Figure 7.19B Different types of screws and screw heads.
Almost all threaded fasteners tighten when the head or nut is rotated clockwise. That is, as a viewer turns a nut clockwise it moves away from her. Such a fastener is said to have a right-hand thread; all screw fasteners are assumed to be right-hand unless otherwise specified. Left-hand threads are usually found only on rotating machinery. For example, the axles of bicycle pedals screw into threaded holes in the cranks. In a pair of pedals one will have a right-hand thread and the other a left-hand thread. That way the rotation of the pedals doesn't tend to unscrew their axles. To designate a left-hand thread, the letters "LH" are placed after the class of fit: 3/8-16 UNC 2B LH (21).
A helix denotes the curve formed on any cylinder by a straight line in a plane that is wrapped around the cylinder with a forward progression. Common objects formed like a helix are a spring, a screw, and a spiral staircase.
Helices can be either right-handed or left-handed. With the line of sight being the helical axis, if clockwise movement of the helix corresponds to axial movement away from the observer, then it is a right-handed helix. If counterclockwise movement corresponds to axial movement away from the observer, it is a left-handed helix.
An external thread, also known as a male thread, is a thread on the outside of a cylinder or cone. An example is the thread of a bolt.
Also known as a female thread, an internal thread is a thread on the inside of a hollow cylinder or bore. An example is the thread inside a nut.
A major diameter is the largest diameter of an external or internal screw thread. A minor diameter is the smallest diameter of an external or internal screw thread; it is also known as the "root diameter."
Screw threads are cut or chased (to cut thread in a lathe rather than with a die); the unthreaded portion of the shank will be equal to the major diameter of the shank.
The axis is the center line running lengthwise through a screw—the line, real or imaginary, passing through the center of an object about which it could rotate, or a point of reference.
The crest is the top or outer surface of the thread joining the two sides (also called "flat"). It is the surface of the thread corresponding to the major diameter of an external thread and the minor diameter of an internal thread.
The root is the surface of the thread corresponding to the minor diameter of an external thread and the major diameter of an internal thread. It represents the bottom or inner surface joining the sides of two adjacent threads.
The depth is the distance from the root of a thread to the crest, measured perpendicularly to the axis. Pitch
The pitch is the distance from any point on the screw thread to a corresponding point on the next thread measured parallel to the axis. For example, nuts and bolts need to have the same pitch and diameter if they are to be used together.
The lead is the distance a screw thread advances in one complete turn, measured parallel to the axis. On a single-thread screw the lead is equal to the pitch; on a double-thread screw the lead is twice the pitch; on a triple-thread screw the lead is three times the pitch.
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