Abstract:
A gear transmission is provided including at least a first and a second shaft with substantially fixed and substantially parallel axes of rotation, a transmission housing with a first and a second wall that extends perpendicular to the axes of rotation, and a plurality of gearwheels that are rotationally fixed or rotationally fixable to the shafts. The shafts include at least one input shaft and at least one output shaft, the shafts being suspended by bearings that are seated in the first and second walls. The first wall is connected to the second wall with at least one rod in order to reduce deflections in operation of the first and second walls.

Description:
BACKGROUND AND SUMMARY 
       [0001]    The present invention relates to gear transmissions, and more particularly to a system for decreased deflection of transmission housing walls that are subjected to bearing loads. 
         [0002]    Fix-axes gear transmissions are used in many fields of technology, for instance in vehicles in manually shifted or automated form. They comprise a number of gearwheels that are located on a number of shafts. The shafts are supported by bearings that are seated in a transmission housing. The rotational axes of the shafts are thereby substantially fixed, hence the designation fix-axes gear transmissions. When torque is transferred from one gearwheel to another, significant gear mesh forces arise. These gear mesh forces will create reaction forces in the bearings. Thereby, in operation the walls of the transmission housings will be subjected to forces that will tend to deflect the walls. For proper function and life of the gearwheels and bearings, it is important that these deflections are limited. 
         [0003]    In order to reduce weight, there is a trend towards using light alloys rather than iron based alloys for the transmission housings. Due to lower inherent stiffness of conventional light alloys, the deflections of the housing walls will increase. Furthermore, cost-efficient casting methods for light alloys do not permit as large wall thickness as is common in cast iron housings. That will further increase the deflections in light-alloy housings. Finally, the significantly larger thermal expansion of light alloys compared to steel will cause increased axial clearance in the bearings at operating temperature, which will give reduced bearing life. 
         [0004]    There are some known ways to reduce the increased deflections of the walls of transmission housings made of light alloys. Firstly, reinforcing ribs will increase the stiffness of the walls while maintaining a small wall thickness. This can be seen in DE-10027375 and DE-10316321. That will, however, in general not give stiffness equal to that of a wall of full thickness unless the ribs are very large, which implies increased length of the transmission. Another way is to have a curved or bell-like shape of the wall instead of a more conventional flat wall. That will, unfortunately, also result in increased transmission length. 
         [0005]    It is desirable to decrease deflection in the walls of a transmission housing. 
         [0006]    So, there is a need for a way to increase the stiffness of fix-axes gear transmission housing walls but without the disadvantages regarding length, weight, and casting ability of prior art. According to an aspect of the present invention, a design is provided with a number of substantially straight rods connected between the housing walls where the bearings that support the shafts are seated. Thereby, the axial deflections of these housing walls will be reduced significantly. 
         [0007]    Moreover, the corresponding misalignment of the bearing seats will be reduced, too. That will increase the bearing life. 
         [0008]    In a preferred embodiment, the housing is made of light alloy. The rods will then compensate for the lower inherent stiffness (modulus of elasticity) of the light alloy. 
         [0009]    In a further preferred embodiment, the rods are made of steel or any other material with lower thermal expansion than the housing material. Then, it is possible to counteract the thermal expansion, from ambient to operating temperature, of the housing relative to the shafts. 
         [0010]    In yet a further preferred embodiment, the rods are preloaded at the assembly. When made of steel (or any other material that has a lower thermal expansion than the material of the housings), the rods will then also at temperatures lower than room temperature be able to reduce the axial deflection of the housing walls. 
         [0011]    In an additional preferred embodiment, rods are located radially close to the intersections, when seen in axial direction, of the diametrically largest rotating parts of the shafts. Thereby, the rods will be close to the parts of the housing walls that would have the largest deflections if there were no rods. That will give a large reduction of the deflections. 
