Pump and turbine wheels (1,2) together enclose a toroidal working space. A shell (5) co-rotatably fixed to the pump wheel (1) surrounds the turbine wheel so their annular clearance (12) communicates radially outwards with the working space. An annular chamber (6) at the radially inner end of the shell concentric to the coupling axis encloses a build-up space (6.1) which connects via pref. four ports (6.3,6.4) to the ring clearance. The cover disc (6.2) demarcates the build-up space from the inside coupling housing and is itself held in place by a safety ring. The cover has an annular gap (6.3) and the connecting ports lie in the radially outer parts of the cover disc.
