Patent Publication Number: US-2015068338-A1

Title: Mobile vehicle gear unit

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of stepping down the speed of a rotary motion from a first speed, supplied to an input shaft, to a second speed delivered by an output shaft. The present invention further relates to a mobile vehicle gear unit comprising an input shaft, and an output shaft substantially parallel with said input shaft, the gear unit being configured for providing a non-unity transmission ratio between said input shaft and said output shaft via a layshaft arrangement. 
     BACKGROUND OF THE INVENTION 
     The awareness and concern of the public with respect to environmental impact of transports is rapidly increasing. To meet such concern and awareness electric drive vehicles and low fuel-consumption vehicles are designed. An important aspect of an electric drive vehicle, and of a low-fuel consumption vehicle, is that the weight of the vehicle should be low, to reduce consumption of power and fuel, respectively. Since the motor typically rotates at a relatively high rpm, whether it is an electric motor or a petrol, diesel, biogas, or ethanol driven engine, a gear unit is needed for stepping down the rotation speed of the motor or engine to a speed suitable for rotating drive wheels of the vehicle. 
     U.S. Pat. No. 4,297,906 discloses a gear box for a vehicle with a somewhat reduced weight. The gear box of U.S. Pat. No. 4,297,906 is, however, still too heavy for efficient use in electric vehicles and low fuel-consumption vehicles, in particular when designed for the relatively high torques typical of electric motors. 
     There is a need for a gear unit offering higher reliability, lower weight, and/or lower cost of manufacture. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to solve, or at least mitigate, parts or all of the above mentioned problems. To this end, there is provided a method of stepping down the speed of a rotary motion from a first speed, supplied to an input shaft, to a second speed delivered by an output shaft, the method comprising 
     transmitting said rotary motion to said output shaft, via a driven gear of said output shaft, the driven gear being helical and having a first helix angle, from a layshaft arrangement, so as to generate a first axial thrust of the output shaft in a first axial direction; 
     transmitting said rotary motion to said layshaft arrangement, via a drive gear of said input shaft, said drive gear being helical and having a second helix angle that is larger than said first helix angle, from said input shaft, so as to generate a second axial thrust of the input shaft in a second direction, said second direction being substantially opposite to said first direction; and 
     applying at least a portion of said first axial thrust and at least a portion of said second axial thrust to the same location of an axially rigid support structure, such that said first and second axial thrusts counter-act and at least partly cancel out in said support structure. 
     The difference in helix angles will operate so as to make the magnitudes of said thrusts more equal, so as to increase the extent to which said axial thrusts at least partly cancel each other out. The resultant force on the axially rigid support structure will thereby be limited. And by applying said axial thrusts to the same location of said support structure, axial movement of the input and output shafts due to varying load conditions can be minimized. As a comparison, in step-down methods of prior art, input and output shafts generally apply their respective dynamic axial thrusts onto opposite end walls of a gear unit housing, such that the gear unit housing flexes or yields somewhat under the axial load, and the input and output shafts are thereby somewhat pushed apart. Such movement may lead to premature failure of bearings, particularly under varying load conditions. Axial movement also requires a somewhat forgiving gear unit design, where substantive tooth clearance or backlash allows some freedom of movement. By instead applying the oppositely directed axial thrusts to the same location of a support structure, it is possible to obtain an axially rigid relation between the input and output shafts, such that the input and output shafts will not move significantly in the axial direction relative to each other. Hence, a gear unit with tighter backlash, and thereby increased lifetime, may be designed for such a step-down method. Throughout this disclosure, “substantially opposite directions” is to be construed as said directions forming an angle of more than 165°. 
     According to an embodiment, said axially rigid support structure is a main thrust bearing arrangement interconnecting said output and input shafts, such that said axial thrusts at least partly cancel out via the main thrust bearing arrangement. The main thrust bearing arrangement may comprise one or several thrust bearings arranged as a “push bearing arrangement”, i.e. for providing support when the input and output shafts are pushed towards each other; as a “pull bearing arrangement”, i.e. for providing support against the input and output shafts being drawn apart; or as a bidirectional bearing arrangement providing support in both axial directions. 
     According to an embodiment, said axially rigid support structure is a main thrust bearing support axially supporting said input shaft and said output shaft, such that said axial thrusts at least partly cancel out via said main thrust bearing support. The main thrust bearing support may be any axially rigid structure holding both the input shaft and the output shaft in respective thrust bearings, so as to allow the respective axial thrusts to cancel out. By way of example, the main thrust bearing support may be a centrally located bracket in a gear unit, said bracket holding the input and output shafts such that the driven gear of the output shaft and the drive gear of the input shaft are located on opposite sides of said bracket. Each of said input and output shafts will thereby either simultaneously pull or simultaneously push onto opposite sides of said main thrust bearing support, such that said thrusts will at least partly cancel out. Alternatively, the main thrust bearing support may be a portion of an end wall of a gear unit housing, with respect to which wall the driven gear of the output shaft and the drive gear of the input shaft may be arranged on the same side. In such a configuration, one of said shafts will apply an axially pulling force onto said wall, while the other shaft will apply an axially pushing force, such that said thrusts will at least partly cancel out in said wall. 
     According to an embodiment, said method further comprises 
     transmitting said rotary motion via at least one layshaft of said layshaft arrangement; and 
     for each layshaft of said layshaft arrangement,
         generating a drive gear axial thrust in a helical drive gear having a drive gear helix angle;   generating a driven gear axial thrust in a helical driven gear having a driven gear helix angle, said driven gear helix angle being larger than said drive gear helix angle;   directing the drive gear axial thrust in said second direction; and   directing the driven gear axial thrust in said first direction, such that the axial thrust of the respective drive and driven gears of each layshaft at least partly cancel out within said layshaft. By canceling also at least a portion of the axial thrusts acting on each layshaft, the total resultant axial loads on a gear unit will be reduced. According to one embodiment the layshaft arrangement comprises a plurality of layshafts, wherein the axial thrusts of each of said plurality of layshafts at least partly cancel out within each layshaft in accordance with the principles described hereinabove.       

