BACKGROUND AND PRIOR ART The present invention relates to a gearbox for vehicles according to the preamble of patent claim 1. The invention also pertains to a vehicle, which comprises such a gearbox, according to the preamble of patent claim 16.

For vehicles, and in particular heavy goods vehicles such as trucks, a gearbox, also called a range gearbox, is often connected to the main gearbox in order to double the number of potential gearings. Such an additional gearbox usually comprises a planetary gearbox, which has a low and a high gear with which the gear possibilities of the main gearbox may be divided into a low range position and a high range position. In the low range position, a gear reduction occurs through the planetary gearbox, and in the high range position no gearing occurs in the planetary gearbox.

The range gearbox is arranged between the main gearbox and a propeller shaft connected to the driving wheels of the vehicle. The range gearbox is housed in a gearbox housing and comprises an input shaft connected to the main gearbox, an output shaft and a planetary gearbox arranged between the input shaft and the output shaft. The planetary gearbox usually comprises three components, which are rotatably arranged in relation to each other, namely a sun wheel, a planetary wheel holder and a ring wheel. With knowledge about the number of teeth in the sun wheel and the ring wheel, the mutual speeds of the three components may be determined during operation. In a range gearbox, the sun wheel may be connected in a non-rotatable manner with the input shaft, a number of planetary wheels may engage in the said sun wheel, the planetary wheels of which are rotatably journalled on the planetary wheel holder, which is connected in a non-rotatable manner with the output shaft, and an axially shiftable ring wheel, which envelops and engages with the planetary wheels. The teeth of the sun wheel, the planetary wheels and the ring wheel may be oblique, i.e. they have an angle in relation to the rotation shaft common to the sun wheel, the planetary wheel holder and the ring wheel. By cutting teeth obliquely, a reaction force is obtained from the cogwheels comprised in the planetary gearbox in the rotation shaft's direction. The direction of the reaction force is dependent on in which direction the cogwheels in the planetary gearbox are obliquely cut. Thus the reaction forces may operate backwards or forwards in the propagation of the rotation shaft.

The low and high gears of the gearbox are achieved by axial sliding of the ring wheel between the low range position, in which the ring wheel is rotation-locked in relation to the gearbox housing, and the high range position, in which the ring wheel is ro- tatable in relation to the gearbox housing. The planetary gearbox comprises two coupling rings arranged on each side of the ring wheel, one high coupling ring and one low coupling ring, and two synchronisation rings which are arranged on each side of the ring wheel. The synchronisation rings are designed to achieve synchronous gear shifting.

In order to achieve a good synchronisation function in this type of range gearbox, the surface of the synchronisation ring's teeth, which faces the ring wheel and which is designed to receive the ring wheel's teeth on synchronisation, must have an angle, a so- called locking angle in relation to the synchronisation ring's rotation shaft, the locking angle of which must be balanced against the braking torque, which the synchronisation ring transfers to the ring wheel in order to achieve a synchronous engine speed. This means that the said locking angle must be designed so that the teeth on the synchronisation ring abut to the part of the ring wheels' teeth which are equipped with a locking angle and impacts sufficiently on the ring wheel in order for a synchronous engine speed to be achieved and, subsequently, to be released from the part of the ring wheel's teeth, which is equipped with the locking angle and when the ring wheel are to engage with the relevant coupling ring when the synchronous engine speed has been achieved. In order to secure that a synchronous engine speed is obtained before the ring wheel goes past the synchronisation ring in an axial direction, the teeth of the synchronisation ring must not let go of the ring wheel's teeth too easily. When the synchronisation ring's teeth have been released from the ring wheel's teeth, when a synchronous engine speed has been achieved between the ring wheel and the coupling ring, the ring wheel will be displaced axially, so that the synchronisation ring is inserted in the ring wheel and remains in an axial position in relation to the ring wheel, the axial position of which is determined by the position where the synchronisation ring meets and abuts against the planetary wheels of the planetary gearbox. The ring wheel's freedom of movement in an axial direction is limited by the geometrical design of the teeth of the ring wheel and the coupling ring. End surfaces meet and abut, at the tips of the ring wheel's teeth, a circumferential end surface of

each coupling ring at the ring wheels' axial end positions, which entails that the ring wheel may no longer be displaced in an axial direction.

