The Ultimate Guide to Roller Bearing Sizing: A Step-by-Step Approach

1. Introduction

Roller bearings are fundamental components in virtually all rotating machinery, from heavy industrial equipment and automotive systems to precision instruments. They are expertly designed to support radial and axial loads while minimizing friction between moving parts. This load transfer and friction reduction are achieved by utilizing cylindrical, tapered, or spherical rolling elements positioned between two rings (races).

What Roller Bearings Are and Their Importance

Rolling bearings are mechanical elements that facilitate relative motion by using rolling elements (such as rollers, needles, or balls) positioned between an inner and an outer ring (races). Compared to plain bearings, rolling bearings offer lower friction coefficients, higher load-carrying capacity, and simpler maintenance.

Their importance lies in several key areas:

• Support and Positioning: They accurately support and position rotating shafts or moving components.
• Friction Reduction: They convert sliding friction into rolling friction, significantly reducing energy loss and heat generation.
• Efficiency Assurance: Optimized friction characteristics help the equipment sustain high speeds and maintain high operational efficiency.

Consequences of Improper Sizing

Improper bearing sizing is a leading cause of mechanical failure and downtime.

• Undersized Bearings: These occur when the bearing’s capacity is lower than the actual working load.
  • Consequences: Lead to excessive stress on the rolling elements and raceways, accelerating fatigue, causing early failure, increased temperatures, and severe damage, ultimately resulting in production interruptions.
• Oversized Bearings: These occur when the bearing’s capacity is far greater than the actual requirement.
  • Consequences: Result in increased initial cost, require larger installation space, add to equipment weight, and sometimes affect the formation of the lubricating oil film due to insufficient working load, which can paradoxically reduce the bearing’s life.

2. Understanding Roller Bearing Types

Selecting the correct bearing type is the critical first step in the sizing process. Each roller bearing type is optimized for specific load directions, rigidity requirements, and speed capabilities. Choosing the wrong type can severely limit performance, regardless of how accurately the size calculations are performed.

Overview of Different Roller Bearing Types

Applications and Load Characteristics

• Cylindrical Roller Bearings (CRB):

  • Load Characteristics: Exceptional capacity for pure radial loads. Their design allows for a large contact area between the roller and the raceway. They are typically unable to support significant axial loads, though some variations (like NJ and NUP series) can handle light axial forces.
  • Applications: Gearboxes, electric motors, traction motors, and pumps where high speed and heavy radial loads are present.

• Needle Roller Bearings (NRB):

  • Load Characteristics: Designed for applications with severely limited radial space. They offer a high radial load capacity due to the large number of long, thin rolling elements, resulting in a low cross-section height. They have a minimal to zero capacity for axial loads.
  • Applications: Automotive transmissions, universal joints, small engines, and certain machine tools where space is at a premium.

• Tapered Roller Bearings (TRB):

  • Load Characteristics: This design is highly effective at carrying combined loads—simultaneous radial and axial forces. The angle of the taper allows them to handle high axial forces. Crucially, they must be mounted in pairs to react to the opposing axial thrusts generated within the bearing.
  • Applications: Wheel hubs, gear drives, differentials, rolling mills, and mining equipment, valued for their stiffness and high load ratings.

• Spherical Roller Bearings (SRB):

  • Load Characteristics: Possess the highest radial and bi-directional axial load capacity among all rolling bearings. Their unique feature is the self-aligning capability, which allows the outer ring raceway to pivot, compensating for significant shaft misalignment or housing deflection that might occur under heavy load.
  • Applications: Industrial fans, large pumps, paper machines, shakers, and mining and construction machinery, where high load, vibration, and potential misalignment are common.

3. Key Factors in Roller Bearing Sizing

Accurate bearing sizing requires a thorough analysis of the operational environment, as the load is only one piece of the puzzle. This section details the critical factors that influence bearing selection and longevity.

Load Requirements

Understanding the forces acting on the bearing is paramount. Loads are categorized by direction and application behavior.

