A dust cap assembly including a semi-trailer (2) and a towing vehicle (3). The dust cap assembly is for closing a king pin (21) which has been hollowed out with a borehole (22) for automatic fifth-wheel coupling. A dust cap device (1) is arranged in contact with the underside (20) of the semi-trailer (2) and such that it can rotate about the king pin (21). In order to mount the dust cap device (11) such that it can rotate about the king pin (21), a circular ring-shaped bearing disc (25) having a circular bearing seat (26) is provided, which is secured coaxially to the king pin (21) on the underside (20) of the semi-trailer (2). The circular bearing seat (26) on the circular ring-shaped bearing disc (25) has a first sliding bearing edge (261) on the inner circular opening and a second sliding bearing edge (262) on the outer rim, in which the dust cap device (1) engages. Also, a dust cap device for this assembly.