Particularly in the automotive field, it is known the use of a gearbox of the above-specified type, in which transmission of the torque from the driving shaft and the driven shaft is effected by means of gears rotating as a single piece with those shafts, and the shifting of gear ratio is achieved by selecting the gears through which the torque is to be trans- mitted. In a conventional gearbox, each gear shifting opera- tion is performed by temporarily uncoupling the primary and secondary shafts. During this uncoupling step, there are se- lected and engaged the gears through which to transmit the torque in order to attain the desired gear ratio. The pres- ence of this inactive step is the main drawback of the tradi- tional gearboxes, since it causes a decrease in the overall mechanical efficiency of the transmission.

SUMMARY OF THE INVENTION It is the object of the present invention to provide a gear- box which is capable of overcoming the above-mentioned draw- back of the prior art, by enabling to shift gear without in- terrupting the transmission of torque between the primary and secondary shafts of the system.

This object is fully achieved according to the present inven- tion by virtue of a gearbox having the characteristics de- fined in independent Claim 1. Further advantageous charac- teristics of a gearbox according to the invention are speci- fied in dependent claims.

BRIEF DESCRIPTION OF DRAWINGS The characteristics and the advantages of the present inven- tion will become apparent from the detailed description which follows, given purely by way of non-limiting example with reference to the appended drawings, in which: Figures 1 and 2 are a front perspective view and a rear perspective view, respectively, of a preferred embodiment of a gearbox according to the invention; Figure 3 is a partially sectioned, plan view of the gearbox of the invention showing the operating condition in which the axes of rotation of a transmission device of a pri- mary unit and of a rotor of a secondary unit are aligned with each other; Figure 4 is a similar view to that of Figure 3, showing the operating condition in which the axes of rotation of the transmission device of the primary unit and the rotor of the secondary unit are spaced apart farthest; Figure 5 and 6 are enlarged perspective views illustrat- ing the transmission device of the primary unit with a pairs of rods in the fully extended position and in the fully re- tracted position, respectively; Figure 7 is an enlarged perspective view which shows a detail of one of a plurality of plates of the rotor of the secondary unit forming a radial seat for the rods of the transmission device of the primary unit; Figures 8a and 8b are schematic views which illustrate the rods of the transmission device of the primary unit en- gaged in radial seats provided on of the rotor of the secon- dary unit, in a first operating condition of the gearbox of the invention corresponding to the engagement of the first gear; Figures 9a and 9b are similar view to those of Figures 8a and 8b, showing a second operating condition of the gear- box corresponding to the engagement of the second gear; Figures 10a and lOb are similar view to those of Figures 8a and 8b, showing a third operating condition of the gearbox corresponding to the engagement of the third gear; Figures lla and lib are similar view to those of Figures 8a and 8b, showing a fourth operating condition of the gear- box corresponding to the engagement of the fourth gear; Figures 12a and 12b are similar view to those of Figures 8a and 8b, showing a fifth operating condition of the gearbox corresponding to the engagement of the fifth gear; Figures 13a and 13b are similar view to those of Figures 8a and 8b, showing a sixth operating condition of the gearbox corresponding to the engagement of the sixth gear; Figure 14 is a schematic view illustrating the geometri- cal construction of the shaped outline of the slide surface of an abutment element of the primary unit; Figure 15 is a schematic view illustrating the geometri- cal construction of the angular interval in which shifting is allowed; and Figures 16 to 21 are front view of the gearbox of the invention illustrating schematically some steps of the shift- ing operation between two consecutive gears.

DESCRIPTION OF A PREFERRED EMBODIMENT In the following description, there will not be mentioned a specific example of application of the gearbox, since the latter can be used indifferently, with the required modifica- tions, in any mechanical system in which it is necessary to transmit torque between a driving shaft and a driven shaft with a discretely variable gear ratio, for example, on a mo- tor vehicle.

By referring initially to Figures 1 to 4, a gearbox according to the invention is generally indicated 10 and comprises a primary unit 11, or driving unit, and a secondary unit 12, or driven unit.

