Vacuum, or suction as less pushing
Why there is no such thing as a pulling force from a vacuum.
Suction never pulls anything. It removes the push on one side of an object so that the ordinary push already present on every other side has nothing left to balance against, and what looks like a vacuum dragging something toward it is the surrounding air, or whatever pressure remains nearby, shoving that object forward on its own.
Pressure from every side, minus one
Air pressure presses on every surface of every object all the time, from every direction at once, and because those pushes are normally equal and opposite they cancel and go unnoticed. A vacuum pump's only action is to remove air, and therefore pressure, from one particular region. The pressure on every other side of whatever sits near that region is unchanged and now unopposed, so the object is pushed toward the low-pressure side by the ordinary push still acting on its far side. Nothing new has been created by removing the air, only an imbalance in a push that was there all along.
How that imbalance shows up depends on how rigid the object is. A stiff metal part moves toward the low-pressure side as a whole, while a thin, flexible sheet can bow and deform under the same unopposed push long before it moves anywhere.
What really lifts a drink up a straw
Drinking through a straw feels exactly like pulling liquid up into the mouth, and the muscles doing the work are indeed the ones creating suction, yet nothing tugs on the liquid itself. Drawing breath through the straw lowers the air pressure inside the mouth and the top of the straw. The ordinary atmospheric pressure still pressing down on the surface of the drink, now facing less resistance from above, pushes the liquid up the straw to fill the space.
Remove that push by drinking from a sealed container with no air able to reach the drink's surface, and no amount of suction will draw up a single drop. Anyone who has sucked hard on a juice carton with its straw hole sealed tightly, getting nothing until a second small hole is pricked in the top to let air back in, has felt this directly. The same logic explains an old trick with an ordinary straw: fill it with liquid, cap the top with a fingertip and lift it clear of the glass, and the liquid stays put. The small pocket of air sealed above it is cut off from the room's pressure and cannot push down hard enough to overcome the atmosphere shoving upward through the open bottom end.
A vacuum fixture is held down by the room
An earlier article in this set described a vacuum workholding fixture holding a part by drawing air out from beneath it, language that invites the same misreading, that the vacuum grips or pulls the part downward. The part is pushed onto the fixture by the atmospheric pressure acting on its exposed top surface, which has nothing to balance against once the air beneath has been pumped away.
This matters because it sets a hard limit. Even a perfect vacuum, with every trace of air removed from beneath the part, can only produce a holding force equal to the atmosphere's push on the opposite side, and that push is about one kilogram for every square centimetre, or roughly ten tonnes spread over a square metre. A part 10 cm square can therefore be held down with the weight of at most about a hundred kilograms, however large the pump. A leaking seal around the fixture's edge leaves that ceiling where it is and stops the system from ever reaching it, letting outside air bleed back beneath the part and rebalance the push the fixture depends on.
How the word "suck" misleads
Everyday speech describes suction as an active pull: a vacuum cleaner sucks up dust, a straw draws a drink, a pump pulls a vacuum. For everyday purposes that language does no harm, since the outcome looks the same whether the object is described as pulled or pushed. The trouble starts when someone reasons quantitatively about a vacuum system with the pulling picture in mind. A pull has no obvious upper limit, and a stronger motor can always be imagined pulling harder. A push borrowed from the atmosphere has a ceiling built into how much air pressure exists, and a designer who keeps thinking in terms of pulling harder is working from a picture that parts company with the physics exactly where the numbers start to matter.
Designing for the push that is available
Engineering a system around suction means engineering around the pressure available to do the pushing, and the pump's specification sheet says little about that. A more powerful pump removes air faster and more completely, and the force it unlocks stops at the same ceiling. A vacuum lifter struggling with a heavy load is very often adequately powered and simply being asked to lift more than the atmosphere's push, spread across its contact area, can supply. The fix is a larger pad or more pads, adding pushing surface, since doubling the pump's flow rate changes only how fast the ceiling is reached and leaves its height untouched.