1. A reversible power transmission comprising a housing, an input and an output shaft journalled coaxially in said housing, an input gear and an output gear drive coupled to said input and output shafts respectively, a planetary gear carrier journalled in said housing coaxial with said shafts, a friction clutch operable to selectively drive couple said input shaft to said planetary gear carrier, a brake operable to selectively lock said carrier stationary, planetary gears journalled on said carrier and arranged to drive said output shaft in the same direction as said input shaft when said carrier is drive coupled to said input gear and to drive said output shaft in opposite direction to said input shaft when said carrier is locked stationary, dog clutch means selectively operable to drive couple said input and output shafts, and actuator means operable to engage said friction and dog clutches in sequence in that order, said planetary gears comprising a pair of first planetary gears driven by said input gear and disposed in diametrically opposite positions in said carrier, each of said first planetary gears being in driving engagement wiTh a respective second planetary gear also supported by said carrier in diametrically opposed relation, each of said second planetary gears driving said output gear, and engagement between said second planetary gears and said output gear being maintained during sliding movement of said output gear to effect engagement of said dog clutch.

2. A reversible power transmission comprising a housing, an input and an output shaft journalled coaxially in said housing, an input gear and an output gear drive coupled to said input and output shafts respectively, a planetary gear carrier journalled in said housing coaxial with said shafts, a friction clutch operable to selectively drive couple said input shaft to said planetary gear carrier, a brake operable to selectively lock said carrier stationary, planetary gears journalled on said carrier and arranged to drive said output shaft in the same direction as said input shaft when said carrier is drive coupled to said input gear and to drive said output shaft in opposite direction to said input shaft when said carrier is locked stationary, dog clutch means selectively operable to drive couple said input and output shafts, and actuator means operable to engage said friction and dog clutches in sequence in that order, said actuator means comprising a sleeve mounted for axial sliding movement relative to said input gear, a plurality of clutch actuating fingers operatively interposed between said friction clutch and said sleeve to engage said friction clutch in response to axial movement of said sleeve in one direction relative to said input gear, and said dog clutch means comprising complementary formations of dog clutch teeth drive coupled to said input and to said output shaft being axially slidable relative thereto in response to axial movement of said sleeve to engage and disengage said tooth formation coupled to said input shaft, the relative arrangement of said dog teeth formations, said friction clutch fingers, and said sleeve being such that axial movement of said sleeve in said one direction first effects engagement of said friction clutch and then effects engagement of said dog teeth formations.

4. A reversible power transmission comprising a housing, an input and an output shaft journalled coaxially in said housing, a planetary gear carrier journalled in said housing coaxial with said shafts, a friction clutch operable to selectively drive couple said input shaft to said planetary gear carrier, a brake operable to selectively lock said carrier stationary, planetary gears journalled on said carrier and arranged to drive said output shaft in the same direction as said input shaft when said carrier is drive coupled to said input shaft by said friction clutch and to drive said output shaft in opposite direction to said input shaft when said carrier is locked stationary, dog clutch means selectively operable to directly drive couple said input and output shafts, and actuator means operable to engage said friction and dog clutches in sequence in that order to first drive couple said input and output shafts through said planetary gear and thereafter direct couple said shafts through said dog clutch.