Counterweight

Simple Crane

A counterweight is a weight that, by exerting an opposite force, provides balance and stability of a mechanical system.[1] Its purpose is to make lifting the load more efficient, which saves energy and is less taxing on the lifting machine.[2]

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.[3]

A counterbalance is a weight or force that balances or offsets another as when two objects of equal weight, power, or influence are acting in opposition to each other. The objects are then said to be in counterbalance.

Counterweights in action

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.[4][5]

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.[6]

Desk lamp: Some balanced arm lamps work with a counterweight to keep the arm and lamp in the desired position.

Counterweights in architecture
César Pelli's Ratner Athletic Center uses cables, masts and underground counterweights as a load-bearing support structure.
Leaning Tower of Pisa has used external counterweights to stabilize the once gradually falling structure.

Elevator: In traction (non-hydraulic) elevators, a heavy counterweight counterbalances the load of the elevator carriage, so the motor lifts much less of the carriage's weight (specifically, the counterweight is the weight of the carriage 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 carriage and the counterweights both have wheel roller guides attached to them to prevent irregular movement and provide a smoother ride for the passengers.[7]

Diagram of a space elevator. At the bottom of the tall diagram is the Earth as viewed from high above the North Pole. About six earth-radii above the Earth an arc is drawn with the same center as the Earth. The arc depicts the level of geosynchronous orbit. About twice as high as the arc and directly above the Earth's center, a counterweight is depicted by a small square. A line depicting the space elevator's cable connects the counterweight to the equator directly below it. The system's center of mass is described as above the level of geosynchronous orbit. The center of mass is shown roughly to be about a quarter of the way up from the geosynchronous arc to the counterweight. The bottom of the cable is indicated to be anchored at the equator. A climber is depicted by a small rounded square. The climber is shown climbing the cable about one third of the way from the ground to the arc. Another note indicates that the cable rotates along with the Earth's daily rotation, and remains vertical.
Space elevator with counterweight at top.

Space elevator: A space elevator is a proposed structure designed to transport material from a celestial body's surface into space. Many variants have been proposed, but the concept most often refers to an elevator that reaches from the surface of the Earth to geostationary outer space, with a counterweight attached at its outer end.

By attaching a counterweight at the end, upward centrifugal force from the Earth's rotation ensures that the cable remains stretched taut, countering the gravitational pull on the lower sections and thereby allowing the elevator to remain upright.

The counterweight itself could assume one of several forms:

  1. a heavy, captured asteroid;
  2. a space dock, space station or spaceport positioned past geostationary orbit; or
  3. an extension of the cable itself far beyond geostationary orbit.
Concrete counterweights on a tower crane
Bascule bridge with concrete counterweight

Metronome: A wind-up mechanical metronome has an adjustable weight and spring mechanism that allows the speed to be adjusted by placement of the weight on the spindle. The tempo speed is decreased by moving the weight to a higher spindle marking or increased by moving it to a lower marking.[8]

Crane: The tower crane (see picture) is a modern form of balance crane that is fixed to the ground. A horizontal boom is balanced asymmetrically across the top of the tower. The long arm carries the lifting gear.[9] The short arm is called the machinery arm; this holds the motors and electronics to operate the crane, as well as the concrete counterweights.[10]

References

  1. ↑ http://www.thefreedictionary.com/counterweight
  2. ↑ How do elevators and lifts work? at explainthatstuff.com
  3. ↑ "Basic Crane Design principles". E-Crane International. 2002.
  4. ↑ "How a Trebuchet Catapult Works". Retrieved 2007-04-07.
  5. ↑ "How does a catapult work?". 2000-04-01. Retrieved 2007-04-07.
  6. ↑ Wan, Mark (2000). "AutoZine Technical School: Engine Smoothness". Retrieved 2007-04-07.
  7. ↑ Bellis, Mary. "The History of the Elevator". Retrieved 2007-04-07.
  8. ↑ Metronome. Wikipedia. Accessed 7 April 2007.
  9. ↑ Crane (machine). Wikipedia. Accessed 7 April 2007.
  10. ↑ Brain, Marshall. "How Tower Cranes Work". Retrieved 2007-04-07.
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