Notch'o ordinary repair

In our last episode your local horologist was vexed by Ken's clock. 

It had started simply enough.

Just a few very minor repairs and adjustments to a movement that was in excellent condition overall. The clock was now running and striking.

Rush Limpbone here had called Ken and said the clock was ready to come home.

Except it wasn't... Something had gummed up the striking. After running fine in the shop, the next morning it was striking the wrong numbers at the wrong times.

Back to the bench.

Your clock doc did a quick review of the wheels the strike train. The power from the strike mainspring runs through those wheels to the striking mechanism. 

Nada. They appeared to be operating fine. So power from the strike mainspring through the wheels of the strike train was sufficient.

Next up: was there any binding (friction) occurring somewhere in the strike works?

First up for this inspective detective was the count wheel which is a device that functions similarly to a rack and snail but its mechanism is somewhat different. Count wheels are the earlier invention for striking but their simplicity and reliability were such that they were never completely replaced by rack and snail mechanisms. This is especially true in American striking clocks which, well into the 20th century, still utilized them for striking functions.

As the count wheel turns a lever rides (slides along) the wheel's outer edge. The wheel turns and the clock strikes until that lever falls into a notch in the edge. When the lever falls it stops the striking.

Short wheel sections (rides along the edge of the wheel) amount to fewer strikes and long sections enable more strikes.

While there are a few candidates for sources of friction on this wheel that were examined the one Hong Kong Foolish noodled on the most was the notion that perhaps the count lever was binding with / getting stuck on the edge of the notches.

Checking each step...

Nope the lever cleared the notch edges just fine and looking closely both one side of each notch and the lever itself were beveled to ensure a clean (minimized friction) ride up notch.

YLH looked at several other parts of the strike mechanism.

Those parts are between the plates. Deferring a full disassembly this practical horologist chose to do a deep observation with related tests and, in short, the solution to our mystery became obvious.

Rather than take the patient reader through all the bits and bobs your essayist will focus on the core elements of the conundrum. You're welcome.

Many variants of clocks strike by means of a pin wheel lifting the strike hammer arm and releasing it so that the hammer falls and hits the gong / rod / bell / etc.

How does the pin wheel lift the hammer arm?

Well first take a quick look here at the pin wheel on Nana's clock. And then the one here on the DUFA tall case.

OK. Pin wheel.

As a pin wheel turns its pins engage with a small lever on the hammer arm arbor. A wheel pin lifts the lever and as the wheel continues to turn that lever slides off the pin and the lever and the hammer falls.

The hammer arbor lever is on the right below. It's engaged with a pin on the pin wheel.

OK your visual artist knows the diagram is a bit complex but there's a good reason.

As outlined in our last episode there is another lever on that hammer arbor. It's the one on the left that engages with the hammer spring. Remember that the spring is really just a very stiff wire that by being hooked under the lever creates upward pressure against that lever.

Can you see the problem in this design?

Yeah the pin wheel lever pushes the arbor one way and the hammer arbor spring pushes it the other way.

There is a competition of lifting force.

Is this a flawed design?

No.

By design the power of the pin wheel lift should far exceed the recoil pressure from the hammer arbor spring.

Except it doesn't in our 100 year old clock.

So several attempts were made to lessen the tension of that spring.

Those efforts were insufficient and this clock doc did not want to risk breaking the very tensile spring.

So in the end...

Captain Obvious decided that the clockmakers before him who had serviced this gizmo had it right. Someone recognized that the pressure from that spring was too much and the clock did strike ok (more or less) without it. The weight of the hammer itself was enough to create a medium-ish striking sound.

Now there was another option.

The clock could have been completely disassembled and a much deeper analysis made of the strike. Looking at the mainspring power and considering replacing it, the gearing of the wheels, the tension of the hammer arbor spring, wear or binding in the strike train, the alignment of the pin wheel, etc., etc.

With every effort there is risk of creating problems and complexity.

KISS.

The spring was disengaged when found.

And disengaged for return.

The clock ran fine this way.

Ken was thrilled.

Simple, right?