Sessions Westminster Chime No.3

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.

email:  bobby@greenfieldstore.com


NOT FOR SALE
Sessions Westminster Chime No.3
Sessions Westminster Chime No.3
Sessions Westminster Chime No.3
Sessions Westminster Chime No.3
Sessions Westminster Chime No.3
Sessions Westminster Chime No.3
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.......
Sessions Westminster Chime No.3
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.
Sessions Westminster Chime No.3
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:
  • Don't assume that the clock is assembled or adjusted properly as received. If the clock is not working, there is a good possibility that someone has tried to fix it and given up, leaving it improperly setup.
  • This movement is known for having rough pivots, so plan on taking it apart and doing the required service.
  • There is supposed to be a small spring between the pin drum and the drive arm. The spring must fit inside a recess in the drive arm. If the spring has been replaced with one that's too large to fit into the recess, or one that is too long, the pin drum will not be able to shift far enough out to go from chime to strike mode.
  • There is a locking pin attached to the back plate directly under the pin drum. After the hour chime is complete, the pin drum must slip freely down over this locking pin without binding as the drum shifts inward and into strike mode. The pin location is adjustable, but should not require adjustment unless it has been "messed with". This pin also controls the position where the pin drum stops.
  • If chiming stops with the last hammer still raised, or if a chime hammer begins to raise during the "warning run" the position of the locking pin (the pin between the back plate and the drum NOT the drive pin) may be moved slightly to advance or retard the position where the chime disks come to rest.
  • The chime or pin drum drive arm has an attached pin that engages one of the holes in the outermost disk (It does not matter which hole). The drum MUST be in STRIKE mode before the arm is adjusted - that is, the chime drum MUST be shifted "in" and held by the locking pin on the back plate. Adjust the drive arm so that the drive pin is directly over -not in - one of the holes (does not matter which) in the 4th. disk. The tip of the pin should just clear the 4th. disk as the arm turns the 5th disk while the other four are locked.
  • Any adjustment to the pin drum locking pin will require that the drive arm also be readjusted.
  • It is most important that the chime drum (the inner 4 disks that move as a unit) be able to shift freely onto and off of the locking pin and the drive arm pin without binding, and that the drive arm pin not touch the 4th disk during striking but soundly engages the 4th disk during chiming.
  • The small cam on the back plate shifts the pin drum into chime mode during the 1st. qtr. AFTER the hour. At the 12:00 position the cam follower should be positioned to just begin its journey up the slope. The drop off point is NOT important because the pin drum will stay in chime mode until the hour strike is complete. The cam follower should drop of the cam sometime during the 2nd. qtr. and may be accompanied by a little thump sound. The important thing to remember is that this cam only functions during the first 15 minutes of the hour.
  • This clock has TWO racks and two snails. They work together to count the TOTAL number of teeth to be gathered to advance the chime/strike drum the required amount. The short rack adds the required number of teeth for the quarter chime (1 tooth for every 4 notes) and the long rack adds the required number of teeth for the strike count, BUT the long rack is gathered first and the short rack last. The first part of the long rack will be gathered as the 4th. qtr. chimes are playing, then the last part of the long rack and all of the short rack will be gathered as the strike is sounding.......However, on the quarters where no strike takes place, all of the teeth are gathered on the short rack. The key to understanding this action is to NOT think of the racks as being chime or strike but just two racks who's gathered teeth do exactly the same thing - advance the drum one position
  • The star wheel on the minute hand has one arm that is a little longer than the other three. The lifting parts must be adjusted such that only the short rack is released at the 1st., 2nd., and 3 rd. quarter hour, and BOTH racks are released on the hour only. There is a popular web site that INCORRECTLY states that both racks are released every quarter.
  • The tail of the short rack on some models can be easily bent out of shape, this can cause the gathering pallets to bind on the ends of the rack teeth and stop the train. The adjustment should be obvious.
  • If the chime/strike sequence does not run to completion for some reason, the small snail can bind on the rack tail and cause the clock to stop, so if the clock seems to have stopping issues, it could really be a strike did not run issue.
  • When installing the large hour snail, it is important to have some running clearance between it and the small snail (which turns at a different speed). The hour tube must have some "end shake" or the clock will not keep running.
  • If the strike train fails to run intermittently, make sure that the levers that raise the hammers have not been bent or are otherwise able to hang on one another as they shift side to side.
  • When assembling the strike hammer unit, a small brass spacer is used with the group of hammers and the group of small activating levers. One spacer is thicker than the other and if they are not in the correct place, things just won't line up right.

  • If you know of any other tips that should be included here, please let me know and I will add to this list.
General disclaimer: the information on this page is believed to the writer to be correct but is presented with no guarantee. Please send any additions or corrections to:

          bobby@greenfieldstore.com