The document WOO 155620 shows a synchronisation device at a planetary gearbox, where the planetary gearbox comprises a sun wheel, a planetary wheel holder and a ring wheel. The sun wheel is connected in a non-rotatable manner with the input shaft and a number of planetary wheels engage with the sun wheel, the planetary wheels of which are rotatably journalled on a plant wheel holder connected in a non-rotatable manner with an output shaft. An axially shiftable ring wheel envelops and engages with the planetary wheels. The low and high gears of the gearbox are obtained by the ring wheel being displaced axially between the low range position and the high range position.

With the objective of reducing the power and energy required on shifting, an axially shiftable coupling sleeve may be arranged around the ring wheel. The coupling sleeve may be designed with a lower weight than the ring wheel, which entails that it becomes easier to shift the sleeve compared to shifting the ring wheel.

The document SE-C2-504063 shows a planetary gear and a supplemental gearbox equipped with coupling rings and synchronisation rings, which are arranged on each side of the ring wheel and a coupling sleeve, arranged outside the coupling rings, the synchronisation rings and the ring wheel, the coupling sleeve of which is shiftable for connection of the ring wheel with either of the coupling rings. The cogwheels of the planetary gearbox are equipped with spurs, which entails that the ring wheel and the coupling sleeve are designed to be adapted to have a function adapted for the cog- wheel's straight teeth. The prior art supplemental gearbox has only two gears where the ring wheel assumes either a high range or a low range position.

SUMMARY OF THE INVENTION

Despite prior art there is a need to develop a gearbox which requires low energy on shifting between the low range position and the high range position. The ring wheel has a considerable mass which requires energy for movements in an axial direction. In order to achieve the axial movement of the ring wheel, manoeuvring forks and equip- ment intended for this purpose must be designed to withstand resulting forces, which increases the weight of the gearbox. The coupling and synchronisation rings' diameters are limited by the diameter of the ring wheel, which entails that the coupling and synchronisation rings' synchronisation function is adversely affected. The traditional range gearbox has a few determined locked positions between the synchronisation rings and the ring wheel, which entails that facets are formed in the surface of the cogwheel's tooth flanks. The number of facets corresponds to the number of locked positions between the synchronisation rings and the ring wheel. These facets entail an uneven operation and noise in the range gearbox and contribute to a shorter service life of the cogwheels, which entails a deterioration of the reliability and operational security of the gearbox. The aim is also to reduce noise from the gearbox and to increase the potential torque transmission in the gearbox. Under certain circumstances, it is desirable when a vehicle is at a standstill to extract force from the vehicle's combustion engine via a main gearbox. Since the prior art supplemental gearbox only takes a high range or a range position, the prior art supplemental gearbox will under all operational cir- cumstances transmit a torque.

The objective of the present invention is to provide a gearbox which requires low energy on shifting. Another objective of this invention is to provide a gearbox which has high reliability and operational security. Another objective of this invention is to pro- vide a gearbox which allows for increased torque transmission. Another objective of this invention is to provide a gearbox which has a low noise level. Another objective of this invention is to provide a gearbox which allows external transmission from a vehi- cle at standstill. Another objective of this invention is to provide a gearbox, with low axial forces, which acts on the thrust bearings of the gearbox. Another objective of the invention is to provide a gearbox which has a good synchronisation function. These objectives are further achieved with a gearbox of the type mentioned above, which is characterised by the features specified in patent claim 1. These objectives are further achieved with a vehicle, which comprises a gearbox of the type mentioned above, which is characterised by the features specified in patent claim 16. By restricting the movement of the ring wheel in an axial direction and instead arranging an axially shiftable sleeve outside and coaxially with the ring wheel, shifting may be carried out by moving the sleeve to a first axial end position, where the sleeve is fixedly connected with the gearbox housing, and to a second axial end position, where the sleeve is connected with the sun wheel. The sleeves may each be designed with less thickness than the ring wheel, which entails that the sleeve mass becomes lower than the mass of the ring wheel.

