Guide to Spherical Plain Bearings Selection and Maintenance

In the vast world of mechanical design, there exists an unassuming yet critical component that silently withstands tremendous pressures while ensuring smooth equipment operation—the spherical plain bearing. Just as human joints enable flexible movement, these bearings serve as the "joints" of machinery, granting them the ability to move freely under complex working conditions.

Spherical plain bearings are ready-to-mount bearing elements consisting of a spherical inner ring and a relatively movable outer ring. Their unique design enables spatial adjustment movements between shafts and housings, including:

• Rotational movement: Allows components to rotate within a certain angle range.
• Tilting movement: Permits components to tilt within specific angles.
• Oscillating movement: Enables reciprocating swinging motions.
• Self-aligning capability: Compensates for shaft misalignment caused by manufacturing or installation errors.

Unlike rolling bearings (such as ball bearings), spherical plain bearings transmit static and dynamic loads (including alternating loads) directly through sliding surfaces, classifying them as sliding bearings. When combined with bearing housings, they form complete ready-to-mount units commonly known as rod ends.

Based on friction pair materials, spherical plain bearings are categorized into several main types:

• Steel/copper alloy: Requires lubrication and maintenance. Suitable for alternating loads, medium-to-large rotational movements, and moderate sliding speeds.
• Steel/steel: Similar to steel/copper alloy, requires lubrication for comparable working conditions.
• Steel/PTFE composite: Maintenance-free due to PTFE's self-lubricating properties. Ideal for unidirectional loads and small-to-medium impact loads. Maximum tilt angles must not be exceeded.

Manufacturers offer specialized spherical plain bearings including versions with widened outer rings (S-type) and various size series (K, E, G, W). Options include maintenance-free (PTFE-lined) or maintenance-required (relubricatable) types, available in stainless steel, bearing steel, free-cutting steel, or high-strength tempered materials, with or without seals.

A unique variant incorporates rolling elements, functioning similarly to self-aligning ball or roller bearings. These follow spherical bearing standards for dimensions while adopting rolling bearing standards for load ratings, making them suitable for alternating loads, large rotational movements at medium-high speeds, and full rotations.

Spherical plain bearings conform to DIN ISO 12240-1, which standardizes size series, dimensions, tolerances, and radial internal clearance. While the standard allows extensive variation in friction pairs, materials, and surface treatments, marking conventions, load ratings, and life calculations remain manufacturer-specific. Despite initial interchangeability challenges, suitable alternatives are typically available across manufacturers.

These bearings serve diverse industries including factory construction, baking machinery, conveyor systems, agricultural equipment, food processing, textile machinery, robotics, road construction, vehicle manufacturing, rail vehicles, medical technology, and feed processing. Custom solutions are available for specialized applications through collaboration with technical partners.

As ready-to-mount sliding bearing elements, spherical plain bearings feature spherical inner and outer rings that enable rotation, tilting, and pivoting without edge pressure. They accommodate structurally necessary misalignment and compensate for manufacturing-related deviations.

Selection criteria include load capacity, operational loads, and requirements for service life and operational safety. Load capacity is indicated by rated loads in dimension tables, though these values vary between manufacturers due to the absence of standardized definitions.

Standard spherical plain bearings operate effectively between -10°C to +80°C. Temperature ranges for sealed versions (RS) and rolling element designs are specified in technical documentation.

• Static load rating (C0): The radial static load causing no permanent deformation when stationary, assuming normal room temperature and adequately supported surrounding components.
• Dynamic load rating (C): Used for estimating service life under dynamic loads, though actual capacity depends on factors like load type, rotation/tilt angles, speed profiles, bearing clearance, friction, lubrication, and temperature.

• Basic static load rating (C0): Corresponds to the load causing 0.0001 times the rolling element diameter in permanent deformation.
• Basic dynamic load rating (C): Represents the load at which 90% of identical bearings reach 1 million revolutions before rolling surface fatigue failure.

Bearing size selection must consider specified loads, direction (radial, axial, or combined), and type. Compared to rod end housings, standalone spherical plain bearings exhibit higher static load capacity. Dynamic loading requires verification through service life calculations.

• Maintenance-required sliding bearings
• Maintenance-free sliding bearings
• Rolling element bearings

Standardized tolerances apply to outer ring dimensions (diameter, width) and inner ring dimensions, noting that split outer rings may temporarily lose circularity until installed in precision housing bores.

Defined as the inner ring's radial and axial freedom of movement, with axial clearance typically exceeding radial clearance. Specific clearance ranges vary between sliding and rolling element designs.

Maximum tilt angles must never be exceeded during installation or operation to prevent bearing damage or seal/washer failure. Designs must restrict movement within specified angles without applying excessive housing forces, particularly ensuring adequate seal clearance (especially for 2RS models).

As precision ready-to-mount components, spherical bearings require careful handling:

1. Maintain original packaging until installation to preserve lubrication.
2. Never transmit installation/removal forces through bearing races to housings.
3. Ensure sliding occurs only between inner/outer ring surfaces—not between bearings and shafts/housings—by using proper interference fits.

Recommended fits balance interference needs while avoiding excessive clearance changes from race expansion/contraction. Verify connection tightness, as standard fits may require adjustment for specific applications.

Installation must respect maximum force limits to ensure proper bearing seating.

Metal friction pair bearings require periodic lubrication for optimal service life. Initial grease suffices for very light loads. Lubrication effectiveness depends on load magnitude/type (constant, pulsating, alternating), rotation angle, and sliding speed. Testing shows small rotation angles and extreme sliding speeds impair lubricant film formation, as does unidirectional loading versus alternating loads.

For optimal distribution, lubricate bearings in unloaded conditions. Note that manufacturers typically supply bearings with anti-corrosion coatings only, requiring initial lubrication before commissioning or immediately after installation.

• Steel/bronze pairs: Use corrosion-resistant lithium-based high-pressure grease (-20°C to +110°C). Solid lubricants improve performance at upper temperature limits.
• Steel/steel pairs: Lithium-soap-based corrosion-resistant high-pressure grease with high-viscosity base oil and molybdenum disulfide additives.
• Steel/PTFE pairs: Naturally low-friction and maintenance-free.
• Rolling element bearings: Pre-lubricated with lithium soap grease.

For centralized lubrication systems, carefully control dosage—especially for sealed versions—to prevent seal displacement from overpressure.