Magdeburg Hemispheres Experiment

How Suction Pads Lift Heavy Objects

March 25, 2020
How a 370-year-old science experiment provides the principles for pick-and-place machines.

We have all seen vacuum pads picking up objects in factories or trade shows and probably give little thought as to how they pick up those objects. Some of us may assume they employ a vacuum to just suck the object up. But science tells us suction does not exist, so how do vacuum pads lift these objects?

A 17th Century experiment might clear things up.

In the 1650s, a German scientist named Otto Von Guericke conducted experiments in Magdeburg to demonstrate the power of atmospheric pressure; it used two halves of a sphere and a vacuum pump which he made. The experiment is known as the Magdeburg Hemispheres Experiment (and the equipment he used is still on display in Munich’s Deutsches Museum).

The two halves of the copper sphere (the two hemispheres) were 50 cm in diameter and had mating rims.

The halves were put together and sealed with grease. The sphere was supported and attached to two teams of horses. A vacuum pump then pulled the air from inside the hemispheres. Once this was done, the two teams of horses—16 in all—tried to pull the two halves apart. They couldn’t.

Here’s why the horses strained in vain: When you put two halves of a sphere together, pressure inside the sphere is equal to the pressure outside. So, a person could easily pull them part with their hands.

But once the air/pressure inside is removed, atmospheric pressure (14.7 psi) acts perpendicularly on all the outside surfaces of the two halves. This force can prevent 16 horses from pulling the two halves apart. This experiment has been repeated many times since the 1650s. (There are kits available for sale if you want to try at home.)

How does this relate to vacuum pads? When we see those pads lifting a workpiece, the sides of the pad flatten against the workpiece when vacuum is applied (air inside the pad is removed). This is not suction doing this flattening; outside atmospheric pressure is pushing the sides of the pad down.

When the robot or machine raises the pad, atmospheric pressure holds the workpiece against the pad by atmospheric pressure. The volume inside the pad is at a much lower pressure and therefore has less force pushing down on the workpiece. The atmospheric pressure on the other surfaces of the workpiece is stronger, which pushes the workpiece up against the pad. This is what keeps it held up. The pad is the interface between the vacuum system and the workpiece.

When the vacuum is removed from the pad and atmospheric pressure refills the pad, pressure above and below the workpiece equalizes, and the pad releases the workpiece. The pad also returns to its original shape.

Just as atmospheric pressure kept the hemispheres stuck together when pressure inside was removed, atmospheric pressure holds a workpiece against a vacuum pad once pressure inside the pad is removed.

Wayne Wilding works in technical sales support at SMC Corp.

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