SE540167C2

SE540167C2 - Changing Gear Ratio in a Gearbox of a Vehicle

Info

Publication number: SE540167C2
Application number: SE1650702A
Authority: SE
Inventors: Selling Tomas
Original assignee: Scania Cv Ab
Priority date: 2016-05-23
Filing date: 2016-05-23
Publication date: 2018-04-17
Also published as: DE102017004681B4; DE102017004681A1; SE1650702A1

Abstract

The invention relates to a method for changing gear ratio in a gearbox (2) of a vehicle (1), said gearbox (2) being arranged to transfer torque in a powertrain (3) between a power source (4) and at least one drive wheel (8) of said vehicle (1), said gearbox (2) being controllable to engage at least two gears having different gear ratios, the method comprising steps of:- applying a first force (F) acting on an engaging element (43) arranged between said gearbox (2) and said at least one drive wheel (8);- decreasing torque (T) generated by the power source (4);- disconnecting said gearbox (2) from said at least one drive wheel (8) by means of the first force (F) acting on the engaging element (43) when the first force (F) overcomes a reaction force (F) acting on the engaging element (43), which reaction force (F) is a result of a torque (T) transferred by the engaging element (43);- changing gear ratio of said gearbox (2) when the gearbox (2) is disconnected from said at least one drive wheel (8); and- connecting said gearbox (2) to said at least one drive wheel (8) when said gear ratio of said gearbox (2) has been changed. The invention also relates to a computer program comprising program code, a system for changing gear ratio in a gearbox (2) of a vehicle (1), and a vehicle (1).

Description

Changing Gear Ratio in a Gearbox of a Vehicle BACKGROUND AND PRIOR ART The invention relates to a method for changing gear ratio in a gearbox of a vehicle, a system for changing gear ratio in a gearbox of a vehicle, a computer program, a computer program product and a vehicle according to the appended claims.

Vehicles, and in particular heavy goods vehicles, e.g. trucks, are usually equipped with a gearbox connected to a power source such as an internal combustion engine, an electric engine or a combination thereof. The gearbox may be automatic, manual or a combination thereof. In an automated manual transmission, a so-called AMT gearbox, the gearbox is controlled by an electronic control device. Such a gearbox may be equipped with a main gearbox device provided with a main shaft. A lay shaft is arranged in parallel with the main shaft. The main shaft is connectible to an input shaft via the lay shaft and to an output shaft in the gearbox via a range gearbox device, if such a range gearbox device is integrated in the gearbox. The range gearbox device may constitute a separate device instead of being integrated in the gearbox. The gearbox may also be equipped with a split gear device, arranged between the input shaft and the lay shaft.

A brake mechanism may be arranged to retard the lay shaft in connection with change of gears in the gearbox, in order to achieve synchronous rotational speeds between the lay shaft and the main shaft, so that the new gear may be engaged without any difference in rotation speed existing between those transmission parts in the gearbox which are brought to engage with each other at the moment the new gear is engaged. The brake mechanism is thus used to retard the lay shaft in relation to the main shaft at a stage during a shifting operation when the main gearbox is in the neutral position, while the lay shaft is disconnected from the main shaft. Also, a similar brake mechanism may be arranged to retard the main shaft in connection with change of gears in the gearbox.

In gearboxes of this type, the synchronization devices, comprising conical synchronization rings and coupling rings, may be replaced by coupling sleeves provided with splines, which are displaced axially in order to engage with gearwheels placed on the main shaft. Each gearwheel placed on the main shaft is engaged with corresponding gearwheel elements, which are firmly attached to the lay shaft. On shifting, the coupling sleeve is displaced axially in order to engage with coupling teeth arranged on a selectable gearwheel, in order to connect the gearwheel to, and rotation lock it, on the main shaft. The synchronization device in the split gearbox device and in the range gearbox device may also be replaced by coupling sleeves.

The synchronization between gearwheels and shafts in the gearbox is for some gear shifting provided by the power source. The power source accelerates the gearwheel or the shaft to be connected with each other. When synchronization has been reached the gearwheel and the shaft are connected by means of the coupling sleeve.

The range gearbox device is usually provided between the main gearbox device and a propeller shaft coupled to the drive wheels of the vehicle. The range gearbox device is accommodated in a gearbox housing and comprises an input shaft coupled to the main gearbox device, an output shaft and between the input shaft and the output shaft is a planetary gear of the range gearbox device disposed. The planetary gear usually comprises three components, which are rotatable arranged relative to each other namely a sun gear wheel, a planet wheel carrier with planet gear wheels and a ring gear wheel. With knowledge of the number of teeth of the sun gear wheel and the ring gear wheel the relative speed of the three components can be determined during operation. In a range gearbox device the sun gear wheel can be rotatable connected to the input shaft, a number of planet gear wheels which engage said sun gear wheel, which planet gear wheels are rotatable mounted on the planet wheel carrier which is fixedly connected to the output shaft, and an axially displaceable ring gear wheel which surrounds and engages the planet gear wheels. The teeth of the sun gear wheel, planet gear wheels and ring gear wheel can be helical, that is, they have an angle to a common rotation axis of the sun gear wheel, planet wheel carrier and ring gearwheel.

There are range gearboxes in which the synchronization devices are replaced with coupling sleeves provided with splines. By controlling the transmission to synchronous speed between the two components to be engaged, an axial displacement of the coupling sleeve is made possible in order to connect them. When the components should be disengaged the transmission is controlled so that torque balance occurs between the components so that the coupling sleeve is not transmitting torque. It then becomes possible to move the coupling sleeve axially in order to disengage the components from each other.

The document US6196944 shows a planetary gear comprising a sun gear wheel, a planet wheel carrier with planet gear wheels and a ring gear wheel. The sun gear wheel may be connected to the input shaft by means of a coupling sleeve in a low range gear position and disengaged from the input shaft in a high range gear position. In the high range gear position the input shaft is connected to the planet wheel carrier by means of the same coupling sleeve. The ring gear wheel is firmly connected to a gearbox housing. The known planetary gear is arranged in an auxiliary gearbox, having only two gear positions.

The document US6196944 shows a gearbox for motor vehicles comprising a planetary gear comprising a first and a second sleeve acting on the planet wheel carrier, the ring gear wheel, the gearbox housing and the output shaft. The first and second sleeves are controlled as one connected unit.

The powertrain may be provided with one or more power take-off to which auxiliary equipment may be connected. Such auxiliary equipment can for example be a tiltable body, a crane or a mixer for concrete, which receives torque from the power take-off. Thus, torque from a power source in the powertrain is transferred to the auxiliary equipment via the power take-off. The power take-off may be arranged on the power source, on an output shaft of the power source or on the gearbox, or on a combination thereof, in the powertrain. However, if the power take-off is arranged downstream that part of the clutch which is fixed to the power source it may be difficult, or even impossible, to shift gears in the gearbox, because torque to the power take-off may not be transferred when the clutch is disengaged. For that reason a vehicle may not be drivable when a power take-off downstream the clutch is activated. If the power take-off is arranged upstream the clutch there may be a limit for the amount of torque transferred to the power take-off when a shifting of gears is allowed.

