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I have recently taken apart a few cassette players to get motors, and I have noticed there is always a metal sleeve around the main drive motor.

Here is an Ebay product photo showing this on a Mabuchi EG-530AD-2B motor: enter image description here

The sleeve is not attached to the motor and can be easily removed by sliding it off. I have not seen them on DC motors anywhere else. What is it there for?

The reason I chose an EG-530AD-2B motor is because one of the cassette players I took apart had a 'Mebochi' (not Mabuchi) brand EG-530AD-2B motor with a very similar label to the one shown below, except that the text was blue. I cannot seem to find any information about this online. Does anyone have any idea what it is?

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    \$\begingroup\$ What's underneath it? \$\endgroup\$ – pjc50 Jan 13 '16 at 16:53
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    \$\begingroup\$ @pjc50: Just the motor casing itself. \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 13 '16 at 16:56
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    \$\begingroup\$ Motors generate rapidly varying magnetic fields. Cassette tapes generate sound from rapidly varying magnetic fields. There may be some connection between these facts and a layer of magnetic screening material around the motor. \$\endgroup\$ – Brian Drummond Jan 13 '16 at 16:57
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    \$\begingroup\$ @jBot-42 see kerrywong.com/2012/01/26/… "Motor’s metal casing typically provides enough shielding capability for reducing the over-the-air RF interference, but an extra metal enclosure should provide much better RF interference reduction capability" \$\endgroup\$ – Kev Jan 13 '16 at 17:00
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    \$\begingroup\$ @jBot-42: Note, I discovered many years ago that cassette player speed regulation is done inside the motor. I modified one with an external pot so I could correct pitch to be in tune with my piano. Don't expect linear speed/voltage as in a plain DC motor. \$\endgroup\$ – Transistor Jan 13 '16 at 17:46
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It's a mu metal shield. This is a special material that 'conducts' low frequency and static magnetic fields very well (but saturates at a low field strength). The basic idea is that any small residual field generated by the motor magnets gets shunted through the mu metal rather than propagating out into the air around the motor and interfering with the tape reader head.

Mu metal has a relative permeability around 25x greater than soft iron, so if they wanted provide the same amount of shielding using the motor body, it would have to be roughly 25x thicker than the thin strip of mu metal.

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    \$\begingroup\$ I wouldn't rely on it being mu-metal instead of much cheaper iron, but that peculiarly matt-looking strip just might be. \$\endgroup\$ – Brian Drummond Jan 13 '16 at 18:02

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