Eben Walker
Chief Engineer
Graphite Metallizing Corp.
Yonkers, N.Y.

Graphalloy bearings and bushings are available as flanged bushings, thrust washers, and pillow blocks in bores to 20 in. Compression strength ranges from 15,000 to 25,000 psi, depending on grade. The bearings are nearly inert and mostly impervious to industrial processing fluids such as petrochemicals, pulp and paper mill liquor, food compounds (some are FDA approved), acids, steam, and certain corrosive gases. Applications include kilns, furnace conveyors, dryer rollers, steady bearings, boiler feed systems, and petrochemical pumps.

Graphalloy bearings and bushings are available as flanged bushings, thrust washers, and pillow blocks in bores to 20 in. Compression strength ranges from 15,000 to 25,000 psi, depending on grade. The bearings are nearly inert and mostly impervious to industrial processing fluids such as petrochemicals, pulp and paper mill liquor, food compounds (some are FDA approved), acids, steam, and certain corrosive gases. Applications include kilns, furnace conveyors, dryer rollers, steady bearings, boiler feed systems, and petrochemical pumps.

Bearings in submerged pumps and agitators found in pulp mills, petrochemical, and waste-treatment plants, are typically lubricated by the working fluids. But intermittent operation and varying working-fluid levels may starve bearings, which is a problem because grease and oil lubricants aren't an option. Graphite-metal alloy bearings, in contrast, can operate dry for over 50 hr in some cases until fluid is reintroduced.

Bearings in submerged pumps and agitators found in pulp mills, petrochemical, and waste-treatment plants, are typically lubricated by the working fluids. But intermittent operation and varying working-fluid levels may starve bearings, which is a problem because grease and oil lubricants aren't an option. Graphite-metal alloy bearings, in contrast, can operate dry for over 50 hr in some cases until fluid is reintroduced.


Agrease-filled ball bearing on an oven-door latch seizes and keeps the door from latching. It turns out the +550°F oven temperatures over time had solidified the grease. A graphite-based dry lubricant better tolerated the high temperatures but made bearing balls skid and damage the race. The ball bearing was eventually replaced with a graphitemetal solid bearing and the problem went away.

Graphite-metal bearings can survive temperatures to 1,000°F (or higher in nonoxidizing environments), beyond the range of liquid and solid lubricants such as PTFE and molydisulfide, and above the melt point of polymer bearings. They also work at cryogenic temperatures down to 450°F.

Graphite alone lacks the durability for bearings so it is often alloyed with a metal such as copper, babbitt, bronze, or nickel. Speed and load limits are set by a conventional PV factor. Here P = bearing load in psi and V = shaft speed in surface ft/min. PV is about 12,000 for dry operation though applications submerged in process fluids can raise it by a factor of 7 to 10. For example, depending on load and shaft diameter metal-graphite bearings can operate at 60,000 rpm submerged in liquid and 200,000 rpm when gas lubricated.

Metal-graphite bearings handle mechanical stresses exceeding 1,000 psi and have a friction coefficient of about 0.15 to 0.3 on cold-rolled steel. Coefficient of thermal expansion is about 50% of steel. This allows running clearances of 0.008 to 0.012 in. or roughly onethird the amount needed for metallic bearings. Closer running clearance in fluid-handling systems, for example, cuts internal leakage and boosts efficiency. It also lowers vibration in pumps with mechanical seals.


TOP FIVE KILLERS OF OIL AND GREASE LUBRICATED BEARINGS

Temperature extremes — High temperatures make lubricants migrate from bearing areas or carbonize on bearing surfaces when temperatures are high enough. High temperatures can also destroy bearing seals. Low temperatures solidify liquid lubricants so they are unable to flow between bearing surfaces, leading to accelerated wear.

High vibration loads can expel bearing lubricants, especially bearings with large diameter-to-width ratios. Even moderate levels of vibration accelerate brinelling of rolling elements and may shorten bearing life by a factor of 10. A certain amount of vibration is inevitable in rotating systems and large operating clearances promote it. Conventional metallic bearings and bushings need sufficient clearances to accommodate thermal expansion. In pumps, for instance, this is typically about 0.030 in. or more between an impeller and seal ring.

Corrosives and washout — Solutions in pickling lines, pulp mills, plating and dye lines, waste treatment plants, and marine environments can harm shafting and metallic bearings or self-lubricated bushings. An associated problem is washout. Fluids and semifluids that go through industrial, foodprocessing, or municipal pumps and mixers are considered solvents to bearing lubricants. Steam is especially problematic because it will defeat most seals. Pumps that rely on process fluids for lubrication obviously cannot use conventional lubricants.

Particulates — Mill scale, silica dust, sawdust, kaolin, coal, fly ash, talc, flour, grain dust, and similar particles are typically small enough to breach bearing seals. In lubricated bearings, particulates can combine with grease or oil to form a lapping compound that damages seals and bearing surfaces.

Inactivity — Over time lubricants migrate from the bearing area and eventually harden, causing metal-tometal contact at start-up.

Graphite-metal bearings make sense when:
  • loads approach 500 psi or are sufficient to squeeze out conventional lubricants.
  • vibration is the result of clearance required for expansion (such as with metallic bearings) and is expected to be 0.5 in.-sec or greater.
  • rotation speeds exceed 50,000 rpm in air or gas-lubricated bearings.
  • speeds are insufficient for proper lubrication more than 10% of operating life; when shafts do not make a complete rotation; or when there are frequent direction reversals.
  • temperatures fall outside the range of 30 to 200°F for more than 20% of operating life, or when rapid thermal shock (on the order of 20°F/sec) is possible.
  • process fluid doubles as bearing lubricant (especially light hydrocarbons), and when there are frequent starts and stops or transients causing the bearing to run dry.
  • bearings sit idle for more than three months without being turned, or more than a month at ambient temperatures exceeding 100°F.
  • bearings operate in hostile chemical environments such as acids, bases, washdown solvents, petrochemicals, process fluids, and steam.
  • food contact is possible (for FDA regs).

 

TYPICAL APPLICATIONS OF GRAPHITE-METAL BEARINGS

MATERIAL

APPLICATIONS

CHARACTERISTICS

OPERATING-TEMPERATURE
LIMIT (°F)

Babbitt graphite

Pump bushings, thrust washers, rotary-seal rings

Medium loads and speeds.
Unaffected by most acids,
bases, petrochemicals, sludge,
or seawater.

300

Copper graphite

Hot conveyor bushings
and stirring shafts, mixing equipment, food processing

High loads and low speeds conveyors, stem bushings

750/1,700*

Bronze graphite

Furnace-cart wheels,
transfer and screw conveyors, drag chains,
cam followers.

Wide load range, variable speed.

750/1,700*

Nickel graphite

Pump and mixer bushings,

Wide load, speed range. Tough chemical and nuclear environments.

750/1,700*

Cast-iron graphite

Pump bushings

Fuel pumps, sulfuric acid,
molten metals

750/1,500*

* In a nonoxidizing environment.