Aggiornato il 22-07-09.

The ball bearings of the rollerskis wheels  

This is not meant to be an engineering article (because I am not an engineer at all) but a simple explanation of what we can learn from the books about mechanics and the ball bearingS catalogues applied to rollerskis.

I will be happy to go into this subject in more depht with everyone who could spot some mistakes and/or have a better knowledge of this subject.

As far as I know every rollerskis manufacturer uses ball bearings. The ball bearings are made up of a couple of concentric rings, made of tempering and rectified steel, which are interposed by steel spheres. In this specific case there are SINGLE ROW DEEP GROOVE BALL BEARINGS and a crown of spheres. Usually the distance beetwen the spheres is kept by plastic or metallic cages.

The bearing can be open or endowed with SHIELDS of two types:

  • RS type or completely protected (in theory) type, is made up of a rubber ring fixed with pressure to the external bearing ring, endowed with a LIP which leans (with more or less friction) against the internal ring.
  • Z type is made up of a moulded steel sheet fixed together (with pressure) with the external ring, which brushes the internal ring.

At present we can find bearings with different dimensions such as 626 and 627, but the most used ball bearings belong to the 608 series: this number stands for a SINGLE ROW DEEP GROOVE BALL BEARINGS with the following dimensions: 8x22x7 MILLIMETERS (hole diameter x external diameter x ball bearing width). You can find several types of ball bearings with different characteristics, but surely the cage material, the type and the number of SHIELDS and the precision in the dimentions and in the installation are all features which determine the bearings life and slide.

How much "energy" could you gain by knowing you are wearing the fastest component under your feet ?

As far as I am concerned I gain a lot of energy, a great deal indeed! This may explain why some ROLLERSKIS manufacturers hates me!

The cage
The ball bearing cage can be made of metalic or plastic material. I think that the polyamide cage is the fastes, even if the difference is minimal. Surely the non-metal cage does not rust and can keep more lubricant.

The protection device is needed only on the external side, at least on the aluminum rim wheels, if it is installed also on the internal side of the wheel, it can be removed.

The best protection SHIELD are the RS types which can guarantee a longer life to the bearing.

The Z SHIELD cause less friction (because they do not have the SLIDING LIP), but they let a lot of dust in.

The rear wheels receive the dust raised by the front wheels, that is why I prefer to use Z type for the front wheels and RS for the rear ones.

When well lubricated the RS SHIELD does not slow you down a lot.

To remove the SHIELDS if some of them are damaged, I avoid to remove the Z types: for RS types it is sufficient to use a needle, once the internal SHIELD is removed (this one will not be installed again), I remove the external one pushing it out from inside the bearing (so that the CAGE is not damaged).

Dimensional precision
The ABEC # suffix (for example ABEC 5) comes from the America norms, the higher the number #, more precise the bearing.

I have compared the American ABEC system to the Internation ISO one, in scheme 1 which follows:

- 0 1
P6 6 3
P5 5 5
P4 4 7
PA9 - 9
Scheme 1

Generally speaking the higher the identification number which follows the ABEC writing, the higher the precision level (for SKF and ISO is viceversa). A low precision level bearing (ABEC 1 Picture 1B) presents a higher distance between the spheres and the rolling tracks obtained from the rings if compared to a high precision level bearing.

Making it simplier: the more precise the bearing, the higher number of spheres bear the weight (Picture 1A), which means that every sphere has a smaller weight to bear, which means less friction and less wear of the rolling tracks.

The rotation precision (eccentricity and ring OUT OF ROTATION AXIS) goes together with precision: suppose you have an oval (I am always talking about fractions of millimeters) internal ring, the rolling bodies would be overloaded with an anomalous cyclical stress.

Picture 1

A) In a high level precision bearing (ABEC 5) the weight is carried by a higher number of spheres;

B) In a less precise bearing (ABEC 1) the weight is carried by a lower number of spheres

Assembling precision
We can even buy ABEC 9 bearing type but if the wheels are not precise, they will be as fast as ABEC 1 bearing or even less; in picture 2 you can see some bad assembling mistakes (which are very frequent)

Picture 2

The BUSH has not the correct dimensions, this implies higher friction degree in the bearings and a faster wear of the rolling tracks.

