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Machine Design

Bronze and Copper-Alloy Bearings

Dozens of copper alloys are available as bearing materials. Most of these can be grouped into five classes: copper lead, copper tin (sometimes called tin bronze), leaded bronze, aluminum bronze, and beryllium copper.

As a general rule in these alloys, a higher lead content promotes compatibility with soft alloy shafts and reduces friction in low-lubrication conditions (start-up, for example) while slightly sacrificing wear resistance. Thus, copper lead and leaded bronzes are often used where compatibility outweighs the effects of lower mechanical properties. Other alloying elements are added to copper to tailor an alloy for user requirements based on load capacity, bearing strength, hardness, wear resistance, and fatigue strength.

Compared with the softer babbitts, copper-alloy bearings provide greater load capacity, better high-temperature operation, greater wear resistance, but poorer scoring resistance.

Copper lead: Since lead is practically insoluble in copper, a cast copper-lead microstructure consists of lead pockets in a copper matrix. These pockets of lead serve as reservoirs for maintaining a continuous lead film on the bearing surface.

With either continuous casting or powder-metallurgy techniques, a steel backing is used with copper-lead bearings for increased strength. These bearings are also frequently used with a babbitt overlay in a three-layer construction. The hardness of copper-lead materials is similar to that of babbitt at room temperature, but is higher at temperatures approaching 300°F. Corrosion of either the lead or copper can be minimized by additives in high-quality automotive and industrial lubricating oils.

Copper-lead systems are used for diesel engines on trucks and off-road vehicles, although aluminum alloys are frequently specified for greater corrosion resistance at a sacrifice of compatibility. Copper lead is used in moderate load and speed applications, such as electric motors, turbine engines, and generators.

Leaded bronze: The 4 to 10% tin content in leaded bronze increases strength, maximum load capacity, fatigue resistance, and hardness above what is available with simple copper leads. Zinc is sometimes used as a replacement for tin, and nickel (or nickel or silver) is often added to improve corrosion resistance and toughness.

Leaded bronzes have better compatibility than tin bronze because the spheroids of lead smear over the bearing surface under conditions of inadequate lubrication. These alloys are generally a first choice for intermediate loads and speeds. They are used in machine tools, home appliances, farm machinery, and pumps.

C93200 alloy is currently the standard cast-bronze bearing material with many suppliers. The 80-10-10 SAE C93700 phosphor bronze is also popular. Its relatively high hardness and good impact resistance make it widely used in steel plants for such applications as roll-neck bearings. It is also used in lathes, instruments, household appliances, diesel rocker-arm bushings, automotive piston-pin bushings, pumps, and trunnion bearings.

Softer C93800, with its higher lead content, offers better compatibility characteristics and good performance where lubrication is doubtful. It is widely used for diesel engine bearings, in cranes, and in railway and earthmoving equipment. It has good antiscoring properties with soft shafts. However, alloys with lead content over 20% are becoming difficult to obtain because they are difficult to cast.

Tin bronze: These alloys have high hardness, thus require reliable lubrication, good alignment, and a minimum Brinell shaft hardness of 300 to 400. They are used in high-load, low-speed applications such as trunnion bearings, gear bushings for off-road vehicles, rolling-mill bearings, and in internal combustion engines for connecting-rod bearings, valve guides, and starters.

Cast-bronze bearings offer good compatibility, casting, easy machining characteristics, low cost, good structural properties and high load capacity. They do not require a separate overlay or a steel backing.

Aluminum bronze: Bronzes of high strength are obtained by using aluminum, iron, manganese, silicon, and nickel as alloying elements. Such bearings have excellent shock and wear resistance. They retain high strength at high temperatures and are used in equipment operating above 500°F. A major use is in high-impact sliding surfaces in aircraft landing gear.

Because aluminum bronze has poor compatibility, embeddability, and conformability, it is best suited for heavy-duty, low-speed applications with plentiful lubrication. Aluminum bronzes require harder shafts than softer bearing materials. Proper alignment is more critical because of low conformability.

Beryllium copper: Adding about 1.8% by weight of beryllium and about 0.2% cobalt to copper provides an alloy with strength comparable to many steels. The high strength, hardness, and thermal conductivity of the alloy promotes its use in high load bearings, especially where reliability is required under occasional overload, impact, high temperature, or marginal lubrication conditions. These alloys are used in electrically conducting applications and are frequently specified for aircraft landing gear and other airframe sliding surfaces.

TAGS: Bearings
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