Abstract:
A crop harvester having a wheeled frame with a tongue pivotally connected to the frame in the vicinity of a longitudinal centerline. The tongue, rotatable about a first generally vertical pivot axis, is swingable between extreme left and right positions on opposing sides of the longitudinal centerline with its distal end adapted for connection to a prime mover. A drive line transmits rotational power from the prime mover. A crop harvesting header, suspended from the frame in a forward disposition, includes operable components for engaging and processing crop material as the harvester is pulled over a field of standing crop. A gearbox receives rotational power from the drive line and transmits it to the operable components. The gearbox, mounted on the crop harvesting header in the vicinity of said longitudinal centerline forwardly of the first generally vertical pivot axis, is journaled for rotation about a second generally vertical pivot axis. A unique electrical control system effects rotational movement of the gearbox in concert with the pivotal movement of the tongue to maintain appropriate alignment between the tongue and the gearbox.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This case is a Continuation-in-part application based on U.S. application Ser. No. 9/036,929, entitled CONTROL APPARATUS FOR ROTATABLE GEARBOX ON CROP HARVESTER, filed Mar. 9, 1998 now U.S. Pat. No. 6,044,633 in the name of Thomas L. Stiefvater, et al. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a pull-type harvesting implement with a pivotal tongue, and more particularly apparatus for controlling in concert with the position of the tongue the position of a rotatable gearbox incorporated in the mechanical drive system for various harvesting components of the implement. 
     BACKGROUND OF THE INVENTION 
     Pull type harvesting implements have been utilized for various harvesting operations for many years. These harvesting implements, supported primarily by ground engaging wheels, are made in a variety of configurations, but normally include components supported on a transverse frame to which is connected a pivotal draft member, commonly called a drawbar or draft tongue. For convenience, any draft member discussed in the following description will be referred to as a tongue. 
     In pull type harvesters, the tongue extends forwardly of the transverse frame for attachment to a prime mover, such as a tractor, which pulls the implement and provides operative power for various harvesting components supported by the main frame. For example, if the implement is a mower-conditioner, cutting and conditioning components mounted on the frame require operative power, which is provided mechanically or hydraulically by the tractor. Hydraulic drive systems are costly and have been fraught with operational problems, not the least of which is power limitations when large implements are involved. Thus, in light of such problems implement designers have turned to the use of a mechanical drive line between the tractor and the harvester. 
     U.S. Pat. No. 4,187,664, issued Feb. 12, 1980 in the name of Nigel W. Meek, et al, and U.S. Pat. No. 5,172,539, issued Dec. 22, 1992 in the name of Richard E. Jennings, et al, show examples of prior art pull type harvesting implements having a pivotal tongue and a mechanical drive system for providing operative power to conventional cutting and conditioning components mounted on a frame. In the implement shown in the &#39;664 patent the tongue pivots about a vertical axis located at the left side of the implement and the mechanical drive system employs a telescopic shaft coupled to a gear box located also on the left side of the implement but forward of the tongue pivot axis. In the &#39;539 patent an implement is shown wherein the vertical axis about which the tongue pivots is centrally located along the frame and the gear box, also centrally located, is located forward of the tongue pivot axis. Among other things, a centrally located tongue permits operation of the implement on both sides of the tractor, as well as superior maneuverability for steering around obstacles and cutting square corners. 
     Other examples of prior art pull type harvesting implements with centrally located pivot tongues and mechanical drive systems with centrally located gear boxes are shown in U.S. Pat. No. 4,899,523, issued Feb. 13, 1990 in the name of Johny Frumholtz, et al, and U.S. Pat. No. 4,719,742, issued Jan. 19, 1988 in the name of Rino Ermacora, et al. In both these implements the drive shaft for the gear box is disposed within the tongue. 
     Another pull-type harvester configuration of the same general nature as those discussed above is the pivot tongue windrower shown in U.S. Pat. No. 5,272,859, issued on Dec. 28, 1993 in the name of Martin E. Pruitt, et al. This patent discloses a central gearbox mounted on the header such that the gearbox is movable relative to the implement frame. The gearbox is rotatable about a vertical axis so that the input shaft into the gearbox can generally follow the pivotal movement of the tongue, which pivots about a rearwardly located vertical axis. This eliminates the occurrence of sharp angles at the gear box under conditions where the tongue swings between its extreme angular positions. 
