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°F (454°C), and the allowable wear is at least 0.050 inches (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-inches (0.2 micron). With counter material surface finishes rougher than about 16 micro-inches (0.4 micron), the asperities on the counter material are too tall and cannot be covered by the graphite-burnished film that is 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 affect wear rate as well. 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.
To avoid cracking, chipping and breaking of the mechanical carbon material, the loading is normally limited to about 1000 psi (70 kg/cm2). This load is less than 10 percent of the compressive strength of most mechanical carbon materials. This high safety factor is required because the actual load on the carbon part is often much higher than the calculated loading. This occurs because of the “line contact” of new carbon bearings with shafts that have the recommended running clearance. This line contact disappears quickly after rotation begins and the shaft “beds into” the carbon bearing. With carbon thrust washers, the safety factor is required because of possible edge loading due to misalignment, as well as possible impact loading from dynamic vibration.
Mechanical carbon parts are limited in temperature mainly because some carbon-graphite materials begin to oxidize in air at a temperature of about 600°F (316°C). Some electrographite grades begin to oxidize in air at about 750°F (400°C). The oxidation reaction is C + O2 = CO2.
Oxidation is a diffusion-controlled reaction, and the solid carbon material is changed to CO2 or CO gas and removed from the outside surface of the carbon material. The oxidation onset temperature can be increased by about 100°F (55°C) by impregnating the base carbon material with oxidation inhibiter salt solutions. The salt solution impregnated carbon material is heated to evaporate the solvent, and the oxidation inhibiter salt is left in the porosity of the carbon. The oxidation inhibiter salts help to create the burnish graphite film on the metal counter surface, and they react chemically with the carbon material to inhibit the oxidation reaction.
In neutral or reducing atmospheres, oxidation is usually not problematic. Carbon-graphite grades will show some shrinkage when heated in a neutral atmosphere above 1800°F (1000°C). Electrographite grades do not show any significant dimensional change even when heated to 5100°F (2800°C) in a non-oxidizing atmosphere. With metal impregnated grades, the melting point of the metal cannot be exceeded. With resin-impregnated materials, the dissociation temperature of the resin cannot be exceeded.
The coefficient of friction of dry running mechanical carbon parts depends on several factors: the load, speed, counter material, and condition of the surfaces. The coefficient of friction of mechanical carbon parts sliding against metals is normally in the range of 0.1 to 0.3, which is higher than the coefficient of friction for oil-grease lubricated metal parts. Oil grease lubricated metal parts can show a coefficient of friction as low as approximately 0.02. Therefore, dry running carbon parts can exhibit up to ten times the amount of friction as oil-grease lubricated metal parts.