Patent Publication Number: US-10781915-B2

Title: Method for controlling a gear shift in a transmission arrangement

Description:
BACKGROUND AND SUMMARY 
     The present invention relates to a method for controlling a gear shift in a transmission arrangement. The invention also relates to a control unit for controlling the gear shift and a vehicle comprising such a transmission arrangement and control unit. The invention is applicable on vehicles, in particularly working machines such as e.g. wheel loaders, articulated haulers, dump trucks, etc. Although the invention will mainly be described in relation to an articulated hauler, it may also be applicable for other type of vehicles such as e.g. trucks. 
     In connection with transportation of heavy loads at construction sites or the like, a working machine is often used. The working machines may be utilized for transportations in connection with road or tunnel building, sand pits, mines, forestry and similar environments. Thus, the working machine is frequently operated with large and heavy loads in rough terrain and on slippery ground where no regular roads are present. 
     In order to fulfil the desired demands from the fields where the working machine is frequently operated, high quality of the vehicle gearbox is necessary. The gearbox is arranged for adjusting the speed and tractive effort of the vehicle in dependency of the specific driving scenario. The gearbox comprises a transmission arrangement and depending on the specific type of gearbox, the transmission arrangement may comprise e.g. ordinary gear sets with cylindrical gear wheels in meshed connection with each other or planetary gear sets comprising a respective sun gear, ring gear and a planet carrier, or a transmission arrangement having a combination of ordinary gear sets and one or more planetary gear sets. 
     SE 527 108 describes a vehicle transmission arrangement, particularly suitable for working machines. The transmission arrangement in SE 527 108 comprises an input shaft and an output shaft. Five planetary gear sets are arranged between the input shaft and the output shaft for providing the desired gear ratios. Also, the transmission arrangement comprises three clutches and five brakes for achieving the desired gear shifts. Hereby, nine forward gears and four reverse gears are provided for the transmission arrangement in SE 527 108. 
     Although the transmission arrangement in SE 527 108 provides for a sufficient number of gears, the transmission arrangement is still in need of further improvements in terms of costs and functionality. 
     It is desirable to provide a method for controlling a gear shift in a transmission arrangement that improves the gear shift when shifting from a forward gear to a reverse gear in comparison to the prior art. 
     According to a first aspect of the present invention, there is provided a method for controlling a gear shift in a transmission arrangement, the transmission arrangement comprising a first, a second, a third, and a fourth planetary gear set comprising a sun gear, a planet carrier and a ring gear, respectively, wherein the transmission arrangement further comprises a transmission housing, an input shaft and an output shaft, wherein the planet carrier of the first planetary gear set is operatively connected to the output shaft of said transmission arrangement; the ring gear of the first planetary gear set and the planet carrier of the second planetary gear set are operatively connected to each other, the sun gear of the first planetary gear set and the sun gear of the second planetary gear set are operatively connected to each other; the ring gear of the second planetary gear set and the planet carrier of the third planetary gear set are operatively connected to each other; two of the sun gear, the planet carrier and the ring gear of the fourth planetary gear set are each operatively connected to a respective one of the input shaft, the sun gear, the planet carrier and the ring gear of the third planetary gear set; a first locking mechanism is arranged for selectively locking the ring gear of the third planetary gear set to the transmission housing; a second locking mechanism is arranged for selectively locking the planet carrier of the second planetary gear set to the transmission housing; and a first connecting mechanism is arranged for selectively connecting the planet carrier of the second planetary gear set to the ring gear of the third planetary gear set; the method, when shifting from a forward gear to a reverse gear, comprising the steps of: positioning the first locking mechanism and the first connecting mechanism at least in a partially engaged state for reducing the rotational speed of the planet carrier of the second planetary gear set; and positioning the second locking mechanism in an engaged state when the rotational speed of the planet carrier of the second planetary gear set is below a predetermined threshold limit. 
     The wording “operatively connected to” should in the following and throughout the entire description be interpreted such that the components thereof are fixedly connected to each other, i.e. the rotational speed of the components which are operatively connected to each other is the same. Hence, no connecting mechanism or the like is arranged between the components that are operatively connected to each other and they can therefore not be disengaged from one another during operation. Accordingly, the planet carrier of the first planetary gear set is always connected to the output shaft of the transmission arrangement. 
     Furthermore, it should be readily understood that the wording “two of the sun gear, the planet carrier and the ring gear of the fourth planetary gear set are each operatively connected to a respective one of the input shaft, the sun gear, the planet carrier and the ring gear of the third planetary gear set” implies that one of the members of the fourth planetary gear set, i.e. the sun gear, the planet carrier or the ring gear, is operatively connected to one of the members of the third planetary gear set or to the input shaft of the transmission arrangement. At the same time, another one of the members of the fourth planetary gear set is operatively connected to another one of the members of the third planetary gear set or to the input shaft of the transmission arrangement. 
     The wording “selectively connectable to”, “selectively connecting” and “selectively locking” should in the following and throughout the entire description be interpreted as an element being connectable at desirable points in time to another element. Hereby, gear shifts of the transmission arrangement can be executed by either connecting or disconnecting elements to/from each other. Components may be selectively connectable to each other by means of connecting mechanisms and controlled by e.g. a control unit or the like. When a connecting mechanism is positioned in an engaged state the components are connected to each other. 
     Still further, it should be readily understood that the step of reducing the rotational speed of the planet carrier of the second planetary gear set can be executed by either locking the first locking mechanism to the transmission housing and slipping, i.e. at least partially engaging, the first connecting mechanism until the rotational speed of the planet carrier of the second planetary gear set is below a predetermined threshold limit, or by positioning the first connecting mechanism in an engaged state and slipping, i.e. at least partially engaging, the first locking mechanism until the rotational speed of the planet carrier of the second planetary gear set is below the predetermined threshold limit. It should also be readily understood that the above described step of reducing the rotational speed of the planet carrier of the second planetary gear set can also be executed by slipping, i.e. partially engaging, both of the first locking mechanism and the first connecting mechanism until the rotational speed of the planet carrier of the second planetary gear set is below the predetermined threshold limit. Hereby, the first connecting mechanism and the first locking mechanism are at least positioned in a partially engaged state. 
