Thread Protrusion of a bolted assembly

 

What should be the minimum threads protruding beyond the nut in fastener assemblies?

Possibly not much attention is given to this aspect. However it is pertinent to note that planning the correct bolt/screw length provides for a joint that ensures safety and saves space, weight and cost.

A typical bolted fastener assembly will have the components as in the below image.

 

The minimum theoretical bolt length should be calculated on
this basis, allowing for the thickness of the nut, washer, grip and bolt protrusion.
While many specs require flush with the face of the nut, this will not provide full engagement with the threads.

Examining the leading end of the bolt or stud reveals a partial starting thread.

In order for there to be full engagement a full thread cross-section must exist for there to be full engagement.

 

The industrial norm suggests that “a bolt should have two full threads beyond the nut”, one for thread engagement and one for tolerances. However, the designer should give further consideration to particular cases where greater than normal tolerances need to be accommodated. One example is where the paint coatings are unexpectedly thick. Another is where galvanizing is also unexpectedly thick on the faces of the plies, as can occur with Silicon rich steel.

Excessive protrusion should be avoided particularly when necessary clearances, accessibility and safety are important. Thread protrusion may be considered excessive if it could cause damage to machinery or harm to personnel or users.

In no case should the thread protrusion exceed ten threads and maintained within five where practicable.

 

To sum up, thread protrusion should be within a range of two to five threads. A good rule to follow is to use the shortest standard length fastener that gives a minimum one thread protrusion.

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