14 replies to this topic

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The description of the three Thompsons got me thinking. Poorly, as I'm out of practice.

The system of all 3 is to an approximation a mass on a spring, with a retarding force( friction or friction/blish lock). A damped oscillator. When the trigger is squeezed and the safety is off, it becomes an oscillator driven by periodic forcing functions, the base of the cartridge pushing on the bolt after the explosion and the reaction force at the rear of the action as well. Right?

Is it possible to get resonance problems?

The natural frequency is the square root of k/m. Going from the '21 to the M1 you increased the mass m of the bolt, but you also decreased the k of the spring, the 'stiffness', thus lowering this natural frequency. Does this effect the ROF? I know nothing of mg theory, sub or otherwise.

oh look, it's time for lunch...

[Z3BigDaddy]

And I thought the other board I post to was wacky sometimes....

Death from the air?

[TAB]

Actually I was trying to understand something. Admittedly a bit tongue in cheek.
It was PK's explanation yesterday that got me thinking. At the risk of being rude, that doesn't happen all that often. Doesn't mean I didn't in the end ask a dumb question, but I've answered my share of them without mocking the person.

Not that i didn't get a laugh out of the story of the klansman.

[Z3BigDaddy]

No risk of being rude with me... On any level.... Mocking you was the last thing on my mind... Trust me... I only made the comment due to the fact it was so over my head and a little on the esoteric(sp?) side... Apologies if I hurt your feelers...

Blaine

aka Z3BigDaddy

PS A least your question was addressed to people who may understand the question...

[TAB]

...times I hate electronic communication...my mistake...no need to apologize, though maybe i should

[Whiskey Brother]

What's a "resonance problem"?

[Mk VII]

most mechanical constructions have a 'natural frequency' of vibration. If an exterior vibrating force (such as a cartridge firing - yes, I know it's not 'external' but it's not generated from inside the steel in the gun) happens to match this natural frequency the construction vibrates with a large amplitude, getting larger and larger until somthing breaks. The Tacoma Narrows Bridge in 1939 is the classic example - when the wind blew at the right speed the road deck oscillated to destruction.

[Whiskey Brother]

Hmmm...
That makes sense. When I first read it I was reminded of music for some reason, and could not make the connection to guns. Just like you get different tones out of different size cymbals, you would get a different "resonance" with a heavier or lighter bolt.

It seems to me the discussion on the 27A's recoil springs would apply here. If you lowered the springs tension, you would get a faster bolt action. So fast in fact that the bolt slamming into the rear of the reciever could set up some bad "resonance". BUT, if you managed to somehow increase the weight of the bolt, you would counteract the effect of the lightened springs.

Or something like that...


Now I just need to know what the "k" and the "m" stands for in the phrase "square root of k/m"...

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k is a measure of the stiffness, the amount of force which the spring will exert with a given extension or compression from it's rest length, so a stiff spring would have a larger k. m is the mass of the bolt. The oscillation comes from the fact that when you extend a spring it pulls back, and when you compress it, it pushes forward. If you were to have no friction, and just pulled the mass away from equilibrium it would oscillate back and forth with a natural frequency(like plucking a guitar string). The square of that frequency is k/m. And a factor of 2 pi, but forget that.

The point is that by changing springs or bolt weights, you are playing with factors that are part of a whole, if that makes sense. The 'damping' is important too- I'm guessing part of the effective purpose of the blish lock was just that, as many screen doors have springs such that the door will close at a reasonable rate, but if you try to slam it, it will resist. But I am guessing.

The resonance is as MkVII pointed out, if you feed energy into such a system in 'tune' with the system, you get a big bang for your buck, even in a real system where there are frictional losses... The Tacoma Narrows Bridge is the classic example of what can happen...and even rebuilt I didn't care for driving over it entirely.

Anyway, what does determine the ROF of an automatic weapon?

[Whiskey Brother]

If I am not mistaken, the Blish lock was a mechanism for causing a delayed blowback action rather than a straight blowback action. The theory was that two different metals actually create an "adhesion" with each other when held at high pressures with each other. When the pressure had decreased to a safe level, this adhesion was lost and the bolt was allowed to open. Without either a blish lock, heavy bolt, heavy springs, or some other method of keeping the bolt closed while ignition occurred, the bolt is prone to open prematurely. This causes a loss of pressure at the breech, and thus loss of bullet velocity. (In other words, all that flame you see coming out of the ejection port would serve a much more useful purpose by devoting it's energy to bullet velocity) Since the bolt is opening sooner, you will see a higher rate of fire, but with a subsequent loss of velocity. Sure it's all neato to try to up your RPM by getting rid of the delayed blowback mechanism, or using lighter springs- but that aint the way Colonel Thompson designed the gun. He wanted the maximum effect of the .45 ACP, and thus incorporated the Blish lock in his gun.

