Spherical Plain Bearings

What a Spherical Plain Bearing Is

A spherical plain bearing is a ready-to-install bearing element made of just two rings. The inner ring has a convex, ball-shaped sliding surface; the outer ring has a matching concave spherical bore. The two slide directly on each other across that spherical interface, with no balls, rollers, or cage in between. Because the contact surface is a section of a sphere, the inner ring can both rotate about the shaft axis and tilt the shaft axis itself within the outer ring, all while transmitting load. That dual freedom, load support plus angular accommodation, is the entire reason the type exists.

The working principle is sliding friction across a large conforming area rather than rolling contact at a point or line. Spreading the load over the full projected sphere area lets the bearing carry static and shock loads that would brinell or fracture a rolling bearing of the same size. The price for that capacity is friction: a spherical plain bearing dissipates more torque than a rolling bearing and is suited to slow, oscillating, or intermittent motion rather than continuous high-speed rotation. In a typical service the inner ring swings back and forth through a limited arc many times per minute, as in a cylinder eye or a linkage pivot, instead of spinning at thousands of revolutions per minute.

It is worth separating the spherical plain bearing from the self-aligning ball bearing, which the two are often confused. Both tolerate misalignment, but the self-aligning ball bearing does so by rolling two ball rows on a common spherical outer raceway, keeping low friction and high speed at the cost of much lower load per unit width and only a few degrees of tilt. The spherical plain bearing slides metal or liner directly on metal, trading speed and friction for a step change in load capacity and tilt angle. In short: rolling for speed, sliding for load and articulation.

The family has a long industrial history. The geometry derives from the simple ball-and-socket joint, and standardisation of the modern bearing matured through the German DIN series and was consolidated internationally in 1998 as ISO 12240, which cancelled and replaced the earlier ISO 6124 and ISO 6125 standards. The work sits under ISO Technical Committee TC 4, Rolling bearings, Subcommittee SC 7, Spherical plain bearings. Today the type is produced by every major bearing house and by aerospace specialists under SAE and military liner specifications, in bores from a few millimetres to over a metre.

Four engineering properties define a spherical plain bearing in selection: load capacity (static, dynamic, and shock), the sliding-contact pairing (which sets friction, maintenance, and wear life), the angle of tilt, and the operating temperature window. These four interact. A maintenance-free PTFE liner buys freedom from greasing but limits temperature, speed, and total wear life; a steel/steel pairing carries the highest static load and runs hot or cold but demands a grease film and periodic relubrication. The rest of this guide unpacks each property so the trade-off can be made deliberately rather than by default.

Bearing Types and ISO 12240

ISO 12240 organises spherical plain bearings into four families, each a separate part of the standard. The split is by the direction of load the spherical interface is shaped to carry, plus the rod-end housing variant. Choosing the wrong family is a structural error, not a tuning detail: a radial bearing asked to carry significant axial thrust, or a thrust bearing asked to take radial load, will edge-load and wear prematurely. The table below maps the four families to their standard part and primary duty.

Radial spherical plain bearings (ISO 12240-1, GE series) are the workhorse. The spherical interface is oriented to carry load mainly perpendicular to the shaft axis. They appear at both ends of hydraulic cylinders, in excavator and crane booms, in vehicle suspension and steering links, and anywhere a pinned joint must swing freely while taking heavy radial load. They are supplied with or without an outer-ring fitting slot (the slot eases assembly of the one-piece outer ring over the integral inner sphere) and with or without integral seals.

Angular contact radial spherical plain bearings (ISO 12240-2, GAC series) place the contact zone at an angle so the bearing carries combined radial and single-direction axial load. They suit joints that must locate a shaft axially as well as radially, and are frequently mounted in opposed pairs to take thrust in both directions.

Thrust (axial) spherical plain bearings (ISO 12240-3, GX series) orient the sphere to carry load mainly along the shaft axis while still permitting angular movement. They are used in axial pivots, screw-jack heads, and tooling where the load line is essentially axial but the seating may not be perfectly square.

