Got off the phone earlier today with a seller, talking about the differences between the two types of lm guides whether it be nsk/thk/hiwin etc..
It came down to him saying that 2 races are very capable, have very little backlash (if any)
He said 4 races typically have maybe a 10% increase in ratings (I'm just starting to look into lm guides), which I haven't backed up myself with research (don't know what values I'm looking for really).
It was also mentioned that many people tout the 4 race bearings as superior due to nsk and thk really hyping them up and using it as a marketing ploy to single out the 2 race types.
Thoughts, opinions, and real world experiences appreciated.
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I'm considering a machine based on 2 race hiwin lm guides. Cutting envelope of ~36"x48". Combination of wood/aluminum/(steel may be ambitious, but would be interesting to try)
At a very basic level here is my thoughts on design (all based on 20-30mm 2 race rails and aluminum extrusion from 80/20. Acme drive)
X axis gantry, 2 rails that support the z axis w/ 1 bearing block on each rail.
Y axis, 1 rail on each side mounted to the moving gantry via 2 bearing blocks (per side)
I'm undecided on what size extrusions to use. I was going back and forth between 25-5010 and 3060 for the gantry w/ .75" aluminum side plates for the gantry
For anyone who is on the zone alot and can think of a good build log that goes along those lines, drop me a link
If you are putting 100 kW through the shaft and driving several tonnes, then a 4-race system might be worth looking at. Mind you, roller bearings would probably be far more appropriate.
For a router with an aluminium frame? Fergedit. Way way overkill.
Cheers
Roger
EDIT: I think I screwed up here. I was talking about circular ball races, not linear rails. Total stuff-up by me.
Yes, I suspect linear bearings with 4 rows of balls are going to be a bit better than ones with just 2 rows, at least under heavy variable loads. Less room for slop - although none of them have much
Whether 80/20 rail by itself is strong enough is another question. I would add some 12 mm plate myself.
I own a set of THK RSR rails, wide low profile, only have 2 ball tracks, they seem to have no play, very solid. I also own a set of Bosch Rexroth Rc1651 rails, 4 ball tracks. What the extra tracks provide is a larger contact area, translates to less wear at the same load. Download the books from Bosch, on Linear Motion Guides, products guide, and also they have a great book available for download, Linear Motion Handbook. I prefer the Bosch 4 track approach over THK or anyone else's 4 track design.
Profile Rail, 2 race vs 4 race lm guides
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