     The predetermined threshold limit should be understood as a rotational speed below which it is suitable to engage the second locking mechanism. 
     An advantage of the present invention is that the above method enables for the second locking mechanism to be designed as a so-called dog clutch. Thus, according to an example embodiment, the second locking mechanism may be a dog clutch. A dog clutch is advantageous to use since it does not use friction to connect the planet carrier of the second planetary gear set to the transmission housing. Rather, a dog clutch uses interference between connecting parts. Hence, the second locking mechanism is more durable in comparison to e.g. a friction brake. Also, lower drag losses occur when the locking mechanism is disengaged. 
     According to an example embodiment of the present invention, the method may comprise the step of positioning the first locking mechanism in a disengaged state when the second locking mechanism has been positioned in the engaged state. 
     Hereby, when the engagement process of the second locking mechanism is ended, the first locking mechanism can be positioned in the disengaged state, thus completing the gear change process. 
     According to an example embodiment of the present invention, the transmission arrangement may comprise a third locking mechanism for selectively locking the sun gear of the first planetary gear set and the sun gear of the second planetary gear set to the transmission housing; and a second connecting mechanism for selectively connecting the sun gear of the third planetary gear set to the planet carrier of the second planetary gear set and to the ring gear of the first planetary gear set. Accordingly, also components of the first planetary gear set may be selectively connectable to the transmission housing. 
     According to an example embodiment of the present invention, the method may comprise the step of positioning the third locking mechanism in a disengaged state when reducing the speed of the planet carrier of the second planetary gear set. 
     Hereby, the sun gear of the first planetary gear set will not be fixated to the transmission housing and the output shaft will not be in mechanical connection with the planet carrier of the second planetary gear set which will enable the output shaft to continuously rotate when the second locking mechanism is positioned in the engaged state. 
     According to an example embodiment of the present invention, the method may comprise the step of positioning the second connecting mechanism in a disengaged state when reducing the speed of the planet carrier of the second planetary gear set. 
     Hereby, the input shaft can continuously rotate when the second locking mechanism is positioned in the engaged state since no mechanical connection will be present between the input shaft and the planet carrier of the second planetary gear set. 
     According to an example embodiment of the present invention, the sun gear of the fourth planetary gear set may be operatively connected to the ring gear of the third planetary gear set; the ring gear of the fourth planetary gear set may be operatively connected to the sun gear of the third planetary gear set; wherein the transmission arrangement may further comprise a third connecting mechanism for selectively connecting the input shaft to the ring gear of the fourth planetary gear set and to the sun gear of the third planetary gear set; and a fourth connecting mechanism for selectively connecting the input shaft to the planet carrier of the fourth planetary gear set. 
     The transmission arrangement comprises four connecting mechanisms and three locking mechanisms, i.e. a reduced total number of connecting and locking mechanisms in comparison to the prior art. 
     According to an example embodiment of the present invention, the planet carrier of the fourth planetary gear set may be operatively connected to the input shaft; the sun gear of the fourth planetary gear set may be operatively connected to the ring gear of the third planetary gear set; wherein the transmission arrangement may further comprise a third connecting mechanism for selectively connecting the input shaft to the sun gear of the third planetary gear set; and a fourth connecting mechanism for selectively connecting the ring gear of the fourth planetary gear set to the sun gear of the third planetary gear set. 
     The transmission arrangement comprises four connecting mechanisms and three locking mechanisms, i.e. a reduced total number of connecting and locking mechanisms in comparison to the prior art. 
     According to an example embodiment of the present invention, the input shaft may be operatively connected to the planet carrier of the fourth planetary gear set; the ring gear of the fourth planetary gear set may be operatively connected to the sun gear of the third planetary gear set; wherein the transmission arrangement may further comprise a third connecting mechanism for selectively connecting the sun gear of the fourth planetary gear set to the input shaft and the planet carrier of the fourth planetary gear set; and a fourth connecting mechanism for selectively connecting the sun gear of the fourth planetary gear set to the ring gear of the third planetary gear set. 
     The transmission arrangement comprises four connecting mechanisms and three locking mechanisms, i.e. a reduced total number of connecting and locking mechanisms in comparison to the prior art. 
     According to an example embodiment of the present invention, the planet carrier of the fourth planetary gear set may be operatively connected to the sun gear of the third planetary gear set; the sun gear of the fourth planetary gear set may be operatively connected to the planet carrier of the third planetary gear set; wherein the transmission arrangement may further comprise a third connecting mechanism for selectively connecting the input shaft to the planet carrier of the fourth planetary gear set and to the sun gear of the third planetary gear set; and a fourth connecting mechanism for selectively connecting the input shaft to the ring gear of the fourth planetary gear set. 
     The transmission arrangement comprises four connecting mechanisms and three locking mechanisms, i.e. a reduced total number of connecting and locking mechanisms in comparison to the prior art. 
     According to an example embodiment of the present invention, the planet carrier of the fourth planetary gear set may be operatively connected to the planet carrier of the third planetary gear set; the sun gear of the fourth planetary gear set may be operatively connected to the sun gear of the third planetary gear set; wherein the transmission arrangement may further comprise a third connecting mechanism for selectively connecting the input shaft to the sun gear of the fourth planetary gear set and to the sun gear of the third planetary gear set; and a fourth connecting mechanism for selectively connecting the input shaft to the ring gear of the fourth planetary gear set. 
     The transmission arrangement comprises four connecting mechanisms and three locking mechanisms, i.e. a reduced total number of connecting and locking mechanisms in comparison to the prior art. 
     According to an example embodiment of the present invention, the method may, for each of the gears of the transmission arrangement, comprise the steps of positioning three of the locking and connecting mechanisms in an engaged state; and positioning four of the locking and connecting mechanisms in a disengaged state. 
