Patent Description:
The differential is a power transmission member available in vehicles to distribute the torque coming from an input shaft to a pair of output shafts connected to respective wheels. In particular, as it is known, the differential allows different velocities to be delivered to the wheels, so as to allow each one of them to follow the relative trajectory during paths covering a bend or paths with different grips.

In its simplest configuration, a differential comprises a carrier, to which two shafts are constrained, and two satellites, which are placed on the shafts and mesh with two driven planetary gears, which are integral to the axle shafts, each connected to the respective wheel.

Known differentials are also provided with a locking function, which is configured to cause the two axle shafts to be integral to one another. This function is useful in case of a surface allowing for a reduced grip of the tyres, in which case one of the two wheels, if it were not constrained to the other one, would tend to skid, thus dispersing the torque.

However, known differentials are subjected to a constant need for improvements, so as to ensure a versatile use thereof and increase the functions thereof.

Examples of such known differentials are disclosed in <CIT>, <CIT>, <CIT>, <CIT> or <CIT>.

Therefore, existing vehicle differentials need to be improved.

The object of the invention is to fulfil the needs discussed above in an economic fashion.

According to the invention, this object is solved by a differential group according to claim <NUM>, the dependent claims show further advantageous embodiments of the invention.

The invention will be best understood upon perusal of the following detailed description of a preferred embodiment, which is provided by way of non-limiting example, with reference to the accompanying drawings, wherein:.

In the accompanying figures, number <NUM> indicates a differential group <NUM> according to the invention housed inside a casing <NUM> configured to delimit an inner space <NUM> suited for the purpose. The casing <NUM> further is advantageously configured to house part of the axle shafts <NUM>, <NUM> of an axle <NUM>, as shown in <FIG>, <FIG> or <FIG>. To this aim, the casing <NUM> can be manufactured as one single piece or in different parts depending on the assembling needs of the differential group <NUM> and of the axle <NUM>.

The differential group <NUM> is operatively interposed between the axle shafts <NUM>, <NUM> and a drive source <NUM>, such as a transmission shaft configured to convey the torque of an internal combustion engine or an electric motor, which is configured to transmit a torque to the axle shafts <NUM>, <NUM>.

The axle shafts <NUM>, <NUM>, as it is known, are advantageously concentric to one another around a common longitudinal axis A, cooperate - with a respective inner end 4a, 5a - with the differential group <NUM> and define a wheel hub at outer ends 4b, 5b of theirs.

The differential group <NUM> comprises a differential <NUM> and a transmission <NUM>, the differential <NUM> being operatively interposed between the axle shafts <NUM>, <NUM> and the transmission <NUM> and the latter being operatively interposed between the differential <NUM> and the drive source <NUM>.

The differential <NUM> is of a known type and comprises a carrier <NUM>, which carries a plurality of satellites <NUM>, for example four satellites, which are angularly equally spaced apart from one another by <NUM>° and are supported by a cross-shaped support <NUM>, which is rigidly carried by the carrier <NUM> and around whose arms the satellites <NUM> can rotate. In particular, the satellites rotate on the support <NUM> around axes contained in a plane, which is perpendicular to the axis A.

The satellites <NUM> cooperate with a planetary comprising a left bevel gear 15a and a right bevel gear 16b, respectively, which are rigidly carried by the axle shaft <NUM> and by the axle shaft <NUM>, respectively, on the respective inner portion.

The transmission <NUM> basically comprises a pair of bevel gears <NUM>, <NUM>, which are operatively interposed between the drive source <NUM> and the differential <NUM> so as to alternatively connect the drive source <NUM> and the differential <NUM>, defining different torque transmission ratios between them.

In particular, each bevel gear <NUM>, <NUM> comprises a first and a second bevel wheels 17a, 17b, 18a, 18b meshing with one another and, advantageously, the first and the second bevel wheels 17a, 17b, 18a, 18b are coaxial to one another and are sized so that a gear <NUM> defines a pair of toothed wheels 17a, 17b having a greater primitive diameter than the other gear <NUM>.

In detail, the pinions 18b, 18b and the crowns 18a, 18a of the gears <NUM>, <NUM> axially face one another and, advantageously, a crown 17a is supported on the other crown 18a in a rotary free manner, whereas the pinions 17b, 18b are integral to one another in the rotation or, vice versa, a pinion 17b is supported on the other pinion 18b in a rotary free manner, whereas the crowns 17a, 18a are integral to one another in the rotation.

