Tweaks and valleys

As mentioned in our last episode, several years ago this clock collector was lucky to have acquired a beautiful French silk suspension clock, circa 1860 (very late in the era of silk suspensions), and learned much about the idiosyncrasies of its movement during restoration.

A welcome comment to that last episode posed a question about affixing the brass block to the rod of the silk suspension pendulum that your cross-country horologist had acquired specifically for the Tole clock.

And to answer that... well... 

It's all about power.

Some backstory...

Your correspondent greatly enjoys the field of horology, especially the workings of mechanical clock movements. Working on older clocks full of history and mystery... very gratifying indeed.

Now clocks can break down in myriad ways but most clockmakers will tell you that the most common reason an older clock has slowed or stopped working is that the clock is no longer transmitting enough power to operate well or at all.

And there are whole categories of issues regarding why a clock is not transmitting power well. Chasing down the underlying cause of a loss of power can be a prolonged, frustrating and frequent exercise for the horologist. 

Moreover there are numerous, considerable hurdles in sorting out how to configure, much less repair, complex mechanisms engineered and built somewhere between the Enlightenment and the advent of electricity.

Did you know that Thomas Edison did not invent the light bulb?

And the French did not invent the silk suspension for clock pendulums.

That was the Dutch, and in particular, the father of the pendulum clock, Christiaan Huygens, a man considered one of the greatest scientific minds of the 17th century.

Galileo proposed the idea of a pendulum for timekeeping but Huygens did as well (later insisting that he knew nothing of Galileo's observations about pendulums at the time) and he built the first pendulum clocks.

Did you know that in 1610 Galileo observed Saturn but did not correctly identify the rings around it (he thought it was two moons).

Guess who did.

   

Moons    -     Rings

Uh professor... what does this all have to do with that question from the comment?

Right!

In the Smithsonian you can find a very early and beautiful example of a Huygens silk suspension clock.

And the Tole clock presents another fine example of a silk suspension for us to take pleasure in today. Yes made approximately 180 years later but fundamentally operating the same way. 

Silk suspensions, while elegant in their simplicity, have a few deficiencies that accelerated their disuse when better solutions came along. 

For example they are susceptible to speed errors due to changes in humidity affecting the length of the silk string. 

But perhaps their most notable flaw is that they are inefficient in the transmission of power from the movement to the pendulum. 

An improperly configured pendulum in a silk suspension movement can cause problems or even stop an otherwise healthy clock.

Even a silk suspension in good shape, well-maintained and properly set up will lose more power to its pendulum swing than will a metal spring suspension.

Here's why:

Pendulums regulate the time of the clock by their swing but that swing must be powered by the movement in little taps. The ticks and tocks.

Most pendulum clocks have an extended lever called a crutch. The crutch transmits power from the movement to the pendulum. The crutch fork taps the pendulum in each direction as it swings and that's what keeps pendulums swinging.

Power is transmitted like this:

Power source (wound mainsprings or weights) --> movement wheels --> escape wheel --> anchor  --> anchor arbor --> crutch --> crutch forks --> pendulum rod

When the crutch fork taps the pendulum rod, energy runs down the rod to the bob and the pendulum swings just a bit further than gravity would, by itself, allow. Just enough extra lift to keep the pendulum swinging. 

However energy from that tap also goes up the rod towards its top. When the top is held in place directly to an arbor (in some clocks) or by a metal suspension spring (most clocks), much of the energy is retained in the motion of the pendulum. 

If the pendulum rod is suspended by a silk thread, the top of the rod will lose some of that upward energy as the rod will wobble slightly where it meets the far less dense thread.

Here are two views of the exterior of the Tole clock showing the parts that are outside of the plates.

A closer look.

The silk thread is delightfully simple but... power inefficient.

Wait, wait, wait professor!

You didn't get to the question from the comment! What about the block!?

Ah yes.

So that block must be affixed to the rod very tightly. Even the tiny wiggle room it has now between rod and block (just enough to slide it up and down the rod) can generate some power loss during a tap from the crutch.

Furthermore the hook of the pendulum rod and the bob must be aligned absolutely in parallel with the movement itself so that the pendulum will swing perfectly parallel to the movement AND the pendulum rod block must be affixed so it is absolutely parallel to the crutch fork.

Geometry!

And now back to the question: how to affix that block to the pendulum rod?

Answer: Not sure yet.

It is essential that the block is at the correct height on the pendulum rod, allowing sufficient play both up and down for speed adjustment.

Furthermore, as research deepens, it appears that shortening the size of the silk thread loop will help reduce wobble and what is known as circular error on the pendulum (fair warning, this goes deep). But reducing the loop size too much creates different problems. 

Therefore getting the clock timing / loop length / block location just right is key BEFORE adhesion.

Also crucial... the adhesion must be sufficiently resilient so as it will hold up to 1 jillion taps by a crutch fork and stay tightly attached to the rod.

However in the off chance that butter fingers does not perfectly align the block, perhaps utilize a compound that could possibly be reversed in its adhesion with heat or some such. 

Not entirely sure that is essential. Go with confidence!

First impulse was some variant of cyanoacrylate glue. Perhaps one of the many variants of Loctite.

Research is happening.

Suggestions are welcome.

This horology hound has learned a lot about setting up a silk suspension pendulum.