An actuator (1) is fixed on a valve (2) through a fixing joint (3) which comprises a tubular sleeve (32) on which a tightening nut (30) is screwed; the nut (30) and the sleeve (32) are provided with a conical thread connection (34, 300); by screwing the nut (30), a plurality of longitudinal prolongations (40), arranged in a crown around the axis of the sleeve (32), tightens on a fixing head (20) of the valve (2), firmly fixing the actuator (1) to the valve (2); the joint at issue provides an alternative to a flange connection, equally solid and resistant to unscrewing due to vibrations; by resting against a mechanical stop surface (24) the nut (30), being unscrewed, can act as an extractor that easily removes the joint (3) from the head (20).