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Double roller 992 info Please

Billman730

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Jul 29, 2010
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My OLDER 992's have the Double Roller engraving on the movements. And on some of those movements there is a drum with 3 or 4 screws through the washer on the movement. I have one DR engraved movement with just a single center screw thru the washer.

My question is if the drum has only a center screw and a washer has the DR drum been removed and a swapped part put in its place for some reason?
 

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ben_hutcherson

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Jul 15, 2009
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Bill,

The part you're referring to-the crown wheel-has nothing to do with whether or not the watch has a double roller.

Rather, double roller refers to different designs of the escapement-specifically how the balance wheel interacts with the pallet fork. I have some photos showing the differences in construction between the two which I will post later today.
 
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doug sinclair

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I suspect the parts you are refering to would be the smaller of the two flat steel gears that turn when you wind the watch. That wheel is called the crown wheel. The "washer" you refer to I suspect is the crown wheel core, or hub the crown wheel turns on. The older 992s had two screws holding the crown wheel core, while the later ones (as in your picture) have only one screw. The double roller has nothing whatsoever to do with these parts.
 

ben_hutcherson

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Okay, here's the long explanation now that I'm on a computer and not an iphone :).

Okay, here's the bottom of a single roller balance wheel

134.jpg

And the corresponding pallet fork

135.jpg

If you look at the bottom of the balance wheel, you'll see a pin sticking straight up from the round part. The round part is called the roller table. The pin is normally made of ruby, and is called the roller jewel or by several other names(ruby pin, impulse jewel, impulse pin, etc). This is the part that interacts with the slot in the pallet fork. It's responsible for transferring the "kick" from the pallet fork to the balance wheel(making it move), and in turn moving the pallet fork in order to allow the gear train to advance.

You'll also notice that, sticking up from the pallet fork is a brass pin, called the safety pin or safety dart. What this does is prevent the pallet fork from moving any time the roller jewel isn't directly in line with the pallet fork. If this weren't present, a jolt to the watch could cause the pallet fork to jump to the other side while the roller jewel is out of the way, leading to a condition where the roller jewel is resting against the side of the pallet fork rather than going into the slot. This will stop the watch, and would require removing the balance in order to correct it.

A double roller, rather than combining both the impulse and safety functions, separates them into two separate parts.

Here's a double roller balance wheel

137.jpg

And the corresponding pallet fork

139.jpg

Notice that the roller table is in two "levels", and that the safety dart sticks out parallel to the balance wheel rather than being perpendicular to it as in the single roller.

In the double roller, the part with the impulse pin serves only one function-to deliver the impulse to the balance wheel. Similarly, the lower roller-called the safety roller-serves only to keep the pallet fork from moving when it shouldn't.

The advantage of a double roller is that it's more resistant to going out of action with a shock.

Also, many double roller watches will use something other than hardened steel for the upper(impulse) roller. I know that the 992B is this way, as is the Waltham Crescent Street in my pocket at the moment. Presumably, this particular alloy-whatever it is(it's a bronze color)-is lighter than steel and thus reduces the total mass of the escapement.
 

Dano4734

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Absolutely wonderful explanation .. I was just going to ask what exactly does a double roller means and you answered it for me before I asked. Wonderful thank you for that information ... I always heard that term but never understood it.

:) Dan
 

terry hall

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Apr 12, 2001
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And on some of those movements there is a drum with 3 or 4 screws through the washer on the movement.
What Doug describes on the winding wheel is two screws (on the first model 992, regardless of single or double roller)....

What the original poster is describing is 3 or 4 screws....:confused:

Please clarify....

Also...

Great post there Ben!!!

is the single roller pallet not called a 'poised pallet'?
 

Billman730

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Thanks for the great info. I was mixed up as to what the term double roller was referring to. Should I be able to look at a double roller while in a watch movement and identify it?
 

Kevin W.

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Ben,:):) great explanation and with pictures helps alot.Thanks.
 

ben_hutcherson

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Terry and Dan,

Thanks for the complements.

Terry,

The fork I show for a single roller is in fact a counterpoised fork. This one is out of an 18s Elgin, and, as far as I know, all 18s FP Elgins with an inline escapement have this style of fork(at least it's present on both my SN 1986, and on my 1928 18 size).

It should, however, be mentioned that one finds counterpoised forks on both SR and DR escapements.
-> posts merged by system <-
Bill,

Whether or not you can tell without taking anything apart depends a lot on the movement design. I just looked at my Model 1908 Crescent Street, for example(only one I had handy), and, from just the right angle under strong light and looking through a glass I can see the safety roller and tell that it's a double roller.

In many cases you can tell by taking the movement out of the case, removing the dust ring(if present) and looking under the balance again with strong light and a glass. This works best on 18 size watches, in my experience, and can be hit or miss on 16 and larger.

The easiest way to see is to take the balance out of the watch and look at the underside of it, although I realize this isn't always an option.
 

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