• Radial Load:
  • Definition: The force acting perpendicular to the center axis of the shaft. This force is typically generated by gravity, belt tension, or gear separation forces.
  • Calculation: Must be determined precisely from the mechanical system analysis, often using a free-body diagram.
• Axial Load (Thrust Load):
  • Definition: The force acting parallel to the shaft axis. This is generated by components like helical gears, worm gears, or propeller thrust.
  • Calculation: Requires analysis of thrust components from the machine’s design.
• Combined Loads (Equivalent Dynamic Load):
  • Definition: Most bearings experience both radial and axial forces simultaneously. The combined effect of these forces is converted into a single, theoretical Equivalent Dynamic Load. This standardized load represents the magnitude of a purely radial load that would result in the same fatigue life as the actual combined load scenario.
  • Determination: Calculated by applying specific factors obtained from bearing manufacturer data to the radial and axial load values.
• Dynamic vs. Static Loads:
  • Dynamic Load: Occurs when the bearing is rotating. Sizing is based on fatigue life prediction (L10 life) and the Basic Dynamic Load Rating.
  • Static Load: Occurs when the bearing is stationary or oscillating at very low speeds. Sizing is based on preventing permanent plastic deformation of the raceways under maximum stress and the Basic Static Load Rating.

Speed and Operating Conditions

Operating speed and temperature dramatically affect bearing performance and lubrication.

• Rotational Speed (RPM):
  • Impact on Bearing Life and Lubrication: High speeds increase operating temperature and demand higher-performance lubrication. Every bearing type and size has a published limiting speed, which must not be exceeded to prevent premature failure due to excessive heat or centrifugal forces on the rolling elements.
• Operating Temperature:
  • Effects on Material and Lubrication: High temperatures degrade the effectiveness of the lubricant, accelerating its failure and demanding specialized high-temperature grease or oil. High heat also reduces the hardness of bearing steel, which directly reduces its load-carrying capacity.
• Lubrication Type:
  • Grease vs. Oil: Grease is generally favored for simpler sealing and lower speeds, while oil is typically required for high speeds and high operating temperatures where continuous heat dissipation is necessary.
  • Selection Criteria: Determined by operating speed, temperature, and environmental contamination levels. The viscosity of the lubricant is a crucial factor for achieving adequate separation between rolling surfaces.
• Environmental Factors:
  • Contamination and Corrosion: The presence of abrasive particles, moisture, or corrosive chemicals can drastically accelerate wear and failure. Selection of appropriate sealing arrangements is mandatory in harsh environments to maintain bearing integrity.

Bearing Life and Reliability

Sizing is fundamentally a life-prediction exercise, based on fatigue analysis.

• Desired Bearing Life (L10 Life):
  • Definition: The life, standardized in millions of revolutions or hours, that 90 percent of a large group of seemingly identical bearings will reach or exceed under specified operating conditions. This is the global standard for dynamic sizing.
• Reliability Factor:
  • Adjusting for Desired Reliability Levels: The standard L10 life corresponds to 90 percent reliability. For mission-critical applications requiring higher assurance (e.g., 95 percent or 99 percent reliability), a reliability adjustment factor is used. This factor necessitates the selection of a larger, more robust bearing to meet the stricter life requirement.
• Safety Factors:
  • Incorporating Safety Margins: Design engineers frequently apply a safety margin to the calculated or estimated loads. This accounts for uncertainties, momentary high peak loads, or shock loading conditions that may not be fully represented in the steady-state load analysis.

Shaft and Housing Considerations

The surrounding components dictate the bearing’s physical boundaries and mounting integrity.

