The present invention relates to an i nter-axle differential assembly, to a driving axle system comprising such an inter-axle differential assembly, and to a motor vehicle. In particular, but not exclusively, the invention relates to an i nter-axle differential assembly for use i n a driving axle system in the form of a tandem bogie or a tridem bogie, i .e. a bogie with two and three driving axles, respectively.

BACKG ROU N D AN D PRIOR ART

A conventional driving axle system in the form of e.g . a tandem or tridem bogie of a motor veh icle comprises two or more driving axles arranged to be driven by a single input shaft con nected to a power source. In the case of a tandem bogie, the bogie comprises two driving axles which may be referred to as a forward-rear and a rear-rear driving axle. The forward-rear and rear-rear drivi ng axle each include a pair of drive shafts on which one or more wheels of the motor vehicle are mounted. Each of the forward-rear and rear-rear driving axles further i ncludes a differential gear set that allows the vehicle wheels on each driving axle to rotate at different speeds. Further, an axle gear system i ncluding means for distributing torque between the forward-rear and the rear-rear driving axles, such as an i nter-axle differential assembly, is usually provided.

The inter-axle differential assembly allows the wheels of the two driving axles to rotate with mutually different rotational speeds and thereby compensates for slippage, cornering , mismatched tires, etc. Such an inter-axle differential assembly comprises a driving input shaft around which a forward differential wheel is rotationally mounted. It further comprises a rear differential wheel and a differential spider, mounted arou nd and connected for common rotation with the i nput shaft, between the forward and the rear differential wheels. On the differential spider, differential pi nions configured to meshingly engage with the differential wheels are rotatably mounted. The forward-rear driving axle is connected to the forward differential wheels by means of a set of different components, such that torque drivi ng this axle is transferred via the forward differential gear. The output shaft is connected to the rear differential wheel , so that torque is transferred to the rear- rear driving axle via the rear differential wheel .

A locki ng mechanism may also be included in the inter-axle differential assembly. The locking mechanism can be used to lock e.g . the rear differential wheel and the differential spider together, thus forci ng the rear differential wheel , and thereby also the forward differential wheel , to rotate at the same rotational speed as the input shaft. Thus, a selection can be made by a driver of the motor veh icle whether to al low the wheels of the rear-rear and the forward-rear driving axles to be driven at different rotational speeds or not. EP2574490 discloses a driving axle system that comprises an inter-axle differential assembly includi ng a locking mechanism such as described above, and further comprisi ng a disengagement system for disengaging the rear-rear driving axle. The disengagement assembly comprises an intermediate shaft, con nected as the output shaft of the inter-axle differential assembly. The intermediate shaft can , by means of a control device, be moved between a position in which it transmits torque to the output shaft driving the rear-rear driving axle, and a position in which it does not transmit any torque. One control device is used to control the locki ng mechanism and a separate control device is used to control the disengagement assembly.

SUMMARY OF TH E I NVENTION

It is a primary objective of the present invention to provide an , i n at least some aspect, improved i nter-axle differential assembly and driving axle system . Another objective is to provide an inter- axle differential assembly which enables disengagement of a rear driving axle i n a tandem or a tridem drive system . Yet another objective is to provide an inter-axle differential assembly which allows selection between an open mode in which an input shaft and an output shaft are allowed to be driven at different rotational speeds, a locked mode in which the input shaft and the output shaft are forced to rotate with the same rotational speed, and a disconnected mode in which only a forward drivi ng axle is driven , which inter-axle differential assembly is less complex and more robust than previously known such assemblies. At least the primary objective is, according to a first aspect of the invention , achieved by means of an inter-axle differential assembly for distribution of torque between a forward driving axle and a rear driving axle i n a motor vehicle according to claim 1 . The inter-axle differential assembly comprises:

- a driving input shaft,

- a forward differential wheel rotationally mounted around the input shaft,

- a rear differential wheel ,

- a differential spider mounted between the differential wheels and being con nected for common rotation with the input shaft, on which differential spider differential pinions configured to meshingly engage with said differential wheels are rotatably mou nted ,

- a differential housing to which the differential spider is con nected for common rotation ,

wherein torque is transferable to the forward driving axle via the differential spider, the differential pinions and the forward differential wheel , and wherein the assembly further comprises an output wheel sharing a common axis of rotation with the input shaft, via which output wheel torque is transferable to the rear driving axle, and a connection means movable between :

- an open position in which it connects the output wheel to the rear differential wheel for common rotation , and in which the differential housi ng can rotate independently of the rear differential wheel ,

- a locking position in which it con nects both of the differential housing and the output wheel to the rear differential wheel for common rotation , and - a disconnecting position in which it connects the differential housing to the rear differential wheel for common rotation , and in wh ich the output wheel can rotate independently of the rear differential wheel .

