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Pumps & Systems, May 2008

For more than 100 years, machine parts composed of mechanical carbon have provided an alternative solution in applications where temperature and atmosphere prevent the use of oil-grease lubricants. Mechanical carbon materials contain graphite, which is relied on for its self-lubricating characteristics. 

Mechanical carbon materials can be an effective solution-and sometimes the only workable solution-for moving/movable machine parts where rubbing must occur with low wear and low friction, and oil-grease lubrication cannot be used.

There are two categories into which mechanical carbon applications can be divided: dry running applications, where the carbon parts are running in a gas; and submerged applications, where the carbon parts are running in a liquid. This article takes a closer look at some of the challenges and solutions for each category.

Compositions

Bonding fine graphite particles with a hard, strong, amorphous carbon binder produces a mechanical carbon material called "carbon-graphite." Further heat-treating, to approximately 5,100-deg F (2,800-deg C), causes the amorphous carbon binder to become graphitized. This material is called "electrographite." 

The electrographite material is generally softer and weaker than the carbon-graphite material, but has superior chemical resistance, oxidation resistance and thermal conductivity compared to the carbon-graphite material.

Impregnation

Both carbon-graphite and electrographite are normally produced to contain approximately 15 percent porosity by volume. To produce mechanical carbon grades with enhanced properties, the porosity in the carbon-graphite and electrographite materials can be impregnated by vacuum-pressure with thermal setting resins, metals or inorganic salts, as explained below:

• Resins. The most common thermal setting resins used are phenolics, polyesters, epoxies and furan resins. Resin impregnation produces materials that are impermeable and have improved lubricating characteristics.
• Metals. The most common metal impregnations are babbitt, copper, antimony, bronze, nickel-chrome and silver. Metal impregnation produces materials that are harder, stronger and impermeable, with improved lubricating qualities, and better thermal and electrical conductivity.
• Inorganic Salt. The inorganic salt impregnations are proprietary formulations with enhanced lubricating qualities and improved oxidation resistance of the carbon-graphite or electrographite base material.

Dry Running Applications

If two metal parts are rubbed together without oil-grease lubrication between them, the oxide film on the metal parts will quickly wear off and the two metals will exhibit strong atomic attraction. The atomic attraction results in high friction, high wear, and-at higher speed or loads-galling and seizing. 

On the other hand, when carbon materials are rubbed against metal, oil-grease lubricants are not needed. Since no strong atomic attraction exists between carbon and metals, a thin film of graphite is automatically burnished onto the metal surface when mechanical carbon materials are rubbed against metals. This thin layer permits rubbing with low friction and low wear.

For many dry running applications, oil-grease lubrication is excluded as an option because the machines operate at elevated temperatures. At temperatures exceeding 300-deg F (150-deg C), oil-grease lubricants can lose their viscosity, volatilize or carbonize, which makes them ineffective for lubricating metal parts.

Another problem occurs at low temperatures. At temperatures between -30-deg F and -450-deg F (-22-deg C and -268-deg C), oil-grease lubricants can become too thick or even solidify. In a vacuum or partial vacuum, oil-grease lubricants can volatilize and contaminate the environment. In abrasive dust environments, oil-grease lubricants can attract abrasive dust to form a grinding compound that can increase the wear rate. Additionally, oil-grease lubricants are not permitted in some gas compressors and air pumps because the pumped gas must be kept oil-grease free.

Because of its ability to function without oil-grease lubrication, mechanical carbon is used for many dry running applications, such as bearings and thrust washers for high temperature conveyers; bearings for hot air dampers; bearings, vanes and endplates for rotary air and vacuum pumps; and radial and axial seal rings for steam turbines, blowers and jet engines. Other typical mechanical carbon applications include seal rings for rotary steam joints, faces for dry running mechanical seals, piston rings and guide rings for gas compressors, and seats for high temperature gas valves.

Wear

The primary limitation for dry running mechanical carbon parts is wear. Mechanical carbons are softer than the metal parts they rub against; therefore, the mechanical carbon parts wear and the metal parts do not.

The wear rate of the carbon part is roughly proportional to the rubbing speed, V (ft/min), multiplied by the face loading, P (psi). This product, or PV factor, represents the intensity of rubbing. If the PV factor is less than 500-psi X ft/min (0.19-kg/cm² m/sec), the temperature is less than 850-deg F (454-deg C) and the allowable wear is at least 0.050-in (1.3-mm) per year, then it is usually possible to specify a mechanical carbon and counter material combination that will meet the wear requirement. If the PV factor or the temperature is lower, the wear rate will also be lower.

Other factors that affect the wear rate are counter material and counter material surface finish. Counter material should be at least Rc 20 hard, and even harder counter material gives better wear rates. The counter material should have at least a 16 micro-inch (0.4 micron) surface finish. Wear rates continue to improve until surface finish reaches about 8 micro-inch (0.2 micron). With counter material surface finishes rougher than about 16 micro-inch (0.4 micron), the asperities on the counter material are too tall and cannot be covered by the graphite-burnished film essential for a low dry-running wear rate. The uncoated asperities on the counter material can "grind" the softer mechanical carbon material and cause a higher wear rate.

Temperature and atmosphere can also affect wear rate. Low wear rates for mechanical carbons require condensable vapors in the surrounding atmosphere. In atmospheres with no condensable vapors, such as in vacuum, dry nitrogen or high altitude air, the mechanical carbon material can be impregnated with solid lubricants that do not require condensable vapors.

The most accurate way to determine the wear rate of mechanical carbon is to test run sample mechanical carbon parts in a prototype machine at the proposed operating conditions.