Thread Systems Simplified

It’s true: variety is the spice of life – at least in most cases. When it comes to thread systems, however, a variety of forms, terms, and use cases can leave some with a bad taste. In this brief overview, we’ll break down some of the most common categories of thread systems as well as a brief history of the thread we know today.

 

Thread System Categories

Let’s begin with two broad categories of screw thread systems: symmetrical threads and translation threads.

Symmetrical threads have a few advantages up their sleeves (or in their grooves). Symmetrical threads are easier to manufacture and inspect. They can be matched with general-purpose threaded fasteners of all types, which makes them common in many industrial applications.

There are some threads that are quite literally more capable of doing the heavy lifting. For threads that are used to repeatedly move or translate machine parts against heavy loads, aptly-named “translation” threads are needed. A common example is the square thread which is the most efficient but also the most difficult to manufacture given its squared, parallel sides. Moreover the Acme form is another common type of translation thread, and while it is not as efficient as the square thread, it is stronger and can be adjusted to compensate for wear (unlike the square thread). Lastly the non-symmetrical form of the buttress thread appropriately lends a hand when used to translate loads in one direction.

 

A History of Threads

They say you can’t rush perfection, and that applies even to screws. The common threads we see today are the product of centuries of innovation and testing.

 

V-Thread and Sharp V-Thread

The V-Thread isn’t as common today, and in fact it is usually only made per request for special applications. However, it is an ancestor of sorts to modern-day threads. This thread gets its name from the sharp, 60-degree angle its threads form. While excellent for situations where steam-tight joints are necessary, this thread was discontinued in 1909 due to the fact that its simple design was actually difficult to produce.

 

United States (American) Standard Screw Thread

Successor to the V-Thread in many ways, the American Standard thread was commonplace in manufacturing for many years. Dating all the way back to 1864, this thread was specifically designed for the US Navy, which is later how the thread earned its namesake.

 

Unified Screw Thread

The Unified Thread Standard (UTS) defines a standard thread form and series commonly used in the US and Canada.Today it is the main standard for bolts, nuts, and a wide variety of other threaded fasteners.The outer diameter and pitch dimensions were designed relative to an inch fraction rather than a millimeter value like its ISO Metric cousin.

 

ISO Metric Thread System

Unlike the Unified Screw Thread, which is mostly common in the US and Canada, the ISO metric screw thread is the most commonly used type of general-purpose screw thread worldwide. This thread gets its name from the International Organization for Standardization (ISO) which established its design in 1947. You may often notice a "M" designation for metric screws, which indicates the nominal outer diameter of the screw thread in millimeters. For example, an M6 screw has a nominal outer diameter of 6 millimeters and will therefore be interoperable with a 6mm diameter hole.

The world of thread systems offers a wide variety of fascinating complexities. While overwhelming to newcomers, learning a bit about their broad categories and history can serve as an excellent starting point for the mastery of today’s thread systems. For more information or to work with a consultant, contact us at [email protected]