Why did it break?

[Fred Reiss]

I'm repairing a shelf clock with a Hermle 351-020 movement. The clock is dated 1982. It runs fine, chimes OK, but doesn't strike the hour. I let down the spring on the strike side and removed the barrel to inspect. I discovered a single tooth missing on the #50 barrel and the second wheels arbor bent. The bent arbor caused binding that stopped the train. I'm replacing the barrel and 2nd wheel from a scrap movement. I'm sure that's all it will need.
My question is...how did this happen? Everything else in the movement appears fine with very little wear. What could have broken the tooth and bent the 2nd wheel arbor? Just curious.

[R. Croswell]

I'm repairing a shelf clock with a Hermle 351-020 movement. The clock is dated 1982. It runs fine, chimes OK, but doesn't strike the hour. I let down the spring on the strike side and removed the barrel to inspect. I discovered a single tooth missing on the #50 barrel and the second wheels arbor bent. The bent arbor caused binding that stopped the train. I'm replacing the barrel and 2nd wheel from a scrap movement. I'm sure that's all it will need.
My question is...how did this happen? Everything else in the movement appears fine with very little wear. What could have broken the tooth and bent the 2nd wheel arbor? Just curious.

Either the click failed to hold or the spring came loose from the center arbor or broke. Either way, the sudden unwinding of the spring caused the damage. Not uncommon on these.

RC

[Fred Reiss]

Thanks for the response. The click is fine. The spring is not broken. I guess the spring must have slipped off its arbor and I'm lucky it only broke 1 tooth!.

[doug sinclair]

Check out the state of the click and the teeth on the ratchet wheel. Chances are someone's fingers slipped during winding, and the click was unable to grab the ratchet wheel in the nano-second of time that it took for the spring to expend itself. Typical damage under such circumstancesi.

[Willie X]

I agree with others, click failure.

It is very unusual for the barrel teeth to break but very common for the 2nd wheel arbor to bend.

If you remove the click wheel and examine it closely with magnification, I'm guessing that there will be some damage. Might just be a slight ding on one tooth. You can clean it up with a file but much better to replace it with the newer milled version.

Willie X

[Fred Reiss]

Both the ratchet wheel and click are fine. I popped the cap off the barrel and it looks like the spring was the culprit. It has wierd bends right around where the tang connects into the hole. This had to have been an unusual way to break a barrel tooth and bend the arbor on the 2nd wheel.

[Bob Vance]

I'm kind of surprised it only took out one tooth. Once all that power is released quite often 4or5 will be taken off. Make sure you have enough pressure with the click spring. I see a lot of these fail on these movements.
Bob

[Willie X]

If the spring did simply 'unhook' at the arbor, which would be very unusual, you can bet that it will do it again ...

I would suggest that you check the click spring tension as Bob suggested and replace the stamped click wheel with a milled updated one. Also make sure that the click is held close to the plate by its rivet and that there is no burr at the tip of the click, especially on the back side against the plate.

Willie X

[shutterbug]

Yes. You want a flat surface butting against a flat surface. Otherwise you're just waiting for another bad experience.

[Fred Reiss]

Maybe my eyes aren't the youngest on the block, but I've inspected the ratchet wheel and the click as closely as possible and I don't see anything resembling
a burr, flat spot, wear, dings, or anything. The spring on the other hand, is not concentric...it's bent close to where it hooks onto the barrel.
I've had springs that are hard to start winding because they don't catch easily on the tang. I bend the spring slightly to decrease its' diameter so it will catch hold.

Only one tooth broken on the barrel...but a very obvious bend in the 2nd wheel!

[shutterbug]

If a spring breaks or comes loose from the arbor, the expansion is absorbed by the barrel, and you won't get internal damage to other wheels. If the click fails though, the barrel spins rapidly during the unwinding of the spring and everything in the train is stressed with potential damage resulting. That's one way to determine what has happened even before you open the plates. Wheel damage like that mentioned in this thread is almost certainly caused by a failed click.

[Tinker Dwight]

Hi Sbug
I fail to see the difference between a spring that breaks at the
arbor and a click failure. The only difference is the inertia of the
arbor and key, that is small compared to the spring.
It would be different if the spring broke at the end attached to
the barrel.
Tinker Dwight

[shutterbug]

Hi Sbug
I fail to see the difference between a spring that breaks at the
arbor and a click failure. The only difference is the inertia of the
arbor and key, that is small compared to the spring.
It would be different if the spring broke at the end attached to
the barrel.
Tinker Dwight

I'm willing to be enlightened here, Tinker ... but if the spring breaks at the arbor, the arbor remains stationary. The spring expands into the barrel (also stationary), which is usually damaged, but the other wheels are spared. If, however, the click fails, the spring expansion is passed to the barrel which is still attached to the arbor via the spring. In that case, the only release for the spring is through the barrel to the train. To me, that's a big difference. I don't think I'm wrong .... but I await your explanation

[Fred Reiss]

Now I'm really confused. When I read Tinker Dwight's response, I think he's exactly right and that means the spring slipping off the
arbor did all the damage. Then I read Shutterbug's response and it makes sense and that means the click, click spring or ratchet wheel is the culprit. I'm even drawing pictures trying to figure out who's right.
Can anyone else weigh in??

[Tinker Dwight]

Hi
The damage is done by the inertia of the spring. Two things
are happening when something at the center of the spring
lets go ( arbor or click ).
The spring pushes outwards. This sometimes splits the barrel.
It is also rotating very fast and comes to a suddent stop
when it comes up against the barrel.
Never under estimate rotational inertia. Any engineer that
has worked with rotating machinery can tell you that
it doesn't take much to create a lot of force with a suddent stop.
If the click breaks, the key and the arbor add to the mass but
they also slow the rotational speed of the spring by their
intertia. That amount of energy transfered to the barrel is
the same and slams the gears teeth.
It might be slightly greater with the key and arbor spinning
because there is a slowing effect of the spring coils pushing
the air between turns but I doubt there is enough to make
much difference.
If the spring breaks at the outside of the coil, its intertia
spins the click ratchet and arbor until friction slows it down.
Sbug's mistake is to think that the spring is stationary. It
isn't. When released, it spins very fast and comes to a sudden
stop when against the barrel. The total inertia is essentally
that of the energy stored in the spring. It is the same amount
of energy if the arbor and possibly key come along for the ride.
Conservation of energy says that it has to go someplace.
Tinker Dwight