The motor barrel is an old American system used in the highest-grade railroad watches like the Illinois Bunn Special and the Hamilton 950. The motor barrel allows the mainspring barrel to ride between jewels when the watch is running. The purpose of this arrangement was that if the spring breaks, destructive recoil forces would not be applied to the gear train.

The Illinois Bunn Special and Hamilton 950 are two great examples of high-grade pocket watches that use a motor barrel in their movements

A movement showing barrel arbor

A ratchet wheel in place

Here’s a complete mainspring assembled showing the top jewel and three small screws that hold the jewel in place. The three small screws have always been a problem with this barrel since the threads are very fine and they can easily be stripped.

The system is comprised of many parts compared to a traditional barrel.

All parts of a motor barrel are shown

Traditional barrel and its parts

A motor barrel in a standard size man’s movement

A beautiful Hamilton 950 showing its motor barrel

CHARLEY PHOTO OF THE WEEK: When exactly do the dog days of summer start? Charley would like to know. In the meantime, a fun fact about Charley is she loves sitting in the pantry so she can see what’s going on in the kitchen, basement and workshop while also simultaneously being beneath everyone’s feet.

Check out some previous week’s posts