A ratchet gear consists of a round gear or linear rack with teeth, and a
pivoting, springloaded finger called a pawl that engages the teeth. The
teeth are uniform but asymmetrical, with each tooth having a moderate
slope on one edge and a much steeper slope on the other edge.
When the teeth are moving in the unrestricted forward direction, the pawl
easily slides up and over the gently sloped edges of the teeth, with a spring forcing it (often with an audible 'click') into the depression between the teeth as it passes the tip of each tooth. When the teeth move in the opposite (backward) direction, however, the pawl will catch against the steeply sloped edge of the first tooth it encounters, thereby locking it against the tooth and preventing any further motion in that direction.
B.I.R.S. Machine & Supply supplies ratchet gears to your specification.