Abstract:
Various configurations of twin layshaft gear transmission are disclosed, typically having a large degree of common parts between single and dual clutch variants. In one embodiment reverse ratio is provided via an independent compound idler and is driven from a driving gear also associated with a forward speed ratio.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Great Britain Application No. 0516726.7 filed Aug. 15, 2005 and International Application Serial No. PCT/GB2006/003048 filed Aug. 15, 2006. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to a multi-speed twin layshaft gear transmission suitable for use in a motor vehicle. 
       BACKGROUND 
       [0003]    Conventionally a gear transmission with multiple forward ratios has co-axial input and output shafts, and a single layshaft. Pairs of gears between the layshaft and input/output shafts provide different speed ratios, and are selectable one at a time by the vehicle driver. Multi-speed transmission of this kind are relatively long because the gear pairs lie side by side along the transmission drive axis. 
         [0004]    For reasons of fuel economy, the number of forward speeds has increased in recent years, so that six-speed transmissions are now favoured. Such transmissions allow the vehicle engine to provide adequate acceleration whilst being economical and quiet at cruising speed. However it is difficult to fit a single layshaft six speed transmission into a front wheel drive vehicle with transverse engine/driveline, especially a small vehicle, because the engine/transmission unit is long compared with the available space between the driving wheels. 
         [0005]    Twin layshaft transmissions offer the possibility of reducing transmission length, at the cost of increasing transmission width, and are one potential solution to this problem. 
         [0006]    Another problem faced by vehicle and transmissions engineers is that of providing both manual and automatic variants. Generally speaking these variants have traditionally been very different, but it would be advantageous for both to have a very similar size and shape so as to avoid unnecessary compromise in vehicle design. So called twin-clutch transmissions offer such a possibility since the internal arrangement of both manual and automatic variants can be very similar. Both manual and sequential automatic variants can be offered with almost the same external envelope, and the same number of ratios. 
         [0007]    In order to produce manual and automatic versions at minimum cost, each variant should have the maximum number of common parts. 
       BRIEF SUMMARY 
       [0008]    According to a first aspect of the present invention there is provided a multi-speed gear transmission being one of two variants and comprising an input, an output and two layshafts, pairs of gears being provided between the input and layshafts for selectable drive on demand, and each layshaft being in driving engagement with said output, wherein the transmission provides six selectable gear pairs providing six spaced forward ratios, the input comprises one of 
         [0009]    A: a drive shaft common to all gear pairs, and 
         [0010]    B: dual co-axial drive shafts, one associated with the gear pairs of the odd ratios and one associated with the gear pairs of the even ratios, 
         [0011]    wherein for both A and B, five of said gear pairs are associated with the same layshafts, and speed ratios  3 ,  5 , or  4 ,  6  are at the outboard end of said transmission. 
         [0012]    Such an arrangement allows the input, output and two layshafts to be on the same centres for both A and B variants. Furthermore the said five gear pairs are common to both A and B variants. Preferably the even ratios of B are associated with an outer drive shaft. 
         [0013]    In this specification, common parts are proposed for two transmission variants. By ‘common’ we mean that the respective finished parts are functionally identical and substantially identical in shape save for very minor and inconsequential features. Thus shafts and gears are typically identical in material, size and form. Selector hub components, shafts and forks may also comprise substantially identical finished parts. 
         [0014]    In one preferred embodiment second speed ratio is associated with one layshaft for variant A, and a different layshaft for variant B. 
         [0015]    Typically the layshaft axes will be at a different distance from the input axis; accordingly in the preferred embodiment the second speed ratio is identical in both variants, but the associated gear pairs are of different diameter. 
         [0016]    Preferably the layshafts are substantially identical for variants A and B. Thus in the preferred embodiment each layshaft defines four gear locations to be associated with respective gear pairs. Said locations are in use associated with freely rotatable driven gears engageable on demand with respective layshafts. In the preferred embodiment said locations are identical on both layshafts so that in use the locations lie in four successive transverse planes. The locations are preferably paired and the transmission further comprises a plurality of selector hubs, one each associated with each location pair. Preferably the hubs lie in two successive transverse planes. Typically a selector hub will be axially reciprocal about a respective layshaft axis to engage one of two driven gears associated therewith. 
         [0017]    In the preferred embodiment, the gear pairs are ordered differently on the respective layshafts for variants A and B. Thus in variant A first layshaft is associated with speed ratios  1 ,  2 ,  5 ,  6 , whereas in variant B, the same layshaft is associated with speed ratios  1 , 5 , 6 . 
         [0018]    In the preferred embodiment, the endmost speed ratios for both variants are  3 ,  5 . 
