How to calculate holding force based on Vacuum pressure?

[Metal Flake (clocked in)]

Ok, here's one for those engineering-learned folks. We've got a weak vacuum setup here. It's one of those things that's driven by the shop air line. Not a real vacuum pump. So..we're seeing 15in on the guage. How can I calculate what kind of holding force results for a given area? Seems to me that typical good vacuum is something like 22inches. Is this correct?

 

BTW: We're in the Everett,WA area. About 600'msl.

 

Thanks for any help!!!

 

Dan

[Mito@DeeringBanjos]

a perfect vacume will exert 14.7 pounds per squar in. a real vacume pump is the key to do this in my opinion. do a search this has been covered not to long ago.

[Tom Dorgan from American Precision]

Metal Flake,

I just uploaded a PDF of a Vacuum hold down chart in the Router Files folder on FTP Site. This will give you a good idea of the range of vacuum needed. On our router tables if we fall below 15 we have trouble hold parts but then we are cutting at a minimum of 700 IPM most of the time.

[Tom Dorgan from American Precision]

BTW....I believe at sea level perfect vacuum is 29 inches of mercury.

[Metal Flake (clocked in)]

Most excellent!!

 

Thank you!

 

Dan

[BernieT]

Here is the older post on this subject:

 

Vacuun calcs

 

[MattW]

Hey, maybe I can help on this one... it doesn't involve MC...

 

Standard atmospheric pressure is 14.7 psi, 29.92 in Hg at sea level. You will lose about 1 in Hg every 1000' of elevation. Pressure varies day to day, but not by much more than 1/2 in Hg. My quick and dirty conversion is 1 psi = 2 in Hg.

 

You can get a decent vacuum pump for not a lot of money, 15 in is kind of low. Compare level with the pump blanked to determine how tight your seal is, it doesn't take much of a leak to spoil the vacuum.

 

MattW

Vertex Pharmaceuticals

[George_Gerhard]

+1000 to MattW

One more question MattW, do you know what temperature in necessary?

29.92 is pressure altitude

 

rgds

George

[MattW]

Beware of George, he asks trick questions.

 

Standard Atmospheric Pressure is 29.92 "Hg. Standard Atmospheric conditions are 29.92"Hg, 59 degF, which is of interest when air density is of concern. For the purposes of Vacuum holddown, only local atmospheric pressure is of interest.

 

When I said blank the pump to check for leakage, that is as simple as pinching the hose to the fixture like a garden hose, and the gauge should not move much (not at all would be good). I have a vacuum pump like this Gast unit at home for vacuum bagging composites, it should work well for vacuum holddown of non porous material. The downside is they are a bit noisy, but are small and portable and not expensive.

 

Now if I could just learn to use MC effectively...

 

MattW

Vertex Pharmaceuticals

[Jim at Gentex]

I run a simple and very small vacuum pump on my 5 axis for routing visors for flight helmets. Depending upon the visor blank itself and how well it seals to the fixture, we generally get about 20-22 in Hg on the gage. This is plenty of pressure b/c you can barely break the seal by pulling the visor off the fixture with your fingers. I have had some come loose during machining, but those were always at the lower end of sealing pressure, say around 14-15 in Hg. I don't know how to calculate it exactly, but I do know what works. I'd say anything around 20 or more should be good to go.

 

 

Jim teh 500 posts!! WOOHOO!!!

[notso]

This is a fairly easy calculation:

the force actually comes from the atmospheric pressure pushing down on the part, not from the vacume pulling (but it really makes no diff in the calculation)

1) convert vacume to pressure (ie in hg to PSI). 1in hg = .491 psi

 

2) determin surface area exposed to vacume in square inches.

 

3) multiply the two together. (basic unit conversion: lb/in^2 * in^2 = lb

 

ex: 15in hg vacume, 100 in^2 surface area

15in hg=30.549 psi

 

30.549 lb/in^2 * 100 in^2 = 3054lbs

 

of course that is the total force that is exerted over the entire plate and if it was perfectly rigid it would take that much force (plus the wieght of the plate) to lift it enough to break the seal. Also, this doesn't tell the force necc to slide the plate around (would need to know the static coeficient of friction between the two materials to calculate that).

[Philcott]

Do most of you guys really calculate this out for your parts, or do you do like our newest senior member, Jim, and just have a feel for what will hold and what won't?

 

Phil

[MetalMarvels]

Quick and dirty area calc here. But I never try to hang onto a part with less than about 20 in Hg. If is less than this, I am looking for a leak somewhere..... I have an inexpensive automotive refrigeration vacuum pump that I use, coupled with an 11-gallon Husky air tank that I use as a vacuum reservoir. I can typically run a couple of hours worth of parts before having to fire up the vacuum pump for a minute or two (fire it up at about 20 in Hg and shut it off at about 22 in Hg).

 

I did figure out pretty quickly that I needed to cut "runners" in the surface of the fixture to better distribute the vacuum to all areas. I usually just use an 1/8 ball emill to make a few shallow cuts over to the vacuum hole from the farther areas of the fixture. A fixture typically only takes 4-5 minutes tops, including index shoulders, etc. for flat parts. A good bit longer for 3D parts - but as long as you don't damage them, they last almost forever (you do have to replace the gasketing every now and then).

[CADin & CAMin]

This simple numbers are 15 lb/in^2 holding force at 30 in of vaccum. If you are getting 15 in of mercury vacuum, you will get about 7.5 lb/ square inch of area you are holding.

 

The gasket you use can help also. If you are pulling 10 square inches, you get 75 lb of clamp down force. Next is the question of how slipery is the surface you are pulling on. We use a seal around a rubber surface that is less likely to slip.