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Torque, Gears, and Speed
Gear ratios trade torque for speed and back
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Robots often need to trade speed for strength, especially when motors are small but the job is demanding. Gears let a robot designer change how fast a wheel, arm, or climber turns and how much twisting force it can produce. A small motor pinion gear driving a larger output gear is a common way to increase torque. This matters in robots that must climb, lift, push, or accelerate without stalling the motor.
The gear ratio compares the number of teeth on the output gear to the number of teeth on the input gear. For example, a 12 tooth pinion driving a 36 tooth gear has gear_ratio = 36 / 12 = 3, often written as 1:3 from input to output. In an ideal gear train, the output torque becomes 3 times larger while the output speed becomes 1/3 as large. A slow climber benefits from high torque, while a fast racer usually uses a lower gear ratio to keep wheel speed high.
Key Facts
- Torque is twisting effect: tau = r * F, where r is lever arm distance and F is force.
- Gear ratio can be calculated with gear_ratio = N_out / N_in, where N is the number of gear teeth.
- A 12 tooth input gear driving a 36 tooth output gear gives gear_ratio = 36 / 12 = 3.
- In an ideal 3:1 reduction, output torque is 3 times motor torque and output speed is 1/3 motor speed.
- Increasing torque with gears does not create extra energy, because power is limited by the motor and losses.
- High-torque gearing is useful for climbers and arms, while low-ratio gearing is useful for fast drive trains.
Vocabulary
- Torque
- Torque is the twisting effect of a force around an axis or pivot.
- Gear ratio
- Gear ratio is the comparison between output gear teeth and input gear teeth that predicts speed and torque changes.
- Pinion gear
- A pinion gear is a small driving gear attached to a motor shaft or another rotating input.
- Output gear
- The output gear is the gear that delivers motion and torque to the robot mechanism.
- Speed reduction
- Speed reduction is the decrease in output rotational speed caused by gearing that increases torque.
Common Mistakes to Avoid
- Using diameter instead of tooth count without checking the gears, which can give the wrong ratio if the gears are not from the same gear system.
- Thinking a larger output gear makes the robot faster, which is wrong because it usually increases torque while reducing output speed.
- Ignoring motor limits and friction, which is wrong because real gearboxes lose energy and can still stall or overheat.
- Mixing up input and output in gear_ratio = N_out / N_in, which reverses the predicted torque and speed change.
Practice Questions
- 1 A motor pinion has 10 teeth and drives a 40 tooth output gear. Calculate the gear ratio, the torque multiplier, and the output speed if the motor spins at 600 rpm.
- 2 A robot arm needs at least 6 N·m of torque. The motor produces 1.5 N·m, and losses are ignored. What minimum gear ratio is needed, and what happens to the output speed at that ratio?
- 3 A robot team must choose between a high-torque slow climber and a low-torque fast racer. Explain which gear setup each robot should use and why the same motor cannot maximize torque and speed at the same time.