A fly system, flying system or theatrical rigging system, is a system of lines (e.g. ropes), blocks (pulleys), counterweights and related devices within a theater that enables a stage crew to quickly, quietly and safely fly (hoist) components such as curtains, lights, scenery, stage effects and, sometimes, people (e.g. in Peter Pan). Systems are typically designed to fly components between clear view of the audience and out of view, into the large opening, fly loft, above the stage.
Fly systems are often used in conjunction with other theatre systems, such as scenery wagons, stage lifts and stage turntables, to physically manipulate the mise en scène.
Theatrical rigging is most prevalent in proscenium theatres with stage houses designed specifically to handle the significant dead and live loads associated with fly systems. Building, occupational safety, and fire codes limit the types and quantity of rigging permitted in a theatre based on stage configuration. Theatrical rigging standards are developed and maintained by organizations such as USITT and ESTA (now PLASA).
A counterweight is an equivalent counterbalancing weight that balances a load. Counterweights are often used in traction lifts (elevators), cranes and funfair rides. In these applications, the expected load multiplied by the distance that load will be spaced from the central support (called the "tipping point") must be equal to the counterweight's mass times its distance from the tipping point in order to prevent over-balancing either side. This distance times mass is called the load moment.
Trebuchet: There are five major components of a trebuchet: beam, counterweight, frame, guide chute, and sling. After the counterweight drops from a platform on the frame, gravity pulls the counterweight and pivots the beam. Without the counterweight, the beam could not complete the arc that allows the sling to accurately release the projectile.
Crankshaft: A counterweight is also used in many rotating systems to reduce vibrations due to imbalances in the rotating assembly. A typical example is counterweights on crankshafts in piston engines.
Desk lamp: Some balanced arm lamps work with a counterweight to keep the arm and lamp in the desired position.
Elevator: In traction (non-hydraulic) elevators, a heavy counterweight counterbalances the load of the elevator car, so the motor lifts much less of the car's weight (specifically, the counterweight is the weight of the car plus 40-50% of its rated capacity). The counterweight also increases the ascending acceleration force and decreases the descending acceleration force to reduce the amount of power needed by the motor. The elevator car and the counterweights both have guide rollers attached to them to prevent irregular movement and provide a smoother ride for the passengers.