Abstract:
A multi-regime continuously variable ratio transmission has a transmission input shaft, a transmission output shaft, and a ratio varying unit having a rotating input and a rotating output, the rotational axes of the input and the output being coaxial. A shunt having first and second epicyclic gear sets is connected across the ratio varying unit. One gear set has an input driven by the input shaft and an input driven by one side of the ratio varying unit and the other gear set has an input from the first gear set and an input from one side of the ratio varying unit. The gear sets rotate about a common axis and are offset with respect to, and parallel to, the rotational axes of the input and output of the ratio varying unit. A clutch is operable to selectively connect the output of the second gear set to the output shaft.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national phase of PCT application no. PCT/GB2010/050809 filed May 18, 2010; which claims priority to Great Britain application GB 0908581.2, filed May 19, 2009. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to continuously variable ratio transmissions and in particular, but not exclusively, to continuously variable ratio transmissions used in large vehicles in the construction industry, commonly known as “wheel loaders”. 
     (2) Description of Related Art 
     A wheel loader is a vehicle, often used in construction, that is primarily used to load and move bulk material. It normally comprises a tractor having an adjustable bucket, snow plough or other attachment adjustably connected to its front end. 
     In wheel loaders, the physical constraints imposed on its construction result in the engine normally being mounted well above the driven wheels. Consequently, the transmission needs to provide for a vertical drop, typically of approximately 50 cm, from the engine output shaft to the shaft driving the wheels. In conventional transmissions for wheel loaders, gearing arrangements can be conveniently located in the vertical drop. 
     BRIEF SUMMARY OF THE INVENTION 
     Wheel loaders are commonly required to “shuttle” backwards and forwards during use, which requires repeated engagement and disengagement of clutches at relatively low speeds and high torques. 
     It is an object of the present invention to provide a continuously variable ratio transmission which allows “shuttling” to take place conveniently and yet which accommodates the vertical drop normally present between the output of the engine and the driven wheels in such arrangements. 
     In accordance with the present invention, there is provided a multi-regime continuously variable ratio transmission (CVT) comprising: 
     a transmission input shaft; 
     a transmission output shaft; 
     a ratio varying unit having a rotating input and a rotating output, the rotational axes of the input and the output being coaxial; 
     a shunt comprising first and second epicyclic gear sets connected across the ratio varying unit, one epicyclic gear set comprising an input driven by the transmission input shaft and an input driven by one side of the ratio varying unit and the other epicyclic gear set comprising an input from the first epicyclic gear set and an input from one side of the ratio varying unit, the first and second epicyclic gear sets rotating about a common axis and being offset with respect to, and parallel to, the rotational axes of the input and output of the ratio varying unit; and a clutch for selectively connecting the output of the second epicyclic gear set to the transmission output shaft. 
     Use of a ratio varying unit, such as a variator, allows “shuttling” of the transmission to take place at low speeds, without the requirement for changing clutches. Moreover, the provision of a double shunt in the form of first and second epicyclic gear sets moderates the power recirculated through the variator at low speeds, which is desirable when high torque is applied. 
     Moreover, the present invention allows a construction which accommodates the vertical drop present in the known transmissions. 
     Preferably, the transmission further comprises one or more clutches for selectively connecting the output of the variator to a transmission output shaft. 
     Preferably, selective engagement of the clutches produces a plurality of overlapping ratio ranges. 
     In a preferred embodiment, the ratio varying unit comprises a rotatable input disc, a rotatable output disc mounted coaxially with the input disc and a plurality of rollers of variable inclination transmitting rotation between the input disc and the output disc. 
     The input disc and the output disc are preferably mounted on a hollow shaft and the transmission input shaft passes through the hollow shaft. 
     In one embodiment, the output of each of the clutches is adapted to drive a common rotatable member and wherein the transmission further comprises a forward clutch and reverse clutch located between the common member and the transmission output shaft. 
     This arrangement allows the outputs from the second epicyclic gear set and/or the ratio varying unit to be applied in a forward direction or a reverse direction, as required, by suitable application of the forward or reverse clutch. In particular, it can significantly increase the ratios which can be achieved in reverse. 
