ASME B1.1-2024: Unified Inch Screw Threads
Unified screw thread is the essential feature of the screw as a simple machine as well as on fasteners, which are critical for connecting pieces of wood, steel, concrete, and other materials in the creation of a vast variety of construction projects like buildings, bridges, and other infrastructure. ASME B1.1-2024: Unified Inch Screw Threads (UN, UNR, And UNJ Thread Forms) specifies the thread form, series, class, allowance, tolerance, and designation for unified screw threads.
What Is the Unified Thread Series (UTS)?
In 1949,the Unified Thread Series (UTS) was agreed on by the United States, Great Britain, and Canada to obtain interchangeability. The UTS defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads. It is the main standard for bolts, nuts, and a wide variety of other threaded fasteners.
Unified screw threads are defined by their major diameter and pitch. These threads are symmetrical and have a 30° flank angle, which is the angle from the crest to the groove. A UTS thread, therefore has the same 60° profile to an ISO metric screw thread. The difference between UTS and ISO is that UTS measures pitch as an inch fraction; whereas, ISO measures pitch in millimeters.
What Is the UN Designation for a UTS Thread?
The UN designation for a UTS thread starts with a number indicating the major diameter, followed by a number indicating the pitch, measured in threads per inch (TPI), and then the thread’s coarseness. Further, in UTS, the pitch value is the reciprocal of TPI. For example, if a bolt has 20 TPI, its pitch is 1/20” or 0.05”.
What Is ASME B1.1?
ASME B1.1-2024 applies to the thread form, series, class, allowance, tolerance, and designation for unified screw threads. This standard covers only [Unified] UN and UNR [external threads only] thread forms.
ASME B1.1-2024 specifies that UN applies to both internal and external threads. UNR applies only to external threads; the difference between UN and UNR threads, in addition to designation, is that a flat or rounded root contour due to tool wear is specified for UN threads, while only a defined rounded root contour is specified for UNR threads.
What Are the Classes of Unified Threads?
A thread series is a set of various thread designations which represent different thread sizes to define the thread geometry. It is classified into coarse threads, fine threads, and extra fine threads. In other words, unified threaded bar and threaded fasteners are available as coarse thread (UNC) and fine thread (UNF) series.
What Is the Difference between Fine and Coarse?
Coarse threads have a bigger pitch, meaning the screw advances farther with each full turn, resulting in a faster installation time. Consequently, they are often used in applications where installation speed and ease of usage are critical. Advantage of coarse threads included that they offer an easier and faster assembly of components with less chance of cross threading, and they are less prone to stripping when fastened into lower strength materials.
On the other hand, fine thread has a smaller pitch. This slows down installation time, but the larger number of threads enables more precision and control. Precision industries such as electronics, aerospace, and medical equipment frequently use fine threads. Hence, the advantage of fine threads is that they allow more precise adjustment because of smaller helix angle, and they are easier to tap in harder materials.
ASME B1.1-2024: Unified Inch Screw Threads (UN, UNR, And UNJ Thread Forms) is available on the ANSI Webstore and in the Standards Package, ASME B1.1 / ANSI/ASME B1.2 / ASME B1.20.1 – Unified Screw and Pipe Threads Package.