     According to another aspect of the invention, parts or all of the above mentioned problems are solved, or at least mitigated, by a mobile vehicle gear unit comprising an output shaft, and an input shaft substantially parallel with said output shaft, the gear unit being configured for providing a transmission ratio between said input shaft and said output shaft via a layshaft arrangement, the output shaft being provided with a driven gear in mesh with a drive gear of said layshaft arrangement, and the input shaft being provided with a drive gear in mesh with a driven gear of said layshaft arrangement, the transmission ratio being non-unity such that one gear of the driven gear of the output shaft and the drive gear of the input shaft will be arranged for operating at a relatively lower torque, and the other gear will be arranged for operating at a relatively higher torque, said relatively higher torque being higher than said relatively lower torque; the output shaft being journalled in an output shaft main thrust bearing arrangement, mounted to a main thrust bearing support, and being arranged for limiting axial movement of the output shaft in a first axial direction; 
     the input shaft being journalled in an input shaft main thrust bearing arrangement, said input shaft main thrust bearing arrangement being co-located with said output shaft main thrust bearing arrangement on said main thrust bearing support, said input shaft main thrust bearing arrangement being arranged for limiting axial movement of the input shaft in a second axial direction, said second axial direction being substantially opposite to said first axial direction; said main thrust bearing support rigidly connecting the output shaft main thrust bearing arrangement to the input shaft main thrust bearing arrangement; said driven gear of said output shaft being helical of a first hand; said drive gear of said input shaft being helical of a second hand, said second hand being the same as the first hand for a positive transmission ratio, i.e., when the input and output shafts rotate in the same direction, and opposite to said first hand for a negative transmission ratio, i.e., when the input and output shafts rotate in different directions; and 
     said gear arranged for operating at a relatively lower torque having a helix angle exceeding the helix angle of said gear arranged for operating at a relatively higher torque. 
     In such a gear unit, when torque is supplied to the input shaft in a drive direction, axial thrust generated by the driven gear of the output shaft will be directed in said first axial direction, against said output shaft main thrust bearing arrangement. Axial thrust generated by the drive gear of the input shaft will be directed in a second axial direction, which is substantially opposite to said first axial direction. Hence, input and output shaft axial thrusts will be of opposite directions and applied to the same location, i.e. where the input and output shaft main thrust bearing arrangements are joined by the main thrust bearing support. Hence, such a gear unit may be used for carrying out the method described hereinbefore, and thereby relates to the same inventive concept. The difference in helix angles will operate so as to make the magnitudes of said thrusts more equal, so as to increase the extent to which said axial thrusts at least partly cancel each other out in the main thrust bearing support. The resultant force on the bearing support will thereby be limited. Thanks to the input and output shafts being arranged for applying their respective axial thrusts to the same, axially rigid location, axial movement of the shafts is reduced. This allows for tighter gearing backlash, which prolongs service life of the mobile vehicle gear unit. Furthermore, reduced axial movement reduces so-called skidding, a phenomenon that will be described in more detail below. Similar to “substantially opposite directions”, the term “substantially parallel” is to be construed as forming an angle of less than 15°. 
     According to an embodiment, said main thrust bearing support is fixed to a gear unit housing. Thereby, the main thrust bearing support may perform the additional function of providing radial support. 
     According to an embodiment, said output shaft main thrust bearing arrangement is arranged on a first side of said main thrust bearing support, and said input shaft main thrust bearing arrangement is arranged on a second side of said main thrust bearing support, said second side being opposite to said first side. 
     According to another aspect of the invention, parts or all of the above mentioned problems are solved, or at least mitigated, by a mobile vehicle gear unit comprising an input shaft, and an output shaft substantially parallel with said input shaft, the gear unit being configured for providing a transmission ratio between said input shaft and said output shaft via a layshaft arrangement, the output shaft being provided with a driven gear in mesh with a drive gear of said layshaft arrangement, and the input shaft being provided with a drive gear in mesh with a driven gear of said layshaft arrangement, the transmission ratio being non-unity such that one gear of the driven gear of the output shaft and the drive gear of the input shaft will be arranged for operating at a relatively lower torque, and the other gear will be arranged for operating at a relatively higher torque, said relatively higher torque being higher than said relatively lower torque; the input shaft being journalled to the output shaft in a main thrust bearing arrangement arranged for limiting axial movement of the input shaft relative to the output shaft in a first axial direction; said driven gear of said output shaft being helical of a first hand; said drive gear of said input shaft being helical of a second hand, said second hand being the same as the first hand for a positive transmission ratio and opposite to said first hand for a negative transmission ratio; and said gear arranged for operating at a relatively lower torque having a helix angle exceeding the helix angle of said gear arranged for operating at a relatively higher torque. In such a gear unit, when torque is supplied to the input shaft in an input direction, axial thrust generated by the driven gear of the output shaft will be directed in said first axial direction. Axial thrust generated by the drive gear of the input shaft will be directed in a second axial direction, which is substantially opposite to said first axial direction. Hence, input and output shaft axial thrusts will be of opposite directions and applied to the same location, i.e. where the shafts are joined by the main thrust bearing arrangement. Such a gear unit may be used for carrying out the method described hereinbefore, and thereby relates to the same inventive concept. The difference in helix angles will operate so as to make the magnitudes of said thrusts more equal, so as to increase the extent to which said axial thrusts at least partly cancel each other out in said main thrust bearing arrangement. The resultant axial thrust acting on the input and output shafts will thereby be limited. Thanks to the input and output shafts being arranged for applying their respective axial thrusts to the same, axially non-yielding main thrust bearing arrangement, axial and radial movement of the shafts is reduced. This allows for tighter gearing backlash, which prolongs service life of the gear unit. Furthermore, reduced axial movement reduces rolling element skidding in the axial thrust bearings. 
     According to an embodiment, said output shaft is arranged on a first side of said main thrust bearing arrangement, and said input shaft is arranged on a second side of said main thrust bearing arrangement, said second side being opposite to said first side. 
     According to an embodiment of any of the gear units described hereinbefore, each of said main thrust bearing arrangements is a bidirectional thrust bearing arrangement for limiting axial movement of the respective input or output shaft in two axial directions. Thereby, axial forces cancel out in the main thrust bearing support or the main thrust bearing, as the case may be, regardless of the direction of rotation or input torque of the gear unit. 
     According to an embodiment of any of the gear units described hereinbefore, said input shaft is connected to a power source, such as an electric motor or an internal combustion engine, for driving the input shaft in an input direction of rotation, and the hand of said driven gear of said output shaft is oriented for applying output shaft axial thrust in said first axial direction when the power source transmits torque to the input shaft in said input direction. Implicitly, the axial thrust of the output shaft will thereby be applied in said second axial direction substantially opposite to said first axial direction. By having the mobile vehicle gear unit connected to a power source, for receiving rotary power primarily in said input direction therefrom, it is possible to design the gear unit so as to withstand a larger torque in the predetermined input direction than in a direction of rotation opposite to said input direction. Thereby, weight may be saved and the construction may be simpler, since thrust bearing(s) may be unidirectional, and/or housing walls may be made thinner. 
     According to an embodiment of any of the gear units described hereinbefore, each of said input and output shafts is axially preloaded in a preloading arrangement. Each of said input and output shafts may be preloaded in a preload arrangement either between a pair of auxiliary preload bearings, or between a main thrust bearing arrangement and an auxiliary preload bearing. In the latter case, the main thrust bearing arrangement will have the double function of cancelling out dynamic axial loads, and acting as one of the preload bearings of a preloading arrangement. The auxiliary preload bearings may be located e.g. at the respective end walls of a gear unit housing. By directing the dynamic axial thrust to a single, axially rigid location, where the thrusts at least partly cancel out when the gear unit is driven in a forward, high-load drive direction, the magnitude of the preload force can be selected more freely and with a higher accuracy. In the case of tapered roller bearings, an accurate axial preloading reduces skidding and brings a greater portion of a bearing&#39;s rolling elements in contact with bearing inner and outer races during a greater portion of each turn of the respective shaft, thereby sharing the axial load more accurately between the rolling elements. For this reason, a correctly preloaded thrust bearing generally has a longer life expectancy than a non-preloaded thrust bearing. However, a too highly preloaded thrust bearing generally has a shorter life expectancy than a non-preloaded bearing. By cancelling out a significant portion of the dynamic axial forces, the preload of the shafts can be kept on a lower, more constant, and more accurately selectable level. Furthermore, any housing or support structure carrying the preload thrust bearings may be dimensioned rigid enough for the static preload, without becoming excessively heavy as it would have been if required to take up heavy dynamic axial thrusts originating from operation of helical gears as well. It also becomes possible to arrange the preload thrust bearings in a flexible support so as to provide a selected, constant axial preloading force. Still further, axial preloading also to some extent reduces the axial and radial movement of the respective shafts, thereby prolonging service life expectancy. 
     According to an embodiment of any of the gear units described hereinbefore, said input and output shafts are substantially concentric. Thereby, a minimum of transversal torque will act on the main thrust bearing main thrust bearing support, and minimum of bending force will act on the output shaft. Throughout this disclosure, “substantially concentric” is to be construed as a central axis of at least one of the input shaft and the output shaft extending through a surface defined by the outer boundaries of a thrust bearing supporting the other shaft. 
     According to an embodiment of any of the gear units described hereinbefore, said driven gear of said output shaft has an output shaft driven gear pitch diameter D out  and an output shaft driven gear helix angle ψ out ; 
     said drive gear of said input shaft has an input shaft drive gear pitch diameter D in  and an input shaft drive gear helix angle ψ in ; and 
     said driven gear of said output shaft and said drive gear of said input shaft satisfy a condition corresponding to 
     
       
         
           
             
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     wherein I tot  is the transmission ratio of said gear unit. Thereby, the resultant axial force is even further reduced. Preferably, 
     
       
         
           
             
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     such that input and output axial thrusts to an even greater extent cancel out in the main thrust bearing or the main thrust bearing support, as the case may be. 
     According to an embodiment of any of the gear units described hereinbefore, said layshaft arrangement comprises at least one layshaft, and optionally a plurality of substantially parallel layshafts connected in series, each layshaft being provided with a helical drive gear and a helical driven gear, the driven gear of each layshaft being of the same hand as the drive gear of the same layshaft. Thereby, axial thrusts of each layshaft at least partly cancel out as a compressive or tensile axial force within each respective layshaft. Preferably, for each layshaft i of said at least one layshaft, or of said optional plurality of layshafts, the respective drive gear has a drive gear pitch diameter D drive, i  and a drive gear helix angle ψ drive, i ; the respective driven gear has a driven gear pitch diameter D driven, i  and a driven gear helix angle ψ driven, i , the driven gear helix angle ψ driven, i  being different from the drive gear helix angle ψ drive, i ; and 
       0.2&lt;|( D   drive, i *tan ψ driven, i )/( D   driven, i *tan ψ drive, i )|&lt;5.
 
