That creeping feeling

You know that nagging feeling that something is not quite right? 

The one where you come to a conclusion and tell yourself that it was the right one but it did require some convincing... and yet... you still weren't really sure...

And then you think about it for a while. Sometimes on and off for days.

You can't quite let it go. 

It feels like it's still there. Still following you.

You know. 

When that thing you put behind you is actually creeping up on you...

Well that's where this clock fool found himself a while back.

Let's rewind.

In our last episode your local horologist was rambling on about an intermittent problem with the striking reported by Jane and Paul, the owners of the Peter Green tall case clock. 

The kind reader may recall that YLH, after considerable examination and such, noted that after the adjustment of the rack tail spring, our Dutch friend exhibited no further strike misses. While no guarantee that this solved the problem, there was no other clock adjustment occurring to your horology student that indicated that it might remedy the issue.

Meanwhile, the clock doc continued to work on other issues found with the Peter Green (and will report on some of the exciting results in later posts). Various horological adventures and learnings ensued. 

When your correspondent finally sat down to write about the "repair" of the skipping strike (an unpublished Holmes mystery title) he was resigned to the spring adjustment theory but... not enamored of it. While nothing else considered seemed a likely underlying cause, the idea that the strong recoil of the rack to a tight spring thus making the rack hook slip a tooth made sense. 

But it was far from the proverbial smoking gun. 

This burgeoning clockmaker could not hypothesize any other cause for the irregularly aberrant behavior but it is entirely possible that the adjustment did nothing. 

This did not sit well, but... the thing seemed to be working fine now. Time to move on!

In reviewing the scores of photos of the delightful Peter Green clock YLH was gazing at one photo in particular. That photo was used to explicate the action of the rack tail pin and how it lands on the snail.

Here it is again from the earlier post.

In the effort to see exactly how that rack tail pin fell onto the snail photo boy here took many other snaps including this one taken in the opposite direction from the one above.

At this point there was no light that flashed on. More like a tiny whiff of smoke from a smoldering fire.

Keep in mind that this aspiring mystery novelist (not at all) was well beyond the fireplace and the smell of smoke was unexpected.

Wait a minute.

...

Geometry.

Physics.

Probability.

(In an earlier life your aspiring scientist took many, many statistics classes).

YLH remembered something. There was another adjustment he had made.

The rack tail and its pin. There was something about them. 

Oh yes! 

In the initial examination of the rack YLH noticed that the rack tail was a touch too close to the snail and made a minor adjustment, a gentle bend of the tail resulting in the rack tail head and pin being about 1mm further away from the snail. 

Should land cleanly now. 

Took about 5 seconds. Next issue.

Holy crap!

Epiphany. 

Yes. Yes! 

That could be what caused the skipping strike!

Uh professor. What are you talking about?

Regardez.

Let's go back to another photo from an earlier post of that rack.

The reader can see how the rack tail is essentially a wide but thin strip of brass. That thinness facilitates minor adjustments made by gently bending of the brass. In the image above to the right, a careful viewer might note how the end / head of the rack tail is bent inwards just slightly resulting in a very tight fit to the snail.

Look back further up at the 2 prior photos and see how that rack tail pin head bends inwards towards the snail and is snugged right up to it. Look at the very first movement photo above. Imagine the inward pressure of the rack head pressing up against the snail.

Important aside: Often when working on a clock there are opportunities that present themselves that require simple kinds of tweaks by this clockmaker and serve as good hygiene for the overall care of a timepiece. Little adjustments and fixes and such. This no-longer-an-amateur clockmaker does many of these without even thinking about them.

Here is what was YLH remembered.

At the time spent hunting down the cause of the skipping strike YLH noticed that the rack tail spring was slamming the rack back rather hard during the warning. An adjustment was made. Also observed that same day (and until recently completely forgotten) was the overly close fit of the rack tail pin. As noted earlier, the rack tail is thin and could have easily been bent out of alignment by anyone working on the clock in prior service efforts.