     To accomplish the coordinated movement of the rotation of the gearbox with the pivotal movement of the tongue, the &#39;859 patent discloses a telescoping steering mechanism that interconnects the tongue and the gearbox such that the gearbox is rotated in response to swinging movement of the tongue relative to the implement frame. The telescopic action of the steering mechanism is operable to accommodate the changes in distance of the respective connecting points of the telescoping mechanism between the tongue and the gearbox, i.e., as the tongue swings from side to side, the distance changes between the different horizontally spaced pivot axes of the tongue and the gearbox. The telescopic action of the steering mechanism, along with a horizontal pivot or gimble, also allows the steering mechanism to follow the vertical movement of the header relative to the tongue without restricting movement of the header or the tongue. 
     This telescopic steering mechanism, as taught by the &#39;859 patent, suffers from operational problems associated with vibration due to the loosely fitting telescopic parts and wear of the sliding parts. The implement, as can be seen in the drawings of the &#39;859 patent, is provided with a rotatable cutting mechanism and a crop conditioning mechanism, which are typical of harvester components. The operation of harvesting components, such as these, provides a substantial amount of vibration within the implement. Since the steering mechanism is loosely constructed to permit the necessary relative pivotal movements of the header and the tongue, operational vibrations are not uncommon, leading to noise and wear in other elements and parts of the harvester. Furthermore, the looseness of the elements and the sliding action therebetween also cause substantial wear in the steering mechanism itself, resulting in increased likelihood of malfunction, or even worse, failure leading to costly downtime. 
     Additionally, the mechanism of the &#39;859 patent is subject to side forces from the torque applied to the gearbox by the drive mechanism and the cutterbar forces within the crop harvesting header on which the rotatable gearbox is mounted. These external forces place a side load on the steering mechanism linkage and can result in a binding of the telescoping tubes forming the linkage, which results in a thrust load on the gearbox and the connecting apparatus, particularly when the header rises and the telescopic tubes telescope together. These telescoping tubes also present a hinge-like degree of freedom which can contribute further to vibrations in the implement. 
     SUMMARY OF THE INVENTION 
     Accordingly, an important object of the present invention is to provide a crop harvester that overcomes the above mentioned problems encountered in the prior art. 
     Another important object is to provide a pull-type, pivot-tongue crop harvester having a mechanical drive line coupled to a pivotal gearbox for operating the components of the harvester with improved serviceability, reliability and repairability. 
     A further important object is to provide a pull-type, pivot-tongue harvester having a mechanical drive line coupled to a pivotal gearbox for operating the components of the harvester with a reliable control system for effecting rotational movement of the gearbox in concert with pivotal movement of the tongue. 
     A still further important object is to provide a pull-type, pivot-tongue harvester having a mechanical drive line coupled to a pivotal gearbox for operating the components of the harvester with an electro-mechanical control system for reliably effecting rotational movement of the gearbox in concert with pivotal movement of the tongue. 
     Yet another important object is to provide a pull-type, pivot-tongue harvester having a mechanical drive line coupled to a pivotal gearbox for operating the components of the harvester with an electro-hydraulic control system for reliably effecting rotational movement of the gearbox in concert with pivotal movement of the tongue. 
     In pursuance of these and other important objects the present invention contemplates a crop harvester having a wheeled frame, a tongue pivotally connected to the frame in the vicinity of a longitudinal centerline thereof, the tongue being rotatable about a first generally vertical pivot axis, means for swinging the tongue between extreme left and right positions on opposing sides of the longitudinal centerline and being adapted at a distal end for connection to a prime mover, a drive line for transmitting rotational power from the prime mover, a crop harvesting header suspended from the frame and being forwardly disposed relative thereto, the crop harvesting header having operable components for engaging and processing crop material as the crop harvester is moved forwardly, a gearbox for receiving rotational power from the drive line and transmitting it to the operable components of the crop harvesting header, means for mounting the gearbox on the crop harvesting header in the vicinity of the longitudinal centerline and forwardly of the first generally vertical pivot axis, the means for mounting said gearbox including journal means for supporting the gearbox for rotation thereof about a second generally vertical pivot axis, and unique control means for effecting rotational movement of the gear box in concert with the pivotal movement of the tongue. More particularly, the control means comprise first and second electrical means, and means interconnecting the first and second electrical means in a master and slave relationship, the first electrical means coupled to the tongue for sensing the position thereof, and the second electrical means coupled to the means for mounting the gearbox for providing a force to rotate the gearbox under conditions where the tongue is rotated about the first vertical pivot axis. 
