For the majority of injection moulded cages, polyamide 66 (PA66) is used. This material, with or without glass reinforcement, is characterized by a favourable combination of strength and elasticity. The mechanical properties like strength and elasticity of polymer materials are temperature dependent and subject to ageing. The most important factors that play a role in the ageing process are temperature, time and the medium (lubricant) to which the polymer is exposed. Diagram 1 shows the relationship between these factors for glass fibre reinforced PA66. It shows that cage life decreases with increasing temperature and the aggressiveness of the lubricant.
Therefore, whether polyamide cages are suitable for a specific application depends on the operating conditions and life requirements. The classification of lubricants into “aggressive” and “mild” is reflected by the “permissible operating temperature” for cages made of glass fibre reinforced PA66 with various lubricants table 1. The permissible operating temperature in table 1 is defined as the temperature that provides a cage ageing life of at least 10 000 operating hours.
Some media are even more “aggressive” than those provided in table 1. A typical example is ammonia, used as a refrigerant in compressors. In those cases, cages made of glass fibre reinforced PA66 should not be used at operating temperatures above 70 °C (160 °F).
Polyamide also has a low temperature limit because it loses its elasticity, which can cause cage failures under extremely cold conditions. As a result, cages made of glass fibre reinforced PA66 should not be used in applications where the continuous operating temperature is below –40 °C (–40 °F).
In applications where a high degree of toughness is a critical operational parameter, such as in railway axleboxes, a super-tough modified PA66 can be used. For additional information, contact the SKF application engineering service.
Glass fibre reinforced polyamide 46 (PA46) is the standard cage material for some small and medium-size CARB toroidal roller bearings. The permissible operating temperature is 15 °C (25 °F) higher than for glass fibre reinforced PA66.
Glass glass fibre reinforced polyetheretherketone (PEEK) is popular for demanding applications where there are either high speeds or high temperatures or a need for chemical resistance. The exceptional properties of PEEK provide a superior combination of strength and flexibility, high operating temperature range, high chemical and wear-resistance and good processability. Due to these outstanding features, PEEK cages are available as standard for some ball and cylindrical roller bearings, such as hybrid and/or super-precision bearings. The maximum temperature for high-speed use is limited to 150 °C (300 °F) as this is the softening temperature of the polymer. The material does not show signs of ageing by temperature or oil additives up to 200 °C (390 °F).