         [0012]    In another preferred embodiment of the invention, the rods are screws that have a head in one end and threads in the other end. That will facilitate the assembly, especially if the rods are to be preloaded. Furthermore, if one of the housing walls to be connected by the rods is designed to have lubricating liquid on one side only, the threaded end of the screws can be mounted in a tapered boss in that wall. Thereby, possible risks for lubricating liquid leakage can be eliminated. 
         [0013]    In still another preferred embodiment, the transmission is at least a part of a vehicle transmission for a heavy road vehicle, such as a truck or a bus. In such a transmission, the loads are very high. Thus, rods according to the invention will be of great advantage, especially if the transmission is the main section of a compound (or range) type vehicle transmission, for instance as shown in FIGS. 1, 3 and 5 in EP-1476681. There, rods as long screws can be mounted with the head against the housing wall between the main section and the compound (or range) section. There will be lubricating liquid on both sides of that wall. Thus, a possible lubricating liquid leakage around the screw heads would be irrelevant. The other end of the screws can then be mounted in tapered bosses in the opposite housing wall that faces the clutch. This will prevent leakage of lubricating liquid. 
         [0014]    Finally, in a preferred embodiment, at least one rod is mounted inside a hollow shaft. Thereby, the rod will act very close to the bearings that support the hollow shaft. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The present invention will be described in greater detail below with reference to accompanying drawings which, for the purpose of exemplification, shows further preferred embodiments of the invention and also the technical background. 
           [0016]      FIG. 1  shows a schematic longitudinal section of a typical fix-axes gear transmission according to prior art. 
           [0017]      FIG. 2  shows forces in a typical conventional taper roller bearing according to prior art. 
           [0018]      FIG. 3  shows a taper roller bearing seated in a transmission housing wall with small wall thickness and reinforcing ribs according to prior art. 
           [0019]      FIG. 4  shows deflections of housing walls and shafts of the transmission of  FIG. 1 . 
           [0020]      FIG. 5  shows, according to the invention, a rod that connects the housing walls that carry bearings in a modified variant of the transmission of  FIG. 1 . 
           [0021]      FIG. 6  shows an axial view of a fix-axes gear transmission with rods according to the invention located close to the intersections of the tip circles of the diametrically largest rotating parts of the shafts. 
           [0022]      FIG. 7  shows a rod according to the invention embodied as a long screw. 
           [0023]      FIG. 8  shows a rod according to the invention mounted inside a hollow shaft. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIG. 1  shows a simplified longitudinal section of a fix-axes gear transmission  101  with a transmission housing  102 , input shaft  103  and output shaft  104 . The transmission housing  102  is here shown as a one-piece unit, but it could also have been composed of two or more housing parts. The input shaft  103  is supported by an input shaft bearing  105 , which is seated in a front wall  106  of the transmission housing  102 , and by a flywheel pilot bearing  107 , which is seated in a prime mover output shaft (not shown). Similarly, the output shaft  104  is supported by an output shaft bearing  108  that is seated in a rear wall  109  of the transmission housing  102 . The input shaft  103  is substantially coaxial with the output shaft  104 . A second point of support for the output shaft  104  is a pilot bearing  110  that is carried by the input shaft  103 . Furthermore, there is a countershaft  111  located radially apart from the input shaft  103  and the output shaft  104 . The countershaft  111  is supported by a front countershaft bearing  112 , which is seated in the front wall  106 , and a rear countershaft bearing  113 , which is seated in the rear wall  109 . An input shaft primary gearwheel  114  is fixedly connected to the input shaft  103 . The input shaft primary gearwheel  114  is in mesh with a countershaft primary gearwheel  115  that is fixedly connected to the countershaft  111 . A countershaft secondary gearwheel  116  is also fixedly connected to the countershaft  111 . The countershaft secondary gearwheel  116  is in mesh with a floating gearwheel  117  that is rotationally supported on the output shaft  104 . A tooth clutch  118  can rotationally connect the output shaft  104  to either the floating gearwheel  117  or to the input shaft  103  when an engaging sleeve  119  is moved to the right or to the left, respectively. Thereby, two speed ratios can be obtained. When the floating gearwheel  117  is rotationally connected to the output shaft  104 , a reduction gear is obtained where the output shaft  104  rotates slower than the input shaft  103 . When the input shaft  103  and the output shaft  104  are rotationally connected, their speeds will be equal and a direct gear is obtained. 
