A spherical plain bearing has no rolling elements at all. An inner ring with a convex sphere slides inside an outer ring with a matching concave sphere, so the two can rotate relative to each other and, crucially, tilt. That single feature is why these bearings sit at the end of hydraulic cylinders, in suspension arms, in steering linkages and in bridge dampers - positions where the motion is an oscillation, not a continuous rotation.
This guide explains how the GE families are built and named, how static and dynamic ratings differ, why capacity falls as tilt rises, how maintenance-free liners compare with steel-on-steel, and how to select the right bearing and the right shaft finish for the duty.
- Sliding, not rolling. A spherical plain bearing carries load across a spherical contact area, which is why it accepts tilt but has a much lower speed limit than a rolling bearing.
- C0 and C are not the same measure. The static rating governs shock and peak load, the dynamic rating governs oscillation life, and a bearing is often selected on C0 even though it never rests.
- Tilt derates capacity. As the tilt angle grows, the load spreads over a smaller contact area, so the usable rating falls well below the catalogue figure.
- The mating surface carries half the joint. Steel-on-steel designs need a hard, finely ground and lubricated mating surface or they will wear the shaft instead of themselves.
Sliding contact instead of rolling contact
A rolling bearing separates its surfaces with a rolling element and a lubricant film that is renewed every revolution. A spherical plain bearing does the opposite: the surfaces are in direct sliding contact, separated only by a lubricant film or a self-lubricating liner, and the film is never fully renewed. That is why the two families are chosen for completely different duties.
The payoff is geometry. Because the contact is spherical, the bearing can tilt through a large angle while continuing to carry load - often 5° or more per side, far beyond the fraction of a degree a self-aligning rolling bearing offers. The cost is friction, speed and life. Run a spherical plain bearing as a continuous rotating bearing and the sliding contact generates heat, the film breaks down and the bearing wears out quickly.
The GE families and how to read them
Radial spherical plain: GE…ES, GE…LO, GE…UK
Radial spherical plain bearings follow the GE designation. The ES suffix is the standard steel-on-steel design with a split outer ring and a lubrication groove; LO denotes a maintenance-free variant with a self-lubricating liner; UK marks a design with a wider inner ring and different seal arrangement. The shape of the number after GE is the bore code, so a GE 40 ES takes a 40 mm shaft.
Rod ends: SI…ES and the GEEW / GEG variants
Rod ends combine a spherical plain bearing with a threaded stem, so the bearing can be adjusted in length when it is screwed into a linkage. The SI prefix denotes the metric male rod end; the GEEW and GEG families cover different liner types and load directions. Rod ends dominate steering, control linkage and actuator positions where the adjustment is as valuable as the articulation.
Bore codes and the sizing convention
The bore is written directly in millimetres after the family letters. The outside diameter, the width and the permissible tilt angle all follow from the bore and the series, and the dimensions are set by ISO 12240-1 for radial spherical plain bearings, ISO 12240-2 for angular contact types and ISO 12240-3 for rod ends.
| Family | Type | Contact | Lubrication | Typical duty |
|---|---|---|---|---|
| GE…ES | Radial spherical plain | Steel on steel | Grease, regular | Heavy static and shock load |
| GE…LO | Radial, maintenance free | PTFE fabric liner | None after assembly | Oscillation with limited access |
| GE…UK | Radial, wide inner ring | Steel on steel | Grease, regular | Heavier load, sealed |
| SI…ES | Rod end, male | Steel on steel | Grease, regular | Linkages and actuators |
| GEEW / GEG | Rod end variants | Bronze or PTFE liner | Varies | Control and steering duty |
Static and dynamic ratings: C0 versus C
Two ratings are published for every spherical plain bearing and they answer different questions. The basic static rating C0 is the load at which the contact stress reaches the permissible limit for the material, and it governs peak, shock and holding loads even when the joint never moves. The basic dynamic rating C governs the life of the bearing under oscillating motion, where the failure mode is wear of the sliding surfaces rather than classical rolling fatigue.
