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Unread postAuthor: boyntonstu » Mon Oct 19, 2009 1:48 pm

JoergS wrote:Had to strengthen the welding a bit, the kick is enormous.

Added a longer barrel and attached the much stronger black bands. Much more accurate now!

Image

Image

Jörg

MRR, rubber at the tip of the sled would work, but the force would smash it to pieces in no time.



Jörg,

You read my mind. Fantastic!

Two issues:

The sled must hit the projectile ball and get it to separate before stopping it..

After separation, you can shock absorb the sled's kick.

I am anxious to see the chrono results.

You are fast!
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Unread postAuthor: Ragnarok » Mon Oct 19, 2009 1:56 pm

JoergS wrote:MRR, rubber at the tip of the sled would work, but the force would smash it to pieces in no time.

A possible solution to that is if you can find an old mouse ball in the right size - they're almost invariably steel ball bearings with a fairly tough rubber coating.
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Unread postAuthor: D_Hall » Mon Oct 19, 2009 2:40 pm

Just hardened steel is going to provide a better "kick" than anything rubber coated.

Seriously, have you ever seen hardened steel bounce on a HARD surface? It's amazing. Way better than rubber. The problem in "everyday life" is that the steel ball will crush whatever you're dropping it onto. Say... Concrete. Drop a steel ball on concrete and it doesn't bounce, but you've also dented (however slightly) the concrete.

Do it on something HARD, and you'll be amazed.
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Unread postAuthor: boyntonstu » Mon Oct 19, 2009 6:35 pm

D_Hall wrote:Just hardened steel is going to provide a better "kick" than anything rubber coated.

Seriously, have you ever seen hardened steel bounce on a HARD surface? It's amazing. Way better than rubber. The problem in "everyday life" is that the steel ball will crush whatever you're dropping it onto. Say... Concrete. Drop a steel ball on concrete and it doesn't bounce, but you've also dented (however slightly) the concrete.

Do it on something HARD, and you'll be amazed.


Excellent, you are correct.

Check out the Modulus of Elasticity here:
http://www.engineeringtoolbox.com/young ... d_417.html

And compare rubber to steel.

They are not in the same ballpark.

BoyntonStu
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Unread postAuthor: D_Hall » Mon Oct 19, 2009 11:06 pm

Ummm... Young's Modulus (aka, modulus of elasticity) has to do with how much force it takes to cause a given amount of deformation. It is not a measure of how high a ball made of material X will bounce. I'm sure there IS such an engineering term, but "Modulus of Elasticity" isn't it.
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Unread postAuthor: Ragnarok » Tue Oct 20, 2009 3:54 am

D_Hall wrote:I'm sure there IS such an engineering term, but "Modulus of Elasticity" isn't it.

"Coefficient of restitution", if I'm not mistaken.
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Unread postAuthor: boyntonstu » Tue Oct 20, 2009 6:23 am

From the Web:

Which ball bounces the highest -- one made of rubber, steel, or glass?

A ball of glass will bounce higher than a ball of rubber. A ball of solid steel will bounce higher than one made entirely of glass.


How come steel is more elastic than rubber ?

A steel ball does not bounce as much as a rubber or tennis ball. How do you define elasticity? One fiber of a spider web is stronger than a steel wire of the same dia. - is this true ?

Elasticity is the capability of an object to return to its former shape once a load inducing strain is removed.

If you were to drop a steel ball on a very hard surface, it would probably bounce higher than than rubber. If you drop it on a softer surface, because it would deform less, then the surface it is to bounce off will be the one deforming, so the bounce would be a function of the elasticity of the floor, not of the ball.


I hope that this helps explain elasticity and Modulus of Elasticity.

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Unread postAuthor: JoergS » Tue Oct 20, 2009 9:34 am

Tried to add sturdy rubber, it gets torn apart in no time. I guess I have to live with the kick.
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Unread postAuthor: boyntonstu » Tue Oct 20, 2009 9:51 am

JoergS wrote:Tried to add sturdy rubber, it gets torn apart in no time. I guess I have to live with the kick.


I am sure that we can do something to minimize the kick.

One thought is to trigger a back spring after the projectile is launched.

The slide could engage a second rubber that it stretches to slow it down.
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Unread postAuthor: JoergS » Tue Oct 20, 2009 10:06 am

Yes, the second rubber set is a good idea. I guess it could work.

Needs fine tuning of both band sets.
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Unread postAuthor: boyntonstu » Tue Oct 20, 2009 10:36 am

JoergS wrote:Yes, the second rubber set is a good idea. I guess it could work.

Needs fine tuning of both band sets.


I am sure that you will see improvement.

BTW A flat piece of tungsten carbide would be a good choice for the sled face instead of using sphere vs sphere.

You could use a tungsten carbide tooth from a radial blade.

Keep it attached to the steel and screw the steel to your sled.

Have you seen any speed increase?
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Unread postAuthor: Ragnarok » Tue Oct 20, 2009 2:51 pm

boyntonstu wrote:I hope that this helps explain elasticity and Modulus of Elasticity.

I should note that you're trying to explain engineering to D_Hall... which, as the saying goes, is like teaching your grandmother to suck eggs (not that the saying makes much sense.)

Modulus of Elasticity is stress divided by strain. It tells you (as D_Hall said) the force needed to cause a certain deformation.

It is not a measure of the amount of energy retained in an inelastic collision (and on a macro scale, almost all collisions are to some degree inelastic) - the figure for that is coefficient of restitution.

The two are related however - the energy absorbed under deformation is equal to the area under the stress-strain curve. The energy released as deformation is "recovered" is also equal to the area under a stress-strain curve - but a different one with lesser stress, because energy has been lost to heat.
The coefficient of restitution is equal to the ratio of the area of the two curves for the appropriate time scale (in this case, time of impact, which is obviously dependent on velocity.)
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Unread postAuthor: D_Hall » Tue Oct 20, 2009 8:58 pm

boyntonstu wrote:BTW A flat piece of tungsten carbide would be a good choice for the sled face instead of using sphere vs sphere.


Highly doubtful. While tungsten carbide is very hard, it is also very brittle. Most likely result from the above scenario would be a shattered sled face.
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Unread postAuthor: boyntonstu » Tue Oct 20, 2009 9:10 pm

D_Hall wrote:
boyntonstu wrote:BTW A flat piece of tungsten carbide would be a good choice for the sled face instead of using sphere vs sphere.


Highly doubtful. While tungsten carbide is very hard, it is also very brittle. Most likely result from the above scenario would be a shattered sled face.


Actually some TC is not very brittle:

http://tinyurl.com/yzmcxob

http://cgi.ebay.com/AIR-HAMMER-STONE-CA ... 53dcf82a18
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Unread postAuthor: D_Hall » Wed Oct 21, 2009 8:53 am

Yes, and you'll notice that such tools are carefully designed so that the tip is under compression. You'll see hammer drills and all sorts of tools that carefully focus the stresses into compression only loading. I seriously doubt the original poster has the ability to perform similar fabrication in his home.
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