Electrically conductive ferrofluid bearing and seal apparatus and low-viscosity electrically conductive ferrofluid used therein

A low-viscosity, electrically conductive ferrofluid for use in a ferrofluid hydrodynamic bearing apparatus, which ferrofluid composition comprises:

• (a) a nonvolatile hydrocarbon or ester liquid carrier compound;
• (b) finely-divided ferromagnetic particles in an amount sufficient to impart magnetic properties to the liquid carrier;
• (c) up to 4% by weight of finely-divided carbon-black particles;
• and
• (d) a cationic surfactant composed of a quaternary-ammonium salt comprising a nitrogen-positive head portion and a long-tail portion, which long-tail portion is soluble in the liquid carrier, and which cationic surfactant is present in an amount which, in combination with the carbon particles, provides for a nonthixotropic ferrofluid composition, having an electrical resistivity of about 10.sup.+2 ohms or less, and a low-viscosity of about 25 cps at 25.degree. C. or less.

A ferrofluid bearing apparatus, which bearing apparatus comprises in combination:

• (a) a housing characterized by a shaft-receiving cavity therein and having a one and another end, and having a radial-bearing, inner wall surface;
• (b) a rotary shaft disposed for rotary movement within the shaft-receiving cavity, and defining with the cavity a radial-bearing cavity;
• (c) thrust-bearing means to provide support for the longitudinal movement of the rotary shaft in the housing;
• (d) a ferrofluid-seal-apparatus means at the one or the other or both ends of the shaft-receiving cavity, to provide a ferrofluid O-ring-type seal about the surface of the rotary shaft;
• (e) an electrically conductive ferrofluid composition employed as the ferrofluid in the ferrofluid seal apparatus;
• (f) a low-viscosity, electrically conductive ferrofluid composition in the bearing cavity, the ferrofluid composition comprising a liquid carrier, magnetic particles, and a cationic surfactant, wherein the ratio of the magnetic particles to cationic surfactant ranges from about 0.5:1 to 20:1, and having a viscosity of less than about 50 cps at 27.degree. C. and a resistivity of about 10.sup.2 ohms or less;
• and
• (g) means to ground the bearing apparatus, thereby providing an electrically conductive bearing apparatus which conducts accumulated electrical charges from the shaft to the ground, without the need for a mechanical grounding button in contact with the shaft.

A computer-disc-drive system, which computer-disc-drive system comprises in combination:

• (a) a magnetic head means to receive or impart information from or to a computer disc;
• (b) a computer disc;
• (c) a magnetically permeable rotary-shaft element, the computer disc secured to one end of the shaft for rotation therewith;
• (d) a housing characterized by a shaft-receiving cavity and an internal wall surface, to define a bearing cavity, the shaft-receiving cavity having a one end and another end;
• (e) an electrically conductive, low-viscosity ferrofluid composition disposed in the bearing cavity;
• the ferrofluid composition comprising a liquid carrier, magnetic particles, and a cationic surfactant, wherein the ratio of the magnetic particles to cationic surfactant ranges from about 0.5:1 to 20:1;
• (f) a ferrofluid-seal-apparatus means at the one and the other ends of the shaft-receiving cavity, to define a ferrofluid O-ring-type seals about the surface of the rotary-shaft element;
• (g) thrust-bearing means at the one and the other ends of the shaft-receiving cavity, to provide for support of the longitudinal movement of the rotary-shaft element;
• (h) a motor means to impart rotary motion to the shaft element and the computer disc;
• and
• (i) means to ground the housing, so that accumulated static charges across the magnetic head means and the computer disc may be conducted through the rotating shaft element and the housing to ground, the ferrofluid in the bearing cavity having a viscosity of less than about 25 cps at 27.degree. C., and having a resistance of less than about 50 ohms.