Torque vs. Speed: Motor Selection in Robotics

If you snap a powerful battery onto a bare DC motor, the shaft will spin incredibly fast—sometimes well over 5,000 Rotations Per Minute (RPM)! But if you attach a wheel directly to that bare shaft and set it on the ground, the motor will stall. It simply doesn’t have the strength to push the robot’s weight.

This highlights the fundamental trade-off in mechanical engineering: Torque vs. Speed.

What is the Trade-off?

• Speed: How fast the output shaft rotates.
• Torque: The rotational force or ‘twisting strength’ of the shaft.

You generally cannot have both without buying a massively expensive, heavy, and power-hungry motor. Instead, engineers rely on gearboxes to trade one for the other.

Using Gear Ratios to Find the Sweet Spot

Think of the gears on a bicycle. When you are going up a steep hill, you use a low gear. You have to pedal very fast (high speed input), but you gain the mechanical strength needed to conquer the hill (high torque output).

Robots do the same thing:

• Geared for Speed: A large gear drives a small gear. The torque drops, but the top speed of the robot increases.
• Geared for Torque: A small gear drives a large gear. The robot moves very slowly, but it can push incredible amounts of weight or lift heavy arms.

Managing the Trade-off in FIRST® Robotics

This physical trade-off is the core element of the FIRST® Tech Challenge (FTC) and FIRST® Robotics Competition (FRC).

Should your team build a robot that zips across the field at 15 ft/sec to play offense? Or should you gear it down to become a heavy, high-torque defensive powerhouse? Choosing the right planetary gearbox ratio (like a 20:1 versus a 40:1) defines the entire strategy of the season.

Next time your robot feels sluggish, double-check your gear ratios—you might just need to trade a little bit of that torque back for speed!