Also, the torque transferred to the auxiliary equipment connected to the power takeoff is difficult to assess. In order to avoid oscillations in the powertrain, it is important that no torque is transferred to the driving wheels when shifting gears in the gearbox. Such oscillations will make it difficult to synchronize the gearbox when shifting to the next gear. Also, the driver and the passengers in the vehicle may experience some discomfort due to the oscillations.

Under some circumstances, when auxiliary equipment is not connected to a power take-off, or even though the powertrain is not provided with a power take-off, oscillations may occur in the powertrain when disconnecting the clutch or when a gear is disengaged when shifting gears. Such a situation may e.g. occur when a control unit for the powertrain receive information that the power source not delivers any torque, but the power source in reality delivers torque. Thus, when the clutch thereafter is disconnected based on wrong information from the control unit said oscillations will occur in the powertrain.

SUMMARY OF THE INVENTION Despite the known solutions there is a need to further develop a gearbox in a powertrain, in which gearbox shifting of gears may occur when a power take-off arranged on said gearbox transfers torque. There is also a need to further develop a gearbox in which oscillations are avoided when shifting gears.

An object of the invention is therefore to provide a gearbox, in which gearbox shifting of gears may occur when a power take-off arranged on said gearbox transfers torque.

Another object of the invention is to provide a gearbox in which oscillations are avoided when shifting gears.

The herein mentioned objects and other objects are achieved by a method for changing gear ratio in a gearbox of a vehicle, a system for changing gear ratio in a gearbox of a vehicle, a computer program, a computer program product and a vehicle according to the independent claims.

According to an aspect of the present invention a method for changing gear ratio in a gearbox of a vehicle is provided. The gearbox is arranged to transfer torque between a power source and at least one drive wheel of said vehicle. The gearbox being controllable to engage at least two gears having different gear ratios. The method comprises steps of: - applying a first force acting on an engaging element arranged between said gearbox and said at least one drive wheel; - decreasing torque generated by the power source; - disconnecting said gearbox from said at least one drive wheel by means of the first force acting on the engaging element when the first force overcomes a reaction force acting on the engaging element, which reaction force is a result of a torque transferred by the engaging element; - changing gear ratio of said gearbox when the gearbox is disconnected from said at least one drive wheel; and - connecting said gearbox to said at least one drive wheel when said gear ratio of said gearbox has been changed.

The step of applying a first force acting on an engaging element arranged between said gearbox and said at least one drive wheel; comprises to apply a first force on the engaging element so that oscillations are avoided when shifting gears. The step of decreasing torque generated by the power source will decrease the torque transferred to the driving wheels. The step of disconnecting said gearbox from said at least one drive wheel by means of the first force acting on the engaging element when the first force overcomes a reaction force acting on the engaging element, which reaction force is a result of a torque transferred by the engaging element, will avoid oscillations in the powertrain when said gearbox is disconnected from said at least one drive wheel. Due to the configuration of the engaging element the torque transferred by the engaging element will result in a reaction force. When the reaction force reaches a specific level, the first force will overcome the reaction force and the first force will disconnect the gearbox from said at least one drive wheel. The step of changing gear ratio of said gearbox when the gearbox is disconnected from said at least one drive wheel is performed without oscillations in the powertrain and when no torque is transferred from the gearbox to the least one drive wheel. The step of connecting said gearbox to said at least one drive wheel when said gear ratio of said gearbox has been changed comprises to connect the gearbox to said at least one drive wheel by means of the engaging element. The above method provides a gearbox in which oscillations in the powertrain are avoided when shifting gears.

The method steps may be performed and/or controlled by means of a control unit connected to the power source and the gearbox.

According to an aspect of the invention the step of disconnecting said gearbox from said at least one drive wheel by means of the first force acting on the engaging element when the first force overcomes a reaction force acting on the engaging element, which reaction force is a result of a torque transferred by the engaging element, comprises a step of: - adapting the size of the first force so that oscillations in the powertrain are avoided when the first force overcomes the reaction force and when said gearbox is disconnected from said at least one drive wheel.

The step of adapting the size of the first force so that oscillations in the powertrain are avoided when the first force overcomes the reaction force and when said gearbox is disconnected from said at least one drive wheel comprises applying a first force that is as low as possible, so that the gearbox is disconnected from said at least one drive wheel when the torque transferred by the engaging element is at a level that the powertrain downstream the gearbox is not turned up by torsion forces. Thus, no oscillations in the powertrain will occur.. When applying the first force on the engaging element, before or when the torque generated by the power source decreases, the engaging element will disconnect the gearbox from said at least one drive wheel when the reaction force decreases below the level of the reaction force. The reaction force decreases as a consequence of that the torque which is transferred by the engaging element decreases.

According to an aspect of the invention the at least one power take-off is arranged in the powertrain and driven by the power source, and wherein the step of disconnecting said gearbox from said at least one drive wheel by means of the first force acting on the engaging element when the first force overcomes a reaction force acting on the engaging element, which reaction force is a result of a torque transferred by the engaging element, comprises a step of: - disconnecting said gearbox from said at least one drive wheel by means of said first force when the torque from the power source substantially corresponds to a torque acting on said at least one power take-off.

The step of disconnecting said gearbox from said at least one drive wheel by means of said first force when the torque from the power source substantially corresponds to a torque acting on said at least one power take-off, entails that little or no torque from the power source is transferred to the engaging element, thus minimizing the reaction force due to a decreasing torque transferred by the engaging element. The torque transferred by the engaging element may be substantially zero and thus also the reaction force may be substantially zero when substantially all torque from the power source is transferred to the at least one power take-off.

According to an aspect of the invention the at least one power take-off is arranged upstream of said engaging element in the powertrain. Thus, when disengaging the gearbox from the said at least one drive wheel by means of the engaging element, torque from the power source will still be transferred to the power take-off.

According to an aspect of the invention the step of decreasing torque generated by the power source, comprises a step of: - controlling the torque from the power source to substantially correspond to the torque acting on said at least one power take-off. In order to provide auxiliary equipment connected to the power take-off with torque, the power source is controlled to deliver the torque needed for the equipment connected to the power take-off to be driven.

According to an aspect of the invention the at least one drive wheel is connected to a propeller shaft, and wherein the step of disconnecting said gearbox from said at least one drive wheel by means of the first force acting on the engaging element when the first force overcomes a reaction force acting on the engaging element, which reaction force is a result of a torque transferred by the engaging element, comprises a step of: - disconnecting the gearbox from the propeller shaft by means of the engaging element; and wherein the step of connecting said gearbox to said at least one drive wheel when said gear ratio of said gearbox has been changed, comprises a step of: - connecting the gearbox to the propeller shaft by means of the engaging element.

When disconnecting the gearbox from the propeller shaft by means of the engaging element no torque is transferred to the propeller shaft and to the at least one drive wheel.

When connecting the gearbox to the propeller shaft by means of the engaging element torque may be transferred to the propeller shaft and to the at least one drive wheel.

According to an aspect of the invention the engaging element comprises an axially displaceable coupling sleeve, which in a first position is arranged to disconnect said gearbox from said at least one drive wheel of said vehicle and in a second position is arranged to connect said gearbox to said at least one drive wheel of said vehicle, and wherein the step to connecting said gearbox to said at least one drive wheel when said gear ratio of said gearbox has been changed, comprises a step: - synchronizing the rotational speed between said gearbox and said propeller shaft by means of a synchronizing element before connecting said gearbox to said at least one drive wheel.

