Alternative to round pipe.

[ger21]

While building machines using skate bearings and round pipe (gas pipe, EMT...) is cheap and easy, there are some limitations. A major one that I noticed is that you can't get the bearing really tight because the 45° loading puts too much axial load on the bearings, which greatly increases the rolling friction.

I came up with this idea to eliminate that problem. Use a laminated wood rail for stability, and epoxy steel rails to it for the bearings to ride on. If you own a wood planar, it's easy to get two parallel surfaces for the top and bottom bearings. The bottom bearings need to be mounted with an eccentric bushing to be able to keep the assembly tight to the rails. I think the easiest route is to buy the chinese dual V eccentric bushings, and some 3/8" ID bearings for the bottom. VXB has 3/8"ID bearings 10 for $15.

Any thoughts?

[epineh]

Nice work Gerry, I like it, nice and simple and could be used on the larger machines without any problems. I have filed a copy for later...

Russell.

[Oldmanandhistoy]

Nice design; thank you for sharing.

Do you think there could be a problem with the bolts running through the top and bottom bearings bending with the pressure? I suppose it will depend on how long they needed to be? Just an observation not a criticism.

Wish I had your CAD skills btw.

John

[jsage]

Very interesting idea. You get a lot of rigidity from the ibeam thickness and the lamination improves the stability of the wood and gives dimensional strength. Don't know much about this type of linear system but would trapezoidal give you better offsetting/sharing/dampening of lateral forces.

Oops, maybe I'm looking at it backwards!

[ger21]

Originally Posted by Oldmanandhistoy Nice design; thank you for sharing. Do you think there could be a problem with the bolts running through the top and bottom bearings bending with the pressure? I suppose it will depend on how long they needed to be? Just an observation not a criticism. Wish I had your CAD skills btw. John

I guess it would depend on how heavy the gantry was. As drawn, they're 8mm x 45mm bolts and about 30mm from the angle to the outside of the second bearing. How strong is a class 12.9 Cap screw? tensile strength is 177000 psi.

[WarrenW]

Excellent idea! I know it may be overkill but based on John's comments, I would extend the top and bottom bolts longer and duplicate it on the other side. In otherwords, add a steel plate on the other side and put bearings on that side also. The top and bottom bolts would just extend through the angle on the other side. Unless you could drill the holes just right, you could make then adjustable with the size of the holes so it can be tightened to the steel plates. If you didn't want to add another bearing set on the opposite side, then just use longer bolts on the top and bottom with spacers and put a metal plate on the outside with those bolts connected. This will reinforce it with the weight of the gantry.

Just my two cents...

Warren

[rancherbill]

EPINEH

I saw this the other day and thought you would like it.

[epineh]

Haa! thats funny, now if only I could fit that as an avatar...

Russell.

[Oldmanandhistoy]

Originally Posted by WarrenW Excellent idea! I know it may be overkill but based on John's comments, I would extend the top and bottom bolts longer and duplicate it on the other side. In otherwords, add a steel plate on the other side and put bearings on that side also. The top and bottom bolts would just extend through the angle on the other side. Unless you could drill the holes just right, you could make then adjustable with the size of the holes so it can be tightened to the steel plates. If you didn't want to add another bearing set on the opposite side, then just use longer bolts on the top and bottom with spacers and put a metal plate on the outside with those bolts connected. This will reinforce it with the weight of the gantry. Just my two cents... Warren

Better than a rap around approach which would mean back to the unsupported rail; there is plenty of length to add more bearing to share the load if it were required of course.

John

[ger21]

Originally Posted by Oldmanandhistoy there is plenty of length to add more bearing to share the load if it were required of course. John

The main purpose of this design was to eliminate the axial loads and have the bearings only carrying radial loads. I would think that the 4 bearings should be able to carry far more weight than I would need them to. The only drawback is that the table needs to be suspended, to allow the gantry sides to be pulled together under the table.

[Oldmanandhistoy]

Originally Posted by ger21 The main purpose of this design was to eliminate the axial loads and have the bearings only carrying radial loads. I would think that the 4 bearings should be able to carry far more weight than I would need them to. The only drawback is that the table needs to be suspended, to allow the gantry sides to be pulled together under the table.

What about adding one more set of bearings in kind of a G set up instead of your C; the rail would be an L shape turned 90 degrees clock wise? If that makes sense, you would not need to suspend the table.

John

[ger21]

Originally Posted by Oldmanandhistoy What about adding one more set of bearings is kind of a G set up instead of your C; the rail would be an L shape turned 90 degrees clock wise? If that makes sense, you would not need to suspend the table. John

I've thought of trying to do that, but it gets a lot more complicated, and seems like it would be a bit weaker, having to warp around underneath.

Now you've got me thinking what if I mount the rail near the bottom, and have the gantry side straddle the rail. One problem might be that a "trough" would be created that might fill with dust and chips. I'll think about this for a bit.