The sleeve and the ring wheel are arranged in a non-rotatable manner with each others' splines. The sleeve interacts with the synchronisation rings during shifting, and since the sleeves are arranged outside the ring wheel and thus have a greater diameter than the ring wheel, the synchronisation rings may be made with a greater diameter compared to the synchronisation rings' diameter in a traditional range gearbox.

By making the teeth with an oblique angle in relation to the sleeve's and the ring wheel's rotation shafts, the noise level of the gearbox may be reduced and the transferable torque through the gearbox may be increased.

Since the mass of the sleeve is lower than the mass of the ring wheels, less power and energy is required on shifting. An increased diameter of the synchronisation rings en- tails that good synchronisation on shifting is obtained. Since a large number of splines may be selected on the sleeve and the ring wheel, a large number of determined locked positions between the synchronisation rings and the sleeve is obtained, which entails that a large number of facets is formed on the cogwheel's tooth flanks. The facets will, however, lie close to each other, so that they together are perceived as a mainly even surface of the tooth flanks. The facets therefore do not impact the function of the range gearbox and have no mentionable impact on the service life of the cogwheels.

According to one embodiment of the invention, the bars on the sleeve and on the ring wheel extend at an oblique angle in relation to the rotation shafts of the sleeve and the ring wheel. Thus the sleeve will receive some of the axial forces which arise through the oblique teeth.

According to another embodiment of the invention, an axial stop journalled on the sun wheel is connected with the ring wheel, the axial stop of which prevents the ring wheel from being shifted axially. The axial stop receives axial forces from the ring wheel. Thus the bars on the sleeve and on the ring wheel may expand in parallel with the rota- tion shafts of the sleeve and the ring wheel.

According to another embodiment of the invention, the coupling sleeve may be shifted to a neutral position between both the coupling rings, which entails that it is possible for a vehicle at a standstill to extract force from the vehicle's combustion engine via a main gearbox.

Other advantages of the invention are set out in the detailed description below.

BRIEF DESCRIPTION OF DRAWINGS

Below is a description of, as examples, preferred embodiments of the invention with reference to the enclosed drawings, in which:

Fig. 1 shows a side view of a vehicle with a gearbox according to the present invention,

Fig. 2 shows a section view of a gearbox, according to a first embodiment of the present invention, in a low range position, Fig. 3 shows a section view of the gearbox, according to a second embodiment of the present invention, in a low range position,

Fig. 4 shows a section view of the gearbox, according to a second embodiment of the present invention, in a low range position, and

Fig. 5 shows a section view of the gearbox, according to a second embodiment of the present invention, in a neutral position.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Fig. 1 shows a side view of a vehicle 1, e.g. a truck, comprising a gearbox 2. A combustion engine 4 is connected to a main gearbox 6, which in turn is connected to the gearbox 2. The gearbox 2 is further connected to the driving wheels 8 of the vehicle 1 via a transmission comprising, among others, a propeller shaft 10. The gearbox 2 is also called a range gearbox, and its objective is to double the number of gearing possibilities. The gearbox 2 is surrounded by a gearbox housing 12.

