Balanced arm lamps

A balanced-arm lamp, sometimes called a floating arm lamp, is a lamp with an adjustable folding arm which is constructed so that the force due to gravity is always counteracted by springs, regardless of the position of the arms of the lamp. Many lamp brands (such as the Anglepoise, originator of the concept, and Luxo L-1) as well as other devices, such as drawing boards, use this principle.

A balanced-arm lamp has a base, two connected arms fitted with springs, and a lamp-head. The lamp can be moved into any position, and the springs will maintain the position until moved again. The same mechanism can be employed in other devices with similar requirements, such as copy holders for typists and some computer display holders. This article uses the terminology lamp cap, forearm, upper arm and base for the four basic parts of these lamps.

For the physics and theory behind balanced-arm lamps, see Classical mechanics, linkage and torque. There are different methods to balance the lamp-cap of a balanced-arm lamp. Some lamps have two coil springs working in parallel on both sides of the pivoting arm. (A set of springs functions in the same way as a single spring.) Others are balanced with counterweights (a method frequently used in drawing-boards). Friction between parts of the lamp arm can also be used to maintain balance. There are a number of mechanical solutions (coupling, hydraulics and pneumatic arms) which have occasionally been used for balanced-arm lamps.

There are many variations of construction with springs. Springs can be located on the mechanical equivalent of the forearm or the upper arm, or both, as well as nearer the base. Some lamps use tension springs, and others use compression springs. The image at the left shows (left to right) a compression spring at rest, then under load, followed by a tension spring at rest, and then under load. Springs have a limited lifting capacity and extension length. Some springs can resonate, producing low-level unwanted noise.

This lamp has a stiff flexible tube as its forearm, and a spring-balanced arm to support it near the middle. That way there is a flexible lamp with a long reach.

The wheel to control the forearm is smaller than the wheel that controls the upper arm. The lamp cap rotates the same direction as the upper arm and forearm.

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