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How Long A Bolt ?


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We've been chatting at work about how far in a stud must go into something before it makes no difference ? So was just wondering if there was a general rule of thumb to work it out ?

i.e. an normal M10 bolt will be no better off going into a 20mm nut compared to a 50mm nut etc, but what is the minimum depth it should go in ?

i'm assuming it's related to bolt diameter and pitch ?



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There are a number of rules of thumb depending on the application;

As has already been mentioned a min of 1.5 times OD is a good rule of thumb for bolts into tapped holes- but bear in mind the material the hole is tapped into will have an effect. I.e. the softer the material, the longer the thread engagement required. Also the failure mode (if the bolt is in tension)will change dependent on the length of engagement. For short engagements in soft material, the tapped thread will strip, for longer engagements in a higher strength material the bolt will fail. There are calculations to determine this.

If you threading into a nut then it is normal practice to leave at a minimum of 1 to 2 threads protruding- primarily to ensure that the nut is on a fully formed section of the bolt, not on a taper.

There is a lot of useful info on this site;


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As mentioned, the key here is the material you are screwing in to. Most bolts are made of a much stronger material than the steel plates you will typically use, so if it is a critcal connection then either drill through and use a nut of the corresponding grade to the bolt or do the sums for the material you plan to tap. A rule of thumb could leave you well short of thread engagement.

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