SpudFiles

A spring gun question?

This questions is fundamental:

Why does a spring gun compress air instead of directly pushing the projectile?

How does the spring velocity compare to the projectile velocity?

BoyntonStu

Well, the spring pushes a piston forward quickly in a chamber, after a certain ammount of pressure is built up, the projectile moves forward and shoots out of the barrel. Here is an explanation: http://en.wikipedia.org/wiki/Air_rifle scroll down to "spring piston"

...Why does a spring gun compress air instead of directly pushing the projectile?

How does the spring velocity compare to the projectile velocity?...

The spring compresses rather than directly pushing to better "maximize" it's thrust for lighter weight projectiles.

The spring velocity is probably greater than the projectile when "dry-fired" because a lot of designs can be damaged during a Dry-fire.

As far as the "maximization" it's sort of like the thread mentioning a 60lb. pull weight bow pushing a .22 pellet to 600 fps, The full size arrow at 200+ fps certainly has far more penetration and energy on the target than a .22 pellet impact. But does not have 600 fps...All depends on what the desired effect is I suppose. 8)

BTW, I like your posts/replies boyntonstu, welcome to SF's. 8)

It also shock heats the air. If you're thinking of making a spring gun, don't.

THUNDERLORD wrote:

...Why does a spring gun compress air instead of directly pushing the projectile?

How does the spring velocity compare to the projectile velocity?...

The spring compresses rather than directly pushing to better "maximize" it's thrust for lighter weight projectiles.

The spring velocity is probably greater than the projectile when "dry-fired" because a lot of designs can be damaged during a Dry-fire.

As far as the "maximization" it's sort of like the thread mentioning a 60lb. pull weight bow pushing a .22 pellet to 600 fps, The full size arrow at 200+ fps certainly has far more penetration and energy on the target than a .22 pellet impact. But does not have 600 fps...All depends on what the desired effect is I suppose. 8)

BTW, I like your posts/replies boyntonstu, welcome to SF's. 8)

Thanks,

I asked the question because I am thinking of an alternative to the bow driven Airowgun.


JoergSprave is a master slingshot inventor/builder. He chronos his shots, etc. You will be impressed at what his slingshots are capable of shooting.

see: for an idea of what a single slingshot rubber can do.

After a few, "what ifs?" I thought about a sleeker more powerful version of the Airowbow, by using multiple slingshot rubbers instead of a bow to create a slingshot, air rifle.

We are in contact with each other in designing a 4 rubber 24" stroke 200 lb "spring" trigger mechanism.

Each rubber can be individually cocked to create a very powerful "speargun" mechanism to drive an air compressor.

BoyntonStu

It's a good idea to cock each one separately.

Remember that if the energy comes from a single stroke of the arm, you will not have more energy than that.

With a trigger mechanism you could use a threaded rod to cock the mechanism, allowing for massive energy.

Edit: No different for a single stroke pneumatic idea I had drafted, which used allthread (threaded rod) as a pump shaft. This drove a fairly large pump to high pressure in a single stroke with minimal effort. The first revolutions could be turned by an electronic motor to save time.

inonickname wrote:It's a good idea to cock each one separately.

Remember that if the energy comes from a single stroke of the arm, you will not have more energy than that.

With a trigger mechanism you could use a threaded rod to cock the mechanism, allowing for massive energy.

Edit: No different for a single stroke pneumatic idea I had drafted, which used allthread (threaded rod) as a pump shaft. This drove a fairly large pump to high pressure in a single stroke with minimal effort. The first revolutions could be turned by an electronic motor to save time.

I sure would like to see sketches of your idea.

What was your pressure goal?

I thought of a few things after posting:

If the first part of the rod was not threaded, you could push it as far as your weight will go an then engage the threads.

Another idea is to use a small hydraulic press.

A 1 ton force Hmmmn!

BoyntonStu

Pretty cool slingshot vid you linked to Stu!
(Guys an expert shot though...lots o' practice...worked better than my homemade mammoth...maybe I'll re-visit it w/ better ammo).
My favorite was his roller bearing compound design.

Beeman airguns sold a slingshot with bearings similar years ago, except it had the bands horizontal (to arm), inside a shroud...It had an attachment for small arrows also, an ad pic showed a large ball bearing imbedded in wood.

Also I remember an ad in back of gun magazines and "Soldier of fortune" magazine --( ) for a blowgun dart air pistol, that used latex bands to compress the air in a piston.
(Doubtful that the entire concept is patented though(?)).

inonickname's idea seems interesting, maybe use a piece with notches and use a foot stirrup to cock back like 4+ 150lb.'er bands to each notch, the trigger would already have it locked in place before cocking each individually...

