1.  Field of the Invention
This invention relates to a gear transmission for a vehicle of a synchronous meshing gear type. 
2.  Description of the Prior Art
In a conventional gear transmission for a vehicle, driving gears and driven gears constituting gear trans meshing always with one another are supported on an input shaft connected to a crank shaft of an engine and on an output shaft disposed in parallel with the input shaft.  A desired speed change range can be obtained by connecting selectively and operatively one of these gear trains to the input shaft. 
An example of a transmission axle for an FF vehicle equipped with such a gear transmission is a gear transmission for a vehicle disclosed in Japanese Utility Model Laid-Open No.  169450/1985 as an application of the present inventor.  This gear transmission for a vehicle will be explained with reference to FIG.  7 of the accompanying drawings. 
In FIG.  7, an input shaft 28 disposed in alignment with a crank shaft of an engine is operatively connected to, or is disconnected from, the crank shaft by a clutch 29.  A first range gear 31, a back range gear 36 and a second range gear 32 are supported fixedly in order named on the input shaft 28 and a third range gear 33 and a fourth range gear 34 are supported rotatably.  A fifth range gear 35 is fixedly supported at the end of the input shaft 28.  On the other hand, an output gear 37 is fixedly supported on an output shaft 30, and a gear 38 forming a gear train with the first range gear 31 of the input shaft 28, a gear 39 forming a gear train with the second range gear 32 and a gear 42 forming a gear train with the fifth range gear 35 are rotatably supported on the output shaft 30.  Furthermore, a gear 40 forming a gear train with the third range gear 33 and a gear 41 forming a gear train with the fourth range gear 34 are fixedly supported on the output shaft 30. 
A synchronizing device 43 is disposed in order to operatively connect the output shaft 30 to the gear 38 or to the gear 39.  In other words, when a sleeve 44 of the synchronizing device 43 is slid and moved towards the gear 38, the rotation of the input shaft 28 is transmitted from the first range gear 31 to the output shaft 30 through the gear 38 and the synchronizing device 43.  When the sleeve 44 of the synchronizing device 43 is slid and moved in the opposite direction, the rotation of the input shaft 28 is transmitted from the second range gear 32 to the output shaft 30 through the gear 39 and the synchronizing device 43.  Next, the torque transmitted to the output shaft 30 is transmitted further from the output gear 37 to the right and left gears 49, 50 through a differential gear device 60. 
Similarly, a synchronizing device 45 is disposed between the third range gear 33 and the fourth range gear 34 of the input shaft 28.  When a sleeve 46 of the synchronizing device 45 is slid and moved towards the third range gear 36, the rotation of the input shaft 28 is transmitted from the synchronizing device 45 to the output shaft 30 through the third range gear 33 and the gear 40.  When the sleeve 46 of the synchronizing device 45 is slid and moved in the opposite direction towards the gear 34, the rotation of the input shaft 28 is transmitted from the synchronizing device 45 to the output shaft 30 through the fourth range gear 34 and the gear 41. 
Furthermore, a synchronizing device 47 is disposed at the end of the output shaft 30.  When a sleeve 48 of the synchronizing device 47 is slid and moved towards the gear 42, the rotation of the input shaft 28 is transmitted from the fifth range gear 35 to the output shaft 30 through the gear 42 and the synchronizing device 47. 
The back speed change range of a vehicle is obtained in the following way.  A gear 51 is fixed integrally with the sleeve 44 of the synchronizing device 43 and this gear 51 and a back idle gear 52 meshing with the back range gear 36 of the input shaft 28 are slidably supported by a shaft 59.  Therefore, when the back idle gear 52 is slid and moved and engaged with the gear 51 and with the back range gear 36, the torque of reverse rotation can be obtained at the output gear 37 of the output shaft 30 and a torque transmission system for moving back a vehicle can thus be obtained. 
The differential gear device 60 includes a pinion shaft 55 which is equipped with a pair of pinions 56 and is supported rotatably inside a carrier 54 equipped integrally with a final gear 53 meshing with the output gear 37.  Side gears 57 and 58 mesh with these pinions 56 and are formed integrally with axles 49 and 50.  In the drawing, reference numeral 4 represents a clutch case; 5 is a transmission case; 6 is a cover; 7 is a differential gear case; and 4a, 5a and 6a are case wall members. 
In the gear transmission for a vehicle described above, however, the increase of the outer diameter of the sleeve 44 is limited because the gear 51 is formed integrally with the sleeve 44 of the synchronizing device 43.  Therefore, the speed change operations to the first and second ranges are somewhat unsmooth.  Since the back idle gear 52 is not equipped with any synchronizing device, each tooth surface must be chamfered in order to directly engage the back idle gear 52 with the back range gear 36 and with the gear 51.  Accordingly, the face width becomes smaller as much and moreover, the increase of the face advance angle is limited, too.  This is not desirable for torque transmission strength of the back idle gear 52. 
Therefore, it may be possible, in principle, to employ the construction of the gear transmission wherein a back range gear R is disposed adjacent to the highest range gear of the input shaft 28 such as a fifth range gear 35, a gear meshing with the back range gear R is disposed on the output shaft 30 and a synchronizing device is disposed between the back range gear R and the fifth range gear 35.  In this gear transmission, however, the length of the input shaft 28 becomes greater than that in the gear transmission for a vehicle shown in FIG.  7 by the face width of the back range gear R and by the stroke of the sleeve of the synchronizing device.  Accordingly, in compact vehicles where the distance (i.e. tread) between the centers of the right and left tire tread surfaces of front wheels is small, the transmission gear axle cannot be mounted.