Torque

From Combat Robot

Torque is a concept of physics that measures rotational force. Torque consists of a force unit times a distance unit such as ounce-inches (oz-in), foot-pounds, or newton-meters (N-m, the SI unit). As the units imply, torque is a measure of force at a certain distance from the center of rotation.

In combat robotics, torque is very important as many bot systems involve rotating parts. The good bot builder will understand the definition of torque as well as its implications in robot design.

Examples in the Drivetrain

In the drivetrain, torque is important at several points. Most drivetrains involve a gearing system. One purpose of gearing is to reduce the angular velocity (speed) of a high-speed motors to reasonable levels at the wheel. However, while speed is reduced, torque is increased and this is equally important in creating a workable drivetrain. Acceleration is directly related to torque, meaning that an ungeared motor (high-rpm and low-torque) would produce sluggish acceleration. The inverse relationship between torque and speed comes from the fact that, in a given system at any instant, power will be constant minus frictional losses. For more on the tradeoffs of gear ratios, see optimum gearing.

Torque also comes into play when choosing a wheel diameter. Here, it is very easy to apply the idea that torque is a force at a distance from center. For this exaple assume that the torque at a shaft after gearing is 100 oz-in. We want to figure out how much linear or tangential force various wheel sizes will exert on the ground. This is the force that will accelerate the robot towards its top speed. To find this value, we divide torque by the radius of the wheel. If the wheel has a 5 inch diameter, the radius is half of that or 2.5 inches. 100 oz-in / 2.5 in = 40 oz of force. For a 2 inch wheel, radius is 1 inch and the force equals 100 oz. The higher the force, the quicker the robot will accelerate. Keep in mind that greater acceleration comes at the cost of reduced top speed.

The above example finds the theoretical maximum force that could be produced by the given wheel. In reality, acceleration is limited by the max friction between wheel and arena surface. For a more detailed discussion of this, see friction.