Re: HoverPad anyone?


"Han" <nobody@xxxxxxxxxx> wrote in message
news:Xns9A5C6C92898CFikkezelf@xxxxxxxxxxxxxxx
PCPaul <urd3@xxxxxxxxxxxx> wrote in
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On Sun, 09 Mar 2008 08:47:34 -0500, Upscale wrote:

"J. Clarke" <jclarke.usenet@xxxxxxx> wrote in message

Before you spend 170 bucks you might want to try
http://amasci.com/amateur/hovercft.html.

Could be a possibility as long as one doesn't mind risking their
$1000-$3000 pieces of machinery on some plywood, nuts, bolts and
painter's drop sheets. Of course, it's unlikely that machinery would
tip over or anything, but it would definitely be a jury rig setup and
subject to problems.

I think the major issue applies to the Pro and homebuilt versions -
inertia.

Yes, you can get it moving at some speed by pushing it with a single
finger for a while. *But* when you realise it's heading for somewhere
you really don't want it to go you have to apply thew same force for
the same time to stop it. Or in the likely event of wanting to stop it
10x quicker than that, multiply the force up by x10 too...

I'd think that removing the air pressure would just about instantaneously
stop the contraption. Then inertia and friction to the ground surface
take over. Think Rube Goldberg consequences.

--
Best regards
Han
email address is invalid


The principle behind these isn't new, but this one has to be the flimsiest
design that I've ever seen.

Boeing has been designing and using hover pads to lift and move aircraft
sections into position since WWII. In fact, Boeing formed a subsidiary to
manufacture commercial versions of these for sale. They come in all sizes
(and cost), but the commercial ones are far too expensive for small shops
and hobbyists. A version was available some years back for use under
refrigerators, to be powered by a vacuum cleaner whenever the lady of the
house wanted to clean behind it, but I don't know if they are still
available. If you can find one it would likely be a cheap way to move your
cabinet saw.

Using only one large air chamber in the design produces a very tippy and
unstable unit that would not be very suitable for moving machines (remember
the hover craft show on Myth Busters?). If you look closely at General's
hover pad you will see that it is composed of many small hover pads. The use
of 3 or more give the platform the needed stability. In use, the pad only
needs the thinest of air film between the skirt and the floor for it to
work, but with only a very thin film of air, the floor has to be very
smooth. Increasing the air flow to increases the thickness of the air film
and will allow it to pass over less than perfect floors, but any significant
void in the floor surface will cause all of the air to leak out of the
affected chamber and the pad will bottom out and drag. In other words, they
will work great on a tile or concrete floor, until you try to cross over a
crack or other significant irregularity in the floor surface that the skirt
can't seal. When the air leaks out faster than it's coming in, the pad will
bottom out. A platform with 3 or 4 chambers under it will provide even
lifting and stability. Boeing makes single chamber pads, but they are
intended to be used in groups of 3 or more to distribute the load and
provide the needed stability. It's amazing how much weight they can lift and
move. 4,000 pounds can easily be moved using only 4-8 inch diameter pads
with only 6-11 pounds of air pressure. More air pressure raises the pad
slightly higher, allowing it to cross slight imperfections in the floor more
easily but it isn't necessary for most use, and it wastes a lot of air. It
doesn't take much air loss for the pad to collapse.

I have made some strong and reliable pads by gluing small new lawn tractor
inner tubes to 3/4 thick birch plywood (tractor tubes are available with
center holes as small as 4" dia and overall diameters of about 8"). The air
path for my pads is similar to the one in the link above, where the air
first enters the tube and then exits the tube into the center "hub hole" of
the tube, where it then leaks out between the bottom of the tube's surface
and the floor, forming the "air bearing". You will need to buy new tubes for
this project as their shape needs to be flat and not all stretched out of
shape, so they can be easily glued to the plywood. I use a wide ring of
automotive door gasket cement to bond the inner tube to the plywood and then
place a concrete block on top as a clamp "until the glue dries". The regular
filling stem is cut off and the resulting hole in the tube is glued to the
plywood so that it is in line with the air entrance hole (glue it well - no
leaks allowed around this point). Using a razor knife I then cut 4 slits
about 1/2" long around the center of the tube to allow the air to escape
from the inside of the tube into the center chamber, similar to the design
in the link above. Using rubber tractor inner tubes makes the pads survive
much longer than the 6 mil plastic, as small scratches from a rough floor
surface (pebbles, etc.) won't immediately tear the rubber like it will the
plastic. Many years ago I made some out of hemorroid cushions, but the ones
that are available now aren't very well suited for this. The old ones were
made from a vulcanized canvas material and the ones that are available now
are made of thin vinyl. (I got strange looks from the drugstore cashier
when I bought 4 at the same time - nobody gives you a second look when you
want to buy 4 inner tubes). I use my 18 cfm shop air compressor to power
them, with an air pressure regulator and hose connected to each pad (this
lets me vary the pressure and flow to one pad at a time) and my pads are all
made separate so I can put one under each corner, leg, etc. of whatever I
want to move. I can use 3, 4, or more if necessary (my compressor won't
likely handle more, but that's the plan). For a table saw or other small
machine 3 or 4 pads mounted on a board that's large enough to carry the tool
would work out fine. In a large commercial cabinet shop it would be handy to
have them separate so they could be placed under each corner of a very large
cabinet, etc. for movement around the shop floor.

Once up "on air" your tool will move very easily, so easy in fact, that you
will need to be careful not to get it moving too fast, or you may not be
able to stop it. It will stop very quickly if the air is shut off, but in a
one-man shop you won't have enough hands to hang on and shut off the air
too.

Charley






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