Gearless Differential Mechanism

This is a demonstration of a drive-axle differential mechanism that uses no gears at all.

The idea was the result of idly speculating whether the “Chinese South-Seeking Chariot” had been produced before gears were invented, and if so, how the necessary differential mechanism could have been constructed.

On the model, the inner end of each half-shaft axle rod has a pulley just inside the differential cage. Each of these pulleys drives, via a drive belt, another pulley mounted on a short transverse (i.e. parallel to the half-shafts) axle rod journalled at the outer edge of the cage. These two rods are diametrically opposed to each other, and also each carry another pulley. These latter two pulleys are in turn connected to each other, via a crossed drive belt, so that they rotate in opposite directions. All pulleys are fixed to their axle rods. The differential cage is rotated by another pulley fixed to the side of it, which is driven by a drive belt off a pdu for demonstration purposes. It works pretty well, passing with flying colours the “finger on the tyre” test.

The pulley sizes on this model are dictated by the available sizes of Meccano drive belts, so it could be made much neater by using non-standard parts. Friction of the drive belts on the pulleys, and particularly the friction of the crossed belt on itself, limits the practical application of the arrangement, and the difficulties of precisely measuring and matching the diameters of pulleys would dictate how accurate a “South Seeking Chariot” would be. However, if the Chariot was, as some believe, produced in ancient times just as an “executive toy”, then perhaps this would not have mattered. The pulley arrangement could then have been used (together with a crossed-belt reverser on one of the half-shafts) with no-one at the Chinese Emperor’s court losing their head for producing an unreliable direction indicator!

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