AU2016205869B2

AU2016205869B2 - Reduction system applied to a wheel hub, and more particularly to a wheel hub connected to a differential of a goods vehicle

Info

Publication number: AU2016205869B2
Application number: AU2016205869A
Authority: AU
Inventors: José Francivaldo PEREIRA LEMOS
Original assignee: Iveco SpA
Current assignee: Iveco SpA
Priority date: 2015-01-08
Filing date: 2016-01-08
Publication date: 2019-08-22
Anticipated expiration: 2036-01-08
Also published as: AR103382A1; BR112017014754A2; AU2016205869A1; WO2016110823A1

Abstract

A reduction system applied to the wheel hub, of the type for use in a truck (1) or a goods transport vehicle is described, with at least one pair of side members (2) in the front part there being provided a cab (3) and between said side members the powertrain of the truck (1) consisting of the engine (4), gearbox (5), universal joint (7) which connects in rotation the output of the gearbox to the differential (6), said differential redirecting the rotation of the universal joint to the wheels (8) fixed on the related wheel hubs (12) by means of a pair of drive shafts (19). On each single wheel hub (12) as above a sun/planet type transmission device is inserted which, in the condition of the system being disengaged, causes a reduction of the transmission ratio between the rotation of the drive shaft (19) and the hub (12) and which, in the engaged condition of the system, transmits the rotation of the drive shaft (19) to the wheel hub (12) with a transmission ratio of 1: 1.

Description

REDUCTION SYSTEM APPLIED TO A WHEEL HUB, AND MORE PARTICULARLY TO A WHEEL HUB CONNECTED TO A DIFFERENTIAL OF A GOODS VEHICLE [001] The present invention relates to a reduction system applied to a wheel hub, and more particularly to a wheel hub of the differential of a goods vehicle.

State of the art [002] A reference herein to a patent document or any other matter identified as prior art, is not to be taken as an admission that the document or other matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

[002a] Where any or all of the terms comprise, comprises, comprised or comprising are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components.

[002b] As known, a goods vehicle, such as a truck or a tractor, is a vehicle the chassis of which is substantially defined by a pair of parallel side members. An engine is anchored to the front of the vehicle, typically a diesel cycle engine, the drive axle of which is connected in output to a gearbox. Furthermore, the cab is mounted in the front of the vehicle, usually over the engine, with the commands of the various operating systems (such as the steering wheel, the brakes, the accelerator, the electrical controls, tyres, etc.) as well as the driver's cab of the vehicle.

[003] Depending on the destination, in the rear of the vehicle a bed on side members, an open truck bed or specific devices, as in the case of a concrete

C:\pof\word\SPEC-1089879.docx

1a

2016205869 11 Jul 2019 mixer truck, can be secured. Indeed, in the case in particular of said tractors, the so-called fifth wheel is fixed to the side members, and such structure used to couple or uncouple a trailer.

[004] Lastly, under the aforesaid side members the axles of the vehicle are also anchored, with interposed suspension systems (usually consisting of leaf spring type shock absorbers), said axles being divided into steering axles and drive axles. The steering axles are structures with I-beam profiles arranged transversely in relation to the side members, in the ends of which the wheel hubs and the front brake are anchored and on the latter the wheel/tyre groups

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PCT/IB2016/050086 of the truck. Due to the primary function of distributing the weight of the truck and the load on the road, the wheels and hubs are not mechanically connected to the engine/transmission, but to the steering system, and are therefore considered steering axles.

[005] Instead as regards the drive axles, they present, in an intermediate position, a differential which comprises an entrance in the longitudinal direction for connecting the universal joint and two opposite outputs in the transverse direction. The drive shafts are connected to said opposite transverse elements and the wheel hubs supporting the relative wheel groups and tyres to the relative protruding ends.

[006] Inside the differential, a mechanism composed substantially of the crown and pinion performs, in addition to the mechanical transmission of the rotations of the universal joint to the drive shafts (transverse to the transmission), also a reduction, depending on the size of the crown and pinion. Furthermore, usually the differential connects the two drive shafts by means of a pair of planet gears, positioned on the inner ends of said drive shafts and a pair of sun gears, the axles of which are mechanically coupled to the casing of the satellite and thus to the crown. This way, it is possible to achieve the release between the rotations of each single drive shaft, for example, in the case of curving of the vehicle.

