The rim (A) was solid, rather than being drilled and tapped for balance screws. Poising was done by removing weight under the rim
The balance arms (B), rather than being straight, spiraled out from the center, and were flexible. This was to absorb shocks, so that movements of the heavy balance rim would not be transmitted to the fragile staff pivots. Students of horology will realize that this idea was not new, but rather was lifted from the Wyler Incaflex
The hairspring was a flat spiral, rather than a Breguet overcoil, and was free sprung, meaning that there is no regulator, and the entire length of the spring from collet to stud is free to vibrate. To regulate the rate, Elgin developed an ingenious system that made use of the spiral arms. Rather than changing the free length of the hairspring to regulate the rate as in a standard watch, the Durabalance allows the user to change the moment of inertia of the balance, by moving two weights (D) in or out. By moving the weights inward, the moment of inertia is decreased and the rate increased, and moving the weights out has the opposite effect. The weights move along the spiral arms of the balance, and are coordinated by the use of a flat steel spring (C), which connects them. Think of an ice skater doing a spin - the more tightly she holds her mass, the faster she spins.
It's a very elegant solution, in theory. In practice, it's a little clunky. To regulate the rate, you have to stop the balance and hold it still while you move the spring one way or the other, and despite those nice little marks on the rim in the picture, I've never seen a Durabalance that actually has the marks. It's strictly trial and error.
The first two movements to carry the Durabalance were the 730 and 750, 23 jewel 13/0 Lord Elgins. This watch, a Lord Elgin Fargo, carries a 730.
If you look at the balance cock, you'll see there's no regulator, just indicators of which way to move the spring to change the rate. Also notice the banking plate above the balance. The Durabalance was more flexible than the standard balance, and Elgin took several precautions to keep the wheel from flexing too much.
Note also the little spring holding down the balance cap jewel. This is not a shock spring, but rather simply to retain the unset cap jewel. Unlike a shock resistant jewel system, the hole jewel is fixed. Elgin felt shock protection was unnecessary, and even offered a lifetime guarantee on the Durabalance. Of course, Elgin went out of business 11 years later, so good luck collecting!
The 750 was a sweep seconds movement.
Elgin re-engineered the sweep second drive, presumably based on difficulties with the double sweep second wheels on the 630/716/724. The 750 uses a single sweep wheel and a floating sweep second pinion that rides in a floating jewel that keeps it separated from the tip of the center wheel arbor. The spring both holds the pinion in place and provides the necessary drag to prevent the second hand from skipping. This movement also has a different retaining spring for the cap jewel. Elgin considered these two types of spring interchangeable.
In 1960 Elgin introduced the 760, a completely American made self-winding movement using the Durabalance. I'll post in greater detail on the 760 at another time, but here's one to contemplate till then...
Sometime later (the Elgin Service Manual update from September 1962 is the first mention), Elgin introduced a number of new Durabalance Lord Elgins. The 770 appears to be the replacement for the 730
With the 770, Elgin eliminated the retaining spring for the balance cap jewels, returning to screwed down settings on the cock dome and pillar plate. They also eliminated the banking plate, and the stamping of the grade information on the train bridge. From then on, that information was etched on the mirror-polished ratchet wheel.
Just as the 770 replaced, the 730, so the 775 replaced the 750. This watch is a retirement watch for a steel worker, so it's in a stainless steel case normally reserved for the B.W. Raymond Railroad Chronometer.
The next in the series, the 780, was normally reserved for the B.W. Raymond watches, but I got this one in a retirement watch.
Like the 730A in the early B.W. Raymond wrist watches, the 780 is a hacking movement. There is a spring which rubs on the balance rim when the stem is pulled out to setting position. The tip of the stem pushes it out of the way, and off the balance, when returned to winding position. This allowed the watch to be set to the second.
The next watch, the 784, has so far eluded me. I saw on a few months ago on Ebay, but it went out of my range. Fortunately, the collector who bought it posted it on Watch Talk Forums, and he was kind enough to let me use his pictures. THANKS, DEVIN!!
Devin's watch is a Lord Elgin Futura, usually known as the 'Mystery Dial Watch', because in place of an hour hand, there is a disk with a marker frictioned onto the hour wheel. This is also one of Elgin's 'Horizon Look' watches.
The 784 is in pretty much all respects the same as the 770, except for the shorter 4th wheel post (no second hand).
Now we come to the end of the line for US-built Lord Elgins, the 785. This picture is of a watch with a 785 that I don't own, but rather shamelessly nicked off the Intertubes. Like the 784, it lacks a second hand.
I do own a watch with a 785 movement, however.
It's in a watch labeled 'Elgin' rather than 'Lord Elgin'. I don't know if it's a Franken, or perhaps an artifact of later operations in Elgin IL before the closure of the factory - perhaps they were just using up movements that were already built?
After 1964, Elgin made and sold some Lord Elgins, but they were all just Swiss movements cased over here. No more American-made Lord Elgins.