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.

 

Load

To avoid cracking, chipping and breaking of the mechanical carbon material, the loading is normally limited to about 1,000-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.

 

Temperature

Mechanical carbon parts are limited in temperature mainly because some carbon-graphite materials begin to oxidize in air at a temperature of about 600-deg F (316-deg C). Some electrographite grades begin to oxidize in air at about 750-deg F (400-deg 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-deg F (55°-deg 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 remains in the porosity of the carbon. The oxidation inhibiter salts are beneficial because they help 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 1,800-deg F (1,000-deg C). Electrographite grades do not show any significant dimensional change even when heated to 5100-deg F (2800-deg 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.

 

 

Friction

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. This higher coefficient of friction for mechanical carbon parts must be taken into consideration when designing equipment using dry running mechanical carbon parts.

 

Typical Applications

Dry running A good example of mechanical carbon in dry running is in gypsum board dryers. Copper impregnated carbon-graphite bearings hold powered rollers with radial load of 30-psi (2.1-kg/cm2) and sliding speed of 10-ft/min (0.055-m/sec). The shafts are 1045 steal with a 16 micro-inch (0.4 micron) polished surface finish. In the hottest dryer zones, the bearings run continuously in humid air at temperatures up to 650-deg F (343-deg C), and the average wear rate is approximately 0.030-in (0.76-mm) per year of continuous running.

Submerged An example of mechanical carbon running at high PV with hydrodynamic film lubrication is in the mechanical seal industry. An antimony impregnated, carbon-graphite mechanical seal primary ring, sealing light oil, runs against a polished, solid silicon carbide counter face with a face loading of about 600-psi (42-kg/cm2) and a rubbing speed of about 3400-ft/min (18.7-m/sec). This gives a PV factor of about 2,000,000-psi X ft/sec (785- kg/cm2 X m/sec) and the seal rings normally last for about three years of continuous running.

  

Running Submerged

The coefficient of friction and wear rate of two rubbing metal parts is extremely low when they are separated by a hydrodynamic film of oil or grease. However, when metal parts are rubbed together in low viscosity liquids such as water or gasoline, the hydrodynamic film is too thin and metal-to-metal contact can occur. When metal-to-metal contact occurs, the metal atoms in sliding contact have strong atomic attraction, which results in high friction, wear, galling and seizing.

When carbon is rubbed against metal in a low viscosity liquid, the resulting thin hydrodynamic film is normally adequate to provide lubrication. Since there is no strong atomic attraction between mechanical carbon and metal, a hydrodynamic film only a few microns thick is sufficient to prevent rubbing contact, even for high-speed and high load applications. Since mechanical carbon is a self-polishing material, a polished finish on the counter material will quickly polish the mechanical carbon material. The thin hydrodynamic film created by low viscosity liquids can then separate the two polished surfaces.

Carbon parts for submerged applications include bearings and thrust washers for pumps that handle water, hot water, solvents, acids, alkalis, fuels, heat transfer fluids and liquefied gases. Mechanical carbon is also used extensively for mechanical seal primary rings for sealing these same low viscosity liquids. Other applications include vanes, rotors, and endplates for rotary pumps; ball valve seats handling hot oil; bearings for liquid meters; case wear rings for centrifugal pumps; and radial or axial seal rings for gear boxes and aircraft engines.

 

Wear

The wear rate of mechanical carbons running submerged is negligible under full fluid film, or hydrodynamic, lubricated conditions. To assure fully lubricated conditions, application engineers must consider the application load, speed, counter material, counter material surface finish, liquid viscosity, liquid flow and chemical resistance.

The maximum load that is normally supported by mechanical carbons with full fluid film lubrication is approximately 1,000-psi (70-kg/cm²). Application PV factors of more than 2,000,000-psi X ft/min (773-kg/cm² X m/sec) have been achieved with sliding speeds of over 3600-ft/min (18.7-kg/cm² X m/sec).

The counter material rubbing against the mechanical carbon must meet specifications of hardness, surface finish and corrosion resistance. The hardness should be greater than about Rc 45, but better results are achieved with even harder counter materials.

The surface finish on the counter material should be 16 micro-inch (0.4 micron) or better. Wear rate continues to improve with finer surface finish until an 8 micro-inch (0.2 micron) finish is reached. These high finishes are required because the hydrodynamic film with low viscosity liquids is extremely thin. With courser finishes on the counter material, the asperities on the counter material would break through the hydrodynamic film and "grind away" the mechanical carbon. 

The liquid viscosity should be in the range from about 100 centipoises (light machine oil) to 0.3 centipoises (acetone).

A continuous flow of liquid to the rubbing surface is important to the performance of submerged running mechanical carbon parts. If the flow of liquid is not sufficient, frictional heat will evaporate the liquid and the parts will revert to the dry running condition, where the wear rate is much higher.

An important benefit of mechanical carbon parts is that the parts can run dry without catastrophic failure if the flow of liquid is briefly interrupted.

The chemical composition of the liquid must be considered because chemical attack of the counter material or the mechanical carbon will increase the wear rate. Chemical attack of the counter material is particularly harmful, causing pits and surface roughness that will disrupt the hydrodynamic film and result in a high wear rate.

Abrasive grit in the handled liquid can also be extremely detrimental to mechanical carbon parts. The abrasive grit disrupts the hydrodynamic film, erodes the softer mechanical carbon material and can destroy the fine surface finish on the counter material.

 

 

Applications Engineering

Most mechanical carbon manufacturers can determine if they have a material that can satisfy specific application requirements, and can also recommend their best mechanical carbon grade for each specific application. They should also be able to recommend dimensions and dimensional tolerances for new mechanical carbon parts to ensure proper press-fit or shrink-fit interference and shaft running clearance. It is also critically important to the success of mechanical carbon applications to specify correct mating material and mating material surface finishes.

Mechanical carbon materials have provided solutions to a wide variety of lubrication challenges for more than a century. For example, in recent years a growing concern for the environment and air quality has resulted in an increased usage of mechanical seals that use carbon primary rings because they allow for less leakage compared to other seal types. Today, new mechanical carbon materials are continually being developed to meet more demanding mechanical applications, proving it a "go to" solution in harsh operating conditions. As new applications emerge, mechanical carbon will provide a solution where other materials are not sufficient.