         [0025]    When torque is being transferred in the reduction gear, gear mesh forces will occur between the meshing teeth of the gearwheels  114 ,  115 ,  116  and  117 . In the bearings, these gear mesh forces will cause reaction forces that will be carried by the walls of the transmission housing  102 . In fix-axes gear transmissions, taper roller bearings are often used to support the shafts. This type of bearing offers high load capacity and long life to a competitive cost. 
         [0026]      FIG. 2  shows a taper roller bearing  230  with inner ring  23 Oi, rollers  23 Or and outer ring  23 Oo. The taper roller bearing  230  is seated in a transmission housing wall  231  and supports a shaft  232 . A radial force  233  is transferred by the taper roller bearing  230  from the shaft  232  to the transmission housing wall  231 . In a taper roller bearing, a radial force always corresponds to an axial force. Hence, the radial force  233  will be transferred between the rollers  23 Or and the outer ring  230   o  as a normal load  234   n  that has a radial component  234   r  and an axial component  234   a.    
         [0027]    In order to reduce noise, helical gearing is normally used. Thereby, axial components of the gear mesh forces will arise. When using taper roller bearings in a fix-axes gear transmission like the one in  FIG. 1 , it is usually necessary to spare the pilot bearing  110  from high axial forces. This is solved by selecting hands of helix in such a way that the axial gear mesh force components that act on the gearwheels on the input shaft  103  and the output shaft  104  are not directed towards the pilot bearing  110 . That is indicated by the primary axial gear mesh component  121  and the secondary axial gear mesh component  122 . Thereby, there will be no external axial forces in the pilot bearing  110 . On the other hand, there will be additional axial forces acting on the bearings  105  and  108 . That will increase the housing wall deflections. 
         [0028]    Thus, with taper roller bearings, gear mesh forces will create axial forces that act on the transmission housing walls in which the bearings are seated. These axial forces will deflect the housing walls. The stiffer the housing walls are, the lower that deflection will be. The transmission housing wall  231  in  FIG. 2  has a thickness that is about as large as the width of the bearing  230 . 
         [0029]      FIG. 3  shows a similar bearing  330  that is seated in a die-cast transmission housing wall  331 . Die-casting processes do in general not allow very large wall thickness. So, the transmission housing wall  331  has been designed with a thin part  331   t  and reinforcing ribs  331   r . Still, the transmission housing wall  331  will not be as stiff as the corresponding transmission housing wall  231  in  FIG. 2 , unless the reinforcing ribs  331   r  are significantly higher than the width of the bearing  330 . An alternative solution would be to stiffen the transmission housing wall  331  by making it curved or dome-shaped. Both these solutions would, however, cause an undesired increase in length of the transmission. Furthermore, light alloys, such as aluminium, have a lower stiffness, modulus of elasticity, than cast iron. Hence, a die-cast light-alloy housing wall ( 331 ) will be less stiff than a cast-iron solid wall ( 231 ). 
         [0030]      FIG. 4  shows in a simplified and exaggerated way the deflections in operation of the shafts and transmission housing walls of the fix-axes gear transmission of  FIG. 1 . The thick dash-dotted lines indicate the deflections of the shafts. For instance, for the front countershaft bearing  112  it can be seen that the deflections of both the countershaft  111  and the front wall  106  will contribute to the misalignment. The total misalignment of the front countershaft bearing  112  is the sum of the countershaft misalignment  441  and front wall misalignment  442 . Moreover, the axial deflections of the rear wall  109  and the front wall  106  will give increased axial clearance for at least some of the bearings  105 ,  108 ,  110 ,  112  and  113 . Misalignment and axial clearance both have a negative impact on the life of the bearings. 