The primary unit 11 comprises a transmission device 13 ro- tatably mounted about a first axis of rotation ri and capable of being coupled for rotation with a primary shaft, or driv- ing shaft (not shown). As will be explained in detail in the following part of the description, the device 13 is arranged to transmit the torque from the primary unit to the secondary unit with a gear ratio selectable among a given number of predetermined ratios.

With reference in particular to Figures 5 and 6, the trans- mission device 13 carries a pair of radial, diametrically op- posed rods 14 capable of sliding along their axes (that is, perpendicular to the axis of rotation ri of the device 13) between a fully extended travel-limit position (Figure 5) and a fully retracted travel-limit position (Figure 6). The de- vice 13 also carries a movable member 15 which can slide axi- ally along the axis of rotation rl. At its end facing the secondary unit 12, the member 15 carries a head element 16 formed by an idle roller in the illustrated example.

The radial and axial sliding movements of the rods 14 and the movable member 15, respectively, are interconnected by a linkage comprised of two pairs of links 17 arranged, for ex- ample, in the form of scissors and hinged at their ends to the movable member 15 and to the respective rod 14. Thus, when the movable member 15 is in the fully extended position, the rods 14 are in the fully retracted position. As the mov- able member 15 moves towards the fully retracted position, on the other hand, the links 17 open out and then cause the rods 14 to extend towards the travel-limit position shown in Fig- ure 5.

In rest condition, a helical spring 18 urges the movable mem- ber 15 in the fully extended position and the rods 14 in the fully retracted position (Figure 6). As will be explained in detail further on, the movable member 15 is moved in use along its sliding direction in a number of axial positions, each corresponding to a predetermined gear ratio, between the two travel-limit positions, as a result of the movement of the head element 16 along a slide surface associated to the secondary unit 12. In accordance with the preceding descrip- tion, to each axial position of the movable member 15 corre- sponds a predetermined radial position of the rods 14, which therefore engage corresponding radial seats of the secondary unit 12 and can thus transmit torque to the unit 12 with the gear ratio corresponding to the selected position.

The secondary unit 12 comprises a rotor 18 rotatably mounted about a second axis of rotation r2 parallel to axis ri and coupled for rotation with a secondary shaft 19, or driven shaft, for example, by means of a gearing 20. The secondary unit is mounted for sliding in a direction indicated by arrow t (Figure 4), substantially perpendicular to the axes of ro- tation rl and rl, so that it can be moved relative to the primary unit 11 to shift in reversible manner from a coaxial arrangement (Figure 3), in which the two axes of rotation rl and r2 are substantially aligned with each other, to a maxi- mum-eccentricity arrangement (Figure 4), in which the two axes of rotation ri and r2 are spaced apart farthest.

In the illustrated embodiment, the rotor 18 is bell-shaped, with a frustoconical portion 18a connecting an annular edge 18b facing the primary unit to a flat, round bottom portion 18c. The rotor 18 and the driven shaft 19 are rotatably mounted on a mounting structure 21 supported on a platform 22 which can slide in the transverse direction t on linear guides of known type (not shown) in order to set the distance between the axes of rotation rl and r2.

An abutment element 25 forming a slide surface 25a for the head element 16 of the transmission device 13 of the primary unit 11 is received inside the rotor 18 of the secondary unit 12 so as to move as a single piece with the rotor in its transverse translational movement relative to the primary unit. As illustrated in particular in Figures 3 and 4, the abutment element 25 is formed by a horizontal plate cantile- vered on the mounting structure 21, wherein the end portion of the plate facing the primary unit forms a vertical surface 25a the outline of which is a curvilinear segment, in plan view, running obliquely away from a point placed in the vi- cinity of the axis of rotation r2.

Alternatively, the abutment element 25 may rotate as a single piece with the rotor 18, for example, being fixed to the end portion 18c thereof. In this case, the slide surface 25a of the element 25 has to be a surface of revolution.