If you ask me, why fix it if it aint broke....

[TAB]

Thank you WB

Different than damping altogether then, though once it releases you still have the same basic system. I'm sorry, I should have read a bit on it first. I see its been discussed before. More than some want . It is allowing energy to be bled away that would otherwise go into the bolt velocity and spring compression, but it is not continuous, just an initial retardation.

I now have even more questions, but enough... Things are only simple when I misunderstand how they work.

[Whiskey Brother]

You should see the rather ingenious delayed blowback system used in I believe the German G3? (It's a bit late... It uses an ingenious system of cams and rollerblocks. I think I also read somewhere that the German MG42 was able to achieve such a high rate of fire because it relied on a delayed blowback system rather than a gas system, but It's too late for me to check on the specifics now, I gotta get some sleep...

[john]

Unlike the spring on a screen door, the recoil spring in a firearm has a couple points to it's advantage in regards to reducing harmonic vibration. First, the spring is wrapped around a pilot for about half of it's "at rest" length. This increases as the bolt is driven rearward. This pilot is buffered at the rear by the fiber disk and the mass of the receiver soaks up some of the vibration.
At the front end is the bronze lock with quite a bit of contact area with the heavy receiver.
The bolt basically runs on a thin cushion of oil, another excellent medium for dampening vibration and bleeding it off to the heavy receiver.


BUT the vibration is still present to a small degree.

Those of you who have several thousand rounds through your Thompsons with the same spring pilot, pull the pilot out and look at it carefully. My '28 pilot (a Savage part) shows wear based on either a fairly high frequency oscillation near the front of the pilot OR maybe based on spring twisting as the spring is compressed. Wear rings a bit closer than 1/16" extend back about 1-1/2 inches along the pilot........

I can't see any other evidence of chattering or vibration anywhere else in the gun (an A.O. Bridgeport) but I'm no expert on harmonics.....

I do know that harmonics plays a significant role in accuracy using various velocities, bullet weights and velocities (been handloading varmint gun ammo for many years) so sometimes it must be considered, however, I don't think it really matters much in the operation of a Thompson Subgun. After all, I have spare spring pilots and springs, and they are cheap.

BUT MAYBE this is why some ammo runs better than others in various guns......


food for thought?

john

[giantpanda4]

I think I should be sleeping after all of the turkey I just ate... but this topic is very interesting, and ties into what we were discussing earlier. I would love to take a reciever into work and whack it with a NV hammer, to determine it resonances. But I work at GM, and firearms/parts are Verbottem on GM property. Anyone elsae able to do this? I assume that the resonances would be in the many kilohertz range, at least in a f/a (bolt axis) mode. This is why I have been amazed that increasing the ROF from 600 to about 900 would cause any problems. After all Gen Thompson had his initially at 1200, then dropped it to 900.

When I hear stories of bashed up recievers, and PK has mentioned the Gunmachines folks may have had some damage to their recievers when they were playing/developing their parts (please correct this, PK!) usually the reciever damage is secondary to what may have actually failed. As an example, I have many go kart race engines with windows in the side of the block. Was usually caused by oil dippers failing at 7000 rpm, then rods, then you just have a pile of scrap metal.

I would love to still hear more stories of how parts (especially recievers) are "failing". Usually an autospy will determine what really failed first.

What would you think if a test rig was devised to determine forces at the base of the reciever? Then we could do a fatigue calculation on the reciever. A strain-gaged block of metal with the insides machined out to accept a factory bolt system and barrel. Of course you need some sort of trigger, too.

Honest, ATFman! It is just a test rig!

Gotta go get seconds....

[Dave Janowski]

In a sense you are correct, but you must remember the springs in the gun have a constant pitch, hence making it a CONSTANT rate (pounds required to compress the spring one inch)as the bolt recoils rearward the rate increases, slowing the bolt down dramatically before it actually strikes the rear most position of the receiver, as for vibration like harmonics I don’t think in firearms this is a major problem clearances must be liberal, but adequate, there is no room for vibration inside the receiver, if so EVERYTHING would work harden and be a total disaster, hell if the clearance was so excessive you would never reliably strip a cartridge from the magazine

If you read up on the gun itself the blish lock principle is basically a hoax! I mean it DOES work but not on all cartridges in all occasions THE only cartridge that actually worked in all of the tests was the pistol caliber cartridges, anything with super velocity was just not feasibly actually if I am not mistaken Thompson wanted to have rifle caliber cartridges on his trench broom, 30.06

Maybe I am wrong, but I have spent 12 years of my life designing and making springs from Heavy trucks to patriot Missiles to nuclear submarines all of the factors come into play, but in this instance I don’t think harmonics, and vibration are a major concern

Just my silly opinion

Later

Dave