Fig. 2 shows a section view of a gearbox 2, according to a first embodiment, in a low range position. The gearbox 2 usually comprises a planetary gearbox 14, which has a low and a high gear, with which the main gearing possibilities of the gearbox 6 may be divided into a low range position and a high range position. In the low range position, a gear reduction occurs through the planetary gearbox 14, and in the high range position the gear ratio is 1: 1. The gearbox 2 is housed in a gearbox housing 12 and comprises an input shaft 16, which may consist of an output shaft of the main gearbox 6. The planetary gearbox 14 comprises three main components, which are rotatably arranged in relation to each other, namely a sun wheel 18, a planetary wheel holder 20 and a ring wheel 22. On the planetary wheel holder 20, a number of planetary wheels 24 are arranged. On the planetary wheel holder 20, the output shaft 26 of the gearbox 2 is arranged, which is connected to the vehicle's 1 propeller shaft 10. With knowledge about the number of teeth 27 in the sun wheel 18 and the ring wheel 22, the relative speeds of the three components may be determined during operation. The sun wheel 18 is connected in a non-rotatable manner with the input shaft 16 and the planetary wheels 24 engage in the said sun wheel 18. The ring wheel 22 envelops and engages in the planetary wheels 24. The teeth 27 of the sun wheel 18, the planetary wheels 24 and the ring wheel 22 are, according to the first embodiment, oblique, i.e. they have an angle in relation to the rotation shaft 30 common to the sun wheel 18, the planetary wheel holder 20 and the ring wheel 22. By cutting the teeth 27 obliquely, a reaction force is obtained in the low range position from the cogwheels comprised in the planetary gear 14 in the direction of the rotation shaft. The direction of the reaction force is dependent on in which direc- tion the teeth 27 in the planetary gearbox 14 are obliquely cut. Thus the reaction forces may operate backwards or forwards in the propagation of the rotation shaft 30.

An axially shiftable sleeve 31 is arranged on the ring wheel, the sleeve 31 of which may, alternatively, be connected with the coupling rings 34, 35 for different gear posi- tions. The sleeve 31 is on an internal surface equipped with bars 33, which interact with similar bars 37 arranged on an outer periphery of the ring wheel 22 for the formation of an axially shiftable spline joint.

The bars 33, 37 on the sleeve 31 and on the ring wheel 22 are, according to the first embodiment, arranged at an oblique angle in relation to the rotation shafts of the sleeve 31 and on the ring wheel 22. Thus forces acting axially on the ring wheel 22 from the oblique gears will entail that the ring wheel 22 is fixed axially by the sleeve 31.

The low and high gears of the gearbox 2 are obtained by shifting the sleeve 31 axially between the low range position, in which the sleeve 31 is rotation-locked in relation to the gearbox housing 12, and the high range position, in which the sleeve 31 is rotatable in relation to the gearbox housing 12. The axial shifting of the sleeve 31 is achieved with a shifting fork 29 shown schematically in Fig. 2, which is arranged in a track 32 circumferential on the outside of the sleeve 31. Preferably, the sleeve 31 has a lower mass than the mass of the ring wheel, which entails that less energy and power is required to move the sleeve 31 when shifting. Thus a quick shifting may be carried out during a short time between the different gear positions in the gearbox. The planetary gearbox 14 comprises two coupling rings 34, 35 arranged on each side of the ring wheel 22 and the sleeve 31, one high coupling ring 34 and one low coupling ring 35, and two synchronisation rings 36, which are arranged on each side of the sleeve 31. The synchronisation rings 36 are designed to achieve synchronous gear shifting. The high coupling ring 34 is connected in a non-rotatable manner with the input shaft 16 and the sun wheel 18, so that the low coupling ring 35 is connected in a non-rotatable manner with the gearbox housing 12. The coupling rings 34, 35 and the synchronisation rings 36 are equipped with interacting friction surfaces 38, 39 which preferably have a conical shape. In the displayed embodiment, both the synchronisation rings 36 have the same design, but are mirrored-mounted on each side of the sleeve 31. The coupling rings 34, 35 are externally equipped with teeth 44, which are designed to engage with the bars 33 of the sleeve 31. The barrier elements 50 are essentially ring-shaped and arranged inside each track 52 in the sleeve 31.

In each synchronisation ring 36 there is also arranged an external circumferential track 54 designed for the barrier elements 50.

Preferably, the number of bars on the sleeve 31 and on the ring wheel 22 exceeds the number of teeth 27 on the ring wheel 22. Thus, a large number of determined locked positions between the synchronisation rings 36 and the sleeve 31 is obtained, which entails that a large number of facets will be formed on the cogwheels' tooth flanks. The facets will, however, lie close to each other, so that they together are perceived as a mainly even surface of the tooth flanks. The facets therefore do not impact the function of the range gearbox 2 and have no significant impact on the service life of the cog- wheels.

