Abstract:
An agricultural tractor rear suspension system having a longitudinally extending suspension arm pivotally coupled to the tractor chassis at one end and to a planetary gearbox at the opposing end, a differential connected to the tractor chassis, and a driveshaft rotatably supported at one end by the planetary gearbox and supported at the other end by the differential housing wherein the driveshaft is configured to resist suspension arm flexure by communicating tensile and compressive forces from the suspension arm to the tractor chassis.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of U.S. Patent application Ser. No. 10/911,888, filed Aug. 5, 2004 now U.S. Pat. No. 7,204,340. 

   FIELD OF THE INVENTION 
   The present invention relates generally to suspensions. More particularly, it relates to tractor suspensions, and even more particularly to rear suspensions for agricultural tractors having a drive shaft configured to be tensioned to resist suspension flexural. 
   BACKGROUND OF THE INVENTION 
   Agricultural tractors have traditionally been unsprung. From their earliest beginnings in the late 1800&#39;s they have been supported on fixed axles extending from the chassis. 
   Originally, tractors were used as stationary engines. Located in a fixed position in a field, farmers would gather crops to be threshed and bring them in wagon loads to the tractor and a belt-driven threshing machine. In these early days, the ability to move fast was not important. 
   Tractors were gradually modified to tow implements such as plows, rakes, harrows, planters, and manure spreaders through agricultural fields. These mobile tractors did not need a great deal of speed since they replaced horses or oxen and needed only enough power to tow implements at horse or ox speed. 
   As time passed, engineers designed ever larger and stronger implements. To tow these implements, tractors were also made stronger and larger, with ten to fifty times the horsepower of the early tractors. 
   Eventually, agricultural tractors were capable of towing implements at higher speeds through agricultural fields. To accommodate these greater speeds, manufacturers began to develop front suspensions with springing and shock absorbing capability. These front suspensions were configured to pivot, permitting the front wheels of the tractor to keep a good grip on the ground as the terrain changed. As of today, however, no major manufacturer of tractors sells a commercially accepted agricultural tractor with a sprung rear suspension. 
   A primary reason that tractors with sprung rear suspensions have not been manufactured is due to the reaction forces that arise when a load is placed on the tractor. Traditional agricultural tractors have large rear wheels, typically on the order of approximately 1 to 2.2 meters in diameter. The large rear wheels apply high force to the ground, especially when a ground-engaging implement is ripping furrows through the ground 2 to 18 inches deep. The ground, in turn, applies an equally high (but in the opposite direction) reaction force on the frame of the tractor, and the reaction force can generate a moment great enough to literally lift the front wheels of a tractor without a rear suspension a meter or more off of the ground. 
   The existence of a moment large enough to lift the front wheels is best illustrated with reference to  FIG. 11 , which schematically shows a tractor  700  without a front or rear suspension towing an implement  148 . An implement, resultant-force vector  402  is applied to the implement by the ground as the implement is pulled through the ground by the tractor  700 . Implement force vector  402  can be broken down into two force vectors  404 ,  406  that represent the horizontal force (vector  404 ) acting to drag on the implement during forward motion, and the vertical force (vector  406 ) that pulls downward on the implement. 
   The implement is rigidly coupled to the tractor typically through a three-point hitch. The three-point hitch couples the implement to the tractor frame via a lower point A and an upper point B. The implement force vector  402  applies draft forces on the tractor that can be separated into horizontal and vertical forces F Ax  and F Ay  acting through the lower link  902  (i.e., at point A) and horizontal and vertical forces F Bx  and F By  acting through the upper link  904  (i.e., at point B). As one of ordinary skill will appreciate, the relative magnitudes of the component draft forces F Ax , F Ay , F Bx  and F By  depend upon the geometry of the three-point pitch. 
   Other forces acting on the tractor  700  include weight (depicted in the drawing as mg), which acts on the center of gravity C G . In response to the weight, the ground applies forces F f  and F r  to the tractor through the front and rear axles, respectively. 
   There are torques shown in  FIG. 11  as well. Drive torque T D  is the torque applied by the engine (not shown in  FIG. 11 ) to the axle (also not shown) to drive the rear wheels. When the tractor is being driven forward, the drive torque is clockwise. The rear wheels, as they are being driven, apply a force on the ground, and the ground, in turn, applies an equal and opposite traction force F Tr  on the wheels that is applied to the tractor frame. The traction force of course is responsible for forward movement of the tractor. 
