Modellers who have experimented with planetary gear trains will have found there are limited ratios available when using standard Meccano gears. Using the well-known 3-1/2” ring gear (p/no. 180), the experimenter soon finds there are only two arrangements of sun and planet gears which will mesh with the ring gear’s 95 internal teeth. These are:
19T sun and 38T planets;
57T sun and 19T planets.

By using gears from the extended range, available from some parts suppliers (see below), two other arrangements are possible:
45T sun and 25T planets;
55T sun and 20T planets.

Use of the newer 2-1/2”ring gear (p/no. 180a with 57 internal teeth) allows the use of:
19T sun and 19T planets;
27T sun and 15T planets.

While searching through textbooks or the internet, for formulas to calculate ratios, modellers will also notice that it is usual for there to be three-planet gears shown in the diagrams because this arrangement is common industrial practice. Three-planet configurations appear at first sight to be impossible when using only standard Meccano gears; two planets being the only number which will fit. This is not entirely true because amongst the first two configurations above is one which will accommodate three planets. It is 19T sun and 38T planets. The advantage of a three-planet configuration is that it gives smoother running. It is more fiddly to set up though. Part no. 109b Faceplate with six slots is useful for positioning the planets at the required 120 degree spacing. See below for suppliers.

A knowledge of how to calculate whether a particular arrangement of sun and planet gears will accommodate three planets is useful. All that is necessary is to add the number of teeth in the sun gear to the number of internal teeth in the ring gear, then determine if the sum is evenly divisible by the number of planets; in this case three. Try it with the 19T sun gear and 95 internally toothed large ring gear, mentioned above.

Armed with this knowledge and a list of standard and extended range gears, it is possible to search for more three-planet configurations. The following comply:

3-1/2” ring gear with three planets:
19T sun and 38T planets;
55T sun and 20T planets.

2-1/2” ring gear with three planets:
27T sun and 15T planets. (with minor slotting of holes in a Six-hole Bush Wheel, p/no. 24b.)

It is also possible to assemble four-planet gear assemblies using standard and extended range Meccano gears. The following arrangements will work:

3-1/2” ring gear with four planets:
45T sun and 25T planets;
57T sun and 19T planets.

2-1/2” ring gear with four planets:
19T sun and 19T planets;
27T sun and 15T planets.

Astute readers will have noticed that a 27T sun and 15T planets in a 2-1/2” ring gear is particularly interesting as it can be assembled with 2, 3 or 4 planets!

All of the above extended range gears and Six-slot Faceplates are available from Meccano Spares and Ashok.