Classification of threads

Thread Content

I. Classification of threads 1. Threads are divided into internal threads and external threads； 2. Classified by tooth shape, they can be divided into: 1) triangular thread, 2) trapezoidal thread, 3) rectangular thread, 4) serrated thread ； 3. Single-start threads and multi-start threads classified by the number of threads ； 4. Based on the direction of rotation, threads are divided into left-hand threads and right-hand threads; right-hand threads are not marked with any indication, while left-hand threads are marked with LH, such as M24×1.5LH ； 5. Classified by purpose, they include: metric ordinary threads, pipe threads sealed with threads, pipe threads not sealed with threads, 60° tapered pipe threads, metric tapered threads, etc. II. Metric Ordinary Threads 1. Metric ordinary threads are denoted by the capital letter M; the thread angle is 2α=60° (where α represents the half-thread angle) ； 2. Metric ordinary threads are divided into coarse-thread ordinary threads and fine-thread ordinary threads based on the pitch ； 2.1. The pitch is generally not indicated for coarse-thread regular threads; for example, M20 denotes a coarse-thread thread ； Fine-thread markings must indicate the pitch; for example, M30×1.5 denotes a fine thread with a pitch of 1.5. 2.2. Ordinary threads are used for connecting and fastening mechanical parts; coarse threads are commonly employed for such connections. Fine threads have slightly higher strength than coarse threads of the same nominal diameter, as well as better self-locking properties. 3. Marking for metric ordinary threads: M20-6H, M20×1.5LH-6g-40. Here, M denotes metric ordinary threads; 20 indicates that the nominal diameter of the thread is 20 mm; 1.5 represents the pitch; LH means left-hand thread; 6H and 6g indicate the thread precision grade – uppercase precision grade codes refer to internal threads, while lowercase codes refer to external threads; 40 denotes the engagement length ； 3.1. The pitches of commonly used metric coarse-thread threads are shown in the table below (diameter of the thread bottom hole: for carbon steel, φ = nominal diameter – P) ； Cast iron φ = nominal diameter – 1.05~1.1P ； The diameter of the bare shaft for external threads is calculated as φ = nominal diameter – 0.13P. Table 1 shows the diameters and pitches of common metric coarse-thread threads: Nominal diameter, Pitch P, Drill hole diameter for cast iron, Drill hole diameter for carbon steel, Diameter of the bare shaft for external threads. M5: 0.8, 4.1, 4.2, 4.9; M24: 3, 20.8, 21, 23.7; M6: 1, 4.9, 5, 5.9; M27: 3, 23.8, 24, 26.7; M8: 1.25, 6.6, 6.7, 7.9; M30: 3.5, 26.3, 26.5, 29.6; M10: 1.5, 8.3, 8.5, 9.8; M33: 3.5, 29.3, 29.5, 32.6; M12: 1.75, 10.3, 10.4, 11.8; M36: 4, 31.7, 32, 35.5; M14: 2, 11.7, 12, 13.7; M42: 4.5, 37.2, 37.5, 41.5; M16: 2, 13.8, 14, 15.7; M48: 5, 42.5, 43, 47.5; M18: 2.5, 15.3, 15.5, 17.7; M56: 5.5, 50, 50.5, 55.5; M20: 2.5, 17.3, 17.5, 19.7; M64: 6, 57.5, 58, 63.5. The drill hole diameter for common metric internal threads can be approximated using the formula: d = D – 1.0825P, where D is the nominal diameter and P is the pitch. III. Pipe threads sealed with threads (GB 7306 is identical to ISO 7/1) 1. Pipe threads sealed with threads can prevent leakage without the use of filler or sealants. Pipe threads sealed with threads come in two types of connections: cylindrical internal threads and conical external threads, as well as conical internal threads and conical external threads. When the pressure is below 5×105 Pa, the former type of connection is sufficient to ensure a tight fit; the latter type is generally used only under high temperature and high pressure conditions. 2. The internal pipe threads sealed with threads come in conical and cylindrical forms. External threads come in only one form: conical. The thread profile is as follows: taper of 1:16, thread angle of 55°; example of an old thread standard: ZG3/8 ； 3. Marking examples: Cone internal thread Rc 3/8, Cylinder internal thread Rp3/8, Cone external thread R3/8. When the thread is left-handed, it is denoted as Rc 3/8-LH (LH indicating a left-handed thread). Basic dimensions for common threads (markings: Rc 3/8, Rp3/8, R3/8): Table 2 – Dimension codes, teeth per inch, major diameter, pitch diameter, minor diameter, recommended bottom hole diameter, base distance, effective length. 