Abstract:
A constant mesh gearbox has five parallel rotational axes. The input and output axes are not aligned. Some torque paths are via a sleeve rotatable on a shaft on one of said axes. Up to eight forward speeds are disclosed, with two very low ratios suitable for non-highway travel. The transmission is suitable for two and four wheel drive.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a gear transmission and particularly, but not exclusively, to a transmission having a plurality of selectable speed ratios provided by respective pairs of gear wheels. Aspects of the invention relate to a gearbox, to a powertrain and to a vehicle. 
       BACKGROUND 
       [0002]    Gear transmissions are typically used in motor vehicles, and multiple speed ratios allow the engine characteristics to be best matched to the required gradient launch, acceleration, top speed, fuel economy and other well understood factors. Gear pairs are typically in constant mesh, and are engaged and disengaged by synchromesh mechanisms which incorporate dog clutches. Manual selection of the required speed ratio is common, but some gear transmission variants have automated or assisted selection, or have fully automatic speed ratio selection. 
         [0003]    In order to achieve good acceleration and fuel economy, the number of speed ratios have increased in recent years, and for some vehicles as many as eight or more forward speed ratios may be provided. Inevitably, these additional speed ratios tend to increase the size of the transmission, which is problematic for vehicle designers where the available space envelope is restricted. A particular difficulty with in-line transmission (so called North-South alignment) is of ingress into the passenger compartment. 
         [0004]    Another objective of vehicle designers is to design components for a range of vehicles, so a substantially common base component is modified to suit the particular attributes which are required of each vehicle in the range. In one extreme example a common component, such as a vehicle transmission, may be required to serve in both a sports car and a vehicle having off-road capability. The base component may be simple and have features added thereto, or it may be complex and have features deleted. 
       SUMMARY OF THE INVENTION 
       [0005]    According to an aspect of the present invention there is provided a constant mesh gearbox having a plurality of individually selectable forward speed ratios, and comprising a casing ( 20 ) defining an array of parallel rotational axes ( 31 - 35 ), said axes including an input axis ( 31 ), an output axis ( 35 ) and a plurality of intermediate axes ( 32 - 34 ), and gear pairs for driving between members rotatable on the input axis, the output axis and said intermediate axes, wherein a first shaft ( 12 ) is rotatable on one of said intermediate axes, a second shaft ( 14 ) is rotatable on another of said intermediate axes, said first and second shaft are directly connected by a first gear wheel pair ( 60 ,  80 ), a sleeve ( 74 ) is rotatable on said second shaft ( 14 ), and said sleeve and said first shaft are directly connected by a second gear wheel pair ( 75 ,  76 ), and 
         [0006]    wherein said sleeve ( 74 ) supports a first gear wheel ( 77 ) for rotation thereon, said first gear wheel ( 77 ) being selectively connectable for rotation with said sleeve, and being in driving engagement with an input shaft ( 11 ) on said input axis ( 31 ). 
         [0007]    In an alternative there may be provided a constant mesh gearbox having a plurality of individually selectable speed ratios, and comprising a casing defining five parallel rotational axes, said axes being respectively an input axis, an output axis and three intermediate axes, and gear pairs for driving between members rotatable on the input axis, the output axis and the intermediate axes. 
         [0008]    The gearbox of the invention may be characterized as being wide and short. A feature of embodiments of the invention is that the input and output axes are not aligned which, in the case of an in-line transmission allows an output axis lower than the crankshaft centre line of the engine; this in turn allows a low floor in the passenger compartment with a comparatively negligible transmission tunnel. 
         [0009]    Embodiments of the invention provide six, seven and eight sequential speed ratios in one direction, and one or two sequential speed ratios in the other direction of rotation. The forward speed ratios are at acceptable steps, in the range 60-75% of the preceding speed ratio, and thus give an appropriate ‘feel’ to the vehicle driver, in use. 
         [0010]    A feature of an embodiment of the inventive transmission is that the six highest speed ratios comprise respective pairs of gear wheels which may be adjusted in diameter to obtain a desired variation in the ratio step from one speed to another. 
         [0011]    In use, one or two speed ratios may be omitted if not required in the vehicle for which the transmission is intended; thus may allow omission of gear wheels and gear selection components if not required for other speed ratios. 