Rod ends (ISO 12240-4) are not a different bearing so much as a packaging of one. A radial spherical plain bearing is pressed or swaged into a forged eye carrying an integral threaded shank, giving a self-aligning pivot that screws straight onto a cylinder rod, tie bar, or actuator. They come with male or female thread, left or right hand, and in the same range of liner pairings as the bare bearing. The eye, shank, and thread, not just the bearing, set the rated load, so a rod end is specified as a system.

Sliding-Contact Material Pairings

The single most consequential choice in specifying a spherical plain bearing is the sliding-contact pairing: the two materials that rub at the spherical interface. The pairing sets the coefficient of friction, whether the bearing needs lubrication, the achievable wear life, and a large part of the cost. There are two broad camps. Lubricated metal pairings (steel/steel and steel/bronze) carry the highest static load and run across the widest temperature band but need a grease film and periodic relubrication. Maintenance-free PTFE pairings run dry, eliminate greasing, and cut friction sharply, at the cost of finite liner wear life and tighter temperature limits. The table below compares the mainstream pairings on the metrics that decide between them.

Steel/steel pairs a hardened, ground inner ring against a hardened outer ring, usually with a phosphated or molybdenum-disulfide running-in coating and grease grooves machined into the sliding surface. It carries the highest static and shock load of any pairing because metal-to-metal contact resists crushing, and it tolerates frequent, large-angle oscillation. The penalty is friction near 0.1 to 0.2 and an absolute dependence on lubrication: it must be greased on assembly and relubricated on a schedule through the bore and groove, or it will gall. SKF designates this pairing with the ES suffix, as in GE 50 ES.

Steel/sintered bronze runs a steel inner ring against a sintered bronze outer surface that can hold an oil or grease reserve in its porosity. It retains very high load capacity and slightly improves dry-running tolerance over steel/steel, and is common in heavy mobile machinery. It is still classed as requiring lubrication, not maintenance-free.

Steel/PTFE fabric is the premier maintenance-free liner: a woven PTFE-and-reinforcement fabric is bonded to the outer ring and runs against a hardened inner ring. It works by transfer film. During run-in, PTFE shears off the fabric and burnishes onto the steel, building a self-renewing low-friction layer. Because that transfer is essential, the bearing must run dry; adding grease disturbs the film and shortens life, so these bearings have no grease nipple. Steel/PTFE fabric carries the highest dynamic load of the maintenance-free options and is the standard for heavy linkage and aerospace control duties.

Steel/PTFE composite uses a PTFE film backed by a metal mesh or bronze expanded-metal layer, typically filled with molybdenum disulfide. It carries a permissible specific static load on the order of 250 N/mm² and reaches higher sliding velocities than the fabric type, suiting medium-load duties. It is the one maintenance-free pairing that benefits from lubrication: an initial fill plus occasional relubrication can extend service life by a factor of at least two, which is why the asterisk above marks it as maintenance-free but relube-tolerant. Hard chromium/PTFE fabric swaps the steel inner ring for a hard-chromium-plated one to add corrosion resistance and a harder mating surface for the transfer film, designated FW in the SKF system.

Dimension Series, Tolerances, and Sealing

ISO 12240 standardises not only the four types but the dimensions, tolerances, and internal clearances within each, so that a GE bearing of a given size from one maker is interchangeable with another. For radial bearings, ISO 12240-1 defines dimension series identified by letter: E, G, C, K, and H. The letter encodes the cross-sectional proportions, principally how wide and how heavy-section the rings are for a given bore. Heavier series carry more load and tolerate more abuse in a larger envelope; lighter series save space and weight. The symbols used on the drawings are consistent across the standard, summarised below.