In particular, the pair of bevel wheels 17a, 17b, 18a, 18b, which are free to rotate on one another, can selectively be engaged, alternatively relative to one another, between the drive source <NUM> and the carrier <NUM> of the differential <NUM>, so as to define different gear ratios.

According to the embodiment of <FIG>, the pinions 17b, 18b are coupled to one another in a rotary manner, whereas the crowns 17a, 18a are supported on one another with a free rotation.

In particular, the transmission <NUM> comprises a shaft <NUM> with axis B, perpendicular to the axis A of the axle shafts <NUM>, <NUM>, which is carried by the structure <NUM> in a rotary free manner and is configured to be coupled to the drive source <NUM> so as to allow the torque to be transmitted. In particular, the shaft <NUM> is radially and axially supported, relative to the axis A, by means of support means, such as rolling bearings <NUM>.

The shaft <NUM> defines a first end portion 21a and a second end portion 21b on the opposite side relative to the first one with respect to the axis B. The shaft <NUM> is advantageously configured to be connected to the drive source <NUM> close to the first end portion 21a.

The pinions 17b, 18b are carried by the shaft <NUM>, coupled to it in a rotary manner, for example by means of a grooved coupling <NUM>. More in detail, the pinion 17b of the first bevel gear <NUM> is arranged axially in contact with the pinion 17b of the second bevel gear <NUM>; furthermore, the pinion 17b has a greater diameter than the pinion 18b. Preferably, the pinions 17b, 18b are carried by the shaft <NUM> close to the second end portion 21b; more preferably, the pinion 18b is carried in a more terminal position than the pinion 17b.

The crowns 17a, 18a mesh with the respective pinion 17b, 18b and, hence, are similarly sized, namely the crown 17a has a greater diameter than the crown 18a. In particular, the crowns 17a, 18b are supported, in a rotary free manner radially and axially relative to the axis A, between the carrier <NUM> of the differential <NUM> and the casing <NUM>, preferably thanks to support means, such as rolling bearings <NUM>.

In particular, the two crowns 17a, 18a axially face one another along the axis A and have, interposed between them, a support means of the type described above.

Each crown 17a, 18a can selectively be connected to the carrier <NUM> by means of a selector <NUM>, which is configured to slide on the carrier <NUM> and is coupled to it in a rotary manner and is configured to couple one of the crowns 17a, 18a to the carrier <NUM> in a rotary manner.

In particular, the selector <NUM> is moved by actuator means <NUM>, which are configured to allow one of the crowns 17a, 18a to be selectively coupled to the carrier <NUM>, said actuator means preferably being as described below.

In the embodiment described herein, the carrier <NUM> is manufactured in two distinct tubular portions 11a, 11b, a left one and a right one respectively, which are coupled to one another and surround the left axle shaft <NUM> and the right axle shaft <NUM> respectively. Consequently, each portion 11a, 11b is hollow and defines an opening to allow for the passage of the respective axle shaft <NUM>, <NUM>.

In particular, the right portion 11b comprises a tubular portion 11b' extending along the axis A to a greater extent than the respective tubular portion 11a' of the left portion 11a. Advantageously, the selector <NUM> comprises a sleeve <NUM> with a tubular shape, which is housed around the carrier <NUM> and is carried by the latter so as to slide on the tubular portion 11b' of the right portion 11b along the axis A and be coupled to the tubular portion 11b' in its rotation.

Advantageously, the coupling between the sleeve <NUM> and the tubular portion 11b' is obtained by means of a grooved coupling <NUM>, for example a splined coupling.

On the opposite side relative to the grooved coupling <NUM>, the sleeve <NUM> defines a toothing <NUM>, hence radially extending from the outer surface of the sleeve <NUM>. Said toothing <NUM> is configured to cooperate with a respective toothing <NUM>, <NUM> obtained in the hub of the crowns 17a, 18a so as to make them integral, in their rotation, to the sleeve <NUM>.

The sleeve <NUM> is held in a first operating position (<FIG>) by means of a force operated by elastic means <NUM>, for example a helical spring wound around a pin <NUM> carried by a connecting rod <NUM> connected to the sleeve <NUM>. In this operating position, the toothing <NUM> meshes with the toothing <NUM>, thus making the crown 17a integral to the carrier <NUM>.

