Patent ID: 6935442

Claim:
Stabiliser device for a string of drill rods which can rotate about a longitudinal axis of the drill string and is subjected to a force in the axial direction ensuring the centering and guiding of the drill string in a borehole and has a tubular central body having means for connection to a first and a second component of the string of drill rods at a first and a second axial end and at least one external element for contact of the stabiliser with a wall of the borehole, wherein the external element for contact of the stabiliser with the wall of the borehole has at least one means for activating a means which co-operates with the wall of the borehole in order to limit the friction between the external contact element of the stabiliser and the wall of the borehole, and that it is mounted on the central body in an axial position which remains fixed at least within a range of values of an axial force exerted between the central body and the external contact element, the extent of which can be fixed at any value whatsoever, and wherein the external contact element of the stabiliser is a tubular casing mounted so as to slide in the direction of the axis and to rotate about the axis of the stabiliser on the central body, that the means co-operating with the wall of the borehole is at least one blade integral with the external surface of the external element, and that the means for activation of the means co-operating with the wall of the borehole consists of at least one means for mechanical coupling in rotation between the external casing and the central body of the drill rod, when the external casing is displaced axially relative to the body in the direction of the axis under the effect of forces exerted between the blade of the casing and the wall of the borehole, the casing being retained in a position in which it is freely rotatable on the body about the axis by resilient means for restoring in the direction of the axis which bear by two axial ends on edges perpendicular to the axis both of the central body and of the external casing of the stabiliser and constitute abutments for precise positioning of the external casing relative to the central body of the stabiliser in such a way that resilient restoring means are both compressed simultaneously by displacement of the external casing towards a position of coupling to at least one means for mechanical coupling in rotation.