[007] In addition, each single drive shaft protrudes beyond the differential and is paired in rotation with a relative wheel (or double wheel, depending on the case) of the drive axle of the vehicle. In addition to the hub wheel and tyre group the brake system is placed on the hub so as to enable the braking of the vehicle when the aforesaid brake disc is activated.

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PCT/IB2016/050086 [008] This way between the drive axle of the engine and the wheels of the vehicle, two reduction steps are performed, i.e. the first, variable, on the gearbox (depending on the gear engaged by the driver) and the second, constant, on the differential. These reductions are necessary to ensure that the torque produced by the engine is amplified and transmitted to the wheels, and thus the loss of rotational speed of the wheels, so that the goods shipped can be moved efficiently. Instead as regards the gearbox, this usually permits a selection of a considerable number of gears, i.e. reductions between the drive axle of the engine and the universal joint. The number of gears, which can reach about 20, reflects the need to keep the engine running at a speed of rotation, or rotation per minute, in an optimised range, usually about 1500 to 2000 revolutions per minute. While the engine is running with values close to this rotation speed, the engine works at maximum torque capacity, the diesel combustion is more efficient and pollutant emissions are reduced. An increase in the speed of rotation does not, necessarily entail a proportional increase of engine torque, but generates an increase in consumption and pollutant emissions due to incomplete combustion of the diesel injected into the combustion chambers of the engine.

[009] Modern trucks, essential tools for industrial and commercial logistics are designed primarily in relation to the goods to be transported on road networks. Consequently the reduction ratios both of the differential and the gearbox are predetermined for moving the vehicle in a loaded condition. However, there are situations in which the vehicle is travelling unloaded, i.e. without carrying any goods or rolling stock. In this case, the reduction ratios adopted can be excessive, even when the vehicle uses the highest gears. Consequently, there

2016205869 11 Jul 2019 is an unnecessary increase in fuel consumption and therefore a high level of emissions.

[0010] This condition of vehicle movement is even more negative when the vehicle in question is a road tractor, without a trailer. In this case in particular, the mass of the tractor is very small compared to the load that it can move, so even the highest gear ratios are not efficient in relation to the reduction of fuel and reduction of emissions.

[0011] It is therefore desirable to provide a reduction system applied to the wheel hub, in particular of trucks and the like, able to allow the wheel hub, by means of defined parameters, to perform an independent variation in the rotation ratio between the wheel of the vehicle and the drive shaft, so as to better adapt the differential axle to the load conditions of the vehicle.

[0012] It is also desirable to provide a reduction system, placed on the wheel hub, so that it can be engaged and disengaged according to the load carried by the vehicle.

[0013] It is further desirable to provide a system able to overcome the drawbacks of the prior art.

Summary of the invention [0014] The present invention provides a reduction system applied to the wheel hub by means of a planetary reduction enabling the coupling and uncoupling of the reduction by a pneumatically operated sliding sleeve.

[0015] In particular, an aspect of the present invention provides a wheel hub comprising a revolution reduction system integrated in said hub of the type for use in a truck or a goods vehicle comprising an engine, mechanically connected to a respective gearbox, and a universal joint which connects in rotation the output of the gearbox to the central differential, said central differential redirecting <filename>

2016205869 11 Jul 2019 the rotation of the universal joint to the wheels fixed on the related wheel hubs by means of a pair of drive shafts, wherein said revolution reduction system comprises a sun/planet type differential transmission device which, in a first operating condition causes a reduction of the transmission ratio between the rotation of the drive shaft and the hub and, in a second operating condition, transmits the rotation of the drive shaft to the wheel hub with a transmission ratio of 1:1, and wherein the selection means for determining said transmission ratio consist of a sliding sleeve suitable to mesh in a first planet gear, rotatably mounted and coaxial with respect to an extension axis of said drive shaft and, in said second operating condition, to couple directly with a cross connector integral with the hub and wherein said cross connector integral with the hub consists of a satellite-carrier cage having at least one arm, radial with respect to said longitudinal extension of said drive shaft, on which a sun gear is rotatably fixed, suitable to rotate around said radial axis and to couple with said first planet gear and simultaneously with a second planet gear parallel and coaxial with said first planet gear, wherein said second planet gear defines a fixed point. In particular, and with the transmission device in a disengaged condition, the reduction in the transmission ratio is from 2:1 to 3:1 and preferably from 2.5:1 to 2.8:1.