• Shaft Diameter:
  • Influence on Bearing Selection: This is often the primary physical constraint. The bearing’s inner diameter (bore) must match the shaft diameter, which immediately narrows the selection of available bearing series and sizes.
• Housing Dimensions:
  • Ensuring Proper Fit and Support: The housing defines the maximum allowable bearing outer diameter and width. The correct fit (clearance or interference) between the bearing rings and their respective seating surfaces is critical for uniform load distribution and preventing inner or outer ring movement (creep).
• Material Compatibility:
  • Selecting Appropriate Materials: The materials used for the shaft and housing, combined with the required interference fits, must be able to withstand the internal stresses and operating temperatures without deformation, ensuring long-term stable operation.

4. Step-by-Step Guide to Roller Bearing Sizing

This comprehensive, sequential guide outlines the process for accurately determining the required size and load rating of a roller bearing, ensuring optimal performance and design life.

Step 1: Determine the Application Requirements

The first step is a thorough definition of the operational environment and performance goals.

• Identify Load Types: Clearly define the primary load components: purely radial, purely axial, or a combined load scenario. This dictates the initial selection of the bearing type (e.g., Cylindrical for high radial, Tapered for combined).
• Determine Operating Speed and Temperature: Record the maximum and typical rotational speed in revolutions per minute (RPM) and the stable operating temperature. These factors will be used later to verify speed limits and lubrication suitability.
• Define Desired Bearing Life and Reliability: Specify the required operational life in hours (e.g., 20,000 hours for industrial machinery). Also, define the required reliability level (e.g., standard 90 percent, or 99 percent for critical applications).

Step 2: Calculate Loads

Accurate determination of the forces acting on the bearing is essential.

• Calculate Radial and Axial Loads Accurately: Use mechanical design principles and system analysis to find the precise values of the radial load and the axial load acting on the bearing location.
• Determine Equivalent Dynamic Load §: If the application involves both radial and axial loads, these forces must be converted into a single, theoretical Equivalent Dynamic Load. This conversion uses specific factors provided in bearing catalogs and represents the magnitude of the purely radial load that would consume the bearing life at the same rate as the actual combined load scenario.

Step 3: Calculate Required Basic Dynamic Load Rating

This is the core calculation for dynamic sizing, ensuring the bearing meets the desired life goal.

• Use the Bearing Life Equation Principle: The required load rating is determined by using the standardized bearing life relationship. This relationship connects the Equivalent Dynamic Load §, the Desired Life (L10), and the required Basic Dynamic Load Rating ( C ).
• Solve for C: Based on the life requirement and the calculated operating load, determine the minimum load capacity the selected bearing must possess. This calculated value of the Basic Dynamic Load Rating ( C ) is the benchmark for selecting a suitable bearing from a manufacturer’s catalog.

Step 4: Select a Bearing Series

With the required load rating established, physical selection begins.

• Consult Bearing Catalogs: Refer to the comprehensive product catalogs of major manufacturers (such as SKF, Timken, or NSK). Search for bearing series that have a published Basic Dynamic Load Rating ( C ) greater than or equal to the value calculated in Step 3.
• Consider Dimensional Constraints: Concurrently, the selected bearing must conform to the physical constraints of the machine, including the required bore diameter (to fit the shaft) and the maximum allowable outer diameter and width (to fit the housing).

Step 5: Verify Static Load Capacity

While dynamic load governs fatigue life, static load prevents catastrophic deformation.

• Check the Static Load Rating (C0): Locate the Basic Static Load Rating (C0) of the selected bearing from the catalog.
• Ensure C0 is Sufficient: Verify that C0 is significantly greater than the absolute maximum static load or shock load the bearing will experience, even momentarily, during assembly, handling, or static operation. This prevents permanent plastic deformation of the raceways or rolling elements.

Step 6: Check Speed and Temperature Limits

The physical constraints of the bearing design must meet the operational speed and heat generation.

• Verify Speed Limits: Ensure that the selected bearing’s reference speed or limiting speed, as published by the manufacturer, is higher than the operating speed determined in Step 1. Failing this check requires selecting a smaller bearing size or a different, higher-speed design series.
• Check Temperature Limits: Confirm that the bearing material and any associated components (like cages and seals) can withstand the maximum operating temperature without degradation.

