Machinery's Handbook, 31st Edition
2098 DRYSEAL Pipe Threads Dryseal Special Extra Short Taper Pipe Thread, PTF-SPL EXTRA SHORT: Threads of this series conform in all respects to PTF-SAE SHORT threads except that the full thread length has been further shortened by eliminating two threads at the small end of internal threads or two threads at the large end of external threads. Limitations of Assembly: Table 9 applies where Dryseal Special Short or Extra Short Taper Pipe Threads are to be assembled as special combinations. Table 9. Assembly Limitations for Special Combinations of Dryseal Threads Thread May Assemble with a May Assemble with b
PTF-SAE SHORT INTERNAL NPSF INTERNAL PTF SPL SHORT INTERNAL PTF SPL EXTRA SHORT INTERNAL
PTF SPL SHORT EXTERNAL PTF SPL EXTRA SHORT EXTERNAL
NPTF or NPSI INTERNAL
PTF SPL SHORT INTERNAL
PTF-SAE SHORT EXTERNAL
NPTF EXTERNAL
PTF SPL EXTRA SHORT INTERNAL a Only when the external thread or the internal thread or both are held closer than the standard tolerance, the external thread toward the minimum and the internal thread toward the maximum pitch diameter to provide a minimum of one turn hand engagement. At extreme tolerance limits the shortened full-thread lengths reduce hand engagement and the threads may not start to assemble. b Only when the internal thread or the external thread or both are held closer than the standard tolerance, the internal thread toward the minimum and the external thread toward the maximum pitch diameter to provide a minimum of two turns for wrench make-up and sealing. At extreme tolerance limits the shortened full-thread lengths reduce wrench make-up and the threads may not seal. Dryseal Fine Taper Thread Series, F-PTF: The need for finer pitches for nominal pipe sizes has brought into use applications of 27 threads per inch to 1 ∕ 4 - and 3 ∕ 8 -inch pipe sizes. There may be other needs that require finer pitches for larger pipe sizes. It is recommended that the existing threads per inch be applied to the next larger pipe size for a fine thread series, thus: 1 ∕ 4 -27, 3 ∕ 8 -27, 1 ∕ 2 -18, 3 ∕ 4 -18, 1-14, 1 1 ∕ 4 -14, 1 1 ∕ 2 -14, and 2-14. This series applies to external and internal threads of full length and is suitable for applications where threads finer than NPTF are required. Dryseal Special Diameter-Pitch Combination Series, SPL-PTF: Other applications of diameter-pitch combinations have come into use where taper pipe threads are applied to nominal size thin wall tubing. These combinations are: 1 ∕ 2 -27, 5 ∕ 8 -27, 3 ∕ 4 -27, 7 ∕ 8 -27, and 1-27. This series applies to external and internal threads of full length and is applicable to thin wall nominal diameter outside tubing. Designation of Special Dryseal Pipe Threads: The designations used for these special dryseal pipe threads are as follows: 1 ∕ 8 -27 PTF-SPL SHORT 1 ∕ 8 -27 PTF-SPL EXTRA SHORT 1 ∕ 2 -27 SPL PTF, OD 0.500 Note that in the last designation the OD of tubing is given. British Standard Pipe Threads British Standard Pipe Threads for Non-Pressure-Tight Joints.— The threads in BS 2779:1973, “Specifications for Pipe Threads where Pressure-tight Joints are not Made on the Threads”, are Whitworth form parallel fastening threads that are generally used for fastening purposes such as the mechanical assembly of component parts of fittings, cocks and valves. They are not suitable where pressure-tight joints are made on the threads. The crests of the basic Whitworth thread form may be truncated to certain limits of size given in the Standard except on internal threads, when they are likely to be assembled with external threads conforming to the requirements of BS 21 “British Standard Pipe Threads for Pressure-tight Joints” (see page 2099).
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