Gears can either be obtained as standard components from a manufacturer's catalogue or alternatively specially designed and manufactured. Smaller sized gears, especially instrument gears, tend to be more readily available from catalogues and larger, less used gear types tend to be produced as specials; usage is claimed by leading manufacturers to be approximately equally divided.
For practical reasons, gear catalogues tend to display only geometric and materials data of stock gears rather than specific operational information. This is because functional behaviour will vary with an application and so it is not feasible to give comprehensive data covering all operational conditions within a catalogue for a complete product range.
The guide describes a practical approach which ensures that gears selected from a catalogue are technicaly suitable for a specific application. Technical considerations usually produce a number of viable alternatives so cost may well be the decisive factor influencing the final selection.