Comprehensive Selection Guide for Slewing Bearings
Slewing bearings are critical components in a vast array of machinery, enabling smooth and controlled rotational movement under complex loads. Selecting the correct slewing bearing is paramount to the performance, safety, and longevity of your equipment. This guide provides a detailed, step-by-step approach to understanding and choosing the right slewing bearing for your application.
What is a Slewing Bearing?
A slewing bearing, also known as a slewing ring, turntable bearing, or rotary connection, is a large-scale rolling-element bearing that facilitates rotational movement between two components. Unlike standard bearings that primarily handle radial or axial loads, slewing bearings are uniquely designed to withstand combined loads—simultaneous axial, radial, and tilting moment loads.
The fundamental structure consists of an inner ring and an outer ring, one of which typically features gear teeth (internal or external). Between these rings, rolling elements (balls or cylindrical rollers) are spaced by spacers or cages. Slewing bearings are characterized by their large diameter-to-width ratio and are often manufactured with mounting holes in the rings, allowing for direct bolting to adjacent structures, which simplifies assembly and design.
They are the workhorses in applications where a robust, reliable, and precise rotation is required, forming the fundamental joint in machines from massive cranes to precise solar trackers.
Why the Slewing Bearing Size Chart Is Important?
The slewing bearing size chart is the primary gateway to the selection process. Its importance cannot be overstated for several reasons:
- Ensures Physical Compatibility: The most immediate reason is to ensure the bearing fits within the designated space in your machine’s structure. The chart provides critical dimensions like bore diameter, outer diameter, height, and pitch circle diameter of the mounting holes.
- Prevents Costly Errors: Selecting a bearing that is too small can lead to catastrophic failure, while one that is too large is a waste of material, weight, and capital. The size chart helps you find a bearing that is “just right,” optimizing both performance and cost.
- Facilitates Initial Screening: With hundreds of standard sizes and configurations available, the size chart allows engineers to quickly narrow down potential candidates based on the spatial constraints of their design before diving into complex load capacity calculations.
- Standardization and Interchangeability: Size charts for standard bearings allow for easier sourcing and potential interchangeability between different manufacturers, providing flexibility in the supply chain.
Understanding Slewing Bearing Size Chart
A slewing bearing size chart is more than just a list of part numbers and diameters. To use it effectively, one must understand the key parameters typically listed:
- Model/Part Number: The unique identifier for a specific bearing type and size.
- Inner Diameter (d): The diameter of the central hole.
- Outer Diameter (D): The overall outside diameter of the bearing.
- Height/Thickness (H): The total height or thickness of the bearing assembly.
- Bolt Circle Diameter (BCD): The diameter of the circle on which the mounting holes are positioned. There are usually two BCDs—one for the inner ring and one for the outer ring.
- Number and Size of Mounting Holes: Specifies the quantity, diameter, and thread size of the holes for bolts.
- Gear Information: For geared bearings, the chart will include details like gear type (internal or external), module, number of teeth, and pitch diameter.
- Rolling Element & Raceway Details: Some detailed charts specify the size and number of balls or rollers.
How to Read It: Cross-reference the required internal and external diameters of your design with the chart. The selected bearing must have a suitable outer diameter to fit your housing and an inner diameter that accommodates any necessary components (e.g., hydraulic swivels, cables) passing through the center.
Common Types of Slewing Bearings
Selecting the correct type is the first major technical decision. The four most common types, defined by their rolling element and raceway configuration, are:
- Single-Row Ball Slewing Bearing (Four-Point Contact Ball Bearing): This is the most versatile and common type. A single row of balls contacts the raceways at four points, allowing it to handle combined axial, radial, and tilting moment loads simultaneously. It is compact and highly efficient, ideal for applications like cranes, excavators, and robotic arms.
- Double-Row Ball Slewing Bearing: This design features two separate rows of balls. This configuration provides a significantly higher load-carrying capacity, especially for axial loads and tilting moments, compared to a single-row bearing of a similar size. It is often used in heavier-duty applications such as large stacker reclaimers and port cranes.
- Crossed Roller Slewing Bearing: In this type, cylindrical rollers are arranged alternately at 90° angles between the inner and outer rings. This configuration allows each roller to handle axial and radial loads directly. They offer very high rotational accuracy and rigidity but typically have lower rotational speeds. They are perfect for applications demanding high precision, such as rotary tables on machine tools, industrial robots, and radar antennae.
- Three-Row Roller Slewing Bearing: This is the heaviest-duty option. It utilizes three independent rows of cylindrical rollers—one dedicated to handling axial loads and the other two dedicated to handling radial loads and tilting moments. This design offers the highest possible load capacity and is almost exclusively used in the most demanding applications, such as massive mining shovels and heavy-lift offshore cranes.
Understanding Load Ratings
Load ratings are the heart of the technical selection process. Choosing a bearing based solely on size is a recipe for failure. The key ratings to understand are:
- Static Load Rating (C₀): This is the load that a bearing can withstand without permanent deformation sufficient to impair operation. It is relevant when the bearing is stationary or oscillating slowly under load. Exceeding the static load rating can cause Brinelling (indentations on the raceways).
