Which of the following is not a standard unit used to express shaft speed?

• A. Feet per second per second
• B. Feet per second
• C. Revolutions per second
• D. Revolutions per minute

Answer: (A)

Shaft speed measures how fast something is rotating, an angular rate, so we express it in revolutions per unit time. Feet per second is a linear speed of a point on the shaft’s surface, which depends on the radius and is not the direct measure of rotation. Feet per second per second is linear acceleration, not speed, so it’s not used to express shaft speed. Revolutions per second and revolutions per minute are both standard ways to describe how fast the shaft is turning.