In use, the head element 16 of the transmission device 13 is held against the surface 25a of the abutment element 25 by the resilient force exerted by the spring 18 of the movable member 15. Thus, when the axes of rotation rl and r2 of the primary and secondary units are aligned (Figure 3) and there- fore the transmission device 13 and the rotor 18 rotate coax- ial, the movable member 15 is in the fully retracted position and the rods 14 are in the fully extended position. As the secondary unit is moved in the transverse direction t, its axis of rotation r2 being spaced from the axis of rotation ri of the primary unit, the movable member 15 moves along the outline of the slide surface 25a, through its head element 16, towards the fully extended position, thereby causing the rods 14 to retract progressively (Figure 4).

Clearly, the shape of the slide surface 25a determines the functional connection of the distance between the axes of ro- tation rl and r2 of the primary unit and the secondary unit, respectively, with the axial position of the movable member 15 or the extension of the rods 14 of the transmission device 13. As will be better understood further on, this functional connection determines the amount of variation in the gear ra- tio when shifting between each pair of consecutive gears.

On the annular edge 18b of the rotor 18 are arranged a plu- rality of equally spaced plates 26 (twenty-four plates, in the shown example). As shown in detail in Figure 7, on each plate 26 there is rotatably mounted a pair of idle rollers 27 between which there is defined a radial seat 28 for engaging the end portions of the rods 14 of the transmission device 13 in order to couple the rotor 18 for rotation with the device 13. The seats 28 have a slightly greater width than the di- ameter (or the transverse dimension, in case of a non- circular shape) of the rods 14, in order to receive the rods with a minimum play. By virtue of the rollers 27 being mounted idle, the rods 14 are prevented from dragging on the rollers 27 when they engage in, or disengage from, the seats 28. Moreover, due to the rods being locked between pairs of rollers and thus rotationally constrained in both direction, both a driving torque (that is, a torque directed from the primary unit to the secondary unit) and a driven torque (that is, a torque directed from the secondary unit to the primary unit) can be transmitted by the gearbox.

The plates 26 are tiltably mounted in the edge 18b of the ro- tor 18 and are held by respective springs 23 (partially shown in Figure 7) in a rest position in which the centres of rota- tion of each pair of rollers 27 are tangentially aligned and the respective seats 28 are orientated radially. Thus, the plates 26 can tilt about the above-mentioned rest position to align the respective seats 28 with the rods 14, when the transmission device 13 rotates eccentric relative to the ro- tor 18 and therefore the rods 14 tend to engage the seats 28 in a direction not perpendicular to the axis connecting the centres of rotation of the rollers, that is to say, in a di- rection other than radial.

According to a preferred embodiment of the invention, there is also provided a control device 29 (Figure 2) for control- ling the relative movement of the two units 11,12 in the transverse direction t when the gear ratio is to be varied.

The device 29 may be simply a knob fixed to a portion of the movable unit (in this case, the secondary unit 12), on which knob the user can act directly or indirectly to move the mov- able unit along the direction t. Alternatively, the control device 29 may be a linear actuator, for example, an electro- mechanical actuator.

In order to control the selection of the desired gear ratio, a selection device 30 (Figure 2) is associated to the control device 29. In the illustrated embodiment, the selection de- vice comprises a portion of rack 31, the teeth of which are engaged by an idle roller 32 mounted at the end of a verti- cally sliding control rod 33. The rod 33 is forced downwards by a biasing spring (not shown), so as to hold the roller 32 always in contact with the teeth of the rack 31.

There is also provided an inhibiting device, generally indi- cated 35 (Figure 1), which is arranged to co-operate with the transmission device 13 and the selection device 30 in order to inhibit a shifting operation when the transmission device 13 and its rods 14 are in a predetermined angular interval of rotation, as will be explained in greater detail later. As illustrated in Figure 1, the inhibiting device 35 comprises a rocker lever 36 hinged in an intermediate point thereof to a mounting structure 37. An end of the lever 36 forms a tip 38 arranged to interact with the elliptical outer surface of a cam member 39 rotating as a single piece with the transmis- sion device 13. The opposite end of the lever 36 can be acted upon by an upper plate 40 of the control rod 33 of the selection device 30 in such a manner that, upon the transla- tional movement of the rack 31 during a shifting operation and the engaging of the roller 32 with the teeth of the rack 31, the rod 33 and plate 40 assembly rises and causes the lever 36 to oscillate bringing its tip 38 near the outer sur- face of the cam member 39. A spring 42 arranged between the arm of the lever 36 controlled by the plate 40 and the mount- ing structure 37 permits to hold the tip 38 spaced from the outer surface of the cam member 39 when the control device 29 is not imparted any gear shifting command.

