Sessions Westminster Chime No.3, circa 1930.
This is a Sessions 2-train Westminster chime mantel clock. The chime and strike functions are combined and powered by the same main spring. The chime and strike action is achieved through an interesting, (and for some confusing) systems of cams and wheels. This model has an undeserved reputation as being a real bear to repair. It's actually no more complicated than any other chime clock, and much less complicated than many. The pictures at the bottom of this page show the unusual mechanism and the captions explain its operation in some detail. I recently finished rebuilding the movement and refurbishing the case. I believe that everything is original except the hands, which I believe are correct replacements taken from another Sessions "parts clock". The clock has proven to be reliable, but does tend to run a bit fast right after being wound as is typical of many spring-powered American clocks. This unusual clock is now part of my permanent collection.
NOT FOR SALE
The photo above shows the 5-disk pin drum of the combined chime/strike train, the oversize main spring, and the additional chime drive wheel and pinion. The four disks closest to the back plate activate the chime tones. The fifth disk activates only the strike tone. During "chiming", all 5 disks turn but the fifth disk is held away from the strike hammer and is "silent". When the chime sequence is complete on the hour, the pin drum shifts in (toward the back plate) where the first 4 disks slip off the drive pin and become stationary while the fifth disk moves into position to activate the strike hammer. The fifth disk then continues turning to strike the hour count. During the first quarter after the hour the durm shifts out again, reconnecting the four chime disks to the drive pin, and the "strike" disk is again positioned so it can't engage the strike hammer and on it goes.......
The photo above shows the small cam attached to the "back end" of the minute hand arbor (shaft). The cam is shown as it should appear at 12:00. This cam engages AFTER the hour strike is complete. During the first quarter hour the cam shifts the pin drum out of "strike" position and into "chime" position, which transition must be complete before the first quarter chimes. There is a small locking pin under the drum which is arranged such that once the drum begins to turn, it will stay in the chime position until the clock is ready to strike the next hour count, at which time the locking pin (not shown) lines up with a small hole in the bottom of the first disk, at which point a spring shifts all the disks out of the chime position and into the strike position. Because the pin drum is always positioned just before the first quarter hour chime when the locking pin is engaged, the chime sequence is inherently self-correcting with no additional hardware required.
Probably the most unusual thing about this clock, aside from its having only a single train to perform both chime and strike functions, is that it has TWO racks and TWO snails (see photo above). The "long" rack is positioned by a normal 12-section snail, and the "short" rack is positioned by a 4-section snail. Much of the confusion about how this clock operates comes from applying conventional thinking to an unconventional situation. Because the long rack has 12 positions, one might logically assume that it is only used to count the hour strikes, and that the short 4-section rack is only used to count the quarter chimes. Such an assumption would be incorrect!
This movement is quite ingenious and incredibly simple. The key to understanding it is to stop thinking about "striking" and "chiming" as separate functions because in this clock, they are combined into one! There is only one pin drum to turn and it makes all the sounds. Each rack tooth gathered advances the pin drum one position. It doesn't matter from which rack the tooth was gathered, it will have the exact same effect. What matters is simply how many positions the pin drum needs to advance at each quarter hour.
Each of the four chime disks plays one note when advanced one position, so if one tooth is gathered from the racks, the drum will advance one position and 4 notes will be played on the first quarter hour. Two teeth will need to be gathered on the half-hour to advance the drum two more positions and play 8 notes. Three teeth will need to be gathered at the three- quarter hour to advance the drum three more positions and play 12 notes. On the hour four teeth will need to be gathered to advance the drum four more positions and play 16 notes, plus one tooth for each hour in the hour strike count. The pin drum has a total of 10 positions, 1+2+3+4=10, which completes one revolution, after which the drum slips onto its locking pin and the 4 chime disks stop turning. The fifth disk - the strike disk - continues, advancing one position and striking one note for each remaining tooth gathered from the racks. When all the teeth have been gathered from both racks, the train stops.
The long rack is always gathered first, followed by the short rack, but the long rack is only released on the hour (fourth quarter) so it will have no teeth to gather during the first three quarters. Now it gets interesting (confusing if one apples conventional thinking) when the clock chimes the last quarter and strikes the hour count. Let's assume that the clock will chime the end of the hour and strike 9 O'clock. The drum needs to advance four positions to play 16 notes at the end of the hour plus an additional nine positions for the 9 strikes, or a total of 13 positions. Both racks are released at the end of the last quarter and 4 teeth will be set on the short rack and 9 teeth will be set on the long rack for the required total of 13. All thirteen teeth are gathered, the 16-note end of the hour chime is played and 9 strikes count off the hour. That's all there is to it!
It's interesting to watch this movement in action, and at first it may seem totally counter- intuitive. In the example above, the first four teeth gathered play the 16 note last qtr. chime but these are gathered from the long rack, then the next five teeth gathered from the long rack are the first 5 strikes of the hour count. Now with the long rack fully gathered, 4 teeth from the short rack are gathered to complete the 9-count strike. When both racks are fully gathered the train stops. It's like making a large pot of coffee using 4 cups of water from the bathroom and 9 cups from the kitchen. Once it's in the pot, the pot doesn't really care where it came from
as long as there are 13 cups total.
The comments below may be helpful to anyone to rebuilding or servicing one of these unusual movements for the first time: