than one degree-of-freedom of rotation. In the naming convention used herein, the word “pivot” is used for such more than one degree-of-freedom connections. Within apparatus  10 , these include  82   p ,  84   p ,  86   p ,  88   p ,  112   p  and  116   p , and  FIGS. 15A-15C  are used to illustrate one such pivot. In embodiment  10 , all such pivots are spherical bearings as is shown for pivot  112   p.    
     Referring to  FIG. 15C , proximal tensioning pivot  112   p  includes a spherical bearing which includes a ball  112   b  which rotates in a socket  112   s  on leading suspension arm  24 . A mechanical connector  112   c  holds ball  112   c  in socket  112   s.    
     By using the inventive structure of the various embodiments of track-module apparatus disclosed herein and by selecting the dimensions of the various components, a track-module designer is able to set the load distribution on the ground-engaging wheels to meet the requirements of a particular vehicle application. For example, it may be desirable to have the leading or trailing wheels take somewhat different percentages of the load on the vehicle. And often it is desirable, when the apparatus has more than one bogie-wheel axle, to have each of the bogie-wheel axles support substantially the same vehicle load. A set of linkage dimensions can be chosen to distribute the load supported by the bogies as desired. 
     Kinematic analysis methods well-known to those skilled in the art of mechanical systems can be used to evaluate the load-distribution performance of a specific set of linkage dimensions in apparatus  10 . In the example described below and in  FIGS. 17A and 17B , such analysis was used to compute the load distribution under a set of different load conditions. Referring to the schematic diagram of  FIG. 10 , the dimensions are represented by the following notation. A horizontal dimension includes the letter “H” followed by two reference numbers separated by a colon. Thus, H 44 : 118  is the horizontal distance from suspension axis  44  to trailing-idler axis  118 . The letter “V” indicates a vertical dimension, and the letter “D” a diameter.  FIG. 17A  summarizes a set of dimensions for a representative configuration of track-module apparatus  10  with suspension elements  68  and  70  in common hydraulic circuit  134 . 
       FIG. 17B  summarizes the results of analysis of the representative example of  FIG. 17A . As can be seen, in this example, the loads F 2 , F 3  and F 4  on bogie wheels  56 ,  64  and  60 , respectively, are and remain evenly distributed among the bogie wheels, and the addition of various portions of the total loading from vehicle weight, track tension, braking and pull cause very modest changes to the load distribution percentages. 
     The power source for the track-module apparatus of this invention is not limited to a rotating power shaft of the vehicle. Other power-source configurations are contemplated by this invention, such as a hydraulic motor or other power source on the vehicle or a mechanical, hydraulic or other power source directly mounted on the apparatus itself. 
       FIG. 18  is a schematic diagram showing an embodiment  10   t  of the inventive track-module apparatus in which tensioning element  106  is mounted with respect to trailing idler wheel  20 , and trailing-idler assembly  42  includes additional linkage elements. Also, leading-wheel assembly  30  is simplified accordingly. 
     In embodiment  10   t , tensioning element  106  provides attachment between trailing suspension arm  34  and trailing-idler assembly  42 . Trailing-idler assembly  42  includes trailing idler wheels  20  and trailing-idler axis  118  about which trailing idler wheels  20  rotate. Trailing-idler assembly  42  includes wheel linkage  120  at trailing-idler axis  118 ; in apparatus  10   t , wheel linkage  120  is idler linkage  120 . Trailing-arm distal end  38  is rotatably attached to idler linkage  120  at an idler offset axis  114  which is offset from trailing-idler axis  118 . 
     Tensioning-element first end  108  of tensioning element  106  is rotatably attached to trailing suspension arm  34  at proximal tensioning pivot  112   p  at rearward suspension end  32  between trailing-arm distal end  38  and suspension-arm axis  44 . Tensioning-element second end  110  is rotatably attached to trailing-idler assembly  42  at distal tensioning pivot  116   p  offset from leading-idler axis  118 . Idler offset axis  114  is parallel to leading-idler axis  118  and angularly displaced therearound such that idler linkage  120  is a class  2  lever with idler offset axis  114  being the fulcrum thereof. Tension forces on track  22  are provided through idler wheels  20  by tensioning element  106  through the class  2  lever action of idler linkage  120  acted on by tensioning element  106 . 
     It may be desirable to have a tensioning element related to each of the leading and trailing wheel assemblies when certain performance requirements need to be addressed. 
     While the principles of this invention are shown and described here in connection with specific embodiments, it is to be understood that such embodiments are by way of example and are not limiting.