With the exception of full complement bearings, all rolling bearings contain a cage. The number of cages depends on the number of ball or roller sets within the bearing and on the cage design. The primary purposes of a cage are:
Separate the rolling elements to reduce the frictional moment and frictional heat in the bearing.
Keep the rolling elements evenly spaced to optimize load distribution and enable quiet and uniform operation.
Guide the rolling elements in the unloaded zone, to improve the rolling conditions and to help avoid damaging sliding movements.
Retain the rolling elements of separable bearings when one bearing ring is removed during mounting or dismounting.
Cages are mechanically stressed by frictional, strain and inertial forces. They can also be degraded by high temperatures and chemicals like certain lubricants, lubricant additives or by-products of their ageing, organic solvents or coolants. Therefore, both the design and material of a cage have a significant influence on the suitability of a rolling bearing for a particular application. As a result, SKF has developed a variety of cages, made of different materials, for different bearing types and operating conditions.
Information about standard cages, and possible alternatives, is provided in the sections related to each product type. If a bearing with a non-standard cage is required, check availability prior to ordering.
Cages can be classified according to the manufacturing process and material group into:
Stamped metal cages
Stamped metal cages for SKF bearings are generally made of sheet steel and with some exceptions, of sheet brass. Depending on the bearing type, the following stamped metal cages are available:
Stamped metal cages are lightweight. They provide ample space inside the bearing to maximize the effects of the lubricant.
Machined metal cages
Machined metal cages for SKF bearings are made of brass, steel or light alloy. Depending on the bearing type, design and size, the following machined metal cages are available:
a two-piece machined riveted metal cage (fig. 5)
a two-piece machined metal cage with integral rivets (fig. 6)
a one-piece machined window-type metal cage (fig. 7) and (fig. 8)
a double prong-type machined metal cage (fig. 9)
Machined metal cages, which generally permit higher speeds, are typically used when forces, other than pure rotational forces, are superimposed on the cage.
Polymer cages for SKF bearings are injection moulded. SKF also manufactures a fabric reinforced phenolic resin cage, but only for super-precision bearings
. Depending on the bearing type, design and size, the following polymer cages are available:
a polymer window-type cage (fig. 10)
a polymer snap-type cage (fig. 11)
Polymer cages are characterized by a favourable combination of strength and elasticity. The good sliding properties of the polymer on lubricated steel surfaces and the smoothness of the cage surfaces in contact with the rolling elements produce little friction so that frictional heat and wear in the bearing are minimized. The low density of the material means that the inertial forces generated by the cage are minor. The excellent running properties of polymer cages under poor lubrication conditions permit continued operation of the bearing for some time without the risk of seizure and secondary damage.
Steel pin-type cages need pierced rollers (fig. 12
) and are only used together with large-sized roller bearings. These cages have relative low weight and enable a large number of rollers being incorporated.
Stamped metal cages are typically guided by the rolling elements.
Depending on the bearing type and design, machined metal and polymer cages are radially centred either on:
the rolling elements (fig. 13)
the inner ring (shoulder(s)) (fig. 14)
the outer ring (shoulder(s)) (fig. 15)
Cages guided by the rolling elements permit the lubricant to enter the bearing easily.
Ring guided cages, which provide more precise guidance, are typically used when bearing arrangements must accommodate high speeds, frequent, rapid accelerations or high vibration levels. Suitable steps must be taken to provide a sufficient supply of lubricant to the guiding surfaces of the cage. For higher speeds, SKF recommends oil lubrication (→ Lubrication and/or relevant product sections).
For information about materials used for cages, refer to Cage materials