Rpm stands for rotations per minute and is used to quantify the speed at which an object spins, such as a motor or a centrifuge. Linear speed measures the actual distance traveled, often in feet per minute. Because a rotation always covers the same distance, you can convert from rpm to linear distance if you can find the distance per rotation. To do so, all you need is the diameter of the rotation.
Measure the diameter of the circle that the item spins in. For example, a motor might spin in a circle with a diameter of 1.3 feet.
Multiply the number of rpm by 3.14. For example, if a motor spins at 140 rpm, multiply 140 by 3.14 to get 439.6.
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Multiply the Step 2 result by the diameter of the circle to find the linear speed per minute. Completing the example, multiply 439.6 by 1.3 feet to get a linear speed of 571.48 feet per minute.