     Under this particular condition, the axial thrust components acting on each layshaft cancel each other out to an even greater extent. Even more preferably, 
       0.5&lt;|( D   drive, i *tan ψ driven, i )/( D   driven, i *tan ψ drive, i )|&lt;2,
 
     such that most of the axial thrust components acting on each layshaft cancel out. It will be appreciated that ψ drive, i  does not need to be identical to ψ driven, i−1 ; such may be the case e.g. if the respective shafts are not exactly parallel. 
     According to an embodiment of any of the gear units described hereinbefore, said gear unit is a step-down gear. And according to an embodiment of any of the gear units described hereinbefore, said gear unit has a transmission ratio, between said input shaft and said output shaft, of 30:1 or less. Preferably the transmission ratio is in the range of 20:1 to 0.5:1, and more preferably in the range of 15:1 to 0.5:1. The gear unit designs disclosed herein are particularly well suited for the large output torques of a step-down gear and/or a high-transmission ratio gear unit. 
     According to an embodiment of any of the gear units described hereinbefore, said gear unit has a fixed transmission ratio. Such a design is relatively compact and reliable, making it well suited for electric vehicles having electric motors that do not require any possibility to vary the gear ratio between the electric motor and the drive wheels. 
     According to another embodiment the gear unit has a variable transmission ratio. Such a gear unit is particularly well suited for electric drive vehicles with high top speed, and vehicles with internal combustion engines. 
     According to an embodiment, the driven gear of the output shaft is axially fixed to the output shaft, and the drive gear of the input shaft is axially fixed to the input shaft. 
     According to an embodiment, for each layshaft of said layshaft arrangement, the driven gear of the layshaft is axially fixed relative to the drive gear of the layshaft. 
     According to an embodiment, for each shaft of said gear unit, the angular shaft play e relative to another shaft of said gear unit satisfies the condition 
       θ S &lt;tan −1 (0.11/ n   G )
 
     wherein n G  represents the number of teeth of a gear of said shaft, said gear being in engagement with a gear of said another shaft. 
     According to one embodiment the output shaft is tapering inwardly in a direction toward a main thrust bearing arrangement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, where the same reference numerals will be used for similar elements, wherein: 
         FIG. 1  is a schematic side view of an electric drive vehicle; 
         FIG. 2  is a schematic side view of the drive unit of the electric drive vehicle of  FIG. 1 ; 
         FIG. 3  is a schematic view in perspective of gearing of a mobile vehicle gear unit; 
         FIG. 4  is a schematic top view in section of a mobile vehicle gear unit comprising the gearing of  FIG. 3 ; 
         FIG. 5   a  is schematic cross-section side view, taken along the line V-V, of the gear unit of  FIG. 4 ; 
         FIG. 5   b  is a magnified view of the area of  FIG. 5   a  defined by a dashed rectangle; 
         FIG. 6   a  is schematic cross-section side view of a second embodiment of a gear unit; 
         FIG. 6   b  is a magnified view of the area of  FIG. 6   a  defined by a dashed rectangle; 
         FIG. 7   a  is schematic cross-section side view of a third embodiment of a gear unit; 
         FIG. 7   b  is a magnified view of the area of  FIG. 7   a  defined by a dashed rectangle; 
         FIG. 8   a  is schematic cross-section top view of a fourth embodiment of a gear unit; and 
         FIG. 8   b  is a schematic cross-section side view, taken along the line B-B, of the gear unit of  FIG. 8   a.    
         FIG. 9   a  is schematic side view of a fifth embodiment of a gear unit; 
         FIG. 9   b  is a schematic cross-section side view of the gear unit of  FIG. 9   a.    
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     Mobile vehicle gear units of prior art have a few weaknesses. By way of example, the shafts of a mobile vehicle gear unit are subjected to axial thrust, which is taken up by thrust bearings in the gear unit housing. For example, an electric motor transmits very high levels of torque to the gear unit already at low rpm; hence, the radial and axial load on the input shaft bearings may be very high. The high load may cause the gear unit housing to flex or yield, leading to increased axial bearing play and requiring substantial gearing backlash. Such axial play may limit the lifetime of the mobile vehicle gear unit; hence, the gear unit housing needs to be designed to take up substantial forces, making it heavy. 
     The direction of axial thrust varies with the load condition on the mobile vehicle gear unit; therefore, axially unconstrained shafts may translate in the axial direction depending on the rpm and torque transmitted from the electric motor. Within a bearing, a loss of contact between rolling elements and a race surface may result in intermittent, skidding contact between race surface and rolling elements. This phenomenon is called skidding, and may increase bearing wear. Still further, alternating torsion transients, occurring due to substantial backlash, i.e. clearance between the teeth of meshing gears, also contribute to shorten the lifetime of a mobile vehicle gear unit. 
       FIG. 1  schematically illustrates a vehicle  10  comprising a car body  12  and a drive unit  14 . The drive unit  14  comprises a motor  16 . The motor  16  may, for example, be an electric motor, a displacement engine, for example an internal combustion engine, such as petrol or diesel engine, or a dynamic engine, such as jet engine. The drive unit  14  further comprises an energy storage system  18 , which may be a battery pack if the motor  16  is an electric motor, and a fuel tank if the motor  16  is an internal combustion engine, a drive shaft  20  for transferring rotation to drive wheels  22  of the vehicle  10 , and a mobile vehicle gear unit  24  connecting the motor  16  to the drive shaft  20 . A typical rotation speed of the motor  16  may, by way of example, be between 1000 and 20 000 rpm. 
       FIG. 2  schematically illustrates the mobile vehicle gear unit  24 . The motor  16  is connected to the drive wheels  22  ( FIG. 1 ) via the gear unit  24 , which steps down the high rotation speed of the motor  16  to a lower speed of the drive shaft  20 , and of the drive wheels  22 . A gear unit output shaft  26  interconnects the gear unit  24  with the drive shaft  20  and the drive wheels  22 , and provides a low-speed, high-torque rotary motion, in a driving direction  19 , to the drive wheels  22 , which convert the rotary motion to a forward movement of the vehicle  10 . A gear unit input shaft  28  interconnects the motor  16  with the gear unit  24 , and provides a high-speed, low-torque rotary motion to the gear unit  24 . 
     Typical drive wheels  22  may, by way of example, be designed for operating at a rotation speed of between 100 and 5000 rpm (revolutions per minute). The total transmission ratio I tot  of the gear unit  24  can be defined as the signed rotation speed of the input shaft  28  divided by the signed rotation speed of the output shaft  26 , when both shafts  28 ,  26  are viewed in a direction from the input shaft  28  to the output shaft  26 . When the input shaft  28  rotates in one direction, and the output shaft  26  rotates in the same direction, i.e., the sign of the input shaft  28  rotation is the same as the sign of the output shaft  26  rotation, then the total transmission ratio I tot  will be positive (dividing numbers of the same sign), and when the input shaft  28  rotates in one direction, and the output shaft  26  rotates in another direction, i.e., the sign of the input shaft  28  rotation is different from the sign of the output shaft  26  rotation, then the total transmission ratio I tot  will be negative (dividing numbers of different sign). Hence, the total transmission ratio I tot  of the gear unit  24  is preferably +/−30:1 or less. 
       FIG. 3  illustrates the gearing of the gear unit  24  in greater detail. The output shaft  26  is connected to the input shaft  28  via a layshaft arrangement  29  comprising a first layshaft  30  and a second layshaft  32 , which are connected in series. The output shaft  26  is provided with a driven gear  34 , which is in mesh with a drive gear  36  on the first layshaft  30 . The driven gear  34  of the output shaft  26  has a larger pitch diameter D out  than the corresponding pitch diameter D drive, 1  of the drive gear  36  of the first layshaft  30 , such that the engagement of the driven gear  34  of the output shaft  26  with the drive gear  36  of the first layshaft  30  provides a first step-down, of transmission ratio of the rotary speed from the first layshaft  30  to the output shaft  26 . 
     The first layshaft  30  is provided with a driven gear  38 , which is in mesh with a drive gear  40  on the second layshaft  32 . The driven gear  38  of the first layshaft  30  has a larger pitch diameter D driven, 1  than the corresponding pitch diameter D drive, 2  of the drive gear  40  of the second layshaft  32 , such that the engagement of the driven gear  38  of the first layshaft  30  with the drive gear  40  of the second layshaft  32  provides a second step-down, of transmission ratio I 2 , of the rotary speed from the second layshaft  32  to the first layshaft  30 . 
     The second layshaft  32  is provided with a driven gear  42 , which is in mesh with a drive gear  44  on the input shaft  28 . The driven gear  42  of the second layshaft  32  has a larger pitch diameter D driven, 2  than the corresponding pitch diameter D in  of the drive gear  44  of the input shaft  28 , such that the engagement of the driven gear  42  of the second layshaft  32  with the drive gear  44  of the input shaft  28  provides a third step-down, of transmission ratio I 3 , of the rotary speed from the input shaft  28  to the second layshaft  32 . Hence, the gear unit  24  comprises three gear steps of transmission ratios I 2 , I 3 , which provide a total transmission ratio I tot =I 1 *I 2 *I 3  from the input shaft  28  to the output shaft  26 . 
       FIG. 4  illustrates the mobile vehicle gear unit  24  arranged in a gear unit housing  45 . 
     The driven gear  34  of the output shaft  26  is a helical gear of a first hand, said first hand in this example being right hand, right hand being defined as the teeth twisting clockwise as they recede from an observer looking along the axis A out  of the driven gear  34  of the output shaft  26 . The driven gear  34  of the output shaft  26  further has a helix angle ψ out , defined as the unsigned value of the angle formed between a tangent to the gear&#39;s helix at the pitch circle, and the direction of the central axis A out  of the driven gear  34 . 
     The drive gear  44  of the input shaft  28  is a helical gear of a second hand, said second hand in this example being left hand, defined as the teeth twisting counter-clockwise as they recede from an observer looking along the axis A in  of the drive gear  44  of the input shaft  28 . The drive gear  44  of the input shaft  28  further has a helix angle ψ in , defined as the unsigned value of the angle formed between a tangent to the gear&#39;s helix at the pitch circle, and the direction of the central axis A in  of the drive gear  44 . The same definition of helix angle applies, mutatis mutandis, to the other helical gears of the gear unit  24 . 
     When the output shaft  26  is rotated in the driving direction  19  ( FIG. 3 ), said driving direction being clockwise, as seen in the direction of the central axis A out  of the output shaft  26  towards the input shaft  28 , the helical driven gear  34  of the output shaft  26 , which is axially fixed to the output shaft  26 , will generate an axial thrust F 1  acting on the output shaft  26 . Due to the hand of the driven gear  34  of the output shaft  26 , the axial thrust F 1  will be directed towards the input shaft  28 . 
     The drive gear  44  of the input shaft  28 , which, via said first and second layshafts  30 ,  32 , will rotate in an input direction  47  ( FIG. 3 ) opposite to the driving direction  19 , will generate an axial thrust F 2 . The drive gear  44  of the input shaft  28  is axially fixed to the input shaft  28 , such that the axial force F 2  will act on the input shaft  28 . Due to the hand of the drive gear  44  of the input shaft  28 , the axial thrust F 2  will be directed towards the output shaft  26 . The directions of the axial thrusts F 1 , F 2  are illustrated by arrows. 
     The output shaft  26  and the input shaft  28  are concentric, and meet at a main thrust bearing support  50 . At the main thrust bearing support  50 , the output and input shafts  26 ,  28  are journalled in a manner which will be described in more detail further below. The main thrust bearing support  50  is fixed to the gear unit housing  45  in a non-rotating manner, and in this particular example forms a bracket for holding the output shaft  26  on a first, output side  52  thereof, and the input shaft  28  on a second, input side  54  thereof. The output side  52  of the main thrust bearing support  50  is opposite to the input side  54 . 
     The main thrust bearing support  50  forms an axially rigid support structure, at which the output and input shafts  26 ,  28  are journalled so as to transfer the axial thrusts F 1 , F 2  thereto. Thanks to the respective hands of the driven gear  34  of the output shaft  26 , and of the drive gear  44  of the input shaft  28 , being oriented such that the respective axial thrusts F 1 , F 2  are directed in opposite directions, the axial thrusts F 1 , F 2  will thereby at least partly cancel out in the main thrust bearing support  50 . This reduces movement of the output and input shafts  26 ,  28  and their associated gears  34 ,  44 , which in turn allows for tighter gearing backlash. And thanks to the output and input shafts  26 ,  28  being concentric, the axial thrusts F 1 , F 2  will not result in any substantial transversal torque or bending force onto the main thrust bearing support  50 , or onto the shafts  26 ,  28  themselves. 
     When torque is supplied to the input shaft  28  in the input direction  47  ( FIG. 3 ), the axial thrusts F 1 , F 2  cancel out as a compressive force where the output and input shafts  26 ,  28  meet in the main thrust bearing support  50 . During normal operation of a vehicle  10  ( FIG. 1 ), the average and instantaneous torque supplied to the mobile vehicle gear unit  24  is typically larger in the input direction  47 , i.e. forward movement, than in the opposite direction. The opposite direction may, for example, be used when the vehicle  10  ( FIG. 1 ) is to be reversed. Reversing an electric drive vehicle may be made by simply shifting the rotational direction of the electric motor, to a direction which is opposite to the input direction  47 . Reversing the vehicle is usually made for very short periods of time, and at rather low load. Hence, the mobile vehicle gear unit  24  does not necessarily need to be arranged for taking up heavy axial thrusts in the opposite directions, i.e. thrusts acting so as to urge the output shaft  26  away from the input shaft  28 . 
     According to the definition of transmission ratio I tot  hereinbefore, opposite directions of rotation of the output shaft  26  and the input shaft  28  will yield a negative total transmission ratio I tot . As a general rule, in order to generate an output shaft axial thrust F 2  in a direction opposite to the input shaft axial thrust F 1 , the second hand, said second hand being the hand of the input shaft  28 , should be the same as the first hand, said first hand being the hand of the output shaft  26 , for an even number of layshafts connected in series between the output shaft  26  and the input shaft  28 . For an odd number of layshafts connected in series between the output shaft  26  and the input shaft  28 , the second hand should be the same as the first hand. Furthermore, for a clockwise driving direction  19  of the output shaft  26 , as seen in the axial direction towards the input shaft  28 , a right-hand driven gear  34  of the output shaft  26  should be selected for obtaining axial thrusts F 1 , F 2  that meet at the main thrust bearing support  50 . For a counter-clockwise driving direction  19  of the output shaft  26 , a left-hand driven gear  34  of the output shaft  26  should be selected for obtaining axial thrusts F 1 , F 2  that meet at the main thrust bearing support  50 . 
     Preferably, the driven gear  34  of the output shaft  26  and the drive gear  44  of the input shaft  28  satisfy the condition: 
     
