What is this clock thing for? #3 – the let-down key

Some folks collect clocks without actually working on them. Collecting clocks and repairing them are two very different things. I know people who would gladly send their clocks out for servicing and don’t seem to mind the extra cost. I like to do my own work.

If you work on your own clocks or service clocks for others you know how important it is to ensure safety at all times. Servicing a clock movement requires that you disassemble it completely and safely. There are specific tools designed not only to make life easier as a clock repair person but will ensure your safety at the same time.

Before taking the pins or nuts from the movement plates there are important steps you must follow to ensure your safety working with movements. There is a minimal threat of injury working with weight driven clocks (unless a weight drops on your foot) so the following information pertains to spring driven clocks be they open mainsprings or springs contained in a barrel. The let-down tool is designed specifically for spring driven clocks.

Click replaced
Click, click spring and ratchet
The letdown key
The letdown key is engaged prior to releasing the click

Some clock movements from French, English and German clocks, for example, contain mainsprings within a barrel, safer than open mainsprings. If the spring lets go in a barrel there is still a risk of damage to the gear teeth and wheels however the break will be contained within the barrel itself. The letdown key is used to release the power of the mainspring into its barrel.

The flat clamps (upper left in photo below) are for those open mainsprings which do not leave a lot of working room. Otherwise, the round clamps are good for most applications. Prior to working on a movement the mainsprings must be letdown or restrained in their clamps.

C-clamps used to contain the power of the mainspring

A flat C-clamp is wrapped around the mainspring as you can see in the next photo.

Mainspring contained in a clamp
Mainspring contained in a C-clamp

Once the mainspring is safely contained you can work on it with a spring winder, like this spring-winder from Olie Baker .

Mainspring servicing
Mainspring servicing using a Olie Baker spring winder

Here’s how it works. There are four pieces in the let-down set. There are 3 chucks in sizes #5-6, #7-8 and #10-12 plus the handle. Choose the size that fits the winding arbour of your movement. Then insert the let-down end with chuck into the winding arbour. Turn it until the mainspring is compressed. Fit a clamp around the mainspring. Once the clamp is secure around the mainspring release the click spring with a small screwdriver and push it out of the way. Once the click spring is out of the way move the click away from the ratchet while firmly holding the let-down key. The power of the spring will be transferred to the key. Allow the spring to gradually release its power by letting the key handle spin gently within your hand. There, the mainspring power has been restrained and you will be able to proceed with dis-assembly.

Your journey into the world of clock repair requires essential tools The let-down set should at the top of the list. Always minimize the risk of injury by restraining the power of the mainsprings. The letdown key is the safest method of letting down the mainsprings.

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