Intermittent

In our last episode your local horologist had brought the Peter Green clock back to the shop to (hopefully) remedy some issues that recently presented themselves to Jane and Paul. Those were the intermittent striking (e.g. striking 5 times at 6 o'clock) and the moon dial not turning.

YLH had noted that he might discover new areas of concern when examining the clock but would limit that examination to a couple hours of time and then call to share observations, make recommendations and agree on a plan.

Copacetic. 

Avanti.

Yes those are nails acting as anchors for the weight cables on the wooden movement seat. Yipes.

Ok, ok. 

Once again worth noting that your correspondent is going to limit our review to the principal issues noted and discovered along the way. But horological learnings were bountiful.

In a concurrent life YLH dabbles in the world of computers and one of the great challenges in troubleshooting them is intermittent issues.

The screen is dead. OK. Let's dig in.

The website works great but sometimes it's slow. Oy vay.

In the world of engineering, diagnosing complex and intermittent issues sometimes entails waiting for (or seeing if one can force) the problem to reoccur.

Don't want to force a skipped strike on a 270 year old heirloom.

Many areas of the movement were examined (as pointed out in the last post) but this clock detective ended up focused on the rack.

So many tests were run on the rack and snail mechanism. (A reader interested in more details on the way the rack and strike works can look here.)

When the clock strikes the gathering pallet spins, catching a rack tooth, pushing the rack forward one tooth, and the rack hook drops into the next tooth. This repeats for each hour of the strike.

For reference. 

And here is a video of our rack in action. Watch the gathering pallet turn, catch a tooth, move the rack and then the hook drops.

The careful viewer will note that on the very first turn of the gathering pallet the rack hook moves forward two, 2, TWO teeth on the rack. This was a rabbit hole YLH went down deeply.

Turned out to be a nothing burger.

Sigh.

But many other areas of the rack were examined.

And the snail.

What's this?

Remember I noted this was a double snail?

Wrong. 

It's a triple snail!

That third snail is for the chiming mechanism

Hang on!

That has nothing to do with the strike. 

Ahem.

Back to the strike rack and second snail.

The rack and its tail are tightly connected via a pipe/post and turn in tandem around an arbor on the movement plate.

The rack tail pin makes contact with the snail and its turned position determines the number of strikes. Took this aspiring clockmaker many dealings with rack and snail mechanisms to get a real feel of how this all works.

Meanwhile all looks normal and well aligned with our Dutch friend.

Next!

Well there is one thing that was bothering me. 

The rack gets reset by a spring that pulls it into position at the end of a strike.

In this case that spring is a long strip of metal bent to create a springing effect. 

The tension of the spring pulls the rack hook, which pushes the rack tail and its pin down to the snail and pulls the rack teeth into position for the next strike.

Here's a video of YLH testing the rack snapping into position.

The top of the rack and its teeth are snapping back rather hard.

YLH wondered if this might sometimes cause the rack hook to land on the wrong tooth of the rack. That would alter the number of strikes.

The rack spring was tweaked to tug more gently.

This was the first effort to remedy the strike. Would it work?

Many other areas of the movement were tested and examined for possibly causing the missing strike.

The movement itself was in the shop for about a month of analysis, repairs and testing. More details to come in future posts.

After that rack spring adjustment, the entire time the clock was in the shop... it never missed a strike.

Was that the solution?

Well with intermittent problems you can never be sure.

This horological statistician is going with... probably.

Stay tuned to see the fascinating mechanism and fix of the moon dial!