Patent ID: 7857567

Claim:
A bolt-and-nut fastening mechanism for attaching a fixture to a frame with a desired clearance formed between the fixture and the frame, said bolt-and-nut fastening mechanism comprising: a nut provided with an inverse internal screw thread having a single axial through hole, the single axial through hole having a single diameter and a single cylindrical shape over a whole of the single axial through hole; a spacer provided with an inverse external screw thread capable of engaging with the inverse internal screw thread, and a normal internal screw thread; a bolt provided with a normal external screw thread capable of engaging with the normal internal screw thread of the spacer, and capable of being passed through an opening formed in the frame from the side of the outer surface of the frame and of being turned to engage the normal external screw thread thereof with the normal internal screw thread of the spacer; and screwing restricting means for permitting only a threaded end part of the bolt to engage with the normal internal screw thread of the spacer and restraining a threaded back part of the bolt extending behind the threaded end part from engaging with the normal internal screw thread of the spacer when a torque not higher than a predetermined first threshold torque is exerted on the bolt, and permitting the threaded back part to engage with the normal internal screw thread of the spacer when a torque not lower than a predetermined second threshold torque is exerted on the bolt, wherein the inverse external screw thread of the spacer can engage with the inverse internal screw thread with an end part of the spacer projecting from the inverse internal screw thread, and while the torque not higher than the predetermined first threshold torque is exerted on the bolt, the bolt advances into the spacer and then the bolt stops advancing into the spacer and retracts together with the spacer from the nut until the spacer contacts the frame, and while further the torque not lower than the predetermined second threshold torque is exerted on the bolt, the bolt advances into the spacer and the bolt presses the spacer against the frame by threading the normal external screw thread of the bolt with the normal internal screw thread of the spacer.