         [0012]    In embodiments of the invention one or two speed ratios may be described as low, and intended for use at low vehicle speeds. By selecting a suitable final drive ratio in a vehicle axle, these low ratios may be suitable for an off-road vehicle. 
         [0013]    An embodiment of the invention provides a transmission in which the higher ratios generally have fewer gear wheels in the gear transmission train than the lower ratios, and thus less risk of gear noise at high vehicle speeds. Thus in an eight forward speed embodiment, the highest six ratios have fewer gear wheels in the transmission train than the lowest two ratios. 
         [0014]    A feature of an embodiment of the invention is a sleeve rotatable on an intermediate shaft. 
         [0015]    In one aspect there is provided a constant mesh gearbox having a plurality of individually selectable forward speed ratios, and comprising a casing defining an array of parallel rotational axes, said axes including an input axis, an output axis and a plurality of intermediate axes, and gear pairs for driving between members rotatable on the input axis, the output axis and said intermediate axes, wherein a first shaft is rotatable on one of said intermediate axes, a second shaft is rotatable on another of said intermediate axes, said first and second shaft are directly connected by a first gear wheel pair, a sleeve is rotatable on said second shaft, and said sleeve and said first shaft are directly connected by a second gear wheel pair. 
         [0016]    The sleeve allows additional gearing options and torque flows within a short transmission. 
         [0017]    Aspects of the gear wheel arrangements are enumerated in the claims appended hereto. 
         [0018]    In an embodiment the sleeve is supported by rolling element bearings adjacent the ends thereof, one of said bearings engaging said casing, and the other of said bearings engaging said second shaft. The other of said bearings is in one embodiment disposed between the second shaft and the second gear wheel, the second gear wheel being journalled on the sleeve. 
         [0019]    According to embodiments of the invention the input axis is not aligned with the output axis, thus permitting a relatively low level output (propeller) shaft for a rear wheel drive vehicle having a conventional north/south engine installation. Such a low level output shaft ensures minimum intrusion into the passenger space of the vehicle. In a front wheel drive vehicle with east/west engine installation, a low level output axis is convenient for drive to the front wheels whilst avoiding excessive angularity of drive shafts. 
         [0020]    A differential gear on the output axis may be provided to give drive to opposite vehicle axles, typically front and rear axles. In one embodiment of the invention drive to one of said axles is on a sixth parallel axis radially outboard of said output axis. Means for driving between the output axis and the sixth axis may include gear wheels or a chain/belt and sprockets. This arrangement provides a low forward drive axis for the front axle of a vehicle, and facilitates passage of the usual propeller shaft past a front mounted engine. 
         [0021]    There may be provided a transmission according to the invention having one or more very low ratios incorporated in a single transmission casing. By very low ratios we mean overall ratios lower than that encountered and vehicle designed solely for highway use, and typically as low as 20:1 and/or with a spread of forward ratios exceeding 15. Such ratios are typically provided by a selectable transfer box of a four wheel drive off-road vehicle. 
         [0022]    Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. Features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:— 
           [0024]      FIG. 1  is a schematic elevation of one axial end of a transmission according to an embodiment of the invention; 
           [0025]      FIG. 2  illustrates a variant of the transmission of  FIG. 1  from the side and one end; 
           [0026]      FIG. 3  is a developed section through the transmission of  FIG. 2 , showing all of the shaft axes in the same plane; 
           [0027]      FIG. 4  is an enlarged view of the upper part of  FIG. 3 ; 
           [0028]      FIG. 5  is an enlarged view of the lower part of  FIG. 3 ; 
           [0029]      FIG. 6  illustrates an example gear shift pattern for a manual variant of the transmission of  FIG. 2 ; and 
           [0030]      FIG. 7  is a schematic illustration of a north/south installation of a transmission according to an embodiment of the invention in a four wheel drive vehicle. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    A transmission according to an embodiment of the invention is illustrated in  FIGS. 1-2 , and comprises five shafts having parallel axes supported for rotation in a transmission casing (not shown). The shafts are identified as input shaft  11 , low speed layshaft  12 , high speed layshaft  13 , reverse layshaft  14  and primary output shaft  15 . A secondary output shaft  16  may optionally be provided on a sixth parallel axis, in particular where drive to two vehicle axles is required. Although termed ‘primary’ and ‘secondary’, the output shafts  15 ,  16  may be alone or in combination, and thus a drive connection from either may be omitted if a single driven axis is sufficient. Thus the transmission in a vehicle may provide front wheel drive, rear wheel drive and all wheel drive, in different configurations. 
