How many threads 1

[biff44]

How many threads have to catch in a 1/4-20 bolt to get the full rated bolt strength? I can not find it anywhere. I vaguely remember that 4 threads is all you need. right/wrong?

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[arunmrao]

Minimum thread depth
thread404-149977


Learn the rules,so you know how to break them properly.
Dalai Lama

_____________________________________

 

[racookpe1978]

I have been taught, and read in the various "nuts and bolts" handbooks, that minimum depth (of "good" threads!) is one dia of the threaded rod or bolt.

If you are "blind tapping" into a hole, the last 1.5 thread turns are NOT "good threads" for strength.

For nuts, the bolt or studs should be visible (extend past the end of the nut) by at least 1-1/2 threads. Of course, that's a generic requirement to develope the "full strength" of the male threads in the bolt and female threads in the nut.

 

[KENAT]

thread725-35222 Corypad gives some links etc for calculating it.

http://www.boltplanet.com/

has a calculator that can be usefull to play around with, but as it's free & on the internet - buyer be ware.

I've seen the 'well only 3 threads are doing all the work' argument used in tight situations. However, usually 1 X diameter is a good target.

When you say 1/4 bolt do you mean into an appropriate nut?

The properties of the female half of the joint also affect it. A 1/4 bolt into modelling clay wont give you the same result as 1/4 bolt into 303 SST nut etc.

I believe rated values are given for going into a nut of a comparable grade but I don' know for sure.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[ornerynorsk]

1 to 1-1/2 x diameter is the general rule of thumb.

 

[mrpi]

Sorry to drag up an old thread, but most machinery design handbooks I've seen recommend ensuring engagement length such that the shear strength of the female threads is twice the tensile strength of the male bolt threads. This will ensure that the female threads are protected against pull-out.

Female thread shear area calculation requires engaged thread length, minimum major diameter of external threads, maximum pitch diameter of the internal thread, and thread pitch. These are often found in tables as An/mm (area per unit length) where the length is the engaged thread length.

Male bolt screw thread tensile area is available directly from tables.

Strength is found by using yeild or proof stress of each material (bolt material and female thread material) together with the tensile/shear areas found above.

Also consider the type of loading the connection will experience, and choose a conservative factor of safety.

 

[macthenife]

The first thread in the nut(bearing face end)sustains a stress level 231%(fine thrd),179%(coarse thrd) of applied load.

This tapers back to a much lower level.

Engaging less threads than in a typical nut raises first thrd stress level higher.

Using a nut one grade lower than the bolt distributes
thread loads more evenly.

I still have not answered your question.

 

[Eltron]

Check out this link:

http://school.mech.uwa.edu.au/~dwright/DANotes/threads/mechanics/mechanics.html

Lots of good information about fastener mechanics. Probably anything past 6 threads is not useful, or as ornerynorsk said above a good rule of thumb is 1.25 - 1.5 times the diameter.

Dan

http://www.eltronresearch.com

Dan's Blog

 

[Cockroach]

Would it not depend on your application? I mean if I got 150 lbf axial load on the screw in my situation and only need a thread length of 1/2 inch, how would that be good for you given 775 lbf load?

The point is that you need to push on the pencil and do a proper analysis for your situation. Professional conduct and skilled practice are two concepts that come to mind.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[unclesyd]

Here is an abbreviated version of the thread engagement calculations from Engineers Edge. I like it because it looks at the engagement difference when using different materials.
The is also a more detail analysis of the problem in the Unbrako Fasteners Handbook.

http://www.engineersedge.com/thread_strength/thread_minimum_length_engagement_fed-std-h28.htm