how to select the correct screw?

123123 · Jul 1, 2020

I have to fasten a fixture to a 6061-T6 aluminum plate. The fixture will then be exposed to extreme impact (>1,000 G-force). the current screw used strips through the threading on the Al plate. How do I know what size tap and screw I need to fix this?

Limy Sami · Jul 1, 2020

Start your day by making your first cup of coffee with the urine of a virgin Unicorn instead of water.

theperfessor · Jul 1, 2020

Without knowing anything about the size and weight of what you're trying to anchor makes it hard for anyone to help you. It might not be possible to keep the threads from stripping, you may need to anchor something to the back of the plate for the screw to go in. Aluminum never impresses me at its thread holding ability, we make a lot of milling fixtures out of aluminum with various steel inserts for clamp screws to fit in.

FredC · Jul 1, 2020

Could not begin to recommend anything without knowing how heavy an item are you trying to attach to the aluminum plate. How many screws? What size failed? Photos?

123123 · Jul 1, 2020

I was more looking for an equation or something so I can calculate it. There are 8 screws holding a part about 100lbs to the aluminum plate (about 1 inch thick). That aluminum plate is then subject to extreme impact. 3/8-16 failed. Unfortunately I am not able to provide pictures. I know that's not much but I've spent so much time trying to figure this out that the crap shoot is worth it

plastikdreams · Jul 1, 2020

Have you thought about using threaded inserts like helicoil, or some of the others...they can greatly increase the holding strength of threads in aluminum.

Cole2534 · Jul 1, 2020

plastikdreams said:
Have you thought about using threaded inserts like helicoil, or some of the others...they can greatly increase the holding strength of threads in aluminum.

Exactly what I was thinking. Also, easy to make if COTS doesn't trip your trigger.

Sent from my SM-G981V using Tapatalk

garyhlucas · Jul 1, 2020

So a 3/8-16 socket head cap screw may have a tensile strength of 180,000 psi and 6061-T6 is maybe 40,000 psi. So to develope full screw strength you’d need a thread depth of 180,000/40,000 x 3/8” = 1-11/16” thick aluminum needed. So you can see that this going to be difficult. Thread inserts that give a larger thread area may help.

Rickyb · Jul 1, 2020

123123 said:
I was more looking for an equation or something so I can calculate it. There are 8 screws holding a part about 100lbs to the aluminum plate (about 1 inch thick). That aluminum plate is then subject to extreme impact. 3/8-16 failed. Unfortunately I am not able to provide pictures. I know that's not much but I've spent so much time trying to figure this out that the crap shoot is worth it

The threats in aluminum 1” thick should be about equal to the strength of the 3/8 fastener. That would be a basic pull out strength. As you are not breaking fasteners your focus can be on the shear strength of the aluminum threads at the pitch diameter. There will be a significant increase just moving to 3/8-24. Fine threads are going to be your friend. You will need to subtract the shear already placed on the threads by the installation torque.

You will have strength numbers with one referencing a known failure.

From here I would calculate the impact load on the fasteners. To see how it compares to thread strength. Most people equate a impact g load as # of g’s x weight(mass). This is true sometimes. In your case, if it is the aluminum plate that is being accelerated at 100g, then yes, the g load is 100x the weight of the plate and everything attached to it. This is what the threads must resist.

Last but not least you must determine how equally the fasteners are loaded during the impact. Lots of other nits and shits to worry about but these are the basics.

mjr6550 · Jul 1, 2020

Assuming all the screws are loaded equally, which may be a big assumption, I think the load per screw is 12,500 pounds. Are the screws in tension or shear?

Bill D · Jul 1, 2020

my math 100 pound load times 1,000 G. equals 100,000 pound load. This is not a static load there is a very high shock load involved. So eight screws equals 12,500 pounds per screw assuming that are all equallly sharing the load.
Looking that up you need at least 7/16" bolts with steel backup plates and steel nuts if you use grade 8 hardware. That would be rated at only 12,800 per bolt.
You really need at least 1/2" grade 8 to have any safety reserve at all. If failure will cause any damage or danger you need more bolts or 3/4 or larger.
Bill D.

