To avoid damage and injury, the mainsprings must be let down to relieve them of their power before anything is done to the movement.
You must do this even if the clock appears to have run down and stopped as a result. Some power will still remain in the spring, and this will, at the very least, push the wheels out of place.
You may need an assistant for the next task. Select a strong steel key that accurately fits the winding arbor. Holding the movement in one hand, take up the tension of the spring with the key and gently ease the spring loaded click away from the ratchet wheel.
Allow the key to rotate about a quarter to half a turn and release the click. Ensure that the click has engaged in the ratchet before releasing the key. Repeat the process until each of the springs has been relieved of all its energy. Keys with wide wings have less chance of slipping so never use carriage clock keys for this job.
Great care should be taken when carrying out this task as one slip can destroy a finger as easily as a clock.
Letting down a mainspring using a key.
Let-down keys are purpose made tools to control the letting down of springs. By having a key attached to a round handle, the speed of rotation can be controlled by how tightly it is held.
A typical set of let down keys
Fusee clocks are best let down when the clock has run down of its own accord, as the main ratchet is inaccessible. The external ratchet only needs to be let down about 2-3 turns when the clock is fully unwound. The short arbor used for setting up the fusee is normally not suitable for letting down the whole spring. If a fusee clock is not run down fully but will not tick, remove the pendulum and let it tick away rapidly. If this fails, the escapement can be carefully removed and the escape wheel allowed to spin round slowly. Control the speed with your fingers or an elastic band wrapped around the arbor and slow down to its normal speed as any striking time is approached.
Movements with springs contained in barrels are easily let down, and the barrel will then act like any other clock wheel. Clocks with open mainsprings present the problem that, with nothing to contain them, the springs get larger and larger as they are let down. They can become quite unmanageable unless contained in the steel Mainspring Clamps manufactured for this purpose.
A open spring clock with a mainspring clamp fitted.
Thick wire can be used, but this is inclined to slip and makes re-assembly difficult when powerful springs are being handled. An advantage to purpose made clamps is that the distance between the centre of the loop and the centre of the coiled spring is maintained. This aids assembly. Always treat springs held by clamps or wire with respect.
Repairing Your Own Clocks by Mervyn Passmore