       
         
           
             
               
                 
                   0.2 
                   &lt; 
                   
                      
                     
                       
                         I 
                         tot 
                       
                        
                       
                         
                           
                             D 
                             in 
                           
                            
                           tan 
                            
                           
                               
                           
                            
                           
                             Ψ 
                             out 
                           
                         
                         
                           
                             D 
                             out 
                           
                            
                           tan 
                            
                           
                               
                           
                            
                           
                             Ψ 
                             in 
                           
                         
                       
                     
                      
                   
                   &lt; 
                   5 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     It has been found that if this condition is satisfied, a significant portion of the axial thrusts F 1 , F 2  of the output and input shafts  26 ,  28  cancel out. 
     More preferably, 
     
       
         
           
             
               
                 
                   
                     0.5 
                     &lt; 
                     
                        
                       
                         
                           I 
                           tot 
                         
                          
                         
                           
                             
                               D 
                               in 
                             
                              
                             tan 
                              
                             
                                 
                             
                              
                             
                               Ψ 
                               out 
                             
                           
                           
                             
                               D 
                               out 
                             
                              
                             tan 
                              
                             
                                 
                             
                              
                             
                               Ψ 
                               in 
                             
                           
                         
                       
                        
                     
                     &lt; 
                     2 
                   
                   , 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     and ideally, 
     
       
         
           
             
               
                 
                   
                     
                        
                       
                         
                           I 
                           tot 
                         
                          
                         
                           
                             
                               D 
                               in 
                             
                              
                             tan 
                              
                             
                                 
                             
                              
                             
                               Ψ 
                               out 
                             
                           
                           
                             
                               D 
                               out 
                             
                              
                             tan 
                              
                             
                                 
                             
                              
                             
                               Ψ 
                               in 
                             
                           
                         
                       
                        
