Babbitt metal facts for kids
Babbitt metal, also known simply as Babbitt, is a special type of alloy used to make the smooth, slippery surface inside plain bearings. These bearings help parts of machines spin or slide easily against each other.
This metal was first invented in 1839 by Isaac Babbitt in Taunton, Massachusetts, USA. Over time, other similar mixtures were created. People sometimes call it "babbitt metal" with a small 'b' to show it's a type of material, not just named after a person. It's a better name than "white metal," which can mean many different things.
Babbitt metal is often used as a thin layer on top of other metals. But originally, it was poured right into place to form the whole bearing. It's special because it resists "galling." This means it doesn't easily stick or tear when two metal surfaces rub together. Even though Babbitt metal feels soft and can be damaged easily, it's perfect for bearings. This is because it's made of tiny, hard crystals spread out in a softer metal. Think of it like a metal matrix composite.
As the bearing wears down, the softer metal slowly erodes. This creates tiny paths for lubricant (like oil) to flow. The hard parts are what actually support the moving machine part. If the softer metal is tin, friction can even make the tin melt a little. This melted tin then acts as a lubricant, protecting the bearing if other oils aren't there.
There are many different kinds of Babbitt alloys. Here are some common ones:
- 90% tin, 10% copper
- 89% tin, 7% antimony, 4% copper
- 80% lead, 15% antimony, 5% tin
- 76% copper, 24% lead
- 75% lead, 10% tin
- 67% copper, 28% tin, 5% lead
Engines in cars often use Babbitt metal that is mostly tin. This type can handle the strong pounding forces inside an engine. Lead-based Babbitt can become harder and crack over time. But it works well for tools that spin constantly, like saw blades.
Contents
How Old Bearings Were Made
In the past, Babbitt bearings were made in a special way. First, a cast iron pillow block (a support for the shaft) was placed loosely around a shaft. The inside of this cast iron block often had holes drilled into it. These holes helped hold the Babbitt metal in place once it was poured.
Workers would pack clay at the ends of the bearing. Then, hot, melted Babbitt metal was poured into the space around the shaft. It would fill about half of the pillow block. After the metal cooled, the bearing was taken apart. The metal was then carefully trimmed. Babbitt metal, even when hard, is soft enough to be cut with a knife.
A thin steel shim (a small spacer) was then put in. This protected the bottom part of the bearing and created a space for the top part of the pillow block. After sealing the ends with clay again, more melted metal was poured. This filled the top part of the pillow block through a hole that would later be used for oil.
The two halves of the bearing were then separated at the shim. Any metal blocking the oil holes was removed. Small channels were cut into the surface of the new bearing for oil to flow. The shaft was coated with a special blue dye and rotated in the bearing. When the bearing was taken apart, the blue dye showed where the shaft touched the bearing. High spots were scraped down, and the process was repeated. This continued until the blue dye showed an even pattern, meaning the shaft touched the bearing surface smoothly.
Finally, the bearing was cleaned, oiled, and adjusted with shims. This made sure the shaft was held firmly but could still spin freely. The bearing was then "run in" by being used with lots of oil at low speed and low weight. This helped expose the hard bearing surface completely. After a final adjustment, a very strong and reliable bearing was ready.
Before electric motors were common, factories used central engines. These engines powered machines through long overhead shafts that ran on hundreds of Babbitt bearings. The phrase "run bearing" comes from this time. If a bearing didn't get enough oil, it would get very hot from friction. The Babbitt metal would melt and "run out" of the pillow block.
Modern Babbitt Bearings
Until the mid-1950s, poured Babbitt bearings were common in cars. The Babbitt was poured directly into the engine block or caps using a mold. Tin-based Babbitts were used because they could handle the strong impacts from the connecting rods and crankshaft. These poured Babbitt layers were kept thin. The rods and caps had shims that could be removed as the Babbitt wore down. For example, old Ford Model T cars used two thin shims on each cap and Babbitt that was 86% tin, 7% copper, and 7% antimony. Steel shims were used because brass shims, used today, can compress over time and shorten bearing life. Poured Babbitt bearings often lasted over 50,000 miles. They also tended to fail slowly, allowing a car to be driven for a while without damaging the crankshaft.
Today, the bearings for the crankshaft and connecting rods in a modern car engine are different. They use replaceable steel shells. These shells fit into the bearing caps. The inside of the steel shell has a layer of bronze, which is then coated with a thin layer of Babbitt metal. This Babbitt layer is the actual bearing surface. The process of adding this white metal layer is called "Babbitting."
Other Types of Bearings
In many situations, other types of bearings have replaced Babbitt bearings. These include rolling-element bearings, like ball or roller bearings. While these bearings can have less friction than plain bearings, their main benefit is that they don't always need a constant flow of oil under pressure. Ball and roller bearings can also handle forces that push sideways (radial) and forces that push along the shaft (axial).
However, rolling-element bearings don't absorb shocks or vibrations as well as fluid-film bearings, like those made with Babbitt. Babbitt bearings can "dampen" or reduce these forces, which is a big advantage in some machines.
Images for kids
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Pigs and bars of Grade #2 Babbitt
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Example: Modern Miba Babbitt Thrust Bearing
See also
In Spanish: Babbitt (metal) para niños
