Abstract:
A track drive system for a mobile irrigation system adapts to either centrally pivoted circular or linear mobile systems, as desired. A first embodiment has a single drive wheel with opposed first and second end guide wheels, surrounded by a single narrow track. A single drive motor and gear reduction drive, drives the central wheel. The central wheel is of considerably larger diameter than the two end guide wheels, and simultaneously contacts the track with both its upper and lower peripheries at all times. The mechanism is articulated to allow independent wheel movement relative to one another, thus providing better traction over uneven surfaces. The central drive wheel is always in contact with both the upper and lower track portion to provide continuous drive traction for the device. A second embodiment includes additional idler wheels between the two end wheels and the central drive wheel.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/274,660, filed Mar. 12, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the agricultural field, and more specifically to mobile irrigation systems in which a suspended irrigation pipe is carried over a field in a circular or lateral linear path to irrigate the underlying crops. The present invention is a track driven support tower for such suspended irrigation pipes or lines, which reduces crop damage by means of its narrow track and width and further reduces field damage by means of the large area of the elongate track which spreads the load of the tower and pipe over a larger surface area. 
     2. Description of Related Art 
     The development of mobile irrigation systems has opened up large areas of land which would otherwise be too arid for growing certain crops which require larger amounts of water. These systems comprise a relatively long irrigation pipe or line suspended over the crops by a series of mobile towers, which carry the line in a linear or circular path to irrigate the underlying crops. 
     While such irrigation systems have provided great benefit to large farming operations by allowing them to grow more profitable crops and/or to produce greater yields per acre, such systems are not without their drawbacks. Most conventional mobile irrigation systems use two or more wheels and tires to support the pipe tower(s), rather than a track system as in the present invention. While such wheeled systems function adequately to provide motion for the tower and to carry the overlying irrigation pipe or line, the considerable weight of the overlying irrigation pipe and the water therein (which may comprise a run of over one hundred feet) results in a considerable load on the wheels and tires of the support tower(s). This often results in the wheels and tires forming deep ruts in the underlying surface, particularly when that surface has been softened by irrigation. 
     These ruts and tracks then interfere with other machinery (harvesters, etc.) operated in the field subsequent to the irrigation operations. In some cases, the tracks and ruts produced by the irrigation equipment can be so large or deep that other types of equipment are damaged by repeated use over such areas. Moreover, the production of such ruts and tracks leads to water collection in the ruts and uneven irrigation of the crops in the fields, as well as wasted water which collects within the ruts. Such deep ruts and tracks have also been known to cause the wheels of the irrigation towers to become stuck during later passage through the same area or track, particularly if the track has been filled with water and the ground is excessively soft as a result. 
     Another problem with conventional wheeled mobile irrigation towers is crop damage due to the relatively wide wheel and tire necessary to support the heavy weight of the overlying irrigation pipe and support tower. For example, a pipe having an internal diameter of only six inches and a span of one hundred feet would carry on the order of 150 gallons of water weighing about 1200 pounds. When the weight of the pipe, its support structure, the support towers, and mobile structure (wheels, propulsion motor(s), etc.) are added to the weight of the water, it will be seen that each tower must support a considerable load. Hence, most such towers utilize relatively large diameter and wide tires in an attempt to distribute the load over a relatively wide area. This results in a relatively large percentage of the crop being knocked down or damaged, due to the passage of the wide wheel, tire, and other tower structure thereover. 
     While track driven mobile irrigation systems have been developed in the past in an attempt to overcome at least the problem of the system becoming mired in its own ruts or tracks, as well as to spread the weight of the apparatus over a relatively large area, such prior art systems do nothing to provide the narrowest possible track width in order to reduce damage to crops to the greatest possible extent. While it may seem obvious to reduce the width of the tower structure which passes through the crops, this has not been accomplished to the maximum possible degree in any of the devices of the prior art of which the present inventor is aware. 
     Accordingly, a need will be seen for a track drive for a mobile irrigation system which provides a relatively narrow track and drive system width in order to reduce damage to crops to the greatest extent possible, but which track is relatively long in order to provide the greatest practicable contact area for the track and hence the lowest possible unit pressure for the overlying structure. The present track drive accomplishes this by means of a single, central drive wheel and a pair of outlying support wheels disposed to each end of the track length to provide a relatively long track span or length. Intermediate track support may be provided by additional idler wheels between the central drive wheel and outlying support wheels, if so desired. The single drive wheel precludes need for drive shafts extending alongside the device to provide motive force for plural wheels, thus reducing the overall width of the assembly. The drive wheel is also dished to provide an interior volume which houses the drive motor and gearbox for the assembly, again greatly reducing the overall width of the assembly. 
