Choose a gear ratio from the target rpm

Start with the rpm you need

Suppose input speed is 1200 rpm and the desired output magnitude is 400 rpm. Required reduction is i = 1200/400 = 3. For a fixed-axis gear pair, i = Z2/Z1: the driven gear needs three times the teeth of the driver.

Then choose direction. External contact reverses rotation; a pinion inside a ring rotates in the same direction as the ring. OhmPlay defines positive rotation as counterclockwise viewed from the front.

Several tooth pairs give the same ratio

The speed ratio matches, but the gears may occupy different space or support different loads. At equal module, pitch diameters grow with tooth count. Choosing teeth solves only part of the design, not the full mechanical selection.

If only 22 and 60 teeth are available, output would be −1200 × 22/60 = −440 rpm: 10% faster in magnitude than the desired 400 rpm. This comparison helps judge whether the difference is acceptable before buying parts.

Do not confuse physical compatibility with a numerical ratio

The calculator preserves an ideal ratio. Tooth counts with the right ratio do not guarantee meshing: module, profile, geometry, mounting and interference must also be checked. In internal mode, ring Z2 must be larger than pinion Z1.

The drawing is not a manufacturing profile. It does not evaluate available torque, tooth strength, efficiency or backlash. Reduction changes motion, but moving a load requires those additional design inputs.

Read the numeric rpm when comparing animations: on-screen motion is slowed so the markers can be followed. Angular position is cumulative; 720° means two turns even when a marker returns to its initial orientation.

Check sign, magnitude and number of revolutions

Try 20/60 at 1200 rpm, then change only the mesh type. After one model second, the input accumulates 7200° and the external output −2400°: 20 input revolutions and 6⅔ output revolutions.