to its closed position as is well known in the art while the wing 15 is being lowered closed or raised opened the rods 13 and 14 act as guiding and stabilizing members and are arranged to take a substantially vertical position where their pins 12 are located at a lower level than their respective pins 11 when the wing 15 is closed and a substantially horizontal position when the wing is fully opened once returned and accompanied in its fully closed position wing 15 is snap engaged by hooks 39 40 and held locked by them in position whilst the motor 36 stops e g due to the action of one or more limit switches not shown or other equivalent control means for the opening the reverse procedure occurs on command which can cause automatic polarity reversion of the motor 36 the latter is set in rotation in the opposite direction throughout the first 1 4 to 1 2 rotation the output shaft 35 moves surface 31 so as it abuts against the arm 45 thereby lifting hooks 39 40 and consequently unlocking the wing then the shaft 19 is set in rotation in the direction in which the counterweights 16 are.