What slewing bearings are used in cranes?

I. Functional Positioning of Slewing Bearings in Cranes

Slewing bearings are core load-bearing components of crane slewing mechanisms, undertaking three key functions: first, transmitting vertical loads between the crane's upper structure (such as the boom and cab) and the lower chassis (or traveling mechanism), including the total weight of the crane and the weight of the lifted load; second, withstanding horizontal loads, such as wind forces, slewing inertial forces, and lateral forces generated by the deflection of the lifted load; third, enabling 360° continuous slewing of the upper structure around the vertical axis, providing a foundation for adjusting the working radius. Their performance directly determines the operational stability, slewing accuracy, and service life of the crane.

II. Structural Types and Application Scenarios of Common Slewing Bearings for Cranes

(I) Four-Point Contact Ball Slewing Bearings

1. Structural Features: The inner or outer ring is of an integral type, the raceway cross-section is arc-shaped, and the steel balls make contact with the raceway at four points. A single bearing can simultaneously bear radial loads, bidirectional axial loads, and overturning moments; the cage is mostly made of brass or reinforced nylon to ensure uniform distribution of steel balls.

2. Application Scenarios: Suitable for medium and small-tonnage cranes (rated lifting capacity ≤ 50t), such as the slewing mechanisms of truck cranes and wheeled cranes. These cranes have small load fluctuations during operation and high requirements for slewing flexibility. The low friction coefficient (friction coefficient ≤ 0.0015) of four-point contact ball bearings can reduce the power consumption of the slewing drive, and their compact structure can reduce the installation space of the slewing mechanism.

(II) Double-Row Angular Contact Ball Slewing Bearings

1. Structural Features: Composed of inner ring, outer ring, two rows of steel balls, and cages, the two rows of raceways are at symmetrical angles (contact angles are usually 45° or 60°), bearing axial loads in different directions respectively. Their radial load-bearing capacity is better than that of four-point contact ball bearings; some models adopt split inner or outer rings for easy installation and maintenance.

2. Application Scenarios: Used in medium-tonnage cranes (50t < rated lifting capacity ≤ 150t), such as the slewing parts of all-terrain cranes and tower cranes. These cranes need to balance vertical lifting and horizontal luffing operations. The bidirectional axial load-bearing capacity of double-row angular contact ball bearings can cope with load reversal during luffing, improving operational stability.

(III) Crossed Roller Slewing Bearings

1. Structural Features: The raceway is V-shaped, the rollers are cylindrical, and adjacent rollers are arranged crosswise (at an angle of 90°). The rollers make line contact with the raceway, resulting in a large contact area; they have balanced radial, axial, and overturning moment load-bearing capacities, and high slewing accuracy (radial runout ≤ 0.05mm), suitable for high-precision slewing requirements.

2. Application Scenarios: Used in cranes for high-precision operations, such as the slewing mechanisms of truck-mounted cranes and port portal cranes. Truck-mounted cranes need to be accurately positioned in narrow spaces, and portal cranes need to frequently adjust the working angle. The high-precision characteristics of crossed roller bearings can reduce slewing clearance and prevent the lifted load from shaking.

(IV) Three-Row Cylindrical Roller Slewing Bearings

1. Structural Features: Composed of inner ring, outer ring, three rows of cylindrical rollers, and cages. Two rows of rollers are arranged axially to bear bidirectional axial loads and overturning moments, and one row of rollers is arranged radially to bear radial loads; the raceways undergo hardening treatment (hardness HRC 58-62), and the rollers are of full or semi-full loading design. Their load-bearing capacity is 2-3 times that of ball bearings of the same size.

2. Application Scenarios: Used in large-tonnage and heavy-duty cranes (rated lifting capacity > 150t), such as crawler cranes and offshore floating cranes. Crawler cranes need to lift ultra-heavy components (such as wind turbine towers), and floating cranes need to cope with complex loads generated by offshore wind and waves. The high load-bearing capacity of three-row cylindrical roller bearings can ensure structural safety under extreme working conditions.

III. Key Selection Parameters for Crane Slewing Bearings

1. Rated Load: Including radial rated dynamic load, axial rated dynamic load, and rated overturning moment. It should be calculated based on the crane's maximum lifting capacity, working radius, and total weight. Ensure that the rated load of the selected bearing is ≥ 1.2-1.5 times the actual working load (safety factor).

2. Slewing Speed: Determined according to the crane's operational efficiency requirements. Generally, the slewing speed of medium and small-tonnage cranes is 1-3 r/min, and that of large-tonnage cranes is 0.5-1.5 r/min. It is necessary to match the limit speed of the bearing (to avoid overheating).

3. Installation Dimensions: Including bearing inner diameter, outer diameter, height, and bolt hole circle diameter. They need to match the installation interfaces of the crane's slewing platform and chassis. The number and diameter of bolt holes must meet the connection strength requirements (bolt strength grade ≥ 8.8).

4. Sealing Performance: Cranes mostly operate outdoors, in dusty or humid environments. Bearings need to adopt double-lip sealing or labyrinth sealing structures to prevent the intrusion of impurities and the leakage of lubricating grease. The sealing material must be resistant to high and low temperatures (-30℃-80℃) and aging.

5. Lubrication Method: Lithium-based grease (NLGI Grade 2) is used. Grease filling ports and discharge ports must be reserved. Medium and small-tonnage cranes require supplementary lubrication every 50-100 hours, and large-tonnage cranes every 200-300 hours to ensure a stable oil film is formed between the raceways and rollers.

IV. Special Design Requirements for Crane Slewing Bearings

1. Impact Resistance Design: In response to the impact load at the moment the crane lifts, the bearing raceways need to undergo surface strengthening treatment , and the rollers adopt a fillet design to reduce stress concentration; some large-tonnage bearings use integral outer rings to avoid displacement of the split structure under impact.

2. Anti-Corrosion Design: The outer rings of cranes operating outdoors need to adopt hot-dip galvanizing or anti-rust coating  spraying, and the connection surface between the inner ring and the slewing platform should be coated with anti-rust grease; bearings for cranes in coastal areas need to be made of corrosion-resistant materials (such as martensitic stainless steel) to cope with salt spray corrosion.

3. Lightweight Design: Truck cranes and all-terrain cranes are sensitive to their own weight. Bearings can use lightweight materials or optimize the raceway structure to reduce the bearing weight by 5%-10% while ensuring the load-bearing capacity.

If you are looking for slewing bearings, please contact LTZC.