1/8: 28 teeth per inch; major diameter: 9.728; pitch diameter: 9.147; minor diameter: 8.566; recommended bottom hole diameter: φ8.4; base distance: 4.0; effective length: 6.5. 1/4: 19 teeth per inch; major diameter: 13.157; pitch diameter: 12.301; minor diameter: 11.445; recommended bottom hole diameter: φ11.2; base distance: 6.0; effective length: 9.7. 3/8: 19 teeth per inch; major diameter: 16.662; pitch diameter: 15.806; minor diameter: 14.950; recommended bottom hole diameter: φ14.7; base distance: 6.4; effective length: 10.1. 1/2: 14 teeth per inch; major diameter: 20.955; pitch diameter: 19.793; minor diameter: 18.631; recommended bottom hole diameter: φ18.3; base distance: 8.2; effective length: 13.2. 3/4: 14 teeth per inch; major diameter: 26.411; pitch diameter: 25.279; minor diameter: 24.117; recommended bottom hole diameter: φ23.6; base distance: 9.5; effective length: 14.5. 1: 11 teeth per inch; major diameter: 33.249; pitch diameter: 31.770; minor diameter: 30.291; recommended bottom hole diameter: φ29.7; base distance: 10.4; effective length: 16.8. 1-1/4: 11 teeth per inch; major diameter: 41.910; pitch diameter: 40.431; minor diameter: 38.952; recommended bottom hole diameter: φ38.3; base distance: 12.7; effective length: 19.1. 1-1/2: 11 teeth per inch; major diameter: 47.803; pitch diameter: 46.324; minor diameter: 44.845; recommended bottom hole diameter: φ44.1; base distance: 12.7; effective length: 19.1. 4. The marking methods specified in GB 7306 are the same as those specified in ISO 7/1. Before standardization under ISO 7/1, the marking methods in various countries were as follows: Table 3 Standard Number Thread Type Internal tapered thread Internal cylindrical thread External tapered thread ISO ISO7/1-1982 Rc1/2 Rp1/2 R1/2 United Kingdom BS 21-1973 Rc1/2″ Rp1/2″ R1/2″ France NF E03-004 —— G1/2″J G1/2″ Germany DIN 2999 —— Rp1/2-DIN299 R1/2DIN2999 Japan JIS 130203 PT1/2″ PS1/2″ PT1/2″ Soviet Union ГOCT 6211 KTPYБ1/2″ —— KTPYБ1/2″ GB GB 133-74 ZG1/2″ —— ZG1/2″ IV. 60° tapered pipe threads (GB/T 12716-91) 1. These threads are equivalent to the NPT threads specified in the American standard ANSI B1.20.1. There is only one type of connection: tapered internal threads and tapered external threads. 2. Similarities between 60° tapered pipe threads and 55° tapered pipe threads: 2.1. The threads themselves possess sealing properties. 2.2. Cone angle: 1:16 2.3. The diameter size is controlled by adjusting the distance of the base plane (reference length). 2.4. It is allowed to insert a sealant within the threaded pair. 3. Differences between 60° tapered pipe threads and 55° tapered pipe threads: Table 4 Item 60° Tapered Pipe Thread 55° Tapered Pipe Thread Designation NPT 3/8 Rc 3/8, Rp3/8, R3/8 Thread angle 60° 55° Different pitch Table 5 Table 2 Tolerance requirements High Low Applications Pipeline connections in automobiles, aircraft, and machine tools Low-pressure pipeline systems for water, gas, etc. 4. The code NPT is the former standard code Z: Designations: NPT 3/8, NPT 3/8‑LH, where LH indicates left-handed rotation. Table 5 Basic dimensions of common threads Dimension code Teeth per inch Major diameter Minor diameter Pitch diameter Recommended bottom hole diameter Reference distance Teeth count 1/8 27 10.242 9.489 8.736 φ8.6 4.102 4.36 1/4 18 13.616 12.487 11.358 φ11.1 5.786 4.10 3/8 18 17.055 15.926 14.797 φ14.5 6.096 4.32 1/2 14 21.223 19.772 18.321 φ17.9 8.128 4.48 3/4 14 26.568 25.117 23.666 φ23.2 8.611 4.75 1 11.5 33.228 31.461 29.694 φ29.2 10.160 4.60 1-1/4 11.5 41.985 40.218 38.451 φ37.9 10.668 4.83 1-1/2 11.5 48.054 46.287 44.520 φ43.9 10.668 4.83 V. Pipe threads without threading seals (GB 7307) 1. This thread standard is identical to ISO 228/1. 2. Tooth profile angle: 55°, taper: 1:16. 