PULL OUT FORCE ON 10-32 THREAD IN ALUMINUM 1

[GEARSLY]

I AM USING A STEEL SOC HD CAP SCREW IN ALUMINUM. I AM TRYING TO CALCULATE THE THREAD ENGAGMENT NEEDED TO ENSURE FOUR 10-32 SCREWS WILL WILL HOLD A RETAINING PLATE WITH 1320 POUNDS OF SPRING LOAD OR IF A LARGER SCREW IS NEEDED.

 

[diamondjim]

What are the mechanical properties
of the aluminum that you are using.
What safety factor do you want.
How often is the load applied to the
fastener. A grade 2 screw (55000psi)
proof stress is good for 820 pounds
clamping load.
I would assume your aluminum UTS would
be greater than 1/2 of this value.
From a static case it looks like the
aluminum would be ok, but for the
dynamic case, more information is
required.

 

[GEARSLY]

THanks for the reply. I am usign Aluminum 6061 (tensile 35,000)for the clamp plate. I am compressing twelve die springs to develop 1320 lbs of force. The four Soc Hd Screws are 10-32 (grade 2)low-med carbon steel and the use is static.

I would like to have a safety factor of 2X or 2500 lbs.

 

[NickE]

Wait, is Soc Hd Screw... Socket Head Cap Screw? of the kind normally used in Die Construction? I had always thought that these were far stronger than Grade2.

Nick
I love materials science!

 

[boo1]

MIL-HDBK-5 allowable tables gives you the strength of fastener/material combinations.
the value is the lowest of the following failures modes:
-fastener shear (shear failure of fastener, based on Fsu_fastener)
-bearing (local yielding of adjacent structure, based on Fbrg or Fbru)
-shear-out (shear failure of adjacent structure, based on Fsu_sheet)
-Tension (tension failure of adjacent structure, based on Ftu)

Analysis is based on the material minimum or A-basis (ref Mil-HDBK-5) tensile yield strength and the thread min tinsile stress area (formua in Fed-STD-H28/2A table II.B.1 ), or the minimum shank ccross sectional area, whichever is least.

 

[diamondjim]

I forgot to mention the depth or
length of screws.
I assume this is for 1 diameter
length long to utilize this force.

If 55000psi allows 820 pounds
clamping load, I assume the answer
for the internal threads would be:

55000 equals 820 pounds grade 2 screws
36000 equals 536 pounds internal threads

1320 divided by 4 equals 330 pounds
536/330 yields only a static safety
factor of 1.62
Less than the 2 that you asked for.
Note: a higher strength bolt does
nothing for the limitation of the
internal threads. There is a better
analysis in Machinery's Handbook for
internal threads. There is some gain
by longer lengths of thread but you
will also have to worry about the
stress on the head of the bolt or screw
for a complete analysis for the greater
lengths of screws.

 

[GEARSLY]

Yes, the weak link is definitely the tapped hole in the aluminum. I calculated the "Length of Engagement" to be .479" minimum for the 10-32 thread (based on Shear Area for each thread, external vs. internal).

Shear area of Ext thread * Tensile strength of screw materail / Shear area of Internal thread * Tensils strength of tapped hole material.

This resulted in a raitio of 1.86 which should be less than "1" for acceptable thread engagement.

Thanks for the input...

 

[israelkk]

I am not sure that the factor of safety of 2 is enough if a human injury may occur in case of a failure. You should look for the regulations in you state. In case of a failure of the bolts you will have a "cannon" that will shoot the retaining plate at 1320 pounds and might hurt someone.

 

[DDRSS10]

Have you thought of using helicoils or threaded inserts to increase the pullout strength of the holes. Just a thought.

Nick

"Speed costs money boys, how fast do you want to go?"

 

[diamondjim]

Rather than using helicoils for these little
buggers, it would make more sense to just go
with the next larger 1/4 dia. screw. For the
.06 increase in diameter, the clamp load almost
doubles.

 

[Kenneth]

See thread404-11447 as well for a discussion on threads in aluminum...

 

[Ron]

Gearsly...even in buildings with no repetitive loads, we use a minimum factor of 3. Further, you are putting carbon steel into aluminum....guaranteed corrosion for the screws.

 

[IFRs]

A few comments:
Use SS fasteners, you should be OK corrosion wise. Over tightening can partially strip the threads making it vulnerable to future failure. Temperature swings may eventually loosen them up. Can you put a nut on the backside? If you have a little room on the backside you can use a blind nut. These are SS threaded inserts that swedge into a hole that work really well. Look up RivNut on the web. Home page is

http://www.bollhoff.com/web/usa/en/web.nsf/HTML/index_us.html,

go for RivNutA on the left and then RivNutA again.

 

[yates]

SS screws in aluminum tapped holes should not induce corrosion problems, as IFRs points out. However, galling is a big danger if you ever want to dismantle the joint. Make sure you lubricate well (graphite or Molykote if environmental conditions permit.)

 

[boo1]

Recommend not using graphite or any carbon based products with aluminium. We use Zinc based petroleum. If you are in a marine enviroment then use a sealant like Pro-seal 870.