Bearing retainers or bearing cages are used in rotary and linear bearings to separate and maintain a specified distance between rolling elements, whether they are balls, needle rollers, or rollers. Bearing retainers or bearing cages reduce wear between elements, minimize the build-up of heat, and help lengthen bearing life. There are two basic types of bearing retainers or bearing cages: crown and ribbon. Crown retainers and crown cages feature a single-piece, open-ball pocket design. Ribbon retainers and ribbon cages have a two-piece, open-ball pocket design. Bearing tables indicate the proper type of bearing retainers or bearing cages to use. Note, however, that bearings with retainers are not suitable in some instances. For example, in applications with low speeds and high loads, a full-complement bearing with no retaining mechanism may be preferred. Selecting bearing retainers or bearing cages requires an analysis of materials and product specifications. Some bearing retainers or bearing cages are made of metals such as heat-treated aluminum, sold bronze, or pressed steel. Others are made of non-metal materials such as nylon, Teflon? (DuPont), or phenolic resins. Pressed steel is a common choice. Brass cages have good noise characteristics, and are often used in harsh or corrosive environments. Phenolic or fiber-resin cages are designed for spindle bearings and machine tool applications. These bearing retainers or bearings cages are relatively lightweight and can withstand high speeds and heavy loads. In terms of performance specifications, there are many parameters to consider. Examples include application loads and speeds, environmental conditions such as temperature and contamination, acceleration issues, vibration levels, high-speed running, and lubrication requirements. Bearing retainers or bearing cages are used in many different applications and industries. Some products are used by tool and die makers, stamping-die manufacturers, and metal stamping operations. Bearing retainers or bearing cages are also used in aerospace and automation applications, medical and production equipment, and the automotive industry. Specialized bearing retainers or bearing cages are used in medical and dental products. For example, polyamide and polyimide retainers provide smooth, quiet operation under conditions of extreme wear resistance. They can withstand multiple autoclave cycles, and may be pre-lubricated with food-safe or autoclave-resistant grease. Because proper lubrication affects the performance of antifriction bearings, these lubricants are often filtered.