Practical Automation: The Power of Vacuum
By Ted Grove
Vacuum is an industrial power source that often is misunderstood and therefore seldom used. If you think of vacuum as being the absence or partial absence of atmospheric pressure, it is easier to pict...
By Ted Grove
Vacuum is an industrial power source that often is misunderstood and therefore seldom used. If you think of vacuum as being the absence or partial absence of atmospheric pressure, it is easier to picture the source of its power as being the surrounding atmosphere trying to fill the void.
The atmosphere is made up of many different substances, the majority of which we hope is the air which surrounds the world like a skin many miles in height.
Atmospheric pressure is caused by the weight of this height of air pressing down on each square inch of the earth’s surface. The greater the altitude above the earth’s surface, the thinner the atmospheric skin above it will be, thus the resulting atmospheric pressure will be less.
In Vancouver or Halifax, where the altitude is sea level, the resulting pressure is 14.7 psi. That means that a column of air 1 in. x 1 in. in cross-section, extending as high as the atmosphere goes, would weigh 14.7 lb. However, in Edmonton, where the altitude is about 2,200 ft above sea level, the atmospheric pressure is about 13.55 psi.
The most common way of measuring atmospheric pressure is in “inches of mercury.” That is the height of a column of mercury that would weigh the same as the column of atmosphere being measured when each has the same cross sectional area. Atmospheric pressure of one atmosphere at sea level would exert a pressure of 14.7 psi or would support a column of mercury 29.92 in. in height.
Figure 1 shows a mercury barometer used to measure atmospheric pressure. The bottom of the glass tube holding the mercury is immersed in a dish of mercury whose surface is exposed to the atmospheric pressure. A good rule of thumb is that for every increase of 1,000 ft in altitude, the atmospheric pressure drops about one inch of mercury. In the barometer, the downward push of the weight of the mercury is exactly supported by the upward push exerted by the atmosphere.
The vacuum levels generally used for industrial applications fall between 0 and 27 in. of mercury. (The symbol for mercury is Hg.) Industrial applications for vacuum often use “vacuum cups” as a means of focusing atmospheric pressure on a particular item to exert a holding or lifting force.
When a vacuum pad is placed against a piece of sheet metal or other non-porous surface, without a vacuum being drawn under the pad, the atmospheric forces are balanced (see figure 2a).
However, when the air is withdrawn from under the cup, the atmosphere exerts a force not only on the surface of the cup but also under the sheet metal, thus holding the metal and the cup together. The force is equal to the vacuum level multiplied by the effective area of the vacuum cup (figure 2b). The vacuum cup may now be lifted and the metal sheet is expected to come with it.
Vacuum cups are used in printing machines to transfer sheets of paper or cardboard into the press. They are used in labelling machines to transfer labels to a product and in large palletizing machines to load and unload cartons from skids.
Large sheets of glass or metal are easily handled by lifting equipment fitted with vacuum pads. The vacuum pad is indeed a convenient way to lift large, flat objects quickly and conveniently, without damaging them.
Vacuum forming of thermoplastic materials in a mould is another common industrial use of vacuum. This process generally involves heating a sheet of plastic, then fastening it over a mould cavity while a vacuum is created between the pliable sheet and the cavity. Atmospheric pressure completes the task of forcing the plastic into every nook and cranny. This process is used to create plastic signs, windows, bathtubs and even canoes.
Less frequently used applications for vacuum include holding film tightly to a negative during the film processing operation, evacuating air from bottles or other containers during the filling operation to provide a quicker flow of material, air sampling from chimney flues or other test areas, vacuum packaging of foods to maintain freshness and, of course, the farmers’ best friend, the milking machine, whose gentle vacuum-induced caresses keeps even Bessie happy.
There are, indeed, a multitude of uses for vacuum, only a few of which are touched in this article. A booklet of additional uses is available to you by simply using the reader service number below. MRO
Ted Grove is a widely experienced fluid power trainer and is corporate training manager of Wainbee Limited of Mississauga, Ont. He can be reached at firstname.lastname@example.org. Previous columns can be viewed at www.mro-esource.com.