         [0032]    By ‘high’ and ‘low’ speed layshafts we mean that the shafts are associated with respective higher and lower speed ratios, not that the shafts necessarily have high and low speeds as such. 
         [0033]    The shafts  11 - 15  support pairs of gear wheels in constant mesh, which may be engaged with and disengaged from respective shafts so as to provide drive through the transmission. 
         [0034]    The number and form of teeth on each gear wheel is selected according to well understood parameters and forms no part of this invention as such. The relative diameter of respective pairs of gear wheels is selected to give appropriate speed ratios, and in particular acceptable steps between adjacent ratios, as will become apparent from the following description. Roller and ball bearings are provided to support the shafts within the transmission casing in accordance with well understood principles, and taking into account the torque to be transmitted, thrust loads, bearing life and other conventional factors. 
         [0035]    As illustrated in  FIG. 1  drive between primary and secondary output shafts  15 ,  16  is by chain or belt  17 , in order to preserve a desired rotational direction. Arrow  18  represents the direction of input torque, whereas arrows  19  represent that output torque may be in either rotational direction (.e. both forward and reverse in a vehicle). 
         [0036]    A feature of the transmission, which will be readily apparent from  FIG. 1 , is that the layshafts  12 ,  13  and the reverse shaft  14  can directly drive the primary output shaft  15 . 
         [0037]    The disposition of shafts within the transmission casing is as illustrated in  FIGS. 1 and 2 . However for the purposes of further explanation,  FIG. 3  shows a developed longitudinal section, with all shafts in a common plane. A casing is represented by dotted line  20  in order to show support for respective shaft bearings. 
         [0038]    The main features of the transmission of  FIG. 3  are as follows: 
         [0039]    selectable gear pairs are provided between the shafts in four planes; 
         [0040]    a fifth plane is defined at the left side (as viewed) for gear wheels which couple the layshafts  12  and  14 ; and a sixth plane is defined at the right side (as viewed) for gear wheels which directly drive the output shaft  15 ; 
         [0041]    five pairs of double acting gear selectors  21 - 25  are provided, each consisting of dog clutches which typically incorporate conventional synchromesh couplings; 
         [0042]    an all wheel drive variant is illustrated—thus both output shafts  15 ,  16  are included, and the primary output shaft  15  includes a differential gear  26  within the transmission casing (the centre differential of a vehicle); 
         [0043]    the low speed layshaft  12  defines a torque path for the low speed ratios 1-4; 
         [0044]    the high speed lay shaft  13  defines a torque path for the high speed ratios 5-6; 
         [0045]    a somewhat conventional reverse ratio is provided via the reverse shaft  14 , in order to reverse the direction of rotation of the output shaft  15 ; 
         [0046]    two additional very low speed ratios, and one additional reverse ratio are provided, as will become apparent; 
         [0047]    the shafts are arranged on parallel axes  31 - 36 . It should be noted that axes  35 ,  36  are not co-axial in practice notwithstanding the depiction of  FIG. 3 . 
         [0048]    With reference to  FIG. 4 , the input shaft  11  protrudes through the transmission casing at the input side  27 , and comprises a male spline  41  for driving connection with the female spline of a driven plate (not shown) of friction clutch; this arrangement is conventional. Rolling element bearings  42 ,  43  support the input shaft for rotation in the casing  20 . 
         [0049]    Provided on the shaft for rotation therewith are five gear wheels  44 - 48 . Gear wheels  44 - 47  are respectively associated with forward speeds 2, 1, 4 and 3. As illustrated gear wheel  44  is solid with shaft  11 , whereas gear wheels  45 - 48  slide onto shaft  11  via splines or key ways and are secured in any suitable manner, for example by circlips or the like. Gear wheels  44 - 48  are connected for rotation with the input shaft at all times. 