     The foregoing and other objects, features and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows, in conjunction with the accompanying sheets of drawings wherein a principal embodiment of the present invention and two additional embodiments of structure for accomplishing the same result as the present invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustrative purposes and are not to be construed as defining the limits of the invention. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of a crop harvesting machine incorporating an embodiment of the invention claimed in the parent application referred to in the first paragraph of this application, the harvesting machine being shown connected to a tractor. 
     FIG. 2 is a top plan view of the crop harvesting machine shown in FIG. 1 shifted to a harvesting position rearwardly and to the right of the tractor. This view also depicts in phantom outline the orientation of the tractor, drawbar and drive line under conditions where the crop harvesting machine has been shifted to a harvesting position rearwardly and to the left of the tractor. 
     FIG. 3 is a partial left side elevational view of the crop harvesting machine shown in FIG. 1 with the hitch end of the tongue and the forward portion of the drive line being broken away for purposes of clarity, the header being raised to a transport position. 
     FIGS. 4A,  4 C, and  4 C are three similar schematic diagrams showing key parts of the hydraulic system for the harvesting machine shown in FIGS. 1-3. These diagrams illustrate the transport, and extreme left and right positions of the tongue. 
     FIG. 5 is a top plan view of a crop harvesting machine incorporating another embodiment of the invention claimed in the parent application set forth in the first paragraph of this application, the harvesting machine being shown connected to a tractor. 
     FIG. 6 is a top plan view of the crop harvesting machine shown in FIG. 5 shifted to a harvesting position rearwardly and to the right of the tractor. This view also depicts in phantom outline the orientation of the tractor, drawbar and drive line under conditions where the crop harvesting machine has been shifted to a harvesting position rearwardly and to the left of the tractor. 
     FIG. 7 is a partial left side elevational view of the crop harvesting machine shown in FIG. 5 with the hitch end of the tongue and the forward portion of the drive line being broken away for purposes of clarity, the header being raised to a transport position. 
     FIGS. 8A,  8 B, and  8 C are three similar schematic diagrams showing key parts of the hydraulic system for the harvesting machine shown in FIGS. 5-7. These diagrams illustrate the transport, and extreme left and right positions of the tongue. 
     FIGS. 9A,  9 B, and  9 C are three additional schematic diagrams showing key parts of a control system for the harvesting machine shown in FIG. 5, to which the present invention is directed. These diagrams illustrate the transport, and extreme left and right positions of the tongue. 
    
    
     DESCRIPTION OF THE INVENTION 
     Referring now to the drawings for a more detailed description of the preferred embodiment of the present invention, left and right references are used as a matter of convenience and are determined by standing at the rear of the machine facing the hitch portion of the drawbar, i.e., the normal operative direction of travel. 
     FIGS. 1-3 show a crop harvesting machine, generally designated by reference numeral  10 , in which are incorporated the elements of the preferred embodiment of the invention claimed in the parent application set forth in the first paragraph of this application. More specifically, a centrally pivoted tongue  11  extends forwardly of a transverse implement frame  12 , which is supported by conventional ground engaging wheels  13 ,  14 . A tractor  15  is attached to machine  10 , for towing it over the surface of ground G (see FIG. 3) during transport or field operation. 
     In FIGS. 1 and 3 the inoperative position of the machine is shown, i.e., harvester  10  is raised and directly behind tractor  15  for transport. In FIG. 2 harvester  10  is shown swung to the extreme right operative position in solid lines and it is shown in the extreme left operative position in phantom outline. A tongue swing cylinder  16 , connected between tongue  11  and bracket  17  (integral with frame  12 ), pivots the tongue to these operative positions in a conventional manner. 
     A crop harvesting header  18  is suspended from transverse frame  12  by links  20 ,  21  and springs  22 ,  23  for general vertical movement relative to frame  12  by means of conventional lift cylinders  24  (only one shown). Header  18  includes conventional harvesting components, including a cutterbar  25 , preferably in a known disc configuration, for severing standing crop material, and a conditioning mechanism  26 , preferably of the known counter-rotating roll configuration, the latter receiving severed crop material from cutterbar  25  and then conditioning it by crushing or crimping the stems thereof. U.S. Pat. No. 5,272,859, described above, illustrates a prior art harvester of the general type shown in FIGS. 1-3, and is hereby incorporated by reference. 