3. Marking examples: 11/2 left-hand internal thread G11/2-LH (right-hand thread not marked); 11/2 Class A external thread G11/2A; 11/2 Class B external thread G11/2B; internal and external threads combined G11/2/G11/2A. Table 6 shows common basic dimensions: Dimension code, teeth per inch, outer diameter, middle diameter, inner diameter, recommended bottom hole diameter, reference distance, and effective length. 1/8: 28 teeth per inch; outer diameter: 9.728; middle diameter: 9.147; inner diameter: 8.566; recommended bottom hole diameter: φ8.8; reference distance: 4.0; effective length: 6.5. 1/4: 19 teeth per inch; outer diameter: 13.157; middle diameter: 12.301; inner diameter: 11.445; recommended bottom hole diameter: φ11.7; reference distance: 6.0; effective length: 9.7. 3/8: 19 teeth per inch; outer diameter: 16.662; middle diameter: 15.806; inner diameter: 14.950; recommended bottom hole diameter: φ15.2; reference distance: 6.4; effective length: 10.1. 1/2: 14 teeth per inch; outer diameter: 20.955; middle diameter: 19.793; inner diameter: 18.631; recommended bottom hole diameter: φ18.9; reference distance: 8.2; effective length: 13.2. 3/4: 14 teeth per inch; outer diameter: 26.411; middle diameter: 25.279; inner diameter: 24.117; recommended bottom hole diameter: φ24.4; reference distance: 9.5; effective length: 14.5. 1: 11 teeth per inch; outer diameter: 33.249; middle diameter: 31.770; inner diameter: 30.291; recommended bottom hole diameter: φ30.6; reference distance: 10.4; effective length: 16.8. 1-1/4: 11 teeth per inch; outer diameter: 41.910; middle diameter: 40.431; inner diameter: 38.952; recommended bottom hole diameter: φ39.2; reference distance: 12.7; effective length: 19.1. 1-1/2: 11 teeth per inch; outer diameter: 47.803; middle diameter: 46.324; inner diameter: 44.845; recommended bottom hole diameter: φ45.1; reference distance: 12.7; effective length: 19.1. 4. Applications: Pipe threads without threading are mainly used in gas and water pipelines under low pressure. When connecting such pipes, packing or sealing materials (such as hemp fiber, lead-tin alloy, PTFE tape, etc.) are used to prevent leaks. 5. The common feature of pipe threads without threaded sealing and those with threaded sealing is that both have a tooth profile of 55°, and threads of the same specification have the same pitch, tooth height, and major diameter. 6. The difference between pipe threads with non-threaded sealing and those with threaded sealing is as follows: 6.1. The internal threads of pipe threads with threaded sealing come in two forms – cylindrical threads and tapered threads – while the external threads are only of the tapered type. 6.2. Pipe threads without threaded seals have only the cylindrical thread form, for both the male and female threads. 6.3. The tolerances for the specifications of the two thread types are different. 6.4. Internal cylindrical pipe threads with non-threaded sealing can be connected to external conical pipe threads with threaded sealing, but external cylindrical pipe threads with non-threaded sealing cannot form a thread-sealed connection with internal pipe threads that have threaded sealing. 7. Different marking methods in various countries: Table 7 Standard number Thread type Internal cylindrical thread External cylindrical thread ISO ISO228/1-1982 G1/2 G1/2A B United Kingdom BS 2779-1973 G1/2″ G1/2″A B France NF E03-005 G1/2″H G1/2″A B Germany DIN 259 R1/2 K1/2 Japan JIS 130202 PF1/2″A B PF1/2″A B Soviet Union ГOCT 6357 TPYБ1/2″Kл2 Kл3 TPYБ1/2″Kл2 Kл3 GB GB 133-74 G1/2″ G1/2″ 6. Metric tapered threads (GB1415) 1. Marking: ZM10, ZM10-S, ZM10/ZM10, ZM10/ZM10-S. 2. ZM – Marking code for metric tapered threads ； When the standard reference distance is used, the reference distance code is omitted ； S-Short baseline distance. 3. Purpose: Suitable for connection threads in gas or liquid piping systems that rely on thread sealing (except for pipe threads used in water and gas pipelines); where necessary, sealing filler can be added between the thread mating surfaces to enhance the sealing performance. 4. Nominal diameter/pitch Diameter: 6, 8, 10; 12, 14, 16, 18, 20, 22, 24; 27, 30, 33, 36, 39, 42, 45, 48, 52, 56, 60. Pitch: 1, 1.5, 2