9.2 Rolling Element and Journal Bearings
4 min read•august 6, 2024
Bearings are crucial components in mechanical systems, enabling smooth rotation and load support. This section covers two main types: , which use balls or rollers, and , which rely on fluid films for .
Understanding bearing selection, installation, and failure modes is essential for engineers. Proper bearing choice and maintenance ensure optimal performance and longevity in various applications, from small motors to large .
Rolling Element Bearings
Types of Rolling Element Bearings
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- Ball bearings consist of a series of balls that roll between inner and outer races (rings) to reduce and support radial and axial loads
- Roller bearings use cylindrical rollers instead of balls to support higher radial loads compared to ball bearings
- are designed to handle both radial and axial loads simultaneously
- Common types of roller bearings include cylindrical, spherical, tapered, and needle roller bearings, each with specific load carrying capabilities and applications
Bearing Life and Load Ratings
- Bearing life is the number of revolutions or hours of operation a bearing can endure before the first sign of , typically expressed as L10 life (90% reliability)
- Load rating is the maximum load a bearing can withstand for a specified life, usually defined as the basic (C) and the basic (C0)
- The basic dynamic load rating (C) represents the constant radial load that a bearing can endure for a life of one million revolutions
- The basic static load rating (C0) is the maximum static load a bearing can withstand without causing permanent deformation of the or raceways
Bearing Lubrication
- Lubrication is essential for rolling element bearings to reduce friction, prevent , dissipate heat, and protect against corrosion
- Common lubrication methods for rolling element bearings include grease lubrication and oil lubrication (oil bath, oil mist, or oil-air)
- Grease lubrication is suitable for low to moderate speeds and loads, while oil lubrication is preferred for high-speed applications or when heat dissipation is critical
- Factors influencing the choice of lubricant include operating temperature, speed, load, environment, and bearing type and size
Journal Bearings
Journal Bearing Fundamentals
- Journal bearings, also known as plain or sleeve bearings, consist of a shaft (journal) rotating within a stationary bearing surface (bushing or sleeve)
- Journal bearings rely on a thin film of lubricant between the shaft and the bearing surface to support the load and reduce friction
- The lubricant film in journal bearings is typically generated by the relative motion between the shaft and the bearing surface () or by external pressurization (hydrostatic lubrication)
- Journal bearings are commonly used in applications with high loads and moderate speeds, such as in engines, turbines, and compressors
Hydrodynamic Lubrication in Journal Bearings
- Hydrodynamic lubrication occurs when the relative motion between the shaft and the bearing surface generates a wedge-shaped oil film that separates the surfaces and supports the load
- The formation of the hydrodynamic oil film depends on factors such as the shaft speed, lubricant viscosity, bearing clearance, and eccentricity ratio
- The eccentricity ratio is the ratio of the distance between the shaft and bearing centers to the radial clearance, and it affects the load-carrying capacity and stability of the bearing
- Hydrodynamic journal bearings require careful design and selection of materials, clearances, and lubricants to ensure proper operation and avoid failure modes such as wear, , or instability (oil whirl or oil whip)
Bearing Selection and Installation
Bearing Selection Criteria
- Bearing selection depends on various factors such as load (magnitude and direction), speed, temperature, environment, space constraints, and cost
- The load-carrying capacity, fatigue life, and friction characteristics of the bearing must be considered when selecting a bearing for a specific application
- Other factors influencing bearing selection include the required precision, stiffness, damping, and noise and vibration characteristics
- Compatibility with the shaft and housing materials, as well as the ease of installation and maintenance, should also be considered during bearing selection
Bearing Mounting and Installation
- Proper mounting and installation of bearings are critical for their performance and longevity
- Bearing mounting methods include interference fit (press fit), clearance fit (slip fit), and adhesive bonding, depending on the application requirements and bearing type
- The shaft and housing surfaces must be properly prepared (cleaned and dimensionally accurate) before bearing installation to ensure proper fit and
- Bearing installation techniques such as heating, hydraulic pressure, or mechanical force may be used to achieve the required fit without damaging the bearing or mating components
Bearing Failure Modes
- Common bearing failure modes include fatigue, wear, corrosion, contamination, and improper lubrication or installation
- Fatigue failure occurs due to repeated stress cycles and can manifest as spalling, pitting, or flaking of the bearing surfaces
- Wear can result from abrasive particles, inadequate lubrication, or misalignment, leading to increased clearances and reduced bearing performance
- Corrosion can be caused by moisture, aggressive chemicals, or galvanic effects, weakening the bearing materials and accelerating wear
- Contamination by foreign particles, such as dust or debris, can cause abrasive wear and damage to the bearing surfaces
- Improper lubrication (insufficient, excessive, or incorrect lubricant) and installation errors (misalignment, incorrect fit, or excessive preload) can also lead to premature bearing failure
Key Terms to Review (37)
ABEC Ratings: ABEC ratings are a system used to measure the precision and tolerances of bearings, specifically rolling element bearings, which play a crucial role in minimizing friction in various mechanical systems. This rating scale helps determine the suitability of a bearing for specific applications by indicating how accurately the bearing is manufactured. Higher ABEC ratings generally signify tighter tolerances and better performance in high-speed or high-precision applications, making them essential for understanding the effectiveness of rolling element bearings in machinery.