   Drive torque T D  also generates a reaction torque (that is, traction torque T Tr ) that acts on the frame of the tractor. The traction torque is proportional to the traction force F Tr  and is counterclockwise. 
   The forces and torques generate moments about a point on the tractor that tend to rotate the tractor about that point. For convenience, the point will be called the center of pitch C p . Its location depends upon a number factors one of ordinary skill will appreciate. While the forces and torques may generate moments that cancel each other out to some extent, the net effect of all of the moments is to generate a counterclockwise moment M p  about the center of pitch when the implement force vector  402  increases. The implement force vector increases when the implement  148  hits a stone, compacted soil, or some other such condition. As previously mentioned, the increased implement force vector can be large enough to cause a moment M P  about the center of pitch that is itself large enough to lift the front tires and increase the load on the rear tires. 
   If the rear wheels were suspended on the frame rather than being fixed, the moment M P  will not at first lift the front wheels, but it will tend to cause the rear suspension to squat. Such squatting can be disconcerting to the operator and can also wreak havoc on implement depth-control systems, which typically require a constant relationship between the tractor-frame and implement-frame heights. 
   One of ordinary skill will appreciate that some suspension configurations will cause the tractor to rotate clockwise (rather than counterclockwise, as has been described) when the tractor is subjected to increased loads. However, for the purposes of this discussion, we will consider the more intuitive case where the tractor rotates counterclockwise in response to increased loads. Nevertheless, the basic principles (and the problems with conventional systems) described herein are the same. Moreover, the principle of operation of the preferred embodiments (which will be described below) is the same regardless of whether the suspension tends to squat or sit up. 
   The suspension arrangement of the present invention generates a reaction torque on the vehicle to reduce the moment M P  about the center of pitch. In other words, when the tractor pulls harder on its implement, the suspension in accordance with the present invention generates an increased counteracting, or reaction, force that matches or is proportional to the increased, horizontal force vector  404 . Similarly, when the tractor pulls more gently on its implement, the suspension in accordance with the present invention generates a decreased force that matches the decreased horizontal force vector  404 . 
   The applicant, in his co-pending patent application U.S. patent application Ser. No. 10/911,888, described a tractor that would solve many of these problems. 
   One problem that was not addressed by the tractor of that application was the undesirable flexing of the suspension arms when the tractor is placed under extreme loads. Agricultural tractors are not designed to go fast, but to go slow and generate extremely large pulling forces. As the force diagram in  FIG. 11  illustrates, the forces can be quite large. They can be so large, in fact, that the suspension arms are deflected inward or outward, pulled away from or pressed toward the vehicle. If the forces are large enough, they can bend the suspension arms. They can also damage the pivot joint that couples the suspension arms to the chassis of the tractor. 
   What is needed, therefore, is a suspension arrangement that counters the flexure of the suspension arms and reduces the forces otherwise applied to the suspension arm pivot joint. What is also needed is an apparatus for countering these forces that uses an existing structure coupling between the suspension arm and the chassis. What is also needed is an apparatus for transmitting longitudinal loads though a rotating drive shaft. 
   It is an object of this invention to provide a tractor and tractor suspension that has these benefits. 
   SUMMARY OF THE INVENTION 
   In accordance with a first aspect of the invention, a rear tractor suspension for a tractor chassis having a differential mounted in a differential housing is provided, the suspension comprising a longitudinally extending suspension arm pivotally coupled to the chassis; a planetary gearbox fixed to the suspension arm; and a drive shaft rotatably supported at one end by the planetary gearbox, and rotatably supported at the other end by the differential housing, wherein the drive shaft is configured to reduce additional outward flexure of the suspension arm when the suspension arm is outwardly flexed, and to reduce additional inward flexure of the suspension arm when the suspension arm is inwardly flexed. 
   The drive shaft may further comprise first and second stub shafts coupled together to permit the stub shafts relative longitudinal sliding movement and to communicate a longitudinal tensile load. The drive shaft may be configured to be placed in longitudinal compression by non-plastic deformation of the suspension arm inward toward the chassis and to be placed in longitudinal tension by non-plastic deformation of the suspension arm outward away from the chassis, respectively. The tension and the compression may communicate a load from the suspension arm to the chassis of the vehicle. The drive shaft may reduce outward flexure by being placed in tension and the drive shaft may reduce inward flexure by being placed in compression. The drive shaft may further comprise first and second flexible shaft couplings; and a sliding coupling that is coupled to and between the first and second flexible shaft couplings. 