The inter-axle differential assembly according to the invention has a connection means that allows selecti ng between an open mode in which the input shaft and a rear output shaft driven by the output wheel are allowed to be driven at different rotational speeds, so that the forward driving axle and the rear driving axle can be driven at mutually different rotational speeds, a locked mode i n which the input shaft and the rear output shaft are forced to rotate with the same rotational speed, and a discon nected mode in which only a forward driving axle is driven . All th ree modes can be activated using a single connection means. The inter-axle differential assembly thereby, by using a relatively small n umber of components, provides a robust and space efficient solution for achieving a possibility to select between an open mode, a locked mode and a discon nected mode.

In all three modes, torque is transferred via the spider, the pin ions and the forward differential wheel to the set of components driving the forward drivi ng axle. Such a set of components can e.g . comprise transfer gear members and a forward output shaft, e.g . in the form of a pin ion , extending in parallel with the input shaft.

The output wheel is configured to drive a rear output shaft, which drives the rear driving axle. The output wheel and the rear output shaft may be formed in one piece, the output wheel form ing an end reg ion of the rear output shaft, but they may alternatively be formed as separate components connected for common rotation .

When the connection means is in the open position , torque is transferred from the i nput shaft via the spider, the differential pinions, the rear differential wheel and the con nection means to the output wheel . The driving input shaft and the output wheel are allowed to be driven and to rotate at different rotational speeds. The rear output shaft, which is driven by the output wheel and which drives the rear driving axle, and a forward output shaft, which drives the forward driving axle, are thereby also allowed to rotate at different rotational speeds. Thus, also the wheels of the forward driving axle and the wheels of the rear driving axle are allowed to rotate at different rotational speeds.

When the connection means is in the locking position , torque is transferred from the input shaft via the differential spider, the differential housi ng and the con nection means to the output wheel . The forward differential wheel rotates at the same rotational speed as the rear differential wheel , the differential spider and the input shaft. The drivi ng input shaft and the output wheel , and thereby also the rear output shaft and a forward output shaft, are driven and rotate at the same rotational speed. When the connection means is in the disconnecting position , no torque is transferred to the output shaft. The rear drivi ng axle can be elevated off ground or used as a passive support axle, while the forward driving axle drives the motor veh icle. Since the differential spider is arranged for com mon rotation with the differential housi ng , the differential housi ng may extend only between the differential wheels without overlappi ng the connection means. The differential housing can thereby be efficiently manufactured in one piece.

The connection means may be configured to be moved between the different positions by means of an actuator movable i n an axial direction along the axis of rotation , wherein the actuator is configured to engage with the connection means. Since the actuator does not need to extend through the differential housing , it may be non-rotational . In other words, the actuator may be configured to be fixed with respect to the axis of rotation i n each of the open position , the locking position and the discon necting position . Furthermore, the actuator only needs to have one su rface arranged for relative motion, namely an engagement su rface configured to engage with a correspondi ng engagement su rface within the connection means. The connection means may thereby rotate together with the rear differential wheel wh ile the actuator is fixed. This enables a relatively si mple and robust design of a control device includi ng the actuator.

According to one embodi ment of the invention , the con nection means comprises a sleeve movable i n an axial direction along said axis of rotation . By movi ng the sleeve in the axial direction , it can move to the three different positions.

According to one embodiment, said sleeve comprises internal splines configured to engage with external splines provided on the rear differential wheel and on at least one of the differential housing and the output wheel . Thus, the sleeve is mounted for com mon rotation with the rear differential wheel by means of the spl ines. At least one of the output wheel and the differential housing are also provided with mati ng external spl ines. The spl ines provide a reliable and robust connection . If only one of the output wheel and the differential housi ng are provided with external splines, locking between the other component and the sleeve is achieved by other locki ng means. Accordi ng to one embodiment, both of the differential housi ng and the output wheel have external splines configured to selectively engage with the i nternal splines of the sleeve. The sleeve can thus slide over external ly spli ned portions on the differential housi ng , the differential wheel and the output shaft. This is a robust and space efficient sol ution .

According to one embodiment, the sleeve comprises internal splines configured to engage with external splines provided on the rear differential wheel , and the sleeve and at least one of the differential housing and the output wheel have end faces provided with teeth configured to selectively engage with each other. This is an alternative configuration in the form of a dog cl utch . It is possible to provide end faces with teeth on both of the differential housi ng and the output wheel . The sleeve has in this case teeth provided on both its end faces.

