- 1). Wrap a length of string one time around the pulley where the rope or cable runs.
- 2). Trim the sting so that it wraps around the pulley exactly one time.
- 3). Lay the string flat and measure it with a ruler. This is the circumference of the pulley.
- 4). The diameter of a circle is equal to the circumference divided by pi. Pi is a non-repeating decimal that is commonly rounded to 3.14. Divide the length of the string you measured 3 by 3.14 to determine the diameter of the pulley.
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