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Decimal Digit Storage

Meccano does not provide us with too many examples of tenfold symmetry, so representing decimal numbers is a challenge. Fortunately we can depend on the following two observations. First, by meshing a 95t gear wheel with a 57t gear wheel we get a ratio of 5:3.  This means that 1/10th of a rotation of the larger gear corresponds to 1/6th of a rotation of the smaller one, and we have available the 6 hole bush wheel. This can provide a detent mechanism to hold the smaller gear in one of 6 positions, corresponding to holding the larger one in one of 10 positions. Second, we note that the ratchet wheel has 20 teeth.  Had not this been a multiple of 10, construction of the mechanism may well have been much more difficult.

Digit wheel
The picture to the left shows the construction of the basic storage element.  A pair of wheel flanges are bolted back to back by a short threaded pin. This assembly is then attached by two ½" bolts to a 2½" gear.  Under the wheel flanges the bolts pass through a double arm crank (boss upwards) and a collar each as spacers. The boss of the gear is pointing downwards. The crank forms an extended bush so that the assembly has minimal play when mounted on a rod. The assembly must be free to turn on a rod, so some care is needed in alignment.
A paper label with the digits 9 down to 0 (from left to right) is attached around the periphery of the wheel flanges. These labels can easily be produced on an ink jet printer, printing onto good quality photo paper, and attached using a few drops of hot glue gun glue.  If you happen to have a copy of CorelDraw, you can download a simple script here to produce these, accurately dimensioned to fit, with guide lines to assist in cutting them out. The label should be applied so that the threaded pin falls between the digits 5 and 4 as shown here.

In total, 13 of these assemblies are required, though on four of them (for the result axis) the threaded pin may be omitted.  It is used as part of the readout mechanism which only applies to the digits on the three difference axes.
Digit wheel with label
Detent wheel
The detent wheel is straightforward: six angle brackets are bolted to a six hole bush wheel. Be careful to place these so that the vertical faces of opposite pairs are in the same vertical plane passing through the center of the axle. Also be sure to make sure these are bolted really tightly so as not to work loose under load. As with the digit wheel, a total of 13 of these assemblies is required.
A roller bears against the angle brackets of each detent wheel to provide positive retention in one of six positions (and therefore locate the digit wheel in one of ten positions). It is constructed from a bell crank, with a tension spring attached at one end by a 3/8" bolt, and a 1" pulley with rubber ring mounted on the other end using a pivot bolt.  The boss of the pulley needs to be away from the bell crank. The bell crank needs to be free to turn on the axle it's mounted on, but because of space constraints it's not possible to locate it between two collars. Instead, a single collar is used to locate it and this is trapped between the bell crank and a fish plate bolted to the bell crank with another collar as a spacer.  Be sure to locate the bolt as close to the boss end of the slot in the crank as possible, otherwise the fish plate will foul the spring in operation. As with the other components 13 of these assemblies are required.
Detent roller
Digit wheel assembly
This picture shows the components assembled in the machine. For clarity this is showing one of the digit wheels on the result axis where there is no readout cage to obscure the view, but the rest are similar. Although it's not very clear here, the lower 57t gear meshes with the 95t gear to the left, but not with the one on the right. Spacing washers must be used to raise the digit wheels on alternate difference axes to ensure clearance. The two 57t gears and the detent wheel are fixed to a 5" rod which is carried in the frame at the lower end. At the upper end a coupling fixed to the rod rotates freely on the lower end of the corresponding rod in the next digit position above.

    Arrangement of axes and drive system
 > Decimal digit storage

    Digit readout and addition
    Carry detection and propagation
    Setup and adjustment

    Parts list

Last modified:  4 February 2004

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