Self Aligning Ball Bearings

What is a self-aligning ball bearing?

A self-aligning ball bearing is a rolling bearing with two rows of balls running between an inner ring and an outer ring, designed so that the shaft can tilt slightly relative to the housing without damaging the bearing. The inner ring has two deep raceway grooves, one for each row of balls, while the outer ring has a single, continuous raceway shaped like part of a sphere. Because the outer raceway is spherical, the inner ring, balls, and cage can swivel as one unit inside the outer ring. This allows the bearing to automatically adjust, or “self-align”, to any angular misalignment between the shaft and the housing. 

A self-aligning ball bearing is made up of four main parts:

• Inner ring - fits on the shaft and has two grooved raceways for the two rows of balls
• Outer ring - fits in the housing and has one common spherical raceway shared by both rows of balls
• Two rows of balls - carry the load and roll between the inner and outer raceways
• Cage - keeps the balls evenly spaced and guides them as they roll

Key features:

• Self-aligning - compensates for angular misalignment, typically up to about 2 to 3 degrees under normal conditions, depending on the bearing series
• Low friction - among the lowest friction of all rolling bearings, so they run cool and efficiently
• High speed capability - low friction and heat generation make them well suited to higher speeds
• Radial and light axial loads - designed mainly for radial loads, with limited axial load capacity in both directions
• Cylindrical or tapered bore - available with a straight bore or a tapered bore for mounting with adapter sleeves
• Open or sealed versions - open bearings for oil or grease lubrication, and sealed bearings that are greased for life

In simple terms, a self-aligning ball bearing is a forgiving bearing that keeps running smoothly even when the shaft is bent, flexing, or not perfectly lined up with its housing.

Why are self-aligning ball bearings essential?

Self-aligning ball bearings are essential wherever perfect alignment between the shaft and housing is difficult to achieve or maintain. In real machines, shafts can deflect under load, housings can be mounted on separate supports, frames can flex, and machining or installation errors can occur. With a standard deep groove ball bearing, even small misalignments cause extra stress, heat, noise, and early failure. A self-aligning ball bearing avoids these problems by adjusting itself to the shaft’s angle.

Their main benefits are:

• Misalignment compensation - they tolerate shaft deflection, mounting errors, and frame distortion without extra stress on the bearing
• Low friction and cool running - they generate less heat than most other rolling bearings, which helps extend lubricant life and reduce energy use
• Quiet and smooth operation - low friction and good alignment give low noise and vibration levels
• High speed capability - they are well suited to fast-running shafts such as those in fans and machinery drives
• Easy mounting - tapered bore versions with adapter sleeves can be fitted to plain shafts without the need for shoulders or precise shaft machining
• Reduced maintenance - sealed versions are lubricated for life, cutting down on maintenance needs
• Protects other components - by absorbing misalignment, they reduce stress on shafts, seals, housings and couplings
• Cost-effective - they offer a reliable, economical solution for many general machinery applications

These qualities make self-aligning ball bearings important in a wide range of applications, including:

• Agricultural machinery - combine harvesters, balers, mowers and other machines with long shafts and flexible frames
• Fans and blowers - industrial ventilation fans and air handling units running at relatively high speeds
• Conveyors - belt conveyors, idlers, and transmission shafts in material handling systems
• Textile machinery - spinning, weaving, and processing machines that require smooth, fast rotation
• Woodworking machinery - saws, planers, and other machines with long shafts
• Paper and printing machinery - rollers and drive shafts
• Pumps and compressors - where shaft alignment can vary
• Simple transmission shafts - often mounted in Plummer block housings on separate supports
• Household and light equipment - small motors, exercise equipment, and appliances

The main types of self-aligning ball bearings

Self-aligning ball bearings are usually grouped by their width series, bore type, sealing arrangement, and special designs. 

