All things being equal, how can a unidirectional winding be more efficient than bidirectional?

[CitizenM]

I read the article "Automatic winding: one direction or two" the other day and glanced at it again recently. It says that many companies, including major movement authorities like JLC, say that their unidirectional winding systems are more efficient in the real world. It also suggests that in the initial testing of the 7750 a few decades back, they concluded the same thing and it wasn't merely a price cutting or thinness measure.

Certainly, JLC's horological knowledge outranks mine, but it seems so counterintuitive. Any expert want to explain how this is possible, assuming arguendo that all things are equal (i.e., that JLC is using the ceramic ball bearings in both bidirectional and unidirectional, etc--what is it about unidirectional winding itself that increases efficiency, if it does do it at all).

[factorof2]

Maybe I should compare titles when I read articles. The only reason I can think of in this instance would simply be that in bi-directional winding the rotor encounters resistance in both directions under motion. The rotor is only dampened in one direction, for instance, when it swings in the movement before coming to rest. Theoretically speaking, the winding energy that this motion provides should be equal to the energy lost by the rotor as it comes to rest. The difference to me comes in when we talk about the energy loss due to friction and resistance due to the mass or inertia of the internal components involved in the self-winding mechanism. If you assume for instance that in bi-directional winding you essentially have an additional copy of the mechanism that winds the mainspring for the other direction of motion then you've doubled the energy loss of the rotor by other means than winding. I think the reason that a cyclotest would indicate differently is that a cyclotest allows the rotor to essentially turn constantly and avoids any sort pendulum-like motion of the rotor, where as we move our arms in many different ways during the day and not necessarily in a smooth fashion.

But I'm not a watchmaker so this is just my take on it from a physics standpoint (which might very well be wrong too)

[dnslater]

I agree with the previous poster. Thin about twisting a small propeller between your fingers and imagine it is loose and spinning freely both ways. Would be hard to flick your wrist and get it going. Now if the same propeller only spins in one direction, you can flick your wrist and use the force of the clutch to give it forward momentum.

[James_]

It might be because bi directional is more expensive to make?

Edit; forgot you said all things being equal. I'm guessing it's for safety. I'm guessing again here, but was the bezel initially designed to time divers air? If so then if the bezel were to be accidentally turned, then the diver would know that it's turned the one direction. So for safety.

[Bronte]

It might be because bi directional is more expensive to make?

I hope that's a joke.

[Torrid]

I hope that's a joke.

I would say the same since bi-directional winding is common among affordable Seikos.

[craniotes]

I would be curious to know the answer myself. Perhaps this should be posed in the Watchmaking forum.

Regards,
Adam

[Bronte]

I would say the same since bi-directional winding is common among affordable Seikos.

Yeah, I more just meant that, in a thread about whether unidirectional or bidirectional winding is more energy efficient, offering as an answer the fact that one is cheaper than the other is not helpful. Especially given that the OP took great pains to specify that everything else is held constant for the purposes of this discussion.

Anyway, synthesizing what factoroftwo and dnslater said, I think this is a pretty plausible argument (in my non-sciency mind): On a unidirectional movement, the rotor gets spinning really fast in one direction. Then, when your wrist twists such that forces push it in the opposite direction, all that energy comes to bear on the mainspring, giving it a really good crank. On a bidirectional rotor, since it's harder for the rotor to get going fast in either direction, it never gets those really hefty cranks.

Scientifically sound or shockingly stupid?

[Dancing Fire]

good Q, b/c i'd own both type.

[Dancing Fire]

It might be because bi directional is more expensive to make?

no,i don't think that would be the reason.my guess is that they can make a UNI rotor movement thinner compared to a BI with the same functions.