Lube it for life

Glenn H. Phelps
Technical Director
Metallized Carbon Corp.
Ossining, N.Y.

Sometimes moving parts go into applications that just can’t risk exposure to petroleum lubricants. Nobody wants grease in their corn flakes, for example. Ditto for pharmaceuticals. Mechanical-carbon materials contain self-lubricating graphite and are sometimes the only way of handling such needs. Bonding fine graphite particles with a hard, strong, amorphous carbon binder produces a mechanical- carbon material that is called “carbon-graphite.”

Further heat treating, to approximately 5,100°F (2,800°C), turns the amorphous carbon binder into graphite. This graphitized material is called “electrographite.” Electrographite materials are typically softer and weaker than carbongraphite, but have superior thermal conductivity and better resist chemicals and oxidation.

Both carbon-graphite and electrographite are normally produced with about 15% porosity by volume. To boost mechanical properties the material is impregnated by vacuum pressure with thermal-setting resins, metals, or inorganic salts. All three boost lubricating properties but provide other qualities as well.

The most common thermalsetting resins used are phenolics, polyesters, epoxies, and furan resins. Resin impregnation produces materials that are impermeable. The most common metal impregnations are babbitt, copper, antimony, bronze, nickel-chrome, and silver. Metal impregnation produces harder and stronger materials. In addition, they have better thermal and electrical conductivity. Inorganic salt impregnations are proprietary formulations that improve oxidation resistance of the carbon-graphite or electrographite base material.

There are two categories to which mechanical-carbon applications are divided: dry running and submerged.

Running dry
As two metal parts rub together without oil-grease lubrication, oxide films on the metal surfaces quickly wear off. The two metals will have a strong atomic attraction. Atomic attraction results in high friction and wear, and — at higher speed or loads — galling and seizing.

In contrast, no oil-grease lubricants are needed when carbon materials rub against metal. There is not strong atomic attraction between carbon and metal. A thin graphite film automatically burnishes onto the metal creating a low-friction and low-wear surface.

Operating temperatures can be problematic for many dry running applications. At temperatures above 300°F (150°C), oil-grease lubricants are often ineffective because they lose their viscosity, volatilize, or carbonize. And at temperatures between –30 and –450°F (–22 and –268°C), oil-grease lubricants can thicken and even solidify.

Likewise, in vacuums or partial vacuums, oil-grease lubricants can volatilize and contaminate the environment. Abrasive dust is a problem because lubricants can combine with it to form a grinding compound that can rapidly wear parts. Oil-grease lubricants also can’t serve in some gas compressors and air pumps because the pumped gas must be kept oil-grease-free.

The self-lubed properties of mechanical carbon let it serve in such dry running applications 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 applications include seal rings for rotary steam joints, faces for dryrunning mechanical seals, piston rings and guide rings for gas compressors, and seats for high-temperature gas valves.

The primary limitation for mechanical- carbon parts that run dry is wear. Mechanical carbons are softer than the metal parts they rub against so they wear while the metal parts don’t. Wear rates are roughly proportional to the rubbing speed, V, (fpm) 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-psifpm (0.19 kg/cm²m/sec), the temperature is less than 850°F (454°C), and the allowable wear is at least 0.050 in./yr (1.3 mm/yr), 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 drop.

Other factors that affect wear rates are the counter material and its surface finish. Counter materials should have a hardness of at least 20 R_c — harder materials wear less. The counter material should have at least a 16-μin. (0.4-μm) surface finish. Wear rates continue to improve until surface finishes reach about 8 μin. (0.2 μm). Surface finishes rougher than about 16 μin. have asperities (sharp, rough, or rugged outgrowths) on the counter material too tall to be covered by the graphite-burnished film. The uncoated asperities can “grind” the softer mechanical-carbon material to wear quickly.

Temperature and atmosphere also affect wear rate. Mechanical carbons need condensable vapors in the surrounding atmosphere to wear slowly. Carbon materials used in atmospheres with no condensable vapors (such as in vacuum, dry nitrogen, or high altitude air) can be impregnated with solid lubricants that don’t require condensable vapors. The most accurate way to determine wear rate of mechanical carbon is to test sample parts in a prototype at the proposed operating conditions.

Calculated loads should be less than 10% (1,000 psi, 70 kg/cm²) of the compressive strength of the mechanical carbon. This high safety factor arises because actual loads often greatly exceed those calculated. The “line contact” of new carbon bearings with shafts that have the recommended running clearance will disappear quickly after rotation begins and the shaft “beds into” the carbon bearing. The safety factor is needed with carbon thrust washers because misalignment may cause edge loading. There may also be impact loading from dynamic vibration.