The piston stroke length seems way too long, a bigger diameter shorter stroke, that uses the peak spring range seems more practical for some reason.

I've sort of inverted the concept of your other thread...
been daydreaming of a propane (pneumatic pressure source/non-combusting) powered crossbow, with a piston cocking the string (or pressing a lever to cock the string)...or silenced power tool turning gears to cock the bowstring etc. etc...what to do with the left over pressure(?).
(Cabelas sells an expensive handcrank cocking crossbow that was sold out last I saw).

BTW, wish someone would make THIS image into a funny avatar...
Got to go run errands and look for work 8)

THUNDERLORD wrote:Pretty cool slingshot vid you linked to Stu!
(Guys an expert shot though...lots o' practice...worked better than my homemade mammoth...maybe I'll re-visit it w/ better ammo).
My favorite was his roller bearing compound design.

Beeman airguns sold a slingshot with bearings similar years ago, except it had the bands horizontal (to arm), inside a shroud...It had an attachment for small arrows also, an ad pic showed a large ball bearing imbedded in wood.

Also I remember an ad in back of gun magazines and "Soldier of fortune" magazine --( ) for a blowgun dart air pistol, that used latex bands to compress the air in a piston.
(Doubtful that the entire concept is patented though(?)).

inonickname's idea seems interesting, maybe use a piece with notches and use a foot stirrup to cock back like 4+ 150lb.'er bands to each notch, the trigger would already have it locked in place before cocking each individually...

The piston stroke length seems way too long, a bigger diameter shorter stroke, that uses the peak spring range seems more practical for some reason.

I've sort of inverted the concept of your other thread...
been daydreaming of a propane (pneumatic pressure source/non-combusting) powered crossbow, with a piston cocking the string (or pressing a lever to cock the string)...or silenced power tool turning gears to cock the bowstring etc. etc...what to do with the left over pressure(?).
(Cabelas sells an expensive handcrank cocking crossbow that was sold out last I saw).

BTW, wish someone would make THIS image into a funny avatar...
Got to go run errands and look for work 8)

Jeorg told me that rubber should be stretched 400% for maximum force and maximum life. I trust his opinions.

I imagine a breech loading repeater 'speargun" air rifle with a .375" -.5" barrel. You would have a choice of stirrup cocking "dead lift" or cocking each rubber.

I love your avatar suggestion.

BoyntonStu

boyntonstu wrote:Why does a spring gun compress air instead of directly pushing the projectile?
How does the spring velocity compare to the projectile velocity?

To answer the first, I have to answer the second.

The piston in a spring air rifle reaches about 20-25 m/s. The projectile velocity is about 10 times higher.

The fact is, the spring has a high mass relative to the projectile, so having to get it up to the same speed as the projectile (as would happen if pushing it directly) would be highly inefficient. Air, being much lighter, needs less energy in it to push a projectile up to velocity saving on inefficiency.

More importantly however, the piston comes to a rest by the time the projectile has left the barrel. It surrenders its energy into the compressed air, meaning that the piston's energy is passed into the projectile, rather than remaining in the piston, which would be wasteful.

Even so, spring air rifles are only about 30-40% efficient. Compare that to a bow, where efficiencies of 90% or more are easily possible.

However, believe it or not, the two mechanisms are scarcely different in their principles of operation. Both have a "spring" and a projectile, and a lightweight medium to transfer energy from the spring to the projectile - air, or the bow string.

The "spring" in both surrenders its energy into the lightweight medium (which is capable of travelling faster than the spring, in the bowstring's case because of leverage), which then passes its energy into the projectile because it's light enough to keep up efficiently.

Because of the way they work, it should be noted that neither should be fired without a projectile (because without the energy let out of the system, it will damage the mechanisms) - a similarity between the two that many people never notice.

Ragnarok wrote:...The piston in a spring air rifle reaches about 20-25 m/s. The projectile velocity is about 10 times higher....

I should've used the word "energy" rather than "velocity".

But I don't understand this:

...Even so, spring air rifles are only about 30-40% efficient. Compare that to a bow, where efficiencies of 90% or more are easily possible. ...

So if the spring itself was pushing a heavier projectile it seems it would be 90% effective also then (?) ...Which is it?