     Header  18  further includes a centrally mounted gearbox  27  operable to receive rotational power from a conventional power-take-off driveline  28  (PTO) rotatably supported at bracket  30  by forwardly extending tongue  11  and connectable to tractor  15 , as is well known in the art. The drive line includes a primary PTO  31 , a secondary telescopic PTO  32 , and a slip clutch  33 . In the event the length of the tongue so dictates, a tongue mounted drive shaft can be utilized between the primary and secondary PTO&#39;s in a conventional manner. A shaft of this nature is commonly referred to as a jack shaft. The slip clutch, not shown in detail, is a well known expedient for providing slippage when excessive torque is encountered. This protects the drive components should jamming occur from rocks or excessive debris. The operative combination of gearbox  27  and clutch  33  are adjacently associated in an conventional manner and enclosed in a mounting assembly  34  rotatable about a generally vertical axis  35 , about which an upwardly extending output shaft of the gearbox rotates. Power is provided for driving the harvester components via a belt drive assembly  36  coupled to the output shaft of the gearbox in a well known manner. The reason mounting assembly  34  for the gearbox/clutch is rotatable is so that the universal joint coupling drive line  28  to the clutch/gearbox combination, i.e., the rotatable gearbox assembly, is capable of following the transverse pivotal movements of tongue  11  and thereby keep universal joints  37 ,  38  within acceptable bend angles. To this end, a system is disclosed for maintaining approximate alignment between the drive shaft and tongue during swing operation between the extreme left and right tongue positions. 
     A first hydraulic cylinder  40  is pivotally connected at its rod end to the rear portion of tongue  11  in the vicinity of the pivotal connection  41  of swing cylinder  16 , while at its other end it is pivotally connected to frame  12 . Cylinder  40  is operatively connected via line  42  to a second hydraulic cylinder  43  in a master and slave relationship. Both cylinders,  40  and  43 , are single acting. Second cylinder  43  is pivotally connected at its rod end to the front end portion of pivotal gear box mounting assembly  34 , while at its other end it is pivotally connected to the harvester header frame. Also pivotally affixed to the front end portion of pivotal gear box assembly  34  is one end of compression spring  44 . The other end of compression spring  44  is secured to the header frame. The master slave relationship of single acting cylinders  40 ,  43  is best described by reference to schematic diagrams shown in FIGS. 4A,  4 B and  4 C, where the elements are conveniently associated in close proximity for illustrative purposes. It should be noted that although gearbox  27  is referred to throughout the following description of operation, mounting assembly  34 , clutch  33  and gearbox  27  are integral insofar as rotation about the second pivot axis is concerned. 
     FIG. 4A shows the elements in the transport position, i.e., tongue  11  is directly aligned with an imaginary longitudinal centerline as shown in FIG.  1 . Gearbox  27  is centered about second pivot axis  35 , and the pistons of first and second cylinders  40 ,  43  are midway with communication of hydraulic fluid accommodated by their respective ports. The force of compression spring  44  is such that gearbox  27  is maintained in its centrally aligned position. 
     Referring to FIG. 4B, tongue  11  is pivoted about a first pivot axis  45  in direction a by force F 1  applied by swing cylinder  16  (not shown in FIGS.  4 A- 4 C), piston rod  46  moves in direction b, compression spring  44  forces gearbox  27  to rotate in direction c and force piston rod  47  in direction d, thus maintaining desired relationship of gearbox and tongue when the tongue is swung to one extreme outer position. 
     In FIG. 4C the tongue is shown swung to its other extreme outer position, i.e., a force F 2  in the opposite direction is applied by swing cylinder  11 , piston rod  46  is retracted in direction e, which forces piston rod  47  to extend in direction f, rotating gearbox  27  about axis  35  which compresses compression spring  44 . When the swing tongue reaches it extreme position, rod  46  is maintained in the retracted position and rod  47  in maintained in the extended position with compression spring  44  compressed to a condition where it is ready to force rod  47  in direction d when piston  46  is subsequently moved in a direction b by tongue  11 . 
     Referring now to FIGS. 5-7 for a detailed description of an alternative embodiment of the invention claimed in the parent application set forth in the first paragraph of this application, primed reference numerals are used in instances where the elements being designated are the same or similar to those of the embodiment of FIGS. 1-3. In the embodiment of FIGS. 5-7, crop harvesting machine  10 ′ is shown in which a centrally pivoted tongue  11 ′ extends forwardly of a transverse implement frame  12 ′, which is supported by conventional ground engaging wheels  13 ′,  14 ′. A tractor  15 ′ is attached to machine  10 , for towing it over the surface of the ground, designated generally by reference letter G′ (see FIG.  7 ), during transport or field operation. 
