Diagrams.ink / Motion studies / Gear trains

Teeth set the speed.
The idler only turns it around.

Three gears mesh in a row. Count the teeth to get the speed ratio; count the meshes to get the direction. Turn the orange crank, or pull the idler out and see what really changes.

Fig. 1 — Three-gear train on a mounting plateIdler in · output follows input

INPUT

IDLER

OUTPUT

Input gear: driven by the crankIdler: just passes motion alongOutput gear: pointer shows its turnsBlack arrows: the teeth at each mesh move at the same speed

Drag the orange crank to turn the input by hand

Input speed

Teeth on each gear

Input gear

Idler gear

Output gear

Idler gear

Input direction

Try a case

Speed ratio (output ÷ input)

Output speed

Output direction

Where the ratio comes from

What the idler does

Ideal spur gears with matching tooth size, no friction or backlash. Motion is drawn at ¼ of the real speed; rpm values are the real model numbers.