| 摘要 |
A screw includes a shank portion including a tapered bottom section having a tip, a head on top of the shank portion, and first and second helical threads. The first helical thread extends helically around the shank portion in a first direction between the tip and the head, and has a first end distal from the head, and a second end opposite to the first end. The second helical thread extends helically around the shank portion in a second direction between the first helical thread and the tip, and has a third end proximate to the second end of the first helical thread. The first and second directions are opposite to each other. A non-helical ridge extends around the shank portion between the first and second helical threads. |