[0016] In addition, said wheel hub comprises a cylindrical sleeve, connected in rotation to the drive shaft, since this sleeve functions as rotation axis of a first planet gear and of a cross connector, this cross connector being in a more inward position in relation to said first planet gear, and since said cross connector has an X-shaped structure with four arms projecting radially from a central hole and which are attached to the body of the wheel hub, each of said arms functions as rotation axis of the respective four sun gears, each of said sun gears being simultaneously meshed with the first planet gear and the second planet gear, <filename>

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2016205869 11 Jul 2019 given that the second planet gear is parallel to the first planet gear, and is in a more inward position in relation to the cross connector <filename>

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PCT/IB2016/050086 and is fixed in relation to the seat of the wheel hub. The above wheel hub also consists of a cover with a rotary valve on the outside and a cylindrical groove on the inside, inside which a piston moves, the inner volume of the cylindrical groove and the outer surface of the piston define a chamber, fed pneumatically by the line by means of said rotary valve and since the aforesaid piston is connected to the sleeve so as to be able to carry out a movement in the direction of the longitudinal axle, as a function of the internal pressure of the chamber and of the elastic reaction of a spring, positioned inside said sleeve and against the projecting end of the drive shaft. In addition, the sleeve presents a radial coupling which selectively couples in rotation the first planet gear or the cross connector as a function of the relative position of the sleeve.

[0017] Lastly, the system is activated by means of a button, preferably positioned on the dashboard of the truck cab, electrically connected to a solenoid valve that communicate the air passage to the line coming from a tank of pressurised air, or discharges the pressure on said line. Alternatively, disengagement of the system is automatic, it being equipped with a sensor capable of detecting the “loaded” condition of the truck, said sensor being electrically connected to a solenoid valve that unblocks the passage of air to the line from a pressurised air tank, or discharges the pressure on said line.

Brief description of the drawings [0018] The purpose of the present invention will be more clearly evident from the detailed description below of a preferred embodiment of the invention, made with reference to the drawings, presented by way of a non-limiting example of the invention, of which:

- Figure 1 is a schematic view of a 4 x 2 tractor type truck;

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- Figure 2 is a cross-section view of the wheel hub of the vehicle in Figure 1, in a condition with reduction; and

- Figure 3 is a cross-section view of the wheel hub of the vehicle in Figure 1, in a condition without reduction.

Preferred embodiment of the invention [0019] In accordance with the attached drawings, and in particular with reference to figure 1, a truck 1 is equipped, substantially, with a pair of side members 2, placed parallel to each other, and which define the truck chassis. In the front is the cab 3, which may be articulated in relation to the side-members, so as to be able to tip forward and thereby allow access to the engine 4 and further components of the powertrain of the truck 1.

[0020] With particular focus on the powertrain of the truck 1, the latter consists of the aforementioned engine 4, which is mechanically connected to the gearbox 5, attached to the engine block by means of a flange around the engine output shaft. The rotation given by the gearbox 5 is changed according to the gear selected by the driver and is transmitted to the differential 6 by means of the universal joint 7, on which it undergoes a further reduction (based on the fixed ratio defined by the differential gears), and is redirected in a direction transverse to the direction of the universal joint, therefore to move the drive wheels 8 of the truck 1 by means of the drive shafts 19 positioned inwards of the arms of the differential. In each protruding end of the drive shafts a respective brake disc 10 is also provided which performs braking of the truck 1 when its rotation is obstructed by the calipers 11.

[0021] In addition, at each end of each arm 9 of the differential a dedicated wheel hub 12 is assembled and attached with which, in a medial part thereof,

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PCT/IB2016/050086 the end of the drive shaft 19 is coupled. As proposed in the present invention, the mechanical coupling between said drive shaft 19 and the wheel hub 12 is carried out by means of a sun/planet type transmission group, so as to allow a gear ratio 1:1 and 3:1, depending on the configuration used.