ABMA Standards: ABMA standards refer to a set of guidelines established by the Anti-Ballistic Missile Association to ensure the effective design, manufacturing, and testing of rolling element bearings and journal bearings. These standards provide industry-wide benchmarks for quality, performance, and reliability, promoting uniformity across various applications in mechanical engineering.
Aerospace systems: Aerospace systems refer to the integrated technologies and processes involved in the design, development, and operation of vehicles that travel through the atmosphere and outer space. This term encompasses a wide range of components, including aircraft, spacecraft, propulsion systems, and control mechanisms, all working together to ensure efficient performance and safety during flight. Understanding aerospace systems is crucial for optimizing their performance, reliability, and efficiency in various applications.
Alignment: Alignment refers to the arrangement or positioning of components in relation to one another to ensure proper functioning and efficiency. In mechanical systems, proper alignment is crucial as misalignment can lead to increased wear, failure of components, and inefficiencies. Maintaining correct alignment is not only vital for rolling element and journal bearings but also essential in presenting information clearly and effectively.
Automotive applications: Automotive applications refer to the use of various mechanical components and systems within vehicles to ensure their efficient operation, safety, and performance. This includes the design, development, and integration of parts such as engines, transmissions, suspension systems, and bearings to facilitate smooth driving experiences and enhance vehicle longevity.
Boundary lubrication: Boundary lubrication refers to a lubrication regime where the lubricant film is extremely thin and may not completely separate the sliding surfaces, leading to direct contact between them. In this regime, the performance of lubricants is heavily influenced by the surface properties and additive components, which form protective layers to reduce wear and friction in applications like rolling element and journal bearings.
Bronze: Bronze is an alloy primarily made of copper and tin, known for its strength, corrosion resistance, and excellent machinability. This metal has been utilized in various applications due to its favorable mechanical properties, making it a popular choice in engineering and manufacturing. Its unique characteristics allow it to perform well in environments where other metals may fail, particularly in applications requiring durability and low friction.
Bronze: Bronze is an alloy primarily made of copper and tin, known for its strength, durability, and resistance to corrosion. This material has been widely used throughout history in various applications due to its excellent mechanical properties and ability to withstand wear and tear, making it a popular choice in both engineering and art. The unique combination of elements in bronze allows for a range of uses from tools and weapons to decorative items and bearings.
Ceramic: Ceramics are inorganic, non-metallic materials made by the action of heat and subsequent cooling. They are known for their hardness, brittleness, and resistance to heat and corrosion, making them suitable for a variety of applications, including components in rolling element and journal bearings where low friction and durability are crucial.
Dynamic Load Rating: Dynamic load rating is the maximum load that a bearing can sustain while rotating at a specified speed, typically defined for a life of one million revolutions. It is a critical parameter in the design and selection of rolling element and journal bearings, as it helps determine the bearing's ability to handle varying loads over time. Understanding this rating allows engineers to select bearings that will perform reliably under specific operating conditions without premature failure.