     The present track drive system also includes an independent suspension system for the central drive wheel and outlying wheels, allowing the device to traverse uneven ground while still maintaining maximum contact with the underlying surface to spread the system load and maintain traction. The single, larger diameter central drive wheel is in contact with both the upper and lower track segment at all times, in order to produce continuous drive force to the track. 
     A discussion of the related art of which the present inventor is aware, and its differences and distinctions from the present invention, are discussed below. 
     U.S. Pat. No. 1,541,026 issued on Jun. 9, 1925 to Arthur R. Blewett, titled “Tractor Machine,” describes an endless track and wheel assembly having a single, central sprocketed drive wheel with forward and rearward non-driven wheels. The central drive wheel contacts the track tangent to both its lowermost and uppermost portions of its rim, thus simultaneously driving the track from both its lower and upper peripheries. However, the Blewett track and wheel assembly differs from the present track drive system in several respects. First, it is noted that all three wheels are linked together by a common structure, thus preventing any relative motion between wheels for traversing uneven surfaces more smoothly. Second, the track is considerably wider than the wheels, as would be necessary to spread the load over a relatively large area in view of the short length of the assembly (all three wheels are spaced apart by only a small fraction of their diameters). It is also noted that Blewett does not provide any means of supporting an overlying structure with his track and wheel assembly. 
     U.S. Pat. No. 1,613,598 issued on Jan. 11, 1927 to Edward R. Armstrong, titled “Traction Appliance For Automobiles,” describes an assembly adapted for installation on an automobile or the like. The assembly includes a specially modified wheel with a solid tire having a series of transverse slots, with a rigid frame including a pair of opposed track end wheels and a track passing around the end wheels and central main wheel. The track engages the transverse slots of the main wheel to drive the vehicle. The structure is thus generally like the structure of the Blewett &#39;026 U.S. patent discussed immediately above, with the rigid structure failing to provide relative articulation between the wheels to better accommodate surface irregularities. As in the Blewett &#39;026 apparatus, Armstrong makes no provision for supporting any overlying structure, as the sole attachment to the vehicle is by means of the drive axle to which the wheel is attached. 
     U.S. Pat. No. 1,966,783 issued on Jul. 17, 1934 to Maurice A. Balaam, titled “Portable Overhead Irrigation Plant,” describes an automatically propelled mobile irrigation device having a track drive in at least one embodiment. The Balaam device differs from the present invention in several respects. First, the track of the track drive embodiment is considerably wider than the present relatively long and narrow track, as is clearly shown in FIG. 5 of the Balaam U.S. patent. This would result in considerably more crop damage along the path of the Balaam device than would occur with the present invention, which when considering that the track path extends at least completely across the field for lateral irrigation systems, and much longer for circular systems, results in considerable crop damage by the Balaam device. Secondly, the Balaam tracked device has a central water powered motor driving forwardly and rearwardly disposed drive wheels. As in the other devices discussed above, the Balaam device also uses a rigid frame which does not provide any relative articulation between wheels. 
     U.S. Pat. No. 2,711,615 issued on Jun. 28, 1955 to Nelson R. Boice, Jr., titled “Apparatus For Processing Surface Areas,” describes a mobile irrigation system with the pipe supported by a series of track drive devices. The Boice, Jr. drive apparatus is more closely related to that of the Balaam &#39;783 U.S. patent discussed immediately above than to the present invention, in that Boice, Jr. provides a single forwardly disposed drive wheel driven by a centrally located motor. A series of idler or bogie wheels is positioned between the forward rearward wheels in the track assembly. All wheels of the Boice, Jr. assembly are installed on a rigid framework which does not allow articulation of the wheels, as does the independent suspension system of the present machine. 
     U.S. Pat. No. 2,931,579 issued on Apr. 5, 1960 to Clarence W. Ruddell, titled “Automatic Irrigation System,” describes a linear mobile irrigation system having a track drive device and a series of non-driven tracked supports disposed along the length of the pipe. The track drive assembly comprises two widely spaced narrow tracks extending laterally from a single carriage which supports the irrigation pipe. The relatively wide carriage is so low that it would damage almost any crop over which it passed, thus causing significant crop damage. The drive system comprises an offset motor which drives an upper intermediate wheel, with the track passing around the intermediate wheel and the two track wheels to drive all of the wheels. As in the case of other devices discussed above, the Ruddell device also has a rigid frame precluding articulation and independent movement of the wheels. 
