Pillow bearings don't line up with motors?

Both visually and according to Inventor, pillow bearings atop c-channels don’t line up with motors.

  • This is the assembly I used to find the problem. I know having a bearing next to a motor is unnecessary - this was just used as a test.


  • What many teams have done in the past. Are my inventor parts wrong or do these parts actually not line up mathematically?

What distance are the holes apart from each other? (Centre to centre)

I always thought that the parts were meant to line up and have built robots like your second example with no particular problems. It could be that any difference is just sufficiently minor to not really matter in actual construction.

I took some photos with real parts for comparison.

I can’t prove that the parts aren’t bent or anything, but they don’t seem to be, and the shaft itself is definitely straight. Also, the shaft is not angled due to gravity, the angle stays the same if I turn it upside down.

I am using aluminium FYI.

And what about a delrin bearing?


Shouldn’t the motor and the delrin bearing be at the same height.

In Solidworks I have the center distance is .0020in.

I’m seeing a difference of 0.007" in Inventor with the latest Aluminum parts. Just shim the pillow block if necessary.


Careful guys, the actual vex c-channels are not symmetrical. See for yourself. If you try to fit a bearing block inside the c-channel, parallel and against the lip of the c-channel, it will only fit on one side. This is true for both the 5 hole c-channel and 2 hole c-channel. This will influence the real life test that you performed. It might be a design flaw, as I have no clue why vex would make the channels like this.

The offset should be negligible - the distance (from axle center to bearing center) was only .009 inches with my parts. On my monitor the offset appeared to be huge so I skipped measuring. I was shocked because some of my previous robots used pillow blocks as well with no noticeable problems.
Thanks for the help:o
Also, I have seen what “therealcedz” was describing

All manufactured parts have specified tolerances, generally the tighter the tolerances the more expensive a part will be to produce. I’m not sure how the VEX C channel is made, it could be an extrusion or more likely a flat sheet with the square holes first being punched and then folded to form the channel.

With the latest STEP files for the aluminum parts PDF drawings are included that have tolerances specified, here is one example.


These look quite tight to me (I’m not a mech engineer so what do I know :slight_smile: ) so I’m surprised that you see a difference between each side of the parts. I will measure a few next weekend and see if I can verify your experience.