   In accordance with a second aspect of the invention, a rear suspension for an agricultural tractor having chassis comprised of an engine, transmission and rear differential housing fixed together to form an elongate rigid member is provided, the suspension comprising a longitudinally extending suspension arm coupled to the chassis to pivot with respect thereto about a laterally-extending axis; a planetary gearbox fixed to the suspension arm; a drive shaft rotatably supported at one end by the planetary gearbox and rotatably supported at the other end by the tractor chassis, wherein the drive shaft is configured to be longitudinally compressed to reduce lateral inward flexure of the suspension arm, and configured to be longitudinally tensioned to reduce lateral outward flexure of the suspension arm during normal operation. The drive shaft may further comprise first and second stub shafts coupled together with a sliding coupling to communicate torque from one stub shaft to the other stub shaft, to permit sliding relative longitudinal movement of the stub shafts, to communicate a tensile load from one stub shaft to the other stub shaft when the coupling is collapsed, and to communicate a compressive load from one stub shaft to the other stub shaft when the coupling is extended. The sliding coupling may be configured to be placed in longitudinal tension by flexure of the suspension arm away from the chassis of the tractor and placed in longitudinal compression by flexure of the suspension arm toward the chassis of the tractor. The tension and the compression may communicate a load on the suspension arm to the chassis of the vehicle. The drive shaft may further comprise first and second flexible shaft couplings disposed in the drive shaft to support opposing ends of the sliding coupling. The drive shaft may reduce outward suspension arm flexure by being placed in tension and reduces inward suspension arm flexure by being placed in compression. 
   In accordance with a third aspect of the invention, a rear tractor suspension for a tractor chassis having a differential mounted in a differential housing is provided, the suspension comprising a longitudinally extending suspension arm pivotally coupled to the chassis; a planetary gearbox fixed to the suspension arm; and a means for rotatably driving the planetary gearbox supported for rotation both by the planetary gearbox and by the tractor chassis, wherein the means for rotatably driving is configured to be longitudinally tensioned and compressed during normal operations. 
   The means for driving may further include first flexible coupling means; second flexible coupling means; and slidable coupling means; wherein first flexible coupling means is fixed to a first end of slidable coupling means and second flexible coupling means is fixed to another end of slidable coupling means. The slidable coupling means may include means for telescopically extending and retracting and a means for transmitting torque. The slidable coupling means may be configured to be placed in longitudinal compression by non-plastic deformation of the suspension arm inward toward the chassis and in longitudinal tension by non-plastic deformation of the suspension arm outward away from the chassis, respectively. The tension and the compression may communicate a load on the suspension arm to the chassis of the vehicle. 
   The slidable coupling means may reduce outward arm flexure by being placed in tension and may reduce inward arm flexure by being placed in compression. One end of the slidable coupling means may comprise a cup having internal splines. The other end of the slidable coupling means may comprise a cylinder having external splines. The cylinder may be slidably supported within the cup and the internal splines are engaged with the external splines. 
   Numerous other features and advantages of the present invention will become readily apparent from the following detailed description, the accompanying drawings, and the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a tractor in accordance with the present invention. 
       FIG. 2  is a fragmentary side view of the tractor of  FIG. 1 , showing the chassis and right rear suspension in greater detail. 
       FIG. 3  is a perspective view of the left and right suspension arms, springs, anti-sway linkage and pivot pin of the tractor of the foregoing FIGURES. 
       FIG. 4  is a partial cutaway rear view of the right side planetary gear system taken at section line  5 - 5  in  FIG. 2 . 
       FIG. 5  is a fragmentary longitudinal cross-sectional view of the drive shaft coupling of  FIG. 4  with the coupling in its completely collapsed position, the cross section being taken through the central longitudinal axis of the drive shaft coupling. 
       FIG. 6  is an exploded perspective view of the drive shaft coupling of  FIG. 5 . 
       FIG. 7  is a fragmentary cross-sectional view of an alternative drive shaft coupling to be used in place of the coupling of  FIGS. 4-6 . The coupling is shown in its completely extended position. 
       FIG. 8  is a fragmentary cross-sectional view of the coupling of  FIG. 7 . The coupling is shown in its completely collapsed or retracted position. 
       FIG. 9  is a cross-sectional view of the drive shaft coupling of  FIGS. 7-8  taken along section line B-B in  FIG. 8 . 
       FIG. 10  is a cross-sectional view of the drive shaft coupling of  FIGS. 7-9  taken along section line A-A in  FIG. 7 . 
