counterweight

A counterweight is a heavy mass used to balance another weight or to provide a balancing force in a mechanism driven by a pulley, lever, or gear train. By balancing opposing forces, the system reduces the amount of energy required to lift or move a load, and can tune the system's response, reduce vibration, and improve control.

In elevators, a counterweight is attached to a rope that runs over a pulley and balances the car and its maximum expected load, reducing motor torque. In cranes and hoists, counterweights balance the weight being lifted to minimize motor power and improve stability. In theatre and stage rigging, counterweights balance flown scenery and equipment via a manual or motorized rigging system. In ships and submarines, ballast weights and counterweights help trim, stabilize, or balance mechanisms such as rudders, cranes, or gun mounts. In machinery and flywheels, counterweights can offset the weight of moving parts to reduce vibration and inertia.

Design considerations include mass, location relative to the pivot, movement range, and the relationship to the load. Properly sized counterweights must be matched to the system's geometry to maintain balance while allowing controlled motion. Safety features include locking devices, redundant supports, and compliance with relevant standards.