The screw possesses great industrial utility in pressing bodies together or in raising weights, and may be classed among the simple machines. The screw is an inclined plane, and the effect of a screw is produced when such a plane moves spirally around a cylinder. This movement may be illustrated by cutting out a wedge-shaped piece of paper and wrapping it about a round stick or bolt. The sloping side draws a thread on the stick as in Fig. 29. This thread is called a helix (Fig. 30).

Fig. 28.   Lifting Power of Wedge.

Fig. 28. - Lifting Power of Wedge.

Fig. 29.   Principle of the Screw.

Fig. 29. - Principle of the Screw.

It really makes no difference in the result whether the inclined plane is wound in a spiral or circular path, or left straight; the wedging action will be there just the same. This means that all screw threads, nuts, bolts, etc., are circular or spiral wedges. The ease with which a screw turns and ascends depends on the slowness of the ascent, that is, on the number of turns, or threads, in a given distance.