Abstract: A downhole tool for providing rotary support of a downhole assembly in which the tool is incorporated, the tool also converting rotary contact with the wellbore to a longitudinal force tending to propel the assembly along the wellbore. The tool resembles a roller stabilizer in which the roller axes are skewed to be tangential to a notional helix, such that the natural (non-slipping) paths of roller contact with the wellbore have a longitudinal component in addition to the usual circumferential path. The tool can be used on drill strings and in downhole motor assemblies. The invention has particular advantage in highly deviated wells since it simultaneously compensates for increased bore friction and dynamically enhances weight-on-bit.

Abstract: A two-part investment casting process is described which involves first casting a hard material shell (32) such as stellite, which is threaded or profiled internally to allow attachment to a pre-machined shank (44) such that the advantages of a fluid and wear resistant investment casting of complex shape may be combined with a tough and machinable shank to allow attachment to the drill string.