When designing a robot, selecting the correct motor requires understanding how much torque the motor must generate. Torque determines whether a robot can move its wheels, lift objects, or rotate its joints effectively.

If the motor torque is too low, the robot may stall or fail to move. If the motor is oversized, it can increase cost, weight, and power consumption.

This guide explains how to calculate the torque required for a robot motor and how to select the appropriate motor for robotics applications.

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What Is Torque in Robotics?

Torque is the rotational force that allows a motor to turn an object. In robotics, torque is required for:

• Driving robot wheels

• Moving robotic arms

• Rotating joints

• Lifting payloads

Torque is usually measured in:

• Newton meters (N·m)

• Kilogram-centimeters (kg·cm)

Higher torque allows the robot to move heavier loads.

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Basic Torque Calculation Formula

The basic torque formula is:

Torque = Force × Distance

Where:

• Force = weight or load acting on the motor (N)

• Distance = length of the lever arm or wheel radius (m)

Example:

If a robot arm lifts a 5 kg object with a 0.2 m arm length, the required torque is:

Torque = 5 × 9.8 × 0.2
Torque ≈ 9.8 N·m

This means the motor should produce more than 9.8 N·m torque to operate safely.

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Calculating Torque for Robot Wheels

For mobile robots, torque is needed to rotate the wheels.

The formula is:

Torque = (Robot Weight × Wheel Radius) / Number of Drive Wheels

Example:

Robot weight = 10 kg
Wheel radius = 0.05 m
Drive wheels = 2

Torque per motor:

Torque = (10 × 9.8 × 0.05) / 2
Torque ≈ 2.45 N·m

In practice, engineers usually add 30–50% safety margin.

Recommended torque:

3–4 N·m per motor

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Calculating Torque for Robotic Arms

For robotic arms, torque depends on:

• Payload weight

• Arm length

• Arm weight

Formula:

Torque = (Payload Weight × Gravity × Arm Length)

Example:

Payload = 2 kg
Arm length = 0.3 m

Torque:

Torque = 2 × 9.8 × 0.3
Torque ≈ 5.88 N·m

The motor should be rated above this value.

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Why Gear Motors Are Often Used

Many robot motors rotate at high speed but produce limited torque.

Gearboxes are used to:

• Increase torque

• Reduce speed

• Improve motion control

Common gearbox types include:

• Planetary gearboxes

• Worm gear reducers

• Spur gear reducers

Planetary gear motors are widely used in robotics because they offer high torque in a compact design.

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Adding Safety Margin

Real robotic systems always include a safety factor.

Typical safety factors:

• 1.5× for light robots

• 2× for heavy robots

Example:

Required torque = 5 N·m

Recommended motor torque:

7.5 – 10 N·m

This prevents motor overload and ensures smooth operation.

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Example Torque Requirements for Robots

These values vary depending on robot size and payload.

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Choosing the Right Motor After Calculating Torque

Once torque requirements are known, you can select the motor based on:

• Torque rating

• Speed (RPM)

• Voltage

• Gear reduction ratio

• Motor size

Many robotics applications use:

• DC gear motors

• planetary gear motors

• brushless motors with gearboxes

These motors provide a good balance of torque, efficiency, and reliability.

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Conclusion

Calculating motor torque is a crucial step when designing a robot. By understanding the relationship between force, distance, and load, engineers can select motors that provide the correct amount of power for robotic systems.

Always remember to:

• Calculate the required torque

• Add a safety margin

• Choose a suitable gear motor

This approach ensures reliable performance and prevents motor failure in robotic applications.