How to Calculate Hydraulic Cylinder Displacement

Written by will charpentier
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Power steering on a boat uses hydraulic rams--a hydraulic cylinder and piston--to move the rudder, which steers the boat. The size of the hydraulic cylinder will determine how quickly the rudder will move and, in part because a larger, heavier rudder requires rams with a larger piston and larger hydraulic cylinder displacement. Calculating the displacement of the hydraulic cylinders in the steering system requires that you know the area of the face of the cylinder--the bore--and the depth of the cylinder.

Skill level:
Moderately Easy

Things you need

  • Calculator
  • Vernier caliper
  • Measuring tape
  • Means of recording cylinder dimensions

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  1. 1

    Measure the inside diameter of the hydraulic cylinder with the vernier calipers by inserting the upper, outward-facing jaws of the caliper into the cylinder, then expanding the jaws until they touch both sides of the cylinder.

  2. 2

    Read the diameter of the cylinder directly from the scale on the caliper. The lower main scale on the caliper reads in inches and fractions of an inch.

  3. 3

    Divide the diameter of the cylinder by 2. Record this number as the "radius" of the cylinder.

  4. 4

    Insert the metal measuring tape into the cylinder to measure its depth. Make sure that the tape touches the wall of the cylinder from top to bottom, to avoid an inaccurate measurement resulting from the tape being held at an angle within the cylinder. Record the result of the measurement as the "depth."

  5. 5

    Multiply the radius of the cylinder itself. Multiply the result by 3.14159 to determine area of the opening at the end of the cylinder. This is the number of square inches in the face of the cylinder. Record this result as the "bore."

  6. 6

    Multiply the bore of the cylinder by the depth of the cylinder. The result is the displacement, in cubic inches.

Tips and warnings

  • Be sure to use the same unit of measurement (all inches) when measuring both the depth and area of the cylinder.

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