I have not found where the manufacturer lists a difference in load bearing based on position, although I am sure it has some bearing (no pun). The actual bearing surface area on the rail is identical whether the rails are mounted right side up, upside down or horizontal in either direction.The linear rail is a 25mm wide rail and has a hole for a 1/4"-20 shcs ( Or 6mm to be more correct ) every 1.9". The rail is rated for dynamic loads up to ~4800# and static loads of ~8000 with the right bearings. The bearings we are using only have 2 bolt flanges, so I think they are rated lower, but I think they have just as much bearing surface area on the rail as the 4 bolt flange variety. These parts seem to be obvious overkill to support a 300# gantry, and will hopefully result in many years of trouble free, nearly maintenance free service.
If there is anything I can do to help you let me know, I have appreciated referencing the information contained in your build log and would be glad to be of any assistance to you.
OH BTW... IT LIVES!!! Thanks to some setup help from Russ we made some very nice first cuts wit the machine today. A little more testing and tuning and this thing is going to rock! I'll get some pictures when I get a chance.
Sean