General 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 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 engineers 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.

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 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.

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

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