J. P. Boylan
Director, Product Engineering
Morgan Advanced Materials and Technologies
St. Marys, Pa.
One of the toughest jobs bearings face is running in a pump with little or no lubrication. Known as “running dry,” the friction and resulting heat build-up can destroy bearings and damage other parts of the pump as well. To make matters worse, if fluid suddenly reaches the hot bearing, the drastic temperature change can make it shatter.
Suppliers of engineered materials have been working hand in hand with bearing producers and pump manufacturers to develop products and coatings that stand up to such harsh conditions. Some newer bearing materials offer excellent protection during run-dry conditions, stand up to thermal shock, and prevent damage from chemical attack and abrasives.
One family of materials with excellent run-dry capabilities is self-lubricating mechanical carbon. It’s a combination of amorphous carbon and graphite held together with high-temperature carbonaceous bonding. These composite materials combine the strength, hardness, and wear resistance of carbon with the corrosion resistance and self-lubricating properties of graphite. The resulting structure is completely carbon bound, has excellent compression strength, and will not creep under load. In addition, the porous carbon-graphite structure can be filled with a variety of impregnants, including metals (antimony, babbitt, silver, and nickel chrome) and resins to enhance chemical, mechanical, and tribological properties.
These new grades of carbongraphite promise to improve a pump’s run-dry performance. For example, a manufacturer of magnetic- drive pumps in the chemical processing industry turned to selflubricating, carbon-graphite bushings when it set out to develop a pump with long-term run-dry capabilities. The special formulation deposits a low-friction transfer film that controls temperature rise if the interface becomes dry, and it also features corrosion resistance required for many pump applications.
Diamond is another carbon material being considered for use in marginal lubrication conditions. Considerable research is underway to determine if diamond, applied as a coating, can improve pump performance. Wear and chemical resistance are main advantages. As a coating, however, diamond’s drawbacks currently are cost and adhesion. A diamond coating is typically just millionths of an inch thick. If an upset condition disturbs the coating, resulting debris can quickly damage shaft and bearings.
Other extremely hard wear-resistant coatings include tungsten carbide and chrome carbide. Both run well against hard bearing materials and are frequently used when long life and infrequent maintenance justifies a rather high initial cost.
Another family of materials with great promise for marginal lubrication situations includes newly developed sintered silicon-carbide composites. The materials contain a network of nonconnected pores and free graphite. As a pump bearing, the graphite in the silicon-carbide matrix provides a measure of self-lubrication. Its pores hold a liquid reservoir of process fluid to maintain hydrodynamic lubrication in the event of an upset condition. The result is substantially improved rundry survivability as well as high thermal- shock resistance.
When abrasion resistance is a factor in choosing bearing materials, the best designs combine a hard shaft material running against a hard bearing material. Tungsten carbide or chrome-oxide coatings on shafts provide viable options. Silicon-carbide bearings running against such coatings offer excellent abrasion resistance, as does silicon-carbide bearings running against a silicon-carbide shaft sleeve on the pumps.
Experts recommend sintered silicon carbides in extremely acidic or caustic conditions. For less chemically aggressive or abrasive media, reaction- bonded silicon carbide has proven successful. Reaction-bonded silicon carbide consists of a siliconcarbide matrix infiltrated with molten silicon. The process yields a solid silicon- carbide structure that contains about 10% free silicon. This free silicon reduces friction as it acts in a sacrificial manner during operation, improving the tribology and surface topology. Graphite can be added to the reaction bonded matrix to further enhance performance. This composite combines the abrasion resistance of silicon carbide with an enhanced tribology associated with graphite.
One successful application of reaction- bonded silicon carbide involves an oil refinery that produces paving asphalt. Ingersoll Dresser recently turned to PR-9242 from Morgan Advanced Materials to eliminate premature bearing failures in a pump handling abrasive and corrosive asphalt at 425°F. The original antifriction bearings failed in about a month because asphalt particles loaded the races. A pump retrofitted with reaction- bonded silicon carbide bearings has operated for over a year with no signs of wear. The material manufacturer estimates that the bearings should last about three years in this application.