     Hereby, the majority of elements are disengaged for the different gears of the transmission arrangement, which thus may increase durability of friction components etc. 
     According to an example embodiment of the present invention, the method may comprise the steps of positioning the first and the third locking mechanisms, and the third connecting mechanism in an engaged state when engaging a first forward gear; positioning the first and the third locking mechanisms, and the fourth connecting mechanism in an engaged state when engaging a second forward gear; positioning the third locking mechanism, and the first and fourth connecting mechanisms in an engaged state when engaging a third forward gear; positioning the third locking mechanism, and the third and fourth connecting mechanisms in an engaged state when engaging a fourth forward gear; positioning the third locking mechanism, and the second and fourth connecting mechanisms in an engaged state when engaging a fifth forward gear, positioning the third locking mechanism, and the second and third connecting mechanisms in an engaged state when engaging a sixth forward gear; positioning the second, third and fourth connecting mechanisms in an engaged state when engaging a seventh forward gear, positioning the first locking mechanism, and the second and third connecting mechanisms in an engaged state when engaging an eighth forward gear, and positioning the first locking mechanism, and the second and fourth connecting mechanisms in an engaged state when engaging a ninth forward gear. 
     Thus, well defined forward gears are provided for the transmission arrangement. 
     According to an example embodiment of the present invention, the method may comprise the step of positioning the third locking mechanism, and the first and third connecting mechanisms in an engaged state when engaging an additional forward gear having a gear ratio between the second and third forward gears of the transmission arrangement. 
     Hereby, a further gear is arranged between the second and third forward gears which provides for an additional step between the second and third gears. A gear shift can be executed from the first forward gear to the additional forward gear. This will provide for a smaller step in comparison to a gear shift from the first forward gear to the third forward gear. Also, a simpler gear shift will be executed since only one of the connecting and locking mechanisms are changed from an engaged state to a disengaged state, and only one of the connecting and locking mechanisms are changed from a disengaged state to an engaged state. 
     According to an example embodiment of the present invention, the method may comprise the step of positioning the first, second and third connecting mechanisms in an engaged state when engaging an alternative seventh forward gear having a gear ratio equal to the seventh forward gear of the transmission arrangement. 
     Hereby, an alternative to the above defined seventh forward gear can be provided for the transmission arrangement. The alternative seventh forward gear gives a lower maximum torque on the third connecting mechanism in comparison to the seventh forward gear. 
     According to an example embodiment of the present invention, the method may comprise the steps of positioning the second locking mechanism, and the first and third connecting mechanisms in an engaged state when engaging a first reverse gear, positioning the second locking mechanism, and the first and fourth connecting mechanisms in an engaged state when engaging a second reverse gear; and positioning the second locking mechanism, and the third and fourth connecting mechanisms in an engaged state when engaging a third reverse gear. 
     According to an example embodiment of the present invention, the method may comprise the step of positioning the first locking mechanism, and the first and third connecting mechanisms in an engaged state when engaging a first intermediate reverse gear. According to an example embodiment of the present invention, the method may comprise the step of positioning the first locking mechanism, and the first and fourth connecting mechanisms in an engaged state when engaging a second intermediate reverse gear. 
     Hereby, the transmission arrangement enables for additional reverse gears. The first intermediate reverse gear is used when shifting from a forward gear to the first reverse gear, i.e. the first intermediate reverse gear is engaged prior to engaging the first reverse gear and thus enables for synchronization before engaging the second locking mechanism for the first reverse gear. Likewise, the second intermediate reverse gear is used when shifting from a forward gear to the second reverse gear, i.e. the second intermediate reverse gear is engaged prior to engaging the second reverse gear and thus enables for synchronization before engaging the second locking mechanism for the second reverse gear. 
     According to a second aspect of the present invention, there is provided a control unit for controlling a gear shift in a transmission arrangement, the transmission arrangement comprising a first, a second, a third, and a fourth planetary gear set comprising a sun gear, a planet carrier and a ring gear, respectively, wherein said transmission arrangement further comprises a transmission housing, an input shaft and an output shaft, wherein the planet carrier of the first planetary gear set is operatively connected to the output shaft of said transmission arrangement; the ring gear of the first planetary gear set and the planet carrier of the second planetary gear set are operatively connected to each other, the sun gear of the first planetary gear set and the sun gear of the second planetary gear set are operatively connected to each other; the ring gear of the second planetary gear set and the planet carrier of the third planetary gear set are operatively connected to each other; two of the sun gear, the planet carrier and the ring gear of the fourth planetary gear set are each operatively connected to a respective one of the input shaft, the sun gear, the planet carrier and the ring gear of the third planetary gear set; a first locking mechanism is arranged for selectively locking the ring gear of the third planetary gear set to the transmission housing; a second locking mechanism is arranged for selectively locking the planet carrier of the second planetary gear set to the transmission housing; and a first connecting mechanism is arranged for selectively connecting the planet carrier of the second planetary gear set to the ring gear of the third planetary gear set; wherein the control unit is, when shifting from a forward gear to a reverse gear, configured to position the first locking mechanism and the first connecting mechanism at least in a partially engaged state for reducing the rotational speed of the planet carrier of the second planetary gear set; and position the second locking mechanism in an engaged state when the rotational speed of the planet carrier of the second planetary gear set is below a predetermined threshold limit. 
     Effect and features of the second aspect of the present invention are largely analogous to those described above in relation to the first aspect of the present invention. 
     According to a third aspect of the present invention, there is provided a vehicle comprising a prime mover, a transmission arrangement and control unit as described above in relation to the second aspect of the present invention. 
     Effect and features of the third aspect of the present invention are largely analogous to those described above in relation to the first aspect of the present invention. 
     Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention. 
     Definitions 
     The relationship between the rotational speeds of the different members in a planetary gear set is defined according to the following: 
     
