Sheet steel cages
The majority of pressed sheet steel cages are made from continuously hot-rolled low carbon sheet steel according to (DIN) EN 10111:1998. These lightweight cages have relatively high strength and can be surface treated to further reduce friction and wear.
Pressed cages normally used in stainless steel bearings are made from stainless steel X5CrNi18-10 according to EN 10088-1:1995.
Machined steel cages
Machined steel cages are normally made of non-alloyed structural steel of type S355GT (St 52) according to EN 10 025:1990 + A:1993. To improve sliding and wear resistance properties some machined steel cages are surface treated.
Machined steel cages are used for large-size bearings or in applications where there is a danger that season cracking, caused by a chemical reaction, may occur if a brass cage were used. Steel cages can be used at operating temperatures up to 300 ºC. They are not affected by the mineral or synthetic oil-based lubricants normally used for rolling bearings, or by the organic solvents used to clean bearings.
Sheet brass cages
Pressed sheet brass cages are used for some small and medium-sized bearings. The brass used for these cages conforms to EN 1652:1997. In applications such as compressors for refrigeration using ammonia, season cracking in sheet brass might occur, therefore machined brass or steel cages should be used instead.
Machined brass cages
Most brass cages are machined from a CW612N cast or wrought brass according to EN 1652:1997. They are unaffected by most common bearing lubricants, including synthetic oils and greases, and can be cleaned using normal organic solvents. Brass cages should not be used at temperatures in excess of 250 ºC.
For the majority of injection moulded cages polyamide 66 is used. This material, with glass fibre reinforcement or without, is characterized by a favourable combination of strength and elasticity. The mechanical properties like strength and elasticity of polymeric materials are temperature dependent and subject to permanent changes under operating conditions, called ageing. The most important factors that play a role in this ageing behaviour are temperature, time and the medium (lubricant) to which the polymer is exposed. The relationship between these factors for glass fibre reinforced polyamide 66 is illustrated in diagram 1. It appears that the 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. In table the classification of lubricants into "aggressive" and "mild" is reflected by the "permissible operating temperature" for the use of cages made from glass fibre reinforced polyamide 66 in various lubricants. The permissible operating temperature in this table is defined as the temperature, which gives a cage ageing life of at least 10 000 operating hours.
Some media are even more "aggressive" than those listed in table. A typical example is ammonia applied as refrigerant in compressors. In those cases, cages made from glass fibre reinforced polyamide 66 should not be used at operating temperatures above +70 °C.
Towards the low operating temperature side, also a limit can be set since polyamide loses its elasticity which can result in cage failures. Cages made from glass fibre reinforced polyamide 66 should for this reason not be applied at a continuous operating temperature below –40 °C.
Where a high degree of toughness is a dominant factor, such as in railway axleboxes, a super-tough modification of polyamide 66 is applied. Please consult the SKF application engineering service for cage availability for specific bearing executions.
Glass fibre reinforced polyamide 4,6 is used for some small and medium size CARB toroidal roller bearings as standard. These cages have a 15 °C higher permissible operating temperature than those made from glass fibre reinforced polyamide 66.
The use of the glass fibre reinforced PEEK for cages has become common within SKF for demanding conditions regarding high speeds, chemical attack or high temperatures. The exceptional properties of PEEK are 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, like hybrid and/or high-precision bearings. The material does not show signs of ageing by temperature and oil additives up to +200 °C. However, the maximum temperature for high-speed use is limited to +150 °C as this is the softening temperature of the polymer.
Lightweight, fabric reinforced phenolic resin cages can withstand high centrifugal as well as acceleration forces, but are not able to accommodate high operating temperatures. In most cases, these cages are used as standard in high-precision angular contact ball bearings.
In addition to the materials described above, SKF bearings for special applications may be fitted with cages made of other engineering polymer materials, light alloys or special cast iron. For information on cages made from alternative materials please consult the SKF application engineering service.