       FIG. 11  is a force diagram of the tractor of the foregoing FIGURES showing the forces applied by the tractor to the ground and the reaction forces applied to the tractor. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   While the present invention is susceptible of being made in any of several different forms, the drawings show a particularly preferred form of the invention. One should understand, however, that this is just one of many ways the invention can be made. Nor should any particular feature of the illustrated embodiment be considered a part of the invention, unless that feature is explicitly mentioned in the claims. In the drawings, like reference numerals refer to like parts throughout the several views. 
     FIGS. 1 ,  2  and  3  show a tractor  100  having a chassis  102  to which right and left suspension arms  104 ,  106  ( FIG. 3 ) are coupled. Rear wheels  108 ,  110  are mounted to axles  124  extending from suspension arms  104 ,  106  and support the tractor for movement over the ground. The axles extend laterally, or side to side, with respect to the tractor. The wheels (including tires) preferably have a diameter of at least 1.5 meters, more preferably at least 2 meters, and even more preferably at least 2.5 meters. They may be fixed to axles  124  at several positions along the length of the axle including positions at least 0.25, 0.5, 1.0, and 1.5 meters or more away from the suspension arm. This is quite unlike automobiles or trucks, in which wheels of 0.3 meter diameter are mounted on axles that extend perhaps 0.2 meters from a suspension arm. Two front wheels  112  (only one shown) are coupled to the front portion of the chassis on opposite sides of the front end to support the front of the vehicle. 
   Referring in particular to  FIG. 3 , each suspension arm  104 ,  106  has a front end  114  and a rear end  116 . The suspension arms are oriented generally fore-and-aft and extend longitudinally along the side of the tractor. The suspension arms are trailing links. The front end  114  is pivotally coupled to the chassis and the rear end  116  is supported by a spring  118 . Spring  118  in the preferred embodiment shown here is a hydraulic cylinder that is coupled to a hydraulic circuit including valves and a gas-charged hydraulic accumulator (circuit not shown) to keep the spring  118  extended the appropriate amount. 
   Hydraulic cylinder  118  in the preferred embodiment shown here is coupled to a gas-charged hydraulic accumulator (not shown). As the tractor is loaded and unloaded, the hydraulic cylinders coupled to the accumulator (or accumulators) act as springs. When the load increases on the rear of the tractor, the suspension arms push upward on the cylinder portion of cylinders  118 . This increases the hydraulic pressure in the cylinder and ejects hydraulic fluid into the gas-charged accumulator. This additional hydraulic fluid in the accumulator causes the pressure in the accumulator and the cylinder to increase until the cylinder pressure is just able to counteract the increased force acting on the swing arm. 
   When the load is decreased on the rear of the tractor, the reverse is true. Cylinders  118  gradually extend, pivoting the rear of suspension arms  104 ,  106  downward, permitting hydraulic fluid to escape the accumulator (or accumulators), and permitting the pressure inside the cylinders to decrease until it just balances the reduced load applied to suspension arms  104 ,  106 . 
   Each suspension arm has an associated planetary gear system  120 , which is fixed to the rear end  116  of each suspension arm  104 ,  106 . The planetary gear system  120  supports the axle  124  that extends from the gear system. The left and right rear wheels  108 , 110  are mounted to left and right axles  124 . 
   The front end  114  of each suspension arm  104 ,  106  is preferably coupled to chassis  102  by a pin  126 . Pin  126  extends through an inner eye member  130  and an outer eye member  128  formed in the front end  114  of the suspension arm. Pin  126  also extends through an eye member  132  ( FIG. 3 ) that is fixed to chassis  102  and fits between the inner and outer eye members  128 ,  130  on the suspension arms  104 ,  106 . Pin  126 , eye members  128 ,  130  and eye member  132  are closely toleranced, such that suspension arms  104 ,  106  are constrained by pin  126  to rotate about a laterally extending axis  134  best seen in  FIG. 3 . This arrangement also constrains the rear ends  116  of the two suspension arms to pivot about axis  134  and (in general) to move only up and down with respect to the chassis  102 . 
   Referring to  FIG. 2 , each spring  118  is coupled at its lower end to its associated suspension arm by a pivot pin  136  that extends through the suspension arm and through an eye formed in the lower end of the spring  118 . This arrangement permits the lower end of the spring  118  to pivot with respect to the suspension arm. A similar eye  140  is formed in the upper end of rod  142  extending from the hydraulic cylinder body  144  which is similarly pivotally coupled to a pin  146 . Pin  146  is fixed to chassis  102  preferably via the tractor&#39;s rockshaft. However, the rod  142  may be coupled to the chassis at other locations. 
