In a Prony brake, torque is determined by multiplying the prony arm radius by which quantity?

• A. The load
• B. The current
• C. The speed
• D. The friction coefficient

Answer: (A)

Torque comes from a force acting at a distance from the shaft, so it’s the force on the brake band multiplied by the lever arm length. In a Prony brake, you apply a known load to the brake mechanism to create a tangential braking force on the drum. The prony arm radius is that lever arm through which this braking force acts. Multiply the tangential force (the load) by the arm radius to get the torque the engine must supply or resist. The current, speed, or friction coefficient aren’t used in the basic torque calculation here—the direct measure comes from the load producing a tangential force at a known radius.