Even with a solid bar you could mash or plastically deform flat spots with a strong point load. The radius could decrease slightly even without abrasion or with very thick walls.
I would agree that it should “settle in” to a nonzero but limited amount of deformation since the deformation causes it to spread the load so it’s no longer at a single point. It would effectively have a reduced OD at that point but if the tube started out slightly oversize it might settle in to a proper fit instead of starting at the correct fit and ending up undersize.
It would probably be good to run a job that rolls along the entire working length of both axes many times because if the rails have flats in some areas and not in others it could introduce a bump when the roller crosses the discontinuity.
@jchidley#2 is what we’re seeing. Depending on what you want to replace more often is what you want to be the softer material. With our tubes, we can just rotate them slightly to get a new nonflat spot. Bearings we’d have to disassemble things.
I can see that it’s possible to overtighten the bearings onto the tube (pretty easy to do I’d think, even just using a hand-held screw driver) and therefore crush it, leading to flat spots. However, the load (< 2 kg per tube, I estimate) strikes me as way to little to cause problems by itself.
This does suggest, to me, that I will have to be very careful when tighening up the bearings - just enough to eliminate slop and no more.
Point load of less than a half millimeter square. Realistically these are the wrong bearings to be using on round surfaces. The reason they’re used is because it’s a cheap system to build, and readily available parts in a medium sized city.
I have a local bearing manufacturer, so I know I could get better bearings. Ones with a v or u profile on the outer raceway would be better wouldn’t they? The big issue is I don’t think they would fit inside of Ryan’s trucks.
I know big machines use linear bearings im just wondering if there are bearings that would be better in that they would be less likely to put flat spots in the steel.
Warning – I’m not any sort of engineer nor do I pretend to be one nor did I sleep in a Holiday Inn Express at anytime in the recent past (i.e. I’m just guessing)
a “V” bearing would create half the point load because there would be 2 contact points assuming perfect contact
a “U” bearing would be better assuming perfect contact but the inner radius of the bearing would have to exactly match the outer radius of the rail (i.e. 1in diameter). I haven’t looked for such a bearing but it seems unlikely a bearing of the same thickness would be readily available. You could look at McMasterCarr they seem to have most everything that’s mass produced.
I have seen “U” shaped bearings that would probably work but they are much thicker as they cover the full diameter of the tube. This would cause a redesign of the trucks and core to accommodate
new bearings
Even if you were able to overcome all of this it seems to me that you would still run the risk of deforming an aluminum rail by continuously rolling a steel bearing back and forth over the surface. It would probably take longer but the hardness is just too dissimilar.
If you want to go to all the trouble of doing a redesign then why not keep the current bearings and have them ride on square tubing or in a V configuration on a piece of right angle aluminum stock. I think this would be easier to find parts. I would guess that it would also suffer the same fate as the previous option – the hardness is just too dissimilar.
If we’re going to all that trouble, perhaps someone could design an apparatus that takes an oversize cylinder and grinds flats into it that are the proper radius. I’m envisioning something vaguely resembling a router table with extra fixturing. The hardest part would probably be avoiding any twist in the flats.
If the oversize tube is too large it might not clear the plastic parts but it should be relatively straightforward to modify Ryan’s parts to increase the clearance a bit.
The other issue would be securing the tube ends to the machine. If the tube is oversize then it might need to be turned down, or the plastic would need adjustment, and both of those are a bit more complicated.
…except then its not mostly printed… like I said before I was mostly asking if there was a simple upgrade to the bearings, even if they are harder to source. The way dom is an upgrade even though it is harder to source than conduit. Though I’m not sure what @jamiek was getting at with his grinding rig. Maybe I will bring a truck into my bearing place and see if they have anything that fits the bill.