All about skate bearings
Anatomy of a bearing
The making of a bearing
A bearing is made of 6 elements:
- an inner race
- an outer race
- rolling bodies: the balls. A 608 standard bearing has 7 balls.
- a cage
The ball are often supported by a cage, which ensures a regular spacing and prevents them from rubbing together. The bearings are lubricated with grease or oil . The bearings, two per wheel, are inserted into the hub of the wheel. Their coherence is ensured thanks to a spacer.
The material is one of the most important elements of the bearing
Current ball bearings are made of different materials, chosen according to the destination of the product. The materials used in the manufacturing of bearings are: metal (steel or other), plastic, nylon, ceramics, carbon, bakelite. a "real" bearing is made of 100C6 alloy (1 outer race + 1 inner race + balls). It is tempered and hardened chrome steel. Its main characteristic is its high hardness (between 62 and 64 HRC). As a comparison, some characteristics of the 100C6 are close to those of glass: it is very hard but breakable in case of a shock.
On the market, there are also low range bearings made of alloys of steel and carbon. That material has a higher yield stress than that of the 100C6 bearings. Its rectification grain (which enables to get a "mirror"-type polished gliding surface) will not be as good as that of the 100C6. In other words, the gliding will be less good… avoid it.
That steel-carbon alloy is used on lots of branded skates, because the production cost is 30% cheeper than that of the 100C6… The joys of business!
The admissible charges
The material enables to guarantee the good holding to static and dynamic loads announced by the manufacturers. Obviously the bearing should have the good number of balls too.
In other words, if you reduce the number of balls, you reduce the rubbing but you also reduce the load capacity. Luckily, a 608 is highly over dimensioned for a skating use.
There are lots of standards defining a bearing (designation, manufacturing tolerance, load capacity, radial play standards…) It is quite barbarous. We mainly talk about ISO standards but the most famous is the ABEC (American standard). It goes from 1 to 3, 5, 7, 9 and even 11 for skates.
It defines the execution precision of the ball bearing: tolerance on the inner and outer sides as well as of the false rotation circle. (A bearing smoothly rotates, it does not turn like a potato!)
An ABEC 7 bearing is necessarily better than an ABEC 1 bearing… It is true for industry where precision can be very important for the good working of machines. The higher the ABEC index, the more precise the bearings. But that standard, which is obsolete, is only used for skating. It has been replaced by the ISO standard in the industry. Let's note that the ABEC standard is a reference in the world of inline skating and that it has even become a sales argument used by manufacturers and distributors. Let's note that lubrication plays an important part in the bearings' performance, far more important than the ABEC standard. The use of the ABEC standard results from a strategic choice made by the Asian firms which do not always master the ISO certification. They have thus constrained the manufacturers around the world to align themselves to enable a better comparison between the products, whereas the manufacturers used the ISO standard, which is more draconian.
For example, the INA brand indicates an ABEC standard on its skate bearings although it is ISO certified. Christian Riefsthal, a commercial manager of the brand, explains that the ABEC standard has overflowed the market for several years and that selling bearings with no ABEC certification on them has become impossible.
The tolerances on inner and outer diameters are not important in skating, as the bearing is set up into a wheel made of polyurethane and with a screw going through. These two parts are already far less precise by definition.
Moreover, the false rotation circle (which could be important in skating) is already to the micron on an ABEC 1, whereas a plastic wheel precision is to the millimeter to the most. All in all, the importance of the ABEC standard is very relative.
Rectification: standards do not define the quality of rectification of a bearing
It is nevertheless one of the most important points. To make it simple, the rectification enables to smooth the sliding surfaces of the inner and outer races and of the balls. It ensures the quality of sliding and reduces the heating. The big bearing manufacturers *should* be at the top. The Asian factories created their own standards.
Most of the time, there are 7 of them in 100C6 for a 608. Some products have 8 or 9 balls. For a skating application, it may be interesting to reduce the number of balls down to 6, which thus reduces the resistance and friction.
There are bearings with ceramic balls too (just one or all balls). It gives a very good gliding, the balls are very hard, thus far less sensitive to pollution, dust, sand, lubricant (a little oil or grease is enough).