YLH gently made a small adjustment by bending the rack tail outward by about a millimeter or so. It was just enough to make sure the rack tail pin would still land correctly so that it wouldn't accidentally rub, bind or get caught somewhere along the snail but not overly adjust it such that the pin misses the snail or gets caught up somewhere else in the movement. Just a tweak.

It was a fleeting adjustment. Barely registering on this tweaker's brain (such as it is) at the time. One of many good hygiene types of tweaks that can pop up during most any clock service.

YLH completely forgot about that tweak.

Wait wait wait!

Why is it important to have the rack tail pin NOT snug against the snail?

Well the rack tail and its pin could get stuck and/or not fall completely down correctly to the snail or... (and here is what YLH realized) the head of the rack tail might land on the snail instead of the pin itself.

Wait what does that mean, the rack tail pin landing on the snail?

Rack and Snail quick primer:

As a reference here is another photo from a previous post. There's the rack hook with its tooth at rest behind the 1st tooth of the rack. This movement is not in warning. This is the normal running position, e.g. not right before striking commences.

The snail turns with the hour hand, making a full rotation every 12 hours. The snail has 12 steps, 1 is highest and 12 lowest. The rack has 12 corresponding teeth. See the image below. Note that in the image the snail steps 11 and 12 can't be seen as they are blocked from view by the curved surface of the outer snail.

When the strike goes into "warning", AKA getting ready to strike, the rack is released, the rack tail spring pulls the rack tail down onto the snail at the step corresponding to the current hour on the clock. The height of the snail step (and the distance of the fall of the pin to that step) stops the rack above, making its corresponding pivot to the spot along it with the matching number of rack teeth to the snail step. 

This aligns the rack to a position such that the rack hook tooth will slot to the correct rack tooth. The number of strikes that the clock will make is dependent on the number of the rack teeth that the rack hook tooth is resting on. 

So for example when the rack tail falls on the 5th step of the snail the rack will also have recoiled back to a position of 5 teeth and the clock will strike 5 times.

Below is a photo of the rack in warning (ready to strike) for 1 o'clock. The rack tail pin is resting on the first step of the snail and the rack hook tooth is nested in front of the 1st tooth of the rack (hard to see but it's in there). The clock is ready to strike once. 

Now here's the thing:

If the rack tail pin doesn't land cleanly on the rack it will be too high, perhaps one snail step too high.

And your photographic nut seems to have, unknowingly, caught an example of this exact problem happening in some of the early snaps made during the initial clock examination.

In the photo below the strike is in warning but the rack tail pin is not resting on the snail which is positioned for a 1 o'clock strike. The snail is at its very end which comes to a point. The top of the rack tail head is just touching/resting on the snail. The head was bent too far inwards and it hit the snail stopping the fall of the rack tail before the pin itself could land on the snail. You can see the gap that captain observant missed in reviewing his earliest photos of the movement.

Right there is a shot of the rack and snail that was, were it to run, about to miss the 1 o'clock strike.

And this horology student didn't notice it.

If there's a gap then the rack tail will be sitting higher than it should and the rack won't recoil as far back along the rack hook which might result in it landing a tooth short on the rack. That gap between the rack tail pin and the snail in the shot above is about the height of one step of the snail.

Therefore during the warning the rack hook would land one tooth short on the rack.

And therefore the clock would strike one strike short (strike one hour less than the correct time).

Sometimes the rack tail pin would land on the snail correctly and sometimes the overly snug rack tail head would just catch on the snail as it fell, landing the rack tail about one step too high. It has a lot to do with the intersection of the curved shape of the snail and the shape and bend/angle of the rack tail head.

Something YLH tweaked and forgot about.

This. 

This is why the clock had been missing one strike. Sometimes.

No lurking gremlins (bunnies) behind us now.

Your clock detective is sure this is / was it.

Fixed it and didn't even realize it.

Here’s to good hygiene.