     U.S. Pat. No. 4,365,748 issued on Dec. 28, 1982 to Robert R. Emrich, titled “Self-Driven Mobile Center Pivot Irrigation System,” describes a system having a mobile tractor device for transporting the system and for serving as the central pivot of the system for its operation. One or more radially outlying propulsion devices are also provided, which more closely relate to the present invention. However, the outlying propulsion devices still differ considerably from the present track drive system in that they have two wheels, each of which is driven by a separate independent belt which wraps about the circumference of its respective wheel. The two belts are in turn driven by a single central gearbox powered by a water driven motor. The motor and gear reduction are offset considerably from the plane of the wheels, unlike the present system. Also, while each wheel has a separate brace structure, the two braces are rigidly secured together during operation to preclude relative wheel movement. 
     U.S. Pat. No. 4,452,394 issued on Jun. 5, 1984 to Robert R. Emrich, titled “Self Propelled Irrigation Systems,” describes various embodiments of such a system. One embodiment comprises a pair of wheels driven by a common belt and drive motor positioned between the wheels. This device is more closely related to the device of the &#39;748 U.S. patent to the same inventor discussed immediately above, than to the present invention. The two wheels of the Emrich &#39;394 device are rigidly affixed relative to one another by the frame structure, unlike the present track drive system with its articulated independent suspension for the wheels. 
     U.S. Pat. No. 5,078,326 issued on Jan. 7, 1992 to Hubbart L. Wright, titled “Mobile Irrigation Equipment Belt Traction Apparatus,” describes a system having a series of four wheels, with a single belt extending therearound. At least one of the front and rear wheels is driven by a motor at the hub thereof. The motor(s) and support structure for the tower supported above the wheels, is laterally offset from the plane of the wheels, rather than having the wheels located symmetrically beneath the tower structure as in the present invention. The asymmetric relationship between the wheels and tower, place a significant bending load upon the wheel axles and lower tower components, requiring a significantly heavier structure than that of the present invention. Moreover, the entire wheel support frame of the Wright apparatus is a single rigid structure, with no relative movement of the wheels permitted. While Wright clearly recognizes the problems associated with wheeled mobile irrigation systems, as indicated in his discussion in the Background of the Invention of his U.S. patent, his invention falls short of the solution provided by the present invention. 
     U.S. Pat. No. 6,131,833 issued on Oct. 17, 2000 to John A. Chapman, titled “Tracked Drive Tower For A Self-Propelled Irrigation System,” describes a track drive system having a single forwardly disposed drive wheel and a series of trailing bogie or idler wheels within the track run, with each pair of idler wheels mounted to a beam which is in turn pivotally attached to a single, rigid overlying structure. The only articulation provided is by means of each pair of idler wheels rocking about its common attachment shaft. Chapman does not provide any spring suspension means, nor adjustment means for such a suspension, in his apparatus. Moreover, no central drive wheel is provided, as is the case of the present invention. It is also noted that the drive motor and gear reduction drive are disposed well outside the plane of the wheels and track, unlike the present invention. Such an installation results in an even wider path for the track assembly as it traverses a field, with the motor and gear reduction damaging additional crops due to their relatively low placement and axial extension from the plane of the wheel and track. 
     Australian Patent Publication No. 227,737 published on Aug. 27, 1959 to Machinery Specialties Pty. Limited, titled “Mobile Irrigation Sprayer,” describes an apparatus having a series of drive towers disposed along a length of pipe. Each drive tower comprises a forward, an intermediate, and a rearward wheel, with a track extending around the three wheels. Only the forward wheel is driven by a linear water motor, which alternately extends and retracts a strut which in turn drives a ratchet arrangement on the front wheel; the drive mechanism thus resembles that of the Balaam &#39;783 U.S. patent, discussed further above. As in the other devices known to the present inventor and discussed above, the Machinery Specialties system has a rigid, unitary frame precluding any movement of the wheels relative to one another for more effective travel over uneven terrain, as provided by the present invention. 