                     
                     ≈ 
                     1 
                   
                   , 
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     such that there is an almost complete axial thrust balance between F 1  and F 2 . Thereby, the axial load exerted by the input and output shafts  28 ,  26  onto the axial end walls of the gear unit housing  45  can be essentially eliminated. As is apparent to those skilled in the art, the expressions (1)-(3) above may also be adjusted so as to compensate for the friction of the mobile vehicle gear unit  24 . 
     As a specific example fulfilling all the above conditions, for the mobile vehicle gear unit  24  having a total transmission ratio I tot  of 2,5:1, the driven gear of the output shaft  26  may have a pitch diameter D out  of 110 mm (millimeters) and a helix angle ψ out  of 12°, whereas the drive gear  44  of the input shaft  28  may have a pitch diameter D in  of 60 mm and a helix angle ψ in  of 16°. 
     In order to achieve a minimum total axial load onto the gear unit housing  45 , also the layshafts  30 ,  32  of the layshaft arrangement  29  ( FIG. 3 ) may be axially balanced. This may be achieved by each of the layshafts  30 ,  32  being equipped with a respective driven gear and a respective drive gear of the same hand, preferably with a larger helix angle on its respective larger pitch diameter gear than on its respective smaller pitch diameter gear. In the exemplary mobile vehicle gear unit  24  of  FIG. 4 , the drive gear  36  of the first layshaft  30  has a drive gear helix angle ψ drive, 1  and the driven gear  38  has a driven gear helix angle ψ driven, 1  of the same hand as the drive gear  36 . Thereby, when the output shaft  26  is rotated in said driving direction  19 , at least a portion of the axial thrusts generated by the helical gears  36 ,  38  of the first layshaft  30  will cancel out as a tensile force in the first layshaft  30 . The pitch diameter D drive, 1  of the drive gear  36  is larger than the pitch diameter D driven, 1  of the driven gear  38 ; hence, the driven gear helix angle ψ driven1  preferably exceeds the drive gear helix angle ψ drive, 1 , such that axial balance is still further improved. 
     Preferably, for each layshaft i of a layshaft arrangement, 
       0.2&lt;|( D   drive,i *tan ψ driven,i )/( D   driven,i *tan ψ drive,i )|&lt;5,   (4)
 
     where, in the specific embodiment of  FIG. 4 , i=1 represents the respective properties D driven,1 , ψ driven,1 , D drive,1 , and ψ drive,1  of the first layshaft  30 , and i=2 represents the respective properties of the second layshaft  32 . More preferably, 
       0.5&lt;|( D   drive, i *tan ψ driven, i )/( D   driven, i *tan ψ drive, i )|&lt;2,   (5)
 
     and ideally, 
       |(D drive, i *tan ψ driven, i )/(D driven, i *tan ψ drive, i )|≈1.   (6)
 