       
         
           
             
               
                 
                   
                     
                       
                         ω 
                         S 
                       
                       - 
                       
                         ω 
                         P 
                       
                     
                     
                       
                         ω 
                         R 
                       
                       - 
                       
                         ω 
                         P 
                       
                     
                   
                   = 
                   R 
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     wherein 
     ω S  is the speed of rotation of the sun gear; 
     ω P  is the speed of rotation of the planet carrier; 
     ω R  is the speed of rotation of the ring gear; and 
     R is the stationary gear ratio of the planetary gear set. 
     As used herein, the expression “stationary gear ratio” R for a planetary gear set is defined as the ratio of the speed of rotation of the sun gear to the speed of rotation of the ring gear in a situation in which the planet carrier is stationary, i.e.: 
     
       
         
           
             
               
                 
                   
                     R 
                     = 
                     
                       
                         - 
                         
                           
                             z 
                             R 
                           
                           
                             z 
                             S 
                           
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       for 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       single 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       planet 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       gear 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       wheels 
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   and 
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ) 
                 
               
             
             
               
                 
                   R 
                   = 
                   
                     
                       + 
                       
                         
                           z 
                           R 
                         
                         
                           z 
                           S 
                         
                       
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     for 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     double 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     planet 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     gear 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     wheels 
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ) 
                 
               
             
           
         
       
     