Step 7: Consider Lubrication Requirements

Proper lubrication is as critical as sizing for actual longevity.

• Determine the Appropriate Lubrication Method: Based on speed, temperature, and environment, decide whether the application requires grease (simple, low maintenance) or oil circulation (better cooling, high speeds).
• Select a Lubricant: Choose a specific lubricant (grease type or oil viscosity) that meets the operating conditions, particularly the viscosity required to achieve a satisfactory lubrication film thickness at the running temperature.

Step 8: Verify Mounting and Sealing Arrangements

The final checks ensure the bearing is installed correctly and protected from its environment.

• Verify Mounting: Ensure that the chosen bearing’s rings can be properly mounted on the shaft and in the housing, accounting for necessary interference fits and clearances.
• Select Sealing Arrangements: Select appropriate seals, such as contact seals, non-contact seals, or labyrinth seals, to prevent contamination from external factors (dust, water) and to retain the chosen lubricant.

5. Practical Examples

Applying the step-by-step sizing process to real-world scenarios is essential for understanding the critical decisions involved in bearing selection. These examples illustrate how load type and environmental factors dictate the final choice.

Example 1: Sizing a Roller Bearing for a Conveyor System

Application Scenario: A heavy-duty aggregate conveyor system in a quarry.

Requirements Summary:

• Load: The load is predominantly radial due to the weight of the belt, the material (rocks/gravel), and the tensioning system. There are minimal axial loads.
• Speed: Low to moderate RPM.
• Environment: Extremely dusty and dirty, with high potential for moisture ingress.
• Life Requirement: Long service life (50,000 hours) is required to minimize maintenance in remote locations.
• Shaft Alignment: The long frame of the conveyor is prone to slight misalignment and deflection.

Sizing and Selection Process Highlights:

1. Type Selection: Given the heavy radial load and the high probability of shaft misalignment, a Spherical Roller Bearing (SRB) is the ideal choice. The self-aligning capability of the SRB compensates for the inevitable frame deflection and mounting errors.
2. Load Calculation: The primary focus is on calculating the high Equivalent Dynamic Load based on the weight and acceleration forces.
3. Life Verification: The calculated load P is used to determine the required Basic Dynamic Load Rating C needed to achieve the 50,000-hour life. A bearing is selected from the SRB series that meets this C value.
4. Environmental Consideration: The chosen SRB is specified with integral contact seals or paired with robust external labyrinth seals to effectively protect the internal components from the harsh dust and contamination.

Example 2: Sizing a Roller Bearing for a Gear Reducer

Application Scenario: The input shaft of a compact industrial gear reducer connected to an electric motor.

Requirements Summary:

• Load: High combined loads—significant radial force from the meshing gears and considerable axial thrust force from the helical gear teeth.
• Speed: High RPM (matching the motor speed, e.g., 1800 RPM).
• Environment: Controlled, clean environment inside a gearbox housing, allowing for oil bath lubrication.
• Space Constraint: The housing size is compact, requiring a bearing with high load density relative to its size.

Sizing and Selection Process Highlights:

1. Type Selection: Due to the simultaneous presence of high radial and high axial loads, a Tapered Roller Bearing (TRB) is selected. These bearings are inherently designed to manage combined forces and offer high rigidity, which is crucial for gear mesh precision. TRBs must be mounted in pairs (often one on each end of the shaft) to react to the axial thrust.
2. Load Calculation: The complex step here is accurately determining the axial load component generated by the helical gears, which is often greater than the radial load. This is used to calculate the Equivalent Dynamic Load P.
3. Static Verification: Since gearboxes can experience high torque loads at very low speeds, the Basic Static Load Rating C0 must be verified to prevent damage during high-torque startup or momentary overloads.
4. Speed Verification: The bearing’s limiting speed must be checked against the motor’s high RPM. The oil bath lubrication system is designed to provide cooling and adequate oil film for the high-speed operation.