- Dynamic Load Rating (C): This rating represents the constant radial or axial load that a group of identical bearings can endure for a rating life of 1 million revolutions. It is used to calculate the theoretical fatigue life (L10 life) of a bearing under rotation.
- Static Tilting Moment Rating (M₀): This is the maximum tilting moment the bearing can support while stationary without permanent deformation.
- Dynamic Safety Factor (fₛ): This is not a load rating but a critical calculated value. It is the ratio of the bearing’s static load capacity to the actual applied static load (
fₛ = C₀ / P₀). A minimum safety factor, typically provided by the manufacturer (e.g., 1.25 for cranes, 1.5 for wind turbines), must be maintained to account for dynamic shock loads, uneven load distribution, and other unpredictable operational factors.
How to Use Them: The selection process involves calculating the maximum combined loads (axial force, radial force, and tilting moment) that the bearing will experience in your application. These calculated loads are then compared to the bearing’s rated capacities, ensuring that the static and dynamic safety factors are met.
Popular Brands and Their Size Charts
Several global manufacturers are renowned for their quality and engineering support. While their core product lines are similar, their sizing and part numbering are proprietary.
- SKF: A global leader, SKF offers a wide range of standard slewing bearings (e.g., the SBP, SBV, and SBC series) under the “Slewing Bearing and Wire Race” portfolio. Their size charts are comprehensive and integrated into their online selection tools, which help calculate loads and life.
- ThyssenKrupp Rothe Erde: A specialist in large-diameter bearings, Rothe Erde is a top choice for heavy industrial and wind energy applications. Their catalogs provide extremely detailed technical data, including load ratings for a vast array of standard and custom sizes.
- IMO & IBC (International Bearing Corporation): IBC is a major American manufacturer known for its “W” series of four-point contact ball bearings. Their size charts are widely used as a reference in many industries, particularly in North America.
- La Leonessa / Rollix: This manufacturer offers a broad portfolio and is known for its technical expertise. Their catalogs provide clear size charts and load rating tables.
Important Note: While size charts can be compared for physical dimensions, load ratings from different manufacturers are not directly interchangeable. A bearing from Brand A and Brand B with identical physical dimensions can have significantly different load capacities due to differences in heat treatment, material quality, and internal geometry. Always use the load ratings from the specific manufacturer you are sourcing from.
Additional Critical Selection Factors
Beyond type and load ratings, a complete selection must consider these factors:
- Gear Requirements: Determine if you need a geared bearing (for driven rotation) and whether an internal or external gear is required. The gear module and teeth must be designed to handle the transmitted torque from the pinion.
- Sealing: Effective sealing is crucial to keep contaminants out and grease in. Standard options often include rubber seals (e.g., NBR). For harsh environments (e.g., marine, food industry), special seals like polyurethane or labyrinth seals may be necessary.
- Lubrication: Slewing bearings require regular relubrication. The type and number of lubrication fittings (grease nipples) and the presence of lubrication holes and channels are important design considerations. The correct grease type for the operating temperature and speed must be specified.
- Material and Heat Treatment: Standard bearings are typically made from carbon chromium bearing steel (e.g., 42CrMo4) and hardened. For corrosive environments, materials like stainless steel or coatings like zinc-nickel plating are available. The raceways are almost always induction hardened to a specific depth to withstand high contact pressures.
Installation, Maintenance, and Handling
A perfectly selected bearing will fail prematurely if installed or maintained incorrectly.
- Handling: Slewing bearings are precision components. They must be stored horizontally and handled with care to avoid drops and impacts that can cause Brinelling.
- Installation: The mounting surfaces must be flat, rigid, and properly aligned. The bearing must be mounted using high-strength bolts, tightened to the correct preload torque in a cross-bolting sequence. The structure must be designed to have a higher stiffness than the bearing itself.
- Maintenance: A maintenance schedule must be established for regular inspection and relubrication. The first lubrication interval is often shortly after initial operation to purge any contaminants from the running-in period.
Step-by-Step Selection Procedure
- Define Application & Loads: Identify the machine, its function, and calculate the maximum combined loads (axial force Fa, radial force Fr, tilting moment M).
- Determine Safety Factor (fₛ): Establish the required static safety factor based on application standards and operational criticality.
- Preliminary Size & Type Selection: Use a size chart to find a bearing that fits your spatial constraints. Based on the magnitude of the loads, select a preliminary type (e.g., Single-Row Ball for moderate loads, Crossed Roller for high precision, Three-Row Roller for extreme loads).
- Check Load Ratings: With the candidate bearing’s static load ratings (C₀, M₀), verify that the safety factor
fₛ = C₀ / P₀meets or exceeds your requirement. - Specify Accessories & Options: Finalize requirements for gear, seals, lubrication, material, and mounting holes.
- Consult a Supplier: For critical applications, always consult with the technical engineering team of your chosen slewing bearing manufacturer. They can perform a detailed verification and offer expert recommendations.
Conclusion
The selection of a slewing bearing is a multifaceted engineering process that balances physical dimensions, load capacity, operational requirements, and environmental factors. By systematically working through this guide—from understanding the basic “what” and “why,” to interpreting size charts and load ratings, and finally considering installation and maintenance—you can make an informed and reliable selection. This ensures the slewing bearing will perform as the robust and dependable heart of your rotating machinery for its entire service life.
