First-pass speed ratio
Ignoring slip and using pitch diameters, an open belt drive follows n₁D₁ ≈ n₂D₂. This is useful for layout and speed planning before a specific belt system is selected.
Use pitch diameter, not an arbitrary outside diameter
Timing belts, V-belts and flat belts define the working radius differently. Supplier pitch geometry matters when the speed ratio must be accurate.
Center distance and belt length
An approximate open-belt length can help early packaging, but final belt selection should come from the chosen belt series and available standard lengths. The center distance often needs adjustment or a tensioner.
Power capacity depends on more than ratio
Wrap angle, tension, belt section, pulley diameter, belt speed, duty cycle, shock load, environment and alignment can dominate the design. A geometrically valid belt path is not yet a rated drive.
When a belt drive is attractive
Belts can provide low-cost speed changes, separation between shafts and tolerance to some layout flexibility. Timing belts add positive synchronization; friction belts can intentionally provide some overload slip.
Machine-design verification
Check the full load spectrum, fatigue, lubrication, material data, environment, manufacturing variation and the relevant machine-element standard before release.