The actuator means <NUM> are configured to overcome the force exerted by the elastic means <NUM> in order to move the connecting rod <NUM> and, hence, the sleeve <NUM> to a second operating position (<FIG>), in which the toothing <NUM> meshes with the toothing <NUM>, hence making the crown 18a integral to the carrier <NUM>.

Advantageously, the actuator means <NUM> comprise a pneumatic thrust system. Advantageously, said pneumatic thrust system comprises a chamber <NUM>, where an end portion <NUM>' of the pin <NUM> is housed so as to slide in the chamber <NUM> in a sealing manner. The chamber <NUM> is fluidically connected to a source of air under pressure, so as to be selectively filled with or emptied from said air under pressure.

The end portion <NUM>' can slide in a sealing manner in the chamber <NUM> depending on the force balance between the thrust provided by the elastic means <NUM> to the pin <NUM> and the pressure of the air in the chamber <NUM> exerted on the pin <NUM> by means of the portion <NUM>'.

The differential group <NUM> can further comprise blocking means <NUM>, which are configured to lock the differential <NUM>, namely make the carrier <NUM> integral to the casing <NUM>.

In particular, the blocking means <NUM> comprise a sleeve <NUM>, which is carried close to the left portion 11a of the carrier <NUM>. In the embodiment described herein, the sleeve <NUM> is housed, in a sliding manner, coaxially to the left axle shaft <NUM> and is configured to define a selective coupling with the carrier <NUM>, by means, for example, of a shape coupling, preferably a toothed one, between itself and the annular portion 11a' of the left portion 11a of the carrier <NUM>.

Hence, the sleeve <NUM> is configured to operatively assume a first position, in which it is free relative to the axle shaft <NUM> and to the carrier <NUM>, and a second condition, in which it cooperates in contact, through the shape coupling, with the carrier <NUM>, thus making it integral to the casing <NUM> and, hence, allowing the velocity coming from the carrier <NUM> to be equally divided between the axle shafts <NUM>, <NUM>, basically cancelling the function of the differential <NUM>.

The sleeve <NUM> is operated by means of an actuator means, for example a pneumatic one, preferably similar to the actuator means <NUM> and, hence, comprising a chamber <NUM> in which a piston <NUM> is free to slide in a sealing manner. The chamber <NUM> is fluidically connected to a source of air under pressure, so as to be selectively filled with or emptied from said air under pressure. The piston <NUM> is held in a rest position by elastic means <NUM>, which are configured to exert, upon the piston <NUM>, a force that is contrary to the one exerted by the pressure in the chamber <NUM> upon the piston <NUM> itself.

The latter, hence, can slide in a sealing manner in the chamber <NUM> depending on the force balance between the thrust provided by the elastic means <NUM> and the pressure of the air in the chamber <NUM> exerted on it.

The piston <NUM> is further connected to the sleeve <NUM> so that, when there is no air inside the chamber <NUM>, it allows the sleeve <NUM> to be free relative to the axle shaft <NUM> and to the carrier <NUM>, whereas, when there is air under pressure in the chamber <NUM>, the thrust provided by the elastic means <NUM> acting upon the piston is overcome, so that the latter moves, coupling the sleeve <NUM> to the carrier <NUM> and making it integral to the casing <NUM>.

The operation of the embodiment described above will now be explained with reference to <FIG>.

In the condition of <FIG>, the actuator means <NUM> control the sleeve <NUM> so that the toothing <NUM> meshes with the toothing <NUM>, thus making the crown 18a integral to the carrier <NUM>. The torque coming from the drive source <NUM> is transmitted to the shaft <NUM> and, from there, to the pair of pinions 17b, 18b. Then, the torque is transmitted to the crowns 17a, 18b, the crown 17a rotating in an idle manner, whereas the crown 18a transmits the torque to the carrier <NUM> and, hence, to the differential <NUM>, which divides it, in a known manner, between the axle shafts <NUM>, <NUM>.

In the condition of <FIG>, the actuator means <NUM> control the sleeve <NUM> so that the toothing <NUM> meshes with the toothing <NUM>, thus making the crown 17a integral to the carrier <NUM>. The torque coming from the drive source <NUM> is transmitted to the shaft <NUM> and, from there, to the pair of pinions 17b, 18b. Then, the torque is transmitted to the crowns 17a, 18b, the crown 18a rotating in an idle manner, whereas the crown 17a transmits the torque to the carrier <NUM> and, hence, to the differential <NUM>, which divides it, in a known manner, between the axle shafts <NUM>, <NUM>.

