Field of the Invention

[0001] The present invention relates to a selective gear system, a gearbox comprising said system and a powertrain, in particular for an electrical vehicle, including said gearbox.

Background and Prior Art

[0002] Nowadays, electric driven vehicles (e.g. passenger car) generally comprise a single speed transmission, contrary to vehicles driven by an internal combustion engine, which are provided with a multiple speed gearbox. Meanwhile, it has been observed that a gearbox with, for instance, two gear ratios allows to extend the vehicle performances both to “high speed” and “high load” (for standing-start & slope-climbing), and on top of this increases the efficiency of a purely electrical powertrain, because the operating point of the electrical machine can be kept, to some extent, in the operating zone(s) with a high efficiency. For this purpose, gearboxes based on a dual clutch system have been developed for niche markets (e.g. sport vehicle). Besides the fact that this known solution is too expensive for a mass-market application, a dual-clutch system is a heavy and complex solution. Additional benefits of multi-speed transmissions (compared to 1 -speed) are as follows:

- allowing the downsizing of the e-machine, and therefore allowing cost saving on the electrical machine itself;

- restricting the top speed of the electrical machine, and therefore the noise emissions.

[0003] Some of the above-mentioned shortcomings could be overcome by adapting usual gearboxes of combustion engine to the requirements of electric driven vehicles. This approach however underestimates the significant differences between a combustion engine driven powertrain and an electric motor driven powertrain. For instance, on an internal combustion engines, gearboxes are equipped with synchronisers in order to synchronise the shaft to the appropriate speed ratio, and clutches in order to convert into friction the kinetic energy of the turning masses. Instead, on a pure electrical motor powertrain, a gearbox input shaft can be decelerated by the electric machine operating in a generator mode during upshifting, or accelerated by the electric machine operating in a motor mode, during down-shifting. The advanced control of the e- machine during shifting event allows developing new generations of shifting systems without friction release. These differences show that an electric driven powertrain requires a shifting response time that should be as short as possible to minimize the lag during shifting events. There is thus a demand for gearboxes with a fast response.

[0004] A hydraulic gear shifting mechanism and more preferably an electric gear shifting mechanism has a better response time than a purely mechanical one, and can therefore be a possible solution to overcome the drawback of the prior art.

[0005] The patent literature contains a couple of publications regarding electrically actuated gearbox. For instance, GB 856,625 discloses a gearbox for small internal combustion engines comprising a linear electromagnetic actuator. This system is heavy and slow because the needle stroke is long, the length of which is comparable to the layshaft length. Such a design is clearly not suitable for a short response time required by an electric engine driven power train.

Aims of the Invention

[0006] The invention aims to provide a solution to at least one drawback of the teaching provided by the prior art.

[0007] More specifically, the invention aims to provide a solution to improve the response time of an electric driven gearbox. Summary of the invention

[0008] For the above purpose, the invention is directed to a selective gear system comprising: a shaft extending along an axial direction, a first gearwheel provided about the shaft, a second gearwheel provided about the shaft, said second gearwheel being axially offset from the first gearwheel, a first engaging or disengaging mechanism for coupling first gearwheel with the shaft or causing the first gearwheel to rotate freely around the shaft, a second engaging or disengaging mechanism for coupling the second gearwheel with the shaft or causing the second gearwheel to rotate freely around the shaft, an electromagnetic linear actuator comprising a mobile member movable along the axial direction, said member interacting with the first and second engaging or disengaging mechanisms in such a manner as to couple the first gearwheel with the shaft and to cause the second gearwheel to rotate freely around the shaft when the mobile member is in a first position, and to couple the second gearwheel with the shaft and to cause the first gearwheel to rotate freely around the shaft when the mobile member is in a second position, characterized in that the maximum stroke of the mobile member of the electromagnetic linear actuator in use is less than 25%, in particular 10% of the axial distance between the geometric centres of the first and the second gearwheels and/or the maximum stroke of the mobile member of the electromagnetic linear actuator in use is less than 15 mm, preferably less than 10 mm, in particular less than 5 mm, and more preferably less than 1 mm.

[0009] According to preferred embodiments of the invention, the selective gear system comprises one or any suitable combination of the following technical features:

- the shaft is coaxial with the mobile member;

- at least a portion of the mobile member faces at least a portion of an outer circumferential surface of the shaft;

- each engaging or disengaging mechanism overlaps in axial direction with the corresponding gearwheel tooth portion; - the movable element comprises at least one annular sector extending parallel to the shaft, wherein the annular inner surface of the at least one annular sector is adjacent to the outer shaft surface, the angle of said annular sector being at least 5 degrees;

- the mobile member comprises a hollow needle encircling the shaft;

- the mobile member is guided in translation by the shaft;

- the mobile member is stationary (i.e. does not rotate) or rotates synchronously with the shaft;

- the at least one of the first and second engaging or disengaging mechanism comprises at least one pawl member, preferably the at least one pawl member being mounted in rotation with regard to the shaft;

- the at least one pawl member is positioned in an axial direction between the two faces of the gear teeth of the corresponding gearwheel;

- the at least one pawl member is movable between two end positions, wherein in the extended end position, said pawl member engages with the corresponding gearwheel, causing the coupling of said gearwheel with the shaft, and wherein in the retracted end position, said pawl member is disengaged with said gearwheel, causing said gearwheel to freely rotate around the shaft;

- in the extended end position, the at least one pawl member engages in a form-fitting manner in a recess formed on an inner surface of the corresponding gearwheel;

