Radial ball bearings are friction reduction devices that carry loads radially around its axis. A subtype of ball bearings, they operate through the use of lubricated steel balls placed between two grooved rings. They are frequently called deep-groove bearings or Conrad bearings. While these types of bearings can accommodate some axial thrust, thrust bearings or roller bearings should be used for designs with a high axial load.

Operation

Rolling resistance (friction) is considerably less than sliding resistance. Radial ball bearings consist of a specific number of balls contained within a cavity called a raceway that is formed by two radiused ball races. Although the vast majority of ball bearings have balls made of carbon steel, other materials, such as stainless steel,  ceramic or glass are also available. Grease or oil reduces the friction and has a dampening effect on the contact between the balls and the smooth walls of the raceways, and can be contained within the bearing by the use of rubber seals. The bearings may use a metal-faced shield instead of a seal, though the shield does not provide positive sealing of the bearing. The seals or shields also function to keep dirt and dust from affecting bearing rotation. When under load, the bearings settle into the deepest points on the raceways and the load is transferred at the contact points between the raceways and balls. The load can now spin without exerting torque on the shaft.

Radial ball bearings are manufactured in a variety of styles, including single row bearings, double row bearings, internally self-aligning bearings, externally self-aligning bearings, thin-section bearings and insert (wide inner ring) bearings.  Many options within each bearing type are also available, including open bearings, bearings with seals, bearings with shields, and bearings with snap ring grooves.

Production

Radial ball bearings are manufactured as individual components (inner/outer rings, balls, ball separators) . They must be accurately measured and ground to ensure a smooth and quiet rotation. The components are then assembled to produce a bearing that meets the required operating specifications. Ball bearings may have a very slight deviation in dimensions from bearing to bearing (thousandths of a millimeter).

Video credit: The Big Bearing Store / CC BY-SA 4.0

Quality radial ball bearings are subject to standards, which indicate their precision and efficiency. Very high-speed applications will see the greatest benefit from a more precise bearing. A manufacturer does not have to follow these industrial guidelines. North American radial ball bearings are under edict of the ABEC scale, while other ball bearings adhere to ISO 492 or its regional equivalent (DIN, KS, etc.) There are five accepted levels of the ABEC/ISO 492 scale and the level is not related to the size of the bearing. Boca Bearings Inc. has a tolerance chart on the dimensional variances acceptable for each ABEC/ISO standard, organized by bearing dimension.

Selection tip: Some manufacturers may quote an ABEC rating not listed above. This is an inaccuracy; there are only five ABEC ratings.

Configurations

Radial ball bearings are manufactured in many configurations to suit a variety of applications. By revising the cage materials and assembly, the bearing becomes better suited for certain applications.

Ball Distribution

Bearing Retainers/Cages

There are several common types of steel ball retainers that will be acceptable for most applications.

	A two-piece ribbon design of two halves that are connected by folding tabs. The retainer is guided by the balls. This type is acceptable for low to moderate speeds, which is adjustable depending on the type of crimp connecting the halves. Typically made of stainless steel, carbon steel, or brass.
	A one-piece stainless steel "crown" is used in small bearings. It has excellent low-speed, low-torque characteristics. It snaps in over the balls and is guided by the inner raceway.
	A two-piece ribbon design meant for larger bearings carrying high loads. The halves are riveted together and exhibit good inertia and vibration traits. Also guided by the balls, the material is usually stainless steel or carbon steel.
	A molded plastic cage, this cage type is guided by the balls and is suitable for varying running torques and high speeds. It has an operation temperature of -30° C to +120° C. A glass-fiber reinforced retainer can operate at even higher speeds and temperatures.
	A one-piece cage that is typically machined from brass with holes to contain the balls. These cages are more precise, reliable, and durable than stamped of molded cages.

Other specialty bearing cages and retainers are manufactured, often of supplement styles to receive additional benefits.

Bearing Closures - Seals and Shields

For most radial ball bearings, seals and shields protecting the balls, cages, and lubricants from dust and water will not drastically affect the efficiency of the bearing. Typically, seal and shield styles are designated in the bearing part-number suffix, though this may vary between manufacturers.

