pic: Prototype Drivetrain

[billbo911]

Originally Posted by Michael Corsetto

Instead of using washers to lower your middle wheels, try drilling the holes in all 16 pillow blocks off center by 1/16", and flip the middle ones over so the axle ends up being 1/8" lower than the outside axles. Adding washers isn't as clean/sturdy.

Excellent suggestion. Simple and if a jig is used, very repeatable.

Originally Posted by Michael Corsetto

Also, to lower your CG (which is ALWAYS important), you might want to mount the pillow blocks on the top side of your lower frame members, that way the whole frame is lowered a total of 2 inches. And try mounting your electronics (Battery/CRio/etc) on the lower frame level, not the upper one. This will lower your CG and give the manipulator teams more area to work with. Win Win.

From my many years in the Medical Imaging field performing maintenance on very heavy mobile systems, I have to disagree here, to a point.

With the pillow blocks on the bottom of the main rails, the axles put the pillow blocks and their mounting hardware in compression. The weight of the robot is transfered to the wheels without being suspended by mounting bolts.
Now, if the pillow blocks were moved to the top of the rails, then the mounting hardware would carry the load to the wheels, thus creating an additional point of failure, unless designed properly, which would add weight.

Now to take this a step further, why use pillow blocks at all?
The OP said these were dead axles. If that is the case, then just mount them directly through the frame. Granted, you will not lower the CG quite as far, but I believe, based on the picture and description, it would be more than sufficient.