     Preferably, the first and second epicyclic gear sets are located below the ratio varying unit. This arrangement utilises the vertical drop which is commonly present between the engine output shaft and the driven wheels in transmissions for wheel loaders. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       By way of example only, specific embodiments of the present invention will now be described, with reference to the accompanying drawings, in which: 
         FIG. 1  is an illustration of a first embodiment of infinitely variable transmission in accordance with the present invention; 
         FIG. 2  is a schematic representation of the transmission of  FIG. 1 ; 
         FIG. 3  is an illustration of a second embodiment of infinitely variable transmission in accordance with the present invention; and 
         FIG. 4  is a schematic representation of the transmission of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring firstly to  FIGS. 1 and 2 , a continuously variable ratio transmission system comprises a variator V for a variator Rv as shown in  FIG. 2 ) of the known toroidal race rolling traction type having two toroidally-recessed input discs  10 , one arranged at each end of the variator, and a pair of similar output discs  12 , each facing a respective one of the input discs  10  and rotating with each other. 
     Sets of rollers  14  (typically three rollers) are mounted between the opposing faces of the input and output discs  10 ,  12  to transmit drive from the input discs to the output discs  12  with a ratio which is variable by tilting the rollers  14 . The input discs  10  are mounted at either end of a hollow shaft  16  and an input shaft  18 . Input shaft  18  is driven by an engine or other prime mover (not illustrated), via reduction gearing R 1  shown in  FIG. 2 , and passes coaxially through the hollow shaft  16 . 
     The input discs  10  are rotated by the output of a double shunt across the variator in the form of first and second epicyclic gear sets E 1 , E 2  via reduction gearing R 2 , R 3  shown in  FIG. 2 . The first epicyclic gear set E 1  comprises a sun gear S 1 , a plurality of planet gears P 1  rotatably mounted on a carrier C 1  and an annulus A 1  which engages with an input gear  20  connected to and rotatable with, the input discs  10  of the variator V. The carrier C 1  engages with, and is rotated by, an output gear  22  connected to the input shaft  18  and the sun gear S 1  is connected to, and rotates with, the output discs  12  of the is variator V via an idler gear  24  engaged with an intermediate gear  26 . Consequently, rotation of the annulus A 1  of the first epicyclic gear set E 1  (and consequently the speed of the input discs  10 ) of the variator is dependent on speed of the input shaft  18  and the output of the variator V. As shown in  FIG. 1 , the input shaft  18  and the carrier C 1  of the first epicyclic gear set E 1  can also be used as power take-off sources PTO 1 , PTO 2  if desired. 
     The annulus A 1  of the first epicyclic gear set E 1  is also connected to the carrier C 2  of a second epicyclic gear set E 2 , whose sun gear  52  is connected to, and rotates with, the sun gear S 1  of the first epicyclic gear set and with the output of the variator V. A plurality of planet gears P 2  are rotatably mounted on the carrier C 2  and mesh with the sun gear S 2  and with an annulus A 2  which forms the output of the second epicyclic gear set E 2 . 
     The epicyclic gear sets E 1  and E 2  are located below, i.e. underneath, the variator V. 
     As will be explained, by providing a plurality of clutches, drive can be transmitted from the first and second epicyclic gear sets E 1 , E 2  and from the output discs  12  of the variator to a final output shaft  30  of the transmission arranged in parallel with the input shaft  18 . Optionally, rotation of the final transmission output shaft  30  can also selectively be imparted to a further, coaxial output shaft  32  by means of a further clutch  34 , for example to provide four-wheel drive when desired. 
     The output of the second epicyclic gear set E 2 , formed by the annulus A 2 , is connected to one side of a low-regime dog clutch L. The other side of the low-regime clutch L is secured to an intermediate gear  36  which is engaged via a first idler gear  40  to a further intermediate gear  42  (i.e., reduction gearing R 5  shown in  FIG. 2 ) which rotates with an intermediate output shaft  44  which engages via a further idler gear  46  with a transfer gear  48  (i.e., reduction gearing R 7  shown in  FIG. 2 ) which is connected to the transmission output shaft  30 . By engaging the low-regime clutch L, the output of the second epicyclic gear set E 2  can thereby be transmitted to the transmission output shaft  30  via reduction gearing R 7 . 
