Tachy running fast ?

rchuah · Dec 11, 2020

Hi guys. I've recently noticed that the tachy runs considerably faster than the timekeeping on my Speedmaster DSOTM.

I have a habit of synchronising the tachy hands and the second hands at 12 o clock, and let all the hands run in sync. But after 12 hours or so, the tachy seconds hand will be ahead by 10 to 12 seconds. I suppose that's not normal? It that an easy fix Ie. Calibration?

Thanks!

amfalconer · Dec 11, 2020

Can't help you with why this is happening, just wondering, are you running the chronograph all the time?

rchuah · Dec 11, 2020

amfalconer
Can't help you with why this is happening, just wondering, are you running the chronograph all the time?

I wouldn't say all the time but most of the time yea. I would imagine that both the chrono/tachy and the timekeeping hands are driven by the same hairspring via a fixed gear. Not sure how one can be faster than the other

Archer · Dec 11, 2020

R rchuah
I wouldn't say all the time but most of the time yea. I would imagine that both the chrono/tachy and the timekeeping hands are driven by the same hairspring via a fixed gear. Not sure how one can be faster than the other

In normal operation, it can't.

Off the top of my head, the only thing that comes to mind is that the watch has received a shock of some kind, and the "tachy" hand (the chronograph seconds recording hand) has slipped relative to the rest of the movement.

I would suggest repeating this, but lay the watch down flat dial up for a day while letting the chronograph run, and see if they are still synced.

Dan S · Dec 11, 2020

Archer
In normal operation, it can't.

Off the top of my head, the only thing that comes to mind is that the watch has received a shock of some kind, and the "tachy" hand (the chronograph seconds recording hand) has slipped relative to the rest of the movement.

I would suggest repeating this, but lay the watch down flat dial up for a day while letting the chronograph run, and see if they are still synced.

I also had this thought. Wouldn't this also cause the sweep hand to reset incorrectly? Or maybe it is the running seconds hand that is loose.

Archer · Dec 11, 2020

Dan S
I also had this thought. Wouldn't this also cause the sweep hand to reset incorrectly? Or maybe it is the running seconds hand that is loose.

Not referring to a loose hand. In a vertically coupled chronograph, the "driving" of the chronograph wheel is not done via teeth like it is in a horizontally coupled chronograph. It is done via a friction disk (clutch of a sort) on the chronograph wheel assembly. This could slip given an impact - depending on the direction of the impact it may slip one direction or the other.

Cheers, Al

Dan S · Dec 11, 2020

Archer
Not referring to a loose hand. In a vertically coupled chronograph, the "driving" of the chronograph wheel is not done via teeth like it is in a horizontally coupled chronograph. It is done via a friction disk (clutch of a sort) on the chronograph wheel assembly. This could slip given an impact - depending on the direction of the impact it may slip one direction or the other.

Cheers, Al

Thanks Al, if I understand correctly, that type of slippage event would result in a discrete, instantaneous offset. It could serendipitously be reversed by an opposing impact, but in general that wouldn't happen. So would the sweep hand reset incorrectly if such slippage occurred?

Archer · Dec 11, 2020

Dan S
Thanks Al, if I understand correctly, that type of slippage event would result in a discrete, instantaneous offset. It could serendipitously be reversed by an opposing impact, but in general that wouldn't happen.

Yes, so error could be fast or slow.

Dan S
So would the sweep hand reset incorrectly if such slippage occurred?

No. The chronograph is reset by the hammer impacting a can. The flat part of the cam is what determines "home" position when reset, and there is a fixed connection between the came and the shaft of the chronograph wheel that the hand is attached to. The only way those can move relative to each other, is for the hand to slip (tube or rivet issue) or if the wheel itself is somehow damaged.

Tachy running fast ?

rchuah

amfalconer

rchuah

Archer

Dan S

Archer

Dan S

Archer

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