Hydrodynamic bearing system for a brushless DC motor

United States Patent n<)]

Isaacson

[ii] 4,382,199 [45] May 3,1983

[54] HYDRODYNAMIC BEARING SYSTEM FOR A BRUSHLESS DC MOTOR

[75] Inventor: Milton S. Isaacson, Dayton, Ohio

[73] Assignee: Nu-Tech Industries, Inc., Dayton,
Ohio

[21] Appl. No.: 204,624
[22] Filed: Nov. 6,1980

[51] Int.C1.3 A61F 1/24; H02K 5/12

[52] U.S. CI 310/87; 3/1.7;

310/90; 416/111; 417/423 R; 384/107 [58] Field of Search 310/90, 66, 82, 80,

310/87; 3/1.7; 318/696; 128/1 D, 260; 308/DIG. 3, 36, 122, 123, 9, 10; 74/5.43;

290/52; 415/110-112, 503; 417/423 R

[56] References Cited

U.S. PATENT DOCUMENTS

2,916,642 12/1959 Macks 310/90

2,983,832 5/1961 Macks 310/90

3,433,986 3/1969 Arutunoff 310/90

3,446,150 5/1969 Dee 310/90

3,951,573 4/1976 Dunning et al. 417/424

4,027,215 5/1977 Knight et al 318/341

4,173,796 11/1979 Jarvik 3/1.7

4,277,706 7/1981 Isaacson 310/80

Primary Examiner—J. D. Miller

Assistant Examiner—D. L. Rebsch

Attorney, Agent, or Firm—Wood, Herron & Evans

[57] ABSTRACT

A hydrodynamic bearing system for a motor. The motor is illustrated and described as driving a pump for an artificial heart. The motor stator has a cylindrical bore which is closed at one end. The rotor is slidable and rotatable in the bore. The rotor has affixed to its shaft an impeller with its outside diameter concentric to the rotor outside diameter. Both rotor and impeller are supported hydrodynamically such that the tendency is for the entire rotor/impeller assembly (the only moving element) to be completely suspended by fluid. The rotor can be rapidly reversed to provide heart pumping action or can be driven unidirectionally for artificial heart pumping action of another type. The fluid cannot easily escape from the closed end of the stator, thereby providing a dashpot effect which tends to keep the rotor from changing position. In moving away from the closed end, the rotor brings fluid between the end of the rotor and the closed end of the bore to act as a buffer or bearing fluid when reversal moves the rotor toward the closed end.

4 Claims, 3 Drawing Figures

4,382,

HYDRODYNAMIC BEARING SYSTEM FOR A
BRUSHLESS DC MOTOR

This invention relates to a motor, and more particu- 5 larly, to a hydrodynamic bearing for a brushless DC motor.

The invention will be described in particular relation to its use in an artificial heart or in a left ventricle assist device (LVAD), but it should be understood that the 10 hydrodynamic bearing concept utilized by the invention will have wider application. A LVAD generally of the type contemplated by the present invention is illustrated in application Ser. No. 30,280, filed Apr. 16,1979.

In the currently developing LVAD or artificial heart 15 technology, there is need for a motor driven axial flow pump (energy converter) to hydraulically actuate a blood pump to circulate the blood throughout the body. The package of energy converter and blood pump is for implantation in the body with the intention, or at least 20 the hope, that it remain in the body for the rest of the patient's life. Obviously, it is desirable for the energy converter to be the smallest, lightest, most reliable, longest lasting and most efficient device possible. A sacrifice of any of these requirements will result, at the 25 least, in an inconvenience to the patient and at the most in substantial impairment of the patient's ability to function.

The function of the energy converter is to drive a blood pump in such a way as to simulate heartbeats. At 30 present, it is contemplated that the most appropriate energy converter will contain a brushless DC motor of the type disclosed in U.S. Pat. No. 4,027,215. The present invention contemplates a motor whose dimensions are no greater than approximately 1 inch outside diame- 35 ter and 1.75 inches long. The motor is programmed to reverse typically every 0.3 seconds simulating 100 heartbeats per minute. At each reversal, the motor changes from typically 10,000 rpm in one direction to 10,000 rpm in the opposite direction, the reversal occur- 40 ring in approximately 25 milliseconds. Over 100 million cycles at the 100 beats per minute will be required over a two year period.