     The output of the first epicyclic gear set E 1 , formed by annulus A 1 , is transmitted via intermediate gear  50  to one side of an intermediate-regime dog clutch I via reduction gearing R 4  shown in  FIG. 2 , the other side of the intermediate clutch I being connected to the intermediate output shaft  44 . Consequently, by engagement of the intermediate clutch I, the output of the first epicyclic gear set E 1  (the rotation of the annulus A 1 ) can be transmitted to the transmission output shaft  30  via reduction gearing R 7 . 
     The intermediate gear  26  rotatable by engagement with the output discs  12  of the variator V (via the idler gear  24 ) is also arranged to engage with, and rotate, one common side of a high regime dog clutch H and an intermediate reverse dog clutch IR via reduction gearing R 6  shown in  FIG. 2 . The opposite side of the high regime clutch H is connected directly to the intermediate output shaft  44  and engagement of the high regime clutch thereby transmits the rotation of the variator output directly to the transmission output shaft  30 . 
     The opposite side of the intermediate reverse clutch IR is connected to, and rotates, an intermediate gear  54  which engages with a further transfer gear  56  (i.e., reduction gearing R 8  shown in  FIG. 2 ) connected to the transmission output shaft  30 . 
     In operation, only one of the low-regime clutch L, the intermediate regime clutch I, the intermediate reverse clutch IR and the high-regime clutch H is normally engaged at any one time. 
     In low-regime operation, only the low-regime clutch L is engaged, which provides an output to the transmission output shaft  30  corresponding to the output (the annulus A 2 ) of the second epicyclic gear set E 2  via the intermediate shaft  44 . As the rollers of the variator V move, the ratio of the variator can change from reverse, through geared neutral, to low forward ratio. 
     When the low-regime clutch L is engaged, the highest forward ratio of the variator corresponds to the lowest ratio of the variator when the intermediate clutch I is engaged. Consequently, in order to increase ratio, the low regime clutch L is disengaged and the intermediate clutch I is engaged. This connects the output of the first epicyclic gear set (the annulus A 1 ) to the output shaft  30  via the intermediate shaft  44  and the variator rollers can then sweep in the opposite direction to take the ratio from intermediate forward ratio to the Lower end of the next highest ratio range. 
     When the intermediate regime clutch I is engaged, the highest ratio of the intermediate range corresponds to the lowermost ratio of the next highest regime. In order to increase the effective ratio, the high regime clutch H is engaged and the intermediate regime clutch I is disengaged. This connects the output of the variator V to the output shaft  30  via the intermediate shaft  44  and the rollers of the variator can then sweep in the opposite direction to take the ratio from the lower end of the high regime to an even higher ratio. 
     On the other hand, if the transmission is in low regime with the low regime clutch L engaged, the position of the variator corresponding to the maximum reverse ratio of low regime corresponds to the numerically smallest reverse ratio of the next lowest reverse regime. In order to increase the negative ratio, the intermediate reverse clutch IR is engaged and the low-regime clutch L is disengaged, which connects the output of the variator V to the output shaft  30  via the intermediate gear  54  and transfer gear  56 , which allows the transmission to operate in a higher reverse ratio than the low regime will allow. 
     The transmission can be “shuttled” very quickly between forward and reverse operation, if desired, with a reduced requirement for engagement and disengagement of clutches. The addition of the intermediate reverse ratio also allows a relatively high reverse ratio to be achieved. On the other hand, the incorporation of the high regime ratio allows the vehicle to travel at relatively high speeds in the forward direction. 
     Moreover, the use of a double shunt across the variator recirculates power through the variator and limits the power passing through it. This allows a variator of reduced size to be used, particularly in vehicles such as excavators which are often required to deliver high torque when moving at low or zero road speed. 
     Moreover, the arrangement of the shunt offset to, but parallel to, the variator, permits the transmission system to be constructed in an envelope which is within the envelope of existing, conventional gearing arrangements. 
