| 摘要 |
The concrete screw of the invention reduces the driving torque required, reduces shear failure, and provides increased pull-out resistance. The screw has Hi- Low threads, the major thread being of greater diameter than the minor thread, t he threads having a pitch displacement at least equal to the diameter of the screw shank. Helical grooves of opposite rotational hand from the threads extend t he length of the threaded portion of the screw, and have a pitch displacement a t least two times the diameter of the screw shank. In a limited portion adjacent the tip of the screw, and extending generally no more than 1/3 of the length of the threads , are intermediate helical counter-rotation grooves. The intermediate grooves may be adjacent the principal grooves to produce a W-shaped notch in the threads. Preferably, the W-shaped notch is asymmetrical in the direction of rotation of the grooves. |
