Different Types Of Threads And Their Uses

There are different types of threads, and threads can come in many varieties. The commonly discussed two types of threads are parallel threads, which include BSPP, UN/UNF, and metric parallel.

The second type of thread is tapered threads; there are metric tapered, BSPT, and NPT threads/NPTF threads. Let’s look at each one!

What is Screw Thread?

A screw thread, often shortened to thread, is a helical structure used to convert between rotary and linear movement or forces.

A screw thread is a ridge wrapped around a cylinder or a cone which in turn produces a helix. The ridge around the cylinder is called a straight thread, and the ridge around a cone is a tapered thread.

The screw thread is the fundamental component of the screw as a machine, and threaded fasteners. The mechanical advantage of a screw thread is a function of pitch, pitch is defined as the linear distance of the screw in one revolution.

In most of the applications, the pitch of a screw thread is designed to provide sufficient friction to hold it from converting the linear motion into rotary motion, that is the screw will not slip even with a linear force is applied, unless there is an external torque.

This property is essential for the vast majority of its uses. As can be seen, the action of tightening screw thread is similar to a wedge being forced into a gap until the screw sticks through friction and a small amount of elastic deformation.

Types of Threads

The Unified screw thread system has three standard thread series that are very important for fasteners: UNC (coarse), UNF (fine), and 8-UN (8 thread).

#1. UNR Threads.

The UNR thread form is a modified standard UN with a single difference being a required root radius with limits of 0.108 to 0.144 times the pitch. At the time of its introduction decades ago, UNR (rounded root) threads had to be specified.

Every fastener that is roll-threaded today should have a UNR thread because thread-rolling dies with rounded crests is now industry standard practice in producing most threads.

#2. UNJ Threads.

UNJ thread form has root radius limits of 0.150 to 0.180 times the pitch. These increased radii in essence increase the minor diameters of the external thread so that they encroach outside of the basic UN and UNR thread profile.

With the encroachment of the minor diameters of the UNJ internal threads, it became necessary to increase their minor diameters to avoid encroachment or interference with mating threads. 3A/3B thread tolerances are the standard for

#3. UNJ threads.

UNJ threads have become the standard thread for aerospace fasteners and are used in specialized industrial applications. UNJ bolts are similar to UNR, but the threaded root curve is more gentle, requiring it to be a shallower root.

The threaded root will be so shallow that the bolt thread will not 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.)

B.S.W. is a British Standard thread profile with coarse pitches. This is an asymmetrical V-thread in which the angle between the flanks, measured in an axial plane, is 55°.

These threads are used on bolts and screws and fastenings for special purposes. Various proportions of B.S.W.

The British standard threads with fine pitches (B.S.F.) were used where great strength at the root was required. These threads are also used for line adjustments, as well as where the connected parts are exposed to much greater vibration, 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, threads are specified by the bore of the pipe.

#2. British Association (B.A.) thread.

The B.A. thread is a B.S.W. thread with fine pitches. The proportions of the B.A. thread. These threads are used on instruments and other precision works.

#3. American National Standard Thread.

The American national standard thread or U.S. or Seller’s thread has flat crests and roots. The flat crest can stand more rough usage than the sharp V- threads. These threads are generic, e.g. on bolts, nuts, screws, and tapped holes.

#4. Unified standard thread.

To support the interchangeability of machinery, Great Britain, Canada, and the United States have agreed on a unified screw thread family with a 60-degree included angle. The thread form has rounded crests and roots.

#5. “V” shape threads.

This thread form looks like the letter “V” from the English alphabet.

V-shape threads are the most common thread needing to be cut with a tap, die, lathe machine, or milling machine, which can be made from multiple standards.

#6. Square thread.

Square threads are very effective to transmit power in either direction, and they’re widely used in these applications. Square threads are found on the feed mechanisms of machine tools, valves, spindles, screw jacks, etc.

Square threads are not as strong as V-threads, but they also have less resistance to motion than Whitworth threads. The pitch of the square thread is often considered to be twice that of a B.S.W. thread of the same diameter.

#7. Acme thread.

It is a modified square thread type. They offer greater strength than the square thread and are more easily made. Acme threads are often used in screw cutting lathes, brass valves, and bench vices.

Acme threads, when used with a split nut, as found on the lead screw of a lathe, can easily be engaged and disengaged because of the tapered sides of the treads.

#8. Knuckle thread.

Knuckle threads are also a variation of the square thread and have a rounded top and bottom. Knuckle threads can be easily cast or rolled, but not made economically on machine tools. These threads will, however, serve quite well when being used in production or rough and ready work.