The angle of tilt α is defined by the standard as the angle through which the inner-ring and outer-ring axes may be inclined relative to each other without reducing the projected theoretical contact area when the two axes are parallel. The standard lists an approximate tilt angle for each dimension series; across the radial series the values typically fall in the range of about 3 to 16 degrees depending on proportions. A critical caveat appears in the standard itself: once the bearing is mounted on a shaft and into a housing, adjacent components may restrict the achievable tilt below the rated value, so the installation envelope, not just the bearing, must be checked.

Tolerances and radial internal clearance are specified per series in tables 1 to 6 of ISO 12240-1, using the standard rolling-bearing deviation symbols (deviation of mean bore diameter, deviation of mean outside diameter, ring-width deviation, and so on). Radial internal clearance, the small designed play between the sphered surfaces, governs both the smoothness of articulation and the impact behaviour under reversing load. Too little clearance risks binding and heat; too much allows knock and accelerated wear under shock. For pinned joints that reverse load, clearance class is a real selection parameter, not a default.

Sealing is the other build option that shapes performance. Bare bearings expose the sliding interface to the environment, which is acceptable in clean, lubricated, or sacrificial duties. For dirty or wet service, integral lip seals keep contaminant out and grease in. In the SKF system the 2RS suffix adds double-lip contact seals on both faces and 2LS adds heavy-duty triple-lip seals. Seals matter beyond ingress: because the elastomer lip sets the temperature limit, a sealed bearing is typically rated to a narrower window (about -30 to +130 degrees Celsius) than the same bearing unsealed (about -50 to +150 degrees Celsius). Choose the seal for the dirtiest realistic condition, then confirm the resulting temperature rating.

Reading a designation ties the chapter together. In the common GE system: the prefix gives the type (GE radial, GAC angular contact, GX thrust), the number gives the bore in millimetres, a series or heavy-duty marker may follow (GEH for the wide heavy series, GEZ for inch bores), the liner suffix gives the pairing (ES steel/steel, FW hard-chromium/PTFE fabric, SW steel/PTFE, TXA PTFE composite maintenance-free), and a trailing 2RS or 2LS gives the seal. So GE 50 ES-2RS reads as a 50 mm bore radial bearing, steel/steel relubricatable, with double-lip seals both sides.

Key Specification Parameters

Eight parameters decide a spherical plain bearing selection. Unlike a rolling bearing, where rotational L10 fatigue life dominates, the plain bearing is sized by static and shock load, by the sliding-contact pairing, and by an oscillation-based wear-life calculation. The eight are: basic static and dynamic load ratings, permissible specific bearing load, coefficient of friction, sliding velocity, angle of tilt, operating temperature, and seal type. Each is explained below.

Basic static load rating C0 and basic dynamic load rating C are given in kilonewtons for each catalogue size. C0 is the load the bearing can take at rest or under shock without permanent deformation of the sliding surfaces; for the plain bearing this is usually the governing number, because the duty is slow and the danger is crushing, not fatigue. C feeds the wear-life calculation for oscillating motion. Both are size-specific and read straight from the maker table.

Permissible specific bearing load p is the load per unit projected sliding area, in newtons per square millimetre (equivalently megapascals). It normalises capacity across sizes and is the headline figure for the liner: steel/steel and steel/bronze reach several hundred N/mm²; steel/PTFE composite is rated around 250 N/mm²; steel/PTFE fabric carries higher dynamic load than the composite. Designers check that the actual contact pressure, applied load divided by projected area, sits below the rated p for the chosen pairing, with margin for shock.

Coefficient of friction determines both the actuation torque the joint imposes and the heat generated. Lubricated steel/steel and steel/bronze run at roughly 0.1 to 0.2; maintenance-free PTFE pairings run far lower, roughly 0.03 to 0.10, with the dry-running transfer film at its lowest at low sliding speed and rising as speed increases. Friction is not a single constant: it varies with contact pressure (PTFE friction falls as load rises), surface finish, and velocity, so catalogue figures are typical, not guaranteed.