Fatigue failure: Fatigue failure refers to the progressive structural damage that occurs when a material is subjected to repeated loading and unloading cycles, often leading to unexpected fracture. This phenomenon is critical in understanding how materials behave under various types of stresses, particularly where cyclic loading is prevalent. It involves the initiation and growth of cracks that can result in catastrophic failure long before the material reaches its ultimate strength.
Friction: Friction is the resistance that one surface or object encounters when moving over another. It plays a crucial role in mechanical systems, affecting the performance and efficiency of components like rolling element and journal bearings. Friction can lead to wear and tear in these bearings, impacting their lifespan and effectiveness in reducing motion resistance.
Friction coefficient: The friction coefficient is a numerical value that represents the ratio of the force of friction between two bodies to the normal force pressing them together. It is an important parameter in understanding how different materials interact under load, particularly in applications involving rolling element and journal bearings where friction plays a critical role in performance and efficiency.
Hydrodynamic lubrication: Hydrodynamic lubrication is a mechanism that allows smooth operation between surfaces in relative motion by creating a thin film of lubricant that separates the surfaces. This type of lubrication occurs when a moving surface drags lubricant into the contact area, generating pressure that keeps the surfaces apart. It is particularly crucial in reducing friction and wear in various mechanical components, especially in rolling element and journal bearings.
Industrial machinery: Industrial machinery refers to large and complex equipment used in manufacturing and production processes across various industries. This type of machinery is crucial for performing tasks such as material handling, processing, and assembly, often enabling mass production and improving efficiency. Industrial machinery encompasses a wide range of components, including rolling element and journal bearings for support and rotation, as well as couplings and clutches for power transmission and control.
Inner race: The inner race refers to the inner ring of a rolling element bearing that houses the rolling elements, such as balls or rollers, and provides a surface for them to rotate against. This component plays a crucial role in ensuring the proper alignment and movement of the rolling elements, which helps reduce friction and wear while supporting loads. The design and material of the inner race are critical for the overall performance and longevity of bearings used in various mechanical applications.
ISO 281: ISO 281 is an international standard that outlines the calculation of the dynamic load ratings and service life for rolling bearings. This standard provides a systematic approach to determine the expected lifespan of bearings under specified operating conditions, which is critical for design engineers to ensure reliability and performance in mechanical systems.
ISO Standards: ISO standards are internationally recognized guidelines and specifications developed by the International Organization for Standardization to ensure quality, safety, and efficiency across various industries. These standards help streamline processes and facilitate international trade by providing a common framework for product and service specifications, including those related to fasteners, springs, bearings, and load factors.
Journal bearings: Journal bearings are a type of bearing that supports rotating shafts and allows for smooth rotation by providing a low-friction interface between the shaft and the bearing. They are commonly used in machinery where axial loads are present, helping to reduce wear and tear on moving parts while maintaining alignment. These bearings can be made from various materials, such as bronze or composite materials, depending on the application requirements.
Life Expectancy: Life expectancy refers to the average period a person is expected to live based on statistical data, often influenced by various factors including health, environment, and technology. In the context of mechanical design, life expectancy is crucial for predicting the operational duration and reliability of components like rolling element and journal bearings under specific conditions.
Load capacity: Load capacity refers to the maximum load or weight that a component, such as a bearing, can safely support without failing. This term is critical in the design and selection of rolling element and journal bearings, as it ensures that these components can withstand the forces they encounter during operation without experiencing excessive wear or catastrophic failure.
Lubrication: Lubrication is the process of applying a substance (usually a liquid or grease) between moving surfaces to reduce friction and wear, ensuring smooth operation of mechanical components. Proper lubrication is crucial for the effective functioning of rolling element and journal bearings, as it minimizes metal-to-metal contact, dissipates heat, and protects against corrosion, ultimately extending the life of machinery.
Outer race: The outer race is a crucial component of rolling element bearings that serves as the stationary ring housing the rolling elements, such as balls or rollers, which are located between the inner race and the outer race. This part helps to facilitate smooth rotation and load distribution, while also providing support and alignment for the inner race. The design and material of the outer race play a significant role in the overall performance and durability of the bearing system.
Polymer: A polymer is a large molecule composed of repeating structural units called monomers, which are bonded together through covalent chemical bonds. Polymers can exhibit a range of properties based on their molecular structure, including flexibility, strength, and resistance to chemicals, making them versatile materials in various applications.