   The suspension arms pivot freely with respect to the chassis  102  with only two limits to their movement: the springs  118  and inter-arm, or anti-sway, link  154 . As shown in  FIG. 3 , link  154  is coupled to and extends between both of the suspension arms  104 , 106 . The anti-sway link  154  is essentially an anti-roll bar providing a passive, anti-roll function. The anti-sway link  154  can be used alone or in conjunction with other anti-roll features, such as active control of hydraulic springs  118 . Such active anti-roll will be discussed in more detail below. 
   The anti-sway link  154  operates in the following way. When one wheel goes over a bump causing its suspension arm to pivot upward, the pivoting suspension arm flexes one end of link  154 . The other end of link  154  is connected to the other suspension arm and reacts to this movement by attempting to pivot the other suspension arm upward to the exact same degree that the first suspension arm pivoted. The second suspension arm, however, is resting on ground (via the axle and wheel) at a slightly different height and is held against the ground by its own spring  118 . Spring  118  of the second suspension arm resists the upward movement of the second suspension arm by link  154 , preventing link  154  from moving the second suspension arm into a perfectly parallel relationship with the first suspension arm. As a result, both suspension arms do not move together to the same (i.e. parallel) positions, and the link  154  twists. The link thereby acts as a torsional spring to resist rolling motion by the tractor. 
   The link&#39;s ability to twist is due to its construction. As shown in  FIG. 2 , link  154  is formed as two parallel plates of steel  156 ,  158  that are spaced apart by spacers  160 . Bolts  159  ( FIG. 2 ) are inserted into holes in the ends of the plates and the spacers. These bolts are inserted into threaded holes in the suspension arms  104 ,  106  and tightened. Bolts  159  are located on each end of link  154  to secure left and right ends of link  154  to the left and right suspension arms, respectively. 
   Swing arms  104 ,  106  pivot about pin  126  with respect to the chassis of the tractor. As they pivot, they also flex with respect to the chassis of the tractor. To provide a substantially up-and-down movement, the pivot point defined by pin,  126  must be extended substantially ahead of the axles  124 . This distance, unfortunately, requires that the swing arms extend several feet backward from the pivot point defined by pin  126 . As a result, when large loads are placed on the tractor&#39;s wheels, the swing arms tend to flex laterally inward toward the center line of the vehicle or laterally outward away from the centerline in a direction generally parallel to the longitudinal axis of the axles  124 . 
   These flexing forces put considerable stress on the forward ends  114  of the suspension arms and the eyes  132  and pin  126  to which the forward ends are coupled. 
     FIG. 4  illustrates a preferred planetary gear arrangement of the suspension arms  104 , 106  of  FIGS. 1-3 .  FIG. 4  is a cross section through the planetary gear system  120  of the right side suspension arm. It is taken at section line  5 - 5  of  FIG. 2 . The cutting plane that defines section  5 - 5  passes through the centerline of right side axle  124  to which the right wheel is mounted. 
   The discussion below relates to the right side planetary gear system. The left side planetary gear system is identically disposed and configured as the right side planetary gear system, but in mirror image form and on the opposite side of the vehicle on the left side suspension arm. Since the two are identical in construction and operation, we do not separately discuss the left side planetary gear system. 
   As shown in  FIG. 4 , planetary gear system  120  includes a drive shaft  500  that is coupled to a sun gear  502 . The tractor engine, typically through a drive shafted connected to the engine&#39;s crankshaft and a set of differential gears, drives sun gear  502 , which drives three planetary gears that engage a ring gear  508  (only two planetary gears  504 ,  506  are shown in  FIG. 4 ). 
   In the embodiment of  FIG. 4  the sun gear preferably has  15  teeth and the ring gear preferably has 73 teeth, although one of skill in the art will appreciate that any number of teeth may be used without departing from the scope of the invention. The planetary gears drive planetary gear carrier  510 , which is coupled to and drives right side axle  124 . 
   The gear system according to the embodiment shown in  FIG. 4  comprises two casings  518  and  520 . Casing  518  is bolted to outer wall  514  of suspension arm  104  by bolts  522 . Casing  520  is bolted to casing  518  by bolts  524 . Ring gear  508  is fixed between the two casings  518 ,  520  and is fixed to the two casings to make a rigid casing when bolts  524  are tightened. 
