There are quite a variety of articulated/balanced desklamps that look to have potential as the basis for robotic arms, cranes etc.

A spring ballast, extension arm, desk lamp

This example, Superlux Model C has quite short arms.

The arms are about 23cm long, the lower one constructed from three beams, the upper, 'fore', arm of only two beams. The lower beams forming each arm are on about 22.5cm centres, the upper beams seem slightly shorter, about 22.2cm between centres. The original payload of light fitting and bulb is about 256 grams (including bulb), this is partially counter-balanced by springs, so the arm will tend to maintain whatever position it is moved to. The springs would be unnecessary if using servos to drive the joints, as the servos would maintain position, in fact the springs would just add extra resistance to movement.

To use as a robot arm, there are 5 joints that you would motorise - swivel at the base, up-down of lower arm, up-down of second arm, up-down of hand, swivel of wrist. The base can swivel 360º (no stops), the lower arm can turn through about 150º, the upper arm can turn about 135º. The wrist can go up and down almost 180º and rotate about 285º.

On this particular arm design the 'hand' end of the arm can only reach the ground from a radius of about 33cm from the base out to 50cm with the arms fully extended horizontally. The under beam of the fore-arm prevents it from bending enough to reach the ground at closer distances. Other lamps that have a square or pentagonal plate at the elbow, can bend without obstruction. Removing the under-beam of the forearm allows the 'hand' to reach the ground from the base out to 50cm, unobstructed.

To help me document how all the component parts are connected I photographed it from many angles:

My attempt to convert this to a robot arm is documented here : Robot arm.

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