The step of synchronizing the rotational speed between said gearbox and said propeller shaft by means of a synchronizing element before connecting said gearbox to said at least one drive wheel may comprise to accelerate a rotating member of said gearbox in order synchronize the rotational speed between the rotating member of said gearbox and said propeller shaft.

According to an aspect of the invention the step of disconnecting said gearbox from said at least one drive wheel by means of the first force acting on the engaging element when the first force overcomes a reaction force acting on the engaging element, which reaction force is a result of a torque transferred by the engaging element, comprises a step of: - displacing said coupling sleeve axially to the first position by means of said first force; and wherein the step of connecting said gearbox to said at least one drive wheel when said gear ratio of said gearbox has been changed, comprises a step of: - displacing said coupling sleeve axially to the second position by means of a second force, which second force is greater than the first force.

The step of displacing said coupling sleeve axially to the first position by means of said first force comprises to disconnect the gearbox from the at least one drive wheel and thus prevents oscillations to occur in the powertrain.

The step of displacing said coupling sleeve axially to the second position by means of a second force, which second force is greater than the first force, comprises to connect the gearbox to the at least one drive wheel in a fast and reliable manner as a result of the second force which is greater than the first force.

According to an aspect of the invention a range gearbox is provided with a planetary gear, arranged to provide a high range gear and a low range gear, and wherein the step of disconnecting said gearbox from said at least one drive wheel by means of the first force acting on the engaging element when the first force overcomes a reaction force acting on the engaging element, which reaction force is a result of a torque transferred by the engaging element, comprises a step of: - disconnecting the planetary gear of the range gearbox from the at least one drive wheel by means of the engaging element; and wherein the step of connecting said gearbox to said at least one drive wheel when said gear ratio of said gearbox has been changed, comprises: - connecting the planetary gear of the range gearbox from the at least one drive wheel by means of the engaging element.

The step of disconnecting the planetary gear of the range gearbox from the at least one drive wheel by means of the engaging element, comprises to disconnect rotatable parts of the planetary gear from said at least one drive wheel, so that no torque or rotation may be transferred to the planetary gear from said at least one drive wheel.

When connecting the planetary gear of the range gearbox from the at least one drive wheel by means of the engaging element, torque may be transferred to the at least one drive wheel.

When disconnecting the planetary gear of the range gearbox from the at least one drive wheel the powertrain is controlled so that torque balance occurs between the planetary gear of the range gearbox from the at least one drive wheel, so that the engaging element is not transmitting torque. By controlling the powertrain to a synchronous speed between the planetary gear of the range gearbox and the at least one drive wheel an axial displacement of the engaging element is made possible in order to connect the planetary gear of the range gearbox and the at least one drive wheel.

According to an aspect of the invention the propeller shaft is connected to an output shaft of the range gearbox, and wherein the step of disconnecting said gearbox from said at least one drive wheel by means of the first force acting on the engaging element when the first force overcomes a reaction force acting on the engaging element, which reaction force is a result of a torque transferred by the engaging element, comprises a step of: - disconnecting the planetary gear of the range gearbox from the output shaft by means of the engaging element; and wherein the step of connecting said gearbox to said at least one drive wheel when said gear ratio of said gearbox has been changed, comprises a step of: - connecting the planetary gear of the range gearbox to the output shaft by means of the engaging element.

The step of disconnecting the planetary gear of the range gearbox from the output shaft by means of the engaging element; comprises to disconnect rotatable parts of the planetary gear from the output shaft, so that no torque or rotation from the at least one drive wheel may be transferred to the planetary gear from the output shaft.

Due to the step of connecting the planetary gear of the range gearbox to the output shaft by means of the engaging element, torque may be transferred to the output shaft and the at least one drive wheel.

A control unit is connected to the powertrain, such as the power source, a clutch and the gearbox to achieve the gear shifting above. The control unit may be the engine control unit or may comprise a plurality of different control units. A computer may be connected to the control unit.

According to an aspect of the invention a system for changing gear ratio in a gearbox of a vehicle is provided. The gearbox being arranged to transfer torque in a powertrain between a power source and at least one drive wheel of said vehicle, said gearbox being controllable to engage at least two gears having different gear ratios, the system comprising: - means for applying a first force acting on an engaging element between said gearbox and said at least one drive wheel; - means for decreasing torque generated by the power source; - means for disconnecting said gearbox from said at least one drive wheel by means of the first force acting on the engaging element when the first force overcomes an reaction force acting on the engaging element, which reaction force is a result of a torque transferred by the engaging element; - means for changing gear ratio of said gearbox when the gearbox is disconnected from said at least one drive wheel; and wherein - the means for applying a first force is also arranged for connecting said gearbox to said at least one drive wheel when said gear ratio of said gearbox has been changed.

The means for applying a first force acting on an engaging element between said gearbox and said at least one drive wheel comprises a pneumatic, hydraulic or electrie actuator.

The means for decreasing torque generated by the power source may comprise a control unit connected to the power source.

The means for disconnecting said gearbox from said at least one drive wheel by means of the first force acting on the engaging element when the first force overcomes a reaction force acting on the engaging element, which reaction force is a result of a torque transferred by the engaging element comprises a pneumatic, hydraulie or electric actuator.

The means for changing gear ratio of said gearbox when the gearbox is disconnected from said at least one drive wheel comprises a pneumatic, hydraulic or electric actuator.

The means for applying a first force is also arranged for connecting said gearbox to said at least one drive wheel when said gear ratio of said gearbox has been changed. Using the same means for disconnecting and connecting said gearbox to said at least one drive wheel will increase the cost efficiency and decrease the overall weight of the vehicle.

According to an aspect of the invention the system further comprises: - that the means for applying a first force is arranged to generate a second force, which is greater than the first force when connecting said gearbox to said at least one drive wheel.

The first and second force may be generated by the same means.

Further advantages of the invention appear from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS Below is a description of, as examples, preferred embodiments of the invention with reference to the enclosed drawings, in which: Fig. 1 schematically illustrates a vehicle according to an embodiment of the invention, Fig. 2 schematically illustrates a powertrain according to an embodiment of the invention, Fig. 3 schematically illustrates a powertrain according to an embodiment of the invention in a low range gear position, Fig. 4 schematically illustrates a powertrain according to an embodiment of the invention in a low range gear position, Fig. 5 schematically illustrates a powertrain according to an embodiment of the invention in a high range gear position, Figures 6a and 6b schematically illustrate graphs over a method for changing gear ratio in a gearbox of a vehicle according to an embodiment of the invention, and Fig. 7 schematically illustrates a flow chart for the method for changing gear ratio in a gearbox of a vehicle according to an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Fig. 1 schematically illustrates a vehiclel according to an embodiment of the invention. The vehicle 1 includes a powertrain 3, which comprises a power source 4, such as an internal combustion engine, a gearbox 2 and a propeller shaft 10. The combustion engine 4 is coupled to the gearbox 2. The gearbox 2 is further connected to the drive wheels 8 of the vehicle 1 via the propeller shaft 10. The gearbox 2 of the present invention comprises a range gearbox, and aims to double the number of gear opportunities. Herein reference is made to various positions in/of the powertrain 3. In this context, the term upstream refers to a position towards the power source 4, and the term downstream refers to a position towards the drive wheels 8.