         [0050]    The low speed layshaft  12  is also supported in the casing  20  by rolling element bearings  52 ,  53  at the ends thereof, and includes the mating gears  54 - 57  for forward speed ratios 2, 1, 4, 3. 
         [0051]    Gear wheels  54 ,  55  and  57  are each double gears, and consist of side by side gears of different diameter connected for rotation in common. Thus gear wheel  54  has associated gear wheel  58 ; gear wheel  55  has associate gear wheel  59 ; and gear wheel  57  has associated gear wheel  60 . Each double gear may be integrally manufactured from a single blank, or may be an assembly. 
         [0052]    Each of the gear wheels  54 - 57  is freely rotatable in the low speed layshaft  12 , but connectable for rotation herewith by a respective double acting synchromesh selector  21 ,  22 . As is well understood, the synchromesh system provides a speed synchronizer and dog clutch for coupling one gear wheel at a time to the respective shaft by right or left movement (as viewed) from an unconnected central position. The selectors  21 ,  22  are positioned respectively between gear wheels  54  and  55  and between gear wheels  56  and  57 . Although close together, gear wheels  48  and  60  are not in mesh. 
         [0053]    The higher speed layshaft  13  is supported in the casing  20  by rolling element bearings  62 ,  63  and has independently rotatable gear wheels  64 ,  65  engaged respectively with gear wheels  46  and  48  of the input shaft  11 . Gear pairs  46 ,  64  and  48 ,  65  respectively provide forward speed ratios  6  and  5 , and a synchromesh selector  23  is provided to connect one or other gear pair for rotation with the layshaft  13  on demand. 
         [0054]    Both the low speed layshaft  12  and high speed layshaft  13  include output gear wheels  66 ,  67  coupled for respective rotation herewith, and which engage with a final drive gear wheel  69  of the output shaft  15 . 
         [0055]    The reverse layshaft  14  is supported in the casing by rolling element bearings  72 ,  73 . The reverse layshaft  14  also includes an output gear wheel  68  fast therewith and engaged for rotation with the final drive gear wheel  69 . 
         [0056]    A rotatable sleeve or muff  74  is provided on the reverse layshaft, and is directly connected by transfer gear wheel pair  75 ,  76  to the low speed layshaft  12 , as illustrated. This transfer pair is at the input side  27 , whereas the output gears are at the output side  28  ( FIG. 3 ). 
         [0057]    The muff  74  is coaxial with the axis of the reverse layshaft  14  and supports gear wheels  77 ,  78  for rotation thereon. These gear wheels  77 ,  78  are respectively in mesh with gear wheels  58 ,  59  of the low speed layshaft  12 . A double acting synchromesh selector  24  connects the gear wheels  77 ,  78  for rotation with the muff  74  on demand. 
         [0058]    The innermost gear wheel  78  is also connectable via one side of synchromesh selector  25  to the reverse layshaft  14 . Rolling element bearings  79 ,  81  are provided for the muff  74  as illustrated. 
         [0059]    The other side of the selector  25  is operable to couple a freely rotatable gear wheel  80  of the reverse layshaft  14  to gear wheel  60  of the low speed layshaft  12 . 
         [0060]    The output shaft axis  35  defines the rotational axis of the final drive gear wheel  69 . In the illustrated embodiment the gear wheel  69  is connected to the casing  91  of a conventional differential gear  90  having oppositely directed outputs represented by arrows  92 ,  93 . 
         [0061]    In use, suitable drive shafts engaged within the differential gear provide drive to respective forward and rearward vehicle axles, via respective axle mounted differential gears (not shown). Such an arrangement is conventional. 
         [0062]    In the illustrated embodiment, the forward output  92  is connected to a sprocket  94  which drives another sprocket  95  on the secondary output axis  36  of the secondary output shaft  16  ( FIG. 1 ). 
         [0063]    The sprocket  95  is supported in the casing by rolling element bearings  97 ,  98 . 
         [0064]      FIG. 2  also illustrates suitable universal drive joints  101 ,  102  for coupling the final drive outputs to propeller shafts  103 ,  104  associated respectively with front and rear drive axles of a vehicle. 
