Rack is a different style of toothed transmission component, which is a directly bar type. As proven within the above figure, when the round small gear rotates, the rack does not rotate, but strikes left and right, so the rack can substitute the rotational motion of the tools into linear motion. This effect can be used to make telescopic or reciprocating mechanisms, etc.Rack module: 0.5MMaterial: PlasticRack size: 125 * four * 7mm (length * width * height)Fixed aperture: small hole 2.05mm, wide gap 2.55mmRack size: 65 * four * 4mm (length * width * height)Fixed aperture: 2.05mm