The Shank
The Nail Shank is the part which, as already mentioned, does most of the holding. The nail shank is forced between the fibres of the wood and once in place, the fibres press against the shank. This pressure is the reason why it is so difficult to pull a nail out of the wood.
The shank is one of four types: Smooth, Spiral, Ring or Screw.
Smooth shank nails have exactly that: a smooth appearance. Due to their regular form they can be produced at very high speed and are therefore the most economical type of nail. However, their holding power is the lowest of the group. They require less energy to drive them and the air consumption per nail can be lower. Sometimes it is possible to use a smaller tool to drive them.
Spiral shank nails have either a threaded appearance, like a screw, or they can have a helical twist to them. Either way, the purpose of the deformed shank is to increase the force required to remove the nail from the wood. The wood fibres wrap themselves around the deformed section and the difference in diameters that their shape provides makes it much more difficult for the nail to be withdrawn.
Ring shank nails have a series of rings punched into the surface of the shank. Again a difference in the diameter of the shank is produced making it even more difficult to withdraw the nail.
Both Screw shank and Ring shank nails have to be produced at lower speeds and their cost is generally higher than for a smooth shank nail. The air consumption per nail is also higher, and the energy required to drive these nails into hard wood is very much greater.
The advantage of Screw shank and Ring shank nails is of course in the holding power, which is better than Spiral and far superior than Smooth shank.