Conrad-style ball bearing

Introduction

After watching an excellent video about Conrad style ball bearings on the YouTube channel "Positive Altitude", I started experimenting with the concept. The result is this parametric OpenSCAD file that contains a design based on his description. The balls came from a broken swivel chair and were cleaned with isopropanol.

The balls are only rendered in the preview. In the final render, only the printable parts are shown. I print all the parts in one go.

If you decide to try it, I would love to hear how it worked out for you (or not), sharing experiences is what this is all about.

I did add an STL file for 8mm diameter balls(17-48mm diameter, 16mm wide), but of course the real fun is in firing up OpenSCAD and playing with the model.

Edit 2025-09-25: seam bulge problems

Today I printed a model with a 0.0mm clearance. It worked great were it not for one problem: the seam causes very noticeable breaks at certain locations. If anyone knows how to make the seam more like an indent instead of a bulge using PrusaSlicer, let me know! 

Youtube Video

The video contains a lot of information and shows how to assemble it. Watch it here:

https://www.youtube.com/watch?v=NIIVZcgJWPs

My Findings

• The bearings work well, but his cage design is a bit over the top (as he admits himself in the video :)). The slope on his cage actually works counter-productive imho; when two balls are close to each other, the ramp will actually  make it difficult to separate them. With a flat cage, you can manually push them apart. I did try the slopes but deleted it from the OpenSCAD file because the calculations were too messy.
• The 45° overhang is not needed (Prusa MK3S). If your mileage varies, just increase the gap between the races or modify the model.
• Clearances can be quite tight.
• The more balls, the more difficult it becomes to assemble.

Bearing Glossary

Race / raceway

The tracks in the inner and outer rings where the balls roll.
Inner race → ring that fits onto the shaft.
Outer race → ring that sits in the housing.

Lip / shoulder

The “edge” of the race that keeps the balls from falling
out sideways. When you assemble a Conrad bearing,
these lips limit how far you can shift the races.

Pitch circle radius (ball-path radius)

The radius of the circle traced by the centers of
the balls when the bearing is assembled. Roughly
halfway between the inner and outer grooves.

Ball diameter

The size of one ball.
Standard 608 bearings, for example, have ~3.5 mm balls.

Ball count

How many balls fit inside the bearing. More balls
can carry more load, but too many make assembly harder.

Conrad bearing (deep-groove bearing)

The most common type of radial ball bearing. It has
deep curved grooves in both inner and outer rings.
Assembly is done by shifting (offsetting) the races
to one side to make a “window” where the balls can
be dropped in.

Cage (or separator, retainer)

A part (often plastic, brass, or stamped steel) that
keeps the balls evenly spaced and prevents them from
rubbing directly against each other. Common in Conrad
bearings.

Cage pocket diameter

Size of the holes in the cage that will fit around the
steel balls.

Groove curvature (raceway curvature)

The shape of the groove where the balls sit. Usually
a bit larger than half the ball’s diameter so
the balls don’t get pinched.

Loading window (assembly window)

The angular gap that must be left clear during assembly
so the balls can be inserted. In Conrad bearings, you get 
this window by offsetting the races during assembly.

Assembly offset

How far you can shift the inner and outer races sideways
relative to each other to create that loading window.
Limited by the lips/shoulders.

Full complement

A bearing with all the space filled with balls (no cage).
It carries high loads but has higher friction because
balls rub against each other.