Do bumpers have smaller balances compared to regular automatic movements of the same general size, in order to make room for the rotor? I can't judge from photos and I don't own any bumpers to compare-- but that's the impression I get looking at these movements.

 

Can I ask what prompted this question?

So the movement you have shown is a 332, so that is a 30.10 mm diameter. I don't have that one on hand, but I do have a 354 which is 28.10 mm in diameter, and measuring the rim OD on the balance, it's about 8 mm in diameter (this does not include the screws).

If I compare that to a Cal. 550 series, the movement is 27.90 mm, so a little smaller, and the OD of the smooth balance is approx. 10.9 mm in diameter.

Cheers, Al

 

Depends on the movement you want to compare the balance wheels on. Many automatic wind movements (particularly full rotor movements), the rotor operates in such a way that it passes OVER the balance wheel. So size of the balance isn’t an issue in that regard.

 

Thanks for taking the time to take the measurements and the info.

What prompted the question? Nothing specific...I was looking at pictures of different bumper movements and I noticed they required a lot more real estate than the full rotor type and that the movement design appeared cramped and smaller. I assume the the smaller size balance affects amplitude, and consequently, positional variance?

 

Well this is a very complicated thing to answer, and I don't think you can make a simplistic statement like that and call it accurate. Generally you want a balance with a larger moment of intertia, and a smaller mass. I = m * r2.

So it really depends on a number of factors that include things like what the materials of the balance are (steel, bi-metallic, nickel gilt, beryllium copper), what the balance configuration is (smooth, adjustable mass of some kind with screws or weights), what sort of drag is involved when the balance is rotating, what the balance spring configuration is (material, shape), etc.

Cheers, Al

 

Diameter of the balance wheel would not be a factor in amplitude of the balance wheel. And a fully adjusted movement with a smaller balance wheel should deliver the same performance characteristics as one with a larger balance wheel.

I have uploaded a picture of my Rolex Prince Observatory chronometer (albeit a manual winder), and it has a balance about the same size as the subject Omega. You will note that it is adjusted to 6 positions.