Thread Types and Sizes

Threaded fasteners are used in numerous industrial and commercial applications around the world. Although they are tiny and their value often goes unnoticed, they form a vital component in an array of different products. They are used as fasteners for a multitude of reasons, such as their strength, removability, ease of installation and cost effectiveness.

However, obtaining threaded fasteners for a particular application can become difficult if one does not possess adequate knowledge regarding the various thread types and sizes. The need for certain tools and information are vital to identify which thread type and size is best suited for a given application. Before selecting a particular fastener, it is important to understand some basic terminologies of a standard threaded fastener.

Screw Thread Terminology

Some of the common terms faced when looking into thread types and sizes are summarised below.

llustration of a threaded fastener with useful terms labelled

Figure 1: Illustration of a threaded fastener with useful terms labelled (Ispatguru.com, 2018)

Major Diameter – Largest fastener thread diameter. For example, in a screw the major diameter would be the diameter of the shaft.

Minor Diameter – Smallest fastener thread diameter. For example, in a screw the minor diameter would be diameter of the inner part of the screw with the exception of the crests of the helix.

Pitch – Linear distance between one thread and the other next to it.

Thread Length – The total length of the threaded part in the fastener.

Flank – The angle at which the thread crest is raised from the thread root.

Thread Standards

A sound understanding of the information above will be useful to thoroughly understand and select appropriate thread types and sizes. Generally, there are two thread series, namely The ISO Metric Screw Thread (in millimetres) and The Unified Screw Thread (in inches). For both these series, there are two types of threads – fine thread and coarse thread.

Between these two types of threads, fine threads of the same hardness are generally stronger as they are able to withstand larger forces under tension and in shear. Fine threads can be easily tapped into materials and are therefore ideal for thin sections of walls.

They are widely used for a variety of general applications as well, such as aerospace components where high strength is a priority. Extra fine threads are also available which are used for highly strength orientated applications.

In materials of low tensile strength, the use of coarse threads provides more resistance from stripping when compared with fine or extra fine threads. Coarse threads have more resistance to failure via fatigue and allows for easier and quicker assembly of parts without the risk of cross threading.

They are less susceptible to extremes of temperature and less prone to damage via scratching or cutting. A scratch on a fastener of fine thread is much more problematic than one on a fastener with coarse thread. Coarse threads are mainly used in industrial as well as in military applications. (Norwood, 2018)

The Unified Thread Standard (UTS)

This series classifies the threaded fastener types as follows :

(Thomasnet.com, 2018)

UNC – Unified National Coarse Threads

The most commonly used and preferred fastener thread for general applications. Due to the presence of coarse thread pitch, threads are deeper and allows for easier assembly without the risk of cross threading.

UNF – Unified National Fine Threads

Large minor diameter and lesser thread depth allows for enhanced torque-locking and better load bearing capacity than UNC threads.

UNEF – Unified National Extra Fine

Threads are finer when compared with UNF threads. Ideal for holes tapped in hard material and aerospace components.

UNJC and UNJF Threads

“J” threads consist of two types-internal and external threads. UNJC and UNJF are very similar to UNC and UNF threads with the exception of the external thread having a larger root radius.

UNR Threads

External thread same as UNC with the exception of the root radius being rounded. Most popular choice amongst producers of external threads.

Constant Pitch Thread

Available in various diameter-pitch combinations to accommodate versatile applications wherein other types such as UNC, UNF and UNEF are not suitable.

Unified threads make use of numbered diameters from #0 to #10 with 0 being the smallest possible diameter and 10 the largest. The diameter of the threads can be interpreted using the following formula: 0.060″ + (numbered diameter) * 0.013″. Hence, #2 will have a major diameter of 0.086″ as from the above stated equation, (2) *0.013 + 0.060 = 0.086. Even though odd numbers pop up, even numbers are much more common.

In interpreting thread sizes, unified threads expresses the first number as the major diameter of the fastener. The second number represents the threads per unit length and the third number corresponds to thread length.

For example, a #2-40 x .5 screw has a major diameter of 0.086 inches, with 40 threads per inch and is 0.5 inches long.

The ISO Metric Screw Thread

The ISO metric screw thread system is the most widely used thread series. It is quite simple to identify and comprehend thread sizes that use the ISO metric thread designation.

The sizes start with an “M” denoting that it is measured using metric thread designation. The “M” is followed by the first number, which is the major diameter of the external thread in millimetres. The second number corresponds to the pitch, which is the distance between adjacent threads. Finally, the third number represents the thread length.

So, a screw with the size M9x2.0 x 12 will have a major external thread diameter of 9 millimetres, with threads per 2 millimetres and a thread length of 12 millimetres.

If the pitch is not specified in a metric size, then the fastener it is automatically considered to be of coarse thread series. In all other cases, the size of the pitch needs to be mentioned.

The more one deals with the two different nomenclatures and different applications, the more familiar and easier it becomes to comprehend the ideal type of fastener required and the specific size to be used. Once the specific size is determined, with knowledge of the two thread systems, the desired fastener can be bought for its required application.

References

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