         [0065]    In the event that only rear wheel drive is required, the differential  90  may be omitted along with the components associated with the front drive output  93 ; the final drive gear  69  is coupled directly to a suitable output shaft for connection to the universal joint  102 . 
         [0066]    In the event that only front wheel drive is required, the differential  90  may be omitted along with components associated with the rear drive output  92 ; the final drive gear  69  is coupled directly to sprocket  94  in any suitable manner, or may directly drive to the universal joint  101  on the same axis. 
         [0067]    The transmission of  FIGS. 1-5  provides the following internal speed ratios. In the following description all selectors  21 - 25  are assumed to be in the mid-position (not driving) unless stated otherwise; in this condition the transmission is in neutral. 
         [0068]    Six sequential forward speed ratios are provided, as follows:— 
         [0069]    1 st    
         [0070]    Selector  21  moved rightward. Drive via input shaft  11 ; gear pair  45 ,  55 ; layshaft  12 ; output gears  67 ,  69 . 
         [0071]    2 nd    
         [0072]    Selector  21  moved leftward. Drive via input shaft  11 ; gear pair  44 ,  54 ; layshaft  12 ; output gears  67 ,  69 . 
         [0073]    3 rd    
         [0074]    Selector  22  moved rightward. Drive via input shaft  11 , gear pair  47 ,  57 ; layshaft  12 ; output gears  67 ,  69 . 
         [0075]    4 th    
         [0076]    Selector  22  moved leftward. Drive via input shaft  11 ; gear pair  46 ,  56 ; layshaft  12 ; output gears  67 ,  69 . 
         [0077]    5 th    
         [0078]    Selector  23  moved rightward. Drive via input shaft  11 ; gear pair  48 ,  65 ; layshaft  13 ; output gears  66 ,  69 . 
         [0079]    6 th    
         [0080]    Selector  23  moved leftward. Drive via input shaft  11 ; gear pair  46 ,  64 ; layshaft  13 ; output gears  66 ,  69 . 
         [0081]    It will be understood that individual ratios may be adjusted by changing the diameter of a respective gear pair, save that 4 th  and 6 th  use a common drive gear  46  and are thus inter-dependent. The relative position of axes  31 - 33  permits further adjustment or gear wheel diameter, and thus ratio. 
         [0082]    In one embodiment, the following ratios are provided: 
         [0083]      3 rd  7.13    
         [0084]    1 st —17.74 2 nd —10.82 4 th —5.08 5 th —3.75 6 th —2.78 
         [0085]    Two extra low ratios are provided as follows:— 
         [0086]    L1 Selector  24  moved rightward. Drive via input shaft  11 ; gear pairs  45 ,  55  and  59 ,  78 ; muff  74 ; gear pair  75 ,  76 ; layshaft  12 ; output gears  67 ,  69 . 
         [0087]    L2 Selector  24  moved leftward. Drive via input shaft  11 , gear pairs  44 ,  54 ;  58 ,  77 ; muff  74 ; gear pair  75 ,  76 ; layshaft  12 ; output gears  67 ,  69 . 
         [0088]    The extra low ratios may be adjusted by changing the diameter of the respective gear pair  45 ,  59  and  44 ,  58 , and by altering the relative position of reverse shaft axis  34 . In this embodiment the gear wheels on the low speed layshaft are idlers. 
         [0089]    In one embodiment, the following further sequential low ratios are provided: 
         [0090]    L1—43.83 L2—27.19 
         [0091]    Thus eight sequential forward speed ratios are provided. It should be noted that movement of the selectors  21 - 24  is compatible with a conventional shift pattern of the multiple ‘H’ type—thus movement of a gear lever in one direction corresponds to leftward selector movement (L2, 2 nd , 4 th , 6 th ) whereas movement in the opposite direction corresponds to rightward selector movement (L1, 1 st , 3 rd , 5 th ). 
         [0092]    A generally conventional reverse ratio is provided by shifting selector  25  rightwards. Drive is via input shaft  11 ; gear train  47 ,  60 ,  80 ; reverse layshaft  14 ; output gears  68 ,  69 . The additional gear in the train reverses rotation of the drive. 