     Australian Patent Publication No. 291,021 published on Jul. 13, 1967 to Machinery Specialties Pty. Limited, titled “Mobile Irrigation Sprayer,” describes a modification of the sprayer device of the Australian &#39;737 Patent Publication noted immediately above. The &#39;021 Australian Publication describes a similar track assembly, but the &#39;021 assembly includes a pair of idler wheels disposed between the front and rear wheels of the track, rather than only a single idler wheel. Moreover, the &#39;021 assembly is driven by a conventional electric motor, rather than a linear hydraulic strut motor. Many of the same problems noted further above are evident in the device of the &#39;021 Australian Publication, e.g. the axially extending motor and gearbox which increase the width of crop damage area as the track passes through the crops, and the rigid frame precluding any relative suspension movement between track wheels. 
     Finally, Australian Patent Publication No. 293,781 published on Jul. 13, 1967 to Machinery Specialties Pty. Limited, titled “Mobile Irrigation Sprayer,” describes a tracked apparatus having only single forward and rearward wheels within a track. Drive is provided by a hydraulic (water) cylinder motor, driving a ratchet mechanism to the rear wheel. The frame is a rigid, unitary construction, precluding any relative suspension movement of the wheels. The apparatus of the &#39;781 Australian Patent Publication is thus more closely related to the device of the &#39;737 Australian Publication to the same applicant, than to the present invention. 
     None of the above inventions and patents, either singly or in combination, is seen to describe the instant invention as claimed. 
     SUMMARY OF THE INVENTION 
     The present invention is a track drive apparatus particularly configured for use in driving a center pivot (circular) or linear movement mobile irrigation system. The present track drive provides numerous benefits over earlier systems of the related art, by combining a relatively narrow track and other structure to reduce crop damage with a relatively long track to provide the desired low pressure footprint for the device, to reduce rut formation and other field damage. 
     The present track drive includes a single central drive wheel with opposed first and second end guide wheels which guide the track around the ends of the machine. The central drive wheel has a considerably larger diameter than the two end guide wheels, thus bearing against the track along both its upper and lower peripheral areas to provide positive drive to the track at all times. The present mechanism is also articulated, to allow the two end guide wheels to move upwardly and downwardly independently of one another and relative to the central drive wheel. This allows the track drive to ride smoothly over uneven contours in the field, avoiding or at least greatly reducing any rocking action over ridges or bridging of depressions, which would tend to place the entire weight supported by the track on either one or two relatively short areas of track and reduce traction accordingly. 
     The track itself, as well as the associated componentry, is relatively narrow, in order to avoid lateral damage to crops to either side of the tower insofar as possible. The narrow span of the track is accomplished by providing a dished center to the central drive wheel, and placing the motor and gearbox entirely within this dished center. Thus, the motor and gearbox do not extend laterally from the remainder of the structure, as is the case with most other track configurations. 
     Accordingly, it is a principal object of the invention to provide a track drive for a mobile irrigation system, for use with both circular and linear movement irrigation systems for reducing crop and field damage where used. 
     It is another object of the invention to provide such a track drive having a single central drive wheel with opposed first and second end guide wheels, with the single central drive wheel having a diameter larger than the guide wheels for contacting both the upper and lower sides of the track surrounding the wheels simultaneously. 
     It is a further object of the invention to provide such track drive wherein the track and associated structure is relatively narrow to reduce lateral damage to crops as the track passes therethrough, with the single drive motor and gearbox being installed within the deeply dished center of the drive wheel and contained completely therein. 
     Still another object of the invention is to provide such a track drive wherein each of the wheels is independently suspended relative to the other wheels, thereby allowing the track to conform to ridges and depressions in the underlying surface and reducing bridging of depressions and rocking over ridges in the field. 
     It is an object of the invention to provide improved elements and arrangements thereof in an apparatus for the purposes described which is inexpensive, dependable and fully effective in accomplishing its intended purposes. 
     These and other objects of the present invention will become apparent upon review of the following specification and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a first embodiment of a track drive for a mobile irrigation system according to the present invention, showing its various features. 
     FIG. 2 is a top plan view in section of the present track drive through line  2 — 2  of FIG. 1, showing further details. 
     FIG. 3 is a schematic side elevation view of the present track drive, showing the positioning of the wheels and suspension on a level surface in solid lines and further showing the upward deflection of the central wheel in broken lines. 