Fig. 2 shows schematically a part of the powertrain 3 comprising the combustion engine 4 and a clutch 11. The figure further discloses the gearbox 2, comprising three gearbox units 15A, 15B, 15C. A first gearbox unit 15A constitutes a conventional main gearbox that can be set to three different forward gear ratios. The figure further discloses a second gearbox unit 15B constituting a range gearbox, arranged downstream the main gearbox unit 15A. The range gearbox unit 15B is surrounded by a gearbox housing 12 and is discussed further with reference to figures 3 - 5. A third gearbox unit 15C is located upstream the main gearbox 15A in the direction of torque from combustion engine 4 to drive wheels 8. The third gearbox unit 15C constitutes a split gearbox which provides for each gear of the main gearbox in two gear steps with different gear ratios in order to provide more gear ratios of the gearbox 2.

Instead of a disengageable clutch 11 an arrangement with first and second electrical machines (not disclosed) may be arranged to rotate and brake a planetary gear (not disclosed) arranged in the powertrain 3 and located upstream the gearbox 2. In such an arrangement the first electrical machine should be arranged at a sun gearwheel (not disclosed) of the planetary gear, and the second electrical machine should be arranged at the first ring gearwheel (not disclosed) of the planetary gear. The first and second electrical machines may be the power source or form part of the power source.

With regard to the main gearbox 15A, a lay shaft 202 comprises gear wheels 203A, 204A, 205A that are rotatably fixed to the lay shaft 202. For example, gear wheel 203A represents the first gear, gear wheel 204A the second gear and gear wheel 205A the third gear. A main shaft 206 comprises corresponding gear wheels 203B, 204B, 205B which rotate freely in relation to the main shaft 206, but which can be selectively locked for rotation with the main shaft 206 in order to engage a gear. For example, the first main gearbox gear can be engaged by manoeuvring a first main sleeve 207, arranged to rotate with the main shaft 206, to a position where the gear wheel 203B is engaged, i.e. to the left in the figure, thereby bringing the gear wheel 203B to rotate the main shaft 206 and thereby also engaging the lay shaft 202 to the main shaft 206 via gear wheel 203A. Each pair of gear wheels on the lay shaft 202 and main shaft 206 represents a gear ratio.

The second main gearbox gear can be engaged by disengaging the first main sleeve 207 from gear wheel 203B and instead moving a second main sleeve 208 to a position to the right in the figure where, instead, gear wheel 204B is engaged, thereby bringing the gear wheel 204B to rotate the main shaft 206. Correspondingly, the third main gearbox gear can be engaged by manoeuvring the second main sleeve 208 to a position to the left in the figure where, instead gear wheel 205B is engaged, thereby setting the main gearbox to third gear. Each of the first through third gears is used for a plurality of the total number of gears provided by the gearbox 2 as a whole. For example, the first gear of the main gearbox 15A will be used for first and second gear of the gearbox 2, low and high split, low range, and also for seventh and eighth gear, low and high split, high range, in a manner well known to a person skilled in the art.

Further, with regard to the split gearbox 15C, the lay shaft 202 comprises an additional gear wheel 209A that, similar to the above, is rotatably fixed to the lay shaft 202. An input shaft 201 comprises a corresponding gear wheel 209B rotating freely in relation to the input shaft 201 but which can be selectively locked for rotation with the input shaft 201 through a split sleeve 210, which may be provided with a split synchronising unit. The split sleeve 210 can further be used to connect the gearbox input shaft 201 to gear wheel 205B directly. The gear wheel pair 209A-B and coupling unit 210 can thereby provide two different split gear ratios for each gear of the main gearbox into two parts.

When e.g. the first gear is engaged, the split sleeve 210 is arranged to engage gear wheel 205B. This will have the result that the input shaft 201 is directly connected to gear wheel 205B, which via gear wheel 205A establishes a first gear ratio between input shaft 201 and lay shaft 202. Gear wheel 205B, however, is not connected to the main shaft 206, but the lay shaft 202 is connected to main shaft 206 through gear wheel pair 203A-B.

When the second gear is engaged, i.e. high split of first main gearbox gear, the vehicle 1 is, instead, driven with gear wheel pair 209A-B engaged, resulting in a second gear ratio between an input shaft 201 and lay shaft 202. The gear wheel 203B is still engaged by the first main sleeve 207 according to the above, thereby extending the range of each gear.

This split can be performed for each gear of the main gearbox 15A, where, when coupling unit 210 engages gear wheel 205B and also the second main sleeve 208 engages gearwheel 205B a 1:1 gear ratio through the gearbox units 15C, 15A is obtained.

Also, the main shaft 206 may be provided with a first braking element 212, such as a main shaft brake, which is used to brake the main shaft 206 and also a planetary gear 14 (fig. 3) in the range gearbox 15B to a standstill condition when shifting gears in the range gearbox 15B. The lay shaft 202 may be provided with a second braking element 211, such as a lay shaft brake in order to decrease the speed of the lay shaft 202 during upshifting in the gearbox 2.

So far, gearbox units 15A, 15C have been described. It is to be understood that fig. 2 merely discloses an example of a design of such gearbox units, and that the present invention can be utilised for various different designs of such gearbox units. For example, the split gearbox 15C can be omitted. As is explained below, the invention can be utilized for any gearbox 2 comprising a plurality of gearbox units that can be disconnected both from the power source 4 and the drive wheels 8 of the vehicle 1. The invention can be used in a system where a range gearbox 15B, e.g. comprising a planetary gear 14, is used and can be disconnected from the drive wheels 8 of the vehicle 1.

The powertrain 3 may be provided with a first, second, and/or a third power take-off 220, 222, 224 to which auxiliary equipment 226 is connectable. Such auxiliary equipment 226 can for example be a tiltable body, a crane or a mixer for concrete (not shown). Torque generated by the internal combustion engine 4 is transferred to the auxiliary equipment 226 via the relevant power take-off 220, 222, 224. The first power take-off 220 is arranged on the gearbox 2, the second power take-off 222 is arranged on the clutch 11 and the third power take-off 224 is arranged on the engine 4. The second power take-off 222 is arranged on that part of the clutch 11 which is fixed to an output shaft of the engine 4.

The first power take-off 220 is arranged downstream that part of the clutch 11 which is fixed to the engine 4 and therefore the clutch 11 must be engaged in order to transfer torque from the engine 4 to the first power take-off 220.

Fig. 3 schematically illustrates a powertrain 3 according to an embodiment of the invention in a low range gear position. The gearbox 2 comprises a planetary gear 14 which has a low and a high gear, so that the switching capability of the main gearbox 15A can be divided into a low range gear position and a high range gear position. In a first gear position corresponding to the low range gear position, a lower gear ratio than 1:1 is provided in the planetary gear 14. In the high range gear position the gear ratio is 1:1 in the planetary gear 14. Fig. 3 shows the gearbox 2 in the first gear position, corresponding to the low range gear position. The range gearbox 15B is used to double the number of gear ratios that are available from the main gearbox 15A, and also, as in this case, the number of gears available from the combination of gearbox units 15A, 15C. The gearbox unit 15B is, as shown in the figures, arranged downstream of the main gearbox 15A as seen from the combustion engine 4.

