It may come as a surprise that the screws and bolts you use every day can have different thread types. Each one has its own drawbacks and benefits, as well as its own applications.
Luckily, there are only a handful of major screw thread types. They’re also distinct and follow a nationwide standard. If you ever need a specific type of screw thread, it won’t be too hard to find.
The three main types of screw threads are UNC, UNF, and UNEF. UNC threads are the most common and you can use them for most general construction projects. UNF threads are better when you have specific strength requirements, such as in the automotive industry. Industrial machinery uses UNEF because it holds up in the harshest environments and the thinnest materials.
The 3 Main Screw Thread Types
Screw threads are the sloping ridges that form helices on the outside or inside surface of a cylinder.
The United States and Canada use the Unified Thread Standard, or UTS, to define different thread forms and series. The specifications include the major diameter of the thread, the pitch measure, and the number of threads per inch.
In the UTS series, there are three main screw thread types. UNC and UNF are the two most common types, but UNEF threads are good for the most precise applications.
1. UNC – Unified National Coarse
A coarse screw thread is one with a smaller number of threads per axial distance (the curve of the screw).
These threads work well in brittle materials and have better fatigue resistance than finer threads. They’re also deeper than finer threads which means there’s less cross threading.
Cross threading is when the “male” fastener (e.g., the screw), doesn’t align with the “female” mating partner (like a nut). Less cross threading means faster and easier assembly.
Coarse threads are also less prone to stripping and are more resistant to damage during shipping and handling. Because of this, they’re less likely to fail during your own assembly.
2. UNF – Unified National Fine
A fine screw thread has more threads in the same space than coarse threads. Due to the small helix angles, these threads have better clamping force and better maintain joint tightness than coarser threads.
These same small angles also make UNC threads less prone to loosen under vibration.
UNF threads are also stronger than coarse threads of the same hardness. Because their tensile stress area is slightly larger, they’re also stronger in shear and tension than UNC threads.
Sometimes you may need to tap a hole for your screw. Tapping is when you cut a thread inside a pilot hole.
Then, you’ll be able to thread a cap screw or bolt inside the hole. It’s also the process of making the threads on nuts.
Since fine threads are smaller, there’s less material you have to cut. This makes tapping holes faster when you’re creating them for fine-thread screws.
3. UNEF – Unified National Extra Fine
UNEF threads share some of the same properties as UNF, just on an exaggerated scale.
Extra fine threads are obviously even smaller than UNF, and that increases their tensile strength. In fact, if a piece won’t require disassembly, it’s common to find UNEF threads in use.
While UNF threads are great for small assemblies, sometimes they just won’t cut it. In these very specific situations, a UNEF can do the job.
What Are The Applications Of Each Thread Type?
Now that you know the physical differences between screw thread types, you may be curious about their applications.
UNC – General Purpose Screw Threads
Coarse screw threads are some of the most common and preferable thread types in the UTS series. You can use them in the largest number of applications.
For instance, you can use coarse thread drywall screws if you’re drilling into wood studs. In short, as long as the measurements for your project aren’t too specific, UNC threads are the way to go.
A specific example of UNC preference is in the agricultural industry. Because of the heavy-duty nature of the equipment, agricultural machinery needs the high strength of coarse threads.
They also need to be able to take the equipment apart for repairs. A finer thread could hold too tightly, so a coarse thread works best.
UNF – When Measurements Matter
You can use Unified National Fine threads when your project requires more precision than usual. This includes when you have a smaller length of engagement.
Length of engagement relates to the length of your screw and its mate material (where you’re putting the screw).
For example, the length of engagement in 10 mm of material is more than in 8mm. This is assuming you use the same screw thread in each.
Finer threads are stronger in shorter lengths of engagement. This is because the density of the threads is greater. Therefore, they can clamp to more surfaces in a smaller space.
The shorter depth of UNF threads also means they’re good for thin-wall applications. Since they can withstand more intense vibrations than coarse threads, UNF threads are also better for high-vibration environments.
The aerospace and automotive industries use UNF threads because of the strict requirements for the tightness of their fittings.
UNEF – The Smallest And Strongest Applications
UNEF threads are much less common than the other two types because they’re so specialized. You can use them when a project requires both great strength and extremely specific requirements.
This is why American heavy-duty machinery uses UNEF to hold everything together. Not only does industrial machinery vibrate heavily, it also rarely needs disassembly.
UNEF threads are so fine that they offer the tightest clamping and tensile strength. Once you apply UNEF screws, it will be very hard to get them out again.
You can use UNEF to hold even the thinnest sheet metal together, and they won’t vibrate apart. This makes them great for application in electrical work.
UNEF threads have even greater thread engagement in small spaces than UNF threads.
The length of a screw matters in many of your everyday projects. However, it’s less common to need a specific thread type in general home improvement projects.
But there are various thread types and some applications do take them into account.
For the average home improvement project, UNC threads, or Unified National Coarse threads, are perfect. These threads have the widest range of applications.
UNC threads in particular are great when you don’t have very specific tensile requirements on your project.
If your project does have more specific strength requirements, UNF is the way to go. Unified National Fine threads are stronger than coarse for smaller lengths of engagement situations.
They are also better at withstanding vibrations. This makes them ideal for the precision and high vibrations of the aerospace and automotive industries.
Finally, for the most particular of projects, UNEF threads are ideal. They are even stronger and more vibration resistant than UNF threads and heavy-machinery industries use them often. Once UNEF screws are in place, there’s little that can shake them loose.