How can an elevator detect excessive speed mechanically?

An elevator's speed can be watched by a mechanical device separate from its normal motor control. A governor rope forms a loop connected to the car. As the car moves, it pulls that rope around a governor pulley, turning car travel into rotation.

In one mechanical governor design, rotating weights can move outward against the forces that restrain them. Their position changes with speed. At the designed trip condition, the moving parts release a mechanism that grips the governor rope. The pulley and weights detect motion; they are not the parts that must directly hold the whole car's load.

The distinction becomes clear after the rope is restrained. The car's continuing movement now pulls against a rope that can no longer follow it freely. This relative movement operates a linkage on the car, engaging safety devices that grip the guide rails.

The rails pass the stopping forces into the building structure. A small speed-sensitive motion has therefore triggered a different, much stronger load path. The chain is car movement, governor rotation, rope restraint, linkage movement and rail grip.

Other brakes, sensors and buffers work with this arrangement, and real systems have specific thresholds and operating limits. A mechanism can use the motion being monitored to trigger a response without relying on a passenger to notice anything. The sensing parts initiate the action; the rail-gripping parts carry the stopping forces.