The practical consequence is that a spherical plain bearing is frequently selected on C0, not on C. A bridge damper or a static support strut may never complete a full oscillation, but it will see a design shock load, and C0 is the number that must exceed it. Dynamic ratings are used for the positions where the joint cycles continuously, such as a hydraulic cylinder on a duty cycle.
Contact angle and why tilt derates capacity
The catalogue rating assumes the load is applied roughly along the axis of the spherical contact, with the tilt near zero. As the inner ring tilts, the load-bearing contact patch slides toward the edge of the outer ring and its area shrinks, so the stress rises for the same force. This is the single most misunderstood aspect of spherical plain bearings.
A joint that tilts through its full permissible angle under full catalogue load is overloaded at the edge of the contact patch, and the wear there accelerates. If the design requires a large tilt, either reduce the load or step up to a larger bore. Keeping the working tilt well inside the permissible angle is the simplest way to keep a joint inside its rating.
Maintenance-free options and friction
PTFE fabric liner
A woven PTFE fabric bonded into the outer ring sphere gives the lowest friction of the maintenance-free options, typically a coefficient around 0.05 to 0.10, and requires no grease. The liner is the reason the LO and many rod end variants can run dry in inaccessible positions, but it has a limited load capacity compared with steel on steel.
Bronze and steel-on-steel
Bronze-lined and plain steel-on-steel designs trade a higher friction coefficient - roughly 0.10 to 0.20 - for much higher load capacity. Steel on steel needs regular greasing and a hard mating shaft, and it is the standard choice in heavy construction, mining and agricultural machinery. The needle roller family covers the neighbouring high-radial-load positions where a sliding joint is not appropriate.
Hard chrome and MoS2 treatments
Where corrosion or a risk of fretting exists, the sliding surfaces can be hard chromed and the contact surface treated with molybdenum disulphide. This raises wear resistance and provides a dry film that survives the first few cycles before grease reaches the contact, which matters on a joint that is assembled once and then left. The corrosion protection guide covers the coating options in more detail.
| Option | Friction coeff. | Load capacity | Maintenance | Best for |
|---|---|---|---|---|
| Steel on steel, greased | 0.10 to 0.20 | Highest | Regular greasing | Heavy shock load, construction |
| PTFE fabric liner | 0.05 to 0.10 | Medium | None | Dry oscillation, limited access |
| Bronze liner | 0.10 to 0.15 | Medium to high | Occasional | Steady oscillation, moderate load |
| Hard chrome and MoS2 | 0.10 to 0.18 | High | Reduced | Corrosive or fretting duty |
Lined and steel-on-steel variants from SKF, INA, IKO and ANDGE share the same GE envelope, so a change of contact system does not force a new housing or a new pin.
Where oscillating motion is the whole point
These bearings are specified where the joint rocks back and forth rather than spinning. The list is short and consistent across industries:
- Hydraulic cylinders. The rod and base eyes tilt as the cylinder extends, and the loads are high and often shock loaded.
- Suspension and steering. Vehicle joints oscillate through a limited angle at moderate speed and need long life without frequent service.
- Control linkages and levers. Rod ends give both articulation and length adjustment in one component.
- Bridge dampers and structural joints. Slow oscillation with high static load, where C0 governs.
- Agricultural implements. Oscillating joints in dusty conditions where a sealed, greasable design survives longer than an open one.
In all of these the shaft or pin is part of the load path, not just a mounting feature. A spherical plain bearing is only as good as the surface it slides against, which is why the bearing type selection guide treats the mating surface as a selection input rather than a detail.
Required shaft surface finish and hardness
For a steel-on-steel spherical plain bearing, the mating shaft or pin should be through-hardened to roughly 55 HRC or above and ground to a surface roughness no coarser than about Ra 0.4 µm. A soft or rough shaft wears instead of the bearing, the clearance opens up, and the joint develops play long before the bearing itself is worn out. A maintenance-free lined bearing is more forgiving of the mating surface because the liner carries the wear, but it still relies on a clean, hard surface to avoid fretting.
