Pendulum test

[MechEngineerNT]

Hey Guys,

I'll be running a pendulum test to test gun recoil. The platform is around 100 lbs and will be supported by 2 steel bars about 8 feet long and 20lbs each. Would it be safe to ignore the weight of the metal arms because they are so long?

Thanks,

Alex

 

[EngineerTex]

Maybe. But I wouldn't.

Also, I wouldn't ignore the rotational inertia of the bars.

Also, TWENTY POUNDS???!! Are you testing Howitzers?

Engineering is not the science behind building. It is the science behind not building.

 

[handleman]

When are you ignoring them? Design of the support structure to hold up the pendulum? Or calculation of the recoil energy?

How are you going to take the measurements? Displacement of the pendulum? Potential energy due to the angle of a homogeneous pendulum is a pretty dang simple calculation.

I would definitely ignore the rotational inertia of the bars. It's irrelevant if you're measuring maximum displacement of the pendulum. All that matters is the delta-h of the CG.

Of course, this is all assuming that you're able to transfer all of the energy from the shot into kinetic energy of the platform, and then assuming that frictional loss as the pendulum swings upward (transforming KE into PE) is negligible.

 

[EngineerTex]

I would definitely ignore the rotational inertia of the bars.

I'm changing my answer. Rotational inertia wouldn't be a factor if you are measuring how far the pendulum swings. I would still account for the weight of the bars, though.

Engineering is not the science behind building. It is the science behind not building.

 

[TheTick]

It's just one more calculation. Get it done and be done.

Better to be too accurate and round down on paper than just not accurate.

 

[IRstuff]

"TWENTY POUNDS???!!"

You say you're trying to test gun recoil, but since the weapon will not follow the pendulum, the calculations get much more complicated. Perhaps you might consider how other people measure recoil. The cites below are from about a minute of Googling.

http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1206&context=mesp

http://precisionrifleblog.com/2015/07/01/muzzle-brakes-recoil-primer-test-equipment-rifles/

https://www.youtube.com/watch?v=Hiju-bzvf34

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[handleman]

Huh? Why will the weapon not follow the pendulum?

 

[IRstuff]

The standard ballistic pendulum requires that conservation of momentum be applied, since the round is embedded into the pendulum. In the case of a rifle butt, the rifle will move back, strike the pendulum, and rebound, while the pendulum moves backward. Conservation of momentum now requires figuring out the momentum of the rifle and the pendulum, separately, and if you could do that, you would have done that, instead of using a pendulum.

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[handleman]

I guess this is assuming that you can't sufficiently clamp the rifle to the pendulum bed such that they are considered a single mass?

 

[ornerynorsk]

Back in the day our recoil tests involved how many rounds could be fired before noticeable flinching occurred and subsequent size and duration of bruising. (tongue in cheek)

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[IRstuff]

"I guess this is assuming that you can't sufficiently clamp the rifle to the pendulum bed such that they are considered a single mass?"

possibly could do that, but there appears to be more direct ways of doing the test that doesn't involve having to physically stop the pendulum, reset it, and fire another shot.

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[handleman]

First link: TLDR
Second link: blocked by my organization
Third link: What's the difference between this machine and clamping a rifle to a pendulum? Other than figuring out where to hang a 140lb, 8-foot pendulum... Both of them operate on principle of measuring the distance a weight is raised.

Granted, the readout of the sliding plastic piece is linear, whereas the readout of angle measurement is not. However, that's what Excel is for. One could argue that the non-linearity of the pendulum's response is an advantage. Accuracy is improved for smaller energy values without sacrificing range at higher energy values.

 

[IRstuff]

The smaller energy values result in very small height changes, making them more difficult to read.

I'm not suggesting either of these approaches, as I think that a force sensor is more appropriate, given that what you care about is a transient event that is completely washed out with a pendulum or the last site, and I was merely pointing out that no one else appears to be using a pendulum to do recoil testing.

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[handleman]

The smaller energy values result in very small height changes, making them more difficult to read.

That was exactly my point to the advantage of pendulum vs. sliding method shown in the YouTube video. With a hanging pendulum at the bottom of its swing, small changes in height equate to large changes in angle. As the angle increases, the incremental height increase per degree increases. So (again, assuming you're measuring the change in angle, not trying to measure height), the pendulum is more sensitive at low energy values, yet it can conceivably measure very large energy values (with precision decreasing as the energy increases)

As to the force vs energy, it depends on what you care about. If you care about force, I completely agree that energy is a roundabout (maybe impossible) way to get there. OTOH, if you're trying to measure energy, force is not a great way to get there.