sides as can further be seen between the connecting plate 4 which is depicted in fig3 on the left and the horizontal swivel arm 7 there is arranged a hydraulic piston cylinder unit 13 by means of which a specific angle of rotation of this connecting plate 4 about the rotational axis d 1 relative to the hinged column 5 and the swivel arm 7 can be set and locked in addition there is arranged a further hydraulic piston cylinder unit 14 between the two connecting plates 4 6 by means of which a specific angle of rotation of the two connecting plates 4 6 relative to each other can be set and locked with a possible rotation of the connecting plate 6 relative to the swivel arm 7 and the hinged column 5 about the rotational axis d 2 as is evident from fig5 which shows a vertical section through the hinge assembly in the basic state along the line a a in fig4 the rotational axis d 1 d 2 of the two connecting plates 4 6 in the basic state run parallel to each other in the plane defined by the hinge centers even though the structural elements 4 5 6 7 of the hinge assembly according to the invention which is depicted in the fig1 to 5 exclusively are rigid structural elements with a fixed geometry it is envisaged in further preferred variants of this embodiment to design several of these structural elements in such a manner that the position which the rotational axis d 1 d 2.