Saturday, November 24, 2012

Caution when using magnets around machinery

A magnet is a handy tool for the aircraft mechanic. There are a few cautions when using it because a magnet will magnetize iron that it comes in contact with. Magnetized parts in an engine will collect "fuzz" or small particles of metal. These particles may interfere with the proper operation of the the component or increase wear. You wouldn't want bits of metal attracted to the camshaft lobes!  Two classic items that are damaged by a magnetic field are hydraulic lifters and impulse couplings.

Hydraulic Lifter

Hydraulic lifters: for example, have very tight sliding clearance between the piston and the body. Particles attracted to the lifter can bind the piston thereby preventing proper operation. 

A magnetized hydraulic lifter interferes with the operation of the check valve. Whenever we had a hydraulic lifter who's bleed-down was not working properly we would send it though the Magnaflux Demag Coil and often this would restore its function. We suspect that they became magnetized when Mechanics used a magnet to pull them out of the Lycoming engine. The hydraulic lifter became magnetized as soon as the mechanic touched it with his magnet.
Magneto impulse coupling

Impulse Couplings: 
At low cranking speeds a weak spring and gravity interact to position the flyweight so that it engages the stop pin and retards the spark for engine starting. Magnetized flyweights can overpower the weak spring and gravity and prevent proper operation. The field fix might be to overhaul the magneto or replace the impulse coupling when a simple demag is all that is required.

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