- a portion of the recess comprises a slope to allow a progressive engagement;

- the at least one pawl member comprises a protrusion being arranged to fit into a groove formed within said recess, preferably said protrusion being configured so that said pawl member remains engaged in said recess even when torque transferred from the shaft to the gear engaged is reversed; - the protrusion comprises an element, said element being configured to bias circumferentially said protrusion within the corresponding groove in order to prevent backlash;

- the number of recesses per engaging or disengaging mechanism is equal or less than 12, preferably less than 6, in particular 5, in order to allow synchronisation with significant difference in speed between the shaft and the gearwheel to be engaged;

- the at least one pawl member comprises a plurality of pawl members circumferentially arranged; said pawls being biased in a retracted end position, preferably by a retaining annular spring encircling said pawls;

- the mobile member comprises at least one magnetised member, in particular a permanent magnet, said member interacting by repulsion and/or attraction with an adjacent magnetised member provided in (mounted on) the corresponding engaging or disengaging mechanism, wherein the radial position of the adjacent magnetised member depends on the axial position of the at least one magnetised member;

- the adjacent magnetised member is provided in (mounted on) a radially sliding element or is provided in (mounted (directly) on) the corresponding pawl member, preferably said magnetised member biasing inwardly or outwardly the corresponding pawl member;

- the mobile member comprises at least one (linear) cam member, said member actuating an adjacent cam follower member provided in (mounted on) the corresponding engaging or disengaging mechanism, wherein the radial position of said cam follower member depends on the axial position of the at least one (linear) cam member;

- the at least one (linear) cam member is ramp;

- the at least one (linear) cam member is rigidly attached to the mobile member or the at least one (linear) cam member can rotate relative to the mobile member; - the adjacent cam follower member is provided in (mounted on) a radially sliding element or is provided in (mounted (directly) on) the corresponding pawl member, preferably said adjacent cam follower member biasing outwardly the corresponding pawl member;

- at least the first and second engaging or disengaging mechanisms further comprise axially biased ball positioned between a cam surface of the (linear) cam member and a cam surface of the adjacent cam follower member;

- the electromagnetic linear actuator comprises one or more solenoids, preferably at least two solenoids being axially arranged in series;

- the at least two solenoids comprise an inner solenoid and an outer solenoid;

- the (geometric centre of the) radially sliding element is positioned in axial direction between the two faces of the gear teeth of the corresponding gearwheel;

- the or each solenoid is stationary (i.e. does not rotate) ;

- the or each solenoid is electrically connected to at least two selectable voltage sources;

- the electromagnetic linear actuator comprises one or more springs for biasing the mobile member, preferably at least two springs being arranged coaxially;

- a third gearwheel provided about the shaft axially offset from the first and the gearwheels and a third engaging or disengaging mechanism for coupling the rotation of the third gearwheel with the shaft or causing the third gearwheel to rotate freely around the shaft; the shaft is a hollow shaft or a full shaft; the shaft is mounted on bearings; - the one or more solenoids extend over a first axial portion of the shaft, and the first, the second and optionally the third gearwheels extend over a second axial portion of the shaft, wherein the first axial portion and second axial portion are adjacent;

- the first and the second gearwheels are directly adjacent to each other;

- the mobile member comprises a needle arranged inside the shaft;

- said selective gear system comprises a complementary mobile member;

- the mobile member and the complementary member are arranged so as to define an axial air gap, whose axial length at rest (no current is supplied to the adjacent solenoid) is less than 90% of the mobile member stroke, preferably less than 50%;

- the complementary mobile member slides axially around the mobile member.

[0010] The invention is also directed to a gearbox comprising the selective gear system according to the invention.

[0011] According to preferred embodiments of the invention, the gearbox according to the invention comprises one or any suitable combination of the following technical features:

- the gearbox has a first and second gear ratios, wherein the first gear ratio is selected when the first engaging or disengaging mechanism is engaged with the first gearwheel, and wherein the second engaging or disengaging mechanism is disengaged with the second gearwheel;

- the first gear ratio is selected by default when the linear electromagnetic actuator is not electrified;

- the mobile member can be held in an intermediate position on its way between the first and second position; the gearbox is an offset gearbox; - the gearbox is a layshaft-type gearbox and the shaft comprises a pinion for the final drive;

- the gearbox is a planetary gearbox with a planetary gear set made of a stack of planetary stages disposed axially in series and the first and second, and optionally third gearwheels are sun gears thereof;

- at least one planet of the first planetary stage and an adjacent planet of the second planetary stage are arranged (co)axially in series, preferably, the at least one planet of the first planetary stage and said axially adjacent planet are made in one piece and/or at least two ring gears of the planetary gear set are made in one piece.

[0012] The invention is also directed to a powertrain, in particular for an electric motor, comprising the gearbox according to the invention, wherein at least one planet carrier common to the planetary stages is directly connected to a differential unit.

[0013] According to preferred embodiments of the invention, a powertrain according to the invention comprises an electrical motor and the gearbox according to the invention, wherein the shaft is coaxial or offset with the rotor of the electrical motor.

[0014] The invention can also be directed to a planetary gear set comprising at least two planetary stages, wherein at least one planet of the first planetary stage is coaxially arranged relative to an adjacent planet of the second planetary stage, characterized in that the at least one planet of the first planetary stage and said adjacent planet are made in one piece, or two adjacent ring gears of the planetary gear set are made in one piece. Said planetary gear set can be combined with any one or any combination of the technical features listed in paragraph 9.

