Three methods of bearing cage guidance

As a vital component of a bearing, the cage serves to guide and separate the rolling elements. The guiding function of the cage essentially refers to the correction of the rolling elements’ movement. This correction is achieved through contact between the cage and the surrounding components.

Generally, there are three types of guidance for bearing cages: rolling element guidance, inner ring guidance and outer ring guidance.

Rolling element guidance:

The standard design configuration is rolling element guidance. For example, in short cylindrical roller bearings, rolling element guidance is employed, whereby the cage makes no contact with the flange surfaces of either the inner or outer rings, allowing the cage to be interchangeable. However, at high speeds, as the rotational speed of the rolling elements increases, rotational instability occurs; consequently, rolling element guidance is suitable for medium speeds and moderate loads, such as in gearbox bearings.

In rolling element-guided bearings, the cage is positioned between the rolling elements. The cage does not come into contact with or collide with the inner or outer rings; instead, the interaction between the cage and the rollers corrects their motion whilst maintaining a consistent spacing between them.

Outer ring guidance:

The outer ring is generally stationary; outer ring guidance facilitates the entry of lubricating oil into the guidance surface and raceway. High-speed gearboxes use oil mist lubrication; if a rotating inner ring were used for guidance, it would displace the oil. In outer ring-guided bearings, the cage is positioned on the side of the rolling elements closest to the outer ring. During bearing operation, the cage may collide with the outer ring, thereby correcting its position.

Outer ring guidance is generally used in high-speed, stable load conditions. Taking cylindrical roller bearings as an example, they only bear a fixed axial load; during rotation, the rotational speeds of the rolling elements vary little, and the rotation of the cage does not cause imbalance.

Inner ring guidance:

The inner ring is generally the rotating ring and, whilst rotating, provides the rolling elements with a drag torque. If the bearing load is unstable or light, slippage may occur.

With inner ring guidance, an oil film forms on the guide surface of the cage. Due to the friction of this oil film, the inner ring exerts a drag force on the cage in the non-load zone, thereby increasing the additional driving torque of the cage on the rolling elements and preventing slippage.

In inner ring-guided bearings, the cage is positioned close to the inner ring relative to the rolling elements. During operation, the cage may collide with the inner ring, thereby correcting its position.

All three cage guidance methods may be found in different types of bearings, due to performance considerations as well as bearing design and manufacturing factors. Engineers may select the appropriate method according to requirements; however, in some cases, they have no choice. In any case, attention should be paid to the differing performance characteristics of the various cage guidance methods.

The differences between the three types of cages are primarily reflected in the variations in speed performance under different lubrication conditions.

Bearings with all three types of cages can be used for both oil and grease lubrication.