# Appropriate bearing to spin a dove prism?

I want to spin a dove prism (image rotating prism) around it's long axis at a controlled rate (100 to 300 RPM). The purpose is to get a de-rotated image of a spinning object (by looking through the prism).

The prism is small and light - about 20x20x150 millimeters and maybe 0.1 kg.

My idea is to mount the prism inside a spinning bearing. I tried an inexpensive "pillow block bearing" (20) and mounting the prism inside, but that didn't work well: (a) The friction needed to turn the bearing was much higher than I wanted - I want it to spin freely (for example, if I give it a spin with my fingers, I'd like it to keep spinning for a while). I was driving the rotation with a small DC motor with a belt (controlling motor speed with a knob). The friction was a problem. (b) The axis of rotation didn't seem to be as constrained as I'd like - I want it to really just spin in one single axis that doesn't move around, so the view through the prism stays centered on a fixed point (assuming the prism is properly aligned with the bearing - I'm going to 3D print a fixture to hold it in place). Can I get a recommendation for the appropriate type of bearing for this? (Or, some other way to accomplish what I'm trying to do?) It would be nice if it's not expensive (<100).

• Although not relevant to the question, I would like to understand more how you are planning to "get a de-rotated image of a spinning object (by looking through the prism)." Mar 27 '21 at 4:54
• @NMech A dove prism rotates an image by 2 degrees for every degree you rotate it. So if you rotate it in the opposite direction at half the spin rate of the thing you're looking at, you'll see a stationary view. Mar 27 '21 at 14:48

Mount the prism in a round plate with a hole for the prism base and clamps.

Then mount the plate in something like a lathe steady which can hold the plate by the edge and drive it with one of the rollers or even a belt around all rollers.

The problem with controlling the axis of rotation is that the members must be somewhat rigid, meaning the bearing won't spin freely as long. If you want to mount directly into a bearing, make sure there's nothing extra in the bearing that will inhibit rotation - grease, seals, shields, etc. Trade number 6804 is should work. Just keep everything oiled.

• Thanks. Can you give me a link to a 6804 bearing so I can see what you mean? Should I remove the seals and de-grease it? Mar 30 '21 at 15:30
• mcmaster.com/5972K215 That one comes without seals but you'll need to clean out the grease and add oil
– jko
Mar 30 '21 at 16:33
• why not a roller bearing? If he must use only one bearing (which is not great to have a stable spinning axis without a force fit of the spinning shaft into the bearing) then the roller would be a little more stable but also give more friction...he must decide what he cares most about Aug 26 '21 at 14:39
• OP's point A lists desiring to spin freely, significant more resistance to rolling with rollers vs ball elements. Ball bearings are rigid enough axially under low load
– jko
Aug 26 '21 at 17:46