Vacuum Hold Down and Vacuum Basics
I have had a good deal of experience with vacuum systems. From 1/4 horsepower
5 cfm units, to 50 horsepower 700 cfm monsters. You don't have to believe what I say
but most of my associates do.
Vacuum Basics
When working with vacuum the most important thing to remember is how vacuum
works.
1. Atmospheric pressure at sea level is approximately 15 pounds per square inch
( psi) This means each square inch of surface area that has vacuum under it has 15
pounds of pressing down on it.
2. When you have a box that has no air in it, ( a vacuum ) the top, bottom, and all the
sides have 15 pounds of pressure on every square in of that surface against it.
Lets say you have a box that is 6" x 6" with a vacuum of 27 inches inside it. On any
one side there will be 6 x 6 x 15 pounds of force evenly distributed against it. That is
540 pounds of pressure. If the box is not strong enough it will collapse.
3. Weather you are using a vacuum table or Pod setup the principle is the same. I use
Pods myself, even though I have the ability to convert my table into a full vacuum
table. This I Why: If you want to hold down a 2 ft. x 2 ft. piece on a table you must
evacuate all of the air from the entire void of the table before you actually grab the
work piece. A 4" x 4" pod with a 1/16 inch thick closed cell foam gasket around the
perimeter will grab the part almost instantly.
If you can afford to purchase a 25 horsepower vacuum pump and supply it with many
kilowatts of power just for the sake of pulling a vacuum on the entire table, be my
guest.
4.I make my pods for use with small vacuum pumps. They work exactly the same with
big pumps. Lets also assume that the maximum vacuum force ( not pressure ) is
23 hgm ( inches of Mercury). I run a setup that pumps down to 23 inches and then
shuts off until it reaches 17 " when it comes back on. If there is no air leaking into the
void area, the pump will turn off and never come back on. This a perfect seal, and
that and that is impossible to obtain in most cases. If your pump cycles off and on
without overheating the pump (off for about 4 minutes, on for 30 seconds) you are in
good shape.
5. Now we get to the last point. You have to think about the Pod and the void area
beneath your work piece. You need a void area under the part you are trying to hold
down. That area should be the maximum square inches that your pod can supply.
The void area can be as little as 1/32 inch deep. The supply hose hole in the pod in
most cases doesn't need to be larger than 1/4" diameter. Again keep in mind, the
larger the area is, the more CFM you need to pull until the part sticks in place. Once
the part is stuck there is no air movement. This is not a vacuum cleaner process, it is
a true vacuum hold down.
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