According to one embodiment, the differential spider comprises a central portion arranged around the input shaft and beari ng pins extending from the central portion , the differential pin ions being arranged on said bearing pins, wherein outer ends of said bearing pins are in locking engagement with recesses formed i n the differential housing . The differential housing is thus efficiently locked for common rotation with the differential spider and the components are also easy to assemble and disassemble. Locking engagement can be provided by means of mating geometrical shapes, such as rounded outer ends and corresponding ly rounded recesses. As an alternative or as a complement, fasten ing means in the form of e.g . screws, bolts or similar, may be provided for locking the differential housing and the differential spider together. The differential housing may sometimes be referred to as a differential cage.

According to one embodiment, the assembly further comprises a control device configured to move said connection means between the open position , the locki ng position and the disconnecting position upon receipt of a signal . Thus, a driver of the motor vehicle may easily provide a signal , e.g . by adjusting a knob or similar, that it is desi red to drive in a specific mode and that the connection means should be moved to the correspondi ng position .

According to one embodiment, the i nput shaft is supported in the rear differential wheel by means of a bearing , preferably a tapered rolling bearing . The tapered rolling bearing can absorb both radial and axial forces between the input shaft and the rear differential wheel and is therefore suitable for th is application . Alternatively, a combination of radial and axial bearings can be used.

According to one embodiment, the rear differential wheel is supported in the output wheel by means of a bearing , preferably a tapered rolli ng bearing . Both radial and axial forces can thereby be absorbed. Alternatively, a combination of radial and axial bearings can be used .

Accordi ng to one embodiment, the output wheel is configu red to be connected for common rotation with a rear output shaft. The con nection can be achieved by means of e.g . spli nes. The rear output shaft is in this case received in the output wheel . By forming the output wheel and the rear output shaft as two different components connected for common rotation , the inter-axle differential assembly is easier to handle and mou nt than in the case when the output wheel is integrated with the rear output shaft.

Accordi ng to another aspect of the invention , at least the above mentioned pri mary objective is achieved by means of a drivi ng axle system comprisi ng at least one proposed inter-axle differential assembly, at least one forward driving axle to which torque is transferable via the forward differential wheel , and at least one rear driving axle to which torque is transferable via the output wheel . Advantages and advantageous features of such a driving axle system appear from the above discussion in connection with the proposed inter-axle differential assembly. Of course, it is possible to also have a foremost driving axle, located at a front of the motor veh icle, which does not form part of th is driving axle system .

Accordi ng to one embodiment, the driving axle system is a tandem drive system . Such a system has two driving axles coupled to a single i nter-axle differential assembly. The i nvention also relates to a motor veh icle comprising the proposed driving axle system , preferably wherein the motor veh icle is a heavy motor vehicle such as a bus or a truck, e.g . a long haulage truck adapted to transport cargo on a trai ler, a military truck, a construction or distribution truck, a truck for use in forestry, mini ng , etc. The motor vehicle may be a motor vehicle powered by an i nternal combustion engine, or a motor vehicle entirely or partly powered by an electric power source, such as a hybrid vehicle.

Other advantageous features as well as advantages of the present invention will appear from the followi ng description .

BRI EF D ESCRI PTION OF TH E DRAWINGS

The invention will in the fol lowing be further described by means of example with reference to the appended drawi ngs, wherein

Fig . 1 schematically shows a motor vehicle havi ng a drivi ng axle system according to an embodiment of the invention

Fig . 2 shows a cross section of an inter-axle differential assembly according to an embodiment of the invention in an open mode,

Fig . 3 shows a cross section of the inter-axle differential assembly from fig . 2 in a locked mode

Fig . 4 shows a cross section of the inter-axle differential assembly from fig . 2 in a disconnected mode, and Figs.5a-c schematically show cross sections of parts of a connection means according to an embodiment of the invention. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

An exemplary motor vehicle 1 according to an embodiment of the invention is schematically shown in fig. 1. The motor vehicle 1 comprises a front axle 2 configured to steer the vehicle and a pair of front wheels 3 mounted thereon. It further comprises a powertrain 4 including an engine 5, a transmission 6 and a driving axle system 7. The driving axle system 7 is arranged towards a rear end portion of the vehicle 1. The driving axle system 7 comprises an inter-axle differential assembly 8, a forward driving axle 9 and a rear driving axle 10. To each driving axle 9, 10, a wheel assembly including four wheels 3 is mounted, such that the motor vehicle 1 in total includes ten wheels 3 distributed between three axles 2, 9, 10. Alternatively, the vehicle 1 may have a different number of wheels.

