Which statement about torque measurement for the pinion is true?

• A. Rotating torque is higher than starting torque
• B. Starting torque is higher than rotating torque
• C. They are equal
• D. Torque is independent of rotation speed

Answer: (B)

When a pinion begins to move, the key factor is friction and inertia. At rest, static friction between gears and bearings is higher than the friction encountered once things are moving, and the load’s inertia must be overcome to start rotation. That combination requires more torque to initiate motion, to “get over the hump” and start turning. Once the pinion is rotating, friction becomes kinetic (dynamic) rather than static, which is typically lower, so the torque needed to keep it moving at a given speed is less than the torque required to start it. There can be changes with speed due to other losses, but in general the starting (stall) torque exceeds the running torque.