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Registered Member #186
Joined: Thu Feb 16 2006, 07:22AM
Location:
Posts: 42
In order to reduce friction, I was thinking of ordering a couple of feet of Teflon Flouropolymer pipe (Look up 8547K61 on McMaster-Carr), though I'm a little concerned that it might not be rigid enough for the gun. It seems to be somewhere between 1/2 and 1/4 the rigidity of polycarbonate of the same thickness. Since I'm only planning a single stage at the moment, the length could be less than a foot, with a thickness of 1/16in. Has anyone had any experience with the stuff?
Also, I was thinking of using a ferrite bead as the armature for two reasons. One; to reduce eddy currents, and two; to help a small bit with aerodynamics. I figured with only a .02in total gap between the armature and the barrel, (.01 evenly around) a small amount of pressure would build up at the front of the armature, increasing drag a little. The small hole in the middle of the bead would help reduce that pressure. Basically.... Would a cylindrical ferrite bead, with an opening in the middle, be more efficient than a traditional iron, bundled/slotted iron, or a solid ferrite armature of the same mass?
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Go with a slotted or bundled iron (or mild steel) bead. Ferrite has a low saturation field (of the order 0.25-0.5T), hence does not experience as much force as steel (Bsat: 1.8-2.2T). Bundling of thin steel rods (insulated from each other) reduces eddy currents considerably and maintains the desired high-field force...
Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
Remember that Bsat is the most important factor for choosing the projectile since the force depends mainly on it. Initial permeability isn't so important and can range between various values. No sense to reduce the eddy currents to zero if force will drop a lot. Use laminated projectile or slotted one. The skin effect of iron will confine the currents into the external shell of the projectile,so slotting will reduce them somewhat without much reduction in force (only a slight , because cross-section will reduce)
Registered Member #158
Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282
Pertaining to barrel and armature composition... is there any kind of rule of thumb or way to calculate the crushing affect on the barrel? Like say plastic is good up to a 100J pulse but higher needs a metal barrel (example)? I know polycarbonate is a very strong plastic is there a cheap source for this stuff in tubes? Any do clear tubes cause problems with optical triggering?
As for the projectile material, is there a way to tell how good a projectile will be without having a working gun? For example, I know you should see the inductance rise as the projectile is placed in the center. If I wanted to try 10 different materials before I built a working gun could I pick out the best just by which causes the inductance to raise the most? Maybe there are some other tests too? There are so many variables that I think it would be good to start designing with certain variables already optomized if you get what I am saying.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I believe the general consensus is that there is no crushing effect on the coil, all of the forces go outward... I have seen coils that use no barrel at all.
As to the ferrite, I don't think the hole should affect the coilgun action too much (but am not sure), although it seems like the standard way of just grinding a point on it works pretty well... I think that the amount of energy required to set the air in the barrel in motion is considerably less than the projectile, so it is not really worth it to try and minimise its effects.
Registered Member #186
Joined: Thu Feb 16 2006, 07:22AM
Location:
Posts: 42
... wrote ...
I believe the general consensus is that there is no crushing effect on the coil, all of the forces go outward... I have seen coils that use no barrel at all.
As to the ferrite, I don't think the hole should affect the coilgun action too much (but am not sure), although it seems like the standard way of just grinding a point on it works pretty well... I think that the amount of energy required to set the air in the barrel in motion is considerably less than the projectile, so it is not really worth it to try and minimise its effects.
Me thinks...
Well, if the projectile is attracted to the coil, then the coil will be attracted to the projectile....
But like it's been said so far, ferrite would be too quick to saturate, so I'm probably going to go with a hacked up and slotted iron bolt instead. As for aerodynamics, the slots in the bolt would serve the same purpose as the hole in the middle of the ferrite, so that problem is solved too.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
aahz wrote ...
Well, if the projectile is attracted to the coil, then the coil will be attracted to the projectile....
If I remember correctly the attractive forces were all in the axis the projectile was moving in, and the forces acting perpendicular to the projectile were actually outward forces.
But it is not really worth arguing over, go ahead and give your bolt a try and see how the coil handles it.
Registered Member #186
Joined: Thu Feb 16 2006, 07:22AM
Location:
Posts: 42
My original issues with rigidity weren't related to the crushing force of the coil, rather the flexibility of the barrel. I didn't want something that would bend under it's own weight, or something excessively flexible....
As far as projectile material.... if saturation density is the only real factor, iron or steel would be the best for its availability.. so that's what I'm going to try out first.... http://www.oz.net/~coilgun/theory/materials.htm
Registered Member #158
Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282
... wrote ...
If I remember correctly the attractive forces were all in the axis the projectile was moving in, and the forces acting perpendicular to the projectile were actually outward forces.
I assumed attractive forces were between the coil and a ferrous object nearby, not at some angle, but between them. One attracts the other right? The greatest 'barrel curshing force' would be when the projectile is directly in the center, the coil would try to crush in around the projectile correct? Before the projectile enters the coil the force is more horizontal with the plane of travel. In a misfire or when some problem causes the projectile to not exit and there is still much charge left then all the force would be on the barrel, and I have heard of barrels get crushed this way. In a good working design obviously the discharge should be over by the time the projectile gets dead center.
I saw that website before but thought the barrel-less design had some inherent problems (not to say that a design w/ barrel doesnt have problems too). I wasnt sure just how much force adding superglue to the layers would hold up to. And if you only have 1 or two layers then the super glue coating wouldnt be as strong as opposed to having 8 layers coated with the stuff. I also saw a movie clip on that site before where one coil goes flying off when fired...
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Coilgun Barrel Composition, Armature Composition