[0022] In particular, and according to figure 2, the wheel hub 12 consists of a substantially cylindrical casing 13, having an axis of rotation and symmetry 28, which is attached to the protruding end of the arm 9 of the differential 6. The casing 13 can rotate with respect to the arm 9 together with the wheel 8 and presents a substantially flat cover 15, defining a base of said cylindrical shape, inside which a cylindrical groove 16 is made to house a piston 17. On the outside, the cover 15 is coupled to a rotary valve 18, powered by a pneumatic actuation flow through the line 20. Consequently, the front of the cylindrical groove 16, with the piston 17, defines a chamber 21 of expansion, fed by the pressure of the line 20. The end of the piston 17 opposite the chamber 21 is coupled to a sliding sleeve 14, which encloses the end of the drive shaft 19. In addition, between the sleeve 14 and the end of the drive axle 19 a spring 22 is provided which acts in compression, and which is located inside said sleeve 14. As a result, and since the drive axle 19 does not move in the longitudinal direction 28, said spring 22 acts in response to the internal pressure of the chamber 21.

[0023] In addition, the radial end of the drive shaft 19 is provided with channels (not visible in figures 2 and 3) which support the mechanical coupling in rotation between the drive shaft 19 and the sleeve 14. To such purpose, the sleeve 14 presents, on the inside, corresponding/complementary channels (not visible) which are inserted in the relative channels on the drive shaft 19.

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PCT/IB2016/050086 [0024] In addition, the sleeve 14 presents a radial coupling 30, which projects from the sleeve in the intermediary area between the cross connector 27 and the first planet gear 23, so as to selectively couple with one or the other depending on the longitudinal position of said sleeve 14.

[0025] The so-called cross connector 27 is also known as a satellite-carrier cage.

[0026] In the position shown in Figure 2, the sleeve 14 is mechanically connected to the first planet gear 23, which is positioned transversely and coaxially with respect to the longitudinal axle 28 of the wheel hub 12. The sleeve 14 is free to turn with respect to the casing 13 of the wheel hub 12.

[0027] In a complementary manner, a second planet gear 24 is positioned parallel and coaxially to the first planet gear 23, but in a closer position to the arm 9 of the drive axle. Said second planet gear 24 is fixed relative to the arm 9, so that the satellites are forced to rotate in relation to the second planet gear, dragging, by means of the satellite-carrier cage 27, the wheel hub 12.

[0028] Between the said first and second planet gears 23, 24 one or more, preferably four, sun gears are placed, of which only the sun gears 25 and 26 are visible in figures 2 and 3. Conventionally for a planetary/satellite transmission system, the toothed edges of each of the four sun gears 25, 26 mesh respective teeth in both the first planet gear 23, and the second planet gear 24. In addition, each of the sun gears 25, 26 can turn around a respective shaft defined by the cross connector 27 and radial to the axis 28. In particular, the aforementioned cross connector 27 is a cross-shaped structure, for example when there are four suns, with the four arms 29 protruding starting from a central slot or hole, which surrounds and hinges the sleeve 14, meaning that the

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PCT/IB2016/050086 sleeve 14 can rotate freely relative to the cross connector. Said arms 29 of the cross connector 27 are positioned in a spoke pattern in relation to the longitudinal axis 28, representing the extension axis of the hub, and are fixed on the casing 13 of the wheel hub 12. This way, the rotation shafts of the preferably four sun gears 25, 26 are transverse in relation to the rotation shaft of the first and second planet gears 23, 24, the rotation shaft is coincident with the longitudinal axle 28 of the wheel hub 12.

[0029] Therefore according to the present invention, the planetary and/or satellite type transmission has the input shaft coinciding with the drive shaft, and the output shaft coinciding with the hub casing, coaxial and parallel to each other along the extension axis 28, with a first and a second planet 23 and 24 parallel to each other and coaxial to the axis 28, of which the second planet 24 is fixed to the casing.

[0030] In operation, when the truck 1 is in the condition of transporting goods, then the transmission device is disengaged, i.e. in the condition shown in Figure

2. In this operating condition there is a reduction of the rotation offered to the wheel hub 12 with respect to the rotation of the drive shaft 19. Conversely, when the truck 1 is in the condition without goods, the system of the invention is engaged (as shown in Figure 3) and the rotation of the drive shaft 19 is transmitted to the wheel hub 12 without any reduction, i.e., in a ratio of 1: 1.

