There are different types of threads found, and some main types of threads are parallel threads, including BSPP, UN/UNF, and metric parallel. The second type of thread is tapered threads in which metric tapered, BSPT, and NPT thread/NPTF thread are included. Let’s learn the difference between each one of them.
What is Screw Thread?
A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread.
A screw thread is the essential feature of the screw as a simple machine and also as a threaded fastener. The mechanical advantage of a screw thread depends on its pitch, that is the linear path that the screw covers in one revolution.
In most applications, the pitch of a screw thread is chosen so that there is sufficient friction to prevent the linear motion from being converted into rotary motion, that is so the screw does not slip even with the application of a linear force, as long as there is no external torque.
This property is essential for the vast majority of its applications. Tightening the screw thread of a fastener is comparable to driving a wedge into a gap until it sticks through friction and slight elastic deformation.
Types of Threads
There are different types of threads which divided into six main types: UN/UNF, NPT/NPTF, BSPP (BSP, parallel), BSPT (BSP, tapered), Metric parallel, Metric tapered.
There are three standard thread series in the Unified screw thread system that is highly important for fasteners: UNC (coarse), UNF (fine), and 8-UN (8 thread).
1. UNR Threads
The UNR thread is a modified version of a standard UN thread. The single difference is a mandatory root radius with limits of 0.108 to 0.144 times the thread pitch. When first introduced decades ago, it was necessary to specify UNR (rounded root) threads.
Today, all fasteners that are roll threaded should have a UNR thread because thread rolling dies with rounded crests are now the standard method for manufacturing most threads.
2. UNJ Threads
UNJ thread is a thread form having root radius limits of 0.150 to 0.180 times the thread pitch. With these enlarged radii, minor diameters of external thread increase and intrude beyond the basic profile of the UN and UNR thread forms.
Consequently, to offset the possibility of interference between mating threads, the minor diameters of the UNJ internal threads had to be increased. 3A/3B thread tolerances are the standard for
3. UNJ threads.
UNJ threads are now the standard for aerospace fasteners and have some usage in highly specialized industrial applications. UNJ bolts are like UNR, but the curve of the thread root is gentler which requires that it be shallower.
In fact, the thread root is so shallow that the bolt thread cannot mate with a UN nut, so there is a UNJ nut specification as well.
Different Types of Screw Threads
- British Standard Whitworth (B.S.W.)
- British Association (B.A.) thread
- American national standard thread
- Unified standard thread
- Square thread
- Acme thread
- Knuckle thread
- Buttress thread
- Metric thread
1. British Standard Whitworth (B.S.W.)
British Standard Whitworth (B.S.W.) thread. This is a British standard thread profile and
has coarse pitches. It’s an asymmetrical V-thread in which the angle between the flanks, measured in an axial plane, is 55°.
These threads are found on bolts and screwed fastenings for special purposes. The various proportions of B.S.W.
The British standard threads with fine pitches (B.S.F.) were used where great strength at the root
required. These threads are also used for line adjustments and where the connected parts are
subjected to increased vibrations as in aero and automobile work.
The British standard pipe (B.S.P.) threads with fine pitches are used for steel and iron pipes and
tubes carrying fluids. In external pipe threading, the threads are specified by the bore of the pipe.
2. British Association (B.A.) thread
This is a B.S.W. thread with fine pitches. The proportions of the B.A. thread. These threads are used for instruments and other precision works.
3. American national standard thread
The American national standard or U.S. or Seller’s thread has flat crests and roots. The flat crest can withstand more rough usage than sharp V-threads. These threads are used for general purposes e.g. on bolts, nuts, screws, and tapped holes.
4. Unified standard thread
The three countries i.e., Great Britain, Canada, and United States came to an agreement for a common screw thread system with the included angle of 60°, in order to facilitate the exchange of machinery. The thread has rounded crests and roots.
5. Square thread
The square threads, because of their high efficiency, are widely used for the transmission of power in either direction. Such types of threads are usually found on the feed mechanisms of machine tools, valves, spindles, screw jacks, etc.
The square threads are not so strong as V-threads but they offer less frictional resistance to motion than Whitworth threads. The pitch of the square thread is often taken twice that of a B.S.W. thread of the same diameter.
6. Acme thread
It is a modification of the square thread. It is much stronger than square thread and can be easily produced. These threads are frequently used on screw-cutting lathes, brass valves, and bench vices.
