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 to the base. Some lamps use tension springs, and others use compression springs. The adjacent image 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.
One tension spring Spring-balanced upper arm, flexible forearm 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.
Tension spring over wheels 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.
Two arms, one spring, one toothed wheel With this technique the tension spring has a double function: it controls both the forearm and the upper arm. This is not a common arm lamp system. The lamp cap rotates the same direction as the upper arm and forearm.
Two tension springs ==== Two parallelograms and two tension
spring sets ==== One tension spring set controls the forearm, which is a
parallelogram having two bars. A stronger spring set controls the upper arm, which has three parallel bars (see photo, left). (There is a less stable construction with two parallel bars in the upper arm which is less expensive to manufacture.) Due to the parallel
linkage of the lamp, the lamp cap keeps pointing in the same vertical direction when adjusting the height of the lamp. As with most balanced-arm lamps, the whole assembly can rotate in the horizontal plane, and is mounted on a stable foot.
One parallelogram and two extension spring sets A lamp like the
Anglepoise lamp has one
parallelogram and two extension springs. One spring controls the shortest arm of a parallelogram. Parallel to this shortest arm there is the forearm. To keep these arms parallel there are two other parallel arms (upper arm) that are controlled by a second extension spring. The lamp cap rotates the same direction as the upper arm and forearm.
Single forearm bar, one parallelogram, two tension spring sets The forearm of this lamp consist of a single bar. When it is adjusted, the angle of the lamp cap changes along with the arm.
Tension springs within the arms A lamp like the
Tolomeo desk lamp has no
parallelograms in either arm. In this modern lamp the tension springs are hidden in the arms. The lamp cap rotates the same direction as the upper arm and forearm.
Compression springs Compression springs in the arms The short arms (green) stay parallel. One spring puts pull
force on the blue arm. (If the blue arm is tilted back the second spring on this arm gets push force.) The blue arm controls the two parallel arms (red) that make up the upper arm. The other spring puts pull force on the other blue arm. This arm controls the forearm (magenta). The two springs can be the same size; one spring has to lift more
weight but is more vertical. The other spring lifts less weight but is more horizontal.
Compression springs near the foot One compression spring controls a very short arm (grey). The
Roller chain cable connects this arm parallel to the forearm (yellow). The longer spring controls the (blue) arm. The body (red) can turn in the horizontal plane. The body is connected to a stable foot. The body can be smaller because the pressure springs can protrude below the body.
A compression spring with one parallelogram In this method, just one compression spring controls a
pantographic arm. ==Lamps balanced by pressure and friction==