Everything you need to know about SAE threads

Threads are a key element in mechanical engineering and hydraulics. The most popular standard in Europe is the BSP thread (i.e., G), but one of the popular standards used in some specific industries is the SAE thread. In this article, we will take a closer look at this standard, its applications, and the differences compared to other thread systems.

What is an SAE thread?

SAE (Society of Automotive Engineers) is an organization dealing with standards for the automotive and mechanical industries - and it introduced the SAE thread standard. SAE threads, also known as UN (Unified Thread) threads, are widely used in hydraulics, pneumatics, and mechanical constructions. What interests us the most - they are often used in applications with gaseous media because they are considered to provide the most reliable guarantee of tightness. They are used, for example, in systems with refrigerants, heat pumps, air conditioning systems, etc.

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Basic types of SAE threads

SAE threads are divided into several categories - let's say types, of which the most important are:

• UNF (Unified National Fine) – fine threads, used in precise connections where high tensile strength is important.
• UNC (Unified National Coarse) – coarse threads, commonly used in structural elements due to ease of assembly and greater resistance to damage.
• UNS (Unified National Special) – special threads that do not fit into standard categories, used in specific applications.
• ORB (O-Ring Boss) – threads sealed with an O-ring, often used in hydraulics for tight connections.

Applications of SAE threads - where?

SAE threads are widely used in various industries, including:

• Automotive – bolts and connections in engines and mechanical systems of vehicles.
• Hydraulics – hoses and fittings in hydraulic systems, especially in North America.
• Aviation – applications requiring high precision and mechanical strength.
• Machinery industry – a wide range of applications in machine constructions and production.
• Heat pumps and heat exchangers for freon gas - connections of elements with gaseous media.

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Differences between SAE and metric threads

Although metric threads are dominant in Europe, SAE threads are still more commonly used in the United States and Canada. However, they can also be found in the industries mentioned above. Here are the key differences:

• Units of measurement – SAE threads are specified in inches, while metric threads are in millimeters.
• Thread angle – UN threads have a 60° angle, while some metric threads may have different values.
• Application (geographical differences) – metric threads dominate in European industry, while SAE is the standard in North America.

When was the SAE thread standard introduced?

The SAE thread standard (UN) was introduced after World War II - in 1949 as a unified thread system for the USA, Canada, and the United Kingdom. It was created as a result of collaboration between the Society of Automotive Engineers (SAE), the American Standards Association (ASA, now ANSI), and the British Standards Institution (BSI). It was and remains, as you can see, mainly a domain of Anglo-Saxon countries. It spread worldwide with globalization processes.

The goal of introducing the SAE thread - in post-war times - was to unify standards and simplify the production and maintenance of machines, as different thread systems were used before, e.g., Whitworth thread (BSW) in the UK and American threads (USS and SAE) in the USA. These countries already during World War II (when their technological cooperation tightened due to being in the same political camp) - recognized discrepancies in thread standards as a serious obstacle to the development of cooperation, trade, etc.

SAE thread dimensions - table

You can find the SAE thread dimensions below.

The table contains SAE thread dimensions – divided into external and internal threads and categorized into UNC and UFC.

Summary

SAE threads are an essential component in the mechanical, hydraulic, and automotive industries. Their standardization allows for use in a wide range of applications, although their popularity is limited geographically and by industry. Understanding the differences between SAE and metric threads is crucial when working with mechanical and hydraulic systems.

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