Clearly, in the two conditions, the gear ratio is different because of the different dimensions of the bevel gears <NUM>, <NUM>. The shift between the conditions of <FIG> is obtained through the pressurization of the chamber <NUM>, as described above.

According to the embodiment of <FIG>, the pinions 17b, 18b are supported on one another with a free rotation, whereas the crowns 17a, 18a are coupled to one another in a rotary manner.

The pinions 17b, 18b are carried by the shaft <NUM> in a rotary free manner and can selectively be coupled to the latter through a selector <NUM>, which is configured to slide on the shaft <NUM> and is coupled to the latter in a rotary manner and is also configured to couple one of the pinions 17b, 18b to the shaft <NUM> in a rotary manner.

In particular, the selector <NUM> is moved by actuator means <NUM>, which are configured to allow one of the pinions 17b, 18b to be selectively coupled to the shaft <NUM>, said actuator means preferably being as described below.

More in detail, the pinion 17b has a greater diameter than the pinion 18b and they are supported, in a rotary free manner radially and axially relative to the axis B, between the shaft <NUM> and the casing <NUM>, preferably thanks to support means, such as rolling bearings <NUM>.

The crowns 17a, 18a mesh with the respective pinion 17b, 18b and, hence, are similarly sized, namely the crown 17a has a greater diameter than the crown 18a.

In particular, the transmission <NUM> comprises a further shaft <NUM>, which extends along an axis C perpendicular to the axis A and is supported by the casing <NUM> thanks to support means, such as rolling bearings <NUM>. Advantageously, the two crowns 17a, 18a are rigidly carried, for example thanks to a grooved coupling <NUM>, on the shaft <NUM> and are in axial contact with one another.

The shaft <NUM> is operatively connected to the carrier <NUM>, advantageously by means of a gear <NUM>, which is preferably obtained between a toothed gear <NUM> carried by the shaft <NUM>, for example thanks to the grooved coupling <NUM> connected the crown 7a, 8a, and is configured to cooperate with a toothing <NUM> carried by the carrier <NUM>. Advantageously, the toothed gear <NUM> and the crowns 17a, 18a are in axial contact with one another.

In the embodiment described herein, the carrier <NUM> is manufactured in two distinct portions 11a, 11b, a left one and a right one respectively, which are coupled to one another and surround the left axle shaft <NUM> and the right axle shaft <NUM> respectively. Consequently, each portion 11a, 11b is hollow and defines an opening to allow for the passage of the respective axle shaft <NUM>, <NUM>.

Preferably, the toothing <NUM> is obtained on an annular element, which is separate and is fixed to one of the portions 11a, 11b or, like in the case described herein, is fixed to both of them through threaded means, in particular is "clamped" between them.

Advantageously, the selector <NUM> comprises a sleeve <NUM>, which is carried by the shaft <NUM> so as to slide on it along the axis B and be coupled to it in its rotations. Preferably, the coupling between the sleeve <NUM> and the shaft <NUM> is obtained by means of a grooved coupling <NUM>, for example a splined coupling.

On the opposite side relative to the grooved coupling <NUM>, the sleeve <NUM> defines a toothing <NUM> radially extending from the outer surface of the sleeve <NUM>. Said toothing <NUM> is configured to cooperate with a respective toothing <NUM>, <NUM> obtained in the hub of the pinions 17b, 18b so as to make them integral, in their rotation, to the sleeve <NUM>.

The sleeve <NUM> is held in a first operating position (<FIG>) by means of a force operated by elastic means <NUM>, for example a helical spring wound around a pin <NUM> carried by a connecting rod <NUM> connected to the sleeve <NUM>. In this operating position, the toothing <NUM> meshes with the toothing <NUM>, thus making the pinion 17b integral to the shaft <NUM>.

The actuator means <NUM> are configured to overcome the force exerted by the elastic means <NUM> in order to move the connecting rod <NUM> and, hence, the sleeve <NUM> to a second operating position (<FIG>), in which the toothing <NUM> meshes with the toothing <NUM>, hence making the pinion 18b integral to the shaft <NUM>.

In particular, the blocking means <NUM> are similar to the ones of the embodiment of <FIG> and, therefore, they will not be described for the sake of brevity.

