This is the 2D side-view companion to the 3D gear train simulation: the classic mechanical-engineering-textbook diagram of a line of two to four meshing spur gears, drawn flat with real involute-style teeth. Each gear meshes with its neighbour, so ωout/ωin = −Nin/Nout: the minus sign means meshing gears spin in opposite directions, alternating clockwise and counter-clockwise down the chain, exactly as the arrows show.
Set the gear count, each gear's tooth count, input RPM and input torque, or tie gears B and C into a compound gear on a shared shaft. The live readout reports per-gear RPM, torque and rotation direction plus the overall ratio and torque multiplication — the same trade-off between speed and force found in clocks, gearboxes, drills and bicycle drivetrains.