threegear

Threegear is a term used in mechanical engineering to describe a compact gear train consisting of three gears that transfer rotational motion between two shafts. In its simplest form, the arrangement includes a driver gear A that meshes with a middle gear B, which then meshes with a driven gear C. The middle gear can be mounted on its own shaft or fixed as a compound gear with one of the end gears, enabling different speed and torque characteristics within a small package.

In the basic inline configuration, the three-gear train provides an overall speed ratio determined by the teeth counts of the first and last gears, while the middle gear primarily serves to space the gears and, if used as an idler, to reverse direction without substantially changing the ratio. When the middle gear acts merely as an idler, the direction of rotation between A and C reverses twice and ends up the same direction, while the speed ratio depends mainly on the end gears. Compound variants, where the middle gear shares a shaft with an end gear, offer different effective ratios and can accommodate more compact or specialized layouts.

Threegear assemblies are used in simple educational kits, small timing mechanisms, hobbyist gearboxes, and other compact devices where a modest change in speed or torque is sufficient. They provide simplicity and a low component count but offer a limited range of ratios compared with longer gear trains. Materials commonly include metals and plastics, with tolerances chosen to minimize backlash and binding.