[0015] The invention can also be directed to a selective gear system comprising: a shaft extending along an axial direction, a first gearwheel provided about the shaft, a second gearwheel provided about the shaft, said second gearwheel being axially offset from the first gearwheel, a first engaging or disengaging mechanism for coupling first gearwheel with the shaft or causing the first gearwheel to rotate freely around the shaft, a second engaging or disengaging mechanism for coupling the second gearwheel with the shaft or causing the second gearwheel to rotate freely around the shaft, an electromagnetic linear actuator comprising a mobile member movable along the axial direction, said member interacting with the first and second engaging or disengaging mechanisms in such a manner as to couple the first gearwheel with the shaft and to cause the second gearwheel to rotate freely around the shaft when the mobile member is in a first position, and to couple the second gearwheel with the shaft and to cause the first gearwheel to rotate freely around the shaft when the mobile member is in a second position, characterized in that said system comprises a complementary mobile member, and in that the mobile member and the complementary member are arranged so as to define an axial air gap, whose maximal axial length is less than 90% of the mobile member stroke, less than 50% of said stroke. Said selective gear system can be combined with any one or any combination of the technical features listed in paragraph 9.

[0016] The present invention is advantageous since it provides a simple and cost-effective solution performing a shifting within 5 to 100 ms (depending on gears dimensions, actuator properties and supply voltage level). Furthermore, the present invention can be adapted for a 2-speed planetary or an offset gearbox (with layshaft), but also 3-speed or multi-speed gearboxes. The claimed solution is particularly compact in size and can be positioned around a wheel shaft and/or in the proximity of a differential.

[0017] In general, the preferred embodiments of each subject matter of the invention are also applicable to the other subject matters of the invention. To the most possible extent, each subject matter of the invention is combinable with other subject matters. The features of the invention are also combinable with the embodiments of the description, which in addition are combinable with each other. Brief description of the figures

[0018] Aspects of the present invention will now be described in more details with reference to the appended drawings, wherein same reference numerals illustrate same features and wherein:

[0019] Fig. 1 represents a schematic view of a multi-speed gear box comprising a planetary gear set with 2 stages, in which at least one planet of the first stage is arranged coaxially with an adjacent planet of the second stage;

[0020] Fig. 2 shows a schematic view of a gear wheel in combination with its cam-based engaging or disengaging mechanism;

[0021] Fig. 3A, 3B and 3C exhibit a median view of some elements of the selective gear system for a two-speed gearbox, said elements comprising a cambased pawl member actuation; Fig. 3A, 3B and 3C respectively correspond to the states when the first gear, the neutral position and the second gear are engaged;

[0022] Fig. 4 shows a transversal view of an electric driven gearbox in combination with a differential, wherein the gearbox is a two-speed gearbox made of a planetary gear set;

[0023] Fig. 5A, 5B, 5C, 5D and 5E represent a median view of some elements of the selective gear system for a three-speed gearbox, said elements comprising a cam-based pawl member actuation. Fig. 5A, 5B, 5C, 5D and 5E respectively correspond to the states when the first gear, the first neutral position, second gear, the second neutral position and the third gear position are engaged;

[0024] Fig. 6A, 6B, 6C and 6D illustrate a median view of some elements of the selective gear system for a two-speed gearbox, said elements comprising a magnetic-based pawl member actuation. Fig. 6A, 6B and 6C respectively correspond to the states when the first gear, the neutral position and the second gear are engaged; Fig. 6D shows a schematic view of a gear wheel in combination with its engaging or disengaging magnetic-based mechanism;

[0025] Fig. 7 shows an offset transmission with the selective gear system according to the invention; [0026] Fig. 8 represents a selective gear system according to the invention wherein the transformation of the axial displacement of the mobile member into a radial displacement of at least a portion of the pawl member is ensured by cambased pawl member actuation;

[0027] Fig. 9A and 9B represent a selective gear system according to the invention comprising both a cam-based pawl member actuation and a magneticbased pawl member (that can be used either for permanent attraction, as the cam is repelling the pawls mechanically, or for attraction as the cam is repelling the pawls mechanically when synchronized to the mechanical cams);

[0028] Fig. 10 illustrates a selective gear system according to the invention comprising a single solenoid (that can be monitored by two voltages to allow the mobile part of the actuator to stop at an intermediate position - neutral - as well as an end-of-stroke - 2 ^nd gear);

[0029] Fig. 11 illustrates a selective gear system according to the invention, wherein the pawl members are biased in their retracted position via magnetic attracting forces;

[0030] Fig. 12 represents an alternative design for the pawl member with amplification (wherein the pawls are supported by a shaft - symmetry axis located in point A - completely inserted - together with the pawls - in the wall of the transmission shaft);

[0031] Fig. 13 represents a further embodiment based on the embodiment of Fig. 11 . In this further embodiment, the mobile member (i.e. needle) cooperates with a complementary mobile member;

[0032] Fig. 14 represents another embodiment based on the embodiment of Fig. 13, wherein a single solenoid is used in combination with a driver with two selectable tensions as shown in Fig. 10;

[0033] Fig. 15 shows an alternative embodiment to the embodiment of Fig. 13, wherein an inner solenoid is arranged inside the outer solenoid; [0034] Fig. 16 shows a further embodiment based on the embodiment of Fig. 14, with three gearwheels as well as a complementary member with four magnetic gaps in order to reach four positions;

[0035] Fig. 17 shows a further embodiment similar to that of Fig. 13, in which the mobile member is arranged inside the (hollow) shaft.

