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Engineers and designers, especially those working in the hydropower industry, are generally familiar with two groups of pipe threads: American Threads (NPT/NPTF, often referred to as SAE) and Unified Threads (UN/UNF) . But there seems to be an infinite number of other unknown threads, generally classed as “metric”, regardless of their actual shape. In fact, there are true metric threads as well as BSP (British Standard Pipe) threads.
For identification purposes, hydraulic tube fittings and connectors are divided into six thread types:
. UN/UNF
. NPT/NPTF
. BSPP (BSP, parallel)
. BSPT (BSP, conical)
. Metric parallel
. Tapered Metric
Three of them are parallel (UN/UNF, BSPP, metric parallel) and three are tapered (NPT/NPTF, BSPT, metric taper). Three are pipe threads (NPT/NPTF, BSPT, BSPP) and three are not (UN/UNF, metric parallel, metric taper).
Regardless of the type of thread, there are a few relatively simple steps that engineers, designers, and technicians can follow to correctly identify the threads used on hydraulic fittings.
1. Determine if the thread is tapered or parallel. Sometimes it is enough to examine them. Tapered threads have a diameter that decreases towards the end of the fitting; the parallel threads have the same diameter from one end to the other. If this is not obvious by looking at the fitting, use the parallel jaws of a vernier caliper, for example, to make a more precise comparison. An o-ring or tube nut can usually help show if the threads are parallel.
2. Determine the pitch. Pitch is defined in terms of threads per inch (TPI) or distance between threads. For metric threads, the pitch is the distance between the threads (mm). A pitch gauge is useful for comparing threads. A word of advice: it is easier to compare nets on a lighted background with a pitch gauge. As some thread pitches are nearly identical, experiment with several gauges before deciding on the best fit. (Possible pitches are shown in the second column of the “How to identify threads” table.) You can also go back to the simple but demanding process of measuring the number of threads in an inch.
3. Determine the size. There are two methods for determining the size of threads: one for pipe threads and one for non-pipe threads. For pipe threads, determine the nominal size by comparing the part to a size profile. A good rule of thumb for pipe sizes up to 2 in. nominal is to measure the outside diameter and subtract ¼ in. Round up to get the nominal pipe size. For non-pipe threads, measure the major diameter (OD) of the threads with a vernier caliper.
4. Designate the thread. This last step is not part of the identification process; it is a way to designate rule types in a standard format. Designations shall indicate thread size (nominal or actual), type and, in some cases, pitch.
Here are more tips and little-known facts about threads.
. Standard thread diameters are no larger than the sizes listed. When measured accurately, with a vernier caliper for example, the measured diameter is normally slightly smaller than the actual size of the thread.
. Do not use these steps to identify internal threads as it is difficult to check the fit of the pitch gauge on these. One way around this problem is to find the corresponding external thread and then determine its pitch.
. Tapered outer threads mate with parallel inner threads.
. NPSM threads are parallel but have the same pitch as NPT/NPTF threads. NPSM threads are typically used to hold swivel nuts together. Sealing is provided by contact between the 30 degree internal chamfer of the male thread and the seating area inside the swivel.
