A method of stepping up the speed of a rotary motion from a first speed, supplied to an input shaft (26), to a second speed delivered by an output shaft (28), comprises transmitting said rotary motion from said input shaft (26), via a drive gear (34) of said input shaft (26), the drive gear (34) being
helical and having a first
helix angle (ψin), to a
layshaft arrangement, so as to generate a first
axial thrust (F1) of the input shaft (26) in a first axial direction; transmitting said rotary motion from said
layshaft arrangement, via a driven gear (44) of said output shaft (28), said driven gear (44) being
helical and having a second
helix angle (ψout) that is larger than said first
helix angle (ψin), to said output shaft (28), so as to generate a second
axial thrust (F2) of the output shaft (28) in a second direction, said second direction being substantially opposite to said first direction; and applying said first
axial thrust (F1) and said second axial thrust (F2) to the same location of an axially rigid support structure (50), such that said first and second axial thrusts (F1, F2) counteract and at least partly cancel out in said support structure (50).