Hydrodynamic bearing type rotary device

1. A hydrodynamic bearing type rotary device, comprising:

a sleeve having a bearing hole and an inner peripheral surface and being formed of a first material having a first elastic limit;

a shaft having an outer peripheral surface and being formed of a second material having a second elastic limit, and which is inserted into the bearing hole of the sleeve so as to be relatively rotatable, a gap being formed between the shaft and the sleeve;

a hub rotor which is attached to either of the sleeve or the shaft, and which rotates;

a bearing surface having hydrodynamic grooves formed on at least one of the outer peripheral surface of the shaft or the inner peripheral surface of the sleeve; and

an ester oil held in the gap between the shaft and the sleeve, wherein

a value of a function (Nd) represented by the following relational expression (1) is 1,000,000 or higher, such that surface pressure at the bearing surface when the shaft and the sleeve contact each other is within the first elastic limit of the first material of the sleeve and the second elastic limit of the second material of the shaft, and

an absolute viscosity of the ester oil is in a range from 2 to 5 centipoises (0.002 to 0.005 [N·S/m^2]) at 70° C.:

Nd =η/( S·Ff·P·D )  (1)

S: Material wear rate S=74800 (constant)

η: Absolute viscosity at 70° C. [N·S/m^2]

P: Load applied to one bearing [N]

D: Shaft diameter [m],

the floating rotation corresponding function, Ff, is defined as follows:

Ff=P /( Fs ×ω)  (2)

ω: Constant angular velocity of the rotary device [rad/S], and

the stiffness corresponding function, Fs, is defined as follows:

Fs =(η·ω· D^ 2 ·L^ 2)/ C^ 3 [N/m]   (3)

L: Length of bearing [m]

C: Bearing gap [m].

2. The hydrodynamic bearing type rotary device according to claim 1 , wherein

the gap formed between the sleeve and the shaft has a width in a radial direction of 1 μm or longer and is substantially constant.

3. The hydrodynamic bearing type rotary device according to claim 1 , wherein:

the shaft is formed of any one of stainless steel, high manganese chrome steel, or carbon steel; and

the sleeve is formed of a material obtained by treating stainless steel or copper alloy with electroless nickel plating, or DLC coating, sintered alloy including 90% or more iron, or a material obtained by forming a triiron tetroxide film on a surface of the sintered alloy including 90% or more iron.

4. The hydrodynamic bearing type rotary device according to claim 1 , wherein:

the shaft has a diameter in a range from 2.0 to 6.0 mm and a rotational rate in a range from 360 to 15,000 rpm; and

a length of the bearing hole provided on the inner peripheral surface of the sleeve is in a range from 3.0 to 20.0 mm.

5. The hydrodynamic bearing type rotary device according to claim 1 , wherein surface roughness of the sleeve inner surface opposing the shaft is in a range from 0.01 to 1.60 μm.

6. A recording and reproducing apparatus including a hydrodynamic bearing type rotary device according to claim 1 .

7. The hydrodynamic bearing type rotary device according to claim 1 , wherein

a surface roughness of the sleeve inner surface opposing the shaft is in a range from 0.01 to 1.60 μm; and

a surface roughness of the shaft is in a range from 0.01 to 0.2 μm, so as to obtain a predetermined wear resistance.

8. The hydrodynamic bearing type rotary device according to claim 1 , wherein the sleeve and the shaft are configured to bear a load of up to about 300 grams when the ester oil is held in the gap between the shaft and the sleeve.