What a mainspring does and why it fails

A mainspring is the metal ribbon coiled inside the barrel at the heart of a mechanical clock. It stores the energy that makes the clock run. When you wind a clock, you tighten this spring; as it unwinds, it releases that energy through the gear train to move the hands. After years of winding and unwinding, the spring metal fatigues, loses its tension, or breaks—and the clock stops keeping time or stops running altogether.

You will know a mainspring has failed when the clock runs for only a few hours after winding, or stops entirely and will not restart. Some springs break visibly; others straightforward lose the ability to hold tension. Replacing it is one of the most common clock repairs, and it is a job you can do at home if you have basic tools and patience.

Key Takeaways

  • A mainspring is a metal ribbon coiled in the barrel that powers the entire clock mechanism by releasing stored energy as it unwinds.
  • You will need the clock movement removed from the case, a barrel arbor tool or small punch, a new mainspring of the correct width and length, and a way to safely contain the spring as you remove the old one.
  • The barrel must be opened by removing the arbor (the shaft running through the center), and the old spring removed carefully because it can snap and cause injury.
  • The new spring is wound into the barrel in a specific direction and the arbor is replaced, then the barrel is tested before reassembly.
  • If the spring is broken or you are uncomfortable working inside the barrel, a clock repair shop can replace it for less than the cost of a new clock.

Gather the right tools and parts before you start

You will need a barrel arbor removal tool or a small punch that fits the hole in the center of the barrel. This tool lets you push out the shaft (arbor) that holds the spring in place. A regular hammer and a punch that fits the arbor hole will work if you do not have a dedicated tool. You also need a mainspring that matches your clock's barrel—width, length, and thickness all matter. Measure the old spring or look up your clock's model number to order the correct replacement.

Gather a clean, flat work surface with good light. Have a small container or cloth ready to catch the old spring as it uncoils—a broken spring can snap suddenly and fly across the room, so containment matters for safety. A small screwdriver, tweezers, and a magnifying glass are helpful. If the barrel is stuck or corroded, a small amount of penetrating oil can help, but do not use it near the spring itself.

Before you begin, remove the clock movement from the case and set the case aside. You will be working only with the movement itself. Lay it on a clean cloth to protect it and to keep small parts from rolling away.

Remove the barrel arbor and old spring

Locate the barrel—it is the large cylindrical gear at the center of the movement, usually near the top. The arbor is the shaft running through the middle of the barrel. You will see a small hole or slot at one end of the arbor where the winding key fits. On the opposite end, there is usually a small hole or a flat surface where you can push or tap the arbor out.

Place the barrel arbor removal tool or punch into that hole and tap gently with a hammer. The arbor should slide out toward you. If it resists, do not force it—explore a tiny amount of penetrating oil around the arbor hole, wait a few minutes, and try again. Once the arbor is free, set it aside in a safe place where it will not roll away.

Now the old spring is visible inside the barrel. It will be coiled tightly and may be under tension. Carefully lift one end of the spring with tweezers and let it uncoil slowly into a cloth or container. Do not let it snap free—guide it as it unwinds. If the spring is broken, the pieces may fall out on their own. Once the barrel is empty, wipe the inside clean with a dry cloth. Do not use water or solvents inside the barrel.

Wind the new mainspring into the barrel

Before you install the new spring, check which direction the old one was coiled. Most mainsprings are wound clockwise when viewed from above, with the outer end anchored to the barrel wall and the inner end attached to the arbor. Your replacement spring should come with instructions or a diagram showing the correct direction.

Hold the barrel upright and insert the inner end of the new spring into the center hole where the arbor will go. Begin winding the spring clockwise into the barrel, keeping it flat and even. Work slowly and let the spring settle into the barrel naturally—do not force it or bunch it up. The outer end of the spring should rest against the inside wall of the barrel. Some barrels have a small notch or hook where the outer end anchors; if yours does, seat the spring end there.