     In FIGS. 5 and 7 the inoperative position of the machine is shown, i.e., harvester  10  is raised and directly behind tractor  15 ′ for transport. In FIG. 6 harvester  10 ′ is shown swung to the extreme right operative position in solid lines and it is shown in the extreme left operative position in phantom outline. A tongue swing cylinder  16 ′, connected between tongue  11 ′ and bracket  17 ′ (integral with frame  12 ′), pivots the tongue to these operative positions in a conventional manner. 
     A crop harvesting header  18 ′ is suspended from transverse frame  12 ′ by links  20 ′,  21 ′ and springs  22 ′,  23 ′ for general vertical movement relative to frame  12 ′ by means of conventional lift cylinder  24 ′, (only one shown). Header  18 ′ includes conventional harvesting components, including a cutterbar  25 ′, preferably in a known disc configuration, for severing standing crop material, and a conditioning mechanism  26 ′, preferably of the known counter-rotating roll configuration, the latter receiving severed crop material from cutterbar  25 ′ and then conditioning it by crushing or crimping the stems thereof. 
     Header  18 ′ further includes a centrally mounted gearbox  27 ′ operable to receive rotational power from a conventional power-take-off driveline  28 ′ (PTO) rotatably supported at bracket  30 ′ by forwardly extending tongue  11 ′ and connectable to tractor  15 ′, as is well known in the art. The drive line includes a primary PTO  31 ′, a secondary telescopic PTO  32 ′, and a slip clutch  33 ′. As in the principal embodiment, gearbox  27 ′ and clutch  33 ′, enclosed in a mounting assembly  34 ′, all form an integral unit rotatable about a generally vertical axis  35 , about which an upwardly extending output shaft of the gearbox rotates. Power is provided for driving the harvester components via a belt drive assembly  36 ′ in a well known manner. As in the previous embodiment, the purpose of mounting assembly  34 ′ is so that the integral gearbox/clutch unit is capable of following the transverse pivotal movements of tongue  11 ′ and thereby keep universal joints  37 ′,  38 ′ within acceptable bend angles. To this end, this embodiment contemplates another unique system for maintaining approximate alignment between the drive shaft and tongue during swing operation between the extreme left and right tongue positions. 
     A first hydraulic cylinder  50  is pivotally connected at its rod end to the rear portion of tongue  11 ′ in the vicinity of the pivotal connection  41 ′ of swing cylinder  16 ′, while at its other end it is pivotally connected to frame  12 ′. Cylinder  50  is operatively connected via lines  51 ,  52  to a second hydraulic cylinder  53  in a master and slave relationship. Both cylinders,  50  and  53 , are double acting. Second cylinder  53  is pivotally connected at its rod end to the front end portion of pivotal gear box mounting assembly  34 ′, while at its other end it is pivotally connected to the harvester header frame. The master slave relationship of double acting cylinders  50 ,  53  is best described by reference to schematic diagrams shown in FIGS. 8A,  8 B and  8 C, where the elements are conveniently associated in close proximity for illustrative purposes, while applying the same rationale as above with respect to the integral relationship of the gearbox, clutch and mounting assembly. 
     FIG. 8A shows the elements in the transport position, i.e., tongue  11 ′ is directly aligned with an imaginary longitudinal centerline as shown in FIG.  5 . Gearbox  27 ′ is centered about second pivot axis  35 , and the pistons of cylinders  50 ,  53  are midway with communication of hydraulic fluid accommodated by their respective dual ports. 
     Referring to FIG. 8B when tongue  11 ′ is pivoted about first pivot axis  45 ′ in direction a′ by force F 1 ′ applied by swing cylinder  16 ′ (not shown in FIGS.  8 A- 8 C), piston rod  54  moves in direction b′, and piston rod  55  moves in the opposite direction d′, thus maintaining desired relationship of gearbox and tongue when the tongue is swung to one extreme outer position. 
     In FIG. 8C the tongue is shown swung to its other extreme outer position, i.e., a force F 2 ′ in the opposite direction is applied by swing cylinder  16 ′, piston rod  54  is retracted in direction e′, which forces piston rod  55  to extend in direction f′, rotating gearbox  27 ′ about axis  35 ′ by virtue of the master slave relationship between the cylinders. When the swing tongue reaches it extreme position rod  54  is maintained in the retracted position and rod  55  in maintained in the extended position due to the communication of hydraulic fluid between the interconnected dual ports. 