In order to better understand the operation of the gearbox, it will be illustrated now how the various gear ratios (in this case, six ratios) are achieved by means of the co- operation of the transmission device 13 with the slide sur- face 25a and the radial seats 28 of the secondary unit 12.

To this end, reference will be made to Figures 8a, 8b to 13a, 13b, wherein each pair of figures relates to a given gear ra- tio and shows the rotor and rods assembly in two different positions spaced 90 degrees, that is to say, in a first posi- tion in which the rods are vertically arranged and in a sec- ond position in which the rods are horizontally arranged.

Where the axes of rotation ri and r2 of the primary unit 11 and the secondary unit 12, respectively, are aligned (Figures 8a and 8b), and hence the transmission device 13 and the ro- tor 18 rotate coaxial, the movable member 15 of the device 13 is fully retracted, whereas the rods 14 are fully extended and engaged in the radial seats 28 of the rotor 18 indicated A and O. Thus, the transmission device 13 rotates the rotor 18 at its same angular velocity, that is, in this first oper- ating condition, indicated I and corresponding to the first gear, the gear ratio is equal to 1.

If, starting from this first operating condition, the secon- dary unit 12 is moved in the transverse direction t by means of the control device 29, a second operating condition (Fig- ures 9a and 9b), indicated II and corresponding to the second gear, is achieved. In this condition, the device 13 rotates with an eccentricity e2 relative to the rotor 18 and the rods 14 of the device 13 are slightly retracted with respect to the initial fully extended position, as a result of the move- ment of the head element 16 of the movable member 15 along the slide surface 25a. The rods 14 engage radial seats indi- cated A and N and the gear ratio is then equal to 11/12, that is, it is reduced 1/12 with respect to the preceding operat- ing condition.

By continuing to space the secondary unit 12 apart from the primary unit 11, a third operating condition III (Figures 10a and 10b) is achieved, in which the device 13 rotates with ec- centricity e3 relative to the rotor 18 and the rods 14 of the transmission device 13 engage the radial seats 28 indicated B and N. Thus, a third gear ratio equal to 10/12 is achieved.

In a fourth operating condition IV (Figures lla and lib) the device 13 rotates with an eccentricity e4 relative to the ro- tor 18 and the rods 14 engage the radial seats 28 indicated B and M, thus achieving a fourth gear ratio equal to 9/12.

In a fifth operating condition V (Figures 12a and 12b) the device 13 rotates with an eccentricity e5 relative to the ro- tor 18 and the rods 14 engage the radial seats 28 indicated C and M, thus achieving a fifth gear ratio equal to 8/12.

Finally, in a sixth operating condition VI (Figures 13a and 13b) the device 13 rotates with an eccentricity e6 relative to the rotor 18 and the rods 14 engage the radial seats 28 indicated C and L, thus achieving a sixth gear ratio equal to 7/12.

It is clear, however, that by varying the travel and the op- erating positions of the transmission device 13 in the trans- verse direction t and the number of radial seats 28 on the annular edge 18b of the rotor 18, the number and the spacing of the gears ratios may be varied with respect to those il- lustrated.

Figure 14 illustrates schematically the geometrical construc- tion of the shaped outline of the slide surface 25a of the abutment element 25 of the primary unit. Such outline is de- fined by a line passing through a series of fixed points, each corresponding to a gear or gear ratio.

The first point, corresponding to the first gear I, has to be placed on the axis of rotation r2 of the secondary unit at such a longitudinal level (that is, in a direction parallel to the axes of rotation rl and r2) that, with the roller 16 of the transmission device 13 contacting the surface 25a in this point, the rods 14 of the device 13 are in the fully ex- tended position, in which they engage the seats A and O of the rotor 18 (Figure 8a).

