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This is the first Van de Graaff generator that I built, constructed in 9th grade for an Honors Lang. Arts demonstration speech, giving a 1" to 1.25" spark. (my teacher was terrified-heh, heh!) It was easily constructed, but it did have one major stumbling block, the material I chose to try to use for a belt. My first attempt used a belt cut and spliced from an old bicycle inner tube, and refused to function at all. Since then I've heard that certain rubbers' black dies make them too conductive for use in Van de Graaff's. But don't let that shy you away from grey (I think it looks black) neoprene belts, they are the best material to use. Also very viable are gum rubber belts, which is what I ended up using in this (turned out to be a sort of prototype) generator. For a long time, including the speech, the prototype generator used a belt made from 3 or 4 1/2" rubber bands (the kind they put on brocolli in the grocery store) spliced together. The splices were smooth but joints, with the edges cut at 45 degree angles, to increase the surface area of the joints and to spread the stress along a distance of the belt, to reduce breakage. The joints were then glued with super glue gel, a smooth bead aplied between the edges (do this on one of the bags from the grocery store produce section, one of the ones on the rolls, the glue won't stck to it) and the edges pushed together (put another layer of plastic on top) and then wieghted, and left to dry (give it a good half-hour). Later I reconfigured the generator to accept these huge (reletively speaking) 5/8" by 7"(~14" circumfernece) rubber bands I found at a local office supply store. Most places should special-order these for you. You should design the rest of the generator according to the belt demensions.
The pulleys for a Vande Graaff should be more like rollers, with a diameter about 1/2 to 1 times the width of the belt. One should be insulating, usually plastic, the material should have a good diaelectric value, and a low humididty absorption. The other pulley is conductive, either solid metal, or a plastic lulley covered with a layer of aluminum tape (buy a roll of this from your local hardeware store, you'll find it has a lot of uses in high voltage). Depending on the polarity you want for the terminal, put the conductive one on top and the insulating on the bottom for positive, and the reverse for a negative terminal. Negative voltages leak off a little more easily, but if your terminal is exceedingly fine (pop cans won't cut it for this- we're talking spun aluminum spheriod here) then the negative will give slightly longer sparks. I noticed no spark increase on the prototype, in fact, I kept it positive since the spark repetition seemed faster.
On the prototype both pulleys were made from sections of Bic pen casings. The bottom pulley consisted of allmost an entire casing, one end forced onto the shaft of a c-frame induction motor, (you'll find these powering cooling fans in all sorts of devices. . . . . microwaves come to mind) the other end stuck inside the bearing still fitted in an old-school (the really big ones) skateboard wheel. The bearing side was shimmed up to the correct hight to keep the pulley level by stacking 3"X5" notcards underneath it. Both the motor and skateboard wheel were held doen with plastic cable ties (available at your hardware store). The support column should be made from PVC pipe, and the prototype's was made from 1"(i.d.). At one end, about 1.5" up, the PVC was cut open, leaving about a third intact, with enough room for the pulley to cross dead center.
The bottom was left intact, and was secured to the base.(a piece of 2X4) By drilling a shallow hole in the wood just big enough for the pipe to enter and gluing it in with epoxy. The top pulley spun on a shaft laying cross-wise through two notches I filed out opposite each other on the top surface of the pipe. What I used for the shaft is hard to describe, I can't remember its name-- it's shape is much like a small bolt, with a round head and no slot, except it has no threads, rather, it is hollow and the threads are on the inner surface, it has a sort of short-shafted bolt that functions as a nut and screws into the end of the quasi-bolt, tightning down. This was sized so it would tighten niceley accross the PVC pipe, and the pulley, cut so it would turn freely on the shaft and within the pipe, slipped over this thing, the belt pulled over the whole thing, and then the entire assembly tightened into the notches.
Charge brushes should be set within 1/16" of the belt, be perfectly parallel to the pulley, not touch the belt to avoid scraping, and not be any wider than the belt. On the prototype, the brushes were originally made from small pieces of brass screening, and if this, or aluminum screening, are used, then the edge should be set with super glue and cut with a razor to give an even row of points. Otherwise, a row of points should be carefully cut in a piece of aluminum or brass shimstock (or the bottom of an aluminum pie pan) using a razor. The bottom piece shoud be set directly opposite the point on the bottom pulley where the belt makes no contact. This can be accomplished by making three 90 degree angle bends in the shimstock or screening. The free end should be secured to the base and a ground, done with aluminum tape on the prototype. The top brush should be set in a likewise position. The free end of the top brush must conductively connected to the inner surface of the terminal. An easy way to do this is to secure the brush to the metal terminal support with aluminum tape.
The best terminal is a perfect sphere, with the support column entry hole turned inwards, forming an involute hole. But, such a terminal can get quite expensive, so it wise to make reasonable substitute from cheaper materials. For a very small VDG, like the prototype, two aluminum cans, cut in a certain way, can be used. Larger generators can use terminals made from two aluminum(or steel) bowls, secured edge-to-edge, with a hole cut in the bottom of one for the pipe. To form a terminal out of two aluminum cans, take one and cut its top off, cutting on the widest diameter. The other, cut the most inner portion of the bottom surface out, so that if the can were set down again, one couldn't tell anything had been done to it, untill it was picked up, and then the cut-out bottom could be seen. Then, also on the can which has had its bottom removed, remove the very top portion, the part that has no paint on it. If you did this correctly, then top portion of the second can should fit easily into the cut-off bottom of the first can, leaving you with an oblong, totally hollow terminal with a single opening at one end. Take a screw driver, and using the shaft, turns inwards the edges of the opening in the terminal. Also sand down the edge of the cut you made on the top can. A dimple should be pressed from the inside out on the top surface of the top can. The dimple will ride the support, so make sure it's at dead center.
The support can be cut from a coathanger or other stiff wire (I've used copper welding rod). Drill two holes into the top surface of the PVC pipe, so that of a line were drawn between them, it would lay perpendicular to the pulley shaft, the rod that you use for the terminal support should press-fit into this hole. Bend the rod in half, and cut the ends to equal length, and fit them into the hole drilled for them. Make sure that when the terminal is lowered onto the support, it completely covers the entire upper pulley assembly.
Follow these directions and you should be able to create a passable Van de Graaff genrator. Go ahead and experimint all you like, and in regards to terminal size, what limits the voltage is its smallest radius- a popcan will never do anything more than 1.5" in spark length. The next VDG that I built creates 5" to 6" sparks, and uses a belt bought from Science First. They are a great source for VDG parts and accesories, but I would strongly suggest that you build your own rather tha purchse one: it costs much, much less and is more fun and satisfying when complete. I'm currently working on a VDG that should give foot-long sparks, but it won't be finished until at least next winter (I'm working on other stuff).
Science First
95 Botsford Place, Buffalo, N.Y. 14216
e-mail: info@sciencefirst.com
FAX: 716-874-9853
phone: 1-800-875-3214
call them for a free set of catalouges, and be sure to mention that you are interested in Van de Graaff's.
email: electrophile@juno.com