Sliding velocity is the relative surface speed at the spherical interface. Plain bearings are slow-motion devices: steel/steel runs at low sliding velocity, while steel/PTFE composite tolerates higher velocity (on the order of a couple of metres per second under favourable load). Exceeding the rated velocity overheats the interface and, for PTFE liners, degrades the transfer film. Angle of tilt, covered in Chapter 4, is the rated angular accommodation, typically about 3 to 16 degrees, subject to installation restriction.

Operating temperature sets the usable window. The bare metal-housed forms are commonly rated about -50 to +150 degrees Celsius across steel/steel, steel/bronze, steel/PTFE composite, and steel/PTFE fabric; the integral seal narrows this to about -30 to +130 degrees Celsius. PTFE itself survives well beyond these figures, so the practical ceiling is usually the seal, adhesive, or steel hardness, not the liner. Seal type (none, 2RS double-lip, 2LS triple-lip) trades ingress protection against the temperature window and adds a small friction increment.

Selection Decision Factors

To turn the preceding five chapters into a specific part number, work through the sequence below. Most selection errors are not a single wrong table value but a decision taken at the wrong level, for example choosing a maintenance-free liner before confirming the static load it must survive. These eight steps double as a fixed RFQ template.

1. Load direction and type: Decide radial (GE), angular contact (GAC), thrust (GX), or rod end first. Resolve the joint reaction into radial and axial components; pick the type whose contact geometry matches the dominant component, and use opposed pairs or angular contact where axial location is required.
2. Static and shock load: Size on the basic static load rating C0 and the peak shock load, not the nominal running load. For pinned joints that reverse and impact, this is the governing calculation. Confirm the actual contact pressure stays below the permissible specific load p for the intended liner.
3. Sliding-contact pairing: Choose steel/steel or steel/bronze for the heaviest, most frequently cycled, lubricated joints; steel/PTFE fabric for heavy maintenance-free dynamic load; steel/PTFE composite for medium load where occasional relubrication is acceptable; hard-chromium/PTFE fabric for corrosion-prone dry duty.
4. Maintenance regime: Decide explicitly whether the joint can be relubricated in service. If yes, steel/steel offers the highest capacity. If no (inaccessible, sealed, or contamination-sensitive), commit to a maintenance-free PTFE liner and accept a finite, load-dependent wear life. Never grease a steel/PTFE fabric bearing.
5. Angle of tilt and clearance: Confirm the required swing plus mounting tolerance fits within the series tilt angle (about 3 to 16 degrees), and check that the shaft shoulder and housing bore do not restrict it below the rated value. Select a clearance class suited to the load reversal and impact severity.
6. Sealing and environment: Choose no seal for clean lubricated duty, 2RS double-lip for general dirt and moisture, 2LS triple-lip for aggressive contamination. Remember the seal narrows the temperature window, so reconcile seal choice with the temperature step next.
7. Temperature and dimension series: Verify the rated window (about -50 to +150 degrees Celsius unsealed, -30 to +130 sealed) covers the worst case. Pick the dimension series (E, G, C, K, H for radial) for the available envelope and required section strength, reading exact d, D, B, C, dk from the maker table.
8. Total cost of ownership: Weigh purchase price against relubrication labour, downtime for replacement, and the wear life of a maintenance-free liner under the real duty cycle. A cheap unsealed steel/steel joint in a dirty, inaccessible location can cost more over its life than a sealed maintenance-free unit installed once.

One dimension that is easy to overlook is serviceability and standardisation. Specifying to ISO 12240 keeps the part interchangeable across SKF, Schaeffler INA, ASKUBAL, FLURO, AST Bearings, and others, which protects spare-part supply over a machine life of years. Confirm that the chosen series, bore, and liner are catalogue stock rather than special, that seal materials match the chemical environment, and, for rod ends, that the thread, shank, and eye are matched to the axial load and protected against vibration loosening with a jam nut or thread-locking scheme.