PDA

View Full Version : Strength of material issue or bad design?



Greg Parent
08-13-2009, 01:25 PM
Hello everyone,

Anyone know of a good 'strength of materials' website site?

I am trying to design a 5" diameter thrust plate and need to know if 0.375" thick material held with 0.125" screws is good enough for the application or if I should go up to 0.5" material and larger diameter screws.

Eight 0.125" stainless steel machine screws will be holding this plate from moving inside a cylinder. The 0.375" material leaves 0.125" of material above and below the screws threaded holes.

I have the initial force calculated at 550 pounds spread out over the 8 screws. Is 0.125" of 6061-T6 aluminum plate above the screws strong enough? Are the 0.125" stainless steel machine screws the weakest link?

Sure wish I was an engineer.

Thanks for any input you may have.

Greg

Rich Carlstedt
08-13-2009, 02:25 PM
Greg
You are not giving all the information needed to get a good decision.
let me try to answer, but I may be making an "assumption" that is in error.

First, is the 1/8 aluminum plate threaded and the screws use it, for retaining the 3/8" plate ?
Or,
is the 3/8" plate threaded, and the screw heads mount in the 1/8" plate.
big difference here.
The force on a screw in tension is relegated to the minor diameter. That is the smallest cross-sectional area.
Lets answer that first. You need the strength of the SS you intend to use.
Lets go with 60,000 pounds, a low strength SS
When you stretch a bolt, its called tension and thats where 'tensile strength"(TS)
So the SS has the ability to hold 60 K (K=000) of load.
a 5-40 screw has a minor diameter of about .094"
That is a area of (.047 x.047x 3.14= ).0069 ( lets say .007 square inches !)
So .007 x 60K= 416 pounds.
Since in a stress situation, you want a SF (safety factor) of 3 (or 4), we could say each screw can handle 140 pounds of load.
So Dave your screws are OK, as 5 would handle 700 pounds.
Note Dave, If Human life may be jepordised by this assembly, it is not unknown to see SF's of 10 !

the problem know reverts to the threaded holes.
Since Aluminum has only 10K tensile, the threaded holes become critical
You have no doubt heard that "maximum screw strength is achieved at one times thread depth" ( so a 1/4 thread needs only 1/4" deep to achieve max strength)
This is NOT always true.
It is OK "IF" both materials are the same alloy and tensile strength.
In your example the Aluminum with 10K versus the SS with 60 K is a 6 to 1 ratio.
Therefore to match the full strength (and SF!) of the screw, the depth of thread must be 6 times thread diameter (OD)
so you need 1/8 ( thrd OD)times 6, or 3/4" thread to keep the aluminum from stripping out at full load and with a 3 Safety factor.
Without any Safety factor, you would still need a .25 thread depth

Hope this helps, confusing as it is.
If you used the 1/8 plate to hold the heads and your 3/8 "plate was threaded instead, you could make it work with a 1.5 SF

Rich

Steve Steven
08-13-2009, 03:08 PM
Rich,
I see this as a slightly different problem. I see a circle of 3/8" aluminum plate, held inside a cylinder by 8 ea, 1/8" screws, installed radially. The screws would be in shear, not tension. Your analysis is good however.

Steve

Greg Parent
08-13-2009, 03:30 PM
Steve is correct.

Rich, sorry for my muddy description.

I am worried about two things.

First, that the remaining metal above the screws is not enough to handle the force applied and the plate will fail above the screw holes (strength of the plate material).

Second, that the screws are too small and will shear (strength of the stainless screws). I never knew stainless screws were that strong. Thanks Rich.

I can change the thickness of the plate and the diameter of the screws but not the number of radially placed screws.

Thanks again for the input.

QUOTE=Steve Steven]Rich,
I see this as a slightly different problem. I see a circle of 3/8" aluminum plate, held inside a cylinder by 8 ea, 1/8" screws, installed radially. The screws would be in shear, not tension. Your analysis is good however.

Steve[/QUOTE]

psomero
08-13-2009, 03:52 PM
as steve pointed out, i'd be weary of using stainless screws in a scenario like this. i'm pretty sure there are a few alloys that are suitable, but if you go to a fastener supplier and say "give me stainless screws," you may not end up with the right ones.

the worst encounter i've had with ss screws was when they were used to hold down a thin sheet of stainless in a fixture for an op on a VMC. we had to screw/unscrew 12 10-32 socket head cap screws per part and they'd fail after about 15-20 ops. i'm not sure what alloy it was, but they had really low yield strength and fatigue life and would elongate each time until the point where the heads would just pop off.


what kind of loading will be on this plate? is it a static load or is it cyclic and get applied repeatedly?

darryl
08-13-2009, 04:27 PM
I've been told more than once that stainless steel bolts shouldn't be considered to be any stronger than mild steel. I've popped the heads off some 1/4 inch bolts without a lot of effort, and these days depending on the supplier of such hardware they could be quite weak.