6. Common Mistakes to Avoid

Even with a systematic approach, several critical errors frequently undermine the bearing sizing and selection process. Avoiding these pitfalls is essential for achieving the planned service life and performance.

Ignoring Axial Loads

A common mistake is treating a radial application as purely radial, especially in systems using helical gears, V-belts, or conveyor pulleys with guide flanges.

• The Pitfall: The designer focuses only on the heavy radial force and underestimates or completely neglects the smaller, but still significant, axial thrust.
• The Consequence: The selected bearing may have a high Basic Dynamic Load Rating ( C ), but if it is a type with poor axial capacity (like a standard Cylindrical Roller Bearing without flanges), the axial load will cause rapid end-face wear and premature failure.

Underestimating Dynamic Loads

This mistake often occurs when designers rely solely on steady-state or nominal load conditions.

• The Pitfall: Failure to account for transient loads, such as shock loads, vibration, sudden stops, or high acceleration forces encountered during startup.
• The Consequence: The Equivalent Dynamic Load ( P ) used for sizing is too low. The chosen bearing’s Basic Dynamic Load Rating ( C ) is insufficient for the true operating environment, leading to early fatigue failure due to overload. Incorporating a proper safety factor is necessary to mitigate this risk.

Neglecting Operating Temperature

Temperature is a primary stressor that affects both material properties and lubrication.

• The Pitfall: The designer assumes room temperature or ignores the heat generated by the machine’s own friction, external heat sources (like furnaces), or high rotational speeds.
• The Consequence: High temperatures permanently reduce the hardness of the bearing steel, resulting in a permanent reduction in the actual load-carrying capacity. Furthermore, the lubricant viscosity may drop too low, leading to metal-on-metal contact and rapid wear.

Improper Lubrication

The correct lubricant, in the correct quantity, is crucial; a properly sized bearing will fail quickly with the wrong lubrication.

• The Pitfall: Using the wrong type of lubricant (e.g., using a general-purpose grease instead of a high-viscosity oil for a heavy-duty, slow-speed application) or using too much or too little of the correct lubricant.
• The Consequence: Too little lubrication leads to overheating and surface wear. Too much grease, especially at high speeds, can cause churning, excessive heat generation, and subsequent seal damage.

Incorrect Mounting

Even a perfectly sized bearing can be destroyed during or immediately after installation.

• The Pitfall: Using brute force (hammering) to seat the bearing, applying force through the wrong ring (e.g., pressing the inner ring onto the shaft by hitting the outer ring), or using incorrect fits.
• The Consequence: Improper mounting causes brinelling (indentations) on the raceways or introduces internal preloads that destroy the designed internal clearance. This leads to vibration, noise, and immediate, catastrophic failure, entirely unrelated to the sizing calculation.

Conclusion

Recap of the Key Steps in Roller Bearing Sizing

Accurate roller bearing sizing is a systematic, eight-step process that transitions from defining the environment to verifying the final installation details:

1. Determine Requirements: Define desired life, speed, and environment.
2. Calculate Loads: Accurately determine radial, axial, and Equivalent Dynamic Loads ( P ).
3. Calculate Required Rating: Determine the minimum Basic Dynamic Load Rating ( C ) necessary for the desired life.
4. Select Bearing Series: Choose a bearing type and size from a catalog that meets the required C and dimensional constraints.
5. Verify Static Capacity: Ensure the Basic Static Load Rating (C0) is sufficient for shock and static loads.
6. Check Limits: Verify speed and temperature limits are appropriate.
7. Consider Lubrication: Select the correct lubricant type and viscosity.
8. Verify Mounting: Ensure proper fit and adequate sealing arrangements.

Importance of Accurate Sizing for Optimal Performance and Longevity

Accurate sizing is the cornerstone of mechanical reliability. It ensures that the bearing can withstand the predicted fatigue stress, avoid catastrophic static deformation, and operate within its thermal limits. A properly sized bearing provides the foundation for:

• Maximizing Component Life: Achieving or exceeding the specified L10 life.
• Minimizing Downtime: Reducing unexpected failures and maintenance costs.
• Optimizing Efficiency: Operating with intended low friction and temperature.