In the condition of <FIG>, the actuator means <NUM> control the sleeve <NUM> so that the toothing <NUM> meshes with the toothing <NUM>, thus making the pinion 17b integral to the shaft <NUM>. The torque coming from the drive source <NUM> is transmitted to the shaft <NUM> and, from there, to the sole pinion 17b. Then, the torque is transmitted to the crown 17a and, hence, to the shaft <NUM>. The latter causes the rotation of the toothed gear <NUM> and of the crown 18a, which meshes with the pinion 17b, which rotates in an idle manner. Hence, the torque is transmitted from the toothed gear <NUM> to the carrier <NUM> and, then, to the differential <NUM>, which divides it, in a known manner, between the axle shafts <NUM>, <NUM>.

In the condition of <FIG>, the actuator means <NUM> control the sleeve <NUM> so that the toothing <NUM> meshes with the toothing <NUM>, thus making the pinion 18b integral to the shaft <NUM>. The torque coming from the drive source <NUM> is transmitted to the shaft <NUM> and, from there, to the sole pinion 18b. Then, the torque is transmitted to the crown 18a and, hence, to the shaft <NUM>. The latter causes the rotation of the toothed gear <NUM> and of the crown 17a, which meshes with the pinion 17A, which rotates in an idle manner. Hence, the torque is transmitted from the toothed gear <NUM> to the carrier <NUM> and, then, to the differential <NUM>, which divides it, as known, between the axle shafts <NUM>, <NUM>.

Clearly, in the two conditions, the gear ratio is different because of the different dimensions of the bevel gears <NUM>, <NUM>. Furthermore, the gearing between the toothed gear <NUM> and the toothing <NUM> allows for a further transmission step in addition to the one of the bevel gears <NUM>, <NUM>.

The shift between the conditions of <FIG> is obtained through the pressurization of the chamber <NUM>, as described above.

For the sake of brevity, the structure of the transmission <NUM> in terms of pinions 17b, 18b and of their coupling to the crowns 17a, 18a will not be described again, since it is substantially identical to the one of the embodiment of <FIG>.

In the particular embodiment described herein, the carrier <NUM> is manufactured in two distinct tubular portions 11a, 11b, a left one and a right one respectively, which are coupled to one another and surround the left axle shaft <NUM> and the right axle shaft <NUM> respectively. Consequently, each portion 11a, 11b is hollow and defines an opening to allow for the passage of the respective axle shaft <NUM>, <NUM>.

In particular, the right portion 11b comprises a tubular portion 11b' extending along the axis A to a greater extent than the respective tubular portion 11a' of the left portion 11a.

The crowns 17a 18a are directly rigidly connected to the carrier <NUM>, in particular thanks, for example, to a grooved coupling <NUM> obtained on the left portion 11a.

In particular, the crowns 17a, 18a are integral to the carrier <NUM>; in particular, they are axially locked along the axis A thanks to threaded means <NUM> connecting both crowns 17a, 18a to the left and right portions 11a, 11b. More in detail, both crowns 17a, 18a are axially in contact with one another along the axis A and are carried by the sole left portion 11a of the carrier <NUM>.

More in detail, an abutment <NUM> radially extends from the left tubular portion 11a' in the area of contact with the right portion 11b and is configured to define an axial support for the crowns 17a, 18a.

The selector <NUM> and the actuator means <NUM> connect the pinions 17b, 18b to the shaft <NUM> as described in the embodiment of <FIG> and, therefore, they will not be described for the sake of brevity.

In particular, the blocking means <NUM> are similar to the ones of the embodiment of <FIG> and <FIG> and, therefore, they will not be described for the sake of brevity.

The operation of the embodiment described above will now be explained with reference to <FIG> and is similar to the one of <FIG>.

In the condition of <FIG>, the actuator means <NUM> control the sleeve <NUM> so that the toothing <NUM> meshes with the toothing <NUM>, thus making the pinion 17b integral to the shaft <NUM>. The torque coming from the drive source <NUM> is transmitted to the shaft <NUM> and, from there, to the sole pinion 17b. Then, the torque is transmitted to the crown 17a and, hence, to the carrier <NUM>, to which the crown 17a is directly connected, and, by so doing, to the differential <NUM>, which divides it, in a known manner, between the axle shafts <NUM>, <NUM>. The crown 18a rotates together with the carrier <NUM> and meshes with the pinion 17b, which rotates in an idle manner.

Clearly, in the two conditions, the gear ratio is different because of the different dimensions of the bevel gears <NUM>, <NUM>.

In all embodiments described above, the differential group <NUM> further comprises several functional elements, such as anti-dust rings, spacers, o-rings, threaded elements, splines or bearings configured to allow a correct assembly and operation of the structure schematically disclosed above, which are not discussed in the detail for the sake of brevity.

