Jewellike bearings have been devised to enhance the performance and safety of small rotary pumps that are used to increase or sustain flows of blood in cardiac patients. A pump of this type includes a spinning impeller in an axial- or radial-flow configuration. The impeller shaft is supported at its ends by the jewellike bearings. Similar bearings could be used in other small pumps that are required to operate for long times without need for maintenance.
The jewellike bearings are designed to overcome the deficiencies of older rolling-element, pivot, and journal bearings. One of the chief deficiencies is susceptibility to pump seizure caused by the accumulation of coagulated blood in narrow flow passages and in voids within bearings. In the case of journal bearings, another notable deficiency is damage to red blood cells and generation of microclots in high-shear flows of blood through the narrow journal gaps.
In a pump, the jewellike bearings at both ends of the impeller shaft are identical. Each bearing (see figure) includes the end portion of the shaft, a ring, and an end stone. The rings support the shaft radially, while the end stones sustain thrust (axial) loads and limit axial movement of the shaft. The inner surface of the ring facing the shaft is rounded for line contact with the shaft, and the tips of the shaft are rounded for point contact with the end stones; these line- and point-contact features reduce bearing friction and thereby reduce the power needed to drive the pump.
The radial clearance between the shaft and ring is typically 0.0001 to 0.0002 in. (0.0025 to 0.005 mm). The shaft end play (axial clearance) is typically 0.005 to 0.010 in. (0.13 to 0.25 mm). The bearing components can be made of ceramics or hardened metals.
The jewellike bearings offer several advantages:
- Unlike rolling-element bearings in which shaft seals are used to keep blood from entering voids, these bearings can function without shaft seals. Any blood that enters the small void in either bearing coagulates quickly, but in so doing, it forms a smooth surface that conforms to the end of the shaft and thus does not interfere with the rotation of the shaft. Moreover, the diameter of the shaft can be made very small so that any increase in friction caused by the coagulated blood results in only a small increase in frictional torque.
- The configuration of the rounded inner ring surface and the mating cylindrical lateral shaft surface provides the precise clearance fit needed for precise rotation of the shaft, yet accommodates misalignment of the shaft.
- No axial-preload mechanism is needed because the pump is designed to operate with shaft end play.
- Because the bearing surfaces are in contact with each other, there is no need for the additional pump, reservoir, and/or complex plumbing that would be needed if the design relied on fluid pressure (as in some journal bearings) to carry the bearing loads.
- Blood trauma and potential clotting are reduced in that blood is not forced through narrow gaps.
This work was done by Greg S. Aber of Johnson Space Center.For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Machinery/Automation category.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
Johnson Space Center
Refer to MSC-22721.