Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/388231
44 | September - October 2014 | www.machinerylubrication.com Journal bearings play a critical role in equipment with large and high-speed rotating compo- nents. They primarily are used in machines with more than 500 horsepower and that operate at speeds of more than 3,000 revolutions per minute (rpm). Previously, their most important job was to reduce friction between two surfaces in relative motion. Now, journal bearings not only reduce friction but also provide the means to remove heat and debris while offering protection to more expensive components such as shafts and rotors. Most journal bearings found in modern industrial equipment are built with a surface of Babbitt material. This material has a smooth, slick surface that is easily wetted by liquids. The soft material is resis- tant to galling but easily wears away, protecting the harder surface of the typi- cally steel shaft. The bearing surface is sacrificed, with Babbitt actually becoming the lubricant, and the shaft surface is preserved in case there is a loss of lubrica- tion or other operating anomaly. In the 17th century, Robert Hooke began utilizing "bell" metal as a bearing surface material. Bells were often formed with a bronze alloy that usually included a 4-to-1 mix of copper and tin. In 1839, Isaac Babbitt patented a bearing alloy similar to the mate- rial used today. These alloy formulations became so popular that the name "Babbitt" has become synonymous with the material. The standard Babbitt alloy is comprised primarily of a solid matrix of tin with various amounts of antimony cuboids and/or copper threads. Table 1 shows some of the physical properties of the most common forms of Babbitt classifications in industry. Although tin enters the liquid phase near 450 degrees F, the liquid phase for the Babbitt alloy microstructure does not occur until temperatures exceed 600 degrees F. This allows for machine operation at higher temperatures, which often result from heavier loads and faster speeds. Journal bearings are quite strong and can carry heavy loads as long as they are lubricated properly. The lubrication forms a protective barrier between the shaft and Babbitt surface while also removing friction-gener- ated heat and debris from the bearing. Theoretically, a journal bearing may have an infinite life since there is no contact between the bearing surface and journal. This lubrication barrier also provides damping characteristics when passing through rotor-critical speeds, allowing for stable equipment operation. A journal bearing is easier to remove and repair given the split design. It typically produces less noise than anti-friction bearings and has natural electrical isolation properties. One of the main issues with journal bearings is the requirement of continuous lubrication that is clean and cooled. Contamination is a primary contributor to many bearing failures in industry. The soft By Mark a. tarBet, luMinant Power BEARING LUBRICATION Babbitt: the other Bearing lubrication PARAMETER TYPE 2 TYPE 3 Tin 89% 84% Antimony 7.5% 8% Copper 3.5% 8% Liquidus Point 669°F 792°F Brinell Hardness 24 27 Tensile Strength 12,600 psi at 68°F 10,000 psi at 68°F % Strength at 212°F 52 52 TABLE 1. Common ASTM B-23 Babbitt classifications FIGuRE 1. Illustration of possible bearing damage from debris