         [0093]    A low reverse ratio is provided by shifting selector  25  leftwards. Drive is via input shaft  11 ; gear train  45 ,  59 ,  78 ; reverse layshaft  14 ; output gears  68 ,  69 . 
         [0094]    In one embodiment, the following sequential reverse ratios are provided: 
         [0095]    R1—35.79 R2—17.6 
         [0096]    The ratio of R2 is substantially the same as 1 st , and thus corresponds to the feel of a conventional manual transmission. This ratio may be adjusted by varying the relative size of gear wheels  60  and  80 . 
         [0097]    The ratio of R1 may be varied by selecting appropriate diameters of gear wheels  59  and  78 . 
         [0098]    The reverse ratios require opposite movement of the selector  25 , and are thus compatible with manual shift pattern of the ‘H’ type. In R2 ratio the gear wheel on the low speed layshaft is an idler. A suitable shift pattern is illustrated in  FIG. 6 . Reverse may be provided in any suitable plane, and is illustrated to the right of 5-6; it could alternatively be to the left of L1-L2. 
         [0099]    The transmission thus provides eight forward and two reverse ratios with an acceptable spread of forward ratios, with steps in the range 60-75%. 
         [0100]    It will be understood that a feature of this transmission is that the three layshafts  12 - 14  in driving connection with the output shaft  15  at all times. Thus drive via any of the three layshafts results in the other two layshafts being back driven. The ratios selected in the described embodiment ensure that the speed of individual gear wheels remains within acceptable design limits for conventional engine speeds, for example below 7000 rpm. 
         [0101]    Whilst the transmission of this invention can provide eight forward and two reverse speeds, it need not do so. For example if specified for road use only, a single reverse ratio (R2) may be sufficient, whereas for off-road use both R1 and R2 may be provided. R1 may be omitted by deleting gear wheels  59  and  78 , and making the selector  25  single acting. 
         [0102]    In the same way L1, or L1 and L2 may be omitted for a road use only application. L1 is omitted by deleting gear wheels  59  and  78  (assuming R1 is omitted and L2 retained). 
         [0103]    L2 is omitted by deleting gear wheels  58  and  77 . Since L1 is almost certainly not required if L2 is omitted, gear wheels  59 ,  78  and  75 ,  76  may also be deleted. In this configuration R1 is almost certainly not required, which results in deletion of the muff  74  along with selector  24 . 
         [0104]    It will of course be understood that a different naming of speed ratios might follow from deletion of 5 th  and 6 th  (as described above) whilst retaining L1 and L2. Thus the sequential range L1, L2, 1, 2, 3, 4 might be termed 1 st  to 6 th . In this configuration the high speed layshaft  13  and its associated selector  23 , gear pairs  46 ,  64  and  48 ,  65 , and output gear  66  could be deleted. 
         [0105]    Likewise a sequential 6 speed transmission could comprise L2, 1, 2, 3, 4, 5 and a seven speed transmission could comprise L1, L2, 1, 2, 3, 4, 5 or L2, 1, 2, 3, 4, 5, 6. Whilst not all of these combinations may at first sight be considered useful, they are all practicable in the event that packaging constraints will not allow fitting of the eight speed transmission casing envisaged. Thus omission of one or more forward and reverse ratios will permit the casing to more closely follow the remaining envelope of gear wheels and permit a casing of lesser overall size. 
         [0106]    A notable feature of the invention is that the output shaft is not aligned with the input shaft (see  FIG. 1 ). Thus the relatively high level input axis  31  associated with a conventional internal combustion engine (by virtue of crankshaft swing) is accompanied by relatively low level output axes  35 ,  36 , which may be useful in eliminating intrusion into the passenger accommodation whilst giving propeller shafts which operate through a lesser angle. The arrangement also permits one propeller shaft to pass the engine more easily. 
         [0107]    Yet another feature is integration of the centre differential  26  within the transmission casing at the relatively low level output axes  35 ,  36 . Integration ensures that tolerance build-up, which necessarily results from a separate differential in a separate casing, is avoided. 
         [0108]      FIG. 7  shows an in-line engine configuration  110  with attached transmission  111 , front and rear propeller shafts  112 ,  113 , front and rear axles  114 ,  115 , and wheels  116 .