Bed bearings explanation

Hello guys!

I've got some question regarding bed frame bearings. Why use 3 not 4? Wouldn't it be more stable then?

Hello,
Four bearings can cause binding. See also [forums.reprap.org]

Some commercial printers come with four bearings (Printerbot is one that comes to mind right away). A friend has a RepRap with four on Y and he claims it experiences zero binding. I am designing a RepRap with a Y carriage that can use four or three bearings. First I will try four. If that doesn't work I can easily switch to three. Correctly aligned I believe four should work just fine. At least I want to test if I can align the bearings properly. If not, it will be a painless experiment.

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Technically speaking, 4 bearings is an "over-constrained mechanism". Two bearings on one rail insure motion along the rail's axis. Adding a third bearing on a second rail parallel to the first prevents rotation around the first rail/axis, thus constraining the motion completely. Adding a fourth bearing does nothing to further control the motion unless you consider a carriage that is not perfectly stiff.

Three or four bearings can be made to work fine, though misalignment when using four is less forgiving - i.e. more likely to bind- than the same misalignment with only three bearings.