     wherein 
     z R  is the number of teeth of the ring gear, and 
     z S  is the number of teeth of the sun gear. 
     In a similar manner, the expression “ratio” for a transmission arrangement should be understood to relate to the number of revolutions of the input shaft of the transmission arrangement divided by the number of revolutions of the output shaft of the transmission arrangement. Furthermore, the expression “step” should be understood to mean the quotient achieved when the ratio of a gear is divided by the ratio of an adjacent gear of a transmission arrangement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above, as well as additional features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of exemplary embodiments of the present invention, wherein: 
         FIG. 1  is a lateral side view illustrating a working machine in the form of an articulated hauler; 
         FIGS. 2-6  schematically illustrate transmission arrangements according to example embodiments of the present invention; and 
         FIG. 7  illustrates an example embodiment of a flow chart of a method for controlling a gear shift in the transmission arrangements depicted in  FIGS. 2-6 . 
         FIGS. 8-9  illustrate shift diagrams, gear ratios and steps for the different gears according to example embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness. Like reference character refer to like elements throughout the description. 
       FIG. 1  is a side view of a working machine  201  in the form of an articulated hauler having a tractor unit  202  with a cab  203  for a driver and a trailer unit  204  with a platform having a dump body  205 , here in the form of a container, arranged thereon, for receiving load. The dump body  205  is preferably pivotally connected to the rear section and tiltable by means of a pair of tilting cylinders  206 , for example hydraulic cylinders. The tractor unit  202  has a frame  207  and a pair of wheels  208  suspended from the frame  207 . The trailer unit  204  has a frame  209  and two pair of wheels  210 ,  211  suspended from the frame  209 . 
     The working machine is frame-steered, i.e. there is a joint arrangement  212  connecting the tractor unit  202  and the trailer unit  204  of the working machine  201 . The tractor unit  202  and the trailer unit  204  are pivotally connected to each other for pivoting around a substantially vertical pivot axis  213 . 
     The working machine preferably comprises a hydraulic system having two hydraulic cylinders  214 , steering cylinders, arranged on opposite sides of the working machine for turning the working machine by means of relative movement of the tractor unit  202  and the trailer unit  204 . The hydraulic cylinders can, however, be replaced by any other linear actuator for steering the machine, such as an electromechanical linear actuator. 
     Furthermore, the articulated hauler comprises a prime mover  216 , here illustrated as an internal combustion engine, and a gearbox  218  having a transmission arrangement according to any one of the embodiments described below in relation to  FIGS. 2-6 . Furthermore, the articulated hauler also comprises a control unit  600  connected to the gearbox for controlling the below described transmission arrangement. The control unit  600  may be, or form part of, already existing control units for controlling a gearbox. 
     Now, with reference to  FIG. 2 , an example embodiment of a transmission arrangement  100  according to the present invention is illustrated. The transmission arrangement  100  comprises a first planetary gear set  102  comprising a sun gear  102 S, a planet carrier  102 P and a ring gear  102 R, a second planetary gear set  104  comprising a sun gear  104 S, a planet carrier  104 P and a ring gear  104 R, a third planetary gear set  106  comprising a sun gear  106 S, a planet carrier  106 P and a ring gear  106 R, and a fourth planetary gear set  108  comprising a sun gear  108 S, a planet carrier  108 P and a ring gear  108 R. The transmission arrangement  100  further comprises an input shaft  136  for receiving a rotary motion/torque from the prime mover  216  of the vehicle  201  and an output shaft  112  for providing a rotary motion/torque to the driven wheels of the vehicle  201 . 
     The different members of the planetary gear sets  102 ,  104 ,  106 ,  108  of the transmission arrangement  100 , i.e. the sun gear, the planet carrier and the ring gear, are in the example embodiment depicted in  FIG. 2  configured according to the following. It should be readily understood that the different members described below are connected to each other, either directly, i.e. operatively connected, or via a connecting mechanism, i.e. selectively connectable. The members can be operatively connected to each other by means of e.g. a connector element. Such connector element can be e.g. a solid shaft, a hollow shaft or a drum, or other suitable element for connecting two members to each other, which elements are known to the person skilled in the art. Hence, no explicit explanation is given below in regards to the means connecting the members to each other. 
     The planet carrier  102 P of the first planetary gear set  102  is operatively connected to the output shaft  112  of the transmission arrangement  100 . i.e. the planet carrier  102 P is at all times directly connected to the output shaft  112  of the transmission arrangement  100 . Further, the ring gear  102 R of the first planetary gear set  102  is operatively connected to the planet carrier  104 P of the second planetary gear set  104 . The sun gear  102 S of the first planetary gear set  102  is operatively connected to the sun gear  104 S of the second planetary gear set  104 . Furthermore, the sun gear  102 S of the first planetary gear set  102  and the sun gear  104 S of the second planetary gear set  104  are selectively connectable to a transmission housing  160  of the transmission arrangement  100  by means of a third locking mechanism  142 . Hence, the third locking mechanism  142 , when being engaged, initially reduces the rotational speed of the respective sun gears  102 S,  104 S, and thereafter locks the respective sun gears  102 S,  104 S to the transmission housing  160 . 
     The ring gear  104 R of the second planetary gear set  104  is operatively connected to the planet carrier  106 P of the third planetary gear set  106 . Furthermore, the planet carrier  104 P of the second planetary gear set  104  is selectively connectable to the sun gear  106 S of the third planetary gear set  106  by means of a second connecting mechanism  144 . The second connecting mechanism  144  can be a clutch arrangement which is either a wet clutch or a dry clutch. Furthermore, the planet carrier  104 P of the second planetary gear set  104  is also selectively connectable to the ring gear  106 R of the third planetary gear set  106  by means of a first connecting mechanism  146 . The first connecting mechanism  146  can be a clutch arrangement which is either a wet clutch or a dry clutch. Moreover, the planet carrier  104 P of the second planetary gear set  104  is selectively connectable to the transmission housing  160  by means of a second locking mechanism  140 . Hence, the second locking mechanism  140 , when being engaged, locks the planet carrier  104 P to the transmission housing  160 . The second locking mechanism  140  may, for example, be designed as a dog clutch. 
     The sun gear  106 S of the third planetary gear set  106  is operatively connected to the ring gear  108 R of the fourth planetary gear set  108 . The ring gear  106 R of the third planetary gear set  106  is operatively connected to the sun gear  108 S of the fourth planetary gear set  108 . The ring gear  106 R of the third planetary gear set  106  and the sun gear  108 S of the fourth planetary gear set  108  are selectively connectable to the transmission housing  160  by means of a first locking mechanism  138 . Hence, the first locking mechanism  138 , when being engaged, initially reduces the rotational speed of the ring gear  106 R and the sun gear  108 S, and thereafter locks the ring gear  106 R and the sun gear  108 S to the transmission housing  160 . 
     Finally, the input shaft  136  is selectively connectable to the ring gear  108 R of the fourth planetary gear set  108  and to the sun gear  106 S of the third planetary gear set  106  by means of a third connecting mechanism  148 , and selectively connectable to the planet carrier  108 P of the fourth planetary gear set  108  by means of a fourth connecting mechanism  150 . The third  148  and the fourth  150  connecting mechanisms can be a respective clutch arrangement which is either a wet clutch or a dry clutch. 
     According to the example embodiment depicted in  FIG. 2 , the stationary gear ratio of each one of the first  102 , second  104 , third  106 , and fourth  108  planetary gear sets are negative. According to a non-limiting example, the stationary gear ratio for each of the planetary gear sets may be as described below in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Exemplary stationary gear ratios for the embodiment depicted in FIG. 2. 
               
