James Ferguson, an ingenious experimental philosopher, mechanist, and astronomer, is known for many of his inventions related to astronomy and particularly the Orrery. It is from this invention that the ‘Ferguson Paradox’ originates.
The paradox was how he was able to construct a gear train that had a thick pinion that drove three stacked gears in different directions, to be precise one gear was driven clockwise,one was driven anti clock wise and one stayed still.He did this by giving the 3 driven gears different numbers of teeth but all on the same pitch circle. Bit of a trick this to get the whole thing to work using a much thinner tooth on the pinion, but with some tweaking it is possible. In his final build of the Orrery he actually used gears of slightly different pitches to get it to work in Brass but his initial prototype in wood was able to work with the same pitch gears.
Using this principle it is possible to design a gear train that will give a 12 :1 reduction, so that we can drive the hour hand from the minute hand shaft.This simple model employs this technique to create a Teaching Clock that can be used to teach young children to tell the time from an analogue dial instead of reading from a digital display on their iPhone.
The construction of the Teaching clock can be seen on the drawings , just click the arrow to download the PDF file.
If you require the DXF files click here to download