Once the spring is fully coiled and seated, slide the arbor back through the center hole. The inner end of the spring should grip the arbor as it goes in. Tap the arbor gently with the hammer until it seats fully and the hole at the winding end aligns with the movement. The arbor should not move side to side once it is in place.

Test the barrel before reassembling the clock

Before you put the movement back together, test the new spring. Insert the winding key into the arbor hole and wind the clock slowly. You should feel resistance building as the spring tightens—this is normal. Wind it until you feel it reach full tension, then stop. Do not over-wind or force the key.

Now let the clock run without the hands attached. Listen for smooth, even ticking. The escapement (the part that regulates the tick) should sound regular and not skip or stutter. If the spring is installed correctly, the clock will run steadily for several hours on one winding. If it stops after a few minutes or sounds irregular, the spring may not be seated correctly in the barrel or the arbor may not be fully seated. Remove the arbor again, check the spring position, and reinstall.

Once the barrel is running smoothly, you can reassemble the movement. Reattach the hands, reinstall the movement in the case, and wind the clock fully. It should now keep time and run for the full duration the spring allows—usually 8 to 30 hours depending on the clock design.

When to call a clock repair shop instead

If the barrel is corroded, cracked, or the arbor is bent or stuck, the repair becomes more complex than a spring replacement. If you are uncomfortable working inside the barrel or the spring breaks as you remove it, a professional clock repair shop can handle the job. The cost is usually between $50 and $150 depending on the clock size and condition, which is far less than replacing the clock.

Some clocks have sealed or riveted barrels that cannot be opened without specialized equipment. If your barrel has no visible way to remove the arbor, do not force it—take it to a repair shop. Forcing a sealed barrel can damage the entire movement.

Common mistakes that damage the clock

The most common error is winding the spring in the wrong direction. If you wind it counterclockwise, it will not grip the arbor and will fall out when you try to reinstall it. Always check the old spring's direction before you wind the new one. Another mistake is over-winding the new spring. Wind until you feel resistance, then stop—do not keep turning the key. Over-winding can break the new spring when ready.

Forcing the arbor out or in can bend it, making the clock run unevenly or stop. Tap gently and use penetrating oil if it is stuck. Finally, do not let the old spring snap free uncontrolled—it can damage the barrel or injure your hand. Always guide it into a cloth as it uncoils.

Frequently Asked Questions

How do I know what size mainspring to order?

Measure the width and length of the old spring, or look up your clock's brand and model number online. Clock spring suppliers list springs by barrel diameter, width, and length. If you order the wrong size, it will not fit or will not hold tension. When in doubt, contact the supplier with your clock's model number and they can confirm the correct spring.

Can I reuse the old mainspring if it is not broken?

If the old spring has lost tension but is not visibly cracked, you can try rewinding it tightly and testing it. However, metal fatigue is permanent—a spring that has lost tension will not regain it. Replacing it with a new one is more reliable. A fatigued spring will slow down or stop again within weeks or months.

What if the spring breaks while I am removing it?

Stop when ready and let the pieces fall into the cloth or container. Do not try to grab a snapping spring—it can cut your hand. Once it has stopped moving, carefully remove the pieces and dispose of them. Then proceed with installing the new spring. A broken spring is actually easier to remove because the pieces come out freely.

How tight should I wind the new spring?

Wind until you feel steady resistance, then stop. You should not be able to turn the key any further without significant force. The spring is now at full tension. If you can keep winding easily, the spring is not seated correctly in the barrel or the arbor is not fully inserted. Do not force it—check the installation and try again.

Why does my clock stop after only a few hours even with a new spring?

The most common cause is that the spring is not fully wound into the barrel or the arbor is not fully seated. Remove the arbor, check that the spring is coiled evenly and the outer end is anchored to the barrel wall, then reinstall the arbor firmly. If the problem persists, the escapement or gear train may need cleaning or repair—that is a job for a professional clock repair shop.