• R- indicates a single seal, located on one side of the bearing
• RR- indicates a single seal, located on both sides of the bearing
• Z - indicates a single shield, located on one side of the bearing
• ZZ- indicates a double shield, located on both sides of the bearing

Bearings without shields or seals, called open bearings, should be employed if externally sealed and the operating conditions permit. These bearings are less expensive and easy to maintain.

Specifications

Bearing Torque

Bearing torque can be attributed to the many variables, such as:

• Ball size
• Number of balls
• Bearing retainer type
• Bearing radial play
• Bearing lubricant type and fill percentage
• Applied bearing load.

Furthermore, bearing torque is classified by three ratings, tested with a light load and oil lubrication:

1. Starting torque is the measurement of the torque required to initiate rotation of one raceway of the bearing. This is substantially higher than running torque.
2. Average running torque is the mean level of torque the bearing is subjected to at a consistent RPM.
3. Peak running torque is the maximum amount of torque experienced by a bearing, but can be difficult to determine. This provides a measure of consistency for a batch of bearings.  

Proper lubrication will significantly decrease torque values. Bearing materials influence torque. Lightweight metals and plastic cages/retainers provide little torque at medium speeds with low inertia.

Bearing Lubrication

Effective lubrication for radial ball bearings includes oil, grease, and dry films.

Synthetic oils are most common for quality bearings. Mineral oils are appropriate for high speed use. Synthetic oils are good for moderate to high speeds.  Petroleum exhibits good lubrication under heavy-load/high-speed conditions. Silicone oils offer good heat resistance and do not corrode rubber, but are more appropriate for low speeds. Oils can by dripped, centrifuged, or impregnated into bearings.

Greases are best for moderate to high rotation speeds. Typically, the high-speed torque of a greased bearing is lower than an oiled bearing, but the opposite is true at lower speeds.  These are usually applied via grease pack, but grease plating is also available. Silicone greases offer good heat resistance and do not corrode rubber, but are best for low to moderate speed applications.

Dry films should only be used where 'wet' lubricants will prove unsuitable or will accumulate large amounts of dirt. Dry films are difficult to apply and produce wear flakes as the bearing rotates, possibly obstructing bearing operation.

Bearing Speed

Bearing speed limitations are almost impossible to conclude from application to application. There are several factors that directly correlate to speed capabilities however:

• Size: Higher speeds can be obtained by smaller bearings. These bearings have experience less torque and have better precision. Miniature bearings usually have a thin, weak retainer and may be less capable.
• Load: Heavier loads decrease speed capabilities.
• Ring rotation: An inner ring, which is smaller, can rotate 33% faster than its larger, outer ring counterpart.
• Retainer/cage material: In order of speed capabilities, phenolic and other nonmetallic materials (very high speed), hardened steel (high speed), full race (moderate speed), loose-crimp ribbon (low speed), and PTFE spacers (very low speed).

• Lubricant: See the above section, "Bearing Lubrication," to assess lubrication's effect on bearing speed.

• Lubrication method: Oil impregnation and grease packs are most capable for high speed rotation.

Bearing Load, Fatigue, and Lifespan

Optimum bearing life is obtained when the balls and raceways have absolute minimal surface contact  supplemented with proper lubrication. Loads for ball bearings are subject to static or dynamics loads, as well as axial or radial loads. This means four variables need to be accounted for to determine working loads for a bearing. Ball bearings can handle substantially more radial and dynamic load than axial and static loads. The first sign of non-elastic deformation will be flattened spots on the balls, which will hinder rotation.

Calculated life of a bearing is based on its load, operating speeds, and environmental factors. Industry standards typically require that 90% of bearings are still serviceable after 1 million rotations, and 50% of bearings still be serviceable after 5 million rotations. This is known as bearing fatigue life. An underestimate (for safety) of a bearing's lifespan, as well as the applicable variables to calculate such, is offered with Engineer's Edge online ball bearing fatigue life calculator.

