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Decimal Counter

Decimal counters have long been considered a challenge in Meccano because of the lack of any part with obvious five-fold or ten-fold symmetry. John Westwood has suggested that the range should have been extended to include a 10 hole bush wheel, and a number of the replica parts manufacturers have obliged. However, it turns out there is a very effective way of building a decimal counter using only standard Binns Rd. parts. I first built decimal counters a few years ago for use on a differential analyzer for recording shaft rotations. A recent discussion on Spanner prompted me to think about this problem anew, and as often happens, I was able to come up with an improved design that uses fewer parts and is more pleasing in appearance.

counter top view
Top view of the finished unit

The example described here operates as does the odometer in a car. The right most dial indicates tenths of a rotation of the input shaft and turns continuously with the input. The remaining dials move in discrete steps whenever the dial to the right changes from 9 to 0 (or vice versa). The counter can fairly easily be preset to any value by manually turning the dials and it can count up or down. While this example has a total of five dials, and so can count to 9999.9 there is no reason in principle this range cannot be increased (though beyond some point there will be too much load presented to the input when all the dials have to change simultaneously), and of course it can easily be reduced.

The key observation, which makes this compact design possible, is that a 95t gear can be meshed directly with a 57t gear to provide a ratio of 5:3. This means that one tenth of a revolution of the 95t gear corresponds to one sixth of a revolution of the 57t gear, so that we can hold the 95t gear in on of ten discrete positions by instead holding the 57t gear in one of six discrete positions. This is easily accomplished using a six hole wheel disc with six double brackets bolted to it.  A diametrically opposite pair of these is then bolted to the 57t gear (boss in, with washers as spacers to prevent teeth fouling on the lugs of the double brackets). It is important the assembly runs freely on a rod. A roller consisting of a 1” pulley with rubber ring mounted on a pivot bolt in the end hole of a crank provides the detent, light pressure being applied by a tension spring (be sure to pick cranks with the tapped bore at right angles to the arm of the crank). Four of these assemblies are needed for a 5 digit counter.

counter sub assemblies
The principal sub-assemblies

The dials are constructed from two wheel flanges back to back, sandwiched between a face plate (boss in) and a 95t gear (boss out). A 2” slotted strip is bolted in one of the slotted holes of the face plate with 3/8” of the strip protruding past the edge of the face plate. This arm will engage between two of the double brackets on the detent assembly of the adjacent dial and cause it to advance by one unit. Collars are used as spacers and the whole assembly is held together with ¾” bolts. Make sure that the assembly runs freely on a rod. Note that the dial for the most significant digit does not need the slotted strip, and the dial for the tenths digit can use a second face plate instead of the 95t gear as it will be fixed on and driven by the input shaft. Labels can be printed on an ink jet printer on glossy photo paper to give a very pleasing finish. These are easily attached with a few spots of hot glue gun glue. The digit 7 should be aligned with the slotted strip to ensure that the adjacent dial will be advanced correctly on the 9 to 0 transition.

counter bottom view
Bottom view of the finished unit

The frame is straightforward, consisting of two 5½” x 3½” flat plates, three 5½” x 2½” flanged plats, and a 5½” x 2½” flat plate. It needs to be reasonably rigid, as flexing of the side plates will allow the thin gears to slip out of mesh. To mask the dials so that only a single digit is visible, the Electrikit rectangular core holder is ideal, but if you don’t have enough of these, 1½” strips can be substituted. Four 6½” rods are needed respectively for the main input shaft (carrying the dials), the detent wheels, the detent rollers, and for the anchor to the tension springs.

counter assembly detail
Detent Assembly Detail

Careful assembly is needed to get the gears exactly aligned and operating freely. Everything except the tenths dial is free to turn on the rods, with collars preventing lateral motion. Electrikit thin washers will probably be needed to accommodate part to part variation. I found that two thin washers between each dial assembly gave them exactly 1” spacing, and to match this, collars are needed between each detent assembly, but you may need to adjust this for variation in your particular parts.


Part Number
Perforated strip, 4½" 2
Double bracket, ½" x ½" 24
Angle bracket, ½" x ½" 2
Axle rod, 6½" 4
Pulley, 1" 4
Wheel disc 6 hole, 1 3/8" 4
Gear wheel 57 teeth, 1½" 4
Gear wheel 95 teeth, 2½" 4
Nut 86
Bolt, 7/32" 68
Washer, 3/8" 23
Tension spring 4
Flanged plate, 5½"x 2½" 3
Flat plate, ½" x 3½" 2
Slotted strip, 2" 4
Collar 42
Crank 4
Set screw 1
Grub screw, 5/32" 19
Flat plate, 5½" x 2½" 1
Face plate, 2½" 6
Bolt, ¾" 10
137 Wheel flange 10
Pivot bolt 4
Rubber ring, 1" 4
Core holder for rectangular coil 10
Thin washer (mostly finish protection) 80

Click here to download the parts list in MeccInv format.

Last modified:  12 February 2007

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