Both synchronisation rings 36 are equipped with external ratchet teeth 40 which, during the synchronisation process, stop the shifting movement until a synchronous rotation is achieved between the rotation locked sleeve 31 locked with the ring wheel 22 and the respective coupling rings 34, 35. In order to achieve a good synchronisation function in the gearbox 2, the surface of the synchronisation ring's 36 teeth 40, which faces the sleeves 31 and which is designed to receive the sleeve's 31 bars 33 on syn- chronisation, preferably has an angle, a so-called locking angle, in relation to the synchronisation ring's 36 rotation shaft 30, the locking angle of which must be balanced against the braking torque, which the synchronisation ring 36 transfers to the sleeve 31 in order to achieve a synchronous engine speed. This means that the said locking angle must be shaped so that the teeth on the synchronisation ring 36 abut against the part of the sleeve's 31 bars 33, which is equipped with the locking angle and impacts sufficiently on the sleeve 31 in order for a synchronous engine speed to be obtained and, subsequently, be removed from the part of the sleeve's 31 bars 33, which is equipped with the locking angle, and when the sleeve 31 is to engage with the relevant coupling ring 34, 35 when a synchronous engine speed has been obtained. In order to ensure a synchronous speed is achieved before the sleeve 31 passes the synchronisation ring 36 in an axial direction, the ratchet teeth 40 of the synchronisation ring 36 must not let go of the sleeve's 31 bars 33 too easily. The sleeve 31 is equipped with two internal, circumferential tracks 52, one at each end 48 of the sleeve 31. As mentioned above, the barrier elements 50 are essentially ring- shaped and arranged inside each track 52 in the sleeve 31. The barrier elements 50 are compressible in a radial direction. In each synchronisation ring 36 there is also arranged an external circumferential track 54 designed for the barrier elements 50, in which the barrier element 50 may be moved when the sleeve 31 is shifted. The barrier element 50 is shaped like an open, springing ring, and between its free ends, in the assembled position for the barrier element 50 in the sleeve 31, there is a gap. The barrier element 50 is arranged in a resting position in its track 52 in the sleeve 31 to be resiliently fixed in the track 52. The barrier elements 50 are arranged with the help of the bars 33 on the sleeve 31 to be able to be compressed in a radial direction to a position inside the bars 33, where the sleeve 31 is shifted axially in order to bring the current synchronisation ring 36 into engagement with its coupling ring 34, 35. Preferably, the circumferential tracks 52, 54 have a depth which is adapted to the barrier element's thickness in a radial direction. The barrier element 50 transfers axial powers from the sleeve 31 to the relevant synchronisation ring 36 in order to achieve contact between the friction surfaces 38, 39 on the relevant synchronisation ring 36 and the coupling rings 34, 35. Thus, an oil film formed between the friction surfaces 38, 39 will be dis- placed and an initial torque between the synchronisation ring 36 and the coupling rings 34, 35 will be built up, which forces the synchronisation ring with its ratchet teeth 40 to take up a latch position in front of the sleeve's 31 bars 33. In the embodiment displayed, two synchronisation rings 36 are arranged inside the gearbox 2. It is, however, possible to arrange several synchronisation rings 36 interacting with each other on each side of the ring wheel 22.

The synchronisation rings 36 are on the one hand rotatable to a limited extent in rela- tion to the sleeve 31 and on the other hand equipped with the external ratchet teeth 40, which on a shifting movement stop the sleeve's 31 axial shifting, and connection with the relevant coupling ring 34, 35 before a synchronous rotation is achieved.

The sleeve 31 is at its ends 48 equipped with internal coupling teeth 70, which are in- tended to interact with the teeth 44 on the coupling rings 34, 35. The coupling teeth 70 and the bars on the sleeve 31 are, advantageously, integrated with each other. The sleeve's 31 coupling teeth 70 engage and interact with the teeth 44 on the coupling rings 34, 35, so that axial forces from the oblique gears are assumed through the coupling teeth 70 and the teeth 44 on the coupling rings 34, 35.

As showed in Fig. 2, the synchronisation rings 36 have an inner diameter, which coincides with or exceeds the ring wheel's 22 internal diameter. This may be achieved when the sleeve 31 is arranged outside the ring wheel 22, which thus has a larger diameter than the ring wheel 22. An increased diameter of the synchronisation rings 36 entails that good synchronisation on shifting is obtained.