It can also pe accomplished with the following formula:

L₁₀ = 16667 Xa₁ x a₂ xa₃ [f_BXC_E/P]

           _______________________

N

Where:

L₁₀ = Bearing Life in hours

C_E = Extended Basic Dynamic Load Rating in Pounds or Newtons
f_B = Dynamic Load Rating Adjustment Factor for Number of Adjacently Mounted Bearings
a₁ = Life Adjustment Factor for Reliability
a₂ = Life Adjustment Factor for Ball Bearing Material
a₃ = Life Adjustment Factor for Application Conditions
N = Operating Speed (rpm)
P = Equivalent Radial Load on Bearing in Pounds or Newtons

Bearing Deflection

A bearing's elastic deflection occurs between balls and raceways from a load and should be expected. Deflection increases nonlinearly as the load increases. Typically, the least rigid bearing at light loads becomes the most rigid at heavy loads. For precision applications bearings should have little deflection; the yield can be minimized in one of four ways:

1. Decrease raceway curvatures
2. Increase ball size
3. Increase ball complement (number) 

Bearing Heat

Radial ball bearings generate heat energy from their mechanical energy via friction. Rubbing of the mechanical parts, as well as oil shearing, contribute to a ball bearing's heat generation. High temperatures can have adverse effects on bearing materials if they are not approved for high-heat use. Temperature can be regulated by monitoring the oil flow through the bearing and by external cooling. Frictional heating is a result of bearing pressure, rolling velocity, and the coefficient of friction. If the coefficient of friction remains somewhat stable for a range of loads and speeds, it will be represented by the value of PV. This is especially true for bearings with plastic materials.

Bearing Materials

Radial ball bearings are made of largely through-hardened materials with a minimum Rockwell rating of 58 R_c. 440C stainless steel and SAE 52100 steel are the most common materials for raceway and ball designs, but these alloys are not suitable for operating or friction temperatures above 350° F. Forms of molybdenum steels are excellent for temperature resistance even over 1000° F.

The shields and seals of a bearing do not carry a radial load and only light axial loads, if any. Metal shields are usually of the same material as the bearing raceways. Stamped steel is the most common retainer material; stamped bronze or brass is readily available in many European countries. These materials feature good temperature resistance in high-speed applications. Plastic retainers have a higher speed capability and are limited by temperatures, but plastic retainers for high-speed applications made of phenolic materials (270° F), PTFE (450° F), or polyimide (500° F) are available.

Bearing Noise

The minor inaccuracies in raceways produce sounds that can be amplified by the machine structure. Using higher precision bearings and an axial preload will significantly reduce noise from a bearing, as well as proper lubrication.

Bearing Shafts/Housing

Proper shaft/housing is critical to a long bearing life. The fit, or interference that exists between mating components, can be placed in two classes:

1. A loose fit allows for easy installation, but may also allow the bearing to slip on the shaft or in the sleeve. This can cause excessive wear and vibration, and bearing failure.
2. A tight fit bearing is more difficult to employ, but misaligned press fits will deform raceways and increase friction while shortening bearing life.

The type of load (static/dynamic) and which raceway bears the load (inner/outer), indicates the fit that should be pursued.

Applications

From vehicles to manufacturing processes to toys, radial ball bearings are diverse in their commission; it is impossible to publish an unabridged application listing. Jet engines employ specialty designed ball bearings in their turbines. In fact, many types of turbines will use these bearings. Automobiles depend on ball bearings for locomotion in both their wheels and engines. Conveyor belts will use radial ball bearings to provide frictionless rotation of traction wheels. Many types of pulley systems use a radial ball bearing component. The first recorded patent on ball bearings was granted to Parisian bicycle mechanic Jules Suriray in 1869, which were used on the winning bike of the first bicycle road race, the Paris-Rouen, later that year. Bikes today still use ball bearings. Indeed, radial ball bearings have considerable recreational applications as they are present on skateboards (in a duplex configuration) and inline skates as well.

Resources

Timken - Radial Ball Bearings

The Free Dictionary - Antifriction Bearing

Koyo Deutschland GmbH - Deep Groove Ball Bearings

NASA - Roller Bearing Life Prediction: Past, Present, and Future (.pdf)

Bearing Specialist Association - Seal Selection (.pdf)

Schaeffler Group - medias - Bearing Selection

AST Bearings - Interactive Online Catalogue

NMB - Bearing Engineering

NTN Bearing Coporation of America - Shaft & Housing Fits

eBearing - Glossary of Bearing Industry Terms

RBC Bearings Incorporated - Nice Ball Bearings

GMN Bearing USA Ltd. - Precision Radial Ball Bearings

Engineer's Edge - Bearing Engineering and Application Menu

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