Bayou City Bolt understands that keeping track of the countless variations in threaded hardware can be difficult. However, it is crucial to know how to measure diameter, thread pitch and length correctly.
Having accurate measurements will ensure that your bolt and nut couple together properly. Without this, they could seize or strip.
Coarse
While it might seem odd, the terms coarse and fine when used in reference to bolt threads per inch do not refer to a difference in tolerances or craftsmanship, nor does the term fine imply higher quality. In fact, both coarse and fine threads can be equally high quality and equally precise, as long as the right application is chosen for each. The distinction lies solely in the relative strength and functionality of the two.
In the US, screw threads are typically* classified as either Coarse or Fine based on the number of threads per inch of their major diameter. This is a unique identifying characteristic of imperial (inch-based) fasteners, as metric bolts have a different Thread Pitch classification that cannot be interchanged with US screws.
Coarse bolt threads tend to have a higher pitch and are less forgiving of misalignment and looseness. They provide greater gripping power in a shorter length than their fine counterparts, and are typically used for applications requiring high loads or heavy clamping.
A fine thread has a lower pitch and is much more forgiving of misalignment and looseness. This allows for a smoother movement of the bolt while providing superior holding power. Fine threads are often used for applications requiring low to moderate loads, but they are equally suitable for tightening and loosening purposes.
The best way to understand bolt threads is to look at them, especially when they are in use. If you are able to look at the top of the screw head or the threaded end, you can find the bolt standard and diameter by looking for the UNC and/or UNF designations. If not, you can measure the threaded end to determine its major diameter and the number of threads within an inch to get the TPI. This is the most accurate method, but it can be time consuming and requires an eye for detail.
Fine
Using the right type of threaded fastener for the job can make or break an installation. However, it can be difficult for project managers to keep track of all the different variations of fasteners as they work to meet the needs of each unique project.
One of the most common thread types that is used in construction projects are fine threads. These have a tighter helix than coarse threads and offer superior strength in both shear and tension. Fine threads also have larger minor diameters which improve their resistance to vibration and allow them to hold tighter in thin materials.
Both metric and standard fasteners are available with either fine or coarse threads. When comparing different fasteners, it’s important to know that threads are identified by their TPI (threads per inch) value. For example, a bolt labeled as 1/4-20 has 20 threads per inch while a fastener that is a 1/4-28 will have 28 threads per inch.
Understanding bolt threading and TPI values can help project leaders select the right fastener for the task at hand. For instance, using a coarse threaded bolt in a fine threaded application can cause the fastener to seize or strip. This can result in the need to replace the bolt or complete a lengthy and costly repair process.
Another aspect to consider when comparing different bolt thread types is their corrosion resistance. While the helix of the threads plays an important role in this area, the underlying material of the bolt is more important.
If a fastener is exposed to an environment that has high salt levels or other corrosive elements, the threads may deteriorate and lose their ability to hold together. While this is not an issue in most construction applications, it is worth mentioning when selecting a fastener. To avoid this type of damage, the use of a coating or plating is often recommended.