Chronographs - Drain power reserve?

[fraggle42]

I was thinking about this because I read the comment ("chronos drain power") and thought "The mainspring cannot somehow "unwind faster" or the spring "uncoil faster" as everything is connected by gears to the balance wheel, the gears are fixed in relation to each other by meshing teeth, so WTF are they on about?".

 

Excuse my ramblings if this is of no interest to you

 

 

Having thought about this, unless they are massively simplifying things, or there are multiple clutches at work somewhere, it's simply cobblers!

 

 

A fully wound mainspring can unwind through 'X' rotations, with 'Y' torque available at the start, and 0 torque available when it is completely unwound.

 

The watch movmement requires 'P' torque to run it. The chronograph requires an additional 'Q' torque to run it.

 

 

'P' plus 'Q' (if the chrono is running) is the total amount of power required to run the watch.

 

The power supply has somewhere between 'Y' and 0 torque available, dpending how fully wound it is.

 

If the chrono isn't in use, when 'Y' is greater than 'P' the watch runs.

If the chrono is in use, when 'Y' is greater than 'P'+'Q' the watch (and chrono) runs.

 

 

Consider the case with the chrono running, the watch will run until Y = P+Q. (technically Y < P+Q but we know that the system is only loses power (it unwinds) and making this tiny change doesn't affect the outcome and stops my brain imploding)

 

If you now stopped the chrono, Q can be removed from the equation:-

(Y)-Q = (P+Q)-Q

Y-Q = P

and If Y-Q = P, and Q is positive and not zero, Y must be > P, meaning that there is enough torque available to run the watch part by itself and the watch will start running again.

 

So I would say it is better to say that running the chrono places a greater demand on the mainspring, but if you stopped the chrono at the exact moment the mainspring could not provide the required torque of the watch + chrono, it would remove the extra power demanded by the chrono and so carry on running and run for just as long as if you had never, ever used the chrono.

 

I.E. if your watch has a 48 hour power reserve, playing about / using the chrono as much as you like within the first 36 hours will not change anything, if you then stop using the chrono the watch will still run for the same length, 48 hours.

 

Or put another way, when you fully wind a main spring it has way, way more power than is required to just run the watch, and the chrono is just putting some of that excess power to a good use rather than letting it go to waste. If the excess is not used it is just wasted trying to throw the balance around with far more energy than is needed, with an end result that the energy is converted into heat in the balance spring, impulse pin, oil, wear and tear, etc.

 

[billyfras]

I've been tempted to ask this question before. But you have put forward a very good argument. I was with you the whole way.

[Rolexman]

Isochronism

[sneed12]

You are correct, running the chronograph on a 7750 does not drain the power reserve any faster. It's pretty obvious when you look at how the 7750 operates. This typically does, but may not always, apply to other chrono movements as well. 

[chronoluvvv]

it's a little past 2 so I'm not @ my best

but from what i remember, the stopwatch on the 7750 is always running

 

using the stopwatch start/stop/reset buttons simply engages the gear and lets it catch instead of slipping all along

the 7750 movement doesn't give a flying [censored] if you run the stopwatch or not

it's too busy keeping COSC level time for a fraction of the price while you wasted a full twenty precious seconds of your life reading this

[krpster]

100% correct assessment. Nice to see it written up so well. Being an engineer it made me smile

[fraggle42]

Thanks guys for the confirmations.

 

I'm a software (and hardware) engineer and when someone says something like this that isn't right, part of my mind just goes "Uh? Something not right!" and I'll work through it myself for fun.

[billyfras]

So with this being confirmed, I can now run my chrono without concern. Nothing quite like a sweeping chrono hand to me.

[panerai153]

Here is a question from a totally non-engineer type person. while the power reserve is not affected, what about wear on the chrono mechanism? When the chrono is disengaged, do all the chrono gears stop, or they all still turning, but not engaged? My question would be if they are in fact not turning, what about increased wear to the chrono mechanism if it were left "on" all the time? I know that with some chronos the recommendation is to use them as little as possible, although I have never seen this admonition with genuine watches running an ETA 7750.

[fraggle42]

I would imagine with any gen movement, or serviced rep movement, the chrono can happily be left running and the chrono gears, etc, will just wear at the same rate as the timekeeping gears, etc.

 

I don't imagine in anything but the cheapest chinese clones would they make the chrono components out of a lower quality metal than the timekeeping components.

 

However the part of the mechanism that starts and stops the chrono it will only be designed for so many operations, so if you are constantly starting and stopping the chrono these parts will wear quicker than intended and will wear out before the rest of the watch, and in the case of bad or cheap rep movements with not so good tolerances and cheaper metals being used, (but exactly the same design and thickness of materials as gen), they may fail very quickly.

 

I don't have any experience and am not yet skilled enough to know common problems in rep 7750s for example, but if I get a rep chrono movment I'll try to limit myself to use it once or twice a week. (and I mean pushing the buttons, I'm happy to leave it running)

 

If I know I'll be needing to time something I usually make sure I'm wearing my Bulovia 1/1000s chrono quartz watch, or one of the other gens.

[sneed12]

Here is a question from a totally non-engineer type person. while the power reserve is not affected, what about wear on the chrono mechanism? When the chrono is disengaged, do all the chrono gears stop, or they all still turning, but not engaged? My question would be if they are in fact not turning, what about increased wear to the chrono mechanism if it were left "on" all the time? I know that with some chronos the recommendation is to use them as little as possible, although I have never seen this admonition with genuine watches running an ETA 7750.

 

When you start the chrono in a 7750, two things happen:

 

1) the tilting pinion moves over and engages the center sweep seconds hand

2) the brake that stops the hour totalizer from rotating is disengaged

 

The tilting pinion is always rotating, so starting the chrono doesn't make it wear any "more". The hour totalizer clutch is slipping when the chrono is NOT running, so arguably when the chrono is running the wear is less.