Screw fixing device for structural units of agglomerated material

Claim 1

A screw-fixing device intended to be integrated in a structural member of agglomerated material, such as plaster or concrete, at the moment of manufacturing of said agglomerated material, comprising a screw and an anchoring sheath adapted to be integrated in said structural member, said anchoring sheath being made of two parts of plastic material, the first part comprising an inner part moulded onto the screw, the second part comprising an outer part surrounding said inner part, said inner and outer parts being shaped so as to be restrained from moving axially with respect to each other, and having a free space between said two parts for accommodating expansion of said sheath material, an outer surface of the inner part and an inner surface of the outer part being provided with helical interlocking structure, said restraint from axial movement being provided by said helical interlocking structure of said inner and outer parts, permitting contingently to unscrew and replace the screw with said inner part of the sheath moulded thereto.