Mechanical Carbon Machine Parts Ease Liquid Handling
- Monday, 01 December 2008
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 that is only a few microns thick is sufficient to prevent rubbing contact, even for high-speed and high load applications. Since mechanical carbon is self-polishing, a polished finish on the counter material will quickly polish the mechanical carbon material. The thin hydrodynamic film that is 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.
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 1000 psi (70 kg/cm2). Application PV factors of over 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-inches (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.
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.
The liquid’s chemical composition 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, resulting in a high wear rate.
Abrasive grit in the liquid being handled 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.