The second point, corresponding to the second gear II, has to be at a transverse distance from the first point equal to the distance e2 defined above and at longitudinal distance from the first point equal to the axial displacement which has to be performed by the movable member 15 and roller 16 assembly of the transmission device 13 in order to cause the rods 14 to engage the seats A and N of the rotor 18 (Figure 9a).

The third point, corresponding to the third gear III, has to be at a transverse distance from the first point equal to the distance e3 defined above and at a longitudinal distance from the first point equal to the axial displacement which has to be performed by the movable member 15 and roller 16 assembly of the transmission device 13 in order to cause the rods 14 to engage the seats B and N of the rotor 18 (Figure 10a).

The fourth point, corresponding to the fourth gear IV, has to be at a transverse distance from the first point equal to the distance e4 defined above and at a longitudinal distance from the first point equal to the axial displacement which has to be performed by the movable member 15 and roller 16 assembly of the transmission device 13 to cause the rods 14 to engage the seats B and M of the rotor 18 (Figure lla).

The fifth point, corresponding to the fifth gear V, has to be at a transverse distance from the first point equal to the distance e5 defined above and at a longitudinal distance from the first point equal to the axial displacement which has to be performed by the movable member 15 and roller 16 assembly of the transmission device 13 in order to cause the rods 14 to engage the seats C and M of the rotor 18 (Figure 12a).

The sixth point, corresponding to the sixth gear VI, has to be at a transverse distance from the first point equal to the distance e6 defined above and at a longitudinal distance from the first point equal to the axial displacement which has to be performed by the movable member 15 and roller 16 assembly of the transmission device 13 in order to cause the rods 14 to engage the seats C and L of the rotor 18 (Figure 13a).

By referring now to Figures 15 to 21, the gear shifting op- eration, and in particular the shifting from the first gear I (gear ratio equal to 1) to the second gear II (gear ratio equal to 11/12), will be described.

The gear shifting operation must be completed within an angu- lar interval smaller than 180 degrees, the size of which de- pends on the pair of gears between which the shifting opera- tion is performed. For example, in case of shifting from the fifth to the sixth gear, the gear shift has to start and com- plete within an angular interval equal to 105 degrees, com- prised between 37.5 degrees and 142.5 degrees (that is, 180 degrees-37.5 degrees). Such value derives from the fact that the distance between the axes of the rollers 27 engaged by the rods 14 of the transmission device 13 when this latter is in the position of maximum eccentricity (corresponding to the sixth gear) coincides with the projection of the above- mentioned distance onto the axis of the ordinates when the device 13 is coaxial with the rotor 18 of the secondary unit and is rotated 37.5 degrees or 142.5 degrees, as shown in the geometrical construction illustrated in Figure 15. In case of shifting between two lower gears, such angular interval is greater: for example, in case of shifting from the first to the second gear, the gear shifting can start at 7.5 degrees and last up to 172.5 degrees (that is, 180 degrees-7.5 de- grees). However, in order to ensure the possibility to cor- rectly perform the gear shifting operation between any pairs of consecutive gears, according to the invention the gearbox is arranged to allow gear shifting only within the angular interval comprised between 37.5 degrees and 142.5 degrees, that is, the smallest angular interval. As will be explained further on, the control of the gear shifting interval is ac- complished by a special device which prevents gear shifting to start until the transmission device 13 has reached the 37.5-degree angular position.

In the schematic view of Figure 16 the transmission device 13 is coaxial with the rotor 18 and the rods 14 engage the ra- dial seats A and O of the rotor 18 (gear ratio equal to 1).

In this Figure, the set of rods is rotated 37.5 degrees rela- tive to the vertical direction, that is, it is exactly at the start point of the angular interval in which the gear shift- ing operation is allowed. The roller 32 of the selection de- vice 30 is pushed urged against the bottom of the teeth of the rack 31, in the position corresponding to the engagement of the first gear. Starting from this condition, secondary unit 12 is caused to move relative to the primary unit 11, by acting on the control device 29, in order to shift to the following gear.