Each driving axle 9, 10 may or may not include an axle differential enabling rotation of the left and right wheel assemblies at mutually different rotation speeds. Such configurations are well known in the art and will not be further discussed herein.

An inter-axle differential assembly 8 according to an embodiment of the invention is shown in a longitudinal cross section in figs.2- 4. A driven input shaft 11, hereinafter only referred to as an input shaft, having a longitudinal axis of rotation C, is provided for transmission of torque from the engine 5 to the driving axle system 7. The input shaft 11 is connected to the transmission 6 of the relevant motor vehicle 1 and is thus driven by the engine 5 via the transmission 6. A forward differential wheel 12 is rotationally mounted around the input shaft 11 by means of a bushing 13, and is thereby allowed to rotate around the input shaft 11. Alternatively, a needle roller bearing providing the same function may be used. The forward differential wheel 12 has external cylindrical teeth 14 for transmitting torque to the forward driving axle 9 via a transfer gear (not shown). The cylindrical teeth 14 are thereby configured to meshingly engage with corresponding teeth of the transfer gear, such that driving torque can be transmitted to the forward driving axle via a forward output shaft (not shown) and, if present, a forward axle differential.

Rearward of the forward differential wheel 12 in the direction of travel of the motor vehicle, a differential spider 15 is provided. The differential spider 15 has a central portion 16 mounted for common rotation with the input shaft 11 by means of splines 17. Four bearing pins 18, on which differential pinions 19 are rotationally mounted, are formed in one piece with the central portion 16 and extend therefrom in a radial direction. In other words, the central portion 16 and the bearing pins 18 of the differential spider 15 are configured to rotate together with the input shaft 11 , at the same rotational speed. The differential pinions 19 are configured to rotate around the bearing pins 18, thus having an axis of rotation that extends radially outward from the central portion 16 of the differential spider 15. Outer ends 20 of the bearing pins 16 are in locking engagement with recesses formed in a differential housing 21 mounted for common rotation with the differential spider 15. The differential housi ng 21 , the differential spider 1 5 and the i nput shaft are thereby configured to rotate together around the axis of rotation C. The forward differential wheel 1 2 has a set of teeth configured to meshingly engage with the differential pin ions 1 9 such that a rotation of the differential spider 1 5 around the axis of rotation C leads to a rotation of the forward differential wheel 12 via the differential pin ions 1 9. The differential housing 21 can e.g . be an nular, enclosing the differential spider 1 5. On a rear portion of the differential housi ng 21 , splines 22 are provided around a periphery of the housi ng 21 .

On the rearward side of the differential spider 1 5, a rear differential wheel 23 is provided. The rear differential wheel 23 receives an end portion of the input shaft 1 1 , which is rotationally mounted i n the rear differential wheel 23 by means of a tapered rolli ng bearing 24 having cylindrically shaped rolling elements. The rolling elements may alternatively have a frustocon ical shape. Si milar to the forward differential wheel 1 2, the rear differential wheel 23 has a set of teeth configured to meshingly engage with the differential pin ions 1 9 such that a rotation of the differential spider 1 5 around the axis of rotation C leads to a rotation of the rear differential wheel 23 around the axis of rotation C. The rear differential wheel 23 further has external spl ines provided arou nd its periphery. An output wheel 26 is provided rearward of the rear differential wheel 23 i n the direction of travel of the motor vehicle. A bearing 27 is provided between the rear differential wheel 23 and the output wheel 26, e.g . in the form of a tapered rolli ng bearing , such that the rear differential wheel 23 and the output wheel 26 may rotate with different rotational speeds if not otherwise con nected . The output wheel 26 receives a rear output shaft 28, which is spaced apart from the input shaft 1 1 and which shares the longitudinal axis of rotation C of the i nput shaft 1 1 . On a periphery of the output wheel 26, spl ines 29 are provided (see figs. 3-4) . The rear output shaft 28 is config ured to rotate together with the output wheel 26 at the same rotational speed.

A connection means in the form of a sleeve 30 having internal spl ines 25 is mounted around the rear differential wheel 23, such that the internal splines 25 of the sleeve 30 engage with the external splines of the rear differential wheel 23 and can sim ultaneously engage with the external splines 22 of the differential housing 21 and/or the external splines 29 of the output wheel 26. The sleeve 30 is movable i n the axial direction , i .e. along the axis of rotation C, by means of a control device (not shown) between an open position shown in fig . 2, a locking position shown in fig . 3, and a discon necting position shown in fig . 4.

