Screw Thread Calculations

Threads

• Threads start at or near the tip of the screw and continue to spiral upward toward the head. Some screws have a shank or smooth part of the screw body between the threads and the screw head, while others have threads all the way up the body. As the screw is twisted, the threads bite into the material and hold the screw. If the threads are matched with threads on a nut, the two threads lock together, forming a tight bond.

Shapes

• Threads have different shapes that are seen upon close examination. All have flattened tops with some being taller than others are. The valley between the threads may be shallow or deep; squared, V- or U-shaped at the bottom; and narrow or wide. Forcing a one-quarter inch screw with a shallow, U-shaped, narrow thread into a one-quarter inch nut that is made for a deep, square, wide thread does not work, even though both pieces are the same size.

Pitch

• The number of threads on any one screw is determined by the thread pitch or distance from one thread to another. Unless the screw is marked or it comes from a marked bin, the only way to know the pitch is counting the number of threads in an inch and dividing the number into 1. For example, a thread count of 10 would give a pitch of 0.10. As with the thread’s shape, if the screw’s pitch and the nut’s pitch do not match, the screw cannot go into the nut.

Hand

• The one phrase that even the most inexperienced handymen knows is, "lefty loosy, righty tighty." This refers to the "hand" or direction the threads extend down the screw looking at it from above the head. Almost all screws turn left to insert it or right to remove it. Because some particular types of motors create vibrations that work a normal right-handed screw out of its position there are left-handed screws made for this application.