Threads design factors

One aspect of pressure vessel design which has received considerable attention in recent years is the design of threaded closures where, due to the high stress concentration at the root of the first active thread, a fatigue crack may quickly initiate and propagate in the radial—circumferential plane. Stress intensity factors for this type of crack are difficult to compute (112,113), and more geometries need to be examined before the factors can be used with confidence. 

Nickel and its alloys form another important class of non-ferrous metals (Table 1.3). The superb creep resistance of the nickel-based superalloys is a key factor in designing the modern gas-turbine aero-engine. But nickel alloys even appear in a model steam engine. The flat plates in the firebox must be stayed together to resist the internal steam pressure (see Fig. 1.3). Some model-builders make these stays from pieces of monel rod because it is much stronger than copper, takes threads much better and is very corrosion resistant. 

The worm thread is a major factor in worm ratios and is an important feature in worm design. The speed-reduction ratio is equal to the total number of worm-gear teeth divided by the number of worm threads. 

Pressure design of straight-threaded joints shall be based on calculations consistent with design requirements of this Code. These calculations shall be substantiated by testing in accordance with to-be-determined procedures and protocols. The testing shall consider such factors as assembly and disassembly, cyclic loading, vibration, shock, hydrogen embrittlement, thermal expansion and contraction, and other factors to be determined. 

Design threads with 4 1 safety factor against failure by shear, tension, or compression due to hydrostatic end force. 

Stripping The designer must consider whether the product can be stripped from the threads. Stripping is the easiest (and often the lowest cost) solution for ejecting molded products however, the ease of stripping depends on many equally important factors. 

Coarse threads can be molded into most materials. Threads of 32 or finer pitch should be avoided, along with tapered threads, such as pipe threads. These types of threads can cause excessive stress. Other factors that should be considered when designing molded-in threads are ... 

The real value and need for TLMs is based on the need for system hazard clarity and focus. The use of TLMs helps to resolve some safety programmatic issues, such as (a) hazard abundance, (b) hazard confusion, (c) subsystem confusion, and (d) total mishap risk. As HAs are performed on a system design, many hazards are identified, sometimes in the thousands. With a large number of hazards, it often becomes difficult to maintain hazard visibility. Sometimes hazards are inadvertently repeated sometimes hazards are stated as causal factors rather than hazards. Hazard risk can be assessed against TLMs to determine if a thread exists that creates increased risk for that TLM category. 

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