Bearing Technical Information

Introduction

This guide has been written to provide information and assistance to allow you to replace bearings. The guide is predominantly based around the replacement of bicycle bearings but nonetheless the same principles apply to bearings used in an industrial capacity.

Bearing Codes, Standards and Sizing

Bearings for industrial applications overlap into bicycle applications. A lot of bearings are based on standards from one of the following bodies

• ISO – International Standards Organisation
• DIN – German Standards Institute
• JIS – Japanese Industrial Standard
• ABEC – American Bearing Standard

ISO, DIN and JIS are essentially the same standard, they are all derived from the same source and interchangeable. Some of the terminology varies from standard to standard but the tolerances and performance factors are the same. The most obvious difference is Japanese number is subtly different to ISO/DIN numbering and lacks a 1 digit in some bearing numbers

ABEC is a standard that has proliferated from the skateboard industry. None of the large bearing manufacturers design their bearings to this standard and in most industrial engineer’s opinion it is a mickey mouse standard.

Standard bearing sizing is often a 3,4 or 5 digit number with some letters suffixed to the end. The vast majority of bearings follow a size convention adhering to JIS/DIN/ISO standards.

ISO and DIN Bearing Numbering System (Bearing Code)

• The first digit represents the bearing type (6 for ball bearings, 7 for angular contact bearings)
• The second digit represents how wide the bearing is. This number is ommitted in JIS numbering
• The third digit represents the diameter series and is effectively the outer diameter of the bearing
• The last digit (for 4 digit bearings) or last two digits (for 5 digit bearings) represents the inner diameter of the bearing.

JIS Bearing Numbering System (Bearing Code)

• The first digit represents the bearing type (6 for ball bearings, 7 for angular contact bearings)
• The second digit represents the diameter series and is effectively the outer diameter of the bearing
• The last digit (for 3 digit bearings) or last two digits (for 4 digit bearings) represents the inner diameter of the bearing.

Bearing Material (inner race, outer race, balls)

Steel, stainless steel and some variation in ceramic are all common types of bearing materials.

Steel

Modern industrial bearings have been made in a hardened type of steel (chromium) for many years and are by far the most popular material type in all applications today. All of the major manufacturers use steel as the de facto standard offering. All elements of the bearing (inner races, outer races and balls) are ground to produce a very smooth running track that allows the balls to rotate freely. Standard steel bearings have a useful characteristic in that they respond well to micro damage, they simply bed in some more with running. The friction does increase but it does not prevent a seizure

Stainless Steel

Stainless steel is used as a material when the bearing is likely to be attacked by substances or when the ultimate in cleanliness is required (food/medical industry). Stainless steel is much softer than chrome steel. From a rolling perspective, they have more friction than standard chromium steel bearings. Stainless steel bearings have an unwanted habit of going through a failure mechanism called galling, this is where the material micro delaminates and causes pitting, which generates more heat and eventually causes a seizure. On a bike, this is not a recommended material as the ingress of dirt can cause premature galling. They also have higher rolling friction coefficient.

Ceramic Bearings

Fully ceramic and hybrid ceramics have been popularized by companies like Enduro and Ceramicspeed. These bearings give good initial low friction but have an extremely poor life. They are really only fast for 500-600km. Check out the bearing performance chart. The material characteristics mean that these bearings are inherently brittle. Despite claims that the balls are rounder and therefore faster, the fact is the standard bearings are plenty round enough and do not destroy themselves. Ceramic bearings are extremely expensive.

Bearing cage material

The bearing cage is a guide piece that holds the balls in that separates the inner and outer races. It has light contact with the balls.

Bearing cages in small bearings are generally made in two types. Either a pressed metal cage or an injection moulded plastic cage. At the time of writing SKF and Koyo were using injection moulded cages whilst INA, NTN, NSK, Nachi were using metal cages. From a performance perspective, the plastic cages make the bearings slightly lighter but compromise with a reduced life.

Bearing Measuring and Dimensions

If you wish to purchase new bearings, it is worthwhile measuring the bearings beforehand. Some bearing manufacturers (typically far eastern) produce bearings in non standard sizes but use standard seals to keep the costs down, this results in a missmatch between what the seal says on it and what the bearing measures up at. If in any doubt, always go with the measurements rather than any seal markings.

All bearings are quoted in the size d x D x B, where d is the inner diameter, D is the outer Diameter and B is the thickness. This terminology is universal across literature. On this website, inside diameter (ID), outside diameter (OD) and Width(W) are used for clarity for the non-engineering public

Bearing Clearance

An often overlooked part of purchasing a bearing is the bearing clearance. For a bearing to function correctly and prevent seizure, bearings must have a small radial clearance between the balls and inner/outer races. This clearance is given a C number. Unmarked or unspecified bearings have a clearance of CN which is normal clearance. The bearing clearance is not a fixed number and varies with the size of the bearing, it is defined in JIS/ISO/DIN standards. The diagram below depicts bearing clearance.

A table of bearing clearance cross references is shown below

*WARNING* Enduro and almost all Ceramic/Hybrid bearings are C3 or greater clearance even though they have no markings to show it.

An increase in bearing clearance is associated with an increased ability to accept axial load at the cost of reduced radial capability. In most bicycle applications a CN clearance bearing will be sufficient, the exception to this is in some wheel hubs which have small bearings – in this case C3 may be more appropriate.