In all the embodiments described above and in the relative operating modes, the operation of the actuator means <NUM> can be controlled by means of an electronic control unit (not shown), for example the ECU of the vehicle, which is configured to allow compressed air to flow in and out.

In the operating conditions described above, the blocking means <NUM> are configured in such a way that the sleeve <NUM> is free relative to the axle shaft <NUM> and to the carrier <NUM>. If the differential <NUM> needs to be locked, it is sufficient to control the blocking means <NUM> so that they couple the sleeve <NUM> to the carrier <NUM>. In the case described herein, air just needs to be let into the chamber <NUM> so as to overcome the resistance of the elastic means <NUM> holding the sleeve <NUM> in the configuration shown and move the latter so that it is coupled to the carrier <NUM>, thus making it integral to the casing <NUM>.

Owing to the above, the advantages of a differential group <NUM> according to the invention are evident.

In particular, the differential group <NUM> allows for two possible gear ratios between the drive source <NUM> and the differential <NUM>, thus increasing the versatility of the driving system of the heavy vehicle. In this way, it is possible to provide a versatile differential group for different types of driving styles and loads of the vehicle.

Furthermore, the differential group <NUM> is particularly compact thanks to the specific arrangement of the bevel gears <NUM>, <NUM>. Furthermore, the use of said bevel gears <NUM>, <NUM> allows for a particularly sturdy differential, which is suited to high torque loads.

Moreover, thanks to the addition of a further gear, as shown in <FIG>, the transmission <NUM> allows for a very large transmission ratio between the source <NUM> and the differential <NUM>.

Finally, the differential group <NUM> according to the invention can be subjected to changes and variants, which, though, do not go beyond the scope of protection set forth in the appended claims.

For example, it is clear that the actuator means <NUM> can be of a different type, for example they can be mechanical or electromagnetic or hydraulic actuator means.

Furthermore, the pairs of toothed gears described herein can be of any type and size, in particular the description relates to gears with straight teeth, but they can obviously be of any type.

Claim 1:
Differential group (<NUM>) for a heavy vehicle, said differential group (<NUM>) comprising a casing (<NUM>) defining a space (<NUM>), a differential (<NUM>) and a transmission (<NUM>) housed inside said space (<NUM>),
said differential (<NUM>) being configured to divide an input torque from a drive source (<NUM>) to said differential group (<NUM>) to a torque of axle shafts (<NUM>, <NUM>) of said vehicle coaxial around an axis (A),
said transmission (<NUM>) being operatively interposed between said drive source (<NUM>) and a carrier (<NUM>) of said differential (<NUM>) and comprising a first and a second bevel gear (<NUM>, <NUM>) and a support shaft (<NUM>),
said first and second bevel gear each comprising a crown (17a, 18a) and a pinion (17b, 18b) meshing with each other,
said support shaft (<NUM>) being connectable to said driving source (<NUM>) and being rotationally carried free from said casing (<NUM>),
one between said crowns (17a, 18a) and said pinions (17b, 18b) being rotationally coupled to one between said support shaft (<NUM>) and said carrier (<NUM>) and the other between said crowns (17a, 18a) and said pinions (17b, 18b) being selectively rotationally coupled to the other between said support shaft (<NUM>) and said carrier (<NUM>),
wherein the other between said crowns (17a, 18a) and said pinions (17b, 18b) can be selectively rotationally coupled to the other between said support shaft (<NUM>) and said carrier (<NUM>) by means of a selector (<NUM>) movable between a first operating condition in which it rotationally couples one of said crowns (17a, 18a) to the other between said support shaft (<NUM>) and said carrier (<NUM>) and a second operating condition wherein the other couples said crowns (17a, 18a) to the other between said support shaft (<NUM>) and said carrier (<NUM>), said selector (<NUM>) being controlled by actuator means (<NUM>) configured to carry it from the first to the second operating condition,
wherein said support shaft (<NUM>) carries said pinions (17b, 18b) which are rotationally coupled to it by means of a grooved coupling (<NUM>),
said selector (<NUM>) being configured to slide, coupled in rotation, to said carrier (<NUM>) to alternately couple one of said crowns (17a, 18a) to said carrier (<NUM>),
characterized in that said crowns (17a, 18a) being rotationally supported between said casing (<NUM>), said carrier (<NUM>) and between them by means of rotating support elements (<NUM>).