Spring thing

In our last episode I indicated that I had run into another issue with Nana's clock.

I'll get directly to it. 

When one of the two mainsprings unwinds / uncoils it rubs up against one of the internal wheels of the clock movement, stopping the clock. It's the time train mainspring that is the culprit.

You may recall that your local horologist discovered this issue while timing the movement. Nana's clock had run for several days untroubled.  

But one morning I discovered it had stopped. 

It had been fully wound and stopping after a few days was unexpected. 

A quick look at the clock suggested no obvious reason. The mainsprings seemed to have plenty of room to still uncoil and power the clock. Keep in mind the movement was still fully installed in the case and I had just taken a peek.

It was only after removing the movement and inspecting it that I understood what was happening.

The time train mainspring, as it was uncoiling, had run up against the "shroud" of the lantern pinion of the second wheel.

Here's a diagram of a wheel with a lantern pinion from the NAWCC.

From the wonderful NAWCC message board

And here is what was happening with Nana's time train mainspring.

As Nana's clock ran the time train mainspring uncoiled, expanded and started to rub up against that lantern pinion shroud. Eventually the friction created by the pressure from the spring stopped the second wheel from turning thus stopping the clock.

OK Clock Doc. Why? 

Why was the time train mainspring hitting that shroud of the lantern pinion of the second wheel? 

[What a mouthful]

Well...

Here's a quick mainspring uncoiling primer...

American clocks typically have open (uncased) mainsprings (vs. those cased in barrels like the dog clock). 

As mainsprings slowly unwind / uncoil they expand, growing in circumference. At some point they either need to be restricted from expanding further into the movement by the use of an enclosing mainspring case / barrel or they need to be angled away / deflected from the internal wheels of the movement as they continue to expand.

Here are the mainsprings on Nana's clock, time train on the left and strike on the right. They are both a bit unwound but the strike is definitely uncoiled a bit more. No problem.

See that time train mainspring? It is unwinding largely in the round, in place around its arbor, while the strike mainspring is spooling outwards away from the movement. That outwards expansion is the desired behavior of virtually every American clock mainspring movement design.

Here's another typical American clock, an Ingraham from 1928. Your correspondent was lucky to have captured a fine example of how the mainsprings are supposed to look when fully uncoiled.

And in most American clocks, including Nana's, the cases are designed to accommodate mainspring expansion.

Plenty of uncoiling room in Nana's clock case.

So Clock Fool how do the mainsprings uncoil outwardly like that?

Most clocks with open mainspring designs have rods riveted to the back plate that are designed to protect the springs from expanding into the clock wheels and to deflect the springs outward. 

Nana's clock has these rods.

Here is a side shot of one in the strike train. That rod is riveted into the plate on the left. 

We'll label the rod (A).

You can see how that rod prevents the mainspring from the strike train to uncoil any further upwards thus avoiding a slow speed collision with the lantern arbor shroud on the right.

The strike train actually has two rods for the purpose of protection / deflection. The second rod for the strike we will label (B).

Highlighting from our earlier photo you can see the rivets for both of the rods on for the strike train A & B.

As the strike mainspring unwinds / expands it presses against both rods A & B which restrict any further inward or upward expansion and force uncoiling to proceed outward to the right. With those two rods blocking the spring from expanding upwards or to the left, the only place to go is right.

So here's the rub (sorry).

There is only one rod (C) for the time train and there is no... (D). I looked. There's no trace of a rivet.

Looking back at some earlier photos of Nana's disassembled movement I found one I had used to show the seahorse.

You can see the 3 rods A, B, C clear as day. No D. (Keep in mind this is now an inside out view.)

By itself that rod C doesn't seem to be sufficient to effectively deflect the time train mainspring outwards.

So is this a design error of the Hip style movement? 

Was the hypothetical rod D not considered necessary to the unspooling of the time train mainspring? 

But the corresponding A rod was needed for the time train mainspring... and the springs are identical...

Is the time train mainspring spring somehow misaligned?

Is something else at play?

Furthermore! How to remedy this unwanted collision?

More to come...