     A second embodiment of the present invention is illustrated in  FIGS. 3 and 4 . The construction is similar in many respects to the first embodiment and comprises a variator V (or a variator Rv as shown in  FIG. 4 ) of the known toroidal race rolling traction type having two toroidally-recessed input discs  110 , one arranged at each end of the variator, and a pair of similar output discs  112 , each facing a respective one of the input discs  110  and rotating with each other. 
     Sets of rollers  114  (typically three rollers) are mounted between the opposing faces of the input and output discs  110 ,  112  to transmit drive from the input discs to the output discs  112  with a ratio which is variable by tilting the rollers  114 . The input discs  110  are mounted at either end of a hollow shaft  116  and an input shaft  118 . Input shaft  118  is driven by an engine or other prime mover (not illustrated), via reduction gearing R 1  shown in  FIG. 4 , and passes coaxially through the hollow shaft  116 . 
     The input discs  110  are rotated by the output of a double shunt across the variator in the form of first and second epicyclic gear sets E 1 , E 2  (or first and second epicyclic gear sets Rce, Rmp as shown in  FIG. 4 ) via reduction gearing R 2 , R 3  shown in  FIG. 4 . The first epicyclic gear set E 1  comprises a sun gear S 1 , a plurality of planet gears P 1  rotatably mounted on a carrier C 1  and an annulus A 1  which engages with an input gear  120  connected to and rotatable with, the input discs  110  of the variator V. The carrier C 1  engages with, and is rotated by, an output gear  122  connected to the input shaft  118  and the sun gear S 1  is connected to, and rotates with, the output discs  112  of the variator V via an idler gear  124  engaged with an intermediate gear  126 . Consequently, rotation of the annulus A 1  of the first epicyclic gear set E 1  (and consequently the speed of the input discs  110 ) of the variator is dependent on speed of the input shaft  118  and the output of the variator V. As shown in  FIG. 1 , the input shaft  118  and the carrier C 1  of the first epicyclic gear set E 1  can also be used as power take-off sources PTO 1 , PTO 2  if desired. 
     The annulus A 1  of the first epicyclic gear set E 1  is also connected to the carrier C 2  of a second epicyclic gear set E 2 . The annulus A 2  of the second epicyclic gear set E 2  is connected to and rotates with the output of the variator V via the intermediate gear  126 . A plurality of planet gears P 2  are rotatably mounted on the carrier C 2  and mesh with the sun gear S 2  and with the annulus A 2 . The sun gear S 2  forms the output of the second epicyclic gear set E 2 . 
     The epicyclic gear sets E 1  and E 2  are located below, i.e. underneath, the variator V. 
     As will be explained, by providing a plurality of clutches, drive can be transmitted from the first and second epicyclic gear sets E 1 , E 2  and from the output discs  112  of the variator to a final output shaft  130  of the transmission arranged in parallel with the input shaft  118 . Optionally, rotation of the final transmission output shaft  130  can also selectively be imparted to a further, coaxial output shaft  132  by means of a further clutch  134 , for example to provide four-wheel drive when desired. 
     The output of the second epicyclic gear set E 2 , formed by the sun gear S 2 , is connected to one side of a low-regime dog clutch L. The other side of the low-regime clutch L is secured to an intermediate gear  136  which is engaged via a first idler gear  140  to a further intermediate gear  142  (i.e., reduction gearing R 5  shown in  FIG. 4 ) which rotates with an intermediate output shaft  144  which engages via a further idler gear  146  with a transfer gear  148  (i.e., reduction gearing R 7  shown in  FIG. 4 ) which is connected to one side of the forward clutch F. By engaging the low-regime clutch L, the output of the second epicyclic gear set E 2  can thereby be transmitted to the intermediate output shaft  144  and from there to the output shaft  130 . 
     The output of the first epicyclic gear set E 1 , formed by annulus A 1 , is transmitted via intermediate gear  150  to one side of an intermediate-regime dog clutch I via reduction gearing R 4  shown in  FIG. 4 , the other side of the intermediate clutch I being connected to the intermediate output shaft  144 . Consequently, by engagement of the intermediate clutch I, the output of the first epicyclic gear set E 1  (the rotation of the annulus A 1 ) can be transmitted to the transmission output shaft  130 . 
