Babbitt metal, also known as white metal, is a low-friction alloy designed primarily for use as a bearing surface in machinery. Invented by Isaac Babbitt in 1839, this specialized material minimizes friction between moving parts. Its unique performance comes from its metallurgical structure, which is a composite of soft and hard metallic components. This engineered mixture allows the material to function effectively under load, protecting costly machine components from wear and damage.
Core Composition and Types
The composition of Babbitt metal centers around four elements: Tin (Sn), Lead (Pb), Antimony (Sb), and Copper (Cu). This alloy structure is categorized as a metal matrix composite, where small, hard crystals are dispersed throughout a softer matrix metal. The soft base metal (either Tin or Lead) allows for pliability and adaptability to the shape of the shaft, while the hard compounds provide the actual load-bearing surface.
Antimony and Copper react with the base metal to form microcrystalline intermetallic compounds, which are significantly harder than the surrounding matrix. These hard particles resist the direct pressure of the rotating shaft. Meanwhile, the softer Tin or Lead matrix yields slightly to distribute the load. The precise ratio of these elements determines the alloy’s grade and its suitability for specific applications.
Babbitt metal is classified into two types based on its primary constituent: Tin-based and Lead-based alloys. Tin-based Babbitt, such as ASTM B-23 Grade 2, typically contains a high percentage of Tin, often around 88% to 91%, with the balance being Antimony and Copper. This type offers superior corrosion resistance and higher fatigue strength, making it the preferred choice for high-speed and high-load applications like internal combustion engines and turbines.
Lead-based Babbitt, exemplified by ASTM B-23 Grade 15, contains a much higher percentage of Lead, often exceeding 80%, with a lower Tin content. This alloy is more economical and possesses a higher operating temperature limit compared to its tin-based counterpart. However, due to their lower mechanical strength, Lead-based alloys are suited for lower-speed and moderate-load machinery, such as line shafting and certain compressors. The choice between the two types depends on the operating environment, including factors like load, speed, and temperature.
Essential Material Properties
The utility of Babbitt metal in plain bearings is linked to a specific set of mechanical properties arising from its hard-soft structure. The first property is embeddability, the ability of the soft matrix to absorb small foreign particles, such as dirt or grit, that enter the bearing with the lubricant. By allowing these particles to sink harmlessly below the surface, the Babbitt lining prevents them from scoring or damaging the steel shaft.
Another important property is conformability, which allows the soft alloy to deform plastically and compensate for slight misalignments in the shaft or bearing housing. This yielding ensures that the load is distributed evenly across the bearing surface, preventing localized pressure points that could lead to premature failure. Without this ability to conform, installation errors or shaft deflections could quickly destroy the bearing material.
Babbitt metal demonstrates a naturally low coefficient of friction, particularly when a continuous film of lubrication is present between the bearing and the shaft. Even if the lubricating film temporarily breaks down, the soft matrix of the Babbitt alloy can smear or melt slightly due to frictional heat, providing a temporary, self-lubricating layer. This characteristic contributes to the material’s resistance to galling, which is the destructive welding of two metal surfaces under friction.
The Tin-based alloys generally exhibit better resistance to corrosion than the Lead-based varieties, especially when exposed to certain lubricating oils and their degradation products. The performance of either type is dependent on maintaining the integrity of the lubricating film. These characteristics combine to make Babbitt metal a robust and forgiving material for bearing applications.
Function and Application in Bearings
Babbitt metal functions almost exclusively as the thin lining in plain bearings, which support and guide rotating shafts. The alloy is typically bonded to a much stronger backing shell made of steel, bronze, or cast iron. This shell provides the mechanical strength to contain the material and support operational loads, ensuring the bearing has a strong backbone while presenting the anti-friction surface to the shaft.
The primary function of the Babbitt lining is to establish hydrodynamic lubrication, where the rotating shaft is completely separated from the bearing surface by a pressurized film of oil. The soft and compliant nature of the Babbitt surface facilitates the formation of this oil wedge, which carries the full load during normal operation. The material’s embeddability and conformability act as a safeguard during demanding operational phases.
The soft matrix protects the shaft during periods of boundary lubrication, such as startup, shutdown, or momentary overloads when the oil film is thin or absent. If the shaft makes fleeting contact with the bearing surface, it touches the soft Babbitt rather than the hard backing material, preventing damage to the expensive shaft. This sacrificial layer ensures that wear or damage is confined to the easily replaceable bearing shell.
Babbitt bearings are widely utilized in heavy-duty and large-scale machinery where high reliability and ease of maintenance are valued. Common applications include large electric motors, turbogenerators, industrial compressors, and gearboxes. They remain the material of choice for large marine engine bearings and turbines, where the cost and time associated with repair necessitate a forgiving bearing material.