Description of Preferred Embodiments of the Invention

[0036] Fig. 1 represents a sectional (median) view (left) and a frontal view (right) of a planetary gear set 2 comprising two planetary stages 4A; 4B according to a preferred embodiment. The gearbox 6 according to the invention is not limited to two planetary stages 4A, 4B as shown in Fig. 1 , and can have three planetary stages 4A, 4B, 4C or more depending on the specification requirements of the powertrain. In the planetary gear set 2 shown in Fig.1 , only one planetary stage 4A, 4B can transfer the torque at a time. Thus, only one of the two sun gearwheel 8A, 8B would be engaged at a time. In Fig.1 , each sun gearwheel 8A, 8B is attached on a (common) shaft 10 via a system configured so that the first 8A or the second sun gearwheel 8B can be either coupled with the common shaft 10 or free running. Such a system is called an engaging or disengaging mechanism 12A, 12B, 12C and its details are disclosed below. Advantageously, the planets 14A.1 -4 of the first planetary stage 4A are each arranged in series (and coaxially) with the corresponding adjacent planets 14B.1 -4 of the second planetary stage 4B. In particular, each planet 14A.1 -4 of the first planetary stage and the corresponding adjacent planet 14B.1 -4 can be made in one piece (e.g. same casted/forged piece before machining). In Fig. 1 , each planet 14A.1 -4 of the first planetary stage and the corresponding adjacent planet 14B.1 -4 share the same rotation axis. Equally, each planet 14A.1 -4 of the first planetary stage and the corresponding adjacent planet 14B.1 -4 can be provided about the same planetary shaft 16.1-4 as shown in Fig. 1. The planetary shafts 16.1 -4, about which the planets 14A.1-4, 14B.1 -4 are provided, can be mechanically connected via one or more planetary rings 18A, 18B. Each at least two adjacent planets 14B.1 -4 and their shaft 16.1 -4 can be made in one piece. Furthermore, the first and/or the second ring gears 20A, 20B of the planetary gear set 2 can be made in one piece (e.g. same casted/forged piece before machining).

[0037] Fig. 2 shows a sectional view (in a plan perpendicular to the axis of the gearwheels 8A, 8B) of an advantageous engaging or disengaging mechanism 12A, 12B according to the invention with three planets instead of four as shown in the embodiment in Fig. 1. The engaging or disengaging mechanism 12A, 12B is (radially and/or concentrically) interposed between the sun gearwheel 8A, 8B and the common shaft 10. The engaging or disengaging mechanism 12A, 12B comprises one or more pawl members 24. Generally, at least one pawl member 24 is mounted in rotation on the common shaft 10. Each pawl member 24 can be either in a retracted position where the pawl member 24 is not in contact with the corresponding sun gearwheel 8A, 8B and therefore does not transfer a force, or in an extended position (see Fig. 2) where a portion of the pawl member 24 engages with the corresponding sun gearwheel 8A, 8B. For instance, an upper (radially outwardly) portion of each pawl member 24 is configured so as to be fitted into a corresponding recess 26 formed on an inner surface of the sun gearwheel 8A, 8B. The pawl member 24 can be biased in the direction of the centre of the common shaft 10. For instance, the pawl members 24 within the same plane can be biased (inwardly) via for instance an annular spring (not shown) encircling all pawl members. The use of an annular spring simplifies the mounting of the selective gear system because the pawl members can be held in place by at least one annular spring. The pawl members 24 can also be biased in a retracted position by a plurality of pairs of attracting magnetised (adjacent) members 58B (not shown). For instance, a first magnetised member 58A is mounted on the mobile member 34 and a second (adjacent) magnetised member 58B is mounted on the engaging or disengaging mechanism 12A, 12B or 12C. Furthermore, one or more annular springs and one or more magnetised members can be arranged to achieve a desired biasing behaviour.

[0038] Moreover, the or each pawl member 24 may comprise a protrusion 28 having two distant opposing front faces in the circumferential direction, said protrusion 28 being arranged so as to fit into a slot in a corresponding recess 26 formed in the inner surface of the sun gear-wheel 8A, 8B, so that the or each pawl member 24 remains engaged in the corresponding recess 26 even when the gear system torque is reversed. The recesses 26 located on the inner surface of the sun gearwheel 8A, 8B, which are in contact with the external profiles of the pawl members when extended, are limited in number (max 12) in order to allow an efficient synchronization of the pawl member 24 even when the sun gearwheel 8A, 8B presents a slight difference of rotating speed. The invention is not limited to the above example. Other designs of pawl members 24 and their corresponding recesses 26 are suitable for the purpose of the invention as long as the sun gearwheel 8A, 8B remains engaged when the transmission shaft 10 is changing direction (propulsion & deceleration modes)

[0039] Fig. 2 shows an asymmetric pawl member. However, depending on the circumstances, the pawl member 24 can be designed so as to be balanced on its shaft 10.

[0040] Some features (e.g. pawl member 24) of the disengaging or engaging mechanism 12A, 12B have been illustrated in combination with a planetary gear set 2 in Fig. 1 and Fig. 2. However, the engaging or disengaging mechanism 12 can be used in an offset gearbox 6 (see Fig. 7) where the common shaft 10 with the two gearwheels 8A, 8B illustrated above (in Fig. 1 ) is replaced by a layshaft 10 with two gearwheels 8A, 8B. In Fig. 1 , the number of planets 14A.1 -4, 14B.1 -4 per planetary stage 4A,4B is equal to four. In Fig.2, the number of planets 14A.1 -3, 14B.1 -3 per planetary stage 4A,4B is equal to three. The number of planets 14A.1 -4, 14B.1 -4 per planetary stage 4A,4B is the result of optimisation well-known to the skilled person.

