Abstract:
A tractor includes an engine and a transmission drivingly coupled to the engine. The transmission has a longitudinal dimension and includes at least two pulleys and an epicyclic rotary mechanism including a sun gear with an axis of rotation, the pulleys each having an axis of rotation. The axes of rotation of the pair of pulleys and the sun gear form a triangle in a plane perpendicular to the longitudinal dimension of the transmission.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a CVT transmission for motor vehicles, in particular for farm tractors.  
         [0003]     2. Technical Background  
         [0004]     In the field of tractors, transmissions in which a continuous control of speed is effected, called CVT transmissions, are known. In other words, in these transmissions, one is able continuously to regulate the speed of the motor vehicle, throughout the range between the maximum forward speed and the maximum reverse speed.  
         [0005]     In particular, in known solutions, the CVT transmission comprises an apparatus for the continuous variation of the motion in terms of delivered torque and speed. Such a transmission is known, for example, from the patent EP-B1-0 889 260.  
         [0006]     However, EP-B1-0 889 260 has a disadvantage of not being optimally compact and therefore is not suitable for use in low-power tractors in which the volume of the components constitutes a determining factor. For example, the epicyclic rotary mechanism is positioned at the central axis of one of the two expandable pulleys. This entails larger transverse dimensions than are optimal in the field of low-power tractors.  
         [0007]     An object of the present invention, therefore, is to provide a CVT transmission for motor vehicles, in particular for low-power farm tractors, that permits an optimal placement of transmission components.  
       SUMMARY OF THE INVENTION  
       [0008]     According to an aspect of the invention, a tractor includes an engine and a transmission drivingly coupled to the engine. The transmission has a longitudinal dimension greater and includes at least two pulleys and an epicyclic rotary mechanism including a sun gear with an axis of rotation, the pulleys each having an axis of rotation. The axes of rotation of the pair of pulleys and the sun gear form a triangle in a plane perpendicular to the longitudinal dimension of the transmission.  
         [0009]     According to another aspect of the invention, a tractor includes an engine, a drive shaft driven by the engine, and transmission means for continuously varying drive ratios. The transmission means includes at least two pulleys and an epicyclic rotary mechanism including a sun gear with an axis of rotation, at least one primary pulley being selectively driven by the engine and at least one other pulley being driven by the primary pulley. Each pulley has an axis of rotation. The axis of the drive shaft, the pulleys and the sun gear are offset. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The present invention will now be described further, with reference to the accompanying drawings, which show a non-limiting example, in which:  
         [0011]      FIG. 1  schematically shows a plan view of a CVT transmission for motor vehicles, in particular for farm tractors, according to a preferred embodiment of the present invention; and  
         [0012]      FIG. 2  shows a front view of the transmission of  FIG. 1 , the outer profile of the engine being shown in dashed lines. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]     In a transmission  100  shown in  FIGS. 1 and 2 , the mechanical power produced by an engine M of, e.g., a tractor is transmitted to a first shaft  10 , which represents the drive shaft and sets in rotation the wheels W 1  and W 2  and preferably a rear power take off, RPTO (see below).  
         [0014]     As shown in  FIG. 2 , the axis (a) of the drive shaft  10  lies on a plane of substantial transverse symmetry of the tractor engine. The drive shaft  10  is substantially transversely centered in the outline (shown in  FIG. 2  in broken lines) of the engine.  
         [0015]     As shown in  FIGS. 1 and 2 , on the drive shaft  10  is keyed to a gear wheel  11  meshed with another idle gear wheel  12  on a shaft  13  (having an axis (b)) parallel to the drive shaft  10  itself (with axis (a)). Moreover, on the shaft  13  is idle another gear wheel  14 . The shaft  13  also bears a clutch  15  which, for purpose that will become more readily apparent below, is able to make integral with the shaft  13  either the gear wheel  12  or the gear wheel  14 . The gear wheel  14 , in turn, through an idle gear wheel  16  meshes with a gear wheel  17 , also keyed on the aforementioned drive shaft  10 .  
         [0016]     If the gear wheel  12  is made to rotate with the shaft  13  by the clutch  15  (and, consequently, the gear wheel  14  on the shaft  13  is made idle), motion is transmitted by the drive shaft  10  to the shaft  13  in “direct” fashion. In other words, the motor vehicle will travel “in forward running mode.” On the other hand, if the gear wheel  14  is made to rotate with the shaft  13  (and consequently, the gear wheel  12  on the shaft  13  is made idle) then motion is transmitted by the drive shaft  10  to the shaft  13  in “reverse” fashion. In other words, the tractor will travel “in reverse running mode” owing to the presence of the idle gear wheel  16 .  
