Calculating force from drop test to determine strength of 8-32 thread

[mperegrinefalcon]

I did a test on with some bulkheads and 8-32 threaded rod where I had a mock Av-bay suspended from a beam, and the other eyebolt tied to a 25.4 lb (11.5Kg) weight. I dropped the weight 16.5" (0.4191m) and it held, but the eyebolts pulled open. The 8-32 thread was perfectly fine though. My question is how might I go about calculating the amount of force exerted on the eyebolts by the weight being stopped. I have only taken high school physics and it has been a while, so please be patient.

I used the equation v=(2gh)^1/2 where G=-9.81 m/s^2 and h=.4191 and I got a Velocity of 2.87m/s.

How would I figure out how much time it would take to slow down?
How would I figure out the distance required to slow down?

I calculated the average velocity of 0m/s and 2.87m/s and used that to find .00885 seconds for the time it took to slow down, assuming it slowed down in 0.5"
This would equal 324.3 m/s^2, or 33.07 gees. This would then simply be 33.07x25.4=839.9 lbs of force.

Would this be correct?

Thanks for any help

 

[OverTheTop]

You have an impulse of energy that is applied to the rod. The problem is that the force applied is very dependant on the distance it decelerates over. If you can estimate that well you will have a decent answer. Not easy though.

I would just take the minor diameter of the thread and use the tensile strength of the material to work out the maximum force. Probably a bit more accurate that the other method. Also, because the allthreads are rolled the strength may be a little higher due to work hardening. Ie, conservative answer.

 

[Steve Shannon]

How would I figure out the distance required to slow down?

The distance required to slow down was the distance the eye opened plus any flex.
That’s not perfect, but it’s a reasonable approach.

 

[OverTheTop]

But how do you measure the flex reliably? Can be done but not necessarily straightforward.

 

[cerving]

<deleted... I misunderstood the problem>

 

[OverTheTop]

If you are really keen you can use some lengths of wood and some bolts to make some levers to test the system. Adding mass to the end of the lever increases the force. The advantage of this system is that the load is not dynamic and easier to determine. I have used this method to test interstate adapters to maximum flight load levels. Dont forget to allow for the mass of the wood .

 

[mperegrinefalcon]

If you are really keen you can use some lengths of wood and some bolts to make some levers to test the system. Adding mass to the end of the lever increases the force. The advantage of this system is that the load is not dynamic and easier to determine. I have used this method to test interstate adapters to maximum flight load levels. Dont forget to allow for the mass of the wood .

Ah, a lever problem. Very good idea. I will test it that way next and report back.

 

[Steve Shannon]

But how do you measure the flex reliably? Can be done but not necessarily straightforward.

Just approximate it at one or two percent of the distance the eye opened. It’s not going to be significant since the screw eye exceeded it anyway.

 

[David Schwantz]

Just tie in a scale and drop it. then read the saved weight

 

[mperegrinefalcon]

Just tie in a scale and drop it. then read the saved weight

I do not have a scale that can measure about 1500lbs, which is the goal for this test.

 

[Dotini]

FWIW, a typical 8-32 wood screw can hold about 80 lbs. A high strength (60,000 psi J82 60M or Grade 1 or Class 4.8) 8-32 bolt has a tensile strength of about 840 lbs. The National Electrical Code allows an 8-32 steel screw to support no more than 35 pounds from a ceiling outlet box.

NAS or airframe quality fasteners may have quite a bit more strength.

 

[rocketguy101]

If I understand your original question, you are trying to determine the maximum force that deformed your eye bolt? I assume it was a bent open style similar to this? What material was the eye bolt made from? You could calculate what force it would take to cause the eye bolt to yield (need the yield strength of the eye bolt material)...there are equations on the web for this.

Knowing this force, you can calculate the maximum G from F= ma

 

[mperegrinefalcon]

I just read up some more on thread tensile strength testing here

https://wilsongarner.com/proof-load...trength, or,applied until the fastener breaks.

And here

https://wilsongarner.com/the-basics-of-external-threads-threads-part-1/
"A properly made bolt subject to extreme tensile stress will break across the diameter before the threads shear."

So the steel itself will break before the threads do.

So calculating the area of the 0.138" got me .015in^2, multiplied by the proof load of a grade 1 8-32 bolt (55,000 psi) I got 825lbs for an 8-32 thread proof load strength.
Going up to grade 5 pushes that to 1275lbs, and this is all rounded down.

I will see if I can find grade 5 hardware to use, but since there will be 2 of these threads holding the AV bay together I won't sweat using grade 1 8-32 thread. It should be plenty strong for a 100G deployment on a 7lbs rocket.

 

[OverTheTop]

You need to use the minor diameter of the thread in your calculations. More like 0.125", as the major diameter is reduced by the depth of thread on both sides.

 

[bjphoenix]

You need to use the minor diameter of the thread in your calculations. More like 0.125", as the major diameter is reduced by the depth of thread on both sides.

The minor diameter is a better predicter of the strength but you actually want to use the "tensile stress area" which for 8-32 is 0.0140 square inches. However these discussions about making the prediction more accurate are somewhat pointless since we don't know the material being used. There are various ASTM specifications used for common bolts such as A307 and the 55ksi that someone mentioned for ultimate strength might be reasonable, but even assuming that is the correct material from one bolt manufacturer to the next it could easily vary 5 or 10%.

It is true that a standard nut should develop the tensile strength of a bolt in tension (even though you can't count on tightening a nut with a wrench enough to break a bolt without the threads stripping). I've seen some fairly thin nuts used with 8-32 so I'm not sure I would count on them to develop the strength. You could use a longer nut such as a coupling nut and that would have plenty of threads. The theory of load distribution in screw threads says the first few threads carry most of the tension load but some of those thin 8-32 nuts seem to only have a couple of threads.

Back to the original question- it is very difficult to convert an impact to a maximum force because it depends on the time/distance that the impact acts over. If you had high speed camera footage you might be able to estimate it. It would be easier to rig up a test fixture to actually test tension capacity directly but the calculated values from other posts are probably accurate enough for the OP's purposes.

 

[anbhtblr]

I just read up some more on thread tensile strength testing here

https://wilsongarner.com/proof-load-yield-strength-and-tensile-strength-of-fasteners/#:~:text=A fastener's tensile strength, or,applied until the fastener breaks.

And here

https://wilsongarner.com/the-basics-of-external-threads-threads-part-1/
"A properly made bolt subject to extreme tensile stress will break across the diameter before the threads shear."

So the steel itself will break before the threads do.

So calculating the area of the 0.138" got me .015in^2, multiplied by the proof load of a grade 1 8-32 bolt (55,000 psi) I got 825lbs for an 8-32 thread proof load strength.
Going up to grade 5 pushes that to 1275lbs, and this is all rounded down.

I will see if I can find grade 5 hardware to use, but since there will be 2 of these threads holding the AV bay together I won't sweat using grade 1 8-32 thread. It should be plenty strong for a 100G deployment on a 7lbs rocket.

100g? First time I noticed this in the thread.