The life and load carrying capability of journal bearings is limited by the properties of the materials used in their construction. Hydrodynamic bearings include a strong steel backing member lined on their bearing surfaces with a compliant layer of metal, typically constructed of tin and lead-tin alloys. Such bearings are commonly called bi-metal bearings indicating a metal backing having a compliant metal bearing surface lining. The compliant metal layer can accomodate wear debris accumulated in the lubricating oil but does not have sufficient fatigue strength and tensile strength to support large mechanical loads. The greater the thickness of this compliant layer, the more tolerant the bearing is to wear debris and other foreign particles in the lubricating oil. However, the load carrying capacity of these bearings decreases as the compliant layer thickness increases. As a result, bearing designs have been a com-promise between the capability to accomodate wear debris and the ability to support loads and resist mechanical degradation such as fatigue.
A wide variety of compliant metal layers have been developed in an attempt to reach an effective compromise between wear debris accomodation and strength while also maintaining good temperature resistance, corrosion resistance, and resistance to scoring, while being conformable to shaft distortion and bending. For example, copper-lead and aluminum-tin compliant metals were developed to provide a harder, stronger lining material while attempting to provide sufficient compatability to accomodate wear debris, but such metal layers were found to suffer from relatively poor seizure and score resistance. Tri-metal construction then was developed by applying an overlay of metal on the bi-metal lining, such as by sintering a metal powder lining onto a steel strip or backing member and overlaying approximately 1 mil final metal layer, generally a lead-tin or lead-tin-copper alloy, such as by electroplating to increase fatigue resistance. The tri-metal bearing surfaces provide much higher load-carrying capacity for heavy duty bearings, but do not have as good embedability, capacity to accomodate wear debris, as the more compliant bi-metal bearing surfaces. Further, the tri-metal bearings are of much higher cost than the bi-metals.
Others have attempted to manufacture bearing materials having both strength and wear debris embedability by incorporating the strength characteristics of graphite fibers into soft metal linings to provide a bearing material comprising a metal matrix incorporating graphite (carbon) fibers. Such carbon fiber bearing materials have been manufactured generally in accordance with two known methods, illustrated by the Giltrow et al. U.S. Pat. No. 3,623,981 and the Old et al. U.S. Pat. No. 3,938,579. In accordance with the Giltrow et al. method, the carbon fibers are chopped and metal coated or mixed with powdered metal, to obtain a desired ratio of metal to carbon fiber, and the carbon fibers and metal are pressed into a desired shape to form a bearing having uniformly dispersed, randomly oriented carbon fibers.
The Old et al. method coats the carbon fibers with a metal coating, such as copper or nickel, so the fibers are wettable by the bearing metal, and the metal coated carbon fibers are directed into a trough in the form of a continuous strip. Molten bearing metal is poured into the trough at a temperature sufficiently high to wet the carbon fibers and thereafter the metal-carbon fiber matrix is cooled to solidify the metal-carbon fiber matrix bearing materials.
Graphite-metal matrix bearings manufactured in accordance with the Giltrow et al. and Old et al. methods have not achieved widespread commercial acceptance as a result of a number of problems associated with the processes. In accordance with these prior art processes, there is an insufficient degree of intimate contact, wetting and infiltration of compliant metal between and around the carbon fibers resulting in bearing materials which may include pockets of loosely held carbon fibers, at least on a microscopic scale, and also resulting in insufficient compliant metal infiltrating between the carbon fibers and the backing member to provide sufficient bonding between the bearing lining and the backing member. This insufficient bonding between lining and backing members sometimes results in delamination between the lining and the backing. Another disadvantage of the prior art Giltrow et al. and Old et al. methods is that these methods do not precisely control the fiber placement, the fiber orientation, or the fiber volume fraction in the graphite-metal matrix. All of these disadvantages are overcome in accordance with the principles of the present invention.