[0031] Specifically, and in the system disengaged condition (as in Figure 2), the line 20 is not pressurised, so the piston 17 is kept in its retracted position by the spring action 22. Since the piston 17 is connected to the sleeve 14, it moves axially, therefore reaching a stable condition away from the arm 9. The channels of the sleeve 14 mesh with the corresponding grooves of the drive

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PCT/IB2016/050086 shaft while the radial attachment 30 keeps the sleeve coupled to the first planet gear 23, the movement is thus transferred from the drive shaft 19 to the first planet gear by means of the sleeve 14. Due to the rotation of the first planet gear 23 the sun gears 25, 26 are induced in rotation relative to the second planet gear 24, fixed relative to the casing 13 of the wheel hub, consequently, the wheel hub 12 is driven in rotation by means of a reduction that is determined by the size and number of teeth both of the planet and sun gears. The calculation of the dimensions and teeth of the gears referred to above may be performed by any technician in the sector, so as to be in the desired gear ratio.

[0032] In addition, and with particular focus on figure 3, i.e. when the truck 1 travels with no load (system engaged), and thus with a lower torque demand, compressed air is distributed from the line 20 into the valve 18, thereby filling the chamber 21 up to overcoming the resistance of the spring 22. At this point, the piston 17 is in a lying down position, in which it pushes the sleeve 14 so that the radial coupling 30 of the sleeve 14 disengages from the first planet gear 23 and engages on the cross connector 27. This way every rotation made by the drive axle 19 is transmitted directly to the sleeve and from the latter directly to the cross connector 27. Since each individual arm 29 of the cross connector 27 is directly coupled to the casing 13 of the wheel hub 12, said wheel hub is driven in rotation at a rotation speed equal to the speed of the drive axle 19, or in other words, with a ratio of 1:1.

[0033] Lastly, it being necessary to return to the condition of reduction shown in Figure 2 (system disengaged), it is sufficient to simply remove the pressure on the line 20, i.e. depressurize the line 20, so that the internal pressure in the

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PCT/IB2016/050086 chamber 21 is reduced and for example equal to atmospheric pressure, thereby allowing the spring 22 to move the piston (and therefore also the sleeve 14) towards the cover 15, so that the radial coupling 30 disengages from the cross connector 27 and goes back to meshing the first planet gear 23.

[0034] This feature further ensures that in the event of a system failure, the same automatically resumes the disengaged condition due to the action of the spring 22 on the sleeve 14, in which the truck is capable of operating for goods transport. Therefore, in the case of a possible fault in the electrical or hydraulic system of the truck, the system automatically returns to the disengaged condition, without jeopardising the transportation of any goods loaded.

[0035] The cover 15 preferably consists of a cylindrical cap or a circular plate which occludes the outer base of the cylindrical casing of the hub, i.e. in an opposite position to the arm 9.

[0036] As a result of the above, it is possible to change the gear ratio of rotation between the drive axle 19 and the wheel hub 12 in a simple and direct manner, it being sufficient to feed or not the valve 18 with compressed air. In addition, the proposed coupling and uncoupling mechanism is quite simple and is only in the longitudinal movement of the piston/sleeve group, i.e. with a minimal movement of parts.

[0037] Alternatively, the system can operate with a direct transmission ratio,

i.e. of 1:1 or with a transmission ratio of 2:1 to 3:1. Moreover, even more preferably, the transmission ratio is from about 2.5: 1 to 2.8:1. From a transmission ratio within the range now described, it is possible to operate with the truck 1 in optimum condition for any load condition (with or without goods). As a result, optimum transfer of engine torque to the wheels is achieved with a

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PCT/IB2016/050086 load condition, as well as a reduction in the transmission of torque, with the consequent increase of speed to the wheels when the system is engaged.

[0038] As may be seen, improvements and variants may be made to the system of the present invention while remaining within the scope of protection of the present invention. In particular, for the system described it is possible to provide for both its direct and automatic activation.

[0039] More specifically, the direct activation of the system may be by means of a special activation button (not shown) with two positions and placed, for example on the dashboard of the truck cab. In this case, said activation button is electrically connected to a solenoid valve (not shown) that releases the passage of air to the line 20 from a pressurised tank. In a first position (on), the air is supplied to the chamber 21 and, in the second position (off), the air is discharged.

[0040] Instead in the automatic functioning mode of the system, a sensor (not shown) detects the presence, or not, of the goods loaded, and emits an electrical signal to a solenoid valve which, as in the previous case, enables the entrance of pressurised air into the chamber 21 (engaged) or discharges the pressure from said chamber (disengaged), as required.

