Gearing Changes the Tradeoff

Gearboxes do not manufacture power. They trade rotational speed for torque, or torque for speed, while introducing real losses.

This is one of the most useful ideas in competition robotics because motors often spin much faster than the mechanism should.

Reduction ratio

Imagine a 12-tooth gear driving a 60-tooth gear.

The driven gear rotates more slowly.

A simple ideal reduction is:

60 / 12 = 5

That is a 5:1 reduction. Ideally, the output turns once for every five motor rotations, while output torque increases by roughly the same factor before losses.

If the motor rotates at 5,000 rpm, the ideal output speed would be:

5000 rpm / 5 = 1000 rpm

Real systems lose some performance through friction, flex, efficiency, and load.

Multiple stages multiply

Suppose a mechanism uses a 3:1 stage followed by a 4:1 stage.

3 × 4 = 12

The total reduction is 12:1.

Do not add the ratios. The stages compound.

What the mechanism actually needs

The correct ratio depends on the job.

An intake roller may need enough speed to move game pieces quickly while still producing enough grip force.

An elevator may need much more reduction because it lifts weight against gravity.

A drivetrain ratio must balance acceleration, top speed, traction, current draw, and controllability.

Compare two designs

A motor drives the same mechanism in two configurations:

Neither design is automatically correct.

If the mechanism only needs modest torque and has a tight cycle-time target, A may be reasonable.

If A stalls under the required load, the speed advantage is imaginary because the mechanism cannot carry the required load through its full motion.

Calculate before cutting metal

For your mechanism, choose or invent a motor free speed and one proposed reduction.

Calculate ideal output rpm.

Then write two statements:

1. what the ratio helps you gain;
2. what the ratio makes worse.

Example:

A 10:1 reduction lowers output speed to one tenth of motor speed and increases ideal output torque by about ten times. The mechanism becomes easier to control under load, but its maximum output speed is lower.

Failure mode: ratio without geometry

Gear ratio is only part of the mechanism.

A wheel or drum converts rotation into linear motion. A larger diameter travels farther per revolution but also changes the torque required at the shaft for the same linear force.

An arm introduces changing leverage as its angle changes.

This is why a gear ratio that looks fine in a spreadsheet can still perform poorly on the real machine.

Record the assumptions

Your calculation must include:

• motor speed used;
• reduction;
• output rpm;
• output element size if relevant;
• expected load;
• one assumption that could be wrong.

Engineers do not hide assumptions. They label them so testing knows what to challenge.