Bearings are these sealed metal rings that help your wheels spin and there’s some “ABEC” thing with numbers. The higher the number, the faster they are, right?
Well, at least that is as far as the knowledge of many derby skaters goes. Bearings are a very important component of your skates, but are often taken for granted. They perform a simple task; they help your wheels spin with a minimal amount of friction to the axles. What you don’t see of the bearings is what does all the work, balls. Yes, my balls do all the work in my bearings.
Seriously though, your bearings contain 6 or 7 ball bearings. 7 is more common until you get away from ABEC rated bearings, 6 is more typical of high quality bearings. Before I go too far with this thought, lets talk about the details of the construction of a skate bearing.
When you look at your bearings, you generally see three parts. These parts are as follows:
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| It probably goes without saying what the dust cover/shield does. There is a difference between a dust cover and a dust shield though. Some bearings have only dust shields, while others have both. A dust shield is a metal plate that protects the inside of your bearings from dirt, dust and to a lesser extent, water. Dust covers do the same thing but are thin plastic. What’s the big difference? Dust shields provide an extra bit of protection from impact to the bearing, dust covers don’t. If you have bearings with dust covers, these covers should be facing inward in each wheel. In other words, they should not be exposed once you have your wheels on. |  |
Raceways are a critical part of bearings, in that if machined with imperfections, it can greatly affect performance. Raceways are just metal rings, the full width of the bearing that have groove cut on the internal surfaces. These grooves serve as a track for the ball bearings to roll in (we’ll talk more about this in a moment). They also help to trap the ball bearings.
Most dust covers/shields are held in place with a metal retaining ring against the outer raceway. If you remove this retaining ring, you can now remove that dust cover/shield. You are now looking at the inside of your bearing. Chances are, it’s pretty disgusting looking. It will be filled with lubricant and any debris that may have made its way inside. Besides that nastiness, you will see the ball bearings and a metal (there are some bearings that use some types of plastic for this) ring that is bent around each one of the ball bearings. If you look closely, you will notice the flat areas on this metal ring in between each ball is the same width. Those flats are the key to the primary purpose of the ring. What I have been calling a ring is actually two rings (one on either side of the ball bearings) that are linked together in some fashion. This part is called the cage and its primary purpose is to create the appropriate spacing between ball bearings in order to prevent jams.
I’ve mentioned the ball bearings many times. These are simply metal (with certain exceptions) balls machined to be as close to a perfect sphere as possible. Believe it or not, machining a perfect sphere is something engineers have been struggling with for a very long time, so as close to perfect as possible is what we get. As these balls roll, any imperfections in the surface of the ball, or unevenness in the machining will create chatter. What I mean by chatter is it will roll roughly every time that spot hits the raceway. The more chatter in your ball bearings, the more friction it creates.
Now that we have talked about all the parts, lets put it all together.
The outer raceway is seated tightly in your wheel. It has the dust covers/shields mounted firmly to it with retaining rings. In between the dust covers/shields are the ball bearings inside their cage. For you math geeks, here is how you can understand how a bearing stays together:
X = radius of inner raceway at the bottom of the groove
Y = diameter of one of the ball bearings.
Z = radius of the outer raceway at the bottom of the groove
X + Y ~ Z
The reason for the approximation is that there is a certain amount of slack that varies depending on the tolerances the manufacturer was following.
Because the raceways have a track, the edges are raised. With the space allowed for the ball bearings, they are unable to roll out from between the raceways. Likewise, the inside raceway can’t simply come out.
Now, the inner raceway is tight against the axle and the outer raceway is tight against the wheel. As the wheel rotates, the outer raceway rotates with it. This forces the ball bearings to spin with the rotation of the outer raceway, also rolling in the opposite direction in the inner raceway.
The idea is that all the friction created is a smooth rolling friction. If there are imperfections in the ball bearings, there will be some bouncing or sliding friction. If the friction created is not the smooth rolling friction I described, it requires the skater to work hard to gain and maintain speed. It will also speed the degradation of the bearings.
In the next part of this series on bearings, I’ll talk about understanding ABEC ratings.
An additional, easier formula for the math (stats) geeks to understand how bearings are held together is this:
X = internal radius of the outer raceway at the thickest point
Y = external radius of the inner raceway at the thickest point
Z = diameter of one of the ball bearings
X – Y < Z