   Casings  518 ,  520  support two bearings  526  and  528 , respectively, on their inner surfaces. These two bearings  526 ,  528  support the planetary gear carrier  510 . Bearings  526  and  528  support the entire weight of the right rear side of the vehicle. Since the wheels may be mounted on axle  124  at some distance from bearings  526 ,  528 , there may be a considerable overhanging load acting on these bearings. For this reason, they are preferably spaced apart a distance of several hundred millimeters, preferably at least 460 mm. The spacing of the bearings may be increased or decreased in application as is necessary; but one of ordinary skill will appreciate that greater bearing spacing—as is achieved in the preferred embodiment of this invention—is preferred because it withstands the overhanging load better than relatively narrow spacing. 
   Casings  518  and  520  also support two seals  530  and  532  that are disposed to seal against the inner and outer ends, respectively, of planetary gear carrier  510 . These seals keep gear lubricant inside gear housing  516  and ensure that the gears are bathed in lubricant. 
   Axle  124  is force fit to planetary gear carrier  510  to collectively form a rigid rotating member that is disposed inside housing  516 . As will be described in detail below, the planetary gear carrier  510  is in the form of a hollow cylinder that is configured to receive and support the sun gear for rotation in the inboard end of the carrier  510  and to receive (and be fixed to) the axle  124  in the outboard end of the carrier  510 . 
   The sun gear  502  is supported inside the inboard hollow end of the gear carrier  510  on bearing  534 . Bearing  534  permits free rotation of the sun gear  502  with respect to gear carrier  510 . A seal  536  is fixed on the outboard side of the bearing  534  to ensure that gear lubricant does not leak out of housing  516  between the sun gear shaft and the inner surface of gear carrier  510 . 
   Drive shaft  500  includes a first flexible rotating shaft coupling  538  (e.g. a universal joint or constant velocity joint, flexible link coupling, ball and socket joint or other coupling or joint configured to transmit torque and provide two degrees of rotational freedom) that is coupled to and drives sun gear  502 . Coupling  538  permits the suspension arm  104  to pivot, or travel, up and down with respect to the differential housing  540 . 
   The left end (in the FIGURE) of drive shaft  500  is supported for rotation in differential housing  540  by bearing  541 , which permits the end of the drive shaft to rotate with respect to the differential housing. A seal  543  seals against drive shaft  500  and differential housing  540  to prevent differential-housing lubricant from leaking out of the differential housing. The differential gears to which the left end of drive shaft  500  is coupled have been removed for clarity of illustration in this FIGURE. 
   Drive shaft  500  includes a second flexible coupling  542 . Coupling  542  also permits the suspension arm  104  to pivot up and down with respect to differential housing  540 . 
   Drive shaft  500  includes a central drive shaft coupling  544  that is disposed between and couples stub shafts  548 ,  550  that extend from both flexible couplings, forming a central portion of the drive shaft. This central coupling includes two half-cylinder retainers  546 . Retainers  546  are butted together to form a cylinder that surrounds and couples the free ends  548  and  550 , preventing them from being completely pulled apart and separated. Two fasteners  552  extend through retainers  546  to secure them to stub shaft  548 . 
   When suspension arm  104  is flexed under load and pulled away from differential housing  540  of chassis  102 , it pulls stub shaft  550  (coupled to the suspension arm) away from stub shaft  548  (coupled to the differential housing). This causes the drive shaft to lengthen until it is fully extended and reaches an extension limit. At this point, the two stub shafts cannot be pulled apart further and resist further flexing of arm  104  away from the chassis  102 . This applied force reduces the flexing of arm  104  and provides greater rigidity to the suspension. 
   Gear carrier  510  may be formed as a single cylindrical casting including a flared central portion, which includes machined bearing mounts and three machined slots  554  (two shown in  FIG. 4 ) to receive the three planetary gears. Through holes  556  are machined in the carrier  510  to receive planetary gear axles  558 . Bearings  560  are disposed between the planetary gears and their respective axles to support the planetary gears for rotation on their axles. 
   In an alternative arrangement, however, gear carrier  510  is formed from two castings, rather than a single casting. A flanged, inner cylindrical portion  562  and a flanged, outer cylindrical portion  564  of planetary gear system  120  may be separately cast and subsequently bolted together with their flanged ends facing each other. 
   It should be noted that  FIG. 4  shows two planetary gears  504 ,  506  that are spaced apart by 180 degrees about drive shaft  500 . All three planetary gears are actually disposed at 120 degrees with respect to one another about the longitudinal axis of the planetary gear system  120 . It is for ease of illustration, understanding, and explanation that only two planetary gears are shown in  FIG. 4  and that they are shown spaced 180 degrees apart. 