When used in conjunction with a split nut, as on the lead screw of a lathe, the tapered sides of the thread facilitate ready engagement and disengagement of the halves of the nut when required.
7. Knuckle thread
It is also a modification of the square thread. And it has a rounded top and bottom. It can be cast or rolled easily and can not be economically made on a machine. These threads are used for rough
and ready work.
They are usually found on railway carriage couplings, hydrants, necks of glass bottles, and large molded insulators used in the electrical trade.
8. Buttress thread
It is used for the transmission of power in one direction only. The force is transmitted almost parallel to the axis. This thread unit has the advantage of both square and V-threads.
It has a low frictional resistance characteristic of the square thread and has the same strength as that of a V-thread. The spindles of bench vices are usually provided with buttress threads.
9. Metric thread
It is an Indian standard thread and is similar to B.S.W. threads. It has an included angle of 60° instead of 55°.
Screw Thread Terminology
Knowing the different screw types and sizes is helpful, but to accurately select the right screw for your project, you’ll likely need to know a little about how screw threads work, as well. So, how do screw threads work? Like the screws themselves, screw threads are designed to meet the needs of specific applications.
You can try to find a screw thread chart that outlines all of your different options, but what you really need is a guide to screw thread terminology that answers all your questions and provides a comprehensive rundown of different screw and bolt thread types why they matter.
To help you make the best investments in fasteners for your next project, here’s a thorough guide to screw thread terminology:
- External Threads: External threads (i.e.: male threads) mean the threads are on the bolts or screws. Threads are sometimes on the nuts rather than the bolts or screws.
- Internal Threads: Internal threads (i.e.: female threads) mean the threads are on the nuts rather than the bolts or screws.
- Machine Screw Threads: Machine screw threads are unique in that they are specially designed to mate with threads on nuts or threads present in tapped holes. Not self-tapping threads.
- Spaced Threads: Spaced threads are designed to form their own threads in pre-drilled holes. You’ll most often find spaced threads on self-tapping, wood, and coach screws.
- Lag Screws: Lag screws are just one example of screws that form their own thread in pre-drilled holes. Typically, coach screws are used to fasten metal to wood.
- Self-Tapping Screws: Self-tapping screws are threaded to create their own holes when installed. They form a matching thread in whatever material they are installed into. This makes using self-tapping screws incredibly efficient.
- Thread-Forming Screws: Used with plastic materials, thread-forming screws feature two separate threads: one high and one low. This makes the pullout strength higher while also ensuring plastic does not crack or otherwise break.
- Type U Screws: Type U screws include an unusual spiral thread that is most often driven with a hammer into materials like plastic and metal casings.
- Wood Screws: Wood screws feature a tapered shank with sharp threading.
Additional Fastener Terminology
Here are a few additional terms related screws and threading that are helpful to know:
- Major Diameter: This refers to the diameter of a screw including the raised helix’s height like an imaginary cylinder around the thread. It is measured using a slot gauge or a caliper rule. You can only measure major diameter with an external thread screw
- Minor Diameter: Minor diameter is a screw’s diameter measured at the base or root of the thread at the innermost part of the screw. You need specialized equipment to measure the minor diameter accurately.
- Effective Diameter: The effective diameter is essentially the average of the major and minor diameters. It is measured halfway up the raised helix, and, again, you need specialized equipment to get an accurate sense of a screw’s effective diameter.
- Pitch: The pitch is the distance between two threads on the same screw.
- Crest: The crest is the height of an external thread. Or, you can find the crest by subtracting the minor diameter from the major diameter. The crest is the difference between the two.
- Thread angles: The thread angle is the actual angle of both flanks of a screw. Symmetrical threads indicate that both sides of the thread are angled to the same degree. It’s common for thread angles to be referred to as simply “the flank.”
- Coarse and fine threads: The terms “coarse” and “fine” refer to the distance between the crest of each thread. Smaller gaps create fine threads and larger ones create coarse threads.
Why It’s Important to Get the Right Screw Thread
Not all screw threads are going to work with all applications. Each one is tailored to different tasks and different material sizes.
Some will require pre-drilled holes while others have sharp designs meant for drilling directly into softer materials like wood. Other screws vary in thread spacing, which is important for various characteristics of the screw. If using any bolts or nuts, you’ll need to make sure they match the spacing of the screws.
By paying special attention to these characteristics, you can more readily ensure that your choice of screw provides appropriate strength and performance for the application.