     FIG. 4 is a schematic side elevation view similar to the view of FIG. 3, but illustrating the downward deflection of the central wheel in broken lines. 
     FIG. 5 is a schematic side elevation view of a second embodiment of the present track drive with additional idler wheels. 
     FIG. 6 is a prior art drawing, illustrating the damage done to a field by conventional pipe supports with tire drive systems. 
    
    
     Similar reference characters denote corresponding features consistently throughout the attached drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention comprises a mobile track drive mechanism for supporting an irrigation pipe in a mobile irrigation system, such as used in irrigating very large cultivated areas. Such irrigation systems may comprise either a circular system, in which a long irrigation pipe is suspended over the field by one or more such mobile drive mechanisms and is carried in a circular path about a central origin, or may comprise an irrigation pipe which is carried linearly over the field by two or more such mobile drive systems. The present track drive system may be applied to either of the above types of irrigation systems, and provides numerous novel features and advantages over earlier devices of the related art. 
     FIG. 1 provides a general, overall view of the present track drive system, designated by the reference numeral  10 , and supporting an irrigation pipe I. The present track drive system  10  is based upon a single, centrally disposed drive wheel  12  which includes an axle  14  driven by motor means (illustrated in FIG. 2, and discussed in detail further below). A first and a second guide wheel, respectively  16  and  18 , are provided to either side of the guide wheel  12  in the longitudinal direction of travel of the track drive  10 , and widely spaced from the drive wheel  12  to provide good longitudinal stability for the assembly. The two guide wheels  16  and  18  are referred to as “first” and “second,” rather than forward and rearward, as the present track drive system  10  may be operated in either direction as desired. 
     The central drive wheel  12  has a much larger diameter than the two end guide wheels  16  and  18 , in order to provide continuous peripheral contact of the drive wheel  12  with the endless drive track  20  which extends in close contact around the two end guide wheels  16  and  18  and the drive wheel  12 . The present track drive  10  incorporates an articulating frame structure in order to provide a spring biased suspension for compliance of the track  20  with an uneven underlying surface (ruts, ridges, furrows, etc.). The provision of a single central drive wheel  12  having a considerably larger diameter than the two outlying end guide wheels  16  and  18 , assures that the track inner surface  22  remains in continuous tangential contact with the upper and lower periphery of the central drive wheel  12  at all times, regardless of its upward or downward deflection relative to the two end wheels  16  and  18 . The track  20  is driven by the drive wheel  12  by a series of inwardly disposed lugs  24  along the track  20 , which engage equally spaced track engagement passages  26  in the central drive wheel  12  rim. 
     The above wheel  12 ,  16 , and  18  and track  20  components are supported by an articulating frame, formed of a series of members pivotally connected to one another and to wheels  12 ,  16 , and  18 . First and second upper frame members  28  and  30  extend generally radially outwardly and downwardly from the pipe I. The first upper frame member  28  is secured to the pipe I by a pivot pin  32  through its pipe attachment end  34 , while the pipe attachment end  36  (shown in FIGS. 3 and 4) of the second upper member  30  may be rigidly and conventionally affixed (bolted, welded, etc.) to the pipe I. 
     Each of the two upper frame members  28  and  30  has a lower outboard guide wheel attachment end, respectively  38  and  40 , with a pair of spaced apart plates  42  depending therefrom. These plates  42  provide for the rotational attachment of the end guide wheels  16  and  18  to the upper frame members  28  and  30 , by means of axles  44 . 
     The guide wheel attachment plates  42  also include attachment points for first and second lower frame members, respectively  46  and  48 . Each lower frame member  46  and  48  comprises a pair of arms spaced apart by a series of lateral members, with one arm disposed to each side of the wheels  12 ,  16 , and  18  to straddle the wheels. Each lower member  46  and  48  has a respective guide wheel attachment end  50  and  52 , pivotally secured indirectly to the respective guide wheel attachment end  50  and  52  by its attachment to the respective guide wheel plates  44 . (FIGS. 3 and 4 are simplified, due to their schematic nature, and show the end guide wheel axles  44  and lower frame member attachment ends  50  and  52  as being coincident.) 
     Each lower frame member  46  and  48  also includes a drive wheel attachment end, respectively  54  and  56 . The first lower frame member drive wheel attachment  54  comprises a relatively large plate, to provide for the attachment of the second lower frame member drive wheel end  56  thereto. The drive wheel attachment end plate  54  is rigidly affixed (welded, etc.) to the first lower frame member  46  to preclude relative movement, with the second lower frame member drive wheel attachment end  56  being pivotally attached to the first lower frame member drive wheel attachment plate  54 . The plate  54  is rotationally attached to the drive wheel axle  12 . 
