The function of a retainer is to maintain proper distance between rolling elements. Almost all roller bearings have stamped retainers made of iron-silicon bronze or lead brass. Run-of-the-mill bearings tend to have stamped steel retainers.
Stamped bronze or brass retainers for ball bearings are used more in Europe than the U.S.; here, they are generally an extra-cost option for high-speed applications. Plastic retainers have higher speed capability than metal retainers in most applications, but are temperature limited.
Retainer wear is inversely related to hardness so, in general, the harder retainer materials are more desirable, particularly at high speeds. However, plastics such as phenolic are used at operating temperatures up to 270°F, PTFE to 450°F, and polyimide to 500°F. Common metals for low wear are S-Monel, M-1, and 440C. Common softer metals include cobalt and copper alloys.
For most applications, retainers can ride on the outer or inner race. But for high speeds, they should ride on the outer race and be thin enough to allow lubricant flow between retainer and inner race. For high-speed small-bore bearings, it is often necessary to use a silver-plated semihard tool-steel retainer with oil-mist lubrication.