         [0031]      FIG. 5  shows a longitudinal section of a modified variant according to the invention of the fix-axes gear transmission  101 . There are two additional rods  551   a  and  551   b  (not visible) that connect the front wall  506  and the rear wall  509  of the transmission housing  502 . A straight rod is very stiff in axial direction compared to a conventional transmission housing. Thus, the axial deflections of the front wall  506  and the rear wall  509  can be counteracted very efficiently by the rods  551   a  and  551   b . Then, if the rods  551   a  and  551   b  are made of a material with a lower thermal expansion coefficient than the material of the transmission housing  502 , increased axial clearance in the bearings at operating temperature will be counteracted, too. 
         [0032]    Furthermore, if these rods are preloaded at the assembly, that preload will be reduced at low temperatures. That will spare the bearings at a startup at sub-zero temperatures. 
         [0033]    Ideally, for the best counteraction of the housing deflections, the rods  551   a  and  551   b  should be located right between the shafts of the transmission. That would, however, lead to major interference with gearwheels and other rotating parts. Instead, the rods can be located as close as possible to that idealized position.  FIG. 6  shows an axial view of the fixed axes gear transmission  501  of  FIG. 5 . The rods  551   a  and  551   b  are located close to the intersections  552   a  and  552   b  of the tip circle  515   c  of the largest rotating part of the countershaft  511  and the tip circle  519   c  of the largest rotating part on the coaxial input shaft  503  and output shaft  504 . Preferably, the distances  553  and  554  from the centre of a rod to these tip circles should both be less than the diameter of the rod. If the rod would not be of circular cross section, the largest extension in lateral direction could be used instead of the diameter. 
         [0034]    In heavy road vehicles, such as heavy trucks and buses, transmissions of compound type are often used. In a compound transmission, a fix-axis gear transmission, the main section, is connected in series with a compound section. The gears in the main section can be combined with the gears in the compound section, giving a large number of gears in total. Some examples are shown in EP-1476681. In general, the main and compound sections are integrated in such a way that the rear housing wall (corresponding to  109  in  FIG. 1 ) of the main section is also the front housing wall of the compound section.  FIG. 7  shows a simplified longitudinal section of a compound transmission  770  that is composed of a fix-axes gear transmission  701  as main section and a compound section  771 . For clarity, the rotating parts, such as shafts, clutches and gearwheels, have been left out. The main section  701  has a main housing  702   a  and a clutch housing  702   b . The compound section  771  has a compound housing  772 . There will be lubricating liquid (preferably oil) on both sides of the main housing rear wall  709 . A possible lubricating liquid leakage through that wall will be harmless. On the other hand, the wall  706  of the clutch housing  702   b  has lubricating liquid on the side that faces the internals of the main section  701 . A possible lubricating liquid leakage to the other side, where a dry plate clutch (not shown) is located, would be disastrous. Therefore, the rod  751  according to the invention is embodied as a long screw with threads  751 . t  in one end and a head  751   h  in the other end. The threaded end  751   t  is mounted in a boss  706   b  with mating threads in the clutch housing wall  706 . The head  751   h  is seated against the main housing wall  709  in the compound section  771 . With the screw rod  751  and the boss  706   b , oil leakage to the dry plate clutch is prevented. Furthermore, the assembly is facilitated and it would be straightforward to apply a preload. 
         [0035]      FIG. 8  shows a longitudinal section of a modified variant according to the invention of the fix-axes gear transmission  101 . There is a rod  851  located inside a hollow countershaft  811 . With that location, the deflections of the housing walls  806  and  809  can be counteracted in a very efficient way. A rod end  851   e  could be formed as a cover and serve as a cover for a countershaft bearing  812 . 
         [0036]    In an alternative embodiment the input shaft is not coaxial with the output shaft. Thus, the output shaft could be arranged in parallel to both the input shaft and the countershaft. 
         [0037]    The invention should not be deemed to be limited to the embodiments described above, but rather a number of further variants and modifications are conceivable within the scope of the following patent claims.