Repairing broken clock mainspring
 

Good morning,
I have managed to find a way to repair a snapped clock spring fairly easily and so will share the tip here.
I bought a 60’s Metamic retro looking mantel clock which was listed as not working. After disassembly it was clear that the outer end of the mainspring had snapped off where is hooks over a lug in the brass barrel. Finding a replacement mainspring would have been very difficult.

This is hardened steel and absolutely no chance of drilling a hole in it!! Attempting to set a bend on the broken tip of the spring caused it to snap it was that hard.

What I managed to do to repair the spring was to put the last 1/2” of the spring into the flame on the ring on the gas cooker until it had got hot enough to glow cherry red and then carefully whilst the end of the spring was still in the flame with long nose pliers set a small 180° bend in the spring. At this temperature, the steel is malleable enough to be easily worked without fracturing. This was sufficient to hook over the lug in the brass mainspring barrel and allow the spring to be wound up and only losing less than an inch of the spring steel.

After careful cleaning and reassembly the clock now runs as it should with only a small loss of spring capacity. This method could be used with other small springs or hardened components that need to be worked. The only downside is that you do lose the temper and hardness in the steel at the area being heated.

Christopher Capener

Re: Repairing broken clock mainspring
 

Well done Christopher!

This is a standard horologists' way of doing things, not a botch at all, and keeps the original spring in service which is nice if originality is the key.

This excerpt is from one of Laurie Penman's brilliant books:

Nick.

Re: Repairing broken clock mainspring
 

I've done this a couple of times with complete success. The hard bit is getting the mainspring in and out of the barrel in safety. The stress level is akin to handling a vintage TV crt! Goggles and strong gloves essential!

Re: Repairing broken clock mainspring
 

This works on snapped telephone dial return coil springs too with great success; once you have recovered the spring from the far end of the room when it is removed from its housing. ;D

Re: Repairing broken clock mainspring
 

Nice job, Christopher. I've done this lots of times.

Also worth straightening the spring out apart from the last inner turn, by holding the outer end in a vice; at the same time you can clean it properly with wire wool and lubricate it properly with thick oil or grease, depending on its size.

There's a knack on removing and refitting a spring in the barrel - takes a few minutes but pages to try to explain!

Re: Repairing broken clock mainspring
 

This is the kind of process that Donald De Carle illustrates with parts about twice the size of the elegant hands manipulating them. It's only when one tries oneself that it becomes clear that to see it, one requires a 1" loupe. The fingers, alas, cannot shrink in sympathy.

Well done, Christopher. A good application of physics! Could you have tried tempering it if it had been annealed too much? It seems as though it works just fine now, though.