     Under those conditions, the layshafts  30 ,  32  may be completely balanced in the axial direction, and their journaling may be thrust bearing free, i.e. they may be journalled for substantive support only in the radial direction. 
     As a specific example fulfilling all the above conditions, for the mobile vehicle gear unit  24  of  FIG. 3 , the drive gear  36  of the first layshaft  30  may have a pitch diameter D drive,1  of 80 mm and a helix angle ψ drive,1  of 12°; the driven gear  38  of the first layshaft  30  may have a pitch diameter D driven,1  of 100 mm and a helix angle ψ driven,1  of 15°; the drive gear  40  of the second layshaft  32  may have a pitch diameter D drive,2  of 75 mm and a helix angle ψ drive.2  of 15°; and the driven gear  42  of the second layshaft  32  may have a pitch diameter D driven,2  of 80 mm and a helix angle ψ driven.2  of 16°. 
       FIG. 5   a , showing the section V-V along the central axis A out  of  FIG. 4 , illustrates an exemplary journaling of the output and input shafts  26 ,  28 , and the magnified view of  FIG. 5   b  illustrates the details of how the shafts  26 ,  28  are journalled to the main thrust bearing support  50 . The output shaft  26  is journalled to the main thrust bearing support  50  in an output shaft main thrust bearing arrangement  56 , which comprises a first thrust bearing, embodied as a first tapered roller bearing  58 , and a second thrust bearing, embodied as a second tapered roller bearing  60 . The two tapered roller bearings  58 ,  60  taper in opposite directions, such that they together form a bidirectional thrust bearing arrangement, i.e. the output shaft thrust bearing arrangement  56  is adapted for supporting substantial axial loads in both axial directions. 
     Similarly, the input shaft  28  is journalled to the main thrust bearing support  50  in an input shaft main thrust bearing arrangement  62 , which also comprises two tapered roller bearings  64 ,  66  tapering in opposite directions, thereby forming a bidirectional thrust bearing arrangement. The output and input main thrust bearing arrangements  56 ,  62  are co-located on the main thrust bearing support  50 , which interconnects said main thrust bearing arrangements  56 ,  62  in an axially rigid manner. 
     Thanks to the output and input shaft main thrust bearing arrangements  56 ,  62  being bidirectional, when torque is supplied to the input shaft  28  in a direction opposite to said input direction  47 , the axial thrusts F 1 , F 2  cancel out as a tensile force in the main thrust bearing support  50 . Thereby, axial loads onto axial end walls  78 ,  80  of the gear unit housing  45  are reduced regardless of the direction of operation or the load direction of the input shaft  28 . 
     An auxiliary bearing  68  supports the output shaft  26  in the radial direction. Since the output shaft main thrust bearing arrangement  56  is bidirectional and provides all axial support that is needed, the auxiliary bearing  68  does not need to be arranged for providing any axial support. Hence, the auxiliary bearing  68  may be a simple, radially supporting bearing of e.g. the cylindrical, non-tapered roller bearing type. 
     Alternatively, also the auxiliary bearing  68  may be an axial thrust bearing, e.g. of the tapered roller bearing type, which supports the output shaft  26  in an axial direction. Thereby, the auxiliary bearing  68  may be used as a preload bearing for axially preloading the output shaft  26  between the auxiliary bearing  68  and the output shaft main thrust bearing arrangement  56 . In such a configuration, the auxiliary bearing  68  and the output shaft main thrust bearing arrangement  56  together form a preloading arrangement, which may permanently keep the output shaft  26  under tensile or compressive load. Thereby, the lifetime of the output shaft main thrust bearing arrangement  56  may be extended. Furthermore, a minimum of backlash can be designed into the mating of the teeth of the gears of the mobile vehicle gear unit  24 , such that the operational lifetime of the entire gear unit  24  is increased. 
     Also the input shaft  28  may be preloaded in a preloading arrangement formed by an auxiliary axial thrust bearing  70  and the input shaft main thrust bearing arrangement  62 . It is also possible to accurately preload the layshafts  30 ,  32 , which may be axially balanced in line with what has been described hereinbefore with reference to  FIG. 4 , in similar preloading arrangements of thrust bearings. 
       FIGS. 6   a - b  illustrate an alternative configuration of the bearings of the mobile vehicle gear unit  24 , according to which configuration the output shaft  26  and the input shaft  28  are arranged for transferring axial thrusts F 1 , F 2  more directly to each other. In the particular example illustrated in  FIG. 6   a - b,  the input shaft  28  is journalled to the output shaft  26  via a main thrust bearing arrangement  156 , which allows the output and input shafts  26 ,  28  to rotate independently of each other. The main thrust bearing arrangement  156  comprises a first tapered roller bearing  158  and a second tapered roller bearing  160 , which are arranged in an axial recess  174  of the output shaft  26 . The two tapered roller bearings  158 ,  160  taper in opposite directions, such that they together form a bidirectional thrust bearing arrangement. Thereby, the axial thrusts F 1 , F 2  of the output and input shafts  26 ,  28  meet and at least partly cancel each other out at the main thrust bearing arrangement  156  irrespective of the input direction of the input shaft  28 . The main thrust bearing arrangement  156  thereby forms an axially rigid support structure for non-yieldingly receiving axial loads from both the output shaft  26  and the input shaft  28 . 
     The output shaft  26  is journalled in an auxiliary bearing  176 , which is mounted on a bearing support  150 . The auxiliary bearing  176  does not need to be a thrust bearing, since axial thrust will mainly be borne by the main thrust bearing arrangement  156 . Similar to what has been described above with reference to  FIG. 5   a - b,  however, the auxiliary bearing  176  may, as an alternative, be a thrust bearing that can be used for preloading the output shaft  26  against e.g. a second auxiliary bearing  68 . 
     Neither the bearing support  150  nor the auxiliary bearing  176  mounted thereto are necessary for balancing the dynamic axial thrusts that occur when operating the gear unit  24 ; hence, they can be dispensed with, and radial support may be provided by other means available to the person skilled in the art. 
     As an alternative to incorporating into the mobile vehicle gear unit  24  a main thrust bearing arrangement  156  that is bidirectional, for a gear unit that is intended for an application in which it is mainly exposed to high torques in a single, predetermined input direction  47  ( FIG. 3 ), it would be sufficient to use a unidirectional main thrust bearing arrangement as has been described in the foregoing. 
       FIGS. 7   a - b  illustrate yet an alternative configuration of the bearings of the mobile vehicle gear unit  24 , according to which configuration the output shaft  26  is hollow, and the main thrust bearing arrangement  156  is located deep into the recess  174  of the output shaft  26 . Even though, in  FIG. 7   a , the axial thrust F 1  is illustrated on the input shaft  28 , it will be appreciated that the thrust F 1  acts on the hollow output shaft  26  surrounding the input shaft  28 . 
     In the configuration of  FIGS. 7   a - b,  the output and input shafts  26 ,  28  are interconnected via a main thrust bearing arrangement  156 , via which the axial thrusts F 1 , F 2  of the shafts  26 ,  28  cancel out. However, as an alternative (not shown), the output shaft  26  may be journalled to the axial end wall  178  of the gear unit housing  45  in an output shaft main thrust bearing arrangement, which is configured to transfer axial thrust from the output shaft  26  to the axial end wall  178 . Similarly, the input shaft  28  may be journalled to the same axial end wall  178  of the gear unit housing  45  in an input shaft main thrust bearing arrangement, which is configured to transfer axial thrust from the input shaft  28  to the axial end wall  178 . In such a configuration, the axial end wall  178  of the gear unit housing  45  would form a main thrust bearing support similar to what has been described hereinbefore with reference to  FIG. 5   a - b.  Such a configuration would however differ from the arrangement of  FIG. 5   a - b  in that the driven gear  34  of the output shaft  26  and the drive gear  44  of the input shaft  28  would be located on the same side of the main thrust bearing support, such that one of the output and input shaft main thrust bearing arrangements would support an axially pushing force, while the other would support an axially pulling force, when the gear unit  24  is operated. 
       FIGS. 8   a - b  illustrate an alternative configuration of the layshafts, according to which configuration the mobile vehicle gear unit  24  comprises a first layshaft arrangement  29  and a second layshaft arrangement  29 ′. Each of the layshaft arrangements  29 ,  29 ′ comprises a respective first layshaft  30 ,  30 ′ provided with a drive gear  36 ,  36 ′ and a driven gear  38 ,  38 ′, and a respective second layshaft  32 ,  32 ′, said second layshaft  32 ,  32 ′ also being provided with a drive gear  40 ,  40 ′ and a driven gear  42 ,  42 ′. The two layshaft arrangements  29 ,  29 ′ are arranged in parallel and configured to provide an identical transmission ratio I tot  between the input shaft  28  and the output shaft  26 . The output and input shafts  26 ,  28 , as well as the layshafts  30 ,  30 ′,  32 ,  32 ′ of the gear unit  24  will be axially balanced under the same conditions, defined by the appended claims, as any other of the embodiments disclosed hereinbefore with reference to  FIGS. 1-7   b.  In the particular example illustrated in  FIG. 8   b , the axial loads of the output and input shafts  26 ,  28  at least partly cancel out in a main thrust bearing arrangement  156 , in a manner similar to what has been described in the foregoing with reference to  FIGS. 6   a - b.    
     Two parallel layshaft arrangements, which are in mesh with the same driven and drive gears  34 ,  44 , respectively, of the output and input shafts  26 ,  28 , are illustrated in  FIGS. 8   a - b.  However, the gear unit may also be provided with any other number of parallel layshaft arrangements, and the layshafts may be connected between different sets of driven gears of the output shaft  26 , and drive gears of the input shaft  28 ; the general principles of axial thrust balancing disclosed herein will nevertheless apply. 
       FIGS. 9   a - b  illustrate a mobile vehicle gear unit  24  according to a fifth alternative embodiment. The mobile vehicle gear unit  24  of  FIGS. 9   a - b  has a variable total transmission ratio I tot . The mobile vehicle gear unit  24  has a first gear, representing a first total transmission ratio I tot1  and a second gear, representing a second total transmission ratio I tot2 . The mobile vehicle gear unit  24  of this fifth embodiment of  FIGS. 9   a - b  is suitable for electric drive vehicles. An electric drive vehicle is provided with an electric motor  16  ( FIG. 1 ). An electric motor  16  has a high torque already at low rpm, e.g. 1000 rpm. However, in order to obtain a high top speed and/or a high energy efficiency of the electric drive vehicle the rpm span of the electric motor  16  of the electric vehicle  10  may not be sufficient. The first gear of the mobile vehicle gear unit  24  of  FIGS. 9   a - b  may be used at vehicle speeds of, for example, 0 to 120 kmh, and the second gear of the mobile vehicle gear unit  24  of  FIGS. 9   a - b  may be used at vehicle speeds of, for example, 80 to 200 kmh. Shifting between the first gear and the second gear could be made manually, using a gear-change lever, or automatically, employing, for example, the principles of the per se known robotized manual gearbox with electronic clutch. 
     It will be appreciated that alternative mobile vehicle gear units manufactured in accordance with the principles described hereinafter with reference to  FIGS. 9   a - b  may be provided with three, four, five, six, seven, or even more gears, providing a further variability in the total transmission ratio I tot . Furthermore, mobile vehicle gear units manufactured in accordance with the principles described hereinafter with reference to  FIGS. 9   a - b,  and provided with anything from two to ten gears, or even more, may also be used for other types of motors  16 , for example internal combustion engines using, for example, petrol, diesel, biogas, or ethanol as fuel. 
     Returning to  FIGS. 9   a - b  the electric drive vehicle mobile vehicle gear unit  24  comprises an output shaft  26  connected to an input shaft  28  via a layshaft arrangement  29 . The layshaft arrangement  29  comprises, in this embodiment, a single layshaft  30 . It will be appreciated that the gear unit  24  may, in alternative embodiments, be provided with two or more layshafts arranged in series, in accordance with the principles of  FIGS. 3-4 , and/or with two or more parallel layshafts, in accordance with the principles of  FIG. 8   a.    