This isn't to say that there isn't such a thing as stronger SS bolts, but I'm not aware of any. I'd go with an alternative, such as SHCS, socket head cap screws, if you can make those fit.

It does boil down to engineering. I'd be interested myself to see how the various criteria are calculated for bolts used radially through the side of a cylinder into the edge of a disc. Seems it would be quite involved, for one because of the tilting forces the threaded hole would experience, and the amount of restriction to that motion that might be provided by the hole in the cylinder that the fastener would go through. Etc, etc.

ckelloug
08-13-2009, 05:21 PM
If this is a critical part I would suggest getting fasteners from somebody like Unbrako or holochrome that actually test lots and guarantees strengths. I'd also suggest against mystery metal. The part will only be as strong as the weakest link.

psomero
08-13-2009, 05:34 PM
does this assembly need to be disassembled ever?

if not (or not very often) i'd suggest you press a few hardened dowel pins in addition to the screws.

Peter.
08-13-2009, 05:52 PM
A2 stainless fasteners are like cheese. A4 is better.

Black_Moons
08-13-2009, 06:06 PM
rich: thanks for the informative math on screw strength in an easy to digest formual :)
Intresting rule of thumb about thread depth vs strength. makes a lot of sense.


Iv heard about only the first 4 or so threads holding all the force because of the bolt streching. How does that fit in with your math? or is it wrong with 'soft' materials like a SS bolt into aluminum?

Greg Parent
08-14-2009, 10:35 PM
Yes, I agree. Thanks Rich. Very informative.


rich: thanks for the informative math on screw strength in an easy to digest formual :)
Intresting rule of thumb about thread depth vs strength. makes a lot of sense.


Iv heard about only the first 4 or so threads holding all the force because of the bolt streching. How does that fit in with your math? or is it wrong with 'soft' materials like a SS bolt into aluminum?

J Tiers
08-14-2009, 11:29 PM
I don't know what 0.125" screws are..... #4? #5? I saw the 5-40, but 40 tpi either way isn't great with aluminum.

In any case, if the minor diameter is about 0.085", they add up to about 0.045 sq inch. Assuming a shear strength of 15,000 psi (with some built-in de-rating), they are good for about 680 lb total in and of themselves.

Then you get to argue about how much of the depth of engagement is effective, and what the result is with regard to the aluminum.

Personally, the 680 vs 550 lb capability is, while sort of adequate, not overly impressive, even given the inherent de-rating due to using 15k PSI shear strength.

And that is without considering the aluminum at all.

With any variable force, the aluminum is very likely to crush and work loose, egging out the screw holes and leading to a failure. Might be a little different if the cylinder is steel, and much thicker than the screw is in diameter.

The loading with a soft material like aluminum is not really shear, it is a "bending plus shear"..... The aluminum may not develop the full strength of teh screws because they may cock (or bend) due to the force being applied through a soft material. If they were into steel, you'd not expect the same situation of potential bending. it would depend on the grade of aluminum also.

You'd be better off to have a pin clear through (yes I know it isn't practical).

Then also, with aluminum, the coarsest screw thread is best....

Greg Parent
08-16-2009, 09:13 AM
Gentlemen,

Thanks for all the input. It is back to the drawing board for another look at this design.

Greg

Cheeseking
08-16-2009, 03:30 PM
Hi, Greg,
Hows that pintle I made you working?

Rich Carlstedt
08-16-2009, 06:59 PM
rich: thanks for the informative math on screw strength in an easy to digest formual :)
Intresting rule of thumb about thread depth vs strength. makes a lot of sense.
Iv heard about only the first 4 or so threads holding all the force because of the bolt streching. How does that fit in with your math? or is it wrong with 'soft' materials like a SS bolt into aluminum?

Black_moons
It generally applies to all threads. As the screw stretches, greater force is applied to the female threads nearer the surface. As the female threads yield, a portion of the load is transferred back to the deeper threads.
When both materials are the same, the yield or stretch is equal (approx) for both screw and hole threads.

The four thread comment is a generalization . However as the hole material gets softer (compared to Screw mat'l), it does not apply, as the stronger screw needs to transfer load to the deeper threads.
It works for general fastener applications when you are not concerned with maximum fastener application strenght ( bang for the buck)


The comment about SS screws is dead on.
For high strengh applications, do NOT use SS.
If you want good quality SS screws, you must stay with Unbrako,Camcarr, Holo-Chrome, or Allen.

Rich
Thanks for the kind comments guys

Greg Parent
08-16-2009, 08:38 PM
Hello Paul,
PM sent.
Thanks
Greg


Hi, Greg,
Hows that pintle I made you working?