   The two casings  518 ,  520  are preferably generally conical. Casing  518  is preferably in the form of a conical section with its vertex pointing inward toward the differential housing and casing  520  is preferably in the form of a conical section with its vertex pointing away from the differential housing. This conical configuration provides a flaring inner surface on both casings that makes it easy to mount the seals and the bearings. 
     FIGS. 5 and 6  illustrate central drive shaft coupling  544  in greater detail. Coupling  544  includes a cup  570  having internal splines  572  that extend longitudinally on an inner surface of cup  570 . Cup  570  is fixed to and coaxial with an end of shaft  548 . Coupling  544  also includes a cylinder  574  having external splines  576  that is disposed to fit inside cup  570  and mate with the splines  572  of cup  570 . Cylinder  574  is fixed to the end of and is coaxial with shaft  550 . The longitudinal orientation of the splines permits relative sliding axial movement of the cup with respect to the cylinder. The cylinder  574  can telescope into and out of the cup  570 . Yet the splines transmit force in a circumferential direction, permitting shaft  548  to rotate and to drive shaft  550  in rotation. Thus, coupling  544  permits stub shaft  548  and stub shaft  550  to translate with respect to each other along a common longitudinal axis. 
   Retainers  546  are identically constructed and dimensioned. They are in the form of the hollow half-cylinders that extend around and couple stub shaft  548  to stub shaft  550 . Each of retainers  546  includes a cylindrical shell portion  578  to which inwardly extending flanges  580 ,  582  are fixed on either end. Flanges  580 ,  582  are fixed at opposite ends of shell portions  578  and include semicircular reliefs or cut outs  584 ,  586 . When the two retainers  546  are butted together (see  FIGS. 4-5 ), reliefs  584  mate with each other and define a circular opening large enough to permit stub shaft  550  to pass through, but small enough to prevent cylinder  574  from being withdrawn from cup  570 . Stub shaft  550  is dimensioned to slide within the circular opening defined by reliefs  584 . 
   Coupling  544  is configured to transmit both tensile and compressive forces along the longitudinal axis of the drive shaft and hence from arm  104  to chassis  102 . It is designed to do this during normal operation of the vehicle. It is within the normal design and operational limits of the coupling and also within the normal design and operational limits of the bearings  534 ,  541  that support both ends of drive shaft  500 . The coupling  544  transmits these longitudinal loads or forces in order to reduce both the lateral inward and the outward flexure of suspension arm  104  with respect to chassis  102 . 
   It should be noted at this point that the coupling prevents or reduces flexure within the normal operating mode of the tractor. Other drive shaft couplings may exist that transmit tensile or compressive loads, but they are not configured to do so within the normal operating conditions or limits of the tractor. The traditional arrangement has been to permit some extension of a drive shaft to accommodate the relative pivoting movement of a suspension arm or linkage. This extension of these drive shafts do not actually reach a limit in which the drive shaft transmitted tensile or compressive forces within the normal design limits of the suspension were exceeded, however. Not until the suspension components to which the drive shaft was coupled are flexed beyond their normal operating limits, outside of their normal operating range, and bent or otherwise damaged by plastic deformation would such a drive shaft have reached a point at which it would transmit tensile or compressive loads that serve to restrain the movement of a suspension arm or linkage. 
   During heavy load conditions, suspension arm  104  may be flexed outwardly away from chassis  102  and differential housing  540 . As these loads increase, suspension arm  104  is flexed outward and pulled away from chassis  102  until the extension limit of coupling  544  is reached. At this point, shaft  550  is almost withdrawn from cup  570 , but not quite. Cylinder  574  abuts the inner surfaces of flanges  580 , which places the entire drive shaft in axial tension. Further separation of the two shafts is prevented. Drive shaft  500  thereby transfers tensile loads from the rear end of suspension arm  104  to chassis  102 , supporting suspension arm  104  and preventing or reducing further outward flexure under load. Drive shaft  500  is configured to transmit these loads without damage to the drive shaft or the suspension arm. 