It is contemplated that the rotor will carry an impeller which will drive a low viscosity liquid first in one 45 direction and then in the opposite direction, that liquid flowing into a diaphragm and causing an alternate pumping of the patient's blood. The nominal value of the axial thrust load seen by the rotor bearings is calculated to be about 1.25 lbs. when the corresponding aor- 50 tic pressure is 100 millimeters of mercury. For high output, corresponding to an aortic pressure of 125 millimeters of mercury, the thrust force is calculated to be 1.6 lbs. For a reverse flow in which the aortic pressure may be one-fourth the forward pressure, the thrust 55 loads for the nominal low output will be proportionally 0.31 lbs.

An objective of the present invention has been to provide a hydrodynamically stable (no half speed whirl or conical whirl) bearing system between the rotor and 60 stator and between impeller tips and bore which will withstand the thrust loads described as well as the minimal radial loads encountered over the life of the system.

This objective of the invention is attained by providing a stator having a cylindrical bore which is closed at 65 one end. A cylindrical rotor passing through the open end of the stator is axially and rotatably mounted in the bore. In the pumping application which has been dis

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cussed, the rotor carries a bladed impeller that is axially and rotatably mounted in its bore. The whole assembly is immersed in the fluid which is to be pumped by the impeller. The rotor/impeller combination is supported hydrodynamically, seeking a position of equilibrium within the two bores.

The position of equilibrium of the rotor with respect to the stator is maintained by the hydrodynamic effects of the fluid which provides the radial and axial bearing support of the rotor with respect to the stator; the dashpot effect which maintains a film of fluid between the rotor and the closed end of the stator so as to maintain the rotor centered axially as well as resisting axial movement in either direction; and a squeeze film effect which minimizes the possibility of rotor-to-stator contact during those periods of almost infinitesimal time when the rotor is stopped during reversal, all as will be described below.

In summary, whereas it is conventional to mount a rotor in a stator with bearings at each end of the stator, in the present invention the bearings are formed by the fluid in the gap between the rotor and stator.

The control circuit for electronically commutating the motor and reversing it is carried inside the body with the main battery power supply outside the patient's body.

The several features of the invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a cross-sectional view of the motor and pump;

FIG. 2 is a cross-sectional view taken along lines 2—2 of FIG. 1; and

FIG. 3 is a diagrammatic view of the pump in association with a heart assist device.

The pump of the present invention is shown at 10 in FIGS. 1 and 2. It includes a housing 11 within which the motor 12 is mounted. The housing is generally cylindrical and has an outer sleeve 14 and an inner sleeve 15. The outer and inner sleeves are maintained in spaced relation by six vanes 17 spaced equiangularly around the circumference of the sleeve and three struts 18, also spaced equiangularly around the housing. The space between the two sleeves forms a passageway 20 for a fluid which may be a gas or liquid, but preferably a low viscosity hydraulic fluid. The fluid is pumped through the passageway by an impeller 25 which is fixed to a shaft 26 on a rotor 27 of the motor 12.

Adjacent the impeller is a nose cone 30 connected by vanes 31 circumferentially spaced around the nose cone to a ring 32. The nose cone 30 and ring 32 are held securely onto the housing by a flanged collar 33 which is threaded onto the end of the outer sleeve 14 adjacent to the impeller.

In addition to the rotor, the motor includes a stator 35 which is built up in the central portion 36 thereof by laminations of ferro-magnetic material and is provided with coils 37 which create the magnetic field. The stator is secured in the central portion of the housing by means of an epoxy encapsulent 38 at the ends of the stator, the stator being centered in the housing by a yoke ring 39.

The stator has an axial bore 40 which is closed at one end by a plug 41 and sealed by an O-ring 42. The plug may be made of stainless steel. It has a surface 45 facing into the bore 40 which may be planar or may be slotted with something in the nature of Rayleigh steps to facilitate the creation of a hydrodynamic thrust bearing, as