flyweight

A flyweight is a heavy mass, usually made of steel or cast iron, that spins around a central axis and moves outward as rotational speed increases. The outward movement is driven by centrifugal force alone, with no springs or external constraint; gravity and inertia do the rest. Flyweights are found in governors, clutches, and certain types of speed regulators where their radial movement must be proportional to shaft speed.

The most common application is the centrifugal governor, where two or more flyweights swing outward as speed rises, actuating a mechanical linkage that adjusts throttle or fuel supply. Early steam engines used simple two-weight governors with arms pivoting on the rotating shaft itself. Modern versions may have more complex geometries, but the principle is unchanged: faster spin equals greater centrifugal acceleration, which equals greater radial displacement.

Construction and Behavior

A typical flyweight consists of a mass mounted on an arm or pivot that allows it to slide or swing radially. The arm is usually attached to the rotating shaft, and the weight's distance from the axis at any given speed is determined by equilibrium between centrifugal force, the weight of the mass, and any mechanical resistance in the pivot. Adding mass increases the outward force; increasing shaft diameter reduces it. The relationship is not linear because centrifugal force depends on the square of rotational speed.

Friction in the pivot joint and air resistance cause hysteresis, meaning the weight does not return to exactly the same radius when speed decreases as it was when speed increased. This lag must be understood in precision governor work. Wear in the pivot also gradually increases friction, which can cause the governor to respond sluggishly or fail to maintain stable speed control over time.

The term flyweight comes from the machine tool trade, where the weights literally fly outward at high speed. It is distinct from a balance weight, which is fixed in place to counterbalance rotating components. A flyweight is always mobile; a balance weight is always stationary relative to its housing.