Expert: Martin Meyer - 3/2/2009

Question
QUESTION: On a Big Ben #5 Chime Alarm: The second gear in the time train turns around the central shaft that goes from the time set knob on the back of the clock-through the movement-and ends as the front tip of the minute hand's shaft. This shaft moves independently of that second gear in the time train when the time set knob is turned.
What drives the clock?
(I thought it was that second gear)

ANSWER: Hi Glenn,

It sounds like your clock transfers the power from the second wheel to the center wheel. (This differs slightly from my description in your previous question, where I suggested that the power transfer may be from the second wheel to the hour wheel).

Your question, "What drives the clock" is unclear to me. Of course, the mainspring drives the clock. I wonder if you are asking what drives the HANDS? it doesn't matter where the power from the time train transfers to the motion work, whether it is the hour wheel or the center wheel.

The center wheel (and entire motion work) is not really "independent" from the second wheel. The motion work has a clutch to allow the hands to be set. This may be on the center wheel or under the hour wheel. Therefore, when you turn the hands to set the time, the force of your turning overcomes the clutch and allows the hands to move while the center wheel remains still. But, when the clock is running, the clutch holds the center wheel firmly on its shaft, and the power is transferred from the second wheel to the center wheel and then to the rest of the motion work.  http://www.clockmasteronline.com



















---------- FOLLOW-UP ----------

QUESTION: This #5 Big Ben has the second wheel rotating around the very center shaft, so I wonder what is meant by "center wheel?"
The center shaft appears to be the same part as the minute shaft (I think) and it has a small gear on it in front of the front plate of the movement, which gears down through two gears (in front of the front plate) that operate the hour hand's tube.
I wonder if that small gear is the center gear? Either that ot the second gear(which rotates on the center shaft.)

And I wonder what would comprise the clutch? The second gear in the time train has a fairly heavy spring with only one or two coils that bears against it.

Also, if the clock had a second hand, I hope there would be no confusion about "what drives that" and the "second wheel in the time train."
         That's all and thanks a lot!

Answer
Hi Glenn,

As I mentioned before, clock terminology is far from standardized, and uses many very antiquated terms instead of simple modern terms which would be more accurate, for example, "wheel" instead of "gear", and "arbor" instead of "shaft". So I'll do my best to clarify these terms. Additionally, I was at a disadvantage previously not having a Big Ben in my shop to refresh my memory. I just found some photos of the #5 movement on the Web, so this should help.

This movement is somewhat different from the usual mechanical clock movement. The Second Wheel IS the Center Wheel in this clock, or rather, does double duty as both. The rest of the time train consists of a Third Wheel, a Fourth Wheel, and the Escape Wheel. The Center Arbor (shaft) does hold the minute hand, but it is rarely called the "minute shaft".

The small gear on the Center Arbor on the outside of the upper main plate is the Cannon Pinion (named for reasons I won't even get into). The larger gear next to and engaging it is the Minute Wheel (even though this is a confusing term). The gear which is on the Hour Pipe (hollow shaft that mounts the hour hand) engages the pinion which is on the minute wheel. These three parts comprise the Motion Work, and gear the hour and minute hands to work in synch.

The clutch is indeed the coil spring that you see on the center arbor. The Center (Second) Wheel is not riveted to its arbor, as are the other wheels. It would be free to rotate but for the force of the coil spring pushing it against a shoulder on the Center Arbor. When you set the time, you overcome the clutch, and the center wheel turns on its non-moving arbor.

Since your clock does not have a seconds hand, and we've gone into considerable detail already, I will forgo discussion of that topic, except to say that the Seconds Hand is geared to the Time Train, and does not engage the motion work (gears for the other hands) at all. An avoidance of confusion over these terminologies is probably why clockmakers refer to the "Seconds" hand, rather than the "second hand".  http://www.clockmasteronline.com