The Bascule Trunnion Bridge

The Michigan Avenue Bridge is a double-leaf, double-deck bascule trunnion bridge. Bascule means “seesaw” in French, which is how this bridge operates. The bridge’s steel and roadway weigh the same as the concrete and steel counterweight.

The bridge and counterweight balance on a trunnion—the point on which the bridge pivots.

The bridge is in almost perfect balance, which means that a surprisingly small motor lifts it.

The bridge’s control system—the source from which the bridge receives its signals to open and close—consists of a magnetic primary control and a DC auxiliary control.

The motor requires little electricity because power was very expensive when the bridge was built.

The bridge operates on 108-horsepower 550 V motors, one for each leaf (two per side). Each leaf also features a brake for additional safety.

The motor (the grey box marked “Earle”) rotates a shaft, which turns a series of gears—that increase in size to transfer power easily.

The final gear, the bull gear, has wider teeth that actually rotate the rack, opening and closing the bridge.