[0041] Fig. 3A, 3B and 3C illustrate a common shaft 10 in combination with two engaging or disengaging mechanisms 12A, 12B and a linear electromagnetic actuator 30. For this preferred embodiment, the linear actuator 30 comprises two solenoids 32A, 32B mounted on a stator element (not rotating) of the gearbox. A mobile member 34 of the linear electromagnetic actuator 30 can be in the form of a hollow needle 34 that encircles the common shaft 10. The mobile member 34 is a stator element that can move in an axial direction between two abutments. In other words, the mobile member 34 does not rotate. The gearwheels 8A, 8B are not represented in Fig. 3A, 3B and 3C. [0042] In Fig. 3A, the two solenoids 32A, 32B are not supplied with current (default position). The mobile member 34 is pressed against its left abutment surface by at least one of the two springs 36 that is pretensioned. By default, in a preferred embodiment, the engaging or disengaging mechanism 12A on the left hand side corresponding to the first gear ratio is engaged. The corresponding pawl members 24 are extended in their respective recesses 26 (not represented). The other engaging or disengaging mechanism 12B on the right hand side corresponding to the second gear ratio is disengaged. The pawl members 24 are biased in a retracted position by an elastic element such as annular springs 40. The mobile member 34 for the embodiment according to Fig. 3A, 3B and 3C axially displaces two cam members 50A. In Fig. 3A, 3B and 3C, the cam members 50A synchronously rotate with the shaft but are axially shifted by the mobile member 34 (a.k.a. plunger). The mobile member 34 does not rotate in Fig. 3A. In Fig. 3A, the two cam members 50A are rotatably attached to the mobile member 34. The cam member 50A on the left hand side can comprise at least two ramp portions configured to displace their respective pawl members 24 outwardly via axially biased balls 52 interposed between them. When the pawl members 24 are extended and in direct contact with the inner side of their corresponding recesses 26, the torque can be transferred from the common shaft 10 to the selected gearwheel 8A, 8B. The cam member 50A on the right hand side has a similar design and is not further described. The axial positioning of the cam member 50A on the right hand side is selected so that the corresponding pawl member 24 is not displaced by the ramp when the pawl members 24 on the left hand side are engaged.

[0043] Fig. 3B illustrates a situation in which no gear is engaged. The first solenoid 32A is supplied with current (on the left hand side) and generates a magnetic field that shifts the mobile member 34 to the left. The cam members 40 on the left and right hand side do not displace their respective balls 52 outwardly. Therefore, all pawl members are maintained in their retracted position by their respective retaining means (e.g. annular spring 40). None of the pawl members 24 is engaged. This is the neutral position. [0044] Fig. 3C illustrates a situation in which the second gear ratio is engaged. The first solenoid 32A (on the left hand side) and the second solenoid 32B (on the right hand side) are supplied with current. The mobile member 34 is shifted even further to the right because the magnetic force is further increased when both solenoids 32A, 32B are activated. The embodiment according to Fig. 3A, 3B and 3C shows two springs 36 biasing the mobile member 34 and arranged coaxially. The stiffness, the pre-tensioning, as well as other parameters of the springs 36 can be selected depending on the needs. When the two solenoids 32A, 32B are simultaneously activated with a predefined current, the mobile member reaches its right hand side abutment surface. The cam members 50A on the left hand side do not displace their respective balls 52. Therefore, the pawl members are maintained in their retracted position by their respective annular spring 40.

[0045] Fig. 4 shows a transmission comprising a planetary gear set 2 and engaging or disengaging mechanisms 12 for an electric motor transmission. The electric motor 54 drives the common shaft 10. The torque is then transferred through the planetary gear set 2 into the planetary ring, said ring driving the differential unit 56. The planetary ring 18 can drive in rotation the differential unit 56. In Fig. 4, the left wheel shaft passes inside the common shaft 10. When the electric motor 54 is directly and coaxially connected to the common shaft 10, the left wheel can pass through the rotor of the electric motor 54.

[0046] Fig. 5A, 5B, 5C, 5D and 5E illustrate a common shaft 10 in combination with three engaging and disengaging mechanisms 12A, 12B or 12C. and a linear electromagnetic actuator 30. This embodiment is equivalent to that presented in Fig. 3A, 3B and 3C, excepted that there is a third engaging or disengaging mechanism 12C cooperating with a third (sun) gearwheel (not shown). This solution makes it possible to obtain a gearbox 6 with three gear ratios (or 3 speeds).

[0047] Fig. 6A, 6B, 6C, 6D and 6E show an alternative embodiment where the transformation of the axial translation of the mobile member 34 into a radial translation of at least a portion of the pawl member 24 is ensured by magnetic repulsive forces. The magnetic forces can be generated by magnetised members 58A, and adjacent magnetised members 58B such as permanent magnets as shown in Fig. 6A, 6B, 6C, 6D and 6E or by electrified coils (not shown). Fig. 6A, 6B, 6C, 6D shows transversal views of the sifting mechanism without the gear wheels 8A, 8B and 8C. Fig.6E shows a frontal view of a given engaging and disengaging mechanisms 12A, 12B or 12C.

