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Thread Length, Engagement Various Materials

Thread Length, Engagement Various Materials

Thread Length, Engagement Various Materials

I am intersted in information regarding the recommended thread engagement for different sized bolts (coarse thread, grade 8) in various materials...most notably ASTM A36.  I'm bolting two pieces together, on an angle, so my threads will be 7/8" long using a 1/2" bolt, OR about 3/4" long using a 5/8 dia bolt.  I need to decide which to use.
Also, I have a table describing recommended thread engagement w/r/t the Ult. Tens. Strength of the bolt material (ksi) and the "SHEAR STRENGTH" of the parent material...I can't find any data on "shear strength" anywhere...I can find Shear Modulus...how do these relate.  I need some help here Please!

RE: Thread Length, Engagement Various Materials

        In steel, recommended thread engagement length is 1.5D where D is bolt nominal dia. Holes tapped into soft aluminum or magnesium based alloys would be better with 2D.
As for your question on shear strength, this may be measured either by single shear (in a simple two-plate sharp edged guillotine. Double shear testing is preferable in which a sharp-edged blade of finite thickness slides in a stirrup which carries a semi-circular recess to hold a cylindrical test-piece.In this case shear strength is max compressive load divided by twice (two shearing planes) the test-piece cross section. This test is common for aerospace bolts and test is described in MIL-STD-1312-13


RE: Thread Length, Engagement Various Materials

But isn't "shear strength" simply a mechanical property of the material?  The chart I have askes for a quantity w/ the units of ksi.

RE: Thread Length, Engagement Various Materials

Yes, shear strength is a mechanical property characteristic of the material, its microstructure, etc.  The test method that yates described is how you obtain those mechanical properties.  In general, you can estimate the ultimate shear strength of a material as 0.6*UTS where UTS = the ultimate tensile strength of the material.

RE: Thread Length, Engagement Various Materials

YES! this is what I've been looking for.
I recall seeing this once as a similar fact for bolts as well...ie. the shear strength of a bolt is 6/10ths of its tensile.  Does this sound right?

RE: Thread Length, Engagement Various Materials

yates provided good general rules.  If you want to calculate the required axial thread engagement, refer to this previous post:


Ultimate shear strength = 0.6 x ultimate tensile strength sounds right.

RE: Thread Length, Engagement Various Materials


As I said in my previous post, the ultimate shear strength can be approximated using 0.6 * UTS.  It doesn't matter if it is a bolt, a tube, a shaft, or an intricate aluminum alloy sand casting for an engine block.  If you need to know exact values for ultimate shear strength, then you will need to have the tests performed, such as NASM1312-13 (the replacement for the cancelled MIL spec MIL-STD-1312-123).

RE: Thread Length, Engagement Various Materials

thanks again...this is good info for a new engineer.

RE: Thread Length, Engagement Various Materials


RE: Thread Length, Engagement Various Materials

Since this is "nominally" a structural forum, I will make the simplistic suggestion to use ASTM A490 or A325 fasteners along with ASTM A563 nuts and ASTM F436 washers.

You can download the RCSC specifications for joints using these fasteners FREE at http://www.boltcouncil.org/

These specifications address you question about thread engagement.

RE: Thread Length, Engagement Various Materials

thanks...I downloaded it.
I'll have a look.

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