Magnetic Particle Inspection ‘Made Easy’ with the MPS Auto Degauss
For years parts have been inspected via MPI and then placed back into operation, with sometimes a small residual or worse yet, a strong magnetic field. When residual fields are combined during unit assembly they can interact and be the origin of stray shaft currents.
MPS experts, after years of engineering investigations, see that MPI followed by inadequate demagnetizing is one of the most common reasons there is shaft current damage to bearings, shafts, journals, couplings and other parts in rotating machinery. Unfortunately, once a machine has had stray currents problems, it is prone to the same conditions again in the future. To prevent reoccurring shaft currents from damaging parts, a machine is often times safeguarded with a shaft grounding device and monitoring system, to divert the currents harmlessly to ground. This solution is typically satisfactory if consistent grounding device maintenance is performed and continuous monitors, such as the MPS VCM-E, are employed to catch the onset of problems before damage occurs.
Significant savings in time and money can be gained by integrating the MPS Auto Degauss system into NDT Magnetic Particle Inspection areas, to eliminate the source before it becomes problematic. The Auto Degauss uniquely uses full-wave DC during automatic "downcycling" to reliably remove deep-seated residual magnetism. The Auto "Down-Cycling" demagnetizing mode can be done immediately after magnetic particle inspection. This saves time since the same cable wrap is used for both, MPI and demagnetizing. One of the largest benefits is the time saved because the movement of parts from the MPI area to the demagnetizing area is completely eliminated, enabling processes to be streamlined. Attached are the simple steps to perform Magnetic Particle Inspection with the Auto Degauss.
Magnetic Particle Inspection with the MPS Auto Degauss:
- The part should be thoroughly cleansed, free of oil, scale, machining marks and other contaminants.
- The cable should be wrapped around the part. (A loose fit is suggested so that the cables can be pushed aside for inspection underneath.)
- The selector switch set to lowest possible setting (marked as 10% or step #6).
- The Auto Degauss placed into "DC" mode and push the " Start" button.
- Slowly turn the selector switch up to 90% (also marked as step #14) to increase the magnetizing current. (It is recommended that you not exceed this maximum, so that you will not need to "downcycle" demagnetize more than once later on.)
- Measure the magnetic field using the hall effect probe on the MPS Gaussometer. Values should achieve the range of 30 to 60 Gauss peak. These are normally adequate magnetization levels for magnetic particle examination. (Inspections can also be made with just the residual magnetic field, although they may not yield results that are as accurate. This can be done by pushing the stop button to discontinue the flow of current.)
- Apply iron filings, magnetic particle spray or other inspection material.
- Inspect parts for defects and discontinuities.
- Push the "Stop" button.
- Check the polarity of the residual field using the hall effect probe on the MPS Gaussometer and set the starting polarity (Positive or Negative) on the Auto Degauss (See the Auto Degauss Operation Manual "Demagnetizing Practices" Section).
- Put the Auto Degauss into "Auto" mode to automatically " downcycle" demagnetize using full-wave DC.
- Put the selector switch setting to the maximum setting (marked as 100% or step #15). (If the part being demagnetized is small, and/or has little magnetism, then a full setting at step #15 may not be needed.)
- Push Start Button, and wait for the Auto Degauss to buzz indicating it has finished the "downcycling" process.
- Remove the inspection material and clean the inspected part (any remaining inspection material will have either fallen off, or can be wiped or blown off).
- Measure the residual magnetism with an MPS Gaussometer and make sure that levels are less than 2 Gauss (See the Auto Degauss Operation Manual "Demagnetizing Practices" Section).
- Repeat magnetization, inspection and demagnetization steps in the perpendicular direction.
Notes and Advice While Using the Auto Degauss for MPI:
- The Auto Degauss uses one phase full-wave rectified (DC) alternating current, because "it has the deepest possible penetration and must be used for inspection for defects below the surface when using the wet magnetic particle method". MPS field tests have shown that Half-Wave or Alternating Current methods of magnetizing are less effective than using full-wave DC magnetizing current.
- Per the ASTM specifications for MPI, "In order to detect discontinuities in all directions at least two magnetic fields, perpendicular to one another in a plane parallel to the surface being inspected, shall be used..." MPS recommends that this be accomplished by wrapping the cable in one orientation, magnetize, inspect, and auto "downcycle" demagnetize. Then, wrap the cable in the perpendicular orientation, magnetize, inspect and auto "downcycle" demagnetize again. Using this technique, the magnetic field will be removed thoroughly during each "downcycling" because the cable wrap is identical to the wrap used during magnetizing.
- The Auto Degauss uses the cable wrap method of demagnetizing and cannot be used for "head shot" or direct circular magnetization. MPS does not recommend the use of the "head shot method of magnetizing, as field tests showed that many submerged localized poles are created. These can divert to internal paths through an assembled machine disrupting proper performance. It also complicates the demagnetizing process and reduces the effectiveness of thoroughly removing deep seated magnetism. "Whenever possible, parts that have been magnetized circularly shall be magnetized in the longitudinal direction before being demagnetized. After demagnetization a magnetic field probe shall not detect fields with an absolute value above 3 G anywhere on the part".
- If the part under inspection was magnetized with several cable wraps before demagnetizing, or was magnetized via different equipment and methods, several Auto Degauss "downcycles" may be needed to "draw-in" the magnetism that may be ill positioned. This is often the case when working with equipment that has just come out of service and the source of the residual magnetism is not known, or has been generated in a state of self-excitation.
Care should be taken after MPI and demagnetizing are completed to not introduce additional magnetism, such as via magnetic chucks, lifting magnets, or welding. Following these steps for MPI and using care to prevent the future introduction of magnetism into rotating machinery parts, will yield enormous benefits. Especially significant will be the millions of dollars that can be saved by avoiding catastrophic failures due to shaft current damage.
 American Society for Testing And Materials, Standard Practice for Magnetic Particle Examination, Designation E1444-94, available from ASTM 1916 Race St. Philadelphia, PA, 19103, USA.
Reference Article Entitled "Shaft Voltages and Rotating Machinery, " Michael J. Costello, IEEE Transactions On Industry Applications Vol. 29, No. 2 March/April 1993 Section D., Available from MPS, 2135 Highway 35, Holmdel, NJ, 07733, USA.