The Chinese South-pointing Chariot

Bill Brackett, of the New England Model Engineering Society,  writes:

I found the attachment in a book in the Worcester library and thought
it would be a good project. A friend of mine had CNC Sherline
equipment at the time and was interested in making gears, so we teamed
up to build a South Pointing Chariot. I will send pictures in the next
mail messages.

A mechanism of an entirely different kind is the model of the Chinese south-pointing chariot, as shown in the illustration, Fig. 4/3. A full-scale machine of this kind was used in ceremonial processions in China between A.D. 120 and 250, It seems that its purpose must have been simply to cause awe and wonder in the onlookers since whichever way the chariot was turned the figure on the top always pointed in the same direction. This intriguing result was obtained by means of differential gearing, very similar to that used in the back-axle of the present-day motor vehicle. Possibly pin gears were used in the originals, though this is not certain.
'The diagrammatic sketch, Fig. 4/'3, shows how the wheels would be coupled together if the drive were merely friction of the edge of one wheel on the edge of the next, but this arrangement would be unsatisfactory in practice. A working model can however conveniently be made using standard gears, and for the model with two road wheels 3 in in diameter and 3 1.2 in apart, two pairs of spur
gears are required and four pairs of bevel gears all with the same number of teeth. 'The wheels are coupled up through the gearing as shown in the diagram and the figure on the top is supported on a triangular base which is in turn supported from the frame of the carriage. The triangular platform supporting the figure is made that particular shape so as to provide support for the gear-wheels connecting the operating end of the differential to one of the side wheels, the other side wheel is coupled to the lower end of the differential. The figure at the top is connected to the planetary axle of the differential as shown in the diagram.
This model is well worth making by anyone who has the means of obtaining accurately cut gear-wheels. These are essential, and the amateur should not be encouraged to try and manufacture gear-wheels for himself since they will not have the necessary accuracy.

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