Eben Walker
Chief Engineer
Graphite Metallizing Corp.
Yonkers, N.Y.
|
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 BEARINGSTemperature 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.
- 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 |
Babbitt graphite |
Pump bushings, thrust washers, rotary-seal rings |
Medium loads and speeds. |
300 |
Copper graphite |
Hot conveyor bushings |
High loads and low speeds conveyors, stem bushings |
750/1,700* |
Bronze graphite |
Furnace-cart wheels, |
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, |
750/1,500* |
* In a nonoxidizing environment. |