There are two common forms of threads: straight threads and tapered threads. Straight threads have a parallel profile that remains the same diameter throughout the part. Tapered threads taper along with the thread profile and become smaller in diameter as the part moves down.
Generally, there are two thread series, namely The ISO Metric Screw Thread (in millimetres) and The Unified Screw Thread (in inches).
There are two types of threads to be managed in a modern system: User threads and kernel threads. User threads are supported above the kernel, without kernel support. These are the threads that application programmers would put into their programs. Kernel threads are supported within the kernel of the OS itself.
There are three standard thread series in the Unified screw thread system that are highly important for fasteners: UNC (coarse), UNF (fine), and 8-UN (8 thread).
The most common thread types are the inch-based Unified coarse / fine (UNC/UNF) and metric coarse / fine.
Metric thread is the most widely used today, and can otherwise be referred to as 'ISO Metric' or 'M'. Both coarse and fine thread fasteners are available, and can be supplied in a wide range of materials and sizes – with a number of popular DIN standards.
BSP & NPT are two different thread standards that originate from two different countries. BSP stands for British Standard Pipe & NPT stands for National Pipe Taper. BSP, as in the name, is from England while NPT is from the USA.
If you need a tighter connection and higher sealing performance, UNF threads may be a better fit. If you need better sealing performance and adapt to the connection needs of the piping system, NPT thread may be more suitable. Compatibility: It is also important to consider compatibility with other components.
UNC and UNF are two different types of Unified National Coarse (UNC) and Unified National Fine (UNF) thread standards. UNC threads have a larger thread pitch than UNF threads, which means there is more distance between each thread.
Metric threads are designated with a capital M plus an indication to their nominal outer diameter and their pitch: This bolt is M10 x 1.5 - which means the outside diameter is 10mm and the threads are 1.5mm apart (the most common thread pitch for M10).
Heavy duty thread
As the name says, the heavy-duty thread is designed to withstand high tension and wear and it's generally thicker than regular all-purpose thread. It can be cotton, polyester, a combination of the two, or even nylon.
Kevlar(TM) thread is one of the strongest and most fire retardant commercially available threads. It is about 2.5 times stronger than nylon or polyester, has almost no stretch, does not melt, and decomposes at 800F.
Whitworth threads have a steeper flank than metric threads: flank angle 55°, compared to 60° for metric threads. Tapered threads (BSPT) according to DIN EN 10226-1 (formerly DIN 2999) are designated as metallic sealing (R).
Standard fasteners are either Unified National Coarse Thread (UNC) or Unified National Fine Thread (UNT). Standard metric threads, meaning ISO metric threads in this case, are not specified this way, though they are comparable to UNC threads. Standard metric screw threads are designated by pitch in millimeters (mm).
NPT/NPS threads have a 60° angle and have flattened peaks and valleys (Sellers thread form) where as BSP threads have a 55° angle and have rounded peaks and valleys (Whitworth thread form). NPT and BSP thread pitches (threads per inch) are listed below.
The way to tell the difference between the two is the flank angle. The flank angle for NPT is 60° compared to the BSP tapered which is 55° (this can be verified with a thread gauge). BSP threads are measured in imperial measurements such as 1/8, 1/4, 1/2, 3/4 etc.
Metric threads are measured by their pitch. That is the distance from the crest of one thread to the crest of the next measured in millimetres. The pitch of a metric thread can be checked by direct measurement with a metric rule. Imperial threads have their pitch measured indirectly as a given number of teeth per inch.
NPT and BSP threads are generally not compatible due to the differences in their thread forms. NPT threads have a 60° included angle and have flattened peaks and valleys (this is a Sellers thread form); BSP threads have a 55° included angle and have rounded peaks and valleys (this is a Whitworth thread form).
This one is quite simple, a BSP thread is British Standard Pipe and BSPT thread is British Standard Pipe Tapered. You will note our earlier blog about how to measure a BSP thread, which is extracted as below.
As international standard threads for joining fittings and pipes, BSP's popularity is primarily in England and other commonwealth countries, while NPT is the predominant pipe fitting in North America.
UNF threads have better torque-locking and load-carrying ability than UNC threads because of their larger minor diameter. Because of their more specific fit, they have tighter tolerances, finer tension adjustment, and can carry heavier loads.
BSP threads are British standard pipe threads. The profile angle of BSP threads is 55 degrees. UNF are unified fine threads of American standard. Their thread profile angle is 60 degrees.
UNF: Is the symbol for Unified fine pitch threads. UNEF: Is the symbol for United extra fine pitch threads. UNS: Is the symbol for Unified special threads. For instance, if the nominal diameter is a: 1/4" UNC, the number of screw threads is 20 (Coarse thread)