Suspending

In our last episode your local horologist was chewing on the purpose of dog bones in our "butcher" Morbier clock. 

That treatise focused much on the transmission of power to the pendulum. 

Your essayist had noted in an earlier entry that he was able to get the clock up on Scott Johnson's splendid test stand fairly quickly and that the clock ran fairly well right out of the box.

All true.

Yet to reveal itself was the fact that during the initial testing on the stand the clock had an extremely subtle but highly problematic break in a key component!

What?

Regardez.

As shown in our last episode the pendulum has a small hook at the top that allows it to connect to hole at the base of the pendulum rod. 

The hook and pendulum rod were not the issue.

No the problem was further... north.

Traveling up the pendulum rod we come to the point where it connects with the suspension spring. 

Let's take a quick step back. 

What is a suspension spring and what's it for?

You see in a clock the pendulum must connect to and safely hang from something but also be allowed to easily swing back and forth. Thus the creation of suspension springs.

In most clocks the swinging motion of the pendulum is facilitated by a very thin and highly flexible strip of metal (or pair of them) referred to as a spring. The pendulum connects to and is "suspended" from that spring. 

YLH wrote a bit about them with the DUFA tall case clock. Examples and details about them in that post.

Suspension springs are often made with "blocks" on each end (especially those used in European clocks, not so much American varietals). Those blocks are thicker pieces of metal allowing the thin springs to robustly connect to the clock case or movement (on top) and the pendulum rod (on bottom).

Suspension springs connect to the clock via a suspension bridge (or post). That is the part that holds up the pendulum. Suspension springs hang from the suspension bridge.

Sometimes the suspension bridge is effectively just a rod that pokes out 90 degrees from the movement, like the DUFA tall case clock when it had a new suspension spring put in by your horology hack.

Suspension springs and bridges can be relatively simple like many English variants. 

Or a bit more complex like the French Brocot suspension unit here.

And here is the Morbier suspension as found by YLH.

Couple things of note. 

In this case the top of the pendulum rod is pinned to the bottom block of the suspension spring. 

The top block is pinned to the suspension bridge; not easily seen in this photo.

Oh and there's a big problem with the suspension spring. 

OK not so clear but it is definitely visible in the above photo and, to be fair, this clock detective didn't notice it himself until disassembling the beastie.

Here's the suspension unit out of the clock.

See the problem now?

Here's a close up.

Yeah that front spring (this is a two strip spring) is cleanly snapped off the base of the top block. 

And the pendulum had been suspended by the rear of the two springs alone.

Note the problematic hanging angle of the bottom block.

This ain’t a fixit situation. This is a replace it situation. 

Well easy enough to order new springs from the good folks at Timesavers who have scores of examples of suspension springs here.

The highly observant reader might have noticed a certain feature of the Morbier suspension spring... that sort of bullet-shaped hole on the bottom block. It has to do with the pinning of the pendulum rod. More on that later.

Here are examples of Morbier specific suspension springs. 

Our Morbier has twin springs and the strips are relatively short with the top block pin hole close to the top edge. YLH thought this one was the closest option. They're inexpensive so this cautious repairer bought two just in case. 

The location of the hole on the top block is very important. If that block is too big or the hole too low it won't even fit in the suspension bridge without major alteration.

Clearly our model is not a perfect match but it will work...

...with some help.

After checking the alignment of the pins to the various holes in the suspension bridge and the pendulum rod it was clear some adjustments needed to be made.

And zo we ended up with this.

And once sufficiently adjusted, it was pinned into the suspension block.

And reinstalled.

And the pendulum rod pinned in place.

With some breathing room between the bottom block and the top of the pendulum rod. 

Back up on the test stand the new spring worked perfectly and the clock ticked away.

However our butcher beauty would need a bit more... 

massaging. 

Stay tuned!