Encourage Readers to Consult with Bearing Specialists for Complex Applications

While this guide provides a detailed methodology, complex or non-standard applications—such as those involving extreme temperatures, high vibration, critical safety factors, or specialized materials—often require expert input. Readers are strongly encouraged to consult directly with the application engineering teams of reputable bearing manufacturers. Their specialized knowledge and advanced simulation tools can validate complex selections and ensure the highest level of reliability.

Frequently Asked Questions (FAQs) on Roller Bearing Sizing

1. What is the fundamental difference between Basic Dynamic Load Rating ( C ) and Basic Static Load Rating (C0), and when is each used?

• Basic Dynamic Load Rating ( C ): This is the crucial rating used for dynamic sizing (when the bearing is rotating). It is the constant radial load that a group of bearings can theoretically endure for $L_{10}$ life (one million revolutions) before 90% of them fail from material fatigue. It governs the bearing’s expected fatigue life.
• Basic Static Load Rating ( C0 ): This rating is used for static sizing (when the bearing is stationary or oscillating slowly). It represents the maximum static load that a bearing can withstand without causing permanent plastic deformation of the raceways or rolling elements beyond an acceptable limit. It governs the bearing’s resistance to permanent damage under heavy, non-rotating loads or shock.

2. Why is the Equivalent Dynamic Load ( P ) necessary, and how does it relate to the actual loads?

The Equivalent Dynamic Load ( P ) is necessary because most bearings operate under a combination of radial and axial forces. Catalog life equations are standardized to use a single, purely radial load value.

• Function: P converts the actual, combined operating loads into a theoretical single radial load that would produce the same fatigue damage and result in the same predicted life as the real operating conditions.
• Relationship: P is calculated using a weighted formula that considers factors (X and Y) published by the manufacturer. These factors depend on the bearing type, as some types are more sensitive to axial load than others.

3. If I choose an oversized bearing for maximum safety, will I automatically get longer life?

Not necessarily. While an oversized bearing will increase your Basic Dynamic Load Rating ( C ) and extend fatigue life under the same load, it can introduce new problems:

• Cost and Space: It increases material costs and demands larger shaft and housing dimensions.
• Lubrication Issues: A large bearing designed for heavy loads might be operating under a relatively light load (underloaded). This can prevent the rolling elements from achieving the minimum load required to maintain a stable, full-thickness lubricating film, potentially leading to increased surface wear.
• Speed and Heat: Larger bearings have higher mass and generate more friction heat at high speeds, potentially violating the bearing’s speed limit or thermal constraints. Proper sizing aims for the most economical bearing that reliably meets the design life.

4. How does shaft misalignment affect the required bearing size?

Shaft misalignment significantly complicates sizing because it dramatically affects the internal load distribution within the bearing.

• Standard Bearings (e.g., Cylindrical or Tapered): Misalignment causes stress concentrations at the edges of the rolling elements, reducing the effective contact area and rapidly decreasing the load-carrying capacity and fatigue life. This requires the designer to select a much larger bearing to compensate for the stress.
• Self-Aligning Bearings (e.g., Spherical Roller Bearings): These bearings are designed to internally compensate for misalignment up to certain limits. In applications where misalignment is unavoidable (like long shafts), selecting a self-aligning type is often the best solution, allowing you to size the bearing based primarily on load and life, rather than having to oversize it to counteract poor alignment.

5. What is the most critical factor to verify regarding speed limitations?

The most critical factor to verify is the Reference Speed (or Limiting Speed) published in the manufacturer’s catalog.

