Shaft demagnetization requirement


Metallic shaft can have magnetic properties or residual magnetism that is induced
by machining or by nondestructive examination (NDE) such as magnetic particle
examination. It is important that a shaft is free of residual magnetism because
electrical vibration measuring instruments will not function properly and will result
in erroneous vibration signal when a shaft is magnetic. Magnetic shaft will also
attract metallic particles that can scratch the finished surface of the shaft, or get
lodged in tight fits and interfere with the rotor assembly.

(Residual magnetism may interfere with other manufacturing processes, such as
welding and painting, but those processes are rarely done on shafts.)


Demagnetization equipment

Shaft demagnetizing is done after final machining by a shaft cell machinist at the
factory, or at a service center. In a typical demagnetizing set-up the basic
equipment required to demagnetize a shaft are: Calibrated Field Indicator,
Magnetic Degaussing Loop, and Conveyor System to roll the shaft through the
Degaussing Loop.

(The Field Indicator Meter must be calibrated at regular time interval as set forth in
the company’s Quality Assurance program or department.)
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Checking for magnetic properties

To check a shaft for residual magnetism set the calibrated Field Indicator at one
end of the shaft, about one inch away. Note and record the Field Indicator reading.
Then move the Field Indicator along the length of the shaft to the other end, also
about one inch away. Take another reading. If a reading, at either end, exceeds
the allowable limit the shaft must be demagnetized.

A typical allowable limit of magnetism in a shaft is [ * ] Gauss (G) for overhang
pump design and [ * ] Gauss (G) for between-bearing design. A lower value of
acceptable residual magnetism, as low as [ * ] Gauss, is sometimes specified in
special applications, such as those requiring lower vibration levels in critical
service.

(Gauss is a unit of magnetic field strength in the centimeter-gram-second (CGS)
system of units, and is equal to 10^
(-4) Tesla.)


Demagnetization methods

To demagnetize it, set the shaft in the cradle outside of the Degaussing Loop
before turning on the machine. With the shaft lying in the cradle, roll the shaft
through the Degaussing Loop (in the direction of the arrow as indicated in the
machine.) Check the shaft with the Field Indicator. If the reading is within the
specified value, the shaft is considered demagnetized. But if the reading is above
the specified value, the degaussing process must be repeated.

(Shaft burnishing is another method of reducing residual magnetism. See related
article.)


[ * ] Some information are excluded in this Beta article.
Read more.


R: SHDE-0111
C: manufacturing, shaft, demagnetizing
F: shaft_demagnetization
multistage shaft requires demagnetization