What Is a Unified Thread Standard?
A unified thread standard is an inch-based threading system with standardized basic dimensions in inches and a 60° thread angle.
While metric screws are standardized in metric units, the unified thread standard, including the witt screw, represents inch-based screws. For more information, visit inch screws on Metoree.
Uses of Unified Thread Standards
Unified thread standards are commonly used in aircraft, automobiles, and motorcycles. They are also prevalent in measuring instruments and industrial robots manufactured internationally, particularly in the United Kingdom, the United States, and Canada.
These standards are occasionally used in domestic equipment, primarily when the components are manufactured abroad.
Principles of Unified Thread Standards
The principle of the unified thread standard is similar to that of standard screws. Utilizing the screw’s slope, a heavy object can be lifted with minimal tightening torque, creating a significant fastening or axial force.
The screw remains secure due to the frictional force on the slope, which exceeds the gravitational slope component, thereby maintaining the fastening force. The thread angle of both unified and metric screws is 60°. However, the witt screw has a 55° thread angle.
Types of Unified Thread Standards
Similar to metric screws, unified thread standards come in two types: coarse (UNC) and fine (UNF). The primary difference lies in the thread pitch, with fine threads having more threads per inch than coarse threads. The denser arrangement of fine threads makes them less prone to loosening but requires more turning for tightening, making them less suited for mass production.
Other Information on Unified Thread Standards
Notation of Unified Thread Standards
Unified thread standards are denoted by labels such as UNC and UNF, as well as by fractions. Below is a guide to understanding these notations with an example.
Example: 3/8 – 20UNC x 5/8
1. 3/8: Thread Nominal (thickness)
This refers to the screw’s thickness, including the threads. In inch threads, this measurement is typically expressed as a fraction with 8 as the denominator.
2. 20: Number of Threads Per Inch
This represents the number of thread peaks within a one-inch length. For screws shorter than 1, the thread count is extrapolated to a one-inch length.
3. UNC: Thread Type (Coarse or Fine)
This indicates whether the thread is coarse (UNC) or fine (UNF).
4. 5/8: Length Below the Screw Head
Expressed in fractions of an inch, this length is measured from the base of the screw head to the end of the screw. It includes the length of the unthreaded shank. In tapered head screws, this length corresponds to the total length of the screw.
While the notation generally follows this format, there are unique terms for thread sizes. The thread designation, indicating the screw’s outer diameter, starts with the thinnest (e.g., #0, #1) and goes up to #12.
For sizes larger than #12, the diameter is represented by fractions of an inch, divided into eighths or quarters. These fractions are colloquially referred to as follows:
- 1/8: one eighth
- 1/4: two eighths (or one quarter)
- 5/8: five eighths
- 5/32: five thirty-seconds
- 5/16: five sixteenths