In Figure 17 the transmission device 13 is illustrated ro- tated 26.25 degrees (1/4 of 105 degrees) relative to the pre- ceding position and slightly eccentric relative to the axis of rotation r2 of the secondary unit 12, as can be seen from the roller 32 of the selection device 30 being positioned on the flank of the tooth of the rack 31 between the two engage- ment positions corresponding to the first and the second gear ratio, respectively. As a result of the relative movement of the device 13 and the rotor 18, the rods 14 are slightly re- tracted with respect to the fully extended position of Figure 16, so that the sole rod engaging the radial seat A transmits torque to the rotor, whereas the opposite rod is disengaged from the respective radial seat O.

In Figure 18 the transmission device 13 is illustrated fur- ther rotated 26.25 degrees and spaced from the axis of rota- tion r2 of the secondary unit 12 with respect to the position of Figure 17. The spacing of the transmission device 13 relative to the axis r2 compensates for the retraction of the rods 14, so that one of the rods continues to engage the ra- dial seat A thereby assuring the transmission of torque to the rotor 18. In the illustrated operating condition, the roller 32 of the selection device 30 is positioned on the top of the tooth of the rack 31 between the engagement positions of the first and second gear, respectively.

In the operating condition illustrated in Figure 19, one of the rods 14 of the transmission device 13 continues to engage the radial seat A, thereby assuring the transmission of the torque to the secondary unit, whereas the opposite rod has passed over the radial seat O due to the greater angular ve- locity of the device 13 than that of the rotor 18.

In Figure 20 the transmission device 13 is rotated 142.5 de- grees, that is, it is exactly at the end of the angular in- terval in which the gear shifting operation is allowed. The roller 32 of the selection device 30 is engaged against the bottom of the toothed profile of the rack 31 in the position corresponding to the engagement of the second gear (gear ra- tio equal to 11/12). The rods 14 of the transmission device 13 are aligned with the radial seats A and P of the rotor 18.

As it will be clearly understood, throughout the gear shift- ing operation the rod 14 aligned with the radial seat A has been held constantly engaged in that seat, thereby assuring continuous transmission of torque.

Finally, Figure 21 illustrates the condition in which, with the next gear being engaged, the transmission device 13 has rotated 180 degrees. By passing through this angular posi- tion, one of the rods 14 disengages from the radial seat A and the opposite rod engages into the radial seat P. Over the next 180-degree rotation (not shown), the torque is transmitted between the primary unit 11 and the secondary unit 12 through the rod 14 engaging the radial seat P.

In order to allow the gear shifting operation to be performed only within the above angular interval of 105 degrees, the cam member 39 of the inhibiting device 35 is so shaped and orientated that it holds the tip 38 of the lever 36 raised when the transmission device 13 is in an angular interval equal to 37.5 degrees with respect to the two diametrically opposed vertical positions. In this way, the rotation of the lever 36 due to the rising of the control rod 33 of the se- lection device 30 is prevented and then the gear shifting op- eration is inhibited in the above-mentioned angular interval.

It is clear from the above description that a gearbox accord- ing to the present invention enables effectively to eliminate the uncoupling step between the primary unit and the secon- dary unit during gear shifting, since throughout the opera- tion at least one of the two rods of the transmission device of the primary unit remains permanently engaged in the re- spective radial seat of the rotor of the secondary unit, thus assuring continuity in the transmission of the torque.

Naturally, the principle of the invention remaining un- changed, embodiments and manufacturing details may vary widely with respect to those described and illustrated purely by way of non-limitative example.

Clearly, although the primary unit has been illustrated in the description as fixed and the secondary unit as movable, in a fully equivalent way it is possible to provide for a fixed secondary unit and a movable primary unit.

Moreover, although in the description there has been illus- trated an embodiment in which the movable unit (in the pre- sent case, the secondary unit) is operated by hand through the control device 29, the gearbox can be controlled by a servo-assisted control system comprising, for example: an actuator for causing the movable unit to move depend- ing on activation signals sent by an electronic control unit, a position sensor for sensing the transverse displace- ment of the movable unit and sending a corresponding signal to the control unit, whereby it can be established whether the gear shifting operation has been completed, and a position sensor for sensing the angular position of the transmission device and sending a corresponding signal to the control unit, whereby it can be established whether it is allowed to start a gear shifting operation.