By width and load series:

• 12xx series - narrow, light series and one of the most common, suitable for general applications
• 13xx series - narrow, medium series with larger balls and higher load capacity for the same bore
• 22xx series - wide, light series with higher load capacity than the 12xx series, and the usual choice for sealed versions
• 23xx series - wide, medium series with the highest load capacity of the standard ranges
• Small bearings - special series such as 108, 126, 127, 129, and 135 for compact applications and small machines

By bore type:

• Cylindrical bore - the standard design, mounted directly on the shaft with a suitable fit
• Tapered bore - has a slight taper, usually 1:12, identified by the suffix K. These bearings are mounted on adapter sleeves or withdrawal sleeves, allowing them to be fitted on plain shafts and easily adjusted or removed

By sealing and lubrication:

• Open bearings - lubricated with grease or oil, suitable for higher speeds and temperatures, and allowing relubrication
• Sealed bearings - fitted with contact seals on both sides, usually marked 2RS, and filled with grease for life. They keep out dirt and moisture and require no lubrication

Special designs:

• Self-aligning ball bearings with an extended inner ring - the inner ring is wider than the outer ring and fixed to the shaft with a locking device, making them ideal for simple, low-cost mounting on transmission shafts and agricultural machinery
• Bearings with adapter sleeves - supplied as a complete unit with a tapered bore bearing, adapter sleeve, lock nut and lock washer for quick mounting on plain shafts
• Stainless steel versions - for wet, food-processing, marine or corrosive environments

By cage design:

• Polyamide cages - lightweight, quiet and low friction, often marked TN9 or similar, but with temperature limits
• Pressed steel cages - strong and suitable for most general applications
• Machined brass cages - robust and suited to higher temperatures, heavier loads and vibration

How to choose the right self-aligning ball bearing?

Choosing the right self-aligning ball bearing means matching it to the shaft arrangement, loads, speed, mounting method, and working environment. Work through these points in order:

• Dimensions - match the bore to the shaft diameter and check the outside diameter and width against the housing and available space
• Misalignment - confirm the expected amount of shaft tilt or deflection is within the bearing’s permissible misalignment, as listed in the manufacturer’s catalogue
• Load size - check the static and dynamic load ratings. If loads are high, choose a heavier series such as 13xx or 23xx. For very heavy loads, consider a spherical roller bearing instead, which also self-aligns but carries much higher loads
• Load direction - remember these bearings are designed mainly for radial loads. They can only carry limited axial loads, so heavy thrust must be taken by another bearing
• Speed - self-aligning ball bearings are well suited to medium and high speeds, but check the speed ratings for the chosen size, cage, and seal type. Sealed bearings usually have lower speed limits than open ones
• Mounting method - choose a cylindrical bore for direct mounting on a machined shaft, or a tapered bore with an adapter sleeve for plain shafts and easier installation and removal
• Housing - consider whether the bearing will be fitted in a Plummer block, flanged housing, or machine casing, and make sure the housing allows the bearing to align freely
• Sealing and lubrication - choose sealed bearings for clean, maintenance-free operation, or open bearings where relubrication, higher speeds, or higher temperatures are involved
• Cage material - choose polyamide for quiet, low-friction running within its temperature limits, pressed steel for general use, or brass for higher temperatures and tough conditions
• Internal clearance - check whether standard clearance is suitable or whether a greater clearance, such as C3, is needed due to tight fits or high operating temperatures
• Standards and brand - stay with the same series and a reputable manufacturer to ensure correct dimensions, quality, and service life

FAQ - All you need to know about self-aligning ball bearings

What is the difference between a self-aligning ball bearing and a deep groove ball bearing? 

A deep groove ball bearing has one row of balls and cannot tolerate much misalignment, while a self-aligning ball bearing has two rows of balls and a spherical outer raceway that lets it adjust to angular misalignment.

Can they run at high speeds?

Yes, their low friction makes them well suited to medium and high speeds, although sealed versions and certain cage materials have lower speed limits.

How do I know when to replace one?

Signs include noise, rough rotation, excessive play, vibration, overheating, or visible wear, pitting, discolouration, or corrosion on the rings and balls.

Do they need lubrication?

Open bearings need grease or oil and can be relubricated, while sealed bearings are greased for life and need no further lubrication.

How much misalignment can they handle?

Typically up to about 2 to 3 degrees under normal conditions, depending on the bearing series and design. Sealed versions usually allow less misalignment.

Can they carry axial loads?

Only limited axial loads in both directions. Heavy thrust loads should be carried by a separate bearing designed for that purpose.