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This is the part of the bearing that is abutted to a shaft. It does not rotate relative to the shaft. In section the inner race has a semi circular raceway upon which the balls rotate
This is the part of the bearing that is on the very edge of the bearing. This is fixed to the housing by means of an interference fit or some other locking mechanism. This has a semi circular raceway upon which the balls rotate.
The balls provide the relative rotational movement between the inner and outer races. They rotate in the raceways and are held in a relative position by means of a cage.
Bearing Size Convention
Bearings are quoted in sizes as d x D x B as an alternative to the bearing number
d = inner diameter (mm)
D = outer diameter (mm)
B = width (mm) or Breadth (mm) or Thickness (mm)
Bearing Material (inner race, outer race, balls)
Steel, stainless steel and some variation in ceramic are all common types of bearing materials.
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 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). 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, this 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.
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 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, DDU, 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, 2RSD)
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.
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.
Bearing Numbering and standards
There are four prevalent bearing standards. ISO, DIN and JIS are the big standards which all of the large bearing manufacturers adhere to, the three are interchangeable. These standards set limits on key parameters which define bearing performance. The intricate internal geometry of bearings can differ but the boundary dimensions (width, inner diameter outer diameter) are all carefully controlled.
The other standard which has proliferated from America via the skateboard industry is ABEC. This is primarily marketed by Enduro, it places less emphasis on boundary dimensions and more on the roundness of balls.
Of the ISO, DIN and JIS standards, the difference between them is the numbering. JIS omits a dimension number occasionally in some sizes eg 6806 JIS is the same as 61806 in ISO or DIN. The bearings are physically the same size and meet the same performance criteria but the number one is omitted.
The table below shows the relationship between ABEC and JIS/DIN/ISO standards. The JIS, DIN and ISO standards are a direct comparison, the ABEC equivalent is an approximaion. In simple terms, the further right on the table the bearing tolerance class is, the more accurate it will be manufactured to. In practice all bearings are made on the same production line. During the testing of the bearing, it will be determined which class it fits into. NTN, SKF, FAG, NSK, Koyo, Nachi exceed class 6 as a minimum
|JIS (Japanese Industrial Standard)||JIS B1514||Class 0||Class 6||Class 5||Class 4||Class 2|
|ISO (International Standards Organisation)||ISO 492||Normal Clas||Class 6||Class 5||Class 4||Class 2|
|DIN (German Standards Insitute)||DIN 620||P0||P6||P5||P4||P2|
|ABEC (ABMA)||Std. 20||ABEC5||ABEC5||ABEC7||ABEC9||ABEC 9|
In a bearing, there is a small gap between the balls which rotate and the inner and outer races. This small gap exists to allow for thermal expansion and lubricant penetration. Standard bearings have a clearance called CN, these bearings are unmarked. A larger gap is called C3 clearance, whilst a smaller gap is called C2 clearance.
Larger bearing clearances are used when the loads on a bearing increase. Bigger bearing clearances result in increased rolling drag.
Almost all ceramic bearings have C3 clearance.
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The table below shows a cross reference of part numbers for the major brands
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
|Two Seals Non Contact||LLB||2RZ||2RSD||-||-||VV||LLB|
|Two Seals Contact||LLU||2RS||2RSR||2RS||PP||DDU||LLU|
|One Seal Contact||LU||RS||RSR||RS||PP||DU||-|
|Polyamide Cage||T2||TN9||TNH or TVH||TN9||PRB||T1X||-|
|Heat Stabilized||Prefix TS3||S1||S1||S1||-||X28||-|
|Greater than Normal Clearance||C3||C3||C3||C3||P||C3||-|
|Greater Than C3 Clearance||C4||C4||C4||C4||J||C4||-|
|Radial Clearance in Microns||RLXX||-||RLXX||-||CGXX||-||-|