            
           
           
               
               
               
               
            
               
                 First planetary 
                 Second planetary 
                 Third planetary 
                 Fourth planetary 
               
               
                 gear set (102) 
                 gear set (104) 
                 gear set (106) 
                 gear set (108) 
               
               
                   
               
               
                 −2.825 
                 −1.625 
                 −1.700 
                 −2.650 
               
               
                   
               
            
           
         
       
     
     With reference to  FIG. 3 , another example embodiment of the transmission arrangement  200  is depicted. The transmission arrangement  200  of the example embodiment depicted in  FIG. 3  is in many ways similar to the transmission arrangement  100  depicted in  FIG. 2 . Therefore, the following will mainly describe those parts that differ from the transmission arrangement  100  in  FIG. 2 . 
     The planet carrier  108 P of the fourth planetary gear set  108  is operatively connected to the input shaft  136  of the transmission arrangement  200 . Moreover, the ring gear  106 R of the third planetary gear set  106  is operatively connected to the sun gear  108 S of the fourth planetary gear set  108 . Further, the sun gear  106 S of the third planetary gear set  106  is selectively connectable to the input shaft  136  by means of a third connecting mechanism  148 , and selectively connectable to the ring gear  108 R of the fourth planetary gear set  108  by means of a fourth connecting mechanism  150 . 
     According to the example embodiment depicted in  FIG. 3 , the stationary gear ratio of each one of the first  102 , second  104 , third  106 , and fourth  108  planetary gear sets are negative. According to a non-limiting example, the stationary gear ratio for each of the planetary gear sets may be as described below in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Exemplary stationary gear ratios for the embodiment depicted in FIG. 3. 
               
            
           
           
               
               
               
               
            
               
                 First planetary 
                 Second planetary 
                 Third planetary 
                 Fourth planetary 
               
               
                 gear set (102) 
                 gear set (104) 
                 gear set (106) 
                 gear set (108) 
               
               
                   
               
               
                 −2.825 
                 −1.625 
                 −1.700 
                 −2.650 
               
               
                   
               
            
           
         
       
     
     With reference to  FIG. 4 , another example embodiment of the transmission arrangement  300  is depicted. The transmission arrangement  300  of the example embodiment depicted in  FIG. 4  is in many ways similar to the transmission arrangements depicted in  FIGS. 2 and 3 . Therefore, the following will mainly describe the differences compared to those transmission arrangements. 
     The sun gear  106 S of the third planetary gear set  106  is operatively connected to the ring gear  108 R of the fourth planetary gear set  108 . Also, the planet carrier  108 P of the fourth planetary gear set  108  is operatively connected to the input shaft  136  of the transmission arrangement  300 . Furthermore, the sun gear  108 S of the fourth planetary gear set  108  is selectively connectable to the input shaft  136  and to the planet carrier  108 P of the fourth planetary gear set  108  by means of a third connecting mechanism  148 . Finally, the ring gear  106 R of the third planetary gear set  106  is selectively connectable to the sun gear  108 S of the fourth planetary gear set  108  by means of a fourth connecting mechanism  150 . 
     According to the example embodiment depicted in  FIG. 4 , the stationary gear ratio of each one of the first  102 , second  104 , third  106 , and fourth  108  planetary gear sets are negative. According to a non-limiting example, the stationary gear ratio for each of the planetary gear sets may be as described below in Table 3. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Exemplary stationary gear ratios for the embodiment depicted in FIG. 4. 
               
            
           
           
               
               
               
               
            
               
                 First planetary 
                 Second planetary 
                 Third planetary 
                 Fourth planetary 
               
               
                 gear set (102) 
                 gear set (104) 
                 gear set (106) 
                 gear set (108) 
               
               
                   
               
               
                 −2.825 
                 −1.625 
                 −1.700 
                 −2.650 
               
               
                   
               
            
           
         
       
     
     With reference to  FIG. 5 , another example embodiment of the transmission arrangement  400  is depicted. The transmission arrangement  400  of the example embodiment depicted in  FIG. 5  is in many ways similar to the transmission arrangements described above and the following will therefore mainly describe the differences compared to those transmission arrangements. 
     The sun gear  106 S of the third planetary gear set  106  is operatively connected to the planet carrier  108 P of the fourth planetary gear set  108 . Also, the planet carrier  106 P of the third planetary gear set  106  is operatively connected to the sun gear  108 S of the fourth planetary gear set  108 . Furthermore, the input shaft  136  is selectively connectable to the sun gear  106 S of the third planetary gear set  106  and to the planet carrier  108 P of the fourth planetary gear set  108  by means of a third connecting mechanism  148 , and selectively connectable to the ring gear  108 R of the fourth planetary gear set  108  by means of a fourth connecting mechanism  150 . 
     According to the example embodiment depicted in  FIG. 5 , the stationary gear ratio of each one of the first  102 , second  104  and third  106  planetary gear sets are negative. The stationary gear ratio of the fourth planetary gear set  108  is however positive. According to a non-limiting example, the stationary gear ratio for each of the planetary gear sets may be as described below in Table 4. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Exemplary stationary gear ratios for the embodiment depicted in FIG. 5. 
               
            
           
           
               
               
               
               
            
               
                 First planetary 
                 Second planetary 
                 Third planetary 
                 Fourth planetary 
               
               
                 gear set (102) 
                 gear set (104) 
                 gear set (106) 
                 gear set (108) 
               
               
                   
               
               
                 −2.825 
                 −1.625 
                 −1.700 
                 2.300 
               
               
                   
               
            
           
         
       
     
     Finally, with reference to  FIG. 6 , another example embodiment of the transmission arrangement  500  is depicted. The transmission arrangement  500  of the example embodiment depicted in  FIG. 6  is in many ways similar to the transmission arrangements described above and the following will therefore mainly describe the differences compared to those transmission arrangements. 
     The sun gear  106 S of the third planetary gear set  106  is operatively connected to the sun gear  108 S of the fourth planetary gear set  108 . Also, the planet carrier  106 P of the third planetary gear set  106  is operatively connected to the planet carrier  108 P of the fourth planetary gear set  108 . Furthermore, the input shaft  136  is selectively connectable to the sun gear  106 S of the third planetary gear set  106  and to the sun gear  108 S of the fourth planetary gear set  108  by means of a third connecting mechanism  148 , and selectively connectable to the ring gear  108 R of the fourth planetary gear set  108  by means of a fourth connecting mechanism  150 . 
     According to the example embodiment depicted in  FIG. 6 , the stationary gear ratio of each one of the first  102 , second  104  and third  106  planetary gear sets are negative. The stationary gear ratio of the fourth planetary gear set  108  is however positive. According to a non-limiting example, the stationary gear ratio for each of the planetary gear sets may be as described below in Table 5. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Exemplary stationary gear ratios for the embodiment depicted in FIG. 6. 
               
