Parallel Threads vs Tapered Threads
Parallel (Straight Threads) Vs Tapered Threads
Many mechanical end connections have threads. If the threads on two parts don't have the identical diameter and pitch, you can't connect them. Even experienced workers sometimes have difficulty identifying threads, regardless of their thread identification procedure and the quality of their gauges.
Threads have peaks called crests and valleys called roots. The part in between the crest and the root is called the flank. The first thing to check is whether the crests are all of the same diameter. Use calipers and measure the first, fourth and last crests. If the measurements are the same, you have straight threads. If you get three different measurements, then you have tapered threads. Worldwide, there are many end connections available with straight threads. Each end connection, however, will generally have threads that are one of the three most common: ASME B1.1 (unified screw thread), ISO 228/1, or ISO 261 straight threads. Because the threads of the mating fittings are parallel to each other, there is no interference between the flanks, crests, and roots. Consequently, the seal must be made with a gasket, O-ring, or some metal to metal contact. A sealant is not required or recommended on straight threads. Depending on the application and materials, thread lubricant may be used.
Tapered threads also have several standard forms of measurement: NPT (National Pipe Thread), ISO 7/1 (also known as British Standard Pipe), and metric. Tapered threads are designed to seal as the mating threads are drawn together. But they always need a sealant to seal system fluids and reduce the potential for galling of the threads. Products such as Swagelok PTFE tape, SWAK®, anaerobic pipe thread sealant with PTFE, and PTFE Free pipe thread sealant perform both the lubricating and sealing functions. If the two pieces of metal are forced against each other without lubrication, galling is possible.
If you want to learn about how to identify thread pitch and size, watch the below video.
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