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The science
Two meshing gears of the same module m roll on their pitch circles r = m z / 2, so the pitch-line speed is shared: w1 r1 = w2 r2 and w2/w1 = -z1/z2. An idler only reverses the direction; a compound (clock-style) train puts a pinion and a wheel on one shaft so each stage multiplies the ratio. Torque scales the opposite way, T2 = T1 (z2/z1) eta, and power drops by the mesh efficiency at every mesh.
Teeth are involutes of the base circle rb = r cos(phi); the flank length from the base circle to radius r is (r^2 - rb^2)/(2 rb), and the generating string (the line of action) has length sqrt(r^2 - rb^2). The contact ratio eps = (sqrt(ra1^2-rb1^2) + sqrt(ra2^2-rb2^2) - a sin(phi)) / (pi m cos(phi)) must exceed 1 so a new tooth pair engages before the old one leaves; gears with fewer than 2/sin^2(phi) teeth (17 at 20 degrees) are undercut.
Simplifications: rigid teeth, no backlash, friction as one constant efficiency per mesh, no tooth load or lubrication model. The 3D page shows the same default eight-gear chain.
What to try
- Select Clock minute-hour (12:1) and read the overall ratio and output torque.
- Choose Too few teeth: the contact ratio drops and the undercut warning appears.
- Lower the mesh efficiency to see how many meshes the power survives.