Metallized Carbon Corporation recently announced Metcar carbon-graphite bushings for use in gear pumps that pump aviation fuel for aircraft engines. The carbon-graphite bushings are used to support both the drive gear shaft and the idler gear shaft. Metcar carbon-graphite bushings are preferred for this application because they can use aviation fuel as the bushing lubricant. Aviation fuel is a low viscosity liquid that produces only an extremely thin hydrodynamic film, too thin to provide adequate lubrication for traditional metallic bushings. But since Metcar’s carbon-graphite material -- in contrast -- has no atomic attraction to a metallic shaft, the thin fuel film is sufficient to lubricate metallic shafts running in the carbon-graphite bushings.

A second major advantage of carbon-graphite bushings is that they are self-lubricating -- they can run dry for short periods of time without catastrophic pump failure or significant wear. In addition, Metcar carbon-graphite bushings are dimensionally stable, which permits the close bushing to shaft running clearances that are required in gear pump applications. Finally, these carbon-graphite bushings have a relatively low elastic modulus. While elastic enough that they can be press-fit into the metallic gear pump housing with no difficulty, after being properly press-fit, the Metcar bushings are pre-stressed in compression. Therefore, they exhibit nearly the same coefficient of thermal expansion as the metallic gear pump housing material. This assures that the running clearance between the shafts and the bushings will remain the same throughout the entire operating temperature range of the pump.