Copper Alloys for Bearing Applications: Part One

Sommario:

Bronzes are unquestionably the most versatile class of bearing materials, offering a broad range of properties from a wide selection of alloys and compositions. Bearing bronzes offer broad ranges of strength, ductility, hardness, wear resistance, anti-seizing properties, low friction and the ability to conform to irregularities, tolerate dirty operating environments and contaminated lubricants.

Copper and copper alloys are one of the major groups of commercial metals. They offer a wide range of properties, including excellent electrical and thermal conductivity, outstanding corrosion resistance, good strength and fatigue resistance, and appearance. They can be readily worked, brazed and welded.

Bronzes are unquestionably the most versatile class of bearing materials, offering a broad range of properties from a wide selection of alloys and compositions. Bearing bronzes offer broad ranges of strength, ductility, hardness, wear resistance, anti-seizing properties, low friction and the ability to conform to irregularities, tolerate dirty operating environments and contaminated lubricants.

The corrosion resistance of bearing bronzes is generally superior to other bearing materials, and can be selected to meet particular ambient conditions. Bronzes permit easy and economical manufacture, allowing bearings to be made in special and one-of-a-kind configurations simply and at low cost. No bearing metals have better machinability than the leaded and high-leaded bearing bronzes. Almost without exception, a bearing bronze can be selected to satisfy any bearing application that exists.

It is not at all unusual to come across a bronze sleeve bearing that has been performing satisfactorily for decades, even under severe operating conditions. In fact, a properly designed and maintained bronze bearing often outlasts the equipment it serves. Achieving such performance is not difficult, but it requires sound design, the right bearing material, accurate manufacture and, as with any mechanical equipment, diligent maintenance.

Good bearing design involves three fundamental elements: understanding the service environment, designing for proper lubrication and selecting the best bearing material for the job. Accurately assessing expected service conditions cannot be overemphasized; it is the basis for all subsequent decisions. Creating or at least identifying the lubrication mode in which the bearing will operate is equally important.

Service Conditions

The most important prerequisite to assure optimum bearing performance is knowing or accurately predicting service conditions. Major areas of consideration are:

  • Load, steady and impact,
  • Speed at design load,
  • Oscillating motion, i.e., less than full revolutions,
  • Corrosive environments,
  • Dirty environments and/or lubricants,
  • Temperature,
  • Frequent start-stop operation,
  • Questionable or interruptible lubricant supply,
  • Shaft or journal misalignment,
  • Hardness differential, bearing vs. shaft.

Many millions of bearings operate successfully in the boundary and mixed-film modes for their entire service lives. The only penalty this entails is an increase in friction compared to hydrodynamically lubricated bearings and consequently higher energy expenditure.

Bearing life, however, will depend very heavily on the choice of bearing material. Even hydrodynamic bearings pass through boundary and mixed-film modes during start-up and shut down or when faced with transient upset conditions. This means that material selection is an important design consideration for all sleeve bearings, no matter what their operating mode. The general attributes of a good bearing material are:

  • A low coefficient of friction versus hard shaft materials,
  • Good wear behavior against steel journals (scoring resistance),
  • The ability to absorb and discard small contaminant particles (embeddibility),
  • The ability to adapt and adjust to the shaft roughness and misalignment (conformability),
  • High compressive strength,
  • High fatigue strength,
  • Corrosion resistance,
  • Low shear strength (at the bearing-to shaft interface),
  • Structural uniformity, and
  • Reasonable cost and ready availability.

The need for adequate corrosion resistance is especially important in bearings that operate in aggressive environments, or for those bearings which stand idle for long periods of time. Good corrosion resistance therefore increases both service life and shelf life.

A bearing material should have structural uniformity and its properties should not change as surface layers wear away. On the other hand, alloys such as the leaded bronzes are used because they provide a lubricating film of lead at the bearing/ journal interface. Lead has low shear strength, and is able to fill in irregularities in the shaft and act as an emergency lubricant if the oil supply is temporarily interrupted.

Finally, a bearing material should be cost-effective and available on short notice. No single bearing material excels in all these properties and that is one of the reasons bearing design always involves a compromise. However the bronze bearing alloys provide such a broad selection of material properties that one of them can almost always fit the needs of a particular design.

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.

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 150°C. Corrosion of either the lead or copper can be minimized by additives in high-quality automotive and industrial lubricating oils. Copper lead is used in moderate load and speed applications, such as electric motors, turbine engines, and generators.

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