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Stage 1 |
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The first stage of the astronomical section is mounted horizontally
and is designed to be driven by the output shaft from the timepiece. Its
function is to convert mean solar time to sidereal time. When the input shaft turns 365¼ times the output wheel must turn 366¼ times. |
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The input shaft is attached to a large bracket which supports a transported
gear train. When the input shaft is rotated it drags pinion 8 around wheel
115. Pinion 8 cannot rotate, since it is connected to wheel 24 which is
meshed with the worm. Consequently wheel 115 is forced to rotate with the
bracket. At the same time bevel wheel 18 is taken around bevel wheel 17, which
cannot move as it is bolted to the frame. The result is that wheel 18
rotates, causing the worm to drive wheel 24 which turns pinion 8. Thus as
pinion 8 drags wheel 115 round it also rotates slowly, adding a fraction of a
turn to wheel 115’s motion. Consequently wheel 115 and the output shaft rotate fractionally faster
than the input shaft. |
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The calculation is: For one
revolution of the input shaft and bracket, wheel 18 is dragged once around
static wheel 17, so wheel 18 makes 17/18 turn, as does the worm. The worm
has an equivalent of one tooth, so wheel 24 makes 17/18 x 1/24 turns. This motion
is transmitted to pinion 8 which therefore produces 17/18 x 1/24
x 8/115 turns of wheel 115.
This is 0.00273752 turns. This must be added to the one turn due to the
movement of the bracket, so for every turn of the input shaft there is
1.00273752 turns of the output shaft. Hence for 365.25 turns of the input shaft there
are 365.25 x 1.00273752 = 366.25 turns of the output shaft. |
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Stage 2 |
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The purpose of stage 2 is to connect the horizontal output shaft of
stage 1 to wheel 360 of stage 3. The overall ratio is 1:1 i.e. wheel 360
makes one revolution for each output turn of stage 1. This stage would be unnecessary if the axis of the main astronomical
section were horizontal. Some people believe that this was in fact the case.
A scale model made for the Our replica clock is based on the design of the model by Eric Watson
and so copies its layout. Wheel 48 is on the output shaft of stage 1. It meshes with pinion 12
which is connected to bevel wheel 36. The latter meshes with bevel pinion 12
which connects to wheel 30 and this drives wheel 360.
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