Conventional journal, rolling-element, and oil-impregnated bearings are limited by their lubrication. For this reason, they often don’t provide satisfactory life in applications hostile to lubricants. Extreme temperatures, grit, vibration, corrosives, and high loads all compromise bearing lubrication, and allow the metal-to-metal contact that causes sudden failure. But eliminating lubricants from bearings eliminates all of the environmental limitations associated with them. In fact, for nearly a century graphite-metal compounds have done just that, replacing conventional bearing mechanisms to operate in extreme environments.
Graphite was one of the earliest materials to be recognized as a solid lubricant, and is still used on applications that are too hot. Where other solid lubricants such as PTFE or moly-disulfide might fail at temperatures above 500°F, graphite keeps rolling. Its low-friction characteristics — 0.15 to 0.3 on coldrolled steel — results from its planar structure; individual plates tend to slip over each other for cool, smooth motion. But because graphite has limited durability by itself, to be used as a bearing it is combined into an alloy with metals such as copper, Babbitt, bronze, nickel, or silver. Usually the application determines which material is most appropriate. (More expensive nickel and silver are obviously reserved for more exotic applications.) Over 100 grades of this lowfriction matrix are offered for bearing, bushing, washer, and seal applications.
These materials are available in the standard shapes of bearings and bushings used today. In fact, many of these compounds are offered as off-the-shelf replacements for conventional bearings, including flanged bushings, thrust washers, and pillow blocks. Bore sizes reach 20 in. and standard units are usually available with up to 6-in. diameters. In addition, custom sizes and shapes are often produced.
Journal bearing sizing
Speeds and loads can be determined by conventional PV limits, where P is the bearing load (in psi) and V is the shaft speed (in feet per min.) Bearings give satisfactory service if designed within the limits set by this formula:
|rpm X W||< 46.00|
Where rpm = Shaft speed W= Total shaft load (not psi) L = Bushing length, in.
For dry-running conditions the PV limit is nominally 12,000; this is the limit on which the formula is based. However, for applications submerged in process fluids, this can be increased by a factor of seven to ten. (Certain combinations of loads and speeds can exceed the value given by the formulas.) Graphite-alloy bushings have operated at 60,000 rpm when submerged and to 200,000 rpm when gas-lubricated. (Load-carrying properties of these bearings exceed 1,000 psi depending on speed and operating conditions.) Compression strength ranges from 15,000 to 25,000 psi depending on the bearing grade. Typical applications where these load-carrying abilities come in handy include hot material handling devices, furnace conveyors, and cartwheels.
There are several common industrial environments that are too tough for conventional bearings. High temperatures are probably the most common cause of premature lubricated bearing failure. They cause lubricants to migrate from bearing areas. Or, if temperatures are high enough to carbonize oil onto bearing surfaces, it can prevent them from turning. High temperature operation can also destroy seals often used on ball bearings to keep lubes in. Low temperatures are equally inhospitable to bearing lubricants, causing solidification; this decreases a lube’s ability to flow between bearing surfaces.
Graphite-metal alloy bearings do not melt or cold flow. They can operate in temperatures ranging from -450° to 1,000°F, well beyond the range of traditional lubricants and above the melt point of polymer bearings. (In non-oxidizing atmospheres, certain grades can even operate above 1,000°F.) Also, properties of these bearing materials change little over the temperature range at which they can be used.
Applications include bearings for kilns, furnace conveyors, rollers for dryers, steady bearings, boiler feed systems, and pumps for petrochemicals. For example, in one baking application bearings on an oven door latch see temperatures to 550°F. The roller bearing initially specified kept seizing because the grease would solidify, preventing the door from latching. A graphitebased dry lubricant was substituted, but then the rollers skidding in the race damaged it. Finally, a graphitemetal alloy bearing was substituted for the original and the problem was eliminated.
Vibrations can be radial (from loads or other irregularities) or torsional from coupling misalignment or resonance. Vibration loads press bearing lubricants out of critical bearing areas. The greater the diameter/width ratio of a bearing, the faster this occurs. (Vibrations nearer to the system resonant frequency also speed lube migration.) Vibration also accelerates brinelling of bearing rolling elements and dramatically accelerates fatigue failure; this effectively multiplies loads. A bearing designed to last for a specified life at a specified load may have life shortened by a factor of 10 under conditions of moderate vibration.
What often goes unrecognized is that vibration is somewhat inevitable in rotating system designs. Because conventional metallic bearings and bushings must operate with clearances that are sufficient to accommodate thermal expansion, a degree of play is necessary between mating parts. In pumps, for instance, this is typically can be 0.030 in. or more between impeller and seal ring.
Vibration-prone applications, such as pumps with mechanical seals, can benefit from graphitemetal alloy bearings because their running clearances can be made tighter than those for other bearing materials. The coefficient of expansion of these materials is about half that of steel, so shaft clearances are typically in the range of 0.008 to 0.012 in. — less than one-third of that required for metallic parts. In fluid handling systems, this has the added benefit of less leakage and greater efficiency.
Corrosives and washout
Solutions in pickling lines, pulp mills, plating and dye lines, waste treatment plants, and marine environments can turn perfectly good ferrous metal parts into pitted relics in a matter of months. This includes metallic bearings or self-lubricated bushings.
An associated problem is washout. Fluids and semi-fluids that go through industrial, food-processing, or municipal pumps and mixers can be considered solvents for bearing lubricants. Though they differ in degree — milk obviously being less of a solvent than xylene — all tend to wash lubricants from bearings. Steam especially is a problem; as a hot fluid it washes out conventional bearing lubes. In addition, intentional washdowns eventually strip bearings, regardless of how good seals are. Finally, some pumps rely on process fluids for lubrication; these obviously cannot use conventional lubricants.
Graphite-metal alloy bearings are very nearly inert, so they’re unaffected by industrial processing fluids such as petrochemicals, pulp and paper mill liquor, food compounds, acids, steam, and most corrosive gases. As such, a common use of graphite-alloy bearings is in pumps, particularly in applications where a remote location makes maintenance difficult. Another common application is on food and beverage equipment, as many of these sleeve bearings are FDA-approved.
Mill scale, silica dust, sawdust, kaolin, coal, fly ash, talc, flour, grain dust, and similar particles are often small enough to get past any seal on bearings. In lubricated bearings, particulate combines with grease or oil to form a lapping compound; this can eventually destroy seals and bearing surfaces.
Graphite-metal alloy bearings are more tolerant of foreign particles than are lubricated bearings. Because they employ no oil or grease, they do not attract or hold grit. As such they are often used in kilns and agitators where dust is present.
Oddly enough, one of the worst things that can be done to a conventional or self-lubricated bearing is nothing. Inactivity results in migration of the lubricant away from critical bearing areas; it also results in eventual hardening. When the machine is finally restarted, metal-tometal contact occurs. Conversely, graphite-metal alloy lubricant cannot slide away.
Graphite Metallizing Corp. >(914)968-8400