Due to the unique self-lubricating characteristics of P/M bearings, these products have additional design guidelines. After conventional die compaction and sintering, bearings are vacuum-oil impregnated to fill the interconnected porosity with oil. The design of P/M bearings and selection of the optimal alloy depend primarily on shaft velocity and bearing load. Other design factors include type of operation (stop-start versus continuous), shaft surface finish, type of lubricant, and conditions of uneven loading. The MPIF bearing standard (Ref 9) provides a systematic guide for these design issues:

• Recommended load as a function of shaft velocity

• Environmental factors that can reduce permissible loads

• Recommended press fits

• Running clearances

• Dimensional tolerances

The selection of a bearing alloy normally considers the following factors:

• Chemical compatibility with the lubricant

• Thermal conductivity to dissipate heat

• Mechanical properties to maintain structural integrity and withstand press-fit installation forces

• Antigalling with the shaft material

• Wear characteristics with the shaft material

Six commercial alloys dominate the self-lubricating bearing designs: bronze, diluted bronze, plain carbon steel, iron-copper alloys, copper steels, and iron-graphite. Graphite can be used as a solid lubricant and is added to the powder mix prior to compacting. More information on these materials can be found in Ref 9 and the article "Powder Metallurgy Bearings" in this Volume.

In some products the designer can incorporate bearing surfaces into the part, thereby eliminating the need for separately fabricated and installed bearings. The following design restrictions must be followed:

• Bearing areas should have a minimum of 10% interconnected porosity to provide sufficient oil capacity.

• Infiltration is not allowed in the bearing areas because the infiltration will fill the open pores.

• Machining on the bearing surfaces must be performed to avoid sealing or closing off the open porosity.

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