            
           
           
               
               
               
               
            
               
                 First planetary 
                 Second planetary 
                 Third planetary 
                 Fourth planetary 
               
               
                 gear set (102) 
                 gear set (104) 
                 gear set (106) 
                 gear set (108) 
               
               
                   
               
               
                 −2.825 
                 −1.625 
                 −1.700 
                 1.770 
               
               
                   
               
            
           
         
       
     
     Since the fourth planetary gear set  108  has a positive stationary gear ratio and the fact that the planet carrier  108 P of the fourth planetary gear set  108  is operatively connected to the planet carrier  106 P of the third planetary gear set  106 , and that the sun gear  108 S of the fourth planetary gear set  108  is operatively connected to the sun gear  106 S of the third planetary gear set  106 , the third  106  and fourth  108  planetary gear sets can be designed as a compound planetary gear set of Ravigneaux type. 
     The above described example embodiments depicted in  FIGS. 2-6  are adapted to assume the gears as presented in  FIG. 8 . As indicated, the transmission arrangements depicted in  FIGS. 2-6  assume nine forward gears and three reverse gears. In  FIG. 8 , the locking mechanisms are denoted simply as “Brakes” while the connecting mechanisms are denoted simply as “Clutches”. A cell marked with a dot indicates an engaged state and a blank cell indicates a disengaged state. Furthermore,  FIG. 8  also indicates non-limiting examples of the gear ratios and steps obtainable by the transmission arrangement for the various gears. The gear ratios and steps presented in  FIG. 8  are derived from the exemplary stationary gear ratios presented in Tables 1-5 above. 
       FIG. 8  illustrates a shift diagram, gear ratios and steps for the different gears. 
     As depicted in  FIG. 8 , the transmission arrangements in  FIGS. 2-6  comprise nine forward gears and three reverse gears (indicated with an R). The switching of gears can preferably be executed by one-step gear shifts or with two-step gear shifts. One-step gear shift should be understood to mean that a gear shift is executed from one gear to the next coming consecutive gear, for example, gear shift from the first gear to the second gear, from the second gear to the third gear, from the third gear to the second gear, etc. Two-step gear shift should be understood to mean that a gear shift is executed to exclude a next coming consecutive gear, for example, gear shift from the first gear to the third gear, from the second gear to the fourth gear, from the third gear to the first gear, etc. 
     As can be seen from  FIG. 8 , one-step gear shifting includes only single shifts of the connecting mechanisms and the locking mechanisms, i.e. when executing one-step gear shifts, only one of the connecting mechanisms/locking mechanisms is shifted from an engaged state to a disengaged state, and only one of the connecting mechanisms/locking mechanisms is shifted from a disengaged state to an engaged state. As an example, when shifting from the first gear to the second gear, it is only the third connecting mechanism  148  that is changed from an engaged state to a disengaged state, and only the fourth connecting mechanism  150  that is changed from a disengaged state to an engaged state. 
     Furthermore, and as is depicted in  FIG. 8 , there is only one occasion during two-step gear shifts where double shift occurs. Double shift should be understood to mean that two connecting mechanisms/locking mechanisms are changed from an engaged state to a disengaged state, and two connecting mechanisms/locking mechanisms are changed from a disengaged state to an engaged state. For two-step gear shifts, this occurs when shifting from the first gear to the third gear, or vice versa from the third gear to the first gear. When shifting from the first gear to the third gear, the first locking mechanism  138  and the third connecting mechanism  148  are changed from an engaged state to a disengaged state, and the first  146  and the fourth  150  connecting mechanisms are changed from a disengaged state to an engaged state. 
     An advantage of the transmission arrangement is hence that the shiftability is improved since a reduced number of connecting mechanisms/locking mechanisms need activation/deactivation during gear shifting. In detail, during one-step gear shifting only single shifts occur and during two-step gear shifting only one double shift occurs, which is when shifting gears between the first and third forward gears of the transmission arrangement. 
     Furthermore, it should also be noted from  FIG. 8  that the second locking mechanism  140  is positioned in a disengaged state for all the forward gears and positioned in an engaged state for all of the reverse gears. Hence, the second locking mechanism  140  may be designed as a dog clutch. 
     Moreover, with the above described example embodiments of the transmission arrangement, further alternative gears are obtainable.  FIG. 9  illustrates alternative gears which are possible to obtain by the connecting mechanisms and locking mechanisms depicted and described above. 
     In  FIG. 9 , gear  2 . 5 * is an additional gear with a gear ratio between the gear ratios of the second and third gears. Also, reverse gear R 4 * is an additional gear having a reduced gear ratio compared to the third reverse gear, i.e. reverse gear R 4 * is a faster reverse gear compared to the third reverse gear. The remaining gears marked with one or more stars in  FIG. 9  are alternative gears for the already present gears. For example, gears  7 *,  7 ** and  7 *** all have the same gear ratio as the seventh gear and are thus alternative gears to the seventh gear. Similarly, the alternative gears R 1 * and R 1 ** have the same gear ratio as the first reverse gear R 1  and R 2 * and R 2 ** have the same gear ratio as the second reverse gear R 2 . 