         [0019]    Preferably the layshafts/output connection is axially at the input side of the transmission. 
         [0020]    The transmission of the invention has common driving gears for a plurality of said gear pairs. Thus in both variants common driving gears are provided for ratios  4 ,  6  and  3 ,  5 . 
         [0021]    In the preferred embodiment selector rails are provided for selector forks, one each of which is associated with a respective selector hub. The selector shafts and forks are identical for both A and B variants and have axes in the same transverse positions with respect to the input axis. Preferably the selector hubs are substantially identical. 
         [0022]    Preferably a reverse ratio is provided and comprising an input gear associated with a forward ratio, and sequential driven gears one on each layshaft, wherein the intermediate gear is an idler. 
         [0023]    A transmission according to the invention provides six forward and one reverse ratio from gear trains arranged in four successive transverse planes along said input axis. Furthermore four selector hubs can be arranged in two transverse planes between the first and second, and third and fourth gear planes. 
         [0024]    In an alternative aspect of the invention a twin layshaft transmission has a reverse ratio provided via a compound gear comprising two gear profiles side by side for rotation together. Such an arrangement preferably utilises a common input gear, such as the input gear of first or second ratio, and an output gear associated with one of said layshafts and selectable via a single acting selector hub. The compound gear is provided on an additional layshaft and provides for variation in speed ratio according to the ratio with both the input gear and the output gear. In the preferred embodiment the compound gear comprises gear profiles of different diameter. Other advantages of commonality between single and twin layshaft transmissions are preserved. 
         [0025]    Such an arrangement allows the reverse ratio output gear to be in a different plane from the forward speed ratios, which may confer advantages in space utilization. 
         [0026]    In a preferred embodiment, reverse ratio is driven from first speed input gear via an independent layshaft and compound gear thereon, said first speed input gear being immediately adjacent the input end of said input shaft, and the plane of reverse ratio output gear being distal thereof. 
         [0027]    Preferably speed ratios  3 ,  4 ,  5 ,  6  are provided in two gear planes distal with respect to the input end of said input shaft from gear planes associated with speed ratios  1 , 2 . Thus four gear planes are provided for forward ratios, and a fifth gear plane for the output gear of reverse gear train, the input gears thereof lying in one of the planes associated with a forward speed ratio. T he reverse output gear plane preferably lies substantially in the neutral plane of the selector hub of two forward speed ratios, preferably the forward ratios associated with first and second speeds. 
         [0028]    Other features of the invention will be apparent from the following description of preferred embodiments shown by way of example only in the accompanying drawings in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1  is a developed schematic plan of a six speed, twin layshaft single clutch transmission. 
           [0030]      FIG. 2  is a representative end view showing the true spatial relationship of shafts. 
           [0031]      FIG. 3  corresponds to  FIG. 1  and shows a six speed; twin layshaft dual clutch transmission. 
           [0032]      FIG. 4  corresponds to  FIG. 2 , but illustrates the true arrangement of shafts of  FIG. 3 . 
           [0033]      FIG. 5  corresponds to  FIG. 1  and shows reverse ratio via independent layshaft. 
           [0034]      FIG. 6  corresponds to  FIG. 5  and shows reverse ratio via wide first ratio input gear. 
           [0035]      FIG. 7  is an alternative embodiment according to a second aspect of the invention and shows a single clutch transmission with compound reverse idler. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0036]    With reference to the drawings  FIGS. 1 and 2  illustrates a single clutch manual transmission having six forward speeds and reverse. An input shaft  10  and two layshafts  11 ,  12  are provided with pairs of gears therebetween. Each layshaft  11 ,  12  has an output gear  13 ,  14  coupled to a common final drive gear  15 , as best illustrated in  FIG. 2 . 
         [0037]    Six forward speeds are provided by respective gear pairs and identified by the numbers ‘ 1 - 6 ’. Each pair comprises a driving wheel of the input shaft  10 , and a respective driven wheel rotatable on one or other layshaft  11 , 12 , and connectable thereto by conventional selector hubs  16 . The hubs  16  are shifted, from the neutral position indicated in  FIG. 1 , either to the left or right to couple a respective gear wheel for rotation with the layshaft which is co-axial therewith. 
         [0038]    Reverse gear (‘R’) is provided via two driven gears, one being provided on each layshaft, and the intermediate gear acting as an idler; in this way a separate layshaft for reverse is avoided. 
         [0039]    The physical diameters of the gears illustrated in the drawings is representative; 
         [0040]    the skilled man will select diameters according to the ratios required, and to avoid clashing of neighbouring gears. Likewise the respective centres of the shafts, and axial spacing of the gears thereon will be determined according to the design requirements. 