Radial bearings: Radial bearings are mechanical components that support rotating shafts, allowing them to spin freely while minimizing friction. They work by distributing the load of the rotating element evenly across a surface, which reduces wear and tear and enhances efficiency. Commonly found in various machines, radial bearings play a crucial role in enabling smooth motion and prolonging the lifespan of mechanical systems.
Rolling element bearings: Rolling element bearings are mechanical components that reduce friction between moving parts by utilizing rolling elements, such as balls or rollers, positioned between two races. They play a crucial role in supporting radial and axial loads while allowing smooth rotation or linear motion in machinery. These bearings are widely used in various applications due to their efficiency, load-carrying capacity, and ability to operate at high speeds.
Rolling Element Bearings: Rolling element bearings are mechanical devices that reduce friction between moving parts by using rolling elements, such as balls or rollers, to support loads. These bearings are crucial in allowing smooth rotation and minimizing wear and tear in machines, thereby enhancing efficiency and extending service life.
Rolling Elements: Rolling elements are mechanical components that facilitate motion between two surfaces by rolling instead of sliding, reducing friction and wear. Commonly found in bearings, these elements can be balls, rollers, or other shapes designed to carry loads while enabling rotational or linear movement. They play a crucial role in enhancing the efficiency and lifespan of machinery by minimizing energy loss due to friction.
Seizure: Seizure, in the context of mechanical engineering, refers to the phenomenon where two surfaces in contact experience excessive friction, leading to a complete stop of relative motion between them. This condition often occurs in bearings, where insufficient lubrication, high temperatures, or misalignment can create extreme pressure on the bearing surfaces, causing them to weld together or bind.
Speed Rating: Speed rating refers to the maximum speed at which a rolling element bearing can operate safely without experiencing failure. It is an important factor in selecting bearings for machinery, as different applications require different speed capabilities to ensure reliability and performance. Understanding speed ratings helps in preventing premature wear, overheating, and potential catastrophic failures.
Static Load Rating: Static load rating is a measure of the maximum load that a rolling element bearing can support while at rest without any motion. This rating indicates the bearing's ability to bear load without experiencing permanent deformation or failure under static conditions. It’s crucial for understanding how bearings will perform in applications where they may not be subjected to dynamic loads immediately.
Steel: Steel is an alloy made primarily of iron and carbon, known for its strength, durability, and versatility. It plays a crucial role in various engineering applications, especially due to its mechanical properties that can be modified through heat treatment and alloying elements. These characteristics make steel a preferred material for components like springs, bearings, shafts, and in detailed designs where performance and reliability are essential.
Thrust bearings: Thrust bearings are specialized components designed to support axial loads, providing a low-friction surface for rotary motion. They are crucial in applications where high-speed rotation and axial forces are present, effectively preventing misalignment and excessive wear. Thrust bearings are essential in various mechanical systems, playing a key role in ensuring smooth operation and extending the lifespan of machinery.
Tolerance: Tolerance refers to the allowable deviation from a specified dimension or value in manufacturing and engineering processes. It ensures parts fit and function correctly despite variations in size, shape, or position, and is crucial for the performance of components like bearings, where precise alignment and movement are essential for efficiency and longevity.
Wear: Wear refers to the gradual removal of material from a solid surface due to mechanical action, such as friction or contact with another surface. This phenomenon is critical in understanding how components perform over time, especially in terms of their longevity and reliability. Different types of wear mechanisms can lead to various failure modes, significantly affecting the performance of elements like bearings and other moving parts.
Wear failure: Wear failure refers to the progressive removal of material from a solid surface due to mechanical action, which can lead to a breakdown in function or structural integrity. This phenomenon is particularly important in applications involving rolling element and journal bearings, where constant motion and load can result in wear, affecting performance, longevity, and safety of mechanical systems.
Wear Resistance: Wear resistance is the ability of a material to withstand the gradual removal of its surface due to mechanical action, such as friction, abrasion, or erosion. This property is crucial for components subjected to repetitive contact and movement, ensuring their longevity and performance. High wear resistance minimizes maintenance costs and downtime, making it an essential characteristic for materials used in various engineering applications.