Sealing and contamination. Plain bearings have no seals in the classical sense, so a felt or rubber wiper is used to keep dust out of the sliding gap. In dusty duty this wiper is what separates a long-lived joint from one that grinds itself to scrap, and the spherical plain range lists the sealed variants separately. Wherever the joint is inaccessible, a sealed maintenance-free design is usually cheaper over its life than a greasable one that cannot be reached.
How to select a spherical plain bearing
- Establish the load type. Separate static or shock load, which is checked against C0, from cyclic oscillating load, which is checked against C.
- Measure the required tilt. Work out the maximum tilt angle through the full travel, then compare it with the permissible angle and derate the capacity if the tilt is large.
- Choose the contact system. Steel on steel for maximum load, PTFE liner for maintenance-free dry duty, bronze for a middle ground, coated surfaces where corrosion is a risk.
- Fix the mating surface. Specify hardness, roughness and lubrication for the pin or shaft, and confirm the wiper or seal arrangement.
- Pick radial or rod end. Radial spherical plain bearings for a clevis or a housing bore, rod ends where the linkage must be adjustable in length.
- Confirm the required clearance and the fit. Use the bearing selector to match bore and series, and check the pin fit for the load direction.
The glossary defines the terms used above, and if the joint is expected to spin rather than oscillate, the angular contact ball family or a self-aligning ball bearing is the better starting point. Since the load passes through the pin, torque values and retaining arrangements follow the same logic as lock nut and adapter practice.
Common mistakes
- Running a spherical plain bearing as a continuous rotation. Sliding contact does not tolerate sustained rotation; heat builds and the surfaces wear rapidly.
- Ignoring tilt derating. Selecting on the catalogue rating while the joint tilts through its full travel overloads the edge of the contact patch.
- Using a soft mating shaft. The pin wears, not the bearing, and the joint develops play early.
- Choosing a maintenance-free liner for a heavy shock load. Lined bearings have lower capacity than steel on steel and are not built for impact.
- Forgetting the wiper in dusty duty. Without a felt or rubber seal, abrasive dust reaches the sliding gap and grinds the surfaces.
Frequently asked questions
What does GE mean in a spherical plain bearing?
GE is the designation prefix for a radial spherical plain bearing, followed by the bore in millimetres and a suffix that identifies the design. GE 40 ES is a 40 mm bore steel-on-steel radial bearing. The suffix matters: ES is the standard greased design, LO is a maintenance-free lined version and UK is a wider, sealed variant.
What is the difference between C and C0 for a spherical plain bearing?
C0 is the basic static load rating, which is the load at which the permissible contact stress is reached and which governs shock, holding and peak loads. C is the basic dynamic rating, which governs life under oscillating motion where the failure mode is wear. Many joints are selected on C0 because they never complete a full rotation.
Can a spherical plain bearing rotate continuously?
It is not designed for it. The contact is sliding rather than rolling, and the lubricant film is not renewed each revolution, so continuous rotation generates heat and rapid wear. Spherical plain bearings are intended for oscillating, tilting and slow rocking motion, which is why they appear on cylinder eyes, linkages and suspension joints.
Why does capacity fall as the tilt angle increases?
The load is carried by a contact patch on the spherical surfaces. As the inner ring tilts, that patch moves toward the edge of the outer ring and its area shrinks, so the stress rises for the same force. The published rating assumes a near-axial load, so a joint that works through a large tilt must be derated or upsized.
What shaft hardness and finish do I need?
For steel-on-steel designs the pin or shaft is normally through-hardened to about 55 HRC or above and ground to roughly Ra 0.4 micrometre or finer. A soft or rough mating surface wears instead of the bearing and the joint develops play. Maintenance-free lined bearings tolerate a wider surface finish but still need a clean, hard mating surface.
Steel on steel or a PTFE liner?
Steel on steel carries the highest load and suits shock and heavy construction duty, but it must be greased regularly. A PTFE fabric liner gives lower friction and needs no maintenance, which suits inaccessible joints, but its load capacity is lower. Choose on load first, then on whether the joint can be reached for relubrication.