You walk toward the entrance.

The door is already moving.

Without stopping, you step inside, rotate through the building, and emerge on the other side.

No one has to hold the door.

No blast of outside air rushes into the lobby.

And people can enter and leave almost continuously.

It seems simple.

But revolving doors are one of the most cleverly engineered entrances ever designed.

We examine the basic mechanics of revolving doors, including the central shaft, rotating wings, bearings, drive systems, and enclosure that allow people to move through the entrance while maintaining a nearly constant seal between the inside and outside of the building.

But revolving doors do much more than move people.

They dramatically reduce the exchange of indoor and outdoor air.

We explore how this hel ...