Propeller shaft - MPI Inspection
The Propeller shaft is forged and machined to the specified dimension. It is a single piece and hence the critical areas must be carefully examined.
MPI or Magnetic Particle Inspection is a Non-Destructive Test (NDT) for detecting surface defects or defects close to the surface (up to 6mm with a 5 Kg magnetic field) of ferrous materials. An external magnetic field is applied, and defects are clearly marked (as the magnetic resistance is greater). A surface defect or one close to the surface causes a distortion in the magnetic flux through it, which can be made visible by means of iron or iron oxide particles floating in a suitable liquid. Sometimes the particles are coated with a fluorescent dye that is made visible with application of UV light.
To check if there are defects in the propeller shaft -surface cracks, and near surface cracks. To check if the dimension is as per the design / drawing, especially the diameter at location where the bearing rest and propeller is fitted.
Before the inspection is done, the MPI is done on a test piece to confirm the operation of the test equipment. The test piece is an octagonal shaped, 3 mm thick ferrous material with crack on one side and the smooth face on the other. It is kept with the smooth face on the top and the field is applied by the yoke. When the ferrous liquid spray is applied, the particles align on the smooth surface to take the shape of the crack underneath. This confirms the test.
For the test the main areas to be checked are - the neck where the intermediate shaft is connected, where the propeller is fitted, and the thread where the propeller nut is fitted. The MPI test is done all around these areas.
The dimension of the propeller shaft is checked using a screw gauge. The gauge is calibrated using a test rod first. All dimensions need not be checked. The critical ones like the diameter of the shaft at the propeller fit-up area, bearing contact area are checked.
The roughness of the machined surface has to meet the specification. In this case the specification is usually N6 (Korean standard) that correspond to Ra of 0.8.
The outcome of the test has to be noted down and any suggestion has to be mentioned in the inspection report.