The gearbox 2 is accommodated in the gearbox housing 12 and comprises an input shaft 16 which may be a main shaft 26 of the main gearbox 15A. The planetary gear 14 comprises three main components which are rotatably arranged in relation to each other, namely a sun gear wheel 18, a planet wheel carrier 20 and a ring gear wheel 22. A number of planet gear wheels 24 are rotatably arranged with bearings on the planet wheel carrier 20. With knowledge of the number of teeth 32 of sun gear wheel 18 and the ring gear wheel 22, the relative gear ratio of the three components can be determined. The sun gear wheel 18 is rotatably connected to the input shaft 16 and the planet gear wheels 24 engage the sun gear wheel 18. The ring gear wheel 22 surrounds and engages the planet gear wheels 24. The teeth 32 of the sun gear wheel 18, the planet gear wheels 24 and the ring gear wheel 22 can be bevelled, so that they have an angle relative to a common axis of rotation 30 of the sun gear wheel 18, the planet gear carrier 20 and the ring gear wheel 22. The input shaft 16 may be connected to the sun gearwheel 18 by means of a splines connection 34.

A first axially displaceable coupling sleeve 42 is in a first gear position arranged to connect the gearbox housing 12 with the ring gear wheel 22 and in a second gear position arranged to disconnect the gearbox housing 12 from the ring gear wheel 22. The first axially movable coupling sleeve 42 is in the first gear position arranged to disconnect the input shaft 16 from the planet wheel carrier 20.

An engaging element in form of a second axially displaceable coupling sleeve 43 is in a first position arranged to disconnect the planet wheel carrier 20 from an output shaft 28 of the gearbox 2, which output shaft 28 is coupled to the propeller shaft 10 of the vehicle 1. In a second position the second axially displaceable coupling sleeve 43 is arranged to connect said gearbox 2 to said at least one drive wheel 8 of said vehicle 1. In that position, disclosed in fig. 3, the second axially displaceable coupling sleeve 43 transfers torque from the planet wheel carrier 20 to the propeller shaft 10 and also to the driving wheels of the vehicle 1. The second axially displaceable coupling sleeve 43 thus forms an engaging element between the gearbox 2 and the driving wheels 8.

The second axially displaceable coupling sleeve 43 may in a third gear position couple the ring gear wheel 22 with the output shaft 28 of the gearbox 2 and thus achieve a reverse gear. In the third gear position, corresponding to the reverse gear, the first axially displaceable coupling sleeve 42 is arranged to disconnect the input shaft 16 from the planet wheel carrier 20 and instead is arranged to interconnect the planet wheel carrier 20 with the gearbox housing 12.

The first coupling sleeve 42 is provided with first splines 50 on an inner periphery of the sleeve 42 and second splines 51 on an outer periphery of the sleeve 42. The first splines 50 arranged on the inner periphery of the sleeve 42 interact with corresponding first cooperating splines 50’ arranged on the input shaft 16. Corresponding first cooperating splines 50’ disposed on the input shaft 16 are made on the periphery of a first sprocket 46 which is mounted on the input shaft 16. The first splines 50 on the inner periphery of the sleeve 42 are also arranged to cooperate with corresponding first cooperating splines 50” arranged on the planet wheel carrier 20. Corresponding first cooperating splines 50” disposed on the planet wheel carrier 20 are made on the periphery of a second sprocket 44 which is mounted on the planet wheel carrier 20. The second splines 51 arranged on an outer periphery of the sleeve 42 interact with corresponding second cooperating splines 51’ arranged on the projection 52 which is fixedly connected to the gearbox housing 12.

The second axially displaceable coupling sleeve 43 is on an inner surface provided with third splines 59 which are arranged to cooperate with corresponding second cooperating splines 59’, 59”, 59’” arranged on the ring gear wheel 22, the planet wheel carrier 20 and the output shaft 28, respectively. The corresponding second cooperating splines 59” arranged on the planet wheel carrier 20 are formed on the periphery of a third sprocket 49 which is mounted on the planet wheel carrier 20. The corresponding second cooperating splines 59’” provided on the output shaft 28 are formed on the periphery of a fourth sprocket 53 which is mounted on the output shaft 28.

An axial stop 54 arranged on the planet wheel carrier 20 is adapted to abut against the ring gear wheel 22, which axial stop 54 prevents the ring gear wheel 22 to be moved axially. The axial stop 54 may comprise a disc-shaped plate, which by means of a first thrust bearing 56 is mounted on the planet wheel carrier 20. The axial stop 54 is rotatable relative to the planet wheel carrier 20 and the input shaft 16, and follows the rotation of the ring gear wheel 22. The axial stop 54 fixes the ring gear wheel 22 axially, and leads to that an axial bearing of the input shaft 16 is subjected to less stress when the gears 18, 22, 24 are provided with helical teeth. However, instead of, or in combination with the axial stop 54 a pair of thrust bearings 56 may be arranged on both axial surfaces connected to the ring gear wheel 22. Thus, the thrust bearings 56 are arranged between the ring gear wheel and the planet wheel carrier 20. A second thrust bearing 57 may be disposed between the shaft 38 of the sun gear wheel 18 and the planet wheel carrier 20 to accommodate axial forces generated by the sun gear wheel 18.

The low gear in the gearbox 2 is obtained by displacing the second coupling sleeve 42, so that the ring gearwheel 22 is connected to the transmission housing 12. The axial displacement of the first and second coupling sleeves 42, 43 are provided with a first and second shift fork 60, 61 arranged in an outside circumferential groove 62 in the respective coupling sleeve 42, 43. The first shift fork 60 is influenced by a first actuator 66 and the second shift fork 61 is influenced by a second actuator 67. The first and second actuator 66, 67 may be a pneumatic or hydraulic cylinder. The shift forks 60, 61 and actuators 66, 67 are schematically shown in Fig. 3.

Fig. 4 schematically illustrates a powertrain 3 according to an embodiment of the invention in a low range gear position and with the second coupling sleeve 43 in the first position. In this position the output shaft 28 is disconnected from the planetary gear 14, and thus no torque may be transferred from the gearbox 2 to the output shaft 28, the propeller shaft 10 and the driving wheels 8. The second coupling sleeve 43 is in this position only connected to the output shaft 28 and not to the ring gear wheel 22 or the planet wheel carrier 20. When the output shaft 28 is disconnected from the planetary gear 14 and the clutch 11 is disengaged, so that no torque is transferred from the internal combustion engine 4 to the gearbox 2, gears may be shifted in the gearbox 2.

Fig. 5 schematically illustrates a powertrain 3 according to an embodiment of the invention in a high range gear position. When the output shaft 28 was disconnected from the planetary gear 14 and the clutch 11 was disengaged, the first coupling sleeve 42 was axially displaced to the second gear position in order to disconnect the gearbox housing 12 from the ring gearwheel 22, and instead connect the sun gear wheel 18 with the planet wheel carrier 20. Thereafter the second coupling sleeve 43 was axially displaced to its first position so that the second coupling sleeve 43 transfers torque from the planet wheel carrier 20 to the output shaft 28. The transmission of torque from the input shaft 16 to the output shaft 28 is in the high range gear position via the input shaft 16 and planet wheel carrier 20 and further to the output shaft 28 via the second coupling sleeve 43, so that the gear ratio through the planetary gear 14 becomes 1:1. The second axially movable coupling sleeve 43 may in another gear position be arranged to engage the ring gear wheel 22 with the output shaft 28. Thus, another gear ratio over the planetary gear is achieved in this position.