Knuckle threads are often found on railway carriage couplings and fittings, hydrants, the necks of glass bottles, and large insulators commonly molded into shape for the electrical trade.

#9. Buttress thread.

The buttress thread is used for the transmission of force in one direction only. In this type of thread , the force is applied nearly parallel to the axis. The buttress thread has advantages of both square thread and V-threads.

The buttress thread has a lower resistance to motion like a square thread and it’s strength is similar to that of a V-thread. Buttress threads are often found on the spindles of bench vices.

#10. Metric thread.

The Metric thread is an Indian standard thread type and follows a similar design to a B.S.W. thread. The only difference is the metric standard has a angle of 60 degrees instead of a 55 degree angle.

Screw Thread Terminology

While having an awareness of the many screw sizes and types is helpful, to select the correct screw for your project, you’ll likely also need some knowledge about how screw threads work.

So, how do screw threads work? Similar to the screws themselves, screw threads are formed to suit the requirements of specific applications.

You can search for a screw thread chart that shows you all of the screw types and their options, but what you need is a guide to screw thread terminology that will answer all of your questions and give you a breakdown of different types of screw and bolt threads and threads and how they matter.

To assist you in making wise fastener purchases for your next project, here is a full guide to screw thread terminology:

• External Threads: External threads (also referred to as male threads) specifically indicate that the threads are present on the bolts or screws. Occasionally, the threads may be located on the nuts as opposed to the bolts or screws.
• Internal Threads: Internal threads (also referred to as female threads) identify the threads are located on the nuts as opposed to the bolts or screws.
• Machine Screw Threads: Machine screw threads are distinctive in that they are made to mate with threads located on nuts or threads cut into pre-tapped holes. They are not self-tapping threads.
• Spaced Threads: Spaced threads indicate a threaded product made to cut their own threads in previously drilled holes. The most common use of spaced threads will be with self-tapping screws, wood screws, and coach screws.
• Lag Screws: Lag screws are simply one type of screw that cuts its own threads in pre-drilled holes. Coach screws are used to fasten metal to wood.
• Self-Tapping Screws: Self-tapping screws are threaded to form their own holes upon installation, and form corresponding threads to whatever substrate they are installed into. This can be very time efficient when using self-tapping screws for the installer.
• Thread-forming Screws: Thread-forming screws feature two separate threads (one high one low) and are often used with plastic materials. This is to provide more pull-out strength while avoiding cracking or breaking the plastic.
• Type U Screws: Type U screws feature an unusual spiral thread and are most often knocked into plastics and metal casings with a hammer.
• Wood Screws: Wood screws feature a tapered shank with sharp threads.

Additional Fastener Terminology

The following are some additional terms associated to screws and threading that will be beneficial to be familiar with:

• Major Diameter: Major diameter is defined as the diameter of a screw measured at the height of the raised helix similar to an imaginary cylinder around the thread. The major diameter is measured by either a slot gauge or a caliper rule. Major diameter can only be measured in an external thread screw. 
• Minor Diameter: Minor diameter is defined as the diameter of a screw measured at the base or root of the thread (innermost part of the screw). Minor diameter must be measured using specialized equipment.
• Effective Diameter: The effective diameter is defined as the average of the major and minor diameters. The effective diameter is measured halfway up the raised helix, and again, requires used specialized equipment to appreciate the true nature of a screw’s effective diameter.
• Pitch: Pitch is defined as the distance between the two threads on the same screw.
• Crest: The crest is defined as height of an external thread. Alternatively, the Crest can be calculated by subtracting the minor diameter from the major diameter. Crest is defined as the difference between the two.
• Thread angles: The thread angle describes the actual angle of both flanks of a screw. When threads are symmetrical, it means that both sides of the screw thread are angled at the same degree. Thread angles are often referred to as simply “the flank.”
• Coarse and fine threads: The terms coarse and fine refer to the distance between the crests of each thread. A screw with smaller gaps refers to fine threads, and a screw with larger gaps refers to coarse threads.

Why Getting the Right Screw Thread Matters?

Not all screw threads work for all applications; screw threads are designed for various tasks and material sizes.

Some screws need drilled holes while others have sharp designs for drilling right into soft materials like wood. Other screws vary in spacing between threads, which is important for different characteristics of the screw.

If using any bolts/nuts, ensure they match the spacing of the screw(s).

By focusing on these characteristics, you will more easily ensure that you obtain a screw that has acceptable strength for the application and reliable performance.

FAQ.