My old simple "Thunderpup" (in pneumatic showcase) pushed an arrow into foam arrow stop same distance as a 65lb. compound bow in a little test one time.
Thing is, it still wasn't maximized for efficiency (probably) because when I loaded two (edit:same time) they penetrated nearly as deep.
Seems Sort of a "black art"...surely there's some scientific applicable formula, BUT at some point I just tend to ask, "What do you want it to do???" (specs?)...
BTW:

I love your avatar suggestion.

You could use it then, If you wanted.
When I click on larger image it comes up a seperate window and square, but I don't know/ have time to convert that to an avatar myself.
(I took still frame shots of vids before but I always get mssg. about too large with my camera shots(???)) 8)

Ragnarok wrote:

boyntonstu wrote:Why does a spring gun compress air instead of directly pushing the projectile?
How does the spring velocity compare to the projectile velocity?

To answer the first, I have to answer the second.

The piston in a spring air rifle reaches about 20-25 m/s. The projectile velocity is about 10 times higher.

The fact is, the spring has a high mass relative to the projectile, so having to get it up to the same speed as the projectile (as would happen if pushing it directly) would be highly inefficient. Air, being much lighter, needs less energy in it to push a projectile up to velocity saving on inefficiency.

More importantly however, the piston comes to a rest by the time the projectile has left the barrel. It surrenders its energy into the compressed air, meaning that the piston's energy is passed into the projectile, rather than remaining in the piston, which would be wasteful.

Even so, spring air rifles are only about 30-40% efficient. Compare that to a bow, where efficiencies of 90% or more are easily possible.

However, believe it or not, the two mechanisms are scarcely different in their principles of operation. Both have a "spring" and a projectile, and a lightweight medium to transfer energy from the spring to the projectile - air, or the bow string.

The "spring" in both surrenders its energy into the lightweight medium (which is capable of travelling faster than the spring, in the bowstring's case because of leverage), which then passes its energy into the projectile because it's light enough to keep up efficiently.

Because of the way they work, it should be noted that neither should be fired without a projectile (because without the energy let out of the system, it will damage the mechanisms) - a similarity between the two that many people never notice.

The issue becomes:

Is it better to launch a 100 gr projectile with a 4 band 200 pound slingshot directly or indirectly using compressed air?

Thanks,

BoyntonStu

THUNDERLORD wrote:But I don't understand this:

...Even so, spring air rifles are only about 30-40% efficient. Compare that to a bow, where efficiencies of 90% or more are easily possible. ...

So if the spring itself was pushing a heavier projectile it seems it would be 90% effective also then (?) ...Which is it?

Not at all. The spring is too heavy and absorbs too much energy in its own acceleration. That's why both bows and air rifles use a low mass "propellant" which allows the "spring" to travel more slowly than the projectile, saving energy.
Both the air in an air rifle and the string on a bow travel much faster than the springs which energised them.

Which is better very much depends on the projectile in question. While a bow fires a relatively heavy projectile (50-60 grams) at no more than about 100 m/s, air rifles fire light projectiles (0.5-2 grams) at potentially over 200 m/s.

The air rifle will always be around 30-40% efficient, fairly regardless of velocity.
The bow will rapidly lose efficiency as velocity increases. While it's about 90% efficient at moderately low velocities, it gets worse quite quickly.

If the efficiency gets too bad, the energy is then going into the bow rather than the projectile, which damages the parts of the bow - so they don't make high velocity bows. You won't find a bow that can shoot as fast as a reasonably powerful air rifle.
Then again, most air rifles won't put out the energy of a bow.

If you're looking for low mass/high velocity, then air. If you're looking for high mass/mid velocity, then bow.

So, Boyntonstu, for a 100 grain projectile, my advice would be for compressed air - but I advise warning. I did myself a major mischief with a homemade springer.

Also, I'm sure people would appreciate it if you stopped quoting the entire posts and just cut it down to the parts which are relevant. We seem to have picked up several people who think it's fine to do this, and it's just messy and bad practice.

I agree, people should not underestimate the power of the FORCE....I mean slingshots They are quite powerfull and fun.

sO... I've wondered before, (and now recently remembered) about the idea to use a piston inside a combustion chamber, with it pressing a volume of air into a smaller diameter barrel.
Sort of a springer, except powered by a combustive force rather than a spring.
Seems like it has potential(???).

Also I've wondered about maybe having a lever connected to a piston inside a combustion chamber, so the slow moving, high energy of the piston could be converted to higher velocity...(???)

Any thoughts on that??? 8)