[0048] The use of a selective gear system according to the invention is not restricted to a planetary gear set 2. Indeed, the selective gear system can be used advantageously in an offset transmission where the common shaft 10 of the selective gear system corresponds to the layshaft 10 of the offset transmission (gear box) and the sun gearwheels relates to the operating gearwheels 8A, 8B of the layshaft 10.

[0049] Fig. 7 shows an offset transmission with the selective gear system according to the invention, where the shaft 10 is a layshaft. The gear located on the right side of the shaft 10 is not a free running gear (the gear is attached to the shaft 10 and rotates synchronously with the shaft 10), and is in constant mesh with the final drive wheel attached (e.g. screwed up) on a differential unit 56 housing. This system is compatible with all types of differential units (open differential unit based on bevel gears, or limited slip differentials).

[0050] Fig. 8 represents a selective gear system according to the invention wherein the transformation of the axial displacement of the mobile member 34 into a radial displacement of at least a portion of the pawl member 24 is ensured by at least one (linear) cam member 50A with an axial displacement in direct contact with a driven (linear) (follower) cam member 50B with at least a radial displacement. In the embodiment according to Fig. 8, the at least one (linear) cam member 50A is rigidly attached to the mobile member (34). Therefore the at least one (linear) cam member 50A slides against and/or rotates relative the corresponding driven (linear) (follower) cam member 50B.

[0051] The slope of the ramp of the cam members 40 can be adapted in order to modify the amplification of the axial displacement into a radial displacement of at least a portion of the pawl member 24, in particular a rotation or translation of said pawl member 24. The amplification of the vertical motion can be assessed as followed: dy = TAN (a) . dx, where a is the ample of the ramp.

Multiplication factor: dy/dx = TAN(a): dy/dx= 1 ; a= INV TAN(1)= 45° dy/dx= 2 ; a= I NV TAN(2)= 63° dy/dx= 3 ; a= INV TAN(3)= 72°; dy/dx= 4 ; a= INV TAN(4)= 76°; dy/dx= 5 ; a= INV TAN(5)= 79:° dy/dx= 6 ; a= INV TAN(6)= 81 °.

In this example, the (driving)(linear) cam member 50A is a groove or a ramp formed on the mobile member 34. The (adjacent) (linear) cam follower (a.k.a driven) member 50B is a protrusion formed at one end of the (adjacent) (linear) cam follower member 50B.

[0052] Fig. 8 also shows that the inner spring 36 biasing the mobile member 34 is not in direct contact with an abutment surface formed on the mobile member 34. This example shows that the stiffness curve of the mobile member 34 can be adjusted according to the circumstances, especially in the present case, to the two solenoids 32A, 32B disposed axially in series.

[0053] Fig. 9A and 9B represent a selective gear system according to the invention comprising both cam members 50B and magnetised members 58A to transform the linear displacement of the mobile member 34 into a radial displacement of the pawl member 24. In Fig. 9A, the (adjacent) (linear) cam follower members 50B operate in combination with the adjacent magnetised members 58B to move the pawl members 34 outwardly. In Fig. 9B, the forces exerted by the (adjacent) cam follower members 50B and the (adjacent) magnetised members 58B are opposed.

[0054] Fig. 10 illustrates a selective gear system according to the invention comprising a single solenoid 32A. The control of the displacement of the mobile member 34 can be ensured by a relay supplying two different tensions. The power supply to the single coil should include 2 different power levels in order to compress the 2 springs 36 (low & high stiffness) with 2 different lifting forces. This can be realized using a DC/DC converter that could only be switched (V2 > V1 ) to move from neutral to gear #2, as the common voltage (V1 ) is used from gear #1 to neutral.

[0055] Fig. 11 illustrates a selective gear system according to the invention, where the pawl members 24 can biased in their retracted position via magnetic attracting forces only, to the extent that no retaining means such as an annular spring 40 is foreseen. The magnetic attracting forces take place for one or more predefined positions of the mobile member 34 as shown in Fig.11 .

[0056] Fig. 12 represents an alternative design to the pawl member 24. A multiplication stroke can also be applied on the pawl member 24, in order to allow a bigger engagement of the pawl member 24 inside the running gear, since the pawl is located in an intermediate position (point B) of the arm AD. A repulsion of the north pole of the magnet with another north pole will generate a rotation of the pawl around point A (= axis of the shaft supporting the pawl). If the rotation allows the intermediate position of the pawl to move from position B to C, then the end of the pawl member 24 will move from position D to E, with DE (=AB.tan[3) >BC(= AD.tanP). The shaft supporting the pawl member 24 could be surrounded by a spring element allowing to face the centrifugal force applied to the pawl during the rotation of the shaft 10, and allowing the magnet to just deliver the force needed to attract and repel the pawl member 24 without facing the full centrifugal force.

[0057] The radial displacement of a portion of the driven cam member 50B means that the driven cam member 50B can undergo at least a radial translation but also a rotation or a combination thereof.