         [0017]     On the shaft  13  is also keyed an expandable pulley  18 , coupled by means of belt (or chain)  19  to a second pulley  20 , also expandable, keyed onto a shaft  21  (having an axis (c)). The set of pulleys  18 ,  20 , with the respective shafts  13 ,  21 , and the belt (or chain)  19  constitutes a device  23  for the continuous variation of motion.  
         [0018]     As shall be seen, both shafts  13  and  21  are kinematically united to an epicyclic rotary mechanism  24  having a central shaft  25 , whose axis (d) is parallel to the aforesaid axes (a), (b), and (c). For this purpose, on the shaft  13  is keyed a gear wheel  26 , whilst a gear wheel  27  is integral with the shaft  21 . The gear wheel  26  meshes with the sun gear  28  of the epicyclic rotary mechanism  24  (the sun gear  28  is idle on the central shaft  25 ) whilst the wheel  27  transmits motion to a spider  29  mounted idle on a coupling box  28   a  of the aforesaid sun gear  28 . In a manner known in itself, to the spider  29  is associated with a plurality of satellites  30  (only one is shown in  FIG. 1 ), each of which is pivotally engaged to a respective pivot pin  31  in a single piece with the spider  29 .  
         [0019]     As shown in  FIG. 1 , each planet gear  30  meshes at one side with a gear wheel  32  formed into a single piece with the sun gear  28 , and on the other side with an inner toothing  33  of the crown  34  integral with the central shaft  25 .  
         [0020]     In known fashion, at the epicyclic rotary mechanism  24  the algebraic sum occurs of the motions coming, respectively, from the coupling of the gear wheel  26  with the sun gear  28  (with constant transmission ratio), and from the coupling of the gear wheel  27  with the spider  29  (variable transmission ratio achieved by means of the continuous motion variation device  23 ). By exploiting the combinatory effects of the two motions which occur at the epicyclic rotary mechanism  24  it is possible to obtain the desired continuous speed variation to change in the desired fashion the rotation speed of a bevel pinion C 1  meshed with a ring bevel gear C 2  belonging to a differential gear DIF.  
         [0021]     As is well known, by changing the angular speed of the ring bevel gear C 2  over time it is possible to change over time also the speed of the differential gear DIF and hence of the pair of rear wheels W 1 , W 2  joined thereto by means of two axles AX 1 , AX 2  supported, respectively, by a bearing B 1  and a bearing B 2 .  
         [0022]     Observing  FIGS. 1 and 2  together, it is readily apparent that the axis (d) of the central shaft  25  of the epicyclic rotary mechanism  24  and the axes (b) and (c) of the pulley  18  and the pulley  20  are positioned according to vertices DBC of a triangle TR.  
         [0023]     This physical characteristic translates into a considerable improvement in terms of the compactness of the transmission  100  and in a close proximity of its transverse dimensions with respect to the trace P ( FIG. 2 ). Moreover, the central placement both of the drive shaft  10  and of the triangle TR has the additional advantage that the disposition of the different PTOs (power take offs) of the tractor is particularly convenient.  
         [0024]     It should also be noted that the triangle TR is positioned in substantially balanced fashion relative to the trace P, and the trace P passes at a small distance from the axis (d) of the central shaft  25  of the epicyclic rotary mechanism  24 .  
         [0025]     All this has the consequence of an equilibrium between the transmission members, and an optimal disposition of the parts, to reduce, as much as possible, the transverse dimensions (for example relative to the axis (a) of the transmission shaft  10 ).  
         [0026]     Moreover, if the central shaft  25  comprises an additional gear wheel  35  downstream of the epicyclic rotary mechanism  24 , the gear wheel can be made to mesh with a gear wheel  36  associated with a motion transmission device  37  and a front PTO, FPTO for actuating a pair of front wheels (not shown), a device that will be inserted (with known means) if the tractor is to travel using all four drive wheels.  
         [0027]     Advantageously, the transmission system  100  of the present invention, in addition to the aforesaid FTPO and RPTO (which may be connectable/detachable to/from the shaft  10  by means of a clutch  38 ), can provide for the use of an additional PTO located in intermediate position relative to the first two and designated herein as MPTO.  
         [0028]     The MPTO takes motion from the RPTO by means of a gear train  39  and it serves the purpose of actuating, at the operator&#39;s choice, a series of tools (not shown herein) which are positioned in the lower part of the tractor.