The 7/16 would appear to be capable but only if every joint is properly cleaned and torqued and everything is mated exactly with no misalignment. A 4% safety margin meets no safety standards I can imagine. Not even shade tree mechanics would knowingly be that close to failure.

Rickyb · Jul 1, 2020

Yes the screw body is intension and all the threads are in shear. Don’t lose sight of the fact that a significant % of their strength is already being used by the original torque of the assembly.

Milland · Jul 1, 2020

I'm in "Wut?" mode.

Wut's the purpose of the test?

Wut's the reason you're using an Al plate rather than something more robust, like a steel plate?

Wut's the type of impact - a drop weight, fired from a cannon, an air-gas detonation in a cylinder, etc.?

Wut's with the lack of pictures and better description of the parts involved?

Wut's the limitations for adding or changing screw sizes? Are the screws fitting within the body of the "object"? Are there more clamping points available? Are there secondary clamps that hold the object, and if so, Wut's the leverage they have acted on them when impact occurs?

Etc.

Wut's goin' on? Yeah, Wut's going on...

Milland · Jul 1, 2020

A couple basic points -

I'd rather have more screws of a smaller size than fewer of a larger.

I'd rather have a smaller differential of strength between my fasteners and the anchoring object, with the fastener always the higher strength value because of geometric weakness in a male threat relative to female (gents, this is why Wimen live longer).

If I must have a significant difference in material strength between fastener and base material, I want to have courser threads, and a longer thread engagement (but going beyond ~2X thread diameter doesn't gain you much).

Al fatigues at stress ranges and cycle limits much lower than steel, hence you don't see Detroit selling street cars with Al rods. So it's not great for high-stress fixturing.

If you need kick-ass fasteners I happen to have a metric-poop-ton of A-286 and similar aircraft bolts that cost more than your car originally. Nice for high-strength reliable operation - you don't see low grade fasteners holding jet engine pylons in place do you? Oops, dropped another Trent onto Hillingdon...

EPAIII · Jul 1, 2020

First, this is an engineering question being asked on a machinist board. Some here are engineers, but most are not. This is not to say that you can't get a good answer here, but you will have to use some judgment to sort the answers.

There are tables and equations, but "...subject to extreme impact." is not any kind of measure of the forces involved. If you can not quantify the forces involved, then no equation or table is going to do any good because they will need some kind of numeric entry.

Putting on my engineering hat, I did work as an engineer, but not a mechanical one; I will say if you can not actually measure the force involved, then one approach may be the good old, cut and try. You can simply use a larger screw and see what happens. You could start by doubling or even tripling the diameter and see if it does any good. An insert with the same sized screw may also be an option. That would effectively increase the screw size in the aluminum while allowing you to use the same hole pattern in the part being fastened.

123123 said:
I was more looking for an equation or something so I can calculate it. There are 8 screws holding a part about 100lbs to the aluminum plate (about 1 inch thick). That aluminum plate is then subject to extreme impact. 3/8-16 failed. Unfortunately I am not able to provide pictures. I know that's not much but I've spent so much time trying to figure this out that the crap shoot is worth it

Illinoyance · Jul 1, 2020

Instead of tapped holes can you use through bolts with nuts and hard washers under the plate? The greater length of bolt under preload will give more consistent clamping load for the fixture. I am also in favor of more bolts.

Bill D · Jul 2, 2020

No idea if the bolts are in shear or tension forces. that makes a big difference. My calculations above assumed tension.
Bill D

Rickyb · Jul 2, 2020

Bill D said:
No idea if the bolts are in shear or tension forces. that makes a big difference. My calculations above assumed tension.
Bill D

None if us know any details. That is why all the answers are general in nature. General, just like the query.

Bill D · Jul 2, 2020

Rickyb said:
None if us know any details. That is why all the answers are general in nature. General, just like the query.

At least general is the right forum.
Bil lD

michiganbuck · Jul 2, 2020

Pull out strength in aluminum
https://www.lightmetalage.com/news/...ll-out-strength-old-and-new-design-equations/
Plus, if in a vertical application you have to consider adding the weight of the lower part. And if it is leaning on a tree the wind direction could be a factor.
Accelerating to near the speed of light good to have the latter side securely affixed to the sooner side.

how to select the correct screw?

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