Preferably, the number of bars 33 on the sleeve 31 and on the ring wheel exceeds the number of teeth 27 on the ring wheel 22. Thus, a large number of determined locked positions between the synchronisation rings 36 and the sleeve 31 is obtained, which entails that a large number of facets will be formed on the cogwheels' tooth flanks. The facets will, however, lie close to each other, so that they together are perceived as a mainly even surface of the tooth flanks. The facets therefore do not impact the function of the range gearbox 2 and have no significant impact on the service life of the cogwheels.

As mentioned above, the bars 33, 37 expand on the sleeve 31 and on the ring wheel 22, according to the first embodiment, at an oblique angle in relation to the rotation shafts of the sleeve 31 and the ring wheel 22. Thus, forces acting axially on the ring wheel 22 entail that the ring wheel 22 is fixed axially by the sleeve 31, the synchronisation rings 36 and the barrier elements 50. Fig. 3 shows a section view of the gearbox 2, according to a second embodiment, in a low range position. The second embodiment differs from the first embodiment because the bars 33, 37 on the sleeve 31 and on the ring wheel 22 extend in parallel with the rotation shafts of the sleeve 31 and the ring wheel 22, which entails that an axial stop 56 journalled on the sun wheel 18 is connected with the ring wheel 22, the axial stop 56 of which prevents the ring wheel 22 from being shifted axially and thus uses axial forces from the ring wheel 22. Corresponding features of the second embodiment are specified with similar reference numerals to those in the first embodiment. The axial stop 56 may consist of a disc-shaped plate, which by means of thrust bearings is journalled on the sun wheel 18.

Since the axial stop 56 takes up axial forces from the ring wheel, the bars 33, 37 on the sleeve 31 and on the ring wheel 22 may expand in parallel with the sleeve 31 and the rotation shafts of the ring wheels 22. Thus, the sleeve 31 and the ring wheel 22 do not need to be equipped with oblique bars 33, 37 in the splines joint, which entails a lower manufacturing cost. The axial stop 56 is rotatable in relation to the sun wheel 18 and the input shaft 16, and follows the rotation of the ring wheels 22. The axial stop 56 entails that the thrust bearings of the input shaft 16 of the gearbox 2 are subject to less stress. The combination of oblique teeth 27 on the cogwheels 18, 22, 24 and straight bars 33, 37 in the splines joint entails that the splines joint may be made with more bars 33, 37 compared to oblique bars, which entails that the load may be spread over several bars. The sleeve 31 may also be made lighter for straight bars 33, 37. Fig. 4 shows a section view of the gearbox 2, according to a second embodiment, in a low range position. The sleeve 31 has, with the help of the shift fork 29, been shifted to the high range position where a synchronisation is carried out in the same manner as described above in connection with the first embodiment.

The gearbox 2 functions as follows and will be described in connection with figures 3 and 4. The sleeve 31, the ring wheel 22 and the synchronisation ring 36 are in Fig. 3 at a standstill, while the second coupling ring 34, which is connected with the input shaft 16, rotates. Thus, the gearbox 2 operates in the low range position, at which a down- shift in the gearbox 2 occurs. In order to shift to the high range position, the sleeve 31 must be shifted by the shift fork 29 to the left in Figure 3, to thus be connected with the rotating coupling ring 34. Since the sleeve 31 and the ring wheel 22 are stationary and the coupling ring 34 rotates, the sleeve 31 and the ring wheel 22 must be brought to rotate with substantially the same speed as the coupling ring 34 before the connection occurs between the ring wheel 22 and the coupling ring 34.

