Fine threads are technically stronger under static loading, because they have a larger minor diameter which translates into a larger cross-sectional area A s. If using fine threads to increase strength, it is important to make sure that the mating thread – the nut or tapped hole – can support the additional load.
It's a legitimate question, considering that coarse threads are stronger and have an overall greater resistance to cross-threading and stripping, while the fine thread fasteners have a slightly larger tensile stress area. The short answer is that for general industrial applications, coarse thread bolts present several advantages over fine threads.
Coarse threads have become the standard for most fastener applications, but when should fine threads be considered? Fine threads are technically stronger under static loading, because they have a larger minor diameter which translates into a larger cross-sectional area A s.
The trouble with fine threads are that the threads themselves are much weaker. Repeatedly taking the fastener in and out will damage the threads. The other issue is that it is easy to “cross thread” the nut when installing on a fine threaded bolt.
Size for size, a fine thread is stronger than a coarse thread. This is both in tension (because of the larger stress area) and shear (because of their larger minor diameter). Because of the smaller pitch, they allow finer adjustments in applications that need such a feature.
Pros of Coarse Threads: Greater resistance to stripping and cross-threading. Faster installation than fine threaded fasteners, partially because they aren't prone to seizing during tightening. This can result in significant time and money savings in high volume assembly applications.
For fasteners with the same nominal size, fine thread fasteners are stronger than coarse.
Coarse threads are more durable and have greater resistance to stripping and cross-threading. The height of each thread is greater than the corresponding fine thread so there is more material between each thread making flank engagement greater.
Fine threads also possess larger minor diameters, which provide high shear strengths. Coarse threads tap better into brittle materials and are less likely to cross thread. Coarse threading lends itself to thicker coatings and platings before thread adjustments need to be made.
Coarse drywall screws feature coarse threads to secure drywall boards to studs. Fine drywall screws feature smaller heads and are used to secure drywall to metal studs. Self-drilling screws and pan-head screws can be used with metal studs or frames. Trim-head screws are used to attach wood trim over drywall boards.
The main difference between the two is in how large is the increment in each step. With coarse, a small movement results in a large jump, while the opposite is true in fine.
Standard Thread Pitch ChartCoarse Thread Series – UNCNominal Size and Threads Per In.Basic Pitch Dia.Section at Minor Dia.3⁄8 – 160.33440.06787⁄16 – 140.39110.09331⁄2 – 130.450.125722 more rows•Jul 2, 2015
To be classified as coarse, the threading must be deep set and wide. On the other hand, fine threaded fasteners will have narrower, shallow ridges and a smaller pitch, with a higher number of threads per axial distance than coarse threaded bolts.
A 1/4 UNF thread is about 14% stronger than its UNC counterpart. Coarse threads are slightly stronger (against stripping) per length of engagement than finer threads (see thread strength section below).
It is caused by excessive friction or adhesion between mating threads. Material can remained attached, as if it were partially sheared off, when the threads are unscrewed. So, one thread will have material removed while the mating thread may have some of this material stuck or welded to it.
Fine Thread. While coarse threading fasteners are better suited for general purpose applications, fine thread applications tend to have many benefits that make them the ideal choice when specifics matter. Fine threading has a higher tensile strength than coarse threading and it can be measured more accurately.
Commonly chosen for construction simply because their threading can be lightly damaged and still work effectively. Coarse threading allows for more material to sit between these threads as they are installed.
This helical shaped threading can be either wide set and deep which are considered coarse thread while a more narrow helical shape with shallow ridges is considered to be fine thread.
Fine thread fasteners are ideal for environments where vibration is considered an issue. Now that you know how to compare coarse thread vs. fine thread fasteners, you can make the right choice when deciding on which fasteners you need for a project.
Fine threading consists of very shallow peaks and valleys when compared to coarse. As a result, tapping a hole for fine threading can be significantly easier as there is much less material to cut to form the threads. Fine threading also has a very shallow pitch (helical angle).
Due to this, it is harder for coarse threaded fasteners to strip or cross thread. Their extra tolerance allows for nicks or marring to occur on the edges of the threading without inhibiting the ability to fasten them. Coarse threading is also much less likely to gall then fine threading.
Coarse threading is also much less likely to gall then fine threading. Fine threading requires many more turns per inch which generates more friction. Excess friction dramatically increases the chance of galling in fasteners. Regardless we recommend using an anti-seize solution especially on stainless steel fasteners.
However, this means that they have a smaller non threaded cylinder section compared to fine pitch threads. Therefore, coarse pitch threads have stronger threads and weaker bodies. Fine pitch threads are the opposite which have weaker threads and stronger bodies.
Coarse pitch threads are ideal when threading into a base material which is equal to or stronger then the fastener. As stated earlier the extra material in the coarse pitch threads means that the threads are stronger then fine pitch.
A bolt on the other hand passes through all of the pieces of material to be clamped and does not interact with any of them. Instead, a nut or other pre-made female thread is applied to the end of the bolt and the material is clamped between the nut and head of the bolt.
Thread pitch deals with how many threads are in a given length of a fastener per its nominal diameter . For example, a fastener could have large and widely spaced threads. This is referred to as having a coarse pitch. A fastener with the same nominal diameter that has more frequent threads per unit of length has what is known as a fine pitch.Coarse ...
Therefore, it is better to use coarse pitch threads in soft materials when the fastener will be repetitively removed and reinstalled.Fine pitch threads give you more threads per length and therefore more load to be distributed into the base material.
Threaded fasteners are almost always intended to clamp (fasten) two or more pieces of material together. Unless specially designed to do so, threaded fasteners are not intended to locate pieces of material relative to each other or prevent pieces of material from sliding. Threaded fasteners are strongest in tension (being pulled apart) ...
The depth of a fastener should be at least two times its nominal diameter. For example a 1/4in (0.25in) fastener should be threaded at least 1/2in (0.50in) deep. Any deeper than this and the threads don’t really do much except add assembly time.Thread pitch also plays a part in how deep a fastener should be.
Fine threads are technically stronger under static loading, because they have a larger minor diameter which translates into a larger cross-sectional area A s.
In joints with high cyclical loading, fatigue failure becomes a concern. Repeated cyclical stress can create cracks which typically occur in the first loaded thread of the joint. Studies have shown that fine threads increase the percentage of load on the first thread, which can lead to a shorter fatigue life.
In conclusion, fine threads should not be used for load bearing joints unless there is a very specific reason and testing is done to validate the joint. Some exceptions could be hard to tap materials or thin wall materials. For non-load bearing joints that require adjustment, fine threads may be your best option.
In the past, vibration resistance was thought to be a benefit of fine threads. The smaller helix angle, at least in theory, slows down the loosening process. A tradeoff of this benefit is the slower assembly time of fine threads; the smaller helix angle requires more angle of rotation to advance, slowing down the assembly process.
When working with stainless steel, aluminum, titanium and other alloys, fine threads are preferable, particularly because of their strength.
Coarse threads are typically called for in most industrial applications, particularly in aviation and military situations, due to a variety of advantages.
When it comes to fasteners, threads are key. Threads determine how well fastener components (like nuts, bolts and screws) hold materials and machines together, and maintain their integrity over time.
Fine threaded bolts can also gall (when metallic surfaces slide against one another and friction and adhesion cause material to be pulled from one surface and then adhered to the other) more easily than coarse threads, and they are often harder to obtain.