Drawbridges operate as they do based on certain principles.  These include the principles of the lever, the pivot and the pulley among others. By using these principles in their designs of drawbridges, engineers are able to successfully move extremely heavy structures with relatively small electric motors.

The Lever

The lever is a simple machine that changes the magnitude and direction of the force applied to move an object. The easiest example is that of the see-saw. Imagine a grownup on one end and a child on the other. Even though the child could weigh enough to prevent the adult from lifting them readily with their arms, it takes almost no effort for the adult to raise the child with the see-saw and keep them there indefinitely.

The weight of the adult is the force pushing down on one end of the pivot, and the lever changes the direction of that force to allow the lifting of the child without difficulty.

Bascule bridges are able to move extremely heavy spans by placing counterweights on one side of the pivot, or fulcrum, and building the main structure of the span on the other end. In this way, relatively small electric motors can raise and lower a bascule span using a small fraction of the horsepower that would otherwise be required.

The Pivot

The pivot is an axis consisting of a short shaft that supports something that turns. The lever is only possible because the pivot allows for the transfer of the direction of force. So bascule bridges use pivots, as do swing span bridges and vertical lift bridges as well. But the most obvious example is the swing span bridge as the entire span rotates upon a pivot in order to open and close the bridge.

The Pulley

The pulley is also a simple machine that changes the direction of an applied force. Rope, or in the case of vertical lift drawbridges, wire rope is run over a grooved wheel that rotates around a pivot. By attaching counterweights to one side of the rope and the bridge span to the other side, the effective weight of the span can be greatly reduced allowing for movement using relatively small motors.

Using all of these principles, engineers are able to design and build massive machines that operate without requiring equally massive drive motors. Without these work-saving principles,  the power infrastructure to supply a drawbridge would be imposing in scale and the costs would be unrealistic for building in most situations.