• Importance: This speed limit is set to prevent the bearing from generating excessive heat due to internal friction, which can lead to rapid lubricant degradation, thermal expansion causing loss of internal clearance, and eventual seizure.
• Verification: You must ensure that the application’s maximum operating speed is safely below the manufacturer’s specified speed limit for the selected bearing size and lubrication type. If the application speed is too high, you may need to switch to a bearing design with specialized cages, different internal geometries, or a more efficient lubrication system (e.g., from grease to oil circulation).

Frequently Asked Questions (FAQs) on Roller Bearing Sizing

1. What is the fundamental difference between Basic Dynamic Load Rating ( C ) and Basic Static Load Rating (C0), and when is each used?

• Basic Dynamic Load Rating ( C ): This is the crucial rating used for dynamic sizing (when the bearing is rotating). It is the constant radial load that a group of bearings can theoretically endure for $L_{10}$ life (one million revolutions) before 90% of them fail from material fatigue. It governs the bearing’s expected fatigue life.
• Basic Static Load Rating ( C0 ): This rating is used for static sizing (when the bearing is stationary or oscillating slowly). It represents the maximum static load that a bearing can withstand without causing permanent plastic deformation of the raceways or rolling elements beyond an acceptable limit. It governs the bearing’s resistance to permanent damage under heavy, non-rotating loads or shock.

2. Why is the Equivalent Dynamic Load ( P ) necessary, and how does it relate to the actual loads?

The Equivalent Dynamic Load ( P ) is necessary because most bearings operate under a combination of radial and axial forces. Catalog life equations are standardized to use a single, purely radial load value.

• Function: P converts the actual, combined operating loads into a theoretical single radial load that would produce the same fatigue damage and result in the same predicted life as the real operating conditions.
• Relationship: P is calculated using a weighted formula that considers factors (X and Y) published by the manufacturer. These factors depend on the bearing type, as some types are more sensitive to axial load than others.

3. If I choose an oversized bearing for maximum safety, will I automatically get longer life?

Not necessarily. While an oversized bearing will increase your Basic Dynamic Load Rating ( C ) and extend fatigue life under the same load, it can introduce new problems:

• Cost and Space: It increases material costs and demands larger shaft and housing dimensions.
• Lubrication Issues: A large bearing designed for heavy loads might be operating under a relatively light load (underloaded). This can prevent the rolling elements from achieving the minimum load required to maintain a stable, full-thickness lubricating film, potentially leading to increased surface wear.
• Speed and Heat: Larger bearings have higher mass and generate more friction heat at high speeds, potentially violating the bearing’s speed limit or thermal constraints. Proper sizing aims for the most economical bearing that reliably meets the design life.

4. How does shaft misalignment affect the required bearing size?

Shaft misalignment significantly complicates sizing because it dramatically affects the internal load distribution within the bearing.

• Standard Bearings (e.g., Cylindrical or Tapered): Misalignment causes stress concentrations at the edges of the rolling elements, reducing the effective contact area and rapidly decreasing the load-carrying capacity and fatigue life. This requires the designer to select a much larger bearing to compensate for the stress.
• Self-Aligning Bearings (e.g., Spherical Roller Bearings): These bearings are designed to internally compensate for misalignment up to certain limits. In applications where misalignment is unavoidable (like long shafts), selecting a self-aligning type is often the best solution, allowing you to size the bearing based primarily on load and life, rather than having to oversize it to counteract poor alignment.

5. What is the most critical factor to verify regarding speed limitations?

The most critical factor to verify is the Reference Speed (or Limiting Speed) published in the manufacturer’s catalog.

• Importance: This speed limit is set to prevent the bearing from generating excessive heat due to internal friction, which can lead to rapid lubricant degradation, thermal expansion causing loss of internal clearance, and eventual seizure.
• Verification: You must ensure that the application’s maximum operating speed is safely below the manufacturer’s specified speed limit for the selected bearing size and lubrication type. If the application speed is too high, you may need to switch to a bearing design with specialized cages, different internal geometries, or a more efficient lubrication system (e.g., from grease to oil circulation).