Drill steel centralizer

In a rock drill apparatus comprising an elongate structure;

• a rock drill;
• a cradle carrying the rock drill and mounted on said elongate structure to be axially movably guided therealong;
• motor means coupled to the cradle for feeding the cradle along said elongate structure;
• a rigid suction hood rigidly mounted on the front end of said elongate structure and arranged to engage the rock face and support said elongate structure when said elongate structure is forced against the rock face;
• and a drill stem centralizer mounted to the rear end of said suction hood;
• the improvement wherein said drill stem centralizer comprises:
• an annular housing;
• a sleeve of an elastomeric material rigidly supported in the annular housing, an annular surface of said annular housing providing a firm radial support for the outer surface of said sleeve, said sleeve surrounding a portion of the length of the drill stem and normally having a clearance to the drill stem;
• and
• a pressure fluid actuated piston coupled to said sleeve for axially compressing said sleeve to cause said sleeve to expand inwardly, said annular surface of said housing radially supporting the outer surface of said sleeve to force said sleeve to expand inwardly during said axial compression thereof to reduce said clearance to firmly engage the drill stem to centralize the drill stem when said sleeve is compressed axially.