Looking to go the other way: Worst mounting conditions before rails bind?

[Drassk]

I'm working on a design project for a client where a very long rail system is required. I'd like to use some HiWin rails I've found because of their load capacity and low friction rather than for precision, but because they will be mounted to a levelled surface over a long concrete floor (though extremely level for such) I'm worried about the possibility of the blocks binding on the rails.

If need be, the plan is to pour an epoxy surface plate material in two tracks on the floor and install bolt anchors in the concrete underneath where the rails will be installed. Getting the anchors in a straight line isn't a big deal. The first rail will be lined up using an aligning laser mounted level and parallel on a bearing block on one end, and with a target on a second bearing block. This will allow the whole rail to be aligned within 0.015 or so (dot size of laser is 1/8" at full rail length) over the whole 25' and better than that locally (1/16" dot at 5').

The second rail would be aligned to the first by mounting the carriage to two blocks on the first rail, two on the second, and then moving it along and fixing the second rail as we go.

So, since I'm not worried about the actual accuracy of the motion on the rails, how bad would my mounting surface need to be before I'll start having binding issues? The carriage is 40" long by 16" wide and the plan is to have it slide along its long axis on four or six bearing blocks. The carriage could be as high as 600lbs or as low as 300lbs (can be redesigned), though a higher weight capacity would be better as most of the weight is used for damping of the sensors on top of the system.

So I have two questions:
1- What are the tolerences needed in mounting rails just to have the blocks continue to roll, rather than to have them present a super accurate guide?

2- If someone has a good suggestion for a guide system that can be set up over 25', can support 600lbs, and has a minimum vertical profile (preferably a rail height under 3/4") then I'm certainly open to the option of using something cheaper. This system only needs to stop the carriage from pitch and yaw movement, and only needs to be as level as the average of the floor. I might end up making one, though I'd much rather buy square rails with given tolerances and mounting holes rather than having to machine and drill a bunch of 8' bars on my Fadal

[spoiledbrat]

I am not the expert in this matter, but I always thought that the stiffness in those blocks is such that they do not flex that much at all without binding. My Z carriage has four hiwin blocks, and with a 40 lb load, flex was around 0.001". Have you thought about angle iron, tabbed and bolted to the floor, and then steel wheels with a V-notch in the center? This would probably save money over the precision rails. Plus, the mill-scale would protect the anle iron from rust for a good while.

Just a though. Hope you find what you need soon.

Rob

[planebuilder]

Drassk
For what it’s worth, You said the car is 40" long and you might use 4 or 6 bearing blocks. The
“6” put up a red flag for me. Looking at one side of the car, I assume you would put one bearing
at each end of the 40" car and one in the middle at 20". A natural thought, but, If you consider a
track 40" long, with a bow up in it, say .020 high in the middle, the middle bearing will be
pushed up .020 relative to the end bearings, maybe (probably) it will bind. If 4 bearings total
won’t carry the weight, you might consider using 8. If you put them in pairs, close together at
each end of the 40", then the deviation from a straight line that the bearings see, over maybe 4 or
6 inches, will be much less than the .020 at 20".

[LeeWay]

Another thought is that using a block on each end to align will do nothing to help align the center of the rail. Much better to use the actual rail for alignment. It's not going to be easy on such a large system. It will certainly take time and money to get anywhere close to good enough.
A center block does not bother me much. If the rails are in alignment, a center block can only be a benefit to help carry the load.

[Drassk]

I'll clear up a couple things in the original post:

The laser target and laser are mounted on the trucks for a reason: it makes them both mobile and maintains alignment with the rail. That means I can leave the laser truck stationary and move the target only, bolting as I go, to get my gross straightness in line. After it's roughly straight from that metric, I can move both trucks together with a gap between them to make sure the rail is aligned locally in any section. I could, for example, measure the straightness between the 10' point and the 20' point by moving the trucks to those locations.

I'm not worried about load deflection as these are only taking a vertical load and they'll be on concrete, so nowhere to deflect to. The question, boiled down, is : 'with two trucks spaced 40" apart on a rigid surface, what is the flatness/distance requirement (or running parallelism expressed angularly) of a rigid mounting surface for the rails such that they won't bind. That could be a simple number like 'if the mounting surface is flat within 0.xxx over 40"'. I suspect it would need to be measured angularly somehow, to account for the fact that the blocks can be far apart.