[0058] Fig. 13 represents a further embodiment based on the embodiment of Fig. 11. In this further embodiment, the mobile member 34 (i.e. needle) cooperates with a complementary mobile member 60. The mobile member 34 and its complementary mobile member 60 can be regarded as a primary plunger and a secondary plunger. This design with two plungers allows a first (Stroke #1 see Fig. 13) and a second (Stroke #2 see Fig. 13) axial magnetic air gaps to facing respectively the corresponding the inner sides of the first 32B and second 32A solenoids. Preferably, each axial gap has a length (in axial direction) ranging from 0.5 to 1 mm. The first axial magnetic air gap is defined by the inner side of the respective solenoid 32B, the corresponding outer contour of the mobile member 34 and an axially oriented face of the complementary mobile member 60. The complementary member 60 is biased by a complementary spring element. In use, once the first solenoid 32B is supplied with current, the mobile member 34 moves to the left in Fig. 13. As soon as the mobile member 34 reaches a first predefined stroke, in particular Stroke #1 , the complementary mobile member 60 moves in unison with the mobile member 34 and is then shifted to the left in Fig. 13. The mobile member 34 reaches its final end stroke when the second gap (see Fig. 13 : stroke #2) is closed. Such a design makes it possible to have reduced axially air gaps, allowing a reduction of the reluctance of the magnetic circuit and therefore an increased magnetic force. The Fig. 13 shows some non-magnetic elements 62 disposed coaxially between the mobile member 34 and the respective solenoids 32A, 32B. These non-magnetic elements 64 allow improve guidance of the magnetic fields. The complementary mobile member 60 can present an annular shape. Moreover, an inner surface portion of the complementary mobile member 60 can slide over an outer surface portion of the mobile member 34. Fig. 13 shows a guiding bushing 64 arranged for improving the guidance of the moving elements.

[0059] Fig. 14 represents another embodiment based on the embodiment of Fig. 13, where a single solenoid 22 is used in combination with a driver with two selectable tensions as shown in Fig. 10. This embodiment shows that the mobile member 34 and the complementary mobile member 60 are configured to form two axial air gaps, the outer one aiming to control the stroke and the inner one to control the magnetic path so as to compensate for the use of a single solenoid 22, which is by definition less optimal than a solution with two or more solenoids. This embodiment is foreseen with a non-magnetic element 62 for the same reasons as those exposed in the previous paragraph. [0060] Fig. 15 shows an alternative embodiment to the embodiment of Fig. 13, where the solenoids 32A, 32B are disposed coaxially, in such a manner that the inner solenoid 32A is arranged inside the outer solenoid 32B.

[0061] Fig. 16 shows a further embodiment based on the embodiment of Fig. 14, with three gearwheels 8A, 8B and 8C instead of two for Fig. 14. This further embodiment comprises a complementary member with four magnetic gaps in order to reach four positions. The number of four positions does not limit the scope of this embodiment, as the number of gaps can be selected depending on the needs.

[0062] Fig. 17 shows a further embodiment similar to that of Fig. 13, in which the mobile member 34 is arranged inside the (hollow) shaft 10. The mobile member 34 is shown as a full shaft but it can also be an hollow shaft. At least one bearing 66 supporting the shaft 10 is arranged between at least one solenoid 32A, 32B and at least one engaging or disengaging mechanism 12A, 12B.

[0063] Fig. 6, 10, 11 , 12, 13, 14, 15, 16 and Fig 17 can be seen as embodiments of a contactless, frictionless electromagnetic shifting mechanism.

[0064] The embodiments disclosed in Fig. 13, 14, 15, 16 and Fig 17 can comprise cam (e.g. ramp) members as an alternative to magnetic members or in combination with magnetic members.

[0065] We understand by “maximum stroke of the mobile member 34”, the stroke of the mobile member 34 defined by the axial displacement of the mobile member between two end positions.

[0066] We understand by “geometric centre of the gearwheel”, the centroid of the gearwheel. The geometric centre corresponds to the mass centre when the body (i.e. the gearwheel) is homogeneous (with constant density).

[0067] Embodiments as discussed above are defined by the following numbered clauses:

1. A selective gear system comprising:

- a shaft (10) extending along an axial direction; - a first gearwheel (8A) provided about the shaft (10);

- a second gearwheel (8B) provided about the shaft (10), said second gearwheel (8B) being axially offset from the first gearwheel (8A);

- a first engaging or disengaging mechanism (12A) for coupling first gearwheel (8A) with the shaft (10) or causing the first gearwheel (8A) to rotate freely around the shaft (10);

- a second engaging or disengaging mechanism (12B) for coupling the second gearwheel (8B) with the shaft (10) or causing the second gearwheel (8B) to rotate freely around the shaft (10);

- an electromagnetic linear actuator (30) comprising a mobile member (34) movable along the axial direction, said member interacting with the first and second engaging or disengaging mechanisms (12A, 12B) in such a manner as to couple the first gearwheel (8A) with the shaft (10) and to cause the second gearwheel (8B) to rotate freely around the shaft (10) when the mobile member (34) is in a first position, and to couple the second gearwheel (8B) with the shaft (10) and to cause the first gearwheel (8A) to rotate freely around the shaft (10) when the mobile member (34) is in a second position; characterized in that the maximum stroke of the mobile member (34) of the electromagnetic linear actuator (30) in use is less than 25%, in particular less than 10% of the axial distance between the geometric centres of the first and the second gearwheels (8A, 8B) and/or in that the maximum stroke of the mobile member (34) of the electromagnetic linear actuator (30) in use is less than 15 mm, preferably less than 10 mm, in particular less than 5 mm, and more preferably less than 1 mm.

2. A selective gear system according to clause 1 , characterized in that the mobile member (34) comprises a hollow needle (34) encircling the shaft (10).

3. A selective gear system according to clause 1 , characterized in that the shaft (10) is hollow and the mobile member (34) comprises a needle (34) arranged inside said shaft (10).