A common problem in bicycles are undersized holes. Often caused by worn cutting tools or poor QA. A bearing that is pushed into an undersized hole will display symptoms of rough running and often feeling gritty. Upon removal, the bearing will usually become smooth again. Often bearings which are left in will bed in but this is a less than ideal scenario.

This problem is extremely common in bottom brackets and some hubs.

Bearings are a ground finish (they are hardened) and thus the manufacturing tolerances on the is a few magnitudes higher than the apertures they are pushed into.

Bearing Seals

Bearing seals contribute the vast majority of friction to a rolling element bearing. Unlike bearing numbers which are universal across manufacturers, seal types carry designations that are manufacturer specific.

Contacting Seals (2RS, LLU, DD, Enduro LLUMAX)

These seals have the best ingress against dirt and moisture but they also have the highest rolling resistance. 2RS is the most common terminology for this type of seal and it literally stands for 2 rubber seals. Some manufacturers (Enduro, FSA) badge their non contacting seal bearings as 2RS. Generally speaking, contacting seals across all manufacturers are made from the same materials. There is subtle difference in geometry and consequently rolling resistance. FAG/INA seals in 2RS are widely regarded as being the best for suppressing moisture and dirt ingress.

Non Contacting Seals (VV, LLB, 2RZ, 2BRS)

Non contacting seals have the lowest friction available of any seal type. They are sold on the premise that the seal never actually touches between the static and rotating parts. This might be the case but the reality is there is often a small layer of dirt that ends up trapped between the seal and the rotating part, this causes a small amount of friction.

All high end ceramic bearings use this type of seal.

Open bearings (no suffix)

These bearings carry no suffix after the bearing number. These bearings are only suitable where this is some sort of external seal or when they are used in a clean environment. On a bike, the pedals are where open bearings are commonly found.

Seal Cross Reference

Unlike bearing numbers, the seal designation is not consistent across manufacturers, the table below shows a broad comparison and cross reference

*Special order

Bearing Class

Bearings come in different classes based on their running characteristics and dimensional accuracy. JIS/DIN and ISO use an interchangeable scale. The scale is almost inverse (from worst to best, it goes class 0, 6, 5, 4, 2), so having a lower number indicates a superior bearing. The German DIN standard uses P instead of class. ABEC bearings have a grade 3,5,7,9 – in this system a higher number indicates a superior bearing but it bears little resemblance to JIS/DIN/ISO standards. The table below shows the JIS/DIN/ISO relationship

The practical implications of bearing class are real. Super precision or grade 2 bearings can be easily distinguished against an unclassified bearing of the same manufacturer. However it should be pointed out that super precision bearings are often made on the same production line as standard bearings, they are simply tested and found to have superior characteristics to justify the grade rating. In reality, an unclassified bearing from one of the large manufacturers and specifically NTN tends to be class 4 as a minimum.

Bearing Brands

The list below gives an overview of the three broad categories of brands that exist in marketplace.

Industrial Brands

Some major industrial bearing brands are; NTN, SKF, FAG, INA, NSK, Timken, Koyo, NTE and Nachi. All of these companies have been making bearings for many years and they have refined their processes and manufacturing systems to make their products to extremely tight tolerances. Fitting these bearings almost eliminates the possiblity of a fit issue with the bearings and given the kinds of clients these companies have, they are unlikely to use substandard materials. The disadvantage with these suppliers is that bikes sometimes have non standard bearings which are unavailable from them.

Bike Specific Brands

Ceramicspeed and Enduro are two boutique brands which are cycling focused, their offerings revolve around standard bearings and exotic ceramic bearings. In thorough testing, these brands have been found to have performance that is on a par with the industrial brands but cost significantly more. The premise of lower friction is essentially a myth as conventional steel bearings outperform them after only a few 100km, therefore they do not make cost effective sense. The advantage with Enduro/Ceramicspeed is they produce sizes that the major brands do not, so if you have a non-standard sized bearing, ceramicspeed/enduro offer an ideal choice albeit at a price

Unbranded Bearings

Unbranded bearings are commonplace on eBay, Amazon and some bike retailers. They can be completely hit and miss, most of the time they are a miss. Tolerances are poor and materials are also very poor. Clicking, creaking and general unwanted noises are commonplace. My reccomendation is to avoid these unless you have no choice. The reason these bearings are popular is because they are cheap. Retailers make huge margins on these bearings.

Counterfeit Bearings and B grade Bearings

A recent problem that has come about in recent years is the abundance of counterfeit bearings. You can read mroe about this here. Counterfeit Bearings and B Grade

Bearing Cross Reference

The table below is a complete cross reference between manufacturers. Please use it as a guide!

Usage: eg, you would like a 6806 bearing with fully contacting seals on both sides, C3 clearance and a brass cage

NTN: 6806 LLU L1 C3

SKF: 6806 2RS M C3

FAG: 6806 2RSR Y C3

Bearing Quality

There is a lot of talk on the internet about which bearings are good and bad. A list of top tier, mid tier and budget bearing brands is shown below. It is not an exhaustive list

Top Tier

NTN, FAG, INA, NSK, SKF, Koyo, JTEKT, Nachi, Toyama, Fujikoshi

Mid Tier

Zen, RHP, Timken, NTE

Budget Tier

CXN, HUX, HBK, NBK, EZO, SPI, TPI

Download Manufacturer Catalogues

A selection of manufacturer bearing manuals are here for download. They are provided as is for reference

• NTN Bearing Manual
• NSK Bearing Manual
• NSK Bearing Cross Reference Chart