Classification and Functions of Bearing Cage

Bearing cage (also known as bearing retainers) is one of the core components of bearings. Their functions include separating rolling elements (steel balls, rollers), guiding the uniform distribution of rolling elements, reducing friction and collision between rolling elements, and preventing rolling elements from falling off. The material, structure, and manufacturing process of cages directly affect the rotational speed, load-carrying capacity, service life, and applicable working conditions of bearings. The following is a detailed analysis of common types of cages and their characteristics:

I. Classification by Material

The choice of bearing cage material must match the bearing's working conditions (such as temperature, corrosiveness, rotational speed, etc.). Common materials and their properties are as follows:

1. Metal Cages

Steel

The most commonly used material, including low-carbon steel, high-carbon steel, and alloy steel, formed by stamping or turning:

• Advantages: High strength, impact resistance, and wide applicability (medium rotational speed, heavy load conditions);
• Disadvantages: Relatively heavy weight, large centrifugal force at high speeds, and prone to frictional heating with rolling elements;
• Applicable scenarios: General machinery (e.g., motors, reducers), medium rotational speed (≤3000r/min), and normal temperature environments.

Copper Alloys

Mainly brass and bronze, mostly formed by turning:

• Advantages: Good wear resistance, excellent thermal conductivity, low friction coefficient with rolling elements, suitable for high-speed operation;
• Disadvantages: Higher cost, not resistant to strong alkali and ammonia corrosion;
• Applicable scenarios: High-speed bearings (e.g., machine tool spindles, automobile gearboxes), high-temperature environments (≤250℃).

2. Non-Metal Cages

Phenolic Resin (Bakelite)

Made by pressing cotton cloth or glass fiber impregnated with phenolic resin:

• Advantages: Good insulation, light weight, low friction coefficient, wear resistance, and no chemical reaction with lubricants;
• Disadvantages: Limited temperature resistance (≤120℃) and poor impact resistance;
• Applicable scenarios: Motor bearings (requiring insulation), household equipment, medium-low speed and light load conditions.

Nylon (Polyamide, e.g., PA66)

Nylon materials containing reinforcing fibers (glass fiber, carbon fiber):

• Advantages: Good toughness, impact resistance, strong self-lubrication, and tolerance to minor lubrication deficiencies;
• Disadvantages: High hygroscopicity (ambient humidity must be controlled), prone to aging at high temperatures (generally ≤120℃; special types such as PA46 can reach 150℃);
• Applicable scenarios: Automobile hub bearings, water pump bearings, humid environments (lubricants must be added to prevent water absorption).

Polytetrafluoroethylene (PTFE)

A fluoroplastic with extremely strong corrosion resistance:

• Advantages: Resistance to acids and alkalis, wide temperature range (-200℃~260℃), extremely low friction coefficient (self-lubricating);
• Disadvantages: Low strength and high cost;
• Applicable scenarios: Chemical equipment, high-temperature or strong corrosion environments.

Cermet/Ceramic

Such as zirconia (ZrO₂) and silicon nitride (Si₃N₄):

• Advantages: Extreme temperature resistance (up to 800℃ or higher), insulation, non-magnetic properties, and wear resistance;
• Disadvantages: High brittleness and extremely high cost;
• Applicable scenarios: High-temperature furnaces, nuclear industry, and non-magnetic environments.

II. Classification by Structure and Manufacturing Process

The structure of a bearing cage determines its guiding method (inner guidance, outer guidance, rolling element guidance) and its ability to constrain rolling elements. Common types are as follows:

1. Stamped Cages

• Characteristics: Formed by stamping thin metal sheets (steel, copper), with simple structures (e.g., crown-shaped, bowl-shaped, wave-shaped), light weight, low cost, and suitable for mass production;
• Guidance method: Mostly rolling element-guided or inner ring-guided, with general high-speed performance;
• Applicable scenarios: Small and medium-sized bearings (e.g., deep groove ball bearings, self-aligning ball bearings), general machinery (motors, water pumps).

2. Machined Cages

• Characteristics: Processed from bars or pipes by turning, with precise structures (e.g., solid ring type, pillar type), high dimensional accuracy, and uniform spacing between rolling elements;
• Guidance method: Mostly outer-guided (contact with outer ring ribs), with good high-speed stability;
• Applicable scenarios: Large bearings, high-precision bearings (e.g., machine tool spindle bearings, angular contact ball bearings), high-speed and heavy-load conditions.

3. Riveted/Welded Cages

• Characteristics: Composed of two half-cages joined by riveting (rivets) or welding, suitable for bearings with a large number of large-sized rolling elements (e.g., self-aligning roller bearings);
• Advantages: Firm assembly, capable of accommodating large-sized rolling elements, and high load-carrying capacity;
• Notes: Welding may cause local stress concentration, requiring strict process control to avoid cracking.

4. Solid Machined Cages

• Characteristics: Integral structure (mostly used for non-metallic materials), seamless, high strength, and suitable for centrifugal force impact during high-speed rotation;
• Typical materials: Nylon, phenolic resin, cermet;
• Applicable scenarios: High-speed bearings, conditions requiring prevention of rolling element detachment (e.g., vertical motors).

III. Key Functions of Cages and Impact of Failure

Core Functions

• Separate rolling elements to avoid wear and heat accumulation caused by direct contact;
• Guide rolling elements to move smoothly along raceways, reducing vibration and noise;
• Assist rolling elements in adjusting positions when the bearing is tilted or misaligned (e.g., self-aligning bearings) to maintain uniform force distribution.

Consequences of Failure

• Cage fracture: Causes rolling elements to become disorganized and stuck, leading to immediate bearing failure and even equipment shutdown;
• Cage wear: Generates metal or non-metal debris, contaminates lubricants, and accelerates overall bearing wear;
• Wear of guiding surfaces: Causes deviation in the movement trajectory of rolling elements, increases friction and heat generation, and reduces bearing life.

IV. Considerations for Selection

1. Matching working conditions: Choose lightweight materials (aluminum alloy, nylon) for high speeds; metal or ceramic for high temperatures; PTFE or titanium alloy for corrosive environments;

2. Load type: Steel or copper alloy cages for heavy loads; nylon or phenolic resin for light loads;

3. Installation environment: Avoid nylon in humid environments (to prevent water absorption); choose phenolic resin or ceramic for insulation requirements;

4. Reference manufacturer data: Cage designs vary by brand (e.g., SKF, NSK), so specific model technical manuals should be consulted for selection.

A reasonable selection of bearing cage can significantly improve bearing reliability and service life. Cages are particularly critical in extreme conditions (high speed, high temperature, corrosion).