Proper way to insert a bearing into a shaft

Hi folks, this question isn’t directly related to CNCing so excuse me if it doesn’t belong here.

After hours of searching the internet and ruining about half a dozen bearings, I figured I’d ask what seems to be a pretty basic question. I have a 4mm shaft with 4x8mm bearings. Naturally the bearing needs to be tight on the shaft so it doesn’t slip, which makes sliding it in further than the tip of the shaft pretty difficult. In the past I managed to “softly” hammer the bearings inside, but they were bigger. When I tried it with one of these 4mm by tapping it against a table, they barely went in and it also resulted in messing them inside I think. I can now feel the balls as it turns. At some point I even tried heating the bearing, hoping it will expand as a result, but then as it started blowing smoke I recalled it might have some oil or grease inside. :slight_smile: So that didn’t work either.

Clearly I have no clue what I’m doing. What’s the proper way to insert a round ball bearing into a shaft?

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The proper way is to use a press with a die that put the force directly and uniformly on the race that is being pressed so no force is applied through the balls.

In the absence of a press and dies, the proper size wrench socket can be used, or a properly sized length of pipe or tubing, or you can make a die (I have a bunch that have been made for various purposes) and press the bearing using a vise or a dead blow hammer against the die. If using a hammer, hit just hard enough to move the bearing and don’t let the die bounce.

Fit should usually be pressed for the part that rotates, and snug sliding for the stationary part to minimize the bearing walking. A good way to remember this is to think about roller blade wheels: The outer race is a press fit in the wheel, and rotates with it, while the non-rotating axle is a snug slide fit through the inner race.

Pressing a bearing INSIDE the rotating element is usually easier when you don’t have the proper tooling, as the outer race is larger and you don’t need to worry about it having a hole clear for the shaft to go into. Key thing for either is to start straight, as if you don’t, it is easy to damage the bore or distort the bearing race.


I don’t like hammering. I use my drill press as a press to press bearings. It gives an even pressure around the entire bearing.


Good ideas, but here’s a little something we did when assembling parts onto shafts using what’s called a “interference” fit.

The shaft we would place in the freezer overnight. The bearing (in this example) would be gently warmed with a torch. Not enough to smoke, just hot enough to feel warm.

The resulting change in sizes would let us use a light press to install the item, and when the parts equalized in temperature, the parts were inseperable.

In a normal “press fit” situation, the effect is more dramatic. You chill the shaft, warm the bearing, and just slide it in place with your hands. :slight_smile:


Thanks everyone. Useful tips here all around.

If anyone has drawings or videos of any of these processes, that would be greatly appreciated (English isn’t my native tongue so some of these terms are foreign to me).

It might be a good idea to also check the actual dimensions of your shaft with a micrometer or look at the specificaton form the suplier. In my experience it’s a little bit pointless to start looking at the class of fit you have for smaller bearings like this but if you wanted to do that this would be a good place to start looking in general it’s a fantastic resource with laminated pages so you can wipe the grease off :slight_smile:

I used to use a little arbor press and either a socket (as enl_public mentioned) or turned up a little tool. The important thing is the bearing goes on straight, so it doesn’t pick up on the shaft, and that you press only on the inner race or both races equally, if it’s going on an internal shaft. If you press on the outer race you will definately damage the bearing. These little bearings a very easy to break so it pays to have a few spares.

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