When the sleeve 30 is in the open position shown i n fig . 2, it con nects the rear differential wheel 23 and the output wheel 26 and forces them to rotate at the same rotational speed. The differential housing 21 , and thereby also the differential spider 1 5, are not connected to the rear differential wheel 23 via the sleeve 30 and may therefore rotate at a different rotational speed. The forward differential wheel 1 2, driving the forward drivi ng axle 9, and the rear differential wheel 23 , drivi ng the rear drivi ng axle 1 0, can thereby also rotate at mutually different rotational speeds, allowi ng compensation for sl ippage etc. between the wheels 3 of the forward driving axle 9 and the rear driving axle 1 0, respectively. When the sleeve 30 is in the locki ng position shown in fig . 3, it con nects the rear differential wheel 23 and both of the output wheel 26 and the differential housing 21 . The sleeve 30 thereby forces the output wheel 26 and the rear differential wheel 23 to rotate at the same speed as the differential spider 1 5, and consequently also as the i nput shaft 1 1 . Since the rear differential wheel 26 is locked for common rotation with the differential spider 15, the forward differential wheel 1 2 is also forced to rotate at the same rotational speed. Thus, the forward output shaft driving the forward drivi ng axle 9 and the rear output shaft 28 driving the rear driving axle 1 0 are forced to rotate with mutually equal rotational speeds. In th is mode, no compensation for slippage etc. between the wheels 3 of the forward driving axle 9 and the rear drivi ng axle 1 0 via the inter-axle differential assembly 8 is possible. A forward and a rear axle differential (not shown) allowi ng the left and right wheel assemblies to rotate at mutual ly different rotational speeds may however be provided. When the sleeve 30 is in the disconnecting position shown in fig . 4, it con nects the rear differential wheel 23 and the differential housi ng 21 , but allows the output wheel 26 to rotate independently of the rear differential wheel 23 and the differential housi ng 21 , and thereby also of the i nput shaft 1 1 . No torque is thus transferred to the rear output shaft 28 and the rear driving axle 1 0. I nstead, all torque is transmitted to the forward driving axle 9. The rear driving axle 1 0 can in this mode be elevated off g round or used as a passive support axle. The output wheel 26 and the input shaft 1 1 are rotationally mounted in an assembly housing (not shown) by means of tapered roll ing bearings 31 , 32. A collar 33 provided on the input shaft 1 1 between the bearing 32 and the forward differential wheel 1 2 prevents the input shaft 1 1 from axial movement. A bearing 34 is provided between the collar 33 and the forward differential wheel 1 2.

A connection means according to a different embodi ment of the invention is schematically shown i n figs. 5a-c. The connection means comprises a sleeve 30 havi ng internal splines 25 configured to engage with external splines 35 provided on the rear differential wheel 23. The sleeve 30 fu rther has end faces in the axial direction provided with teeth 36, 37. The differential housi ng 21 has an end face provided with teeth 38, with which the teeth 37 of the sleeve are configured to be brought into locki ng engagement upon axial movement of the sleeve 30. The output wheel 26 also has an end face provided with teeth 39, with wh ich the teeth 36 of the sleeve are config ured to be brought into locki ng engagement upon axial movement of the sleeve 30.

In fig . 5a, the connection means is shown in the open position , in which the teeth 36 of the sleeve 30 are in locking engagement with the teeth 39 of the output wheel 26, thus locking the output wheel 26 and the rear differential wheel 23 together for common rotation , whi le the differential spider 21 is allowed to rotate at a different rotational speed.

In fig . 5b, the con nection means is shown i n the locking position , in which the teeth 36 of the sleeve 30 are in locki ng engagement with the teeth 39 of the output wheel 26 and the teeth 37 of the sleeve 30 are in locking engagement with the teeth 38 of the differential housi ng 21 , so that all of the output wheel 26, the rear differential wheel 23 and the differential spider 21 are forced to rotate with the same rotational speed. In this position , the teeth 36, 37 of the sleeve 30 are engaged half-ways with the teeth 38 of the differential housing 21 and the teeth 39 of the output wheel 26, respectively. In fig . 5c, the con nection means is shown in the disconnecting position , in wh ich the teeth 37 of the sleeve 30 are in locking engagement with the teeth 38 of the differential housing 21 , forcing the differential housi ng 21 and the rear differential wheel 23 to rotate together, and i n wh ich the output wheel 26 is disconnected.

The front axle config ured to steer the vehicle may also be configured as a driving axle, i n which case the vehicle comprises at least three drivi ng axles. The invention is of course not in any way restricted to the embodiments described above, but many possibilities to modifications thereof would be apparent to a person with skill in the art without departi ng from the scope of the i nvention as defined in the appended claims.