When shopping for fasteners, you may come across the term “class threads.” Many screws, bolts and other types of fasteners have threads. Consisting of helical grooves or ridges, they allow fasteners to connect with each other. The tolerances of these connections, however, may vary depending on the class threads.
What Are Class Threads?
Class threads represent the tolerance of a threaded fastener. Threaded fasteners, of course, are used to secure two or more parts together. You can use a bolt and nut, for instance, to hold a pair of objects together. The class threads will determine how loosely or tight the bolt and nut fit together.
UTS Class Threads
In the United States, thread specifications are defined by the United Thread Standard (UTS). Established by the American National Standards Institute (ANSI) and the American Society of Mechanical Engineers (ASME), it plays a key role in the size, form and tolerance of threaded fasteners.
The UTS features specifications for class threads. Fasteners that use the UTS specification — UTS fasteners are typically measured in inches — often have a class thread label. UTS class threads consist of a number, followed by a letter. Some UTS fasteners have 1A class threads, for instance, whereas others have 3A class threads.
The three most common UTS class threads include the following:
A vs B Class Threads
There are A class threads, and there are B class threads. The former represents external threads, whereas the latter represents internal threads.
Fasteners can have internal or external threads. Internal threads are found inside of fasteners like nuts and threaded inserts. External threads, on the other hand, are found on the exterior of fasteners like bolts and screws. If a fastener’s class thread has the letter A in it, the fastener features external threads. If a fastener’s class thread has the letter B in it, the fastener features internal threads.
Why Class Threads Are Important
Class threads are important for several reasons. First, they can affect load requirements. High-stress applications involving heavy loads often require fasteners with tight tolerances. Tight tolerances allow fasteners to withstand greater forces.
Tight tolerances, though, can cause problems in applications where temperature changes are prevalent. If two fasteners fit tightly together, temperature changes may weaken them and jeopardize the parts with which they are used. For environments where temperature changes are common, fasteners with loose tolerances may be recommended.
