Spherical Plain Bearings

What is a spherical plain bearing?

A spherical plain bearing is a ready-to-mount bearing made of an inner ring and an outer ring with matching curved sliding surfaces. The inner ring has a convex, ball-shaped outer surface, and the outer ring has a concave inner surface that fits around it. Instead of rolling elements such as balls or rollers, the two rings slide against each other, allowing the shaft or pin to tilt, swivel, and rotate slowly within the housing.

Key features:

• Sliding contact - no balls or rollers; the rings slide directly over each other or over a self-lubricating liner
• Self-aligning - compensates for angular misalignment, shaft deflection, and installation errors
• High load capacity - the large contact area lets them carry very heavy static and shock loads in a compact size
• Suited to oscillating movement - ideal for tilting and swivelling rather than continuous high-speed rotation
• Huge size range - from small bores of a few millimetres up to very large bearings of over a metre in diameter

In simple terms, a spherical plain bearing is a heavy-duty pivot that lets connected parts move at changing angles while carrying large forces.

Why are spherical plain bearings essential?

Spherical plain bearings are essential wherever heavy loads must be carried through a joint that tilts, swings, or flexes. Rigid bearings in these situations would suffer edge loading, uneven wear, and early failure. 

Their main benefits are:

• Misalignment compensation - they absorb angular movement and structural flexing without binding or overloading other components
• Very high load capacity - they cope well with heavy, alternating, and shock loads that would damage many rolling bearings
• Robustness - their simple design makes them resistant to vibration, impacts, and harsh working conditions.
• Compact design - they handle large forces in a small space, saving weight and simplifying construction
• Low maintenance options - maintenance-free versions can run for long periods without relubrication

These qualities make them vital in hydraulic and pneumatic cylinders, construction and earthmoving machinery, cranes and lifting equipment, agricultural machinery, truck and trailer suspensions, steering linkages, forestry equipment, mining machinery, bridges, wind turbines, and many types of industrial machines.

The main types of spherical plain bearings

Spherical plain bearings are usually grouped by the direction of load they carry and by their sliding materials.

By load direction:

• Radial spherical plain bearings - the most common type, designed mainly for radial loads with some axial  load in both directions
• Angular contact spherical plain bearings - have tilted contact surfaces so they can carry combined radial and axial loads, often used in pairs
• Thrust spherical plain bearings - designed mainly for axial loads, with some combined radial load capacity, often paired with a radial bearing

By sliding contact and maintenance requirement:

• Requiring maintenance (steel/steel) - both rings are hardened steel, giving high load capacity and good resistance to shock and alternating loads. They need regular lubrication, usually through lubrication holes and grooves in the rings
• Maintenance-free (steel/PTFE fabric or composite) - a self-lubricating liner in the outer ring removes the need for greasing; ideal where access is difficult, where cleanliness matters, or where maintenance must be minimised
• Steel/bronze - a middle option with good wear resistance and moderate lubrication needs

Other variations include:

• Sealed versions - with lip seals on both slides to keep out dirt, water, and abrasive particles; also to retain grease
• Stainless steel versions - for wet, food-processing, marine, or corrosive environments
• Split outer ring designs - which allow the inner ring to be fitted into the outer ring during manufacture and simplify assembly in some larger sizes

How to choose the right spherical plain bearing?

Choosing the right spherical plain bearing means matching it to the loads, movement, and operating conditions. Work through these points in order:

• Dimensions - match the bore to the shaft or pin, and the outside diameter and width to the housing
• Load direction - choose radial, angular contact, or thrust types depending on whether loads are mainly radial, combined, or mainly axial
• Load size and type - check static and dynamic load ratings in the catalogue, and consider whether loads are steady, alternating, or include shocks
• Movement - consider the tilt angle needed, the oscillating angle and frequency, and the sliding speed
• Maintenance - choose maintenance-free types where greasing is difficult or undesirable, and steel/steel types for heavy shock loads where regular lubrication is possible
• Environment - select sealed or stainless steel versions for dirt, water, or corrosive conditions, and check temperature limits, especially for PTFE-based liners
• Fits and mounting - follow the recommended shaft and housing tolerances, as incorrect fits can cause the rings to creep or seize
• Standards and interchangeability - stay with the same ISO dimension series and a reputable manufacturer so that replacements fit correctly

FAQ - All you need to know about spherical plain bearings

What is the difference between a spherical plain bearing and a rod end?

A spherical plain bearing is the bearing alone and must be fitted into a housing; a rod end combines a spherical plain bearing with a housing and shank, ready to attach to a rod.

Can they be used for continuous rotation?

They are best suited to oscillating and tilting movement at low speeds; for continuous high-speed rotation, a rolling bearing is usually the better choice. 

Do they need lubrication? 

Steel/steel types need regular lubrication; maintenance-free types with PTFE or composite liners do not. 

When do I know when to replace one?

Signs include excessive play, knocking or squeaking noises, stiffness, seizing, visible wear on the sliding surfaces, or damage to seals.

How do I find the right reference?

Check the marking on the bearing, the machine documentation, or the manufacturer’s catalogue; if the marking is worn, measure the bore, outside diameter, and width, and ask a bearing supplier to identify it.