When the sleeve 31 is shifted to the left in Fig. 4, the barrier element 50 will interact with the synchronisation ring 36 so that the synchronisation ring 36 is shifted in a direction toward the coupling ring 34. During further shifting of the sleeve 31 the syn- chronisation ring's 36 conical friction surface 39 will be pressed toward the coupling ring's 34 conical friction surface 38, which entails that the rotating coupling ring 34 slowly starts to drag the synchronisation ring 36, which thus starts to rotate, which in turn entails that also the sleeve 31 and the ring wheel 22 start to rotate. Further shifting of the sleeve 31 entails that the barrier element 50 will be compressed radially, so that the gap between its ends is reduced, thus reducing the barrier element's 50 diameter. Thus the barrier element 50 will be pressed down in the synchronisation ring's 36 circumferential track 54, as showed in Fig. 4. The compression of the barrier element 50 is achieved by the circumferential track 52 in the ring wheel 22. When the sleeve 31 is shifted axially to the left in Fig. 4, the ring wheel 22 is prevented from being shifted axially with the help of the axial stop 56 arranged according to the second embodiment. In the event the bars 33, 37 on the sleeve 31 and on the ring wheel 22 expand at an angle in relation to the rotation shaft, as in the first embodiment, an axial reaction force will act on the ring wheel 22, the axial reaction force of which prevents the ring wheel 22 from being shifted axially when the sleeve 31 is shifted axially toward the high range position.

The external ratchet teeth 40 on the synchronisation rings 36 stop the shifting movement until a synchronous rotation has been achieved between the sleeve 31 and the coupling ring 34, following which the coupling teeth 70 on the sleeve 31, once synchronisation has been achieved, may be inserted between the teeth 44 on the coupling ring 34, so that the sleeve 31, the synchronisation ring 36 and the coupling ring 34 end up in the position showed in Fig. 4.

On connection of the high range position described above, the sleeve 31, the ring wheel 22, the synchronisation ring 36 and the coupling ring 34 will rotate as one inter- connected unit and the axial force from the sleeve 31 will be transferred via the conical friction surfaces 38, 39 between the synchronisation and the coupling rings 36, 34.

The shifting process on the low range side occurs in a similar manner, but differs in that the sleeve 31 and the ring wheel 22, with the help of the synchronisation ring 36, brake from a rotating to a stationary state.

The torques, which during a shifting process impact both the synchronisation rings 36, are directed in the same direction with a certain rotational direction of the input shaft 30, e.g. when driving forwards when the input shaft 30 has a specific rotational direc- tion. When reversing the vehicle 1, the rotational direction of the input shaft 30 will be reversed, with the consequence that the torque acting on the synchronisation rings 36 also changes direction. In order to facilitate synchronised shifting when driving forwards and reversing, it is desirable that the ratchet teeth 40 on the synchronisation rings 36 have an appropriate shape, i.e. they have such a shape that the synchronisation rings 36, once synchronisation is achieved, may be turned to a position allowing gear selection through completed axial movement for the sleeve 31. This embodiment may be selected as needed and desired. Fig. 5 shows a section view of the gearbox 2, according to a second embodiment, in a neutral position. The sleeve 31 may, with the shifting fork 29, be shifted to a neutral position between both the coupling rings 34, 35, which entails that it is possible for a stationary vehicle 1 to extract power from the vehicle's 1 combustion engine 4 via the main gearbox 6. In the neutral position the sleeve 31 does not engage with any of the coupling rings 34, 35, which entails that no torque is transmitted through the gearbox 2. This entails that the vehicle 1 comes to a standstill even though torque is transmitted from the combustion engine 4 to the main gearbox 6. By arranging a transmission, which is not displayed on the main gearbox 6, a stand-alone component from the vehicle 1, such as a compressor, may be operated. A similar neutral position may be installed in the gearbox 2, according to the first embodiment shown above.

The above described gearbox 2 is advantageous from a manufacturing and assembly point of view, since the required processing of component parts is simple and the number of component parts small. The design is such that the need for space in an axial as well as radial direction is small. The described gearbox 2 may be used also in other contexts than those described above. Thus, it is possible to use it for e.g. hydraulic automatic gearboxes, where several gearboxes with planetary gearboxes are con- nected with each other. The invention may further be used for the type of synchronisation devices where several synchronisation rings are arranged on each side of the planetary gearbox.

The components and features specified above may within the framework of the inven- tion be combined between different embodiments specified.