These bearings offer less friction thus less heating. The main drawback is the price that can go up to 250€ for 20 bearings.
The cage serves to space out the rolling bodies (balls). It can be made of carbon steel (never 100C6: the cage should wear out, not the balls), nylon (plastic), or bronze. Nylon cages are the best: they are lighter. On the other hand, they have less inertia. The friction coefficient will be better, as nylon opposes less resistance.
The seal enables to limit the effects of pollution and avoids the lubricant to go out. To make it simple, there are two types of seals: with two metal shields (ZZ type: the most common) and with two synthetic joints (2RS type). In fact, there is a small dozen types of seals! the 2RS type is not the best suited for skating because the joint runs on the inner race and slows down the rotation of the bearing. On the other hand, the seal is perfect. Then you can use it in challenging conditions. Better seals exist but you will not find them on the skating market.
Lubrication meets lots of aims:
- reducing friction and wearing
- better cooling of the bearing
- preventing or delaying the pollution of the bearing
- increasing its life expectancy
Two lubrication types are frequently used: grease or oil. Oil has the advantage to be changed regularly and opposes less resistance than grease. Nevertheless, it is often used with ZZ shields. These shields do not hold oil, after a few minutes of skating there is not much oil left. Some greases have sliding and friction characteristics which suit better the needs of skating. Moreover, they prevent dust and water to penetrate into the bearing better. It is all about dosing the grade percentage. Just a little grease will favor the gliding.
Correlation table between ABEC and ISO standards
The bearings used for skating are type 608 and 688 bearing, with few exceptions: few 800-27 bearings can be found too. These are deep groove ball bearings (600 series). A 608 bearing has a 8 mm bore (diameter of the central hole). The "6" in "600" is the space in millimeters between the outer sides of the 2 cylinders.
There are bearings that cannot be dismantled and others that are "reconditionnable" (that can be dismantled), but it is not recommended to dismantle bearings too often as it is often a damaging operation. The nature, quantity and viscosity of the lubricant are the only noticeable differences compared to industrial bearings.
A standard bearing weighs around 12 g and can bear a maximal rotation speed of 40.000 to 48.000 rpm, which would be equivalent to a speed between 500 and 700 kph and above by skate for a 80 mm wheel.
Classification table of 608 bearings
One steel shield
Protects from dust on one side (non-sealed)
Two steel shields
Protects from dust on two sides (non-sealed)
One elastomere shield
Protects from dust and humidity on one side
Two elastomere shields
Protects from dust and humidity on two sides
Joint without contact
Sealed and interchangeable
Sealed with low friction and interchangeable
Double lips seal
Each manufacturer uses its own seal name. For example, D and RS are theoretically the same thing. It is all a question of patent and designation.
The 608 bearings used for quad skates, skateboards and inline skates are classified into different categories: presence or absence of shields, nature of the shields. The shields are membranes which protect the balls and the inside of the bearing. They can be made of metal or plastic. There are bearings without shields, other have only one shielded side.
The types of bearings presented in that table show the diversity of products you can find on the market. We could simplify the table in making two categories: open and closed bearings. Some closed bearing offer but a few maintenance possibilities to their user. If the latter tries to dismantle them, they may break. They have the advantage not to take on dust and water much but once they are jammed, they are good to be thrown away. On the other hand, most of the elastomere joints (plastic) can be taken off very easily with a little screwdriver and can be snapped back just as easily.
The open bearings are divided into two sub-categories: the open and semi-open bearings. In the first case, the two shields are removable, in the second case, only one is removable. You can dismantle your equipment to ensure its maintenance.
They are metal parts which are inserted in-between two bearings, inside the hub of a wheel (see picture above). Most of the time, they are made of nylon on low end skates, and of metal on mid and high range models. They can be replaced by aluminum spacers, offering a better rendering of the bearings in order to have more coherence.
Their rigidity ensures a better coherence of the bearings between them and with the axle. There are spacers for 6, 7 and 8 mm axles.
As you can notice, there is a great variety of spacer shapes. Their dimensions are determined by the size of the axles, but also by the type of bearings. indeed, micro bearings need specific spacers.
Useful linksTexte : Alfathor & Stéphane R.
Photos : Alfathor & droits réservés