Abstract: A transmission system for a four wheel drive vehicle includes an input shaft (2) connected to a differential mechanism (4, 6, 10), which has two output shafts (12, 20), which are connected, in use, to respective pairs of driven wheels of the vehicle. The two output shafts carry first and second coaxially mounted sun wheels (S1, S2, respectively), of an epicyclic gear system which mesh with first and second sets of planet wheels (P1, P2, respectively), the epicyclic gear system including a third sun wheel (S3), which is mounted coaxially with the first and second sun wheels and is in mesh with a third set of planet wheels (P3). The gear ratios of the three meshing sun wheels and sets of planet wheels are different.

Abstract: A transmission system for a four wheel drive vehicle includes an input shaft (2) connected to a differential mechanism (4, 6, 10), which has two output shafts (12, 20), which are connected, in use, to respective pairs of driven wheels of the vehicle. The two output shafts carry first and second coaxially mounted sun wheels (S1, S2, respectively), of an epicyclic gear system which mesh with first and second sets of planet wheels (P1, P2, respectively), the epicyclic gear system including a third sun wheel (S3), which is mounted coaxially with the first and second sun wheels and is in mesh with a third set of planet wheels (P3). The gear ratios of the three meshing sun wheels and sets of planet wheels are different. Each first planet wheel (P1) is connected to respective second and third planet wheels (P2, P3) to rotate therewith about a respective common planet shaft, the planet shafts being connected to a common carrier (24) which is rotatably mounted coaxially with the three sun wheels.

Abstract: At least one gearwheel of each meshing pair of gearwheels (10, 12, 14, 16, 20, 22, 24, 26) includes two respective selectively operable rotary locks which are adapted to lock the gearwheels to the associated shaft so that it rotates therewith in respective opposite directions. Each rotary lock comprises a plurality of rollers (34), whose positions relative to one another are maintained by a respective cage (36, 38) and which are received in respective spaced defined between an internal cylindrical surface on the associated gearwheel and a respective portion (32) of the surface of the associated shaft. The spaces associated with the two sets of rolling bodies (34) have a width in the radial direction of the shaft which progressively decreases from a maximum value in opposite circumferential directions of the shaft, whereby the two cages (36, 38) are movable in respective opposite circumferential directions between a freewheel position and a locked position.