   Heavy loads may also flex suspension arm  104  inward, toward chassis  102  and differential housing  540 . As these loads approach the design limit, suspension arm  104  is flexed inward and pushed toward chassis  102  until the fully retracted or collapsed position ( FIG. 5 ) of coupling  544  is reached. At this point, shaft cylinder  574  is fully inserted into cup  570 . Cylinder  574  abuts the end of shaft  548 , which places the entire drive shaft in axial compression. Further collapse of one shaft toward the other is prevented. Drive shaft  500  thereby transfers compressive loads from the rear end of suspension arm  104  to chassis  102 , supporting suspension arm  104  and preventing or reducing further inward flexure of arm  104  under load. Drive shaft  500  is configured to transmit these loads without damage to the drive shaft or the suspension arm. 
   The inward and outward extension limits of the two drive shafts with respect to each other are preferably reached only under extreme load conditions. During normal unloaded or lightly loaded operation, coupling  544  is not positioned at either of these two extreme positions. All the suspension components are preferably dimensioned such that cylinder  574  is disposed in the central region of cup  570  during normal operation and therefore transmit no axial loads either inward (compression) or outward (tension) between the suspension arm  104  and the chassis  102 . This reduces component wear during normal operation. 
     FIGS. 7 through 10  illustrate a second alternative coupling  544 ′ they can be used in place of coupling  544  in any of the embodiments shown or described herein. 
   Coupling  544 ′ comprises a cup  585  having internal splines  587  that extend longitudinally and inwardly. Coupling  544 ′ also includes a cylinder  588  having longitudinally and outwardly extending external spines  590 . Splines  590  mate with splines  587 , permitting cylinder  588  to slide freely in a longitudinal direction with respect to cup  585 . Cylinder  588  can be telescopically extended from and retracted into cup  585 . Cup  585  is fixed to the end of and is coaxial to stub shaft  548 . Cylinder  588  is fixed to the end of and is coaxial to stub shaft  550 . Thus, coupling  544 ′ permits stub shaft  548  and stub shaft  550  to translate with respect to each other along a common longitudinal axis. 
   Coupling  544 ′ also comprises cylinder retainer  592 . Cylinder  588  is retained within cup  585  by cylinder retainer  592  (shown herein as two plates  594 ,  596 ) that is fixed with ten threaded fasteners  598  to a mating and outwardly extending flange  600  that is fixed to, extends outward from, and defines the open end of cup  585 . 
   Plates  594 ,  596  are both semicircular and have semicircular reliefs or recesses  602  formed in one edge. These reliefs or recesses  602  are arranged to abut each other and form a circular opening. The circular opening is large enough to permit stub shaft  550  to slide freely in and out of cup  585 . The circular opening is small enough to prevent cylinder  588  from being withdrawn from cup  585 . 
   The relative movement of cup  585  with respect to cylinder  588 , and stub shaft  548  with respect to stub shaft  550  is restricted in both directions, however. Inward movement of cylinder  588  within cup  585  is prevented when cylinder  588  abuts the end of stub shaft  548 . This completely collapsed or retracted position is shown in  FIG. 8 . Likewise, the outward movement of cylinder  588  with respect to cup  585  is prevented when the outer peripheral edge of cylinder  588  abuts inwardly extending flanges  590 . 
   Coupling  544 ′ functions the same as coupling  544 . Whenever suspension arm  104  is flexed outwards under load, coupling  544 ′ extends to its fullest extent (see e.g.  FIG. 7 ) and then resists further extension. Whenever suspension arm  104  is flexed inwards under load, coupling  544 ′ collapses or retracts to its fullest extent (see e.g.  FIG. 8 ) and then resists further retraction. In both cases, coupling  544 ′ communicates tensile and compressive forces from stub shaft  548  to stub shaft  550  and thereby reduces both the inward and outward flexure of suspension arm  104  under heavy loads. 
   The fully extended and fully retracted positions of coupling  544  and  544 ′ are within the standard operational limits of the tractor. In other words, the coupling  544 ′ is intended to be placed in tension and compression in the identical manner and for the same purpose as coupling  544 . 
   The length of relative sliding movement of the coupling and the rigidity of the suspension arm are selected such that the suspension arm can be flexed outward and inward under design operating conditions to tension or compress the coupling. In this manner, the suspension arm can be made thinner, lighter and more flexible than would normally be the case, with the knowledge that the coupling  544 ,  544 ′ will support the suspension arm when needed. 
   From the foregoing detailed description of the preferred embodiments, it will be observed that numerous modifications and variations can be effected without departing from the true spirit and scope of the novel concept of the present invention. It will be appreciated that the present disclosure is intended as an exemplification of the invention, and is not intended to limit the invention to the specific embodiment illustrated. The disclosure is intended to cover by the appended claims all such modifications as fall within the scope of the claims.