However, since the planet wheel carrier 20 is in a stand still position during gear shifting and the output shaft 28 is rotating due to the rotation of the driving wheels 8, the speed between the planet wheel carrier 20 and the output shaft 28 must be synchronized before the second coupling sleeve 43 is axially displaced in order to connect the planet wheel carrier 20 with the output shaft 28. Therefore, a synchronizing element 74 is arranged between the planet wheel carrier 20 and the output shaft 28. The synchronizing element 74 comprises first and second friction surfaces 76, 78, which may be engaged and disengaged by means of an actuation arrangement 80. The actuation arrangement 80 comprises an electric, a pneumatic or a hydraulic controlled cylinder 82, wherein a piston 84 in the cylinder 82 is axially displaceable in order to engage and disengage the first and second friction surfaces 76, 78. The first friction surface 76 may be arranged on the planet wheel carrier 20 and the second friction surface 78 may be arranged axially displaceable by the piston 84. According to the embodiment in fig. 5 and also in figures 3 and 4 the first and second friction surfaces 76, 78.According to another embodiment the cylinder 82 and piston 84 may be circular and arranged substantially concentric to the common axis of rotation 30. The second friction surface 78 is arranged on a circular sleeve 86. The actuation arrangement 80 may be mounted at the gearbox housing 12. For that reason the actuation arrangement 80 is not rotatable, which simplifies the connection of wires and fluid pipes to the actuation arrangement 80. A radial bearing 90 is arranged between the circular sleeve 86 and the piston 84, so that the axial displacement of the circular sleeve 86 and the second friction surface 78 can be made without rotating the cylinder 82 and the piston 84. The first and second friction surfaces 76, 78 may be conical in order to increase the friction force between the friction surfaces 76, 78 and therefore also transfer enough torque for accelerating the planet wheel carrier 20 when synchronizing the speed between the output shaft 28 and the planet wheel carrier 20.

An electronic control unit 70 is coupled to the gearbox 2, the combustion engine 4 and the clutch 11 to achieve the gear shifting above. Speed sensors in the gearbox 2, in the main gear box 6 and in the internal combustion engine 4 may be connected to the control unit 70. Another computer 72 may also be connected to the control unit 70. The control unit 70 may be a computer with appropriate software for this purpose. The control unit 70 and/or the computer 72 comprise a computer program P, which can include routines to control the gearbox 2 of the invention. The program P may be stored in an executable form or compressed form in a memory M and/or in a read/write memory. A computer program product is provided, which comprises a program code stored on a, by a computer readable medium for performing the gear shifting above, when said program is run on the control unit 70 or another computer 72 connected to the control unit 70. Said code may be non-volatile, stored in said computer readable medium.

Figures 6a and 6b schematically illustrate graphs over a method for changing gear ratio in a gearbox of a vehicle according to the invention. Fig. 6a illustrates a first force FA1, a second force FA2, and a reaction force FRacting on the second axially displaceable coupling sleeve 43 vary during changing of gear ratio. Fig. 6b illustrates a torque TStransferred by the second axially displaceable coupling sleeve 43 and a torque TPSgenerated by the power source 4 during gear shifting during changing of gear ratio. The reaction force FRacting on the second axially displaceable coupling sleeve 43 is a result of the torque TStransferred by the second axially displaceable coupling sleeve 43. The torque TSis generated by the torque TPSgenerated by the power source 4. At time t1 torque TPSgenerated by the power source 4 is decreased, which leads to the torque TStransferred by the second axially displaceable coupling sleeve 43 also decreasing. Before or at the time t1, a first force FA1acting on the second axially displaceable coupling sleeve 43 is generated by the second actuator 67. Since the torque TStransferred by the second axially displaceable coupling sleeve 43 decreases, also the reaction force FRacting on the second axially displaceable coupling sleeve 43 decreases.

At time t2 the first force FA1overcomes the reaction force FRand the second coupling sleeve 43 is axially displaced which results in that said gearbox 2 is disconnected from the drive wheels 8. When the gearbox 2 is disconnected from the drive wheels 8 no torque TSis transferred by the second coupling sleeve 43. On or after time t2 gear ratio of said gearbox 2 is changed.

At time t3 the gear ratio of the gearbox 2 has been changed and the gearbox 2 is connected to the drive wheels 8 by displacing the second coupling sleeve 43 axially by means of a second force FA2, which second force FA2is greater than the first force FA1. Using a second force FA2which is greater than the first force FA1, a fast and reliable connection between the gearbox 2 and the drive wheels 8 is achieved. The second force FA2is generated by the second actuator 67. Thus, the second actuator 67 can generate different forces directed in different directions.

At time t4 the gearbox 2 and the drive wheels 8 are connected by means of the second coupling sleeve 43. At t4 the clutch is engaged and the power source 4 generates torque TPSto the gearbox 2, which leads to torque TSbeing transferred by the engaging element 43 to the drive wheels 8. At time t5 the torque TShas reached a new level corresponding to the new gear that the gearbox 2 has been shifted to.

Figure 7 shows a flowchart for a method for changing gear ratio in a gearbox 2 of a vehicle 1. The gearbox 2 is according to the embodiment configured as described in Figures 2 - 5. The method comprises the steps of; - applying s101 a first force FA1acting on an engaging element 43 arranged between said gearbox 2 and said at least one drive wheel 8; - decreasing s102 torque TPSgenerated by the power source 4; - disconnecting s103 said gearbox 2 from said at least one drive wheel 8 by means of the first force acting on the engaging element 43 when the first force FA1overcomes a reaction force FRacting on the engaging element 43, which reaction force FRis a result of a torque TStransferred by the engaging element 43; - changing s104 gear ratio of said gearbox 2 when the gearbox 2 is disconnected from said at least one drive wheel 8; and - connecting s105 said gearbox 2 to said at least one drive wheel 8 when said gear ratio of said gearbox 2 has been changed.

The engaging element 43 may be a second coupling sleeve, as discussed in connection with figures 2-5. The engaging element 43 may form part of the gearbox 2. Accordingly, the expression “arranged between said gearbox 2 and said at least one drive wheel 8” entails that the engaging element 43 is arranged downstream of all gears of the gearbox 2.

The step of applying s101 a first force F1 may be initiated prior to, or simultaneously with the step of decreasing s102 torque TPS. A further alternative may be to initiate the step of applying s101 a first force FA1after the step of decreasing s102 torque TPShas been initiated.

The step of disconnecting s103 said gearbox 2 from said at least one drive wheel 8 by means of the first force acting on the engaging element 43 when the first force FA1overcomes a reaction force FRacting on the engaging element 43, which reaction force FRis a result of a torque TStransferred by the engaging element 43, comprises a step of: - adapting s106 the size of the first force FA1so that oscillations in the powertrain are avoided when the first force FA1overcomes the reaction force FRand when said gearbox 2 is disconnected from said at least one drive wheel 8. Powertrain oscillations will thus not affect any gear changes in the gearbox 2. A size of the first force FA1so that oscillations in the powertrain 3 are avoided depends inter alia on the relevant power source 4 and gearbox 2, and the type of vehicle 1. However, the size of the first force FA1so that oscillations in the powertrain 3 are avoided may also be assessed in trails.

