Oilers/Plant Tenders (HHC) Civil Service Practice Exam

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If a journal has a diameter of 7" and turns at a rate of 600 RPM, approximately what is the rubbing speed of the bearing in feet per minute?

800 feet per minute
1100 feet per minute
1300 feet per minute
1500 feet per minute

The correct answer is: 1100 feet per minute

To determine the rubbing speed of the bearing in feet per minute, first, we need to calculate the circumference of the journal, which is given by the formula: Circumference = π × Diameter. With a diameter of 7 inches, the circumference can be calculated as: Circumference = π × 7 inches ≈ 21.99 inches. Next, to find the rubbing speed, we multiply the circumference by the number of revolutions per minute (RPM). Since the journal turns at a rate of 600 RPM, the formula becomes: Rubbing Speed = Circumference × RPM. Substituting the values we calculated: Rubbing Speed = 21.99 inches × 600 RPM = 13194 inches per minute. Now, we need to convert this speed from inches to feet. There are 12 inches in a foot, so: Rubbing Speed in feet per minute = 13194 inches per minute ÷ 12 ≈ 1099.5 feet per minute. This result rounds to approximately 1100 feet per minute. Thus, the choice that reflects this calculation is accurate, confirming that the correct answer is indeed approximately 1100 feet per minute.