Temperature limitations arise mainly because some carbon-graphite materials begin to oxidize in air at 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. The solid-carbon material changes to CO₂ or CO gas which comes off the outside surface of the carbon material. Impregnating the base carbon material with oxidation-inhibiter salt solutions can boost the oxidation onset temperature by about 100°F (55°C). Here carbon materials impregnated with salt solutions are heated to evaporate the solvent. The oxidation-inhibiter salt remains in the porosity of the carbon. The oxidation-inhibiter salts create the burnish graphite film on the metal counter surface, and react chemically with the carbon material to inhibit oxidation.

In neutral or reducing atmospheres, oxidation isn’t typically a problem. Carbon-graphite grades will shrink some when heated in a neutral atmosphere above 1,800°F (1,000°C). Electrographite grades don’t show significant dimensional change even at 5,100°F (2,800°C) in a nonoxidizing atmosphere. For metal and resin-impregnated grades, the melting point of the metal and the dissociation temperature of the resin can’t be exceeded.

The coefficient of friction (COF) on mechanical-carbon parts that run dry depends on several factors: the load, speed, counter material, and condition of the surfaces. The COF of mechanical carbon parts sliding against metals is normally in the range of 0.1 to 0.3. This is about 10 times the COF of metal parts lubed with oil or grease. So designers must factor in the higher COF when designing equipment that runs dry.

Running submerged
The COF and wear rate of two rubbing metal parts is extremely low when a hydrodynamic film of oil or grease separates them. However, the hydrodynamic film is too thin when metal parts rub together in low-viscosity liquids such as water or gasoline. Metalto- metal contact results. When this happens, the metal atoms in sliding contact have strong atomic attraction, which brings high friction, wear, galling, and seizing.

Contrast this behavior with that of carbon rubbing against metal in a low-viscosity liquid. Here the resulting thin hydrodynamic film is normally enough to provide lubrication. There is no strong atomic attraction between mechanical carbon and metal, so a hydrodynamic film only a few microns thick is sufficient to prevent rubbing contact, even for high speeds and high loads. Mechanical-carbon surfaces get polished by the materials they touch. And the thin hydrodynamic film created by low-viscosity liquids separates the two polished surfaces.

Carbon parts go into submerged applications that include bearings and thrust washers for liquid pumps that handle hot water, solvents, acids, alkalis, fuels, heat-transfer fluids, and liquefied gases. Mechanical carbon is also used extensively in rings for sealing these same lowviscosity 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 gearboxes and aircraft engines.

Mechanical carbons running submerged have negligible wear under full fluid film, or hydrodynamic lubrication. Mechanical carbons with full fluid-film lubrication normally support a maximum load of about 1,000 psi (70 kg/cm²). Application PV factors of over 2,000 kpsi fpm (773 kg/cm² m/sec) are possible with sliding speeds of over 3,600 fpm (18.7 kg/cm² m/sec).

The material rubbing against the mechanical carbon must meet specifications of hardness, surface finish, and corrosion resistance. The hardness should exceed about 45 R_c, but harder counter materials can bring better results.

The liquid viscosity should be in the range from about 100 centipoises (light machine oil) to 0.3 centipoises (acetone). It’s important that submerged running mechanical-carbon parts have a continuous flow of liquid to the rubbing surface. Otherwise frictional heat will evaporate the liquid. The parts will revert to a dry running condition where the wear rate is much higher. Fortunately mechanical-carbon parts can run dry without catastrophic failure if the flow of liquid is just briefly interrupted.

The chemical composition of the liquid is important because chemicals that attack the counter material or the mechanical carbon will increase the wear rate. Chemical attack of the counter material is particularly harmful. It can cause pits and surface roughness that will disrupt the hydrodynamic film, resulting in a high wear rate.

Of course abrasive grit in the liquid can also be extremely detrimental. It disrupts the hydrodynamic film, erodes the softer mechanical carbon material, and can destroy the fine surface finish on the counter material.

Most mechanical-carbon manufacturers can determine what material will best satisfy specific applications. They should also be able to recommend dimensions and tolerances for parts to ensure proper press-fit or shrink-fit interference and shaft running clearance. Correct mating material and mating material surface finishes are critically important as well.

In recent years a growing concern for the environment and air quality have led to more use of mechanical seals with carbon primary rings because they don’t leak much compared to other seals. Today, new mechanical-carbon materials are proving it to be the “go-to” solution in harsh operating conditions. These new applications show mechanical carbon will be important when other materials fall short.

Make Contact
Metallized Carbon Corp.,
(914) 941-3738, metcar.com