At least one power take-off 220, 222, 224 is arranged in the powertrain and driven by the power source 4, and wherein the step of disconnecting said gearbox 2 from said at least one drive wheel 8 by means of the first force acting on the engaging element 43 when the first force FA1overcomes a reaction force FRacting on the engaging element 43, which reaction force FRis a result of a torque TStransferred by the engaging element 43, comprises a step of: - disconnecting s107 said gearbox 2 from said at least one drive wheel 8 by means of said first force FA1when the torque TPSfrom the power source 4 substantially corresponds to a torque TPTOacting on said at least one power take-off 220, 222, 224.

In this manner oscillations in the power train 3 may be avoided during gear shifting when an auxiliary equipment 226 is connected to the at least one power take-off 220, 222, 224.

Said at least one power take-off 220, 222, 224 is arranged upstream of said engaging element 43 in the powertrain 3.

In this manner the reaction force FRmay be substantially equal to zero when the force from the power source 4 substantially corresponds to a torque acting on the at least one power take-off 220, 222, 224.

The step of decreasing torque TPSgenerated by the power source 4, comprises a step of: - controlling s108 the torque TPSfrom the power source 4 to substantially correspond to the torque TPTOacting on said at least one power take-off 220, 222, 224.

Said at least one drive wheel 8 is connected to a propeller shaft 10, and wherein the step of disconnecting s103 said gearbox 2 from said at least one drive wheel 8 by means of the first force FA1acting on the engaging element 43 when the first force FA1overcomes a reaction force FRacting on the engaging element 43, which reaction force FRis a result of a torque TStransferred by the engaging element 43, comprises a step of: - disconnecting s109 the gearbox 2 from the propeller shaft 10 by means of the engaging element 43; and wherein the step of connecting s105 said gearbox 2 to said at least one drive wheel 8 when said gear ratio of said gearbox 2 has been changed, comprises a step of: - connecting s110 the gearbox 2 to the propeller shaft 10 by means of the engaging element 43.

Said engaging element 43 comprises an axially displaceable coupling sleeve 43, which in a first position is arranged to disconnect said gearbox 2 from said at least one drive wheel 8 of said vehicle 1 and in a second position is arranged to connect said gearbox 2 to said at least one drive wheel 8 of said vehicle 1, and wherein the step of connecting s105 said gearbox 2 to said at least one drive wheel 8 when said gear ratio of said gearbox 2 has been changed, comprises a step of: - synchronizing s111 the rotational speed between said gearbox 2 and said propeller shaft 10 by means of a synchronizing element 74 before connecting said gearbox 2 to said at least one drive wheel 8.

The synchronizing element 74, may comprise the first and second friction surfaces 76, 78. However, the synchronizing element may also be an electrical machine (not disclosed) connected to the gearbox 2 or the internal combustion engine 4, which may synchronize the rotational speed between said gearbox 2 and said propeller shaft 10. When the gearbox 2 and propeller shaft 10 are to be connected, the electrical machine accelerates a shaft of the gearbox 2 so that the shaft of the gearbox 2 reaches a rotational speed synchronised with a rotational speed of the propeller shaft 10 before connecting the gearbox 2 and propeller shaft 10 by means of the engaging element 43.

The step of disconnecting s103 said gearbox 2 from said at least one drive wheel 8 by means of the first force acting on the engaging element 43 when the first force FA1overcomes a reaction force FRacting on the engaging element 43, which reaction force FRis a result of a torque TStransferred by the engaging element 43, comprises a step of: - displacing s112 said coupling sleeve 43 axially to the first position by means of said first force FA1; and wherein the step of connecting s105 said gearbox 2 to said at least one drive wheel 8 when said gear ratio of said gearbox 2 has been changed, comprises a step of: - displacing s113 said coupling sleeve 43 axially to the second position by means of a second force FA2, which second force FA2is greater than the first force FA1.

The gearbox 2 comprises a range gearbox 15B provided with a planetary gear 14, arranged to provide a high range gear and a low range gear, and wherein the step of disconnecting s103 said gearbox 2 from said at least one drive wheel 8 by means of the first force acting on the engaging element 43 when the first force FA1overcomes a reaction force FRacting on the engaging element 43, which reaction force FRis a result of a torque TStransferred by the engaging element 43, comprises a step of: - disconnecting s114 the planetary gear 14 of the range gearbox 15B from the at least one drive wheel 8 by means of the engaging element 43; and wherein the step of connecting s105 said gearbox 2 to said at least one drive wheel 8 when said gear ratio of said gearbox 2 has been changed, comprises a step of: - connecting s115 the planetary gear of the range gearbox 15B to the at least one drive wheel 8 by means of the engaging element 43.

The propeller shaft 10 is connected to an output shaft of the range gearbox 15B, and wherein the step of disconnecting s103 said gearbox 2 from said at least one drive wheel 8 by means of the first force acting on the engaging element 43 when the first force FA1overcomes a reaction force FRacting on the engaging element 43, which reaction force FRis a result of a torque TStransferred by the engaging element 43, comprises a step of: - disconnecting s116 the planetary gear 14 of the range gearbox 15B from the output shaft 28 by means of the engaging element 43; and wherein the step of connecting s105 said gearbox 2 to said at least one drive wheel 8 when said gear ratio of said gearbox 2 has been changed, comprises a step of: - connecting s117 the planetary gear 14 of the range gearbox 15B to the output shaft 28 by means of the engaging element 43.

The foregoing description of the preferred embodiments of the present invention is provided for illustrative and descriptive purposes. It is not intended to be exhaustive or to restrict the invention to the variants described. Many modifications and variations will obviously be apparent to one skilled in the art. The embodiments have been chosen and described in order best to explain the principles of the invention and its practical applications and hence make it possible for specialists to understand the invention for various embodiments and with the various modifications appropriate to the intended use. The components and features specified above may within the framework of the invention be combined between the different embodiments specified.

Claims

1. A method for changing gear ratio in a gearbox (2) of a vehicle (1), said gearbox (2) being arranged to transfer torque in a powertrain (3) between a power source (4) and at least one drive wheel (8) of said vehicle (1), said gearbox (2) being controllable to engage at least two gears having different gear ratios, the method comprising steps of: - applying (s101) a first force (FA1) acting on an engaging element (43) arranged between said gearbox (2) and said at least one drive wheel (8); - decreasing (s102) torque (TPS) generated by the power source (4); - disconnecting (s103) said gearbox (2) from said at least one drive wheel (8) by means of the first force (FA1) acting on the engaging element (43) when the first force (FA1) overcomes a reaction force (FR) acting on the engaging element (43), which reaction force (FR) is a result of a torque (TS) transferred by the engaging element (43); - changing (s104) gear ratio of said gearbox (2) when the gearbox (2) is disconnected from said at least one drive wheel (8); and - connecting (s105) said gearbox (2) to said at least one drive wheel (8) when said gear ratio of said gearbox (2) has been changed.