Claims

The claims defining the invention are as follows:

1. A wheel hub comprising a revolution reduction system integrated in said hub of the type for use in a truck or a goods vehicle comprising an engine, mechanically connected to a respective gearbox, and a universal joint which connects in rotation the output of the gearbox to the central differential, said central differential redirecting the rotation of the universal joint to the wheels fixed on the related wheel hubs by means of a pair of drive shafts, wherein said revolution reduction system comprises a sun/planet type differential transmission device which, in a first operating condition causes a reduction of the transmission ratio between the rotation of the drive shaft and the hub and, in a second operating condition, transmits the rotation of the drive shaft to the wheel hub with a transmission ratio of 1:1, and wherein the selection means for determining said transmission ratio consist of a sliding sleeve suitable to mesh in a first planet gear, rotatably mounted and coaxial with respect to an extension axis of said drive shaft and, in said second operating condition, to couple directly with a cross connector integral with the hub and wherein said cross connector integral with the hub consists of a satellite-carrier cage having at least one arm, radial with respect to said longitudinal extension of said drive shaft, on which a sun gear is rotatably fixed, suitable to rotate around said radial axis and to couple with said first planet gear and simultaneously with a second planet gear parallel and coaxial with said first planet gear, wherein said second planet gear defines a fixed point.
2. A hub according to claim 1, wherein in said first operating condition, the reduction of the transmission ratio is from 2:1 to 3:1.
3. A hub according to claim 2, wherein in said first operating condition, the reduction of the transmission ratio is from 2.5: 1 to 2.8: 1.

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4. A hub according to any one of the claims from 1 to 4, wherein it comprises a cover, a rotary valve being inserted on the outside and a cylindrical groove being made on the inside communicating with said rotary valve, inside the groove a piston being arranged to form a pneumatic actuator, wherein said piston is suitable to move in the direction coaxial with said drive shaft, to activate the selection means of said transmission ratio to achieve said first or second operating conditions.
5. A hub according to claim 4, wherein said groove and said piston define a chamber, suitable to be fed pneumatically by a pneumatic line by means of said rotary valve and wherein an opposite end of the piston, with respect to said groove is operatively associated with said selection means.
6. A hub according to claim 5, wherein said selection means comprise a sliding sleeve keyed so as to slide and fixed in rotation on said drive shaft, and a spring interposed axially between said drive shaft and said sleeve so as to induce said piston to retract in response to a depressurisation of said chamber.
7. A hub according to claim 6, wherein said sleeve has a cylindrical shape and said spring is surrounded radially by said sleeve.
8. A hub according to any one of claims 1 to 7, wherein said satellitecarrier cage comprises a through hole crossed without interference by said drive shaft.
9. A hub according to any one of the claims from 4 to 7, wherein said sleeve presents a radial coupling suitable to selectively couple the first planet gear or said fixed cross connector of the hub, as a function of the related axial position of the sleeve to achieve respectively said first or second operating conditions.

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10. A hub according to any one of the preceding claims, wherein said differential transmission device comprises a second planet gear permanently connected with a tubular arm crossed coaxially by said drive shaft and wherein said second planet gear is configured to axially lock a bearing which rotatably supports said hub on said arm.
11. A transmission system comprising a hub according to any one of the preceding claims and a key connected electrically to a solenoid valve which controls the pressurisation or depressurisation of the line.
12. A transmission system according to claim 11 wherein the key is positioned on the dashboard of the cab of the truck.
13. A transmission system comprising a hub according to any one of the claims from 4 to 7, wherein a switching between said first and second operating conditions is operated automatically, by means of a weight sensor capable of detecting a load condition of the truck, wherein said sensor is electrically connected to a solenoid valve responsible for the control of said pneumatic actuator.
14. A truck or goods transport vehicle comprising at least one pair of side members positioned parallel to one another, in the front part there being 17 provided a cab and between said side members the powertrain consisting of the engine, which is mechanically connected to the gearbox, anchored to the engine block by means of a flange around the engine output shaft and a universal joint which connects in rotation the output of the gearbox to the central differential, said central differential being suitable to redirect a rotation of the universal joint to the wheels fixed on the related wheel hubs by means of a pair of drive shafts, and wherein a wheel hub complies with any of the claims from 1 to 10.