Pullout strength of threads in Aluminum 3

[58berlyl]

I have a large mold made of 6061-T6 aluminum and need to be able to pull the mold apart. I need to make sure that the amount of bolts I am putting in the backside of the mold will hold. How can I calculate the thread strength of the threads put into the aluminum block?

 

[diamondjim]

You will probably have to use Grade 5 bolts
or Grade 2 bolt torque values. I am thinking
that the tensile and yield are around
78000 psi and 50000 psi but have not info
here at home for the tensile and yield of the
aluminum. One thing you will have to consider is
that the bolts may have to be 1.5 times as long
as the diameter (the threaded portion in contact)
to achieve the value of the bolts. You might want
to consider using the bolts as jacking screws, i.e. to
have threads in only one half of the mold. Machineries
Handbook has a section for length of thread engagement for
different material strengths of bolts verses apparent
materials that are being threaded.

 

[Bryex]

Personally i'd increase the threads depth to 2 1/2 times the diameter, and probably insert heli coils, just to be on the safe side.

 

[Kenneth]

A potential problem with many thread length-of-engagement "rules of thumb" is that they are typically based on an assumption that the mating internal and external threads are made from materials of approximately the same strength level.

You can calculate the length of engagement required to prevent shear or "stripping" of the lower strength internal aluminum thread at the anticipated design load. This calculation is based on the geometric shear area of the internal thread at minimum material condition (equal to the area of that thread which is intersected by a cylinder with a diameter equal to the minimum major diameter of the mating external thread over the length of engagement) and shear strength of the parent thread material. An excellent source of formulas, information and examples for performing these (and other) thread strength calculations are found in Appendix B of FED-STD-H28/2B -- available for free at

http://astimage.daps.dla.mil/docimages/0001/34/77/H28-2B.PD9

These formulas are relatively simple, accurate and readily lend themselves to entry and reuse in a spreadsheet. If you don’t know the shear strength of your materials, two possible sources of free online information are MatWeb (usually “typical” values) at

http://www.matweb.com/

or in MIL-HDBK-5 (usually statistically adjusted values to a given confidence level) at

http://astimage.daps.dla.mil/docimages/0001/73/51/MILHBK5H.PD5

Alternately, you wish to use an applicable material specification “minimum guaranteed” value.

As suggested by a previous poster, externally threaded inserts in “low strength” materials will not only prevent thread damage resulting from repeated reuse but will also increase the thread shear load in your aluminum material for a given bolt diameter/thread (insert internal diameter/thread) and length of engagement due to the greater thread shear area provided by the greater external insert thread shear area. Conversely, for a given bolt diameter/thread, they permit a shorter length of engagement in the aluminum material to achieve a given strength level. Well designed inserts for use in aluminum are typically designed with external thread lengths (insert heights) long enough to develop sufficient “pull out” resistance to ensure tensile failure of a high strength bolt fully engaged with the internal insert thread. For threads that are frequently disassembled , it is a good practice to increase the calculated length of engagement to allow for expected wear of the flanks of the threads (and the resulting decrease in tensile capability) over the useful life of the components -- another good reason to use threaded inserts in aluminum for high reuse threads.

 

[sbi]

As a foundry engineer in a Aluminnum foundry we fight this with customers all the time.
#1- Aluminum has roughly 1/2 the Brinell of iron ( hardness)
thiss means if you used 6 - 3/8X16 x 1 bolts on a iron part you better look at
using 10 1/2 x 13 x 1.5 in aluminum

We also push Heli coils

 

[GRPatel]

You can perhaps use helicoils and consider the material strength that of steel.

 

Instead of downloading MIL-HDBK-5H as the full 40 MB file mentioned by Kenneth (a slow download if you only need to see a couple of pages), I downloaded any MIL-HDBK-5H section(s) I wanted, separately, at

http://euler9.tripod.com

under "MIL-HDBK-5H." You just look in the table of contents for whichever metal you need and click. This might save you some time.

Also, there's a little bit of information on computing thread shear on same site, though it says it's still in work, at

http://euler9.tripod.com/bolt-database/bolt-database11.html

(toward bottom of page). Appears to contain simplified formulas for thread shear, but looks more like formulas for similar metals. Claims thread engagement should be 1.5 times bolt diameter for aluminum. Of course more is better.