8 Day Gear Train

[jltrent]

An 8 day going train that I cooked up (or possibly re-invented, as I'm sure someone else has come up with something similar in the history of horology)...


Assuming a pendulum with a 2-second period, the revolution of the arbors are as follows:


Arbor                                                         Wheel    Pinion       Seconds/Rev        Hours/Rev
===========================    =====   =====   ===========     =========
Escapement  (Seconds Arbor)                     30         12                 60
Arbor 5                                                        90         12                450
Arbor 4 (Minutes Hand Arbor)                      96         24              3600                   1.00
Arbor 3                                                        96         30            14400                   4.00
Arbor 2  (Hour Hand Arbor)                         90         40            43200                  12.00
Arbor 1  (Day Arbor)                                    80                         86400                  24.00
===========================    =====   =====   ===========     =========



This assumes the following barrel (attached to arbor 1) and chain characteristics...


Barrel Dia            Chain Length         Run Time        Run Time
  Inches                    Inches                   Hrs                Days
==========   ===========   ========   ========
   2.65                          67                     193                 8.05
==========   ===========   ========   ========


This was designed such that the going train consisted of arbors that rotated once a day, once every 12 hours, once an hour, and once a minute, thereby eliminating the need for the motion work (ie, the 12:1 reduction typically found between the minute and hour hand)...

[Edited to use more accurate terminology.]

[steve323]

jltrent,

That does seem like a lot of teeth, especially for a wooden gear clock.  Many of them could be reduced.  For example, wheel 3 drives wheel 2 with a 30:90 pinion:wheel ratio.  10:30, 12:36, or 15:45 ratios would also work.

I like the simple way you describe the gear ratios.  Here are the teeth counts for the clock I plan to build whenever I can find the time:

Arbor                                         Wheel    Pinion       Seconds/Rev        Hours/Rev
====================    =====   =====   ===========     =========
Escapement                                  18         10                  36
Wheel 6                                         50         10                180
Wheel 5                                         50         12                900
Wheel 4 (Hour Arbor)                    48         12              3600                   1.00
Wheel 3                                         48         15            14400                   4.00
Wheel 2  (12 Hour Arbor)              45         20             43200                 12.00
Wheel 1  (Day Arbor)                     40                          86400                 24.00

This design adds one more wheel to keep the tooth counts lower.  It does lose the second hand and the escapement rotates "backwards".  The 18 tooth escapement should be easier to build and also has a slightly smaller diameter.

The reason I like these ratios is that all gears can have the same spacing.  The ratios are 10:50, 12:48, 15:45, and 20:40.  The tooth counts always add up to 60 so the spacing can be the same with the same tooth pitch.  I may use a double size tooth on wheel 1 (10:20 ratio) since this wheel holds back the entire driving weight.  There is a lot of pressure on this gear. 

Steve

[jltrent]

Steve,

Thanks for the input...

Am trying to recollect why I used so many teeth when I could have reduced the ratios, and I think my motivation was similar to yours:  I wanted to have a consistent diametral pitch on all the gears, for a more pleasing look.

The other factor that falls out from a consistent pitch is that a wheel/pinion pair with the same total number of teeth has the same spacing between their arbors.  So, in order to have the hour hand arbor drive arbor 3, and in turn drive the minute hand arbor, the distance between arbor hour-to-3 and 3-to-minute must be the same.  By having wheel/pinion counts of 90:30 and 96:24, I met the requirement (ie, both total to 120 teeth).  If wheel/pinion pairs in this instance didn't have the same total number of teeth, I'd have to adjust the pitch accordingly for one wheel/pinion pair...

Additionally, I was trying to avoid small pinions, as my intuition was telling me that this reduces the accuracy needed for the shape of the cycloidal teeth.  Ie, the rolling action of the teeth against each other is minimized...

Ed's 8 day train is interesting in that it involves more favorable ratios of wheel-to-pinion...

Arbor                                                       Wheel      Pinion       Seconds / Rev          Hours / Rev
========================          =====    ======   ==========          ========
Escapement Arbor (Seconds Arbor)         30            16            60
Arbor 4                                                    60             15            225
Arbor 3                                                    60             15            900
Arbor 2 (Minute Hand)                             60             15            3600                        1.0000
Weight Arbor                                           60                            14400                       4.0000
========================          =====    ======   ==========          ========


That is, the 60:15 pairs (4:1 ratio) are far better than, say, the 90:12 (7.5:1 ratio), in that they're easier to drive individually.  Additionally, his train doesn't collapse the gear train and the motion train, which should put less pressure on the weight arbor...

Interestingly enough, Ed's train also leaves the escapement wheel moving counterclockwise, but this apparent disadvantage is turned into an advantage by the addition of a motion train gear pair back to the main arbor (ie, placing the second hand on the same axis as the minute and hour hands), leaving the second hand rotating clockwise again.  With an 18 tooth escape wheel on the 8 day train you proposed, this same concept could be applied and a second hand could actually be introduced.  (Ie, a motion train consisting of a 30 tooth wheel on the escapement wheel arbor driving a 18 tooth pinion on the seconds hand arbor).

Much to consider.  But an 8 day wooden works train definitely appears feasible, and provides more motivation than the high maintenance 1 day train...

Jon