     First and second suspension arms, respectively  58  and  60 , have bifurcated drive wheel attachment ends, respectively  62  and  64 , which attach pivotally to the drive wheel  12  by means of the drive wheel attachment plate  54  of the first lower frame member arm  46 . (The opposite attachment ends from those shown in FIG. 1 attach pivotally to the reduction drive gearbox or to a plate attached thereto, generally as shown in the alternate embodiment of FIG. 5 of the drawings.) 
     These suspension arms  58  and  60  each have an opposite upper frame member attachment end, respectively  66  and  68 , comprising a single threaded rod which passes generally medially through a bushing or passage  70  formed through the respective upper frame member  28  and  30 . A coil spring  72  is provided concentric with each of the upper attachment ends  66  and  68  of the two suspension arms  58  and  60 , with an adjustment nut  74  installed on each upper end  66  and  68  of the two suspension arms  58  and  60  below the springs  72 . A stop nut  76  is installed on each suspension arm upper end  66  and  68  to the opposite sides of the upper frame members  28  and  30  from the compression springs  72 . These springs  72  tend to urge the drive wheel  12  downwardly relative to the two upper frame members  28  and  30 , simultaneously spreading the upper frame members  28  and  30 , which along with the weight of the irrigation pipe I and track drive structure  10 , tends to spread the upper and lower members  28 ,  30 ,  46 , and  48  to provide good tension on the drive track  20  and a resilient ride over uneven terrain. 
     The adjustable spring retainer nuts  74  and stop nuts  76  provide for the adjustment of the pressure and operating range of the suspension of the present track drive  10 . If the spring retainer nuts  74  are tightened (i.e., adjusted toward their respective upper frame members  28  and  30 ), the springs  72  are compressed, thereby producing a greater load on the central drive wheel  12  and resisting articulation as the mechanism  10  encounters uneven terrain. Such relatively tight or close adjustment would normally be used in relatively level fields, without excessive ruts, furrows, ridges, etc. On the other hand, loosening the spring retainer nuts  74  reduces pressure on the central drive wheel  12 . This setting would be preferable when operating the present track drive  10  over relatively uneven terrain. The two stop nuts  76  do not affect suspension actuation, other than limiting the extension of the two upper ends  66  and  68  of the suspension members  58  and  60 , to prevent their unseating from their respective bushings  70  in the upper frame members  28  and  30 . 
     FIGS. 3 and 4 provide somewhat schematic side elevation views illustrating the articulation of the present track drive when encountering uneven terrain. In FIG. 3, the central drive wheel  12  is deflected upwardly to a deflected position  12   a , shown in broken lines. This draws the two end guide wheels  16  and  18  inwardly to the positions  16   a  and  18   a  shown in broken lines, due to the fixed length of the endless drive track  20 . The lower portion of the drive track  20  is shown as  20   a , with the inwardly deflected upper frame members  28   a  and  30   a  shown in broken lines in FIG.  3 . 
     The lifting of the wheel  12  is due to the deflection of the belt  20  as it travels over a rise in the underlying terrain. Thus, the track  20  remains in constant contact with the upper and lower peripheries of the drive wheel  12  to provide positive drive for the track  20  at all times. The raising of the central drive wheel  12  relative to the two end guide wheels  16  and  18  deflects the drive wheel attachment ends  54  and  56  of the two lower frame members  46  and  48  upwardly, which also draws their guide wheel attachment ends  50  and  52  inwardly a corresponding amount to that produced by the track  20  deflection described above. The deflected lower arms  46  and  48  are shown in broken lines as arms  46   a  and  48   a  in FIG.  3 . 
     The upward deflection of the lower arms  46  and  48  toward their respective upper arms  28  and  30 , results in the compression of the suspension springs  72  and extension of the upper ends  66  and  68  of the two suspension arms  58  and  60  through their respective bushings  70  in the two upper frame members  28  and  30 . The springs resist the upward deflection of the central drive wheel  12  as described further above, and urge the entire wheel assembly back toward a level position as illustrated in solid lines in FIGS. 3 and 4. 