     The output shaft  26  is provided with a first driven gear  34 , which is in mesh with a first drive gear  36  on the layshaft  30 , and a second driven gear  35  which is in mesh with a second drive gear  37  on the layshaft  30 . In neutral position, as illustrated in  FIGS. 9   a - b,  the first and second driven gears  34 ,  35  are free-wheeling on the output shaft  26 . A gear-change lever  84  may be connected to an engagement ring  86  arranged on the output shaft  26 , between the first and second driven gears  34 ,  35 . The engagement ring  86  is axially movable along the output shaft  26 , in the axial direction thereof, as illustrated by arrows in  FIG. 9   a . The engagement ring  86  is radially locked to the output shaft  26  and rotates together with the output shaft  26 . By moving the engagement ring  86  towards the first driven gear  34 , i.e., by moving the engagement ring to the right in  FIG. 9   a , the engagement ring  86  may engage the first driven gear  34  and radially lock the first driven gear  34 , such that the first driven gear  34  starts to rotate the output shaft  26 . Such radial locking of the first driven gear  34  to the output shaft  26  is the same as putting in the first gear of the gear unit  24 . By moving the engagement ring  86  towards the second driven gear  35 , i.e., by moving the engagement ring to the left in  FIG. 9   a , the engagement ring  86  may engage the second driven gear  35  and radially lock the second driven gear  35 , such that the second driven gear  35  starts to rotate the output shaft  26 . Such radial locking of the second driven gear  35  to the output shaft  26  is the same as putting in the second gear of the gear unit  24 . The engagement of the engagement ring  86  to one of the first and second driven gears  34 ,  35  may be made using the per se known principles of synchromesh or dogg engagement. 
     The first driven gear  34  of the output shaft  26  has a larger pitch diameter D out1  than the corresponding pitch diameter D drive, G1  of the first drive gear  36  of the layshaft  30 , such that the engagement of the first driven gear  34  of the output shaft  26  with the first drive gear  36  of the layshaft  30  provides a first gear step-down, of transmission ratio of the rotary speed from the layshaft  30  to the output shaft  26 . 
     The second driven gear  35  of the output shaft  26  has substantially the same pitch diameter D out2  as the corresponding pitch diameter D drive, G2  of the second drive gear  37  of the layshaft  30 , such that the engagement of the second driven gear  35  of the output shaft  26  with the second drive gear  37  of the layshaft  30  provides a neutral gear, of transmission ratio I G2 =1, of the rotary speed from the layshaft  30  to the output shaft  26 . 
     The layshaft  30  is provided with a driven gear  38 , which is in mesh with a drive gear  44  on the input shaft  28 . All gears  36 ,  37 ,  38  of the layshaft  30  are axially locked to the layshaft  30 , and transfer axial thrust to the layshaft  30 . The driven gear  38  of the layshaft  30  has a larger pitch diameter D driven  than the corresponding pitch diameter D in  of the drive gear  44  of the input shaft  28 , such that the engagement of the driven gear  38  of the layshaft  30  with the drive gear  44  of the input shaft  28  provides a step-down, of transmission ratio I 3 , of the rotary speed from the input shaft  28  to the layshaft  30 . 
     Hence, in the first gear the mobile vehicle gear unit  24  comprises two gear steps of transmission ratios I G1 , and I 3 , which provide a total step-down transmission ratio I tot =I G1 *I 3  from the input shaft  28  to the output shaft  26 , when the gear unit  24  is set in the first gear. In the second gear the mobile vehicle gear unit  24  comprises a single gear step, since I G2 =1, of transmission ratio I 3 , which provides a total step-down transmission ratio I tot =I 3  from the input shaft  28  to the output shaft  26 , when the gear unit  24  is set in the second gear. 
     The driven gears  34 ,  35  of the output shaft  26  are each helical gears of a first hand, said first hand in this example being right hand, right hand being defined as the teeth twisting clockwise as they recede from an observer looking along the axis A out  of the driven gears  34 ,  35  of the output shaft  26 . The first driven gear  34  of the output shaft  26  further has a helix angle ψ out1 , defined as the unsigned value of the angle formed between a tangent to the gear&#39;s helix at the pitch circle, and the direction of the central axis A out  of the first driven gear  34 , and the second driven gear  35  has a helix angle ψ out2  defined in accordance with the same principles. 
     The drive gear  44  of the input shaft  28  is a helical gear of a first hand, said first hand in this example being right hand, defined as the teeth twisting clockwise as they recede from an observer looking along the axis A in  of the drive gear  44  of the input shaft  28 . The drive gear  44  of the input shaft  28  further has a helix angle ψ in , defined as the unsigned value of the angle formed between a tangent to the gear&#39;s helix at the pitch circle, and the direction of the central axis A in  of the drive gear  44 . The same definition of helix angle applies, mutatis mutandis, to the other helical gears of the gear unit  24 . 
     When the output shaft  26  is, via the layshaft  30 , rotated in the driving direction  19  ( FIG. 1 ), said driving direction being counter-clockwise, as seen in the direction of the central axis A in  of the input shaft  28  towards the output shaft  26 , the helical first driven gear  34 , or the helical second driven gear  35 , as the case may be, which are axially fixed to the output shaft  26 , will generate an axial thrust F 1  acting on the output shaft  26 . Due to the hand of the driven gears  34  and  35  of the output shaft  26 , the axial thrust F 1  will be directed away from the input shaft  28 . 
     The drive gear  44  of the input shaft  28  will rotate in an input direction  47  which is the same as the driving direction  19 , and will generate an axial thrust F 2 . The drive gear  44  of the input shaft  28  is axially fixed to the input shaft  28 , such that the axial force F 2  will act on the input shaft  28 . Due to the hand of the drive gear  44  of the input shaft  28 , the axial thrust F 2  will be directed away from the output shaft  26 . The directions of the axial thrusts F 1 , F 2  are illustrated by arrows. According to an alternative embodiment a gear unit could be designed with the axial thrusts F 1 , F 2  directed towards each other. 
     As shown in  FIG. 9   b , the output shaft  26  has an inner portion  25  of lower diameter than an outer portion  27 . Hence, the output shaft  26  is a tapering shaft. The second driven gear  35 , forming part of the second gear of the gear unit  24 , transmits a lower torque than the first driven gear  34 . Hence, a lower diameter of inner portion  25  is sufficient for the torques transmitted by the second driving gear  35 , which saves weight. A notch  31  at the transition from the inner portion  25  to the outer portion  27  serves as an axial direction support against which the second driven gear  35  may rest. 
     The output shaft  26  and the input shaft  28  are concentric, and meet at a main thrust bearing support  150 , as best shown in  FIG. 9   b . The input shaft  28  is journalled to the output shaft  26  via a main thrust bearing arrangement  156  arranged in a support wall  55  of a gear unit housing  45 , which allows the output and input shafts  26 ,  28  to rotate independently of each other. The support wall  55  is located between the axial end walls  78 ,  80  of the housing  45 . The layshaft  30  extends through an opening  57  in the support wall  55 . Optionally, if radial support is required, a bearing may be arranged in the opening  57  for supporting the layshaft  30 . The fact that the main thrust bearing arrangement  156  is arranged the support wall  55  provides radial support to the bearing arrangement  156 . The main thrust bearing arrangement  156  of  FIG. 9   b  is similar to the arrangement  156  described hereinbefore with reference to  FIG. 6   b  and comprises a first tapered roller bearing  158  and a second tapered roller bearing  160 , which are arranged in an axial recess  174  of the output shaft  26 . The two tapered roller bearings  158 ,  160  taper in opposite directions, such that they together form a bidirectional thrust bearing arrangement. Thereby, the axial thrusts F 1 , F 2  of the output and input shafts  26 ,  28  at least partly cancel each other out at the main thrust bearing arrangement  156  irrespective of the input direction of the input shaft  28 . The main thrust bearing arrangement  156  thereby forms an axially rigid support structure for non-yieldingly receiving axial loads from both the output shaft  26  and the input shaft  28 . 
     The output shaft  26  is journalled in an auxiliary bearing  176 , which is mounted on a bearing support  150 . The auxiliary bearing  176  does not need to be a thrust bearing, since axial thrust will mainly be borne by the main thrust bearing arrangement  156 . Similar to what has been described above with reference to  FIG. 5   a - b,  however, the auxiliary bearing  176  may, as an alternative, be a thrust bearing that can be used for preloading the output shaft  26  against e.g. a second auxiliary bearing  68 . Also the input shaft  28  may be preloaded in a preloading arrangement formed by an auxiliary axial thrust bearing  70  and the main thrust bearing arrangement  156 . 
     Neither the bearing support  150  nor the auxiliary bearing  176  mounted thereto are necessary for balancing the dynamic axial thrusts that occur when operating the gear unit  24 ; hence, they can be dispensed with, and radial support may be provided by other means available to the person skilled in the art. 
     When torque is supplied to the input shaft  28  in the input direction  47 , the axial thrusts F 1 , F 2  cancel out as a tensile force where the output and input shafts  26 ,  28  meet in the main thrust bearing support  150 . During normal operation of a vehicle  10  ( FIG. 1 ), the average and instantaneous torque supplied to the mobile vehicle gear unit  24  is typically larger in the input direction  47 , i.e. forward movement of the vehicle, than in the opposite direction. The opposite direction may, for example, be used when the vehicle  10  ( FIG. 1 ) is to be reversed. Reversing of an electric drive vehicle may be made by simply shifting the rotational direction of the electric motor, to a direction which is opposite to the input direction  47 . When the vehicle is reversed the axial thrusts F 1 , F 2  cancel out as a compression force where the output and input shafts  26 ,  28  meet in the main thrust bearing support  150 . The conditions described hereinbefore with reference to expressions (1)-(3) may be applied also to the output and input shafts  26 ,  28  of  FIGS. 9   a - b  to obtain the best possible cancelling out of axial thrusts F 1  and F 2 . 
     In the embodiment of  FIGS. 9   a - b  the output shaft  26  and the input shaft  28  are concentric, and meet at a main thrust bearing arrangement  156  being similar to the main thrust bearing arrangement  156  described hereinbefore with reference to  FIG. 6   b . It will be appreciated that other types of bearing arrangements may also be combined with the mobile vehicle gear unit  24  of  FIGS. 9   a - b.  For example, the output shaft  26  and the input shaft  28  of the gear unit  24  of  FIGS. 9   a - b  could be arranged in an output shaft main thrust bearing arrangement  56  and an input shaft main thrust bearing arrangement  62  of the type described hereinbefore with reference to  FIG. 5   b , or in a main thrust bearing arrangement  156  of the type described hereinbefore with reference to  FIG. 7   b.    
     In order to achieve a minimum total axial load onto the gear unit housing  45 , also the layshaft  30  may be axially balanced. This may be achieved by the layshaft  30  being equipped with a respective driven gear  38  and respective drive gears  36 ,  37  of the same hand, preferably with a larger helix angle on its respective larger pitch diameter gear than on its respective smaller pitch diameter gear. In the exemplary mobile vehicle gear unit  24  of  FIG. 9   b , the first drive gear  36  of the layshaft  30  has a first drive gear helix angle ψ drive,G1  the second drive gear  37  has a second drive gear helix angle ψ driveG2 , and the driven gear  38  has a driven gear helix angle ψ driven  of the same hand as the first and second drive gears  36 ,  37 . Thereby, when the output shaft  26  is rotated in said driving direction  19 , at least a portion of the axial thrusts generated by the helical gears  36 ,  37 ,  38  of the layshaft  30  will cancel out as a compression force in the layshaft  30 . It will be appreciated that when the gear unit  24  is set in first gear, the forces of the first drive gear  36  should cancel out, at least partly, the axial thrust of the driven gear  38 , and when the gear unit  24  is set in second gear, the forces of the second drive gear  37  should cancel out, at least partly, the axial thrust of the driven gear  38 . 
     The pitch diameter D drive,G1  of the first drive gear  36  is smaller than the pitch diameter D driven  of the driven gear  38 ; hence, the driven gear helix angle ψ driven  is preferably larger than the first drive gear helix angle ψ drive,G1  such that axial balance is still further improved. A similar reasoning can be made with respect to the pitch diameter and gear helix angle of the second drive gear  37 . The conditions described hereinbefore with reference to expressions (4)-(6) may be applied also for balancing axial thrusts of the layshaft  30  of  FIG. 9   b . The layshaft  30  is journalled in bearings  71 ,  73  in the respective end walls  78 ,  80  of the housing  45 . The bearings  71 ,  73  need only, when the axial thrusts of the layshaft  30  are at least partly cancelled out, as described hereinbefore, take up radial forces, and rather small axial thrusts. 
     In all embodiments described hereinbefore, the at least partial cancelling out of axial forces F 1 , F 2  in an axially rigid support structure results in reduced movement of at least the output and input shafts  26 ,  28  and their associated gears  34 ,  44 . This allows for a tighter backlash compared to what is possible to obtain in gear units of prior art. In other words, the mobile vehicle gear unit  24  may be designed as a low-backlash gear unit. As a rule of thumb, the angular gear play θ G1  of a gear G 1  having n G1  teeth, the gear G 1  mating with another gear G G2 , which is held fixed, may be obtained through the relation 
       θ G1 =tan −1 ( k/n   G1 )   (7)
 
