John R. Bentley 2007.
The Æolipile
A Replica of the World's First Rotating Steam Engine
Actually a steam reaction turbine, the Aeolipile was invented by Heron of Alexandria in the first century A.D. In modern times it is often referred to simply as "Hero's Ball". This ingenious device was described in detail in Heron's book Pneumatica. A fire beneath the cauldron boils water, producing steam which is conducted through one of the copper supports to the pivoted brass sphere. This steam issues from nozzles at the ends of the two small opposing arms on the sphere causing it to rotate. This little replica spins almost silently at 1500 RPM with a steam pressure of only 1.8 pounds per square inch, producing surprisingly little visible exhaust.
The secret of Heron's invention was in the bearings (the only high speed bearings of his day were probably on the wheels of a cart). In the Aeolipile only one of the supporting arms carries steam into the sphere, through a sleeve bearing. Since there is no thrust bearing on this arm, steam pressure pushes the sphere toward the other arm containing the thrust bearing. That arm is solid and has a conical point, bearing against a conical indentation on the surface of the sphere. Hence the rotating sphere is held in proper position.
Testing with a small propane torch
A Sequence of Photos: getting underway from a standstill
Initial water droplets will be soon blown away
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© John R. Bentley 2007.
