Follow the explanation
Back to the model ↑How the parts work together
Follow one tooth
At the contact point, the teeth move together. These two external gears rotate in opposite directions.
Count the turns
The driver has half as many teeth, so it turns twice while the driven gear turns once: a 2:1 reduction.
Trade speed for torque
Torque is a turning effect. With losses ignored, the output has twice the torque at half the speed. No extra power appears.
The main parts
- Pitch point & contact path
- The dashed pitch circles roll together without slip at the marked pitch point. Tooth contact travels along the straight line of action, inclined 25° from the tangent. The contact ratio is about 1.43, so one or two tooth pairs carry ideal contact.
- Driver · 16 teeth
- The input gear supplies the motion. Two of its full turns move 32 teeth past the contact point.
- Driven gear · 32 teeth
- The output gear completes one turn for every two input turns. Its larger radius increases the turning effect of the tooth force.
Questions worth exploring
How do you calculate the output speed of two gears?
Multiply the input speed by the driver’s tooth count divided by the driven gear’s tooth count. Here, 60 rpm × 16 ÷ 32 gives 30 rpm. Calling this a 2:1 reduction means the input makes two revolutions for each output revolution. Count the turns in the model to check it.
Why do these gears turn in opposite directions?
The two external gears touch on opposite sides of their centers. As the driver’s teeth pass through the mesh, they push the other gear around the other way: clockwise input gives counterclockwise output. Slowing the animation makes the direction change easier to follow. This explanation applies to the external gear pair shown here.
Does a gear reduction create extra power?
No. Rotational power is torque multiplied by angular speed. In an ideal 2:1 reduction, halving the speed allows twice the output torque while preserving power. Real losses reduce the power delivered. This model illustrates the ideal relationship; it does not calculate friction, tooth deformation, strength or the load a manufactured gear can withstand.