Problem Solved | Stern Tube Measurement on an Oil Tanker

March 14, 2012

On a recent alignment consultation, we were requested to verify the straightness of a stern tube on an oil tanker. It is critical that the stern tube be bored so that it matches the line of the propulsion shaft that will be passing through the center of it. The reason is that the next operation will involve the hydraulic insertion of Babbitt bearings into the stern tube. It would be a very costly operation to undo this is if it were not done correctly the first time.

Stern Tube MeasurementUpon arrival, the customer’s engineer on site noted that the surface was just machined, but not prepped for accurate measurement by the removal of burrs and machining marks.  He wondered if this would affect our measurement. We told him that it could affect any bore measurement, but some unique features of CENTRALIGN ULTRA would let us know if it did. CENTRALIGN ULTRA offers the following to perform this job AS-IS:

  • Single point bracket –The single point brackets are designed to give extremely accurate readings that are independent of the user’s skill and ability. Springs consistently position the plunger on each point around the bore – accomplishing a more accurate translation of the bore surface to the sensor. The result is a true measurement of out-of-roundness and averaged surface finish.
  • Standard Deviation – It takes a minimum of three points on a bore to obtain its center position with tightwire, laser or optical bore measurements.  CENTRALIGN allows for more than three points to be taken. With this, the points can now automatically be fitted to a virtual circle that can reveal not only a center position, but also the quality of the measurement. In essence, Standard Deviation lets us know, “Is what we measured round? If so, on average, how round was it?”

If the measurement had a low Standard Deviation, then we would know the bore that was measured was round. If a nick or a burr was accidentally measured or if the surface finish was rough, you would see higher Standard Deviation results. It turns out there was no need for any further surface preparation as CENTRALIGN indicated that the measurements were excellent and the single point bracket had not measured on a nick or bump. Standard Deviation was under 0.5 thousandths and repeatability averaged 0.2 thousandths per measurement. The entire measurement took under 1 hour from arrival to departure of the job location.

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