Magnetic fluid sealed bearing

A magnetic fluid sealed bearing, comprising:

• an inner ring and an outer ring, both formed of a magnetic material;
• a plurality of rolling elements disposed between the inner ring and the outer ring;
• a ring-shaped polar plate formed of a magnetic material, the ring-shaped polar plate being mounted on an inner circumferential surface of the outer ring such that a gap is formed between the ring-shaped polar plate and an outer circumferential surface of the inner ring;
• a ring-shaped magnet attached to an axially inner surface of the ring-shaped polar plate, the ring-shaped magnet being magnetized such that magnetic poles thereof face in an axial direction, to form magnetic circuits on both an outer ring side and an inner ring side;
• an inner ring magnetic fluid for sealing the gap, the inner ring magnetic fluid being retained in the magnetic circuit formed on the inner ring side, and
• a magnetic field intensity increasing portion for increasing magnetic field intensity of the magnetic circuit formed on the inner ring side to a level higher than that of the magnetic circuit formed on the outer ring side;
• wherein the ring-shaped polar plate includes a thicker portion and a thinner portion, the thicker portion configured to be thicker than the thinner portion in the axial direction, the thicker portion being disposed inside of the thinner portion in a radial direction;
• and
• wherein at least a part of the thicker portion is disposed on an axially outer side of the ring-shaped magnet;
• and
• wherein the magnetic field intensity increasing portion includes the ring-shaped polar plate that is thicker in the inner ring side and is thinner in the outer ring side.