Abstract: A looseness resisting screw formed with an external thread includes a first thread area having a substantially uniform outer diameter and a second thread area extended from the first thread area toward a leading end of the screw. The second thread area has a plurality of fractional lead parts each of which has an outer thread diameter increasing along a helical direction of the fractional lead part from a start point to an end point, and the outer thread diameter is substantially equal to a thread root diameter at the start point, while the outer thread diameter is equal to a thread top diameter at the end point. The fractional lead part has a thread sectional area increasing along the helical direction from the start point to the end point, and the thread top diameter of the fractional lead part is slightly larger at the end point than the substantially uniform outer diameter of the first thread area so that the second thread area provides a tapping thread section.

Abstract: A screw and a screwdriver of the present invention provide an easy insertion of screwdriver bits and the screwdriver conveys a large torque when fastening so that the screwdriver does not rise up from the screw and does not damage recesses to fasten and loosen the screw. The screw has a high strength structure for shear force. A plurality of recesses for the insertion of the bits of the screwdriver are disposed on a top face of a screw head of the screw. Engaging walls are disposed to engage with the bits between the recesses. The whole shape of the recesses has a tapered cross section. Vertical engaging faces are formed inside the recesses near an outer circumference in a fastening direction of the screw and extend to a thickness direction of the screw head. The vertical engaging faces are parallel to bisectors of the width of the engaging walls through a central axis. The screwdriver has the bits having the same shape as the recesses of the screw head with respect to the central axis.

Abstract: A screw and a screwdriver of the present invention provide an easy insertion of screwdriver bits and the screwdriver conveys a large torque when fastening so that the screwdriver does not rise up from the screw and does not damage recesses to fasten and loosen the screw. The screw has a high strength structure for shear force. A plurality of recesses for the insertion of the bits of the screwdriver are disposed on a top face of a screw head of the screw. Engaging walls are disposed to engage with the bits between the recesses. The whole shape of the recesses has a tapered cross section. Vertical engaging faces are formed inside the recesses near an outer circumference in a fastening direction of the screw and extend to a thickness direction of the screw head. The vertical engaging faces are parallel to bisectors of the width of the engaging walls through a central axis. The screwdriver has the bits having the same shape as the recesses of the screw head with respect to the central axis.

Abstract: A fastening screw is provided with a locking function to prevent loosening thereof after being tightened.

Abstract: The screw of this invention can be used for fastening members of a variety of materials and reliably prevent loosening of the screw. The screw shank is formed with two threads, a main thread for driving the screw and a sub-thread which extends in the same direction as the main thread, is smaller in outer diameter than the main thread and has a pitch different from the main thread. The inclination of the slant surface of the sub-thread is set steep on the loosening side and moderate on the fastening side. The pitch of the main thread is set larger than that of the sub-thread. When the tightened screw is applied a loosening torque, the sub-thread bites into the inner wall of the hole, preventing the screw from being loosened.

Abstract: A screw having a main thread (2) to be screwed in and a sub-thread (3) formed in the direction opposite to the main thread, which is characterized in that a slant surface (3b) of the sub-thread on the screw-loosening side has a sharp slope whereas that (3a) on the screw-tightening side has a gentler slope.When such a tapping screw as above is screwed into a pre-drilled hole (4) and the sub-thread (3) advances in the direction opposite to the screwing-in direction thereof while acting like a wedge because the slope of the slant surface (3a) on the screw-in side is gentler.When the screw is subject to torque in the screw-loosening direction after tightened, the torque acts on the sub-thread (3) in the tightening direction thereof and, because of the sharp slope of the slant surface (3b) on the screw-loosening side, the sub-thread resists the unscrewing of the screw as if eating into the surface of the pre-drilled hole and prevents the screw from loosening.

Abstract: The invention is intended to provide a fastening structure made up of a nut and a to-be-tightened member which does not get loose even when used in places where it is subject to large vibrations. The nut has a cylindrical female screw extension portion which projects from the seat of the nut and which has a female screw formed in the inner circumferential surface thereof. The to-be-tightened member which is on the male screw side is formed with a fastening hole into which the female screw extension portion is to be fitted. The inner surface of the fastening hole and the outer surface of the female screw extension portion are tapered so that there is a taper angle difference. As the nut is fastened onto the male screw, the female screw extension portion is pushed inwardly and deformed due to the taper angle difference so that the female screw formed on the female screw extension portion is pressed against the male screw.