Gearbox comprising: - a main unit comprising: a) a first casing; b) a first gear transmission; and c) a first hollow shaft defining a motion input for said first gear transmission; - a secondary unit comprising: a) a second casing; b) a second gear transmission defining a transmission of motion between two axes forming an angle of less than 180°; and c) a second hollow shaft defining a motion output for said second gear transmission; - a drive shaft comprising a first and a second clutch portion (36, 35), which respectively engage said first and second hollow shaft in an angularly fixed and axially slidable manner to transmit motion from said second gear transmission to said first gear transmission; - axial retaining means to prevent said drive shaft from translating axially in at least one of the two directions; said axial retaining means being releasable and comprising an axial shoulder supported in fixed position by a portion which is defined by an axial end of one of said first and second hollow shafts and faces the other of said first and second hollow shafts; - opening means provided between said first and second casing (3, 23) to access said axial end and release said axial retaining means; characterized in that said axial retaining means further comprise an external radial projection, which is part of said drive shaft, is arranged in intermediate position between said first and second clutch portion (35,36) and axially faces said axial shoulder so as to prevent said drive shaft from translating axially towards said axial end; said radial projection and said shoulder being shaped so as to be able to axially slide by the side of each other without interference after a relative rotation. - a main unit comprising: a) a first casing; b) a first gear transmission; and c) a first hollow shaft defining a motion input for said first gear transmission; a) a first casing; b) a first gear transmission; and c) a first hollow shaft defining a motion input for said first gear transmission; - a secondary unit comprising: a) a second casing; b) a second gear transmission defining a transmission of motion between two axes forming an angle of less than 180°; and c) a second hollow shaft defining a motion output for said second gear transmission; a) a second casing; b) a second gear transmission defining a transmission of motion between two axes forming an angle of less than 180°; and c) a second hollow shaft defining a motion output for said second gear transmission; - a drive shaft comprising a first and a second clutch portion (36, 35), which respectively engage said first and second hollow shaft in an angularly fixed and axially slidable manner to transmit motion from said second gear transmission to said first gear transmission; - axial retaining means to prevent said drive shaft from translating axially in at least one of the two directions; said axial retaining means being releasable and comprising an axial shoulder supported in fixed position by a portion which is defined by an axial end of one of said first and second hollow shafts and faces the other of said first and second hollow shafts; - opening means provided between said first and second casing (3, 23) to access said axial end and release said axial retaining means;