     The intermediate gear  126  rotatable by engagement with the output discs  112  of the variator V (via the idler gear  24 ) is also arranged to engage with, and rotate, one side of a high regime dog clutch H via reduction gearing R 8  shown in  FIG. 4 . The opposite side of the high regime clutch H is connected directly to the intermediate output shaft  144  and engagement of the high regime clutch thereby transmits the rotation of the variator output directly to the transmission intermediate output shaft  144 . 
     The output of the intermediate output shaft  144  is transmitted to the transmission output shaft  130  either through a forward direction plate clutch F (via the transfer gear  142  connected to the intermediate output shaft  144 , the idler gear  146  and a transfer gear  148  connected to one side of the forward direction clutch) or through a reverse direction plate clutch R (one side of which rotates with the intermediate output shaft  144 , the other side of which is connected via a first transfer gear  166  which engages with a transfer gear  168  (i.e., reduction gearing R 6  shown in  FIG. 4 ) connected to the transmission output shaft  130 ). Only one of the forward direction clutch and rear direction clutch F, R is engaged at any one time, whereby the output of the intermediate output shaft  144  is connected to the transmission output shaft  130  either in the forward direction (by engagement of clutch F) or in the reverse direction brackets by engagement of clutch R). In this way, the transmission can be arranged to provide identical ratio spreads in both the forward and reverse directions. 
     In operation, only one of the low-regime clutch L, the intermediate regime clutch I, the intermediate reverse clutch IR and the high-regime clutch H is normally engaged at any one time. 
     In low-regime operation, only the low-regime clutch L is engaged, which provides an output to the transmission output shaft  130  corresponding to the output (the annulus A 2 ) of the second epicyclic gear set E 2  via the intermediate shaft  144 . As the rollers of the variator V move, the ratio of the variator can change from reverse, through geared neutral, to low forward ratio. This rotation is transmitted to the intermediate output shaft  144  and thence to the transmission output shaft  130 , the direction of rotation of the output shaft  130  being dependent on which of the forward or reverse clutches F, R is engaged. 
     When the low-regime clutch L is engaged, the highest forward ratio of the variator corresponds to the lowest ratio of the variator when the intermediate clutch I is engaged. Consequently, in order to increase ratio, the low regime clutch L is disengaged and the intermediate clutch I is engaged. This rotation is transmitted to the intermediate output shaft  144  and thence to the transmission output shaft  130 , the direction of rotation of the output shaft  130  being dependent on which of the forward or reverse clutches F, R is engaged. This connects the output of the first epicyclic gear set (the annulus A 1 ) to the output shaft  130  via the intermediate shaft  144  and the variator rollers can then sweep in the opposite direction to take the ratio from intermediate forward ratio to the lower end of the next highest ratio range. 
     When the intermediate regime clutch I is engaged, the highest ratio of the intermediate range corresponds to the lowermost ratio of the next highest regime. In order to increase the effective ratio, the high regime clutch H is engaged and the intermediate regime clutch I is disengaged. This rotation is transmitted to the intermediate output shaft  144  and thence to the transmission output shaft  130 , the direction of rotation of the output shaft  130  being dependent on which of the forward or reverse clutches F, R is engaged, This connects the output of the variator V to the output shaft  130  via the intermediate shaft  144  and the rollers of the variator can then sweep in the opposite direction to take the ratio from the lower end of the high regime to an even higher ratio. 
     The transmission can be “shuttled” very quickly between forward and reverse operation, if desired, by engaging/disengaging the forward and reverse clutches F, R. 
     As for the first embodiment, the use of a double shunt across the variator recirculates power through the variator and limits the power passing through it. This allows a variator of reduced size to be used, particularly in vehicles such as excavators which are often required to deliver high torque when moving at low or zero road speed. 
     Moreover, the arrangement of the shunt offset to, but parallel to, the variator, permits the transmission system to be constructed in an envelope which is within the envelope of existing, conventional gearing arrangements. 
     The invention is not restricted to the details of the foregoing embodiments.