     In operation, adjustment of gearbox tongue alignment is achieved by selection of cylinders having appropriate diameters, or by varying the lever distance of one of the paired cylinders. 
     Attention is now directed to FIGS. 9A,  9 B and  9 C, where an alternative embodiment of the control system for the harvesting machine shown in FIGS. 5-7 is illustrated in schematic form. The key elements of this control system are represented by a block diagram comprising blocks  60  and  61 , with interconnections  62  and  63 . Block  60  consists of components that generate a signal representative of the position of rod  54 , which signal is coupled to the components of block  61  via interconnections  62  and  63 . In conjunction therewith block  62  consists of components that utilize the signal representative of the position of rod  54  to move rod  55 . Thus, as in the above embodiments, gearbox  27 ′ is rotated an amount consistent with the position of tongue  11 ′. 
     More specifically, block  60  comprises a linear displacement transducer, commonly known in the art of feedback control systems. An electrical signal is produced that is proportional to the linear position of rod  54 , which is reciprocated as tongue  11 ′ pivots about pivot axis  45 ′. The signal so produced is utilized by block  62 , which comprises an electrical actuator that responds to the signal and selectively drives rod  55  to position gearbox  27 ′ at its corresponding desired angular position. One commonly known electro-mechanical actuator contemplated for block  62  consists of a motor driving a lead screw in a manner similar to the arrangement used on a control system for controlling the position of a forage harvester discharge spout relative to the receiving receptacle. Another acceptable arrangement for the components of block  62  contemplates an electro-hydraulic actuator commonly used in agricultural implements where an electrical signal is utilized to control the mechanical displacement of a hydraulic cylinder, hydraulic motor, or the like. 
     The following detailed description of the control system of FIGS. 9A-9C is characterized in conjunction with, and therefor refers back to, the harvester of FIGS. 5-7. FIG. 9A shows the elements in the transport position, i.e., tongue  11 ′ is directly aligned with an imaginary longitudinal centerline as shown in FIG.  5 . Gearbox  27 ′ is centered about second pivot axis  35 ′, and the linear displacement of the conventional transducer of block  60  and the rod of the electrical actuator of block  61  are midway with communication of such linear displacement accommodated by interconnections  62  and  63 . 
     Referring to FIG. 9B when tongue  11 ′ is pivoted about first pivot axis  45 ′ in direction a′ by force F 1 ′ applied by swing cylinder  16 ′ (not shown in FIGS.  8 A- 8 C), rod  54  moves in direction b′, and rod  55  moves in the opposite direction d′, thus maintaining desired relationship of gearbox and tongue when the tongue is swung to one extreme outer position. It should be noted that the angular relationship need not be directly proportional, although that may in fact be the case if so desired. 
     In FIG. 9C the tongue is shown swung to its other extreme outer position, i.e., a force F 2 ′ in the opposite direction is applied by swing cylinder  16 ′, rod  54  is retracted in direction e′, which results in rod  55  being extended in direction f′, rotating gearbox  27 ′ about axis  35 ′ by virtue of the master slave relationship between the elements of blocks  60  and  61 . When the swing tongue reaches it extreme position rod  54  is maintained in the left most position and rod  55  in maintained in the right most position due to the feedback control system arrangement of the components of blocks  60  and  61 . It should be further noted that by its very nature this system lends itself to being adapted to common servo control technology. In this regard a microprocessor system could be utilized for the purpose of conforming the desired angular relationships to enhance overall performance. 
     Of the many implicit and explicit advantages of the present invention one of the most important is the provision of a center pivot tongue harvesting machine having a mechanical drive with a rotatable gearbox assembly that tracks the tongue when it swings, in a manner that obviates the need for mechanical steering links. Among other things, gearbox rotation in the manner described above improves component reliability and reduces overall vibration during operation. With prior art mechanical telescoping systems, the gearbox was steered to a point on the tongue, resulting in the gearbox pivot angle always exceeding the tongue pivot angle. In the instant invention, the gearbox angle can be selected for optimum value, i.e., it can understeer, steer equally, or oversteer the tongue. Experience by applicants has demonstrated that optimum conditions are realized when the gearbox is steered equally with the tongue, resulting in equal angles in the secondary PTO with minimal gearbox rotation. 
     While preferred structure in which the principles of the present invention are shown and described is set forth above, it is to be understood that the invention is not limited to such preferred structure, but that, in fact, widely different means of varying scope and configuration may be employed in the practice of the invention.