Gear Ratios Are Design Decisions

A gear ratio is not just something you calculate after picking gears. It determines how the motor and mechanism share speed and torque.

For a simple reduction, if a small driving gear with 12 teeth turns a 60-tooth driven gear:

ratio = driven teeth / driving teeth
ratio = 60 / 12
ratio = 5:1

The output turns about five times slower than the input while ideal torque increases by about five times.

Real systems lose some power to friction and deformation, so the ideal numbers are a starting model.

Same motor, different robot

Imagine a motor turning at 5,000 rpm under a certain operating condition.

The largest reduction is not automatically best.

A 20:1 reduction may produce more torque but make the mechanism too slow. A 1:1 drive may be fast but unable to accelerate the load.

The design target is the operating point that satisfies the mission.

Compound stages

Gear reductions multiply.

A 4:1 stage followed by a 3:1 stage gives:

4 × 3 = 12:1

That is useful because a large ratio can be packaged in several smaller stages. It also adds shafts, bearings, alignment requirements, friction, and backlash.

Every "solution" creates new system responsibilities.

Worked example: wheel speed

A wheel has diameter 0.15 m. Its circumference is approximately:

π × 0.15 ≈ 0.471 m

If the wheel turns at 600 rpm:

600 rev/min × 0.471 m/rev ≈ 282.6 m/min

282.6 / 60 ≈ 4.71 m/s

That is an ideal no-load wheel surface speed. Real robot speed will be lower.

If the design target is 2 m/s, you can work backward toward a useful wheel speed and then toward a gear ratio.

That is more disciplined than choosing gears because they happen to be in the drawer.

Ratio selection note

For one mechanism, record:

• desired output speed;
• estimated output torque requirement;
• motor-side speed assumption;
• candidate ratio;
• one reason the ratio may need revision after testing.

Keep the arithmetic visible. A reviewer should be able to follow the decision without trusting you.