Bearing alloys, also known as bearing metals or babbitt metals, are a class of materials used in the manufacturing of bearings. In this article, I will cover all the topics related to Bearing Alloys.
Any alloy can function as a bearing alloy, provided it possesses the required anti-frictional properties and sufficient strength and corrosion resistance as well as some desirable properties for specific conditions.
Common bearing materials used in sliding bearing are white material alloys, bronzes, copper-lead alloys, silver and lead aluminum alloys, and porous metal bearings.
Such materials as plastics, wood, carbon-graphite, cast iron, steel, various carbides, and sintered ceramics are also used for many special applications.
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Bearings are mechanical components that support the movement of rotating or sliding shafts within machinery, reducing friction and enabling smooth motion. Bearing alloys are designed to provide low friction and wear-resistant surfaces to ensure efficient and long-lasting operation of machinery.
Types of Bearing Alloys
The most common bearing alloys are made up of soft metals, often a combination of tin, copper, and antimony, with some variations depending on the specific application and performance requirements. These metals are chosen for their ability to form a thin layer of low-friction material between the bearing surface and the shaft, reducing metal-to-metal contact and minimizing wear.
A few bearing alloys are described here-
White Metal Bearing Alloys
The white metals that are used as bearing alloys are either tin-base or lead-base alloys and are usually referred to as babbitts.
(a) Tin-base-bearing alloys consist of a soft mixture of a tin-rich solid solution in which numerous hard needles of the copper-tin antimony compound are embedded. A typical composition of the tin base-bearing alloy is as follows-
- Antimony – 75%
- copper – 4.5%
- Lead – 0.25%
- Tin – remaining percentage.
These types of alloy bearings are used in high-speed engines with light loading as in automobiles.
(b) Lead-base-bearing-alloys consist of a soft matrix of pure lead containing hard crystallites of tin and antimony. A typical composition of the lead base-bearing alloys is as follows-
- Antimony – 10%
- Tin – 4.5%
- Copper – 0.5%
- Lead – remaining percentage.
The lead base bearing alloys are cheaper than the tin base bearing alloys and are satisfactory for operation at medium and low speeds of medium and heavy loads. The alloy containing a lower percentage of lead is suitable for heavy loads at low speeds.
The presence of soft matrixes in the alloys provides frictional properties. Its soft matrix is easily extended and smeared out during sliding to form a thin lubricating film. This accounts for the low friction of the bearing since the shearing stress of the soft matrix in low hard crystals dispersed in the soft matrix imparts the hardness to the alloy, but they do not affect its frictional properties.
Copper-Lead Bearing Alloys
Like the Babbitts, the copper-lead alloys consist of a continuous matrix of harder metal, copper, with a small amount of softer metal, lead, and particles, finely tempered it. The copper-lead alloys owe their frictional properties to the spreading of thin films of soft lead over the surface of the hard copper. These alloys are stronger than the babbitts. If higher strength is required they must be fused with steel backing.
Bronze Bearing Alloys
There is a wide variety of bronze-bearing alloys. Some of them are lead-free materials like gun bronze, manganese, and aluminum bronze. Bronze-bearing alloys are relatively hard and strong and do not require steel backing.
Silver-lead Baring Alloys
In this type of bearing a layer of silver, usually 0.50 to 0.75 mm thick, is cast or electro-deposited on the steel backing. Silver has the needed hardness and ductility, good fatigue resistance, and high thermal conductivity. But it has a high coefficient of friction and is difficult to lubricate. Unlike the copper-lead bearing, there is no way to replenish the lead layer if it is accidentally destroyed.
Aluminum Bearing Alloys
Aluminum alloys usually contain 7% tin and a small percentage of copper and nickel. Aluminum alloy bearings are used for heavy-duty in trucks, diesel engines, etc.
Porous Metal Bearing Alloys
Porous metal bearings are made by the process of powder metallurgy. There is sufficient porosity to hold up to 35% of lubricating oil by volume. Porous bronzes containing 90% copper and 10% tin are the most suitable but other compositions, especially, iron base alloys are widely used. No external source of lubrication is required in porous metal bearings.
Properties of Bearing Alloys
Bearing Alloys have their properties. They are-
- A bearing alloy should be composed of a soft matrix with sufficient flexibility to compensate for slight mis alignments of the journal, at the same time the soft matrix should have moderately hard particles embedded in it to withstand wear.
- It should be able to withstand shocks and vibrations.
- It should have a low coefficient of friction.
- It should have high compressive strength.
- It should have high thermal conductivity.
- It should be resistant to corrosion from the lubricating oils.
- It should have the property of retaining a thin film of lubricating oil so that there will be no metal contact.
- It should be able to adhere firmly to the backing piece without cracking or spalling in service.
- It should have a responsible long life.
- It should be reasonably cheap.
- It should not cause an immediate seizure.
- It should not get deformed or squeezed out, especially at operating temperatures.
The selection of the appropriate bearing alloy depends on factors such as the speed, load, temperature, and operating conditions of the machinery in which the bearing will be used.