I have this debate regularly with facilities where I find “short-threading” on their flanges/manways, etc. This phrase is from an old pipe-fitters manual, and it’s what I was taught in 1993 when I entered the chemical process industry and PSM/RMP covered processes. Today, “full engagement” is defined at the bolt/nut being FLUSH. So I asked the all-expert in everything Process Safety (sarcasm) “AI” about the “three-threads showing” and believe it or not, it responded with the following:
In bolted joints, the general recommendation is for 1 to 3 threads to protrude past the nut. This ensures adequate thread engagement and full load-bearing capacity. Having at least three threads protruding is especially important for nylon insert nuts to ensure the locking feature engages properly.
Why 1-3 threads?
- Thread engagement:The protruding threads indicate that the nut is fully engaged with the bolt’s threads, and the bolt is not bottoming out on the shank.
- Optimal load distribution:Having at least three threads engaged helps ensure the load is distributed across multiple threads, preventing stress concentrations and potential failure.
- Locking feature engagement:For nuts with locking features, like nylon inserts, a minimum of three threads is recommended to ensure the locking mechanism engages with the bolt’s threads.
- Bolt length verification:Too many threads protruding may indicate the bolt is too long and could bottom out on the shank, potentially damaging the threads.
Exceptions and Considerations:
- Structural bolts:In some structural applications, like those using A325 or A490 bolts, thread protrusion may be more limited to avoid engaging the thread run-out.
- Project-specific requirements:It’s always best to consult with the project engineer for specific requirements, especially in critical applications.
- Safety:Excessive thread protrusion should be avoided as it can be a hazard or interfere with other components.
In summary, while 1-3 threads is a common guideline, it’s crucial to consider the specific application and consult with relevant standards or engineers for optimal and safe thread engagement.