     If using the above described nine forward gears depicted in  FIG. 8  with the additional gear  2 . 5 * depicted in  FIG. 9 , only single shifts occur for two-step gear shifts and one double shift occurs for one-step gear shifts. The double shift for the one-step gear shift occurs when shifting from the second gear to gear  2 . 5 *, wherein the first locking mechanism  138  and the fourth connecting mechanism  150  are changed from an engaged state to a disengaged state, and the first  146  and the third  148  connecting mechanisms are changed from a disengaged state to an engaged state. 
       FIG. 9  illustrates a shift diagram, gear ratios and steps for the different gears. 
     Hence, with the transmission arrangements depicted and described in relation to  FIGS. 2-6 , a plurality of additional and alternative gears can be achieved. A further advantage is thus that the transmission arrangement according to the present invention has an increased variability when choosing the gears to be used. Thus, a plurality of alternatives is possible when designing gear shifting of the transmission arrangement. 
     Now, reference is made to  FIG. 7  which illustrates an example embodiment of a flow chart of a method for controlling a gear shift in the transmission arrangements described above in relation to  FIGS. 2-6 , and in particularly in a case where the second locking mechanism  140  is designed as a dog clutch. The method is executable when shifting from any forward gear to the first reverse gear or to the second reverse gear of the transmission arrangement. It should be readily understood that the method steps depicted in  FIG. 7  should not be construed as having to be executed in the illustrated consecutive order, which will be described further below. 
     In a first step, the first locking mechanism  138  and the first connecting mechanism  146  are positioned S 1  at least in a partially engaged state. This can be executed by locking the first locking mechanism  138  to the transmission housing  160  and partially engaging, i.e. slipping, the first connecting mechanism  146 . Hereby, the rotational speed of the planet carrier  104 P of the second planetary gear set  104  will be reduced to fall below a predetermined threshold limit. On the other hand, the step can be executed by positioning the first connecting mechanism  146  in an engaged state such that the planet carrier  104 P of the second planetary gear set  104  is mechanically connected to the ring gear  106 R of the third planetary gear set  106 , and positioning the first locking mechanism  138  in a partially engaged state, i.e. a slipping state, such that the rotational speed of the planet carrier  104 P of the second planetary gear set  104  falls below the predetermined threshold limit. Also, the step of reducing the rotational speed of the planet carrier  102 P of the second planetary gear set  102  can also be executed by slipping, i.e. partially engaging, both of the first locking mechanism  138  and the first connecting mechanism  146  until the rotational speed of the planet carrier  102 P of the second planetary gear set  102  falls below the predetermined threshold limit. 
     When the rotational speed of the planet carrier  104 P of the second planetary gear set  104  is below the predetermined threshold limit the second locking mechanism  140  is positioned S 2  in an engaged state. The predetermined threshold limit should be understood as a rotational speed limit when engagement of the second locking mechanism  140  can occur without damaging components thereof. In the specific case, the second locking mechanism  140  is connected to the transmission housing  160  and the rotational speed of the planet carrier  104 P of the second planetary gear set  104  should thus be relatively low in order to appropriately positioning the second locking mechanism  140  in the engaged state. As a non-limiting example, the rotational speed of the planet carrier  104 P should be below 50 revolutions per minute. 
     Furthermore, when the second locking mechanism  140  has been positioned in the engaged state, the first locking mechanism  138  is positioned S 3  in a disengaged state. Hereby, the ring gear  106 R of the third planetary gear set  106  is prevented from rotating by means of the first connecting mechanism  146  and the second locking mechanism  140 , and not by means of the first locking mechanism  138 . Also, positioning the first locking mechanism  138  in the disengaged state enables the gear shift to be completely executed. 
     Still further, when executing the method the third locking mechanism  142  and the second connecting mechanism  144  are both positioned S 4 , S 5  in a respective disengaged state. Hereby, when positioning the third locking mechanism  142  in the disengaged state the sun gear  102 S of the first planetary gear set  102  and the sun gear  104 S of the second planetary gear set  104  are able to rotate since they no longer are fixated to the transmission housing  160 . Hereby, the rotation of the planet carrier  104 P of the second planetary gear set  104  is decoupled from the rotation of the output shaft  112 . Also, by disengaging the second connecting mechanism  144 , the rotation of the planet carrier  104 P of the second planetary gear set  104  is decoupled from the rotation of the input shaft  136 . Hereby, the rotation speed of the planet carrier  104 P of the second planetary gear set  104  can be reduced independently of the rotation of the input shaft  136  and the output shaft  112 . 
     Although  FIG. 7  illustrates consecutive orders of the steps S 1 -S 5 , it should be readily understood that the steps may be executed more or less simultaneously, or in different order, especially steps S 3 -S 5 . For example, the step of positioning S 4  the third locking mechanism  142  in a disengaged state may be executed at approximately the same time as the first step of positioning S 1  the first locking mechanism  138  and the first connecting mechanism  146  in an at least partially engaged state. The same reasoning applies for the step of positioning S 5  the second connecting mechanism  144  in the disengaged state. 
     Also, the wording “positioning” should be understood to also include an element which is already positioned in the engaged or disengaged state when the method step is executed. 
     It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims. For example, although the present invention has mainly been described in relation to an articulated hauler, the invention should be understood to be equally applicable for any type of vehicle.