Sorry if this seems a dumb question. All the builds I have seen using linear bearings have 3 bearings for the Y axis carriage. 4 would seem to me to give less friction and easier setup. 3 could save costs but I can't see another reason. TIA.

Only one of the rails is the guide rail (2 bearings typical), the second rail is a follower (1 bearing typical).
Adding more bearings is done to handle additional loads.
You can review catalogs of bearing suppliers for examples.

Edited 1 time(s). Last edit at 11/30/2013 09:11AM by A2.

4 would provide more stability in y and z direction on y axis, but if mounted correctly and securely attached 3 work ok. 1 rod has 2 and this stabilizes for the most part the play in the y axis, and the other 1 on the other side keeps the part level for the z height. however there are issues with z stability outside of bearing area on the single bearing axis, and no structure to assure z is level throughout the y stage, and there is slight part deformation on this side as well when feed rate is increased.

my best advice is to use 4 bearings, or 4 bushings on y axis, and on x axis for stability and level motion.

Edited 2 time(s). Last edit at 11/30/2013 05:12AM by jamesdanielv.

Depending of the machine type, there is a lot of 4 bushings carriages too.

In theoretical mechanic, 3 bushings is enough without any overguiding.
When you apply a movement, all play should be compensated on the same side and movement is true to the main guide (the one with 2 bushings) with the other only stopping roll movements.
So Y carriages are good, and even if there is play at bushings works well. They are less stable to tilting though.
The point of action of force should however be in the triangle formed by the bushings which is not the case of most Y carriages I have seen, which only reinforce the stability problem.
On course reversing, play is compensated the other side, inducing a small movement in the orthogonal direction. So the less play the better here.

4 bushings carriages fall in 2 categories, 4 fixed bushings or 2 fixed, 2 floating on the direction orthogonal to the rods. The