4. A selective gear system according to any of the previous clauses, characterized in that the at least one of the first and second engaging or disengaging mechanisms (12A, 12B) comprises at least one pawl member, preferably the at least one pawl member (24) being mounted in rotation with regard to the shaft (10).

5. A selective gear system according to any of the previous clauses, characterized in that the at least one pawl member (24) is movable between two end positions, wherein in the extended end position, said pawl member (24) engages with the corresponding gearwheel (8A, 8B), causing the coupling of said gearwheel (8A, 8B) with the shaft (10), and wherein, in the retracted end position, said pawl member (24) is disengaged with said gearwheel (8A, 8B), causing said gearwheel (8A, 8B) to freely rotate around the shaft (10).

6. A selective gear system according to any of the previous clauses, characterized in that, in the extended end position, the at least one pawl member (24) engages in a form-fitting manner in a recess (26) formed on an inner surface of the corresponding gearwheel (8A, 8B), preferably the at least one pawl member (24) comprises a protrusion (28) being arranged to fit into a groove formed within said recess (26), said protrusion (28) being configured so that said pawl member (24) remains engaged in said recess (26) even when the torque transferred from the shaft (10) to the engaged gear is reversed.

7. A selective gear system according to any of the previous clauses, characterized in that the mobile member (34) comprises at least one magnetised member (58A), in particular a permanent magnet, said member interacting by repulsion and/or attraction with an adjacent magnetised member (58B) mounted on the corresponding engaging or disengaging mechanism (12A, 12B), namely the first or second engaging or disengaging mechanism (12A, 12B), wherein the radial position of the adjacent magnetised member (58B) depends on the axial position of the at least one magnetised member (58A), preferably the adjacent magnetised member (58B) being mounted on a radially sliding element or on the corresponding pawl member, more preferably said magnetised member (58B) biasing inwardly or outwardly the corresponding pawl member (24).

8. A selective gear system according to any of the previous clauses, characterized in that the mobile member (34) comprises at least one cam member (50A), in particular a ramp, said member (50A) actuating an adjacent cam follower member (50B) mounted on the corresponding engaging or disengaging mechanism (12A, 12B), namely the first or second engaging or disengaging mechanism (12A, 12B), wherein the radial position of said cam follower member (50B) depends on the axial position of the at least one cam member (50A), preferably the adjacent cam follower member (50B) being mounted on a radially sliding element or on the corresponding pawl member (24), more preferably said cam follower member (50B) biasing outwardly the corresponding pawl member (24).

9. A selective gear system according to any of the previous clauses, characterized in that the electromagnetic linear actuator (30) comprises one or more solenoids (32A, 32B), preferably the or each solenoid (32A, 32B) being electrically connected to at least two selectable voltage sources.

10. A selective gear system according to any of the previous clauses, characterised in that the one or more solenoids (32A, 32B) extend over an axial portion of the shaft, and the first and second gearwheels (8A; 8B) extend over a second axial portion of the shaft (10), wherein the first axial portion and second axial portion are adjacent.

11. A selective gear system according to any of the previous clauses, characterised in that it comprises a complementary mobile member (60), and in that the mobile member (34) and the complementary member (60) are arranged so as to define an axial air gap, whose maximal axial length is less than 90% of the mobile member stroke (34), preferably less than 50% of said stroke.

12. Gearbox (6) comprising the selective gear system according to any of the previous clauses.

13. Gearbox (6) according to clause 12, characterized in that said gearbox (6) is a layshaft-type gearbox, in which the shaft (10) comprises a pinion for the final drive, or a planetary gearbox with a planetary gear set (2) made of a stack of planetary stages (4A, 4B, 4C) disposed axially in series and the first and second gearwheels (8A, 8B, 8C) are sun gears thereof. 14. Gearbox (6) comprising a selective gear system, in particular according to any of the previous clauses 1 -11 , characterized in that the gearbox (6) is a planetary gearbox with a planetary gear set (2) made of a stack of planetary stages (4A, 4B, 4C) disposed axially in series, and optionally the first and second gearwheels (8A, 8B, 8C) being sun gears of the planetary stages (4A, 4B, 4C); in that at least one planet (14A.1 -4) of the first planetary stage (4A) and an adjacent planet (14B.1-4) of the second planetary stage (4B) are arranged coaxially in series; and in that the at least one planet (14A.1 -4) of the first planetary stage (4A) and said axially adjacent planet (14B.1 -4) are made in one piece and/or at least two ring gears (20A, 20B) of the planetary gear set (2) are made in one piece.

15. Gearbox (6) according to any of the previous clauses 12-14, characterized in that the gearbox (6) is an offset gearbox.

List of reference symbols

2 Planetary gear set

4A, 4B, 4C Planetary stages

6 Gearbox

8A, 8B (sun) (first) (second) gearwheel

10 (common) shaft

12A, 12B, 12C Engaging or disengaging mechanism

14A.1-4, 14B.1 -4 Planet

16.1 -4 Planetary shaft

18A,18B Planetary ring

20A,20B Ring gear

24 Pawl member

26 Recess

28 Protrusion

30 Electromagnetic (linear) actuator

32, 32 A, 32 B Solenoid

34 Mobile member, needle

36 Spring (for mobile member)

40 Annular ring

50A, 50B (linear or follower) cam member

52 Ball

54 Electric motor

56 Differential unit

58A, 58B (adjacent) magnetised member

60 Complementary mobile member

62 Non-magnetic member

64 Guiding bushing

66 Bearing