Calculation of pipe thread strength

NPT, PT, and G are all types of pipe threads. NPT stands for National (American) Pipe Thread; it is a 60-degree tapered pipe thread that follows American standards and is used in North America. **The relevant standard can be found in GB/T12716-1991.** PT is an abbreviation for Pipe Thread; it is a 55-degree sealed tapered pipe thread that belongs to the Whitworth thread family and is commonly used in Europe and the Commonwealth countries. It is frequently used in the water and gas piping industries, with a taper ratio of 1:16. **The relevant standard can be found in GB/T7306-2000.** G refers to a 55-degree non-threaded sealed pipe thread that also belongs to the Whitworth thread family; the designation “G” indicates a cylindrical thread. **The relevant standard can be found in GB/T7307-2001.** Additionally, the designations 1/4, 1/2, and 1/8 refer to the diameter of the thread, measured in inches. Industry professionals usually refer to thread sizes in terms of “minutes”; one inch equals 8 minutes, so 1/4 inch is equivalent to 2 minutes, and so on. “G” is simply a general term for pipe threads; the distinctions between 55-degree and 60-degree threads are based on functional considerations, and these are commonly referred to as round pipe threads. That is, the thread is formed by machining a cylindrical surface. ZG is commonly known as pipe thread, meaning that the threads are formed by a conical surface; most water pipe fittings use this type of thread. In national standards, it is denoted as Rc. Metric threads are specified by their pitch, while imperial and British threads are specified by the number of threads per inch – this is their main difference. Metric threads have an equilateral tooth profile at 60 degrees, British threads have an isosceles tooth profile at 55 degrees, and American threads also have a 60-degree tooth profile. Metric threads use metric units, while imperial and American threads use imperial units. Pipe threads are primarily used for connecting pipes; their internal and external threads fit tightly together, and there are two types: straight pipes and tapered pipes. The nominal diameter refers to the diameter of the pipe being connected; obviously, the thread diameter is larger than the nominal diameter. 1/4, 1/2, 1/8 are the nominal diameters of imperial threads, measured in inches. Thread breakage: The weight of the pipe itself, the axial loads generated by the fluid inside the pipe, and the pre-tensioning force used to prevent the threads from loosening all act on the threads of the pipe thread. Corrosive wear further reduces the thickness of these threads and lowers their strength; when the load exceeds the strength limit of the threads, breakage occurs. Preventive measures against failure 1. Measures to prevent thread sticking Wear is the main and direct cause of thread sticking; therefore, the following preventive measures can be taken: (1) Use plating techniques (such as copper brushing) to coat the surface of the pipe thread teeth, thereby improving their wear resistance. (2) Surface induction hardening is applied to the thread surface to increase the strength and hardness of the thread flanks. This method is quite practical; under normal circumstances, as long as the hardness of the thread teeth is properly ensured, such failure as sticking will not occur in those teeth. The actual hardness required for the thread teeth can be determined by calculating its strength. Shear strength condition for the thread teeth: J = F1(πd1bz) ≤ (1)
Bending strength condition for the thread teeth: σw = 3Fh/(πd1b2z) ≤ (2)
Where:
F – the total axial force acting on the thread, in N;
d1 – the minor diameter of the thread, in mm;
z – the number of thread teeth actually in use;
b – the width at the root of the thread tooth, in mm;
h – the actual working height of the thread tooth, in mm;
σw – the allowable shear stress of the thread material, in MPa;
σb – the allowable bending stress of the thread material, in MPa. It can also be expressed as ≈0.6, or ≈σs/(3~5), where σs ≈ (0.52~0.65)σb, and σb ≈ (3.2~3.5)HBS. Thus, the relationship between the bending strength σW of the thread teeth and the Brinell hardness HBS of the thread material is: σW = (0.52~0.65)(3.2~3.5)/4HBS (3) By substituting equation (3) into equation (1), the required strength value for the thread teeth can be determined. This value can be used as a reference standard for induction hardening of the thread surface. (3) When tightening the threads, use a torque monitor to control the tightening torque, in order to prevent the threads from sticking together due to excessive tightening force.