2. A method according to claim 1, wherein the step of disconnecting (s103) said gearbox (2) from said at least one drive wheel (8) by means of the first force (FA1) acting on the engaging element (43) when the first force (FA1) overcomes a reaction force (FR) acting on the engaging element (43), which reaction force (FR) is a result of a torque (TS) transferred by the engaging element (43), comprises a step of: - adapting (s106) the size of the first force (FA1) so that oscillations in the powertrain (3) are avoided when the first force (FA1) overcomes the reaction force (FR) and when said gearbox (2) is disconnected from said at least one drive wheel (8).

3. A method according to claim 1 or 2, wherein at least one power take-off (220, 222, 224) is arranged in the powertrain (3) and driven by the power source (4), and wherein the step of disconnecting (s103) said gearbox (2) from said at least one drive wheel (8) by means of the first force (FA1) acting on the engaging element (43) when the first force (FA1) overcomes a reaction force (FR) acting on the engaging element (43), which reaction force (FR) is a result of a torque (TS) transferred by the engaging element (43), comprises a step of: - disconnecting (s107) said gearbox (2) from said at least one drive wheel (8) by means of said first force (FA1) when the torque (TPS) from the power source (4) substantially corresponds to a torque (TPTO) acting on said at least one power take-off (220, 222, 224).

4. A method according to claim 3, wherein said at least one power take-off (220, 222, 224) is arranged upstream of said engaging element (43) in said powertrain (3).

5. A method according to claim 3 or 4, wherein the step of decreasing (s102) torque (TPS) generated by the power source (4), comprises a step of: - controlling (s108) the torque (TPS) from the power source (4) to substantially correspond to the torque (TPTO) acting on said at least one power take-off (220, 222, 224).

6. A method according to any of the preceding claims, wherein said at least one drive wheel (8) is connected to a propeller shaft (10), and wherein the step of disconnecting (s103) said gearbox (2) from said at least one drive wheel (8) by means of the first force (FA1) acting on the engaging element (43) when the first force (FA1) overcomes a reaction force (FR) acting on the engaging element (43), which reaction force (FR) is a result of a torque (TS) transferred by the engaging element (43), comprises a step of: - disconnecting (s109) the gearbox (2) from the propeller shaft (10) by means of the engaging element (43); and wherein the step of connecting (s105) said gearbox (2) to said at least one drive wheel (8) when said gear ratio of said gearbox (2) has been changed, comprises a step of: - connecting (s110) the gearbox (2) to the propeller shaft (10) by means of the engaging element (43).

7. A method according to claim 6, wherein said engaging element (43) comprises an axially displaceable coupling sleeve (43), which in a first position is arranged to disconnect said gearbox (2) from said at least one drive wheel (8) of said vehicle (1) and in a second position is arranged to connect said gearbox (2) to said at least one drive wheel (8) of said vehicle (1), and wherein the step of connecting said gearbox (2) to said at least one drive wheel (8) when said gear ratio of said gearbox (2) has been changed, comprises a step of: - synchronizing (s111) the rotational speed between said gearbox (2) and said propeller shaft (10) by means of a synchronizing element (74) before connecting said gearbox (2) to said at least one drive wheel (8).

8. A method according to claim 7, wherein the step of disconnecting (s103) said gearbox (2) from said at least one drive wheel (8) by means of the first force acting on the engaging element (43) when the first force (FA1) overcomes a reaction force (FR) acting on the engaging element (43), which reaction force (FR) is a result of a torque (TS) transferred by the engaging element (43), comprises a step of: - displacing (s112) said coupling sleeve (43) axially to the first position by means of said first force (FA1); and wherein the step of connecting (s105) said gearbox (2) to said at least one drive wheel (8) when said gear ratio of said gearbox (2) has been changed, comprises a step of: - displacing (s113) said coupling sleeve (43) axially to the second position by means of a second force (FA2), which second force (FA2) is greater than the first force (FA1).

9. A method according to any of the preceding claims, wherein said gearbox (2) comprises a range gearbox (15B) provided with a planetary gear (14), arranged to provide a high range gear and a low range gear, and wherein the step of disconnecting (s103) said gearbox (2) from said at least one drive wheel (8) by means of the first force acting on the engaging element (43) when the first force (FA1) overcomes a reaction force (FR) acting on the engaging element (43), which reaction force (FR) is a result of a torque (TS) transferred by the engaging element (43), comprises a step of: - disconnecting (s114) the planetary gear (14) of the range gearbox (15B) from the at least one drive wheel (8) by means of the engaging element (43); and wherein the step of connecting (s105) said gearbox (2) to said at least one drive wheel (8) when said gear ratio of said gearbox (2) has been changed, comprises a step of: - connecting (s115) the planetary gear (14) of the range gearbox (15B) to the at least one drive wheel (8) by means of the engaging element (43).

10. A method according to claims 6 and 9, wherein said propeller shaft (10) is connected to an output shaft (28) of the range gearbox (15B), and wherein the step of disconnecting (s103) said gearbox (2) from said at least one drive wheel (8) by means of the first force acting on the engaging element (43) when the first force (FA1) overcomes a reaction force (FR) acting on the engaging element (43), which reaction force (FR) is a result of a torque (TS) transferred by the engaging element (43), comprises a step of: - disconnecting (s116) the planetary gear (14) of the range gearbox (15B) from the output shaft (28) by means of the engaging element (43); and wherein the step of connecting (s105) said gearbox (2) to said at least one drive wheel (8) when said gear ratio of said gearbox (2) has been changed, comprises a step of: - connecting (s117) the planetary gear (14) of the range gearbox (15B) to the output shaft by means of the engaging element (43).

11. A computer program comprising program code that, when said program code is executed in a computer (72), causes said computer (72) to carry out the method according to any of claims 1-10.

12. A computer program product comprising a computer-readable medium and a computer program according to claim 11, wherein said computer program is contained in said computer-readable medium.

13. A system for changing gear ratio in a gearbox (2) of a vehicle (1), said gearbox (2) being arranged to transfer torque in a powertrain (3) between a power source (4) and at least one drive wheel (8) of said vehicle (1), said gearbox (2) being controllable to engage at least two gears having different gear ratios, the system being characterised in: - means (80, 67) for applying a first force (FA1) acting on an engaging element (43) between said gearbox (2) and said at least one drive wheel (8); - means (70, 72) for decreasing torque (TPS) generated by the power source (4); - means (67) for disconnecting said gearbox (2) from said at least one drive wheel (8) by means of the first force (FA1) acting on the engaging element (43) when the first force (FA1) overcomes an reaction force (FR) acting on the engaging element (43), which reaction force (FR) is a result of a torque (TS) transferred by the engaging element (43); - means (66) for changing gear ratio of said gearbox (2) when the gearbox (2) is disconnected from said at least one drive wheel (8); and wherein - the means (67) for applying a first force (FA1) is also arranged for connecting said gearbox (2) to said at least one drive wheel (8) when said gear ratio of said gearbox (2) has been changed.

14. A system according to claim 13, further being characterised in: - that the means (67) for applying a first force (FA1) is arranged to generate a second force (FA2), which is greater than the first force (FA1) when connecting said gearbox (2) to said at least one drive wheel (8).

15. A vehicle, characterised in that it comprises a system for changing gear ratio in a gearbox (2) according to any of claims 13 and 14.