Surely to assemble an ABEC 5 bearing type with the hammer can be very bad, if the wheel are made of aluminum: a wrong hammer blow on the bearing which is badly fitted in its site, causes a deformation of the site.

If we are lucky enough, the wheel can be still used, but on the most of the occasions a hammer blow causes the bearings axis in the wheels to go out. Therefore it is always advisable to assemble the bearings on the press with the proper instruments (HIDRAULIC OR HAND PRESS).

Bearings maintenance
The maintenance methods depend on the bearing types and on the usage conditions.

For the training rollerskis: the water makes the tracks rusty, if you do not eliminate it, it can even block the wheels. This is the reason why after a ride in the rain, it is advisable to disassemble the wheels from the bars and pull out the bearings to eliminate the water which has probably gone in the bearing (this happens also with the RS SHIELDS and above all with the plastic wheels). When I carry out maintenance for the first time, after cleaning the bearing with petrol(1), I personally fill them with grease (I use the MoS2 grease). Then I put the RS SHIELDS back on (in the beginning the bearing does no longer roll), with this method I used the rollerskis in the rain, without cleaning them until the wheels have completely worn out.
If you use compressed air, do not make the bearing rolling under the air jet, because this brings the bearing to a RPM which is too high for the RPM it was built for and as a consequence the cage can deform with the centrifugal force, causing the total jamming of the bearing.

For the competition rollerskis: to carry out maintenance on the rollerskis after every race, lengthens the bearing life, but it can cause also a "contamination". It is better to avoid the maintenance if you cannot assure a total cleaning and a correct disassembling and assembling of the parts. Every blow given with a steel hammer or a similar tool to the bearings shortens their life.

Find here following the correct tips for bearings maintenance:

  • Disassemble to rollerskis wheels
  • Clean the external part of the bearing and the wheel with a cloth
  • Remove the bearings with the proper tool and a piece of wood
  • Put the bearings in a "safe" place protected from dust, soil, stones or filings shavings
  • Remove the SHIELDS (if possible)
  • Clean the bearings putting them in petrol(1) or using a lubricant and water-repellent spray
  • To eliminate all the dirt from the bearings I usually roll them a little between the various immersions and sometimes I give them little blows on a clean white cloth which must not fray.

I carry out this operation until there is no more dirt and the cloth remains clean.

  • Dry the bearings using low pressure compressed air or with a clean cloth.
  • Lubricate with one or two drops of mineral oil (motor oil)
  • Put on the SHIELDS
  • Clean the bearings sites on the wheels
  • Assemble the bearings with proper tools to guarantee the coaxiality with their site
  • Assemble the wheels on the rollerskis using the proper tool (see picture 2)

Picture 2: lining up tool with which it is possible to obtain a prefect lining up of the competition rollerskis wheels.

If the assembly has been carried out correctly, the wheel rolls immediately, (almost) without friction, but there may be a little friction. In this case, using a piece of wood, it is sufficient to lay the rollerskis on a hard surface and blow it (exactly on the head of the locking screw) and the bearings will settle in their site.

Bearing of A row, from the left:
  • SINGLE ROW DEEP GROOVE BALL BEARING 626 with steel cage and withot SHIELDS (open)
  • 627 bearing 7x22x7 with RS SHIELDS
  • SKF 608 ABEC 5 bearing with Z SHIELDS
  • 608 bearing with polyamide cage and one single Z SHIELD (under)
  • 608 bearing with bakelite cage and one single Z SHIELD (under)
  • SKF 608 ABEC 5 bearing with galvanized steel yellow cage
  • SKF 608 ABEC 1 bearing with steel cage

Bearing of C row, from the left:

  • 608 ABEC 5 bearing with Z SHIELD and steel cage
  • SKF 608 ABEC 5 bearing for rollers which does not need maintenance (because they have a special high speed grease)
  • SKF 608 ABEC 7 titanium bearing with Z SHIELD and polyamide cage

Mirco Collavo

(1) When using petrol or water-repellent removers and lubricants, please make sure that the room is well aired and do not smoke !

Aggiornato il 22-07-09.