     wherein the number k is determinative of the angular play. As applied to an exemplary gear engagement of the mobile vehicle gear unit  24  described hereinbefore, the angular gear play θ 34  of the driven gear  34  of the output shaft  26 , having n 34  teeth, may be obtained through the relation 
       θ 34 =tan −1 ( k/n   34 )   (8)
 
     assuming that the drive gear  36  of the first layshaft  30  is held immobile. Typically, a suitable angular gear play of each gear  34 ,  36 ,  38 ,  40 ,  42 ,  44 , relative to its respective mating gear, is obtained for k&lt;0.1. Hence, for a reasonably tight low-backlash gear unit, the majority, and preferably all of the gears  34 ,  36 ,  38 ,  40 ,  42 ,  44  of the gear unit  24  have an angular gear play θ G  fulfilling the relation 
       θ G &lt;tan −1 (0.1 /n )   (9)
 
     wherein n is the number of teeth of the respective gear. An even tighter low-backlash gear unit may be obtained provided that the majority, and preferably all of the gears  34 ,  36 ,  38 ,  40 ,  42 ,  44  of the gear unit  24  have an angular gear θ G  play fulfilling the relation 
       θ G &lt;tan −1 (0.07 /n )   (10).
 
     An axial play allowing a pair of shafts S 1 , S 2 , engaging via helical gears G 1 , G 2 , to move relative to each other results in an angular play between the shafts, since the relative axial translation of the shafts S 1 , S 2  will make the gears G 1 , G 2  turn in their helical engagement. Hence, a reduced axial movement of the shafts in a gear unit also directly results in a reduced angular play between the shafts. 
     For the pair of shafts S 1 , S 2 , the same rule of thumb may be applicable provided that the gears G 1 , G 2  are axially fixed to their respective shaft S 1 , S 2 ; the angular play θ S1  of the shaft S 1  relative to the shaft S 2  may be obtained through the relation 
       θ S1 =tan −1 ( k/n   G1 )   (11)
 
     wherein the gear G 1  has n G1  teeth, the number k again being determinative of the angular play. 
     For a gear unit  24  designed for at least partly balancing axial thrusts, and hence reducing axial movement, according to the guidelines disclosed herein, the angular shaft play θ S  of each shaft  26 ,  28 ,  30 ,  32 , as connected to another shaft via mating gears, preferably corresponds to a number k&lt;0.11, more preferably to a number k&lt;0.08, and even more preferably to a number k&lt;0.07. Thereby, the total angular shaft play between the input and output shafts  26 ,  28  will be low. 
     Although possible in theory, it is in practice, due to e.g. friction, oil viscosity, production tolerances, wear etc., impossible to perfectly cancel out the axial forces F 1 , F 2  to exactly 100%. Therefore it is preferred that both output and input shafts  26 ,  28 , as well as their respective gears  34 ,  44 , be axially fixed relative to the housing  45 , e.g. by means of a thrust bearing arrangement. Thereby, they will not translate axially, as the load conditions change, while the gear unit  24  is operated. This is of particular value in a gear unit for the varying load conditions typical of a mobile vehicle gear unit  24 , since significant axial translation may cause shafts or gears to reach an end position in the gear unit housing  45 , resulting in damage to the gear unit  24 . Axial translation of the output or input shafts  26 ,  28  may also damage any upstream or downstream equipment such as the motor  16  or the drive shaft  20 . 
     Even though not necessary, the output and input shafts  26 ,  28  illustrated in the examples hereinbefore are also radially fixed relative to the gear unit housing  45 . Thereby, they will not translate radially due to changing load conditions while the mobile vehicle gear unit  24  is operated. 
     In order to at least partly balance axial forces within each layshaft  30 ,  32 , the respective driven and drive gears  38 ,  42 ,  36 ,  40  of each layshaft  30 ,  32  are axially fixed relative to each other. By way of example, the drive gear  36  of the first layshaft  30  is axially fixed relative to the driven gear  38  of the same layshaft  30 . Also the layshafts  30 ,  32  may be axially fixed relative to the gear unit housing  45 . The layshafts  30 ,  32  may also be preloaded between respective pairs of axial preload bearings (not shown) in a manner similar to the preloading of the output and input shafts  26 ,  28 . 
     The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims. 
     For example, it will be appreciated that features of the different embodiments disclosed hereinbefore may be combined, so as to form still further embodiments. By way of example, the expressions (1)-(6) disclosed with reference to  FIG. 4  represent preferred relations between transmission ratio, gear pitch diameters, and respective helix angles valid for all embodiments. 
     Moreover, depending on the desired transmission ratio, the gear unit may be provided with any number of layshafts connected in series between the input shaft and the output shaft, for example a single layshaft or three layshafts. In order to obtain the axial thrust balance described hereinbefore, the respective hands, pitch diameters and helix angles of the driven gear of the output shaft and the drive gear of the input shaft should be selected accordingly, as has been described hereinbefore. 
     The gear unit may also be provided with any number of layshaft arrangements connected in parallel between the input and output shafts. The mathematical expressions and conditions above relating to axial balance are still valid, if the parallel layshaft arrangements are similar with respect to helix angles and gear radii. For non-similar parallel layshaft arrangements, the expressions (1)-(6) may need to be adjusted accordingly, as will be appreciated by those skilled in the art. 
     It is not necessary that the mobile vehicle gear unit  24  be placed in a housing so as to form a separate gearbox; alternatively, the gear unit  24  may be built into the motor  16 , into a common structure in which shafts may be journalled, and to which a main thrust bearing support  50  may be attached. 
     The gear unit  24  may form a part of a larger gear unit or system; i.e., the gear unit  24  may be combined with other gearing, connected to the input and/or output shafts  26 ,  28 , so as to form a larger gear system. By way of example, the gear unit  24  may be connected to a planetary gear, which, together with the gear unit  24 , forms a gear system having a total transmission ratio different from that of the gear unit  24  alone. 
     It has been described above how the mobile vehicle gear unit  24  can be used for providing a transmission ratio within a vehicle  10 . However, the field of application for a gear unit according to the invention is not limited to vehicles  10 , such as cars, busses and lorries; by way of example, mobile, axial thrust-balanced gear units may as well be used in other mobile applications, e.g. ships for sea-transportation, for example passenger transportation ships and cargo-ships, in which case the gear unit may be installed for stepping down the rpm of a diesel engine to a suitable rpm for the propeller or other suitable propelling device. Another example of a mobile vehicle application is aircraft, in which the mobile, axial thrust-balanced gear unit may be used for stepping down the rpm of the turbine or engine driving the aircraft. 
     The terms “helical gear” and “helix angle” are to be interpreted broadly, so as to include gears having teeth that are curved, such as spiral gears, but as a whole follow a generally helical path, such that those gears function in a manner equivalent to helical gears.