         [0041]    In use the transmission of  FIG. 1  is operated conventionally in conjunction with the clutch  17 . A selector mechanism not shown, allows selector hubs  16  to be engaged one at a time on demand to define a torque transmission path from engine to road wheels. The speed ratios can be selected in any order. 
         [0042]      FIGS. 3 and 4  show a corresponding twin clutch transmission having co-axial input shafts  18 , 19  driven via one or other of a dual clutch pack  20 . As is apparent from  FIG. 4 , the spatial relationship of the layshafts  21 ,  22  is identical to that of  FIG. 2 , which means that the transmission envelope can be very similar. The gear pairs are differently arranged, as shown by the references ‘ 1 - 6 ’ and ‘R’. 
         [0043]    The transmission of  FIG. 3  is operated by pre-selecting the next desired ratio (which must be associated with the non-driven input shaft) and disengaging one clutch as the other is engaged. Ratio changes are normally under automatic or semi-automatic control. In practice such transmissions tend to be sequential, though several ratios may be missed if desired; for example the transmission may be permitted to change down from 6th speed to 3rd speed if conditions are suitable. 
         [0044]    An important feature of the invention is that the transmissions of  FIGS. 1 and 3  have great commonality of components notwithstanding that in  FIG. 3  the ‘odd’ and ‘even’ ratios must be grouped so as to be associated with the respective input shaft. Thus the following parts are common:
       input gears for 3rd/5th and 4th/6th ratios   all layshaft gears except 2nd   both layshafts   final drive gears (including differential)   selector forks, fork rails, and much of the selector mechanism   selector hub position on the layshafts   selector hub assemblies   minor components such as bearing and seals       
 
         [0053]      FIG. 5  illustrates a variant of  FIG. 1  in which reverse ratio is provided via an additional layshaft  23  on which is supported an idler gear  24 . This reverse ratio no longer uses first output gear as an idler, and this arrangement may reduce the required torque capacity thereof. The same arrangement is possible in a similar modification of the transmission of  FIG. 3 . 
         [0054]    An additional advantage of  FIG. 5  is that the reverse ratio is no longer dependent on first ratio, and thus greater design freedom is available at the cost of an additional layshaft and idler gear. 
         [0055]    It will also be appreciated that the arrangements of  FIGS. 1 ,  3  and  5  are susceptible of further modification to permit reverse ratio to be driven from the second speed input gear rather than the first speed input gear. The alternative arrangement gives additional possibilities for reverse ratio, depending on the physical constraints and the required gears shift pattern. 
         [0056]    Yet a further potential variation is to allow forward and reverse gear ratios of  FIG. 1  in parallel planes, but to be driven by a common, wide, input gear.  FIG. 6  illustrates such a possibility by variation of  FIG. 5 ; the first input gear  25  drives reverse via an idler  24 , but in a plane parallel to the first speed output gear. Such an arrangement is of course also possible using a wide second speed input gear. 
         [0057]    A further option, not illustrated, is to provide two input gears side by side for reverse and first speed ratios. In effect the wide gear  25  of  FIG. 6  is divided to provide a separate torque path for each of first and reverse ratios. Such an arrangement allows further independence of ratio choice. 
         [0058]    In yet a further alternative, reverse gear sleeve of  FIG. 1  is also displaceable to the right, and is thus double acting. In the rightwards position the layshaft  12  is arranged to be locked to the transmission casing, thus giving an anti-rotation feature equivalent to the PARK position in a conventional automatic vehicle transmission. As is well understood, such a feature can replace or compliment a conventional parking brake. 
         [0059]      FIG. 7  illustrates an alternative embodiment in which reverse ratio is provided via an independent layshaft and compound gear  31  which comprises two gear profiles side by side and rotatable in unison. Both first and reverse ratios are driven from a common input gear  32  which is immediately adjacent the clutch  17 . Reverse selector hub  33  engages a loose gear  34 , which is in mesh with the compound gears  31 , with an output shaft  11 . Other forward speed ratios are provided as indicated with ratios  4 , 6  outermost (i.e. at the opposite end to the clutch). 
         [0060]    It will be appreciated that by changing the gear pair positions of the embodiment of  FIG. 7 , the speed ratios  1 ,  3 ,  5 , R may be driven from one input shaft, and the ratios  2 ,  4 ,  6  from a co-axial input shaft, thus providing a dual input transmission of the kind illustrated in  FIG. 3 . Furthermore the ratios  3 ,  5  may be end most. 
         [0061]    Although this description is made with reference to a transmission for a vehicle, it will be appreciated that the arrangements described herein are suitable for other kinds of installation where different speed ratios are required on demand.