I've been meaning to put together some information about one of the features of Speedsters that seems to generate the most questions - the hour counter. I’ve posted details of this in various places over the years, but since questions about this function keep coming up, I wanted to consolidate it all in one thread.
There are often questions about how it’s not centered, why it isn’t accurate, why it creeps when the chronograph isn’t running, so I wanted to show some details of how this counter works, so you can all understand why it might do some of the things it does.
So the first thing to show is how the chronograph functions are operated on a Speedmaster, and for illustration purposes I will show photos from a Cal. 861 below.
Here is the back side of a Cal. 861 Speedmaster:
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This first video will show the start, stop and reset sequence:
There are really two key points here:
1 – The seconds counting hand and the minute counting hand are driven by a wheel that is on an extended post from the fourth wheel of the movement. That wheel in turn drives a wheel on the coupling yoke, and the cam moves that yoke in and out to make contact with the chronograph wheel, to start the chronograph running.
2 – In the reset state, the hammer that resets both the seconds and minute counting hands is
at rest against the cams of those wheels. You can see the hammer swing up and away from the cams when the chronograph has been started, and on the reset the hammer quickly swings down to reset those two wheels. The hammer movement on the reset comes from a long spring, so the hammer speed is determined by that spring.
So the one thing I don’t mention here is the hour counter, and that’s because it has no real direct connection to any of the parts I just mentioned. That’s because the hour counter is driven off the mainspring barrel, not from the wheel that is on the fourth wheel post, so it is driven independently of the seconds and minutes counting wheels.
Here is the dial side of a Cal. 861, and this is only partially assembled:
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And here it is with the remainder of the parts in place:
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The various parts have letters on them, so here is a list with descriptions:
A – mainspring barrel
B – friction spring for driving pinion
C – driving pinion
D – hour counting wheel
E – hour hammer
F – hour counter lock
The mainspring barrel A turns all the time that the watch is running, and so does the friction spring B that is screwed into that barrel cover. That spring will make the driving pinion C turn, if it is able to, and if that pinion is blocked, it will slip against the mainspring barrel.
The driving pinion C will turn the hour counting wheel D, but only if not blocked by the hour counter lock F. The hour hammer E is what resets the hour counting wheel D.
So when the chronograph is switched off, and the watch is reset, the parts are in the positions you see them in this photo I used above:
The hour counter lock F is sitting firmly against the hour counting wheel D, which means that the wheel is not turning, the driving pinion C is not turning. You will note that the hour hammer E is retracted out of the way, so that
the hammer is not resting on the cam of the hour counting wheel. If you recall the seconds and minute counters, when reset the wheels are held in place by the hammer, but that is not the case for the hour counter so this is a major difference.
So when the chronograph has been started, the hour counter lock F is moved, which allows the hour counting wheel D to be driven by the driving pinion C, and the hour counter starts to move. It moves continuously, so there is no jumping feature on these watches for the hour counter. When the chronograph is stopped, the hour counter lock F comes back in contact with the hour counting wheel D, and it holds that wheel in place so that the user can look at the hand to determine the elapsed time.
On reset, the hour hammer E swings down and contacts the hour counter lock F, and pushes it out of the way. The hammer then carries on and strikes the cam on the hour counting wheel D, and resets the wheel to it’s home position. Now remember that the driving pinion C is only connected to the mainspring barrel A, by the friction spring B. This allows the hour counting wheel teeth to stay engaged with the driving pinion, and the driving pinion spins under the friction spring as it’s being reset.
You can see the reset portion of this sequence here – note the hole that is in the hour counting wheel moves when the hammer comes down onto the cam:
Now one thing to note is that the reset sequence as you would use the watch looks to be all one thing, but in reality it's in two parts. Remember the hammer for the seconds and minute counters is spring loaded to reset, so when you push the reset pusher, this will move a part called the stem bolt for hammer out of the way of the hammer, and the spring snaps the hammer down on the cams.
Back to this photo again now:
If you look at the letter E on the hour hammer, there is a part with a hole in it directly to the right of the hammer - this is the hour hammer operating lever, and it is moved by the pusher stem for zero action. These two parts are moved by the pusher that you press.
So as the reset pusher continues to move in towards the case, and after the hammer for the seconds and minute counters has reset, the pusher stem for zero action presses on the hour hammer operating lever, and this moves the hammer down to reset the hour counting wheel. These are two distinct operations, and I showed this is a video I took some time ago:
You can see here that I was able to reset the seconds and minutes, without resetting the hour counter. I sometimes get questions about why the hour counter reset is sluggish, and the speed that it resets is directly related to how quickly you press the pusher in.
So this is an overview of the basic operation, and in subsequent posts I'll go over some of the common problems seen with this function.
Cheers, Al
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