The lamp leans forward, nods, and settles wherever gravity feels like putting it. You tighten the nut, it holds for a day, then it sags again. On most articulated desk lamps that behaviour points to a single component: the tension spring stretched between a fixed pin on the column and a lever on the arm. Replacing it is not a difficult job, but it rewards patience. Measure properly, buy the right spring once, and keep your fingers out of the line of pull. Here is the whole sequence, from working out what you have to testing the finished repair.

First, Work Out Which Spring You Have

Not every jointed lamp uses the same hardware, and ordering the wrong type costs you a week of waiting. Turn the lamp over in your hands and look along the arm before you touch a tool.

• External extension spring. A visible coil running parallel to the upper or lower arm, hooked between a pin on the column and a lever on the arm. This is the classic anglepoise-style arrangement. Some lamps have two, one per arm section.
• Concealed extension spring. Same principle, but the coil runs inside a slot in the arm. You see only the hooks and the anchor pins at each end.
• Compression spring. Sits inside a tube and pushes rather than pulls. Less common on desk lamps, but used in some counterbalance designs.
• Friction-only joint. No spring anywhere, just washers, a bush and a nut doing all the work.

If you find no spring at all, stop here: a worn friction joint or a cracked casting is your culprit. If your lamp carries two springs, replace both at once. A fresh spring paired with a tired one will fight it, and the arm will still droop on one side.

Take the Load Off Before You Take the Spring Off

An extension spring on a lamp arm is holding several kilos of leverage in tension. When that hook slips, the energy goes somewhere, usually into a knuckle or an eye.

• Unplug the lamp and take out the bulb.
• Wear eye protection and thin gloves.
• Photograph both hooks before you disconnect anything, so you know which way the loops face.
• Fold the arm into the position where the spring is shortest. On most designs that is the arm pushed fully down.
• If the spring is still tight, tape or clamp the arm to the column so it cannot spring open while your hands are inside.
• Work over a shallow tray. Anchor pins, circlips and plastic bushes disappear into carpet permanently.

Never cut a spring that is under load. Release the hook with a proper hook tool, a loop of stiff wire, or a small flat screwdriver used as a lever, and keep your fingers as the last resort rather than the first.

The Five Measurements That Matter

Use metric calipers and write everything down. Measure the old spring relaxed and off the lamp, not still dangling from the arm.

1. Hook-to-hook length

Measure from the inside face of one hook to the inside face of the other, with the spring at rest. This is the number that decides whether the arm reaches its stops. If your hooks are the extended type, with a straight leg before the bend, note the total length as well.

2. Outside diameter

Measure across the coil body, not across the hooks. Too wide and the spring rubs the inside of the arm; too narrow and the loops may not clear the anchor pin cleanly.

3. Wire diameter

This is where the strength lives. A few tenths of a millimetre change the pull noticeably. Measure three or four coils and take the average.

4. Coil count and body length

Count the full coils in the wound section and note the body length, which is the hook-to-hook figure minus the hooks. A spring winder uses both to approximate the rate.

5. Hook type and alignment

Full loop, side loop, extended hook, or a loop with a cross pin. Note whether the two loops sit in the same plane or at ninety degrees to each other. A spring whose hooks are out of line will twist the arm sideways no matter how well it fits.

One extra figure is worth having: the distance between the two anchor points with the arm fully extended, and again fully folded. That gives you the working range the spring has to survive without being overstretched.

Sourcing a Replacement Without Guesswork

Start with the manufacturer. Model numbers are often on a sticker under the base or stamped inside the joint, and spare springs for popular designs are still sold. If there is no part available, spring suppliers sell by dimension, and small shops that wind springs to specification will usually quote for a short batch, which is often the only route for an older lamp.

When an exact match is not on the shelf, work down this list in order:

1. Hook-to-hook length within a millimetre or two.
2. Wire diameter, the same or slightly heavier if the lamp sags under its heaviest load. Do not go much lighter.
3. Outside diameter that clears the gap in the arm.
4. Hook type and alignment.

On material: music wire, meaning spring steel, is the normal choice. Stainless is sensible in a kitchen or a damp room. Avoid unbranded springs from assorted hardware boxes for anything carrying real weight, because the steel is often soft and will take a permanent set within weeks. Buy two while you are at it.

Fitting the New Spring

1. If the lamp has a threaded adjuster or turnbuckle, set it to the middle of its range first.
2. Fold the arm to its lowest-tension position and clamp it there.
3. Seat the fixed hook first, usually the one on the column, so the loop sits square on its pin with the hook opening facing away from the pull.
4. Draw the free hook towards its pin with a hook tool or a loop of wire, keeping the spring in line with the pin so it does not twist on.
5. Check that both loops are fully seated. If a hook looks opened out, close it slightly with pliers, but do not work-harden it by bending it back and forth.
6. Cycle the arm slowly through its full range twenty or thirty times, watching for coils rubbing on the arm, the flex, or the housing.
7. Re-check the hooks and the anchor pins after the first dozen cycles.

Balancing the Arm After the Swap

A new spring can easily be too strong, and almost every lamp has a second adjustment that people forget: friction. If the arm drifts down with the lamp fitted, look at the joint nuts before blaming the spring. Turn them a quarter turn at a time. Over-tightening crushes the washers and makes the arm stiff at the top of its travel.

If the arm springs upward on its own, the spring is too powerful for the load. Two fixes work. Move the moving-end hook to an anchor hole nearer the pivot, which shortens the lever arm and reduces the torque the spring can apply, or fit a slightly longer spring. Moving the anchor to a hole further out does the opposite, and it is often the simplest way to fine-tune a lamp that only sags when a heavy shade is fitted.

Lubricate sparingly. A smear of light grease on the pivot pins and on any point where the coil touches metal is enough. Thin oil creeps, and it will find its way onto the shade and the desk.

When the Spring Is Not the Problem

Before you order a third spring, check the pivot itself. Hold the arm and try to rock the joint from side to side. Lateral play means a worn bush, a loose rivet or an oval pin hole, and no spring will cure that. Cracked die-cast collars are common on older lamps, too. Also trace the flex: a cable that has been re-routed can pull against the spring and make a healthy arm behave badly.

Before You Put the Tools Away

Test the finished lamp with its real bulb and shade fitted, running the arm through its full range ten times and letting it settle at three or four heights. Check that the flex is not pinched between the coils and that neither hook has started to open. Then put the old spring in a labelled bag with your measurements on it, and tape a copy of those numbers inside the base if there is room. Next time the arm droops, you will already know what to order, and the repair will take twenty minutes instead of an afternoon.

Photo: StockSnap / Pixabay