Hi Folks.
I'm sorting out the rails and bearing i will use in my cnc router project machine. I have two nice sets of THK HSR35's for the X axis.

I have two sets of THK HSR30's for the Y axis, if needed.
I plan on a gantry style instead of a moving table.

Here is the question: Given the robust spec's of the THK HSR30 units,
wouldn't one (1) rail suffice for use on the Y axis? The y axis is to be about 28" long

My Z axis can be either a THK HSR20, or I also have some small slides which would work nicely. I plan on mounting a small Porter Cable router to the z axis.
The overall wieght should be fairly light. The HSR20 units are rated in the hundreds of pounds if I remember correctly. I understand I should derate due to the weight "hanging" off the side, but again, I think one would work fine.

I know dual rails always help with the load issues, but wouldn't two rails be overkill in this case?
any advice appreciated.
thanks

It depends on how long your Z axis is. This length times the anticipated cutting force (can be tough to calculate -- I use 100 lbs to be conservative) will produce a moment load on your block. Look at the moment rating on the block in that direction, (usually indicated with a curvy arrow in the spec sheet) and you'll have your answer.

generaly geometry beats brute strenght. Also keep in mind any thing that moves has a clearance value and multiply that by the distance.of the arm.... .001" ok not bad x 10 inches arm length now makes it .010 or greater. you will be twisting the bearing instead of pulling and pushing it was designed for.

Just my two cents worth........ I mean five cents worth I forgot the Tax..

Thanks folks,
yes, i can see what you are showing; Even though a load doesn't look like much at first glance, when you multiply the load over the distance, things change a lot.
I have done a lot more searching since my post and have found this question addressed in other threads and on other sites.
I have two rails, so I will use them. my only question now is how best to arrange them. I have many options since i have lots of 80/20 rail, aluminum bar and plate, etc.
I see some folks mount them on the same plane, some put one on top or rail one on bottom of rail, etc.
any suggestions?
thanks again for a great forum

With 30mm rails, I don't think it really matters how you mount them. They can handle far more than you'll ever throw at them.