a heavy duty dog clutch has co - axial driving and driven shaft elements each with a toothed connection to an intermediate sleeve that is axially displaceable to make and release one of said connections . the toothing of the releasable connection is helically formed so that a given direction of driving torque tends to screw together the helical toothing . one of the shaft elements is capable of axial displacement by the reaction to said torque in the helical toothing and this causes a pair of abutment surfaces on the shaft elements to bear against each other to engage the clutch fully .