Cates Clock Repair

A "set" main spring is a spring that will not properly expand and release its stored energy to power the clock

A mechanical clock is powered one of two ways. Either by weights or springs. Without this power a clock cannot function. A main spring is said to be "set" when it cannot expand to its full potential. A spring powers the gears when it  expands and forces the next gear to rotate, and the next gear pass that to rotate, and so on, and so on. When power is lost because the spring does not expand, then the clock stops.

We wind the spring up with a key. The spring then releases its stored energy to the gears of the train, and when the energy of the spring is gone, we rewind the spring and it all starts over again. But if the spring is not allowed to fully expand and release its power, then the spring will become fixed in that coiled up position and become "set". There is no way to "unset" a "set" spring. The spring simply has to be replaced.

You will find this problem mainly in clocks that have barrel springs, but it will occur in all mechanical clocks with springs. It is most common in barrel spring clocks because the spring is encased inside the barrel and is never allowed to expand fully. Barrel springs are used to keep the expanding spring from brushing up against other gears as it releases power. This allows the clock designers to make smaller movements with the gears closer together.

There is no definite shape that will tell the repairman when a "set" spring needs to be replaced. That all comes with experience in clock repairing. The picture below though should help you understand what it is I mean by saying the spring is "set". The spring on the left is new and expands properly, and is the spring that will replace the one on the right, which is, as you can see, "set" and will no longer expand properly.