Flow retarder for bearing assembly of downhole drilling motor

A flow retarder is provided for use in the bearing assembly of a downhole drilling motor. The bearing assembly comprises an inner tubular mandrel and an outer tubular housing. The flow retarder comprises: an inner carbide sleeve connected to the rotating mandrel by a resilient elastomer layer bonded to a driver ring which is locked on the rotating mandrel; an outer carbide sleeve affixed to the stationary tubular housing; the sleeves forming a narrow annular clearance or metering gap between them; a secondary port extending through the mandrel side wall directly above the carbide sleeves; and a housing port extending through the housing side wall below the flow retarder sleeves. Drilling fluid leaks down from the adapter/adjustable housing clearance and passes through the metering gap to lubricate the carbide sleeves. Part of this fluid returns through the secondary port to the main bore of the mandrel. By providing the elastomer layer, the transmission of radial loads from the inner sleeve to the outer sleeve is reduced.