US Patent 7225888 Hydraulic fluid coupling

For use in a wellbore containing a rotary drill string and a drill bit, the combination comprising, a rotary mandrel containing a through opening to pass drilling fluid downwardly from the interior of the drill string, and toward the bit, a non-rotary housing extending about the mandrel, a plenum or plenums in the housing in communication with the interior of the mandrel through an opening to receive fluid pressure used to actuate means associated with drilling activity in the well, and a controlled fluid gap proximate the housing, to slowly leak drilling fluid to the annulus outside the string.