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Discussion Starter #1
I've overhauled a couple of Seiko 8800 stopwatches now, and have some questions. I've got an 8800c running at 18000bph and an 8800d which ticks over at 36000bph. They are both the same base movement, and looking at the parts lists for the two, it seems that the only differences to the movements are the balance bridge, balance and hairspring. I would expect a higher beat movement to use a stronger hairspring and a different escape wheel too, but this isn't the case.

What I notice is that the high-beat 8800d runs at a much lower amplitude, around 190 degrees at full wind, whereas the 8800c runs about 275 degrees. They both show negligible beat error and rate variance in the two positions that I test them, dial up, and hanging from the neck, which is crown up on the 8800c and crown down on the 8800d.

I know that there are many things that can lead to low amplitude, but both movements were pretty clean inside when I started, and since these don't run all the time like a watch, the movements didn't show much wear or damage to my inexperienced eye.

Does anyone have one of these 36000bph 8800d stopwatches that they could share amplitude numbers? Mine is one of the aluminum cased models with crown and pusher gaskets and a screw down case back, but they also come in a similar case to my 8800c. Maybe they just naturally don't have a large amplitude due to using the same pallet fork, escape and mainspring as the low-beat movements?
 

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The high beat is the base calibre, with a strong by design mainspring, so if the same mainspring is used on a low beat movement, it's(the movement) using less energy - as it's beating at half speed. Therefore, some of the excess mainspring torque will be transferred to the impulse jewel, giving higher overall amplitude.
 

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Discussion Starter #3
That's pretty much what I had figured. The high beat must use a stronger hairspring to allow the balance to return to its rest position faster, which would result in a lower amplitude.

Are there any other notable movements that have a high and low beat version, and are there more differences than just the hairspring?
 
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