Rolling Lift Bridge

Scherzer claims a lift bridge whose leaf carries a curved segment that rolls along a flat track instead of turning on a fixed pin. Note the key word: rolling, not hinging.

Hinge a card leaf on a single fixed axle at one end and tip it up. The tip swings on one fixed point and the far end drops below the deck. This is the ordinary bascule — remember how it moves.

Mark a 90° quarter-circle with a protractor and cut it from plywood. This curved segment is the rolling tread — the rocker the whole bridge will roll on.

Glue a straight cardboard deck to the rocker so the curved edge sits under the rear of the leaf. The leaf now stands on its curve, not on a pin.

Set a flat plywood strip as the track on your "pier." The rocker will roll along this flat surface — a curve rolling on a straight line.

Push the leaf up so the rocker rolls back along the track. The leaf tilts up AND draws backward at once. Mark where the tip started and ended — note how far it retreated from the channel.

Mark the spot where rocker meets track. As you roll, that contact point moves along the track — there is no single pin taking the load. The whole curve shares it.

Press a lump of clay onto the leaf behind the rocker, over the tail that lifts as the front rises. This is the counterweight.

Add or trim clay until the leaf sits still, half-open, on its own. Now the weight of the span is almost cancelled — it is self-counterpoising.

Hook a spring scale to the tip and read the force to lift it — first balanced, then with the clay removed. Balanced, it takes a fraction of the force. Record both.

A smooth curve can slide sideways. Cut small teeth into a card strip and press them between rocker and track so the roll is geared, not slipping — a real bridge uses a rack and pinion here.

Roll the leaf right up and measure the clear width over the water. For the same leaf length it opens a wider channel than the pivot flap of step 2, because rolling back pulls the whole span clear.

The patent. US 511,713, granted 26 December 1893 to William Scherzer of Chicago. Scherzer died on 20 July 1893, months before the grant, so the patent issued with his brother Albert H. Scherzer as administrator of his estate; Albert went on to found the Scherzer Rolling Lift Bridge Company, which built rolling lift bridges worldwide. The application date (May 1893) and the printed grant date (December 1893) are different — the 1893 you want is the grant.

What Scherzer actually invented. The bascule — a counter-weighted leaf that tips up to open — is ancient; medieval castles had drawbridges. What Scherzer invented is the rolling lift: instead of turning on a fixed pin (a trunnion), the leaf carries a large curved segment, a quadrant, that rolls backward along a flat track on the pier (steps 3-6). Two things follow. First, the pivot is not a single overloaded pin — the point of contact travels along the curve and the load is spread (step 7). Second, and cleverly, rolling back makes the leaf retreat as it rises, so a given span clears a wider channel and needs no deep pit for a swinging counterweight (step 12). A rack and pinion keeps the curve from slipping (step 11), and the leaf is self-counterpoising — balanced so gravity nearly cancels and a small engine can swing a thousand-tonne span (steps 9-10).

The physics is a curve rolling on a line. A wheel rolling on the ground turns about its moving contact point, its instantaneous centre; Scherzer's quadrant does the same, so the leaf rotates about a centre that is itself travelling. That is why the motion combines lift and retreat, and why there is no fixed hinge to wear out or to carry the whole weight of the span on one pin. Balancing the counterweight over the roll turns the job of opening a huge steel bridge into something a modest motor — or, in this model, two fingers — can do.

Where it went. Rolling lift bridges spread across Chicago and then the world — over rivers and canals, for road and rail — because they open fast, clear a wide channel, and sit on shallow foundations. Many are still working a century on, rocking back on their quadrants exactly as your cardboard-and-plywood model does. It is one of the neatest cases in engineering of replacing a heavily loaded pivot with a rolling contact, and getting a better motion for free.