Elgin Grade 221

Here we can see the center wheel and how it engages, by safety pinion, the mainspring barrel. Next along the train are the third wheel, the fourth wheel (the second hand) and the escape wheel.

This is what they mean by marking the movement "safety pinion". The center wheel pinion is threaded on, rather that riveted. If the mainspring breaks and the mainspring barrel flies in the opposite of normal direction, it rapidly unscrews this pinion which in just a few turns lowers enough to disengage from the barrel. When it does, the train "down stream" is then isolated  and saved from potential damage.

 Assembled and screwed together; the complete center wheel and pinion.

This swing-out case has a stem, and sleeve, then a washer or spacer, then a cap, and then the crown.

Note that the stem on this case is not adjustable.

The movement mounts to a ring that is hinged to the case. The back and middle of this style of case are one solid part.

This movement has a small issue in that when the crown is snapped inward it does not always fully engage winding mode. Vintage American watches like this are negative setting. On these, the snapping in and out is a function of the case entirely. The movement is spring loaded to naturally be in setting mode. pushing the stem in pushes the watch to winding mode and the "snap" holds it there. The snap is a function of a spring in the case neck.

This setup makes cases mostly interchangeable, but to work the stem sometimes has to go further inward, or less inward when snapped in. Cases provide a way to adjust the sleeve in and out so the motion of the stem adjusts to the particular watch.

Here we have a problem. This case is not adjustable, not at all! The sleeve, usually threaded, just sits on a cutout inside the neck of the case. It's position is fixed.

The watch is just going to have to live with the slight engagement issue.

Aside: Swiss style *positive setting" mechanisms, such as modern wristwatches have, avoid this problem by making the snapping mechanism a function of the watch movement. Cases are not as interchangeable, but no adjustment is needed either.

This watch is an Elgin grade 221, 16 size, 15 jewels, made about 1902.