The Sky's the Limit
In any vacuum hold down system you are limited to 15 pounds per square inch, ie. the weight of the sky above. No matter how much grunt your pump has got you cannot go higher than this.
HOWEVER, a pressure chamber that uses the work as part of its seal could increase the effective pressure without limit. Even an old service station compressor can cough out 100 pounds or more.
Well Simon,
One can also have a larger clamping area for the vacuum than the part itself. It is an interesting and can be an involved solution to a problem that may not exist. I have tried it and it works well. The fixture tends to be part specific and won't work for a lot of parts.
Simon, 15 pounds of pressure per square inch does not sound like very much holding pressure but what you are talking about is called one atmosphere I only wish I could get a pump that would pull half an atmosphere, They measure vacuum in inch column feet of mercury. and if you can pull 7 inch of mercury I can tell you you will not be able to break the suction very easy. David in Wyoming
Just an added thought, Back when I was a young and foolish pup I worked at a Uranium Mill (no I don't glow anymore [
] ) Anyway we used super large vacuum pumps to run the yellow cake fillters and the acid Filters to pull out the uranium from a mud slurry, it was capable of pulling a little over 12 inches of mercury and we use to have fun with the new guys by betting them that we could suspend an 80lb steel, ball mill, ball in mid air. Well we used one that was just larger than the intake tube for the pumps and if you got it in just the right spot it would hold the ball in mid air. But if you got it too close to the inlet, Ohboy! you had to hit it with a 60lb Sledge hammer with everthing you had.and at an angle almost 90 degrees to the tube. David in Wyoming
Now Simon Tell me what part of the Story above is a Wyoming Cowpoke's Tell and what is not! David in Wyoming
Senior Member
David,
I know I know, That people in Wyoming even know about angles???? We are just getting these in Cnada Next month, They are said to be great.... Garsh LOL [
]
Senior Member
Simon, 15 pounds per square inch may not sound like much, but have you ever tried to pull a wet rubber suction cup off a glass window? The qualifying words wet/rubber/glass give a hint to the secret - the physical conditions must be perfect.
A garage compressor is designed to produce positive pressure, not vacuum. The "pressure chamber" does not increase the pressure at all - it only prevents the compressor from having to run permanently. The chamber provides a reservoir and it smooths out big surges in demand. A chamber only has value for a table if you need a surge of vacuum to tighten your seals down when you have placed a new part.
Dave, 1 atmosphere pressure is about 30 inches of mercury, or 30 feet (360") of water (roughly speaking). That suction cup on the window has a full 30" mercury holding it because there is no sky (air) between it and the glass.
Specific geavity Mercury is 13.6. 30" Mercury is equal to 408" water. But, for rough guessing an inch a foot is the number I use. And, the ShopBot doesn't seem to know the difference.
Senior Member
I learnt my rough figures on the gold mines near Johannesburg. The mine buildings are 1600m above sea level, while the bottom of the mine is about 500m below sea level. Your ears can pop quite violently during the daily commute - 10% error either way was insignificant.
And the metric equivalent of 1 atmospere is roughly 1 kg per square centimeter (an area 10mm X 10mm)
Gerald I fully agree and understand about atmospheric pressure I am a certified PADI and all you have to do is go down to 90 feet and feel the pressure and you FULLY understand. David in Wyoming
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