Abstract: A locking mechanism for releasably locking a rotatable arm 3 to a generally cylindrical axle or wheel 6. The mechanism comprises a slot 7 extending around a region of the axle 6, the slot having first and second abutment surfaces spaced apart in the direction of said movement. A heel member 17, rigidly fixed to the arm 3, projects from the arm into the slot and provides third and fourth spaced apart abutment surfaces 19,27, said third abutment surface 19 being arranged to engage said first abutment surface 18 to prevent rotation of the axle 6 in a first direction. A locking wedge 21 is provided which is moveable between a first release position in which the wedge lies outside the slot 7 and a second locking position in which the wedge projects into the slot to engage said second abutment surface 10 of the slot and said fourth abutment surface 27 of the heel member 17 to prevent rotation of the axle 6 in a second direction opposite to said first direction.