Speed dial

In our last episode we got Nana's clock movement installed back in the case and it was striking nicely.

It's exciting because those results leave this clock fool no doubt that we will get the clock running well.

Most well-made American clocks that are serviced, maintained and in good shape can often be accurate to within 1 minute a week. Some can do even better. 

Time adjustment is a regular part of any mechanical clocks service and maintenance. Even when in tip top shape a mechanical clock will often need at least tweaks of its timing.

And Nana's clock movement did need time adjustment. 

Furthermore after a full service there's a lot that can be discovered about performance and potential problems by just letting the clock run for several days or even weeks.

Mechanical clock speed is driven by the gearing ratios of the movement wheels and the rate that the pendulum swings back and forth. 

The gearing (wheels in the movement) is essentially fixed and with the right pendulum will keep time very well. 

The pendulum's job is time regulation. Adjusting the pendulum is what underpins clock time adjustment.

Summarily, you adjust clock speed by adjusting the pendulum. Long pendulums swing more slowly. Short pendulums swing more quickly. 

But it's a bit more complicated than that.

Do tell Horology Boy!

Well there's primarily two ways pendulum adjustments are made and it depends on the design of the clock.

Option 1. Raise or lower the pendulum bob

Option 2. Shorten or lengthen the pendulum itself

In Option 1 you are changing the center of gravity of the pendulum by moving the bob, in very small increments, up or down the pendulum rod.

Take a look at this pendulum with a big bob on the end of it. It is from an old English clock that is running on the wall near my desk. 

The round, weighted pendulum bob has a hole/tunnel that runs top to bottom and you slide the pendulum rod through and then screw on the rating nut it to attach the bob. The pendulum rod is threaded at the bottom and the bob rests on the regulating/rating nut. The rating nut can be twisted left or right and that incrementally nudges the bob up or down the threaded pendulum rod. Kinda like a nut on a bolt.

If the heavy bob is adjusted up the pendulum rod it shifts the center of gravity of the entire pendulum upwards as well and in doing so the pendulum swings more quickly. 

And yes twisting the nut the other way lowers the bob and slows the pendulum.

Physics!

This variation of pendulum (with bobs on threaded rods with rating nuts) was very commonly implemented in mechanical clocks. This design for pendulum adjustment goes all the way back to Christiaan Huygens who is credited with inventing the pendulum clock in 1656!

But... 

Nana's clock does not have a threaded pendulum rod and rating nut.

Nana's clock has a bob with a loop top that hangs on the little hook at the end of the pendulum rod. That pendulum design is very common with older American clocks. 

Here's a close up from a photo used previously.

There's no adjusting the pendulum from that hooked end of the pendulum rod.

Zo! Vee go viz Option 2, changing ze length of ze pendulum.

This turns out to be more complex than turning a rating nut but it does come with a clever feature, a front-facing speed adjustment arbor.

A what?

OK let's first understand how you change the length of the pendulum Dr. Science!

Well... you don't exactly. Not literally. You end up adjusting the effective length of the pendulum.

Huh!?

Yeah OK take a look. Here's the movement again with the suspension spring connecting to the pendulum rod with the bob hook on the bottom. The length of the suspension spring is included as part of the full length of the pendulum as the spring is there to facilitate the flexible swing of the pendulum. In other words it swings as part of the pendulum. (No bob in this photo and technically the bob is part of the length of the pendulum but lets try not to over-complicate things.)

Now a closer look up top.

The top of the pendulum is the spot where the suspension spring slides into that slotted rod. That suspension spring can be raised up and down, and only the portion where the spring appears below the slotted rod will flex and act as part of the pendulum. 

That is the magic of Option 2. 

Raising and lowering the suspension spring through that slot has the effect of shortening and lengthening the suspension spring and, in effect, the entire pendulum.

So how do you raise or lower the suspension spring!

Ah! We now return to our old friend(s) the (formerly) bent suspension spring pin(s).

Those pins anchor the very top of the suspension spring. When installed, the very top of the spring is slipped between the pins and is held in place / prevented from sliding downwards by a small wire loop which is punched through the very top of the spring. Essentially the wire is too thick to be pulled through the slot between the pins.

Those pins are actually part of and angle out from a sliding plate which moves up and down by the turn of a threaded screw. The screw is threaded through a section of the plate and when it turns the plate slides up or down, raising or lowering the pins and the suspension spring with slides through the slotted rod.

Here's a side view.

Now further to the way the spring is moved up or down...

The threaded screw has a geared head which meshes with the geared head of the speed adjustment rod. That rod goes through the movement (like an arbor) and comes out on the front of the clock.

Look below and at the top of the 12 on the dial you will see a little squared arbor tip. It looks like a mini version of the two big winding arbors for the time and strike trains. That little arbor is the other end of the speed adjustment rod.

With a dual ended key like the one for Nana's clock, one end is used to wind the two big arbors and the other, smaller end is used to turn the speed adjustment arbor. 

Turning the speed adjustment arbor to the right (clockwise) turns those gears on the back and has the effect of raising the sliding plate and that lifts the suspension spring and effectively shortens the pendulum and speeds the clock. 

And yes turning the arbor to the left does the opposite.

On the dial of many clocks with a speed adjustment arbor you will see an S printed on the left side and an F on the right. Counterclockwise for Slow and clockwise for Fast. Yep, they're there on Nana's clock. 

This design has a considerable advantage over rating nuts: no need to stop the pendulum to make an adjustment. Sometimes clocks that require rating nut adjustments have to be stopped, picked up and turned around the clock to get access to the pendulum.

So despite some of the added complexity, the speed adjustment arbor is widely adopted feature on many clocks. 

Your local horologist made a lot of turns on that speed adjustment arbor on Nana's clock over the course of more than a week of testing.

It's keeping perfect time now and ran nicely for over a week.

Yay!

Done, right?

Wrong. 

We did run into another issue that was discovered only after the clock had been running for about 8 days.

It needs remedy and remains a To Do as of this writing.

More soon!

P.S. Some clocks have BOTH rating nuts and speed adjustment arbors. Like the dog clock.