Explanation of Bi-Metallic
by Tuan Ngo & Michael Friedberg
About a week ago, a message was posted on the Regular Forum asking about the differences between bi-metallic vs. mono-metallic vs. Gyromax balances. I had posted a similiar question in July, and received what seems to be a brilliant analysis by Tuan Ngo. Taking the liberty of publishing his e-mail message -it shouldn’t be buried in obscurity- here goes:
Back more than 50 years ago, the primary reason for the bi-metallic balance is to solve the temperature dependancy of the balance/hairspring combination: with the best spring (unbreakable=reliable) made out of steel (rusty and temperature dependent=bad) and compensated by a bi-metallic balance whereas the moment of inertia of the balance changes with the temperature thereby offsetting the increase (or is it a decrease?) in elasticity of the hairspring. The amount of inertia increase is adjusted by the relative position of each screw in its thread and the material (i.e. density) of the screws itself (brass, gold and platinum): for example the screw closest to the open end of the split rim would have the most temperature dependency effect. Because of this temperature dependency, many watchmakers/metallurgists have tried to find the proper combination of material for the balance and hairspring combination to solve temperature dependency, reliability (hairspring reliability) and rustless type spring problems. Many combination have resulted with compromise on these 3 factors.
In the 30’s, a German spent a lifetime and found the proper combination Glucydur balance with Nivarox hairspring which overall is the best (so far!). Therefore screws are not required anymore for temperature compensation. But the 5/6 positions adjustment would have depended entirely on the manufacturing accuracy of the balance, in addition as mentioned on a previous post, if you adjust the hairspring length to regulate the watch you slightly change the center of gravity and therefore affecting you position adjustment!
The Patek Gyromax I believe solve the above remaining problems: You adjust some of them to have perfect balance (position adjustment), and all of them synchronously to regulate the movement! As a matter of fact I have a 5015 (using the 240 movement) which will hold accuracy to <30S/month average and <2s spread per day (due I believe, to bump in the wrists). I have not tested a 5038 (same movement with perpetual calendar) yet. I have seen much lousier accuracy from 2 AP watches (+/5 s depending in position). A Breguet marine chronograph fares better than the AP but still is position dependent (+/- 3S depending upon position). By the way all the above watches are "adjusted in 5 positions and isochronism".