Spherical plain bearings and rod ends explained

What the joint does

A spherical plain bearing has an inner ring with a convex spherical outer surface that slides inside a matching concave outer ring. The two surfaces make a ball joint: the inner ring can rotate about its bore axis and tilt in any direction within a set angle. Unlike a rolling bearing, there are no balls or rollers. Load passes across the full sliding surface, which gives high load capacity per unit size, good shock tolerance and the ability to take misalignment between connected parts.

These bearings are designed for oscillating or slow tilting motion, such as the pivot of a hydraulic cylinder eye, a linkage, a suspension arm or a steering rod. They are not intended for continuous high-speed rotation, where a rolling bearing is the right tool. For general information on sliding bearings see what is a plain bearing.

Steel-on-steel or maintenance-free

The sliding pair decides how the bearing is maintained, how long it lasts and how much it costs.

Steel-on-steel bearings are usual for construction machinery, agricultural implements and cylinder eyes, where regular greasing is part of the maintenance routine. Maintenance-free types fit applications that are hard to reach, hygienic or that cannot be relubricated, as in some automation, packaging and vehicle linkages. The lubricant matters: choose it for load, temperature and contamination, as set out in the lubrication guide.

Reading the GE designations and nominal sizes

The GE series is the standard catalogue family for spherical plain radial bearings. The number after GE is the bore in millimetres. Suffix letters describe the construction: E for the standard design, S for a lubrication groove and hole in the outer ring, and further suffixes for seals or liners, which vary by plant and should be spelled out in the RFQ.

These are nominal ISO reference values; confirm against the drawing and datasheet. Other series exist with wider or narrower rings and different tilt angles, so the designation alone does not fix every dimension. For other sizes use the size chart.

Rod ends: bearing plus shank

A rod end places a spherical plain bearing in a housing with a threaded shank. It lets a push-pull rod connect to a pivot with misalignment tolerance, and is common on linkages, control rods and hydraulic or pneumatic actuators.

• SI rod ends have an internal (female) thread in the shank.
• SA rod ends have an external (male) thread on the shank.
• Thread hand. Right-hand and left-hand threads exist; left-hand is used in turnbuckle pairs so that the length can be adjusted by turning the rod.
• Thread type. Metric is standard; inch (UNF) is common on imported equipment.

State the thread size, pitch, hand and the thread length in the RFQ. The threaded shank is usually the weaker point in the assembly, so do not assume that the shank carries the same load as the spherical bearing inside. Check shank strength against the datasheet for your thread size.

Material, finish and environment

The housing of a rod end can be steel, stainless steel or plated steel. Zinc plating gives basic protection for general-purpose parts; stainless steel is chosen for wet sites and washdown. Bearing steel inner rings are normally hardened and ground, often with a phosphate or chrome finish to retain lubricant. Sealed versions keep dirt out and grease in, and are the norm for outdoor use on agricultural and construction machinery; see agriculture and conveyor solutions. For wear-sensitive applications, ask whether the sliding surface has any special treatment.

Tilt angle, load direction and life

Two numbers on the datasheet drive the choice more than any other: the permissible tilt angle and the load rating. The tilt angle depends on the series and the ratio of ring width to diameter. Wider rings carry more load but tilt less. Running a bearing at its maximum tilt on every cycle shortens life, so allow some margin in the geometry of the linkage. The load rating differs between static and oscillating duty, and for maintenance-free types it also depends on the direction of the load relative to the liner. Always give the supplier the load direction, the angle swept and the number of cycles per hour, not just a load figure.

Life of a plain bearing is wear-based, not fatigue-based. It depends on contact pressure, sliding speed, lubrication and contamination, and it ends when the clearance becomes too large for the joint to stay accurate. That makes the bearing easy to inspect: play in the joint is visible long before failure. A maintenance plan that includes checking play and regreasing at defined intervals is usually cheaper than oversizing the joint.

Fits also matter. The inner ring is normally clamped on a pin with a close fit, and the outer ring is pressed or staked in the housing eye; a loose outer ring will creep and damage the bore. Specify the housing bore tolerance and retention method, such as a snap ring, staking or a flange, together with the bearing. If the bearing is delivered into an assembly built by a third party, send the drawing of the eye and pin so that the plant can check the fits.

Ordering checklist

1. Designation with bore, series and suffixes for lubrication groove or seals.
2. Construction: steel-on-steel or maintenance-free liner.
3. For rod ends: SI or SA, thread size, hand and housing material.
4. Application: load direction, oscillating angle, frequency, shock and environment.
5. Quantity per designation, marking and packing.

Standard GE bearings and rod ends typically start at about 100 to 500 pieces per designation; custom materials and special sizes follow drawing review and are confirmed in writing per program (see MOQ and lead time). Send the list through the quote form.