Ich bin ein Uhr Narr

The DUFA has been running for a while now. 

It keeps good time and the Largo gong strikes beautifully. That said, our German friend is in great need of an overhaul and not just a simple cleaning. A visual inspection shows some dirt and grime but also dramatic wear of the movement that must be repaired. This will require a complete disassembly.

Here's the movement.

What we have here is a classic "rack and snail" striking movement. Striking meaning, counting the hours, vs. chiming where you'll have a melody play (like Westminster chimes. Think Big Ben.). The rack is the toothy steel bit on the upper left and the snail is in the center. The rack and snail is an ingenious mechanism that basically took over most other forms of striking in mechanical clocks. 

Here's a drawing with some details of a rack and snail striking movement. This one also has a chiming function so it's a bit more complex than our patient today.

I get how the rack and snail works conceptually but haven't put my head completely around the striking works and the various other parts of a striking movement. There are a lot of variations of the rack and strike and more specifically, a lot of other components that go into the full striking of a clock. Suchen. 

There's a bit more of a hill for me to climb before I'll have reached sufficient altitude to assuredly re-assemble, much less effectively repair, our German friend. 

But that's the point, no? This will be a learning adventure.

So there's objective number 1, learn how to correctly rebuild the strike.

Objective number 2 is to advance my learning of bushing. 

Ah bushing. I've had one go so far and didn't screw it up (I screwed up another part) but this clock will test my skills.

The basic components of most clock movements of any age is effectively two plates of brass (sometimes more) with a bunch of wheels (gears) and levers and assorted gizmos. Most wheels spin between the two plates on a rod called an arbor.

Here's another view of that movement in the drawing above.

And here's a fairly typical wheel you'll see in many clocks.

The the tips of the wheel arbors that connect to (are inserted in) the plates are the pivots. They spin in pivot holes in the plates.

Here's a snap of our DUFA

You can see a few wheels here on their arbors. They connect (stick into) to the front and back plates via the pivots. Also there's the top of the rack on the right outside the plate. 

Back to another view of the DUFA movement you can see the tips of some of the pivots protruding from the pivot holes on the front plate. There's two of them along the top on the left (amongst others). You can clearly see the associated wheel behind that second pivot. 

The pivots protrude from what look like little bowl shapes in the plates. Those bowls are to facilitate oiling the pivots. Most clock oiling involves putting a very, very small drop on the pivots in those pivot holes. (Too much oil is actually bad for a clock as it inevitably attracts dust and dirt.)

The opposite plate has corresponding pivot holes.

Here's a close up of one of them.

This particular pivot hole from our DUFA is on an extension of the back plate (it's screwed into the back plate) but that wheel arbor and pivot go through the plate and come out there. The pivot hole has got a bit of dirt/grease but from this angle doesn't look too bad.

Or so we think.

Here's a clear example of a pivot protruding from a pivot hole on a different clock. 

See how it looks... snug in its pivot hole? The hole is in good shape.

Well you don't want it too snug. The pivot hole should allow the pivot to spin smoothly in the hole as this one does. It should have a little wiggle but not much.

But here's the rub (sorry).

You see these wheels spin a lot in a moving clock. Some make hundreds of rotations a day. If the clock is in regular use the wheels can spin almost continuously for years, even decades. And there is, in fact, quite a lot of pressure and tension put on the wheels of clocks in a normal movement. That's how they are powered. 

Even in well-maintained clocks all that spinning can create a lot of friction and ultimately wear, both on the pivot that protrudes from the hole but more typically the pivot hole itself. Those brass plates are softer than the steel arbors and pivots used with the wheels. There's some interesting science about friction and wear and why they use brass and steel like that but suffice it to say sometimes that brass can wear badly with serious consequences.

That same snug pivot in a pivot hole can wear into what looks more like an oval shape. This drawing exemplifies a pivot that is wearing (gouging the plate). The white area shows the location of the original pivot hole centered on the crosshair. The wear in the plate from the pivot is slowing shifting the hole and the center of the hole to the right.

As the pivot gouges deeper into the plate more and more of its surface area is rubbing directly against the brass. This often results in a dramatic increase in friction from this wear can slow or even stop a clock. Moreover the pivot is now drifting away from the original centered location of its pivot hole. This can also lead to problems.

Most pivot holes are on located on the front or back plates directly. This particular pivot hole of concern on this DUFA can be addressed more easily as the plate extension can be unscrewed from the back plate itself. Thusly.

There's the wheel and it's pivot pointing to the pivot hole it uses on that little plate. We're seeing the plate from the inside out.

So far so good??

In fact seen from a different angle it's quite obvious that this particular pivot hole is badly worn. Regard the shape. Just like the diagram above.

The pivot hole needs to be repaired and realigned. But how do you repair/replace the brass?

It needs a brass bushing with a lovely correctly-centered pivot hole. And I happen to have one pictured with it, right below. 

But how to marry the bushing to the plate?

That is objective number 2. Bush the clock where needed.

This is what I would describe as a step beyond the basics and more into intermediate skills for a horologist.

More on both of these objectives in our next installment!