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3. Choose the number of teeth, type and material for the gears
It may not be possible to achieve target speed ratios precisely because gears must, of course, have an integer number of teeth and also because gears in catalogues must be chosen from a preferred list of numbers of teeth. The preferred standard gear teeth numbers are:

12 13 14 15 16 18 20 22 24 25 28 30
32 34 38 40 45 50 54 60 64 70 72 75
80 84 90 96 100 120 140 150 180 200 220 250

Many catalogues display a more comprehensive range than this but many sizes will not be readily available 'off-the-shelf'. The range of available sizes will depend upon the gear module (m). Generally the lower the module, the greater the range of gear teeth sizes available.

The preferred standard module sizes are: 0. 5, 0.8, 1, 1.2 5, 1.5, 2, 2.5, 3, 4, 5, 6

Large speed ratios are achieved in several stages, choosing the number of teeth for each gear so that the overall reduction is as close to target as is possible. Example

Choose the type (helical, bevel, skew, etc.) and select gears from a catalogue for consideration, thus specifying the material, face width, factors and cost for the gears.