Head: Bearings for extreme environments
Deck: Only special bearings can survive the outer limits of temperature, load, and washdown
From:Graphite Metallizing Corp.
Yonkers, NY
Date:November 12, 2003
Conventional journal, rolling-element, and self-lubricating (oil impregnated) bearings are lubrication limited and often do not provide satisfactory life in applications that are hostile to lubricants. Extreme temperatures, grit, vibration, prolonged idle time, corrosives, and high loads all have negative effects on the bearing lubrication. And without the protection of the lubricant, metal-to-metal contact occurs in the bearing and sudden failure is not far behind.
Five environmental killers of conventional bearings
Common industrial environments that are too tough for conventional bearings include:
Temperature extremes—High temperatures are probably the most common cause of premature failure of lubricated bearings. They cause lubricants to migrate from bearing areas, or, if the temperature is high enough, to carbonize on the bearing surfaces, preventing them from turning. High temperature operation can also destroy seals often used on ball bearings.
Low temperatures are inhospitable to bearing lubricants, too, causing them to solidify, losing their ability to flow between bearing surfaces.
Vibration—Vibration loads press bearing lubricants out of the bearing area. The greater the diameter/width ratio of a bearing, and the nearer the system operates to the resonant frequency, the faster this occurs. Vibration can be radial (from loads or other irregularities) or torsional (from coupling misalignment or resonance). Vibration also accelerates the process of brinelling of rolling elements in a bearing and dramatically accelerates fatigue failure, effectively multiplying loads. A bearing that is designed to last for a specified life at a specified load may have its life shortened by a factor of 10 under conditions of moderate vibration.
What often goes unrecognized is that—to a degree—vibration is inevitable in the design of rotating systems. 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.
Corrosives and washout—Solutions in pickling lines, pulp mills, plating lines, dye lines, waste treatment plants, and marine environments can turn perfectly good ferrous metal parts into pitted relics in a matter of months. And 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 to be solvents for bearing lubricants. Although they differ in degree—milk obviously being less of a solvent than xylene—all tend to wash lubricants from bearings. Steam is a special problem in that it is a hot fluid; conventional bearings that are washed down will eventually be washed out, regardless of how good the seal is. Some pumps rely on process fluids for lubrication; these obviously cannot use conventional lubricants.
Particulates—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 in a sealed bearing. In a lubricated bearing, the particulates combine with the grease or oil to form a lapping compound and will eventually destroy the seals and bearing surfaces.
Inactivity—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 the bearing area and eventual hardening. When the machine is eventually restarted, metal-to-metal contact occurs.
Solid solution
Eliminate lubricants from a bearing and you eliminate all of the environmental limitations associated with them.
For over 75 years, graphite-metal compounds have replaced conventional bearings that must operate in extreme environments. Graphite is one of the earliest materials to be recognized as a solid lubricant, and continues to be used on applications that are too hot—above 500° F—for other solid lubricants such as PTFE or moly disulfide. It gains its low friction characteristics (0.15 to 0.3 on cold-rolled steel) from its planar structure; individual planes tend to slip over each other.
By itself, graphite has limited durability, so to be used as a bearing, it is combined into an alloy with a metal such as copper, babbitt, bronze, nickel, or silver. These materials are available in the standard shapes of bearings and bushings, including flanged bushings, thrust washers, and pillow blocks, in bore sizes up to 20 in., standard sizes up to 6 in. in diameter. In addition, custom sizes and shapes can be produced.
Properties
These bearings are very nearly inert, and as such are almost unaffected by industrial processing fluids such as petrochemicals, pulp & paper mill liquor, food compounds (some are FDA approved), acids, steam, or most corrosive gases. As such, one of the most common use is in pumps, particularly in applications where a remote location makes maintenance difficult.
In addition, graphite/metal alloy bearings do not melt or cold flow. They can operate in temperatures that range from –450 to 1000° F, temperatures well beyond the range of traditional lubricants and above the melt point of polymer bearings. Certain grades are available to operate at well in excess of 1000° F in non-oxidizing atmospheres. 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.
Speeds and loads can be determined by conventional PV limits. P is the psi bearing load and V is the shaft speed in feet per minute. For dry running conditions the PV is nominally 12,000; however, for applications that are 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. Some of these bushings have operated at 60,000 rpm when submerged and 200,000 rpm when gas lubricated. Load-carrying properties range up to more than 1000 psi, depending on the speed and operating condition. Compression strength ranges from 15,000 to 25,000 psi. depending on the grade. Typical applications where these bearings are selected for their load carrying abilities include hot material handling devices, furnace conveyors, and cart wheels
Vibration-prone applications, such as pumps with mechanical seals, can benefit from these bearings because their running clearances can be made tighter than those for other bearing materials. The coefficient of expansion of these materials is about 1/2 that of steel, so shaft clearances are typically in the range of .008 to .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.
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.
Some of the typical families of applications are shown in the table below.
Applications of Graphite-metal bearings
MaterialCategory / Bearing Configurations / Load Characteristics / Operating Temperature
Limit (°F) / Applications and
Special Features
Babbitt-graphite /
- Pump bushings
- Thrust washers
- Rotary seal rings
- Medium load/medium speed
- Medium load/high speed
Copper-graphite /
- Hot conveyor bushings
- Hot stirring shafts
- Stem bushings
- Mixing equipment
- Food processing conveyors
Bronze-graphite /
- Furnace cart wheels
- Transfer conveyors
- Screw Conveyors
- Drag Chains
- Cam followers
- Ideal for large and special applications
Nickel-graphite /
- Pump bushings
- Mixer bushings
- For toughest chemical applications
- Nuclear applications
Silver-graphite /
- Steam turbine packing rings
- Vacuum applications
- Vacuum environments
- Packing rings
- Slip Rings
- Electrical applications
Cast iron –graphite /
- Pump Bushings
- Fuel Pumps
- Sulfuric acid
- Molten metals
Rules of thumb for applying graphite-metal bearings
Consider applying these bearings when one or more of the following operating conditions are present:
Operating Condition / Consider Graphite-metal Bearings When:Loads / Loads will approach 500 psi—or high enough to squeeze out conventional lubricants.
Vibration / Vibration is the result of clearance required for expansion when using metallic bearings and is expected to be 0.5 in.-sec, or greater. Typical running clearances for graphite-metal bearings ranges from 0.004 to 0.010 in., depending on the application.
High speeds / Rotating speeds in air or gas-lubricated bearings will exceed 50,000 rpm.
Low speeds / When operating speeds are too low for good lubrication for more than 10% of the operating life, when shafts do not make a complete rotation, or when there are frequent reversals of direction.
Temperature / Temperatures on bearings are outside the range of –30 to +200° F for more than 20% of its operational life, or when rapid thermal shock (on the order of 20 degrees/sec) is possible.
Submerged / A bearing will operate in a process fluid, and particularly when the process fluid serves as a lubricant for the bearing; when the process fluid is a light hydrocarbon; and when there are frequent starts and stops or transient upsets causing the bearing to run dry.
Static / A bearing will sit idle for more than three months without being turned; or when a bearing will sit idle for more than a month at ambient temperatures of more than 100° F.
Chemical environments / Acids, bases, washdown solvents, petrochemicals, process fluids, and steam are encountered.
FDA acceptance / Food contact is possible
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