     The geometry of the two suspension arms  58  and  60  relative to the two upper frame members  28  and  30  results in an angular deflection of the two suspension arms  58  and  60  through their bushings  70  as the structure articulates over uneven terrain. Hence, the bushings may be slotted in the plane of the length of the upper members  28  and  30 , to preclude any bending stresses on the upper ends  66  and  68  of the two suspension arms  58  and  60 . 
     FIG. 4 illustrates the geometry of the present track drive  10  when the central drive wheel  12  encounters a depression (or the equivalent, where the lead end guide wheel encounters a rise). In this case, the drive wheel  12  is forced downwardly to a position  12   b , relative to the two end guide wheels  16  and  18 . The lower portion of the track  20  is correspondingly deflected downwardly, to a position  20   b  wrapping about the lower periphery of the downwardly deflected wheel  12   b . The stop nuts  76  on the suspension arm upper ends  66  and  68  limit the downward travel of the suspension arms  58  and  60 , thus limiting the downward travel of the central drive wheel  12  to ensure that the upper and lower peripheries of the drive wheel  12  remain in constant contact with the drive track  20 . 
     As the downward angular deflection of the two lower frame members  46   b  and  48   b  is about the same as the upward angular deflection of those two arms  46  and  48  when the lower peripheries of all the wheels  12 ,  16 , and  18  are level, the distance between the guide end wheels  16  and  18  remains about the same. Thus, little or no relative angular displacement of the two upper frame members  28  and  30  is present once the downward drive wheel  12  deflection has stabilized as shown by the wheel  12   b  in FIG.  4 . 
     The downward deflection of the wheel  12  as shown in FIG. 4, will also tend to withdraw the upper ends  66  and  68  of the two suspension arms  58  and  60  from their respective bushings  70  in the two upper frame members  28  and  30 . The adjustable upper stop nuts  76  limit suspension travel and prevent complete withdrawal of the suspension arm upper ends  66  and  68  from their bushings  70 . 
     In addition to the above described suspension system, another major advantage of the present track drive mechanism  10  is its relatively narrow width in comparison to earlier developed devices of the related art. Yet, the elongate track and widespread first and second end wheels  16  and  18  provide a relatively large track bearing area or “footprint” for the machine, thereby greatly reducing the tendency for the device to form ruts and depressions in the underlying surface, particularly in soft or muddy ground. FIG. 2 clearly illustrates the relatively narrow width of the present machine  10 , with the drive wheel  12  and first end wheel  16  having a width substantially equal to that of the track  20 . (The second end guide wheel  18 , not shown in FIG. 2, is identical to the first end guide wheel  16 .) 
     The frame structure is also relatively narrow, as can be seen clearly in FIG. 1 of the drawings. The upper frame members  28  and  30  are preferably only slightly wider (if any) than the two end guide wheels  16  and  18 , with the opposed guide wheel attachment plates  42  extending downwardly alongside each guide wheel  16  and  18  to capture the respective guide wheel therebetween. The two end guide wheels  16  and  18  are each formed of an outwardly flanged portion, with the two flanges serving as a bearing or contact surface for the drive track  20 . The two flanges are separated by a series of lateral spacers  78 , which provide a gap between the two wheel portions for passage of the track lugs  24  therebetween. The guide wheel attachment plates  42  are bent inwardly to conform to the relatively narrow space between each end guide wheel portion. 
     It will be seen in FIGS. 1 and 2 that the frame members (e.g., lower frame members  46  and  48 , shown in FIG. 2) and the two suspension arms  58  and  60  define a frame plane for the present mechanism  10 . The drive wheel  12 , two end guide wheels  14  and  16 , and the track  20 , are also coplanar with one another, and define a wheel plane. The placement of the central drive wheel  12 , the two end guide wheels  16  and  18 , and the track  20  directly coplanar with the frame members  28 ,  30 ,  46 , and  48  and suspension arms  58  and  60 , results in these two planes, i.e., the plane of the frame and the plane defined by the wheels, being coplanar with one another. This greatly reduces bending loads upon the axles of the wheels, as well as other stresses, as the attachment of the frame members to both sides of the wheels obviates any cantilevered elements and thus provides a much stronger and sturdier construction. 
     The rim of the central drive wheel  12  is also relatively narrow, having a width substantially equal to that of the narrow track  20 . However, provision must be made for the drive means (motor  80  and reduction drive gearbox  82 ) used to provide the motive power for the present track drive  10 . Accordingly, the drive wheel  12  is asymmetric, with the central disc portion  84  being offset toward one side (i.e., to the right, in FIG.  2 ). A series of radially disposed reinforcement braces  86  may be provided to strengthen the drive wheel  12  assembly, as desired. The open construction of one side of the drive wheel, and spacing between the guide wheel flanges, also assists the machine in the dispersal of debris which might otherwise collect in the wheels. 
     This central portion  84  thus provides a dished out interior  88 , which provides sufficient lateral depth for installing the gear reduction drive  82  and motor  80  (electric, fluid powered, etc., as desired) therein, with the gearbox  82  and motor  80  essentially completely seated concentrically within the interior  88  of the wheel  12  with no componentry extending axially outwardly therefrom. This location for the motor  80  and gearbox  82  provides further benefits in terms of providing a narrow structure for the present track drive machine  10 . This is critical to the efficiency of such mechanisms, as the narrower the machine, the less damage occurs to crops as the drive passes therethrough during operation. While machines with relatively narrow tracks have been used in the past, they universally used one or two wheels, which produce a small and high pressure “footprint.” The present machine&#39;s elongate track produces a relatively low pressure “footprint,” thus greatly reducing damage to the underlying surface, as noted further above. 
     Further means for reducing crop damage may be provided by a crop guide  90  extending from the leading end of the machine  10 , e.g., the first end guide wheel attachment end  38  of the first upper frame member  28 . The crop guide  90  may be applied to either end, or both ends, of the machine  10 , as desired, depending upon the direction of travel of the machine. This guide  90  serves to smoothly divide standing crops and gently urge them aside as the first end guide wheel  16  approaches, thus greatly reducing crop runover by the guide wheel  16 . Other solid or open shapes and configurations for the crop guide  90  may be provided as desired. 
     FIGS. 1 through 4 clearly illustrate the wide spacing between the central drive wheel  12  and the two outlying end guide wheels  16  and  18 . This space provides sufficient room for additional idler wheels, if so desired, to provide additional wheel bearing surface against the inner surface of the track. FIG. 5 provides a side elevation view of a track drive embodiment  100 , equipped with such intermediate idler wheels  102  and  104  between the central drive wheel  112  and the two outlying end guide wheels  116  and  118 . The idler wheels  102  and  104  are carried upon depending lower subframe members  106  and  108  (with the configuration being laterally symmetrical, as noted above for the first embodiment frame), in order to place the lower peripheries of the idler wheels  102  and  104  in the same plane as the other wheels of the device. Reference numerals for the various components of the embodiment  100  of FIG. 5 are one hundred higher than corresponding components of the embodiment  10  of FIGS. 1 through 4, e.g., first and second upper frame members  128  and  130  of the embodiment  100  of FIG. 5 correspond to first and second frame members  28  and  30  of FIGS. 1 through 4, etc. 
     In conclusion, the present track drive for mobile irrigation systems provides a significant advance over earlier devices of the related art, by providing a much gentler “ride” over the underlying surface. The suspension system of the present machine greatly reduces any tendency for the track to “rock” over higher ridges or “bridge” low areas and furrows. The suspension places more of the track area upon the underlying surface at all times, thus greatly reducing the chances of the present track drive becoming stuck or mired in the field, particularly in soft conditions. 
     A further advantage of the present machine is easily noted when the machine of FIGS. 1 through 5 is compared to the prior art irrigation pipe tower T of FIG.  6 . Such prior art devices generally use forwardly and rearwardly disposed wheel W in tandem, in an attempt to avoid an excessively wide track and crop damage. However, only two wheels W rolling over the underlying surface, results in a relatively high local pressure beneath each wheel and tire W, producing ruts and other damage to the underlying surface, as shown in FIG.  6 . 
     Moreover, crop damage is not greatly reduced by such prior art towers T, particularly in high crops (corn, etc.), due to the axially offset, cantilevered attachment of the wheel W to the base of the tower T and the motor M and gear reduction box G extending axially from the base of the tower T and wheel W axle. This relatively low, laterally spread structure breaks down a relatively wide swath of crops, which along with the field damage produced by the high contact pressure of the wheels and tires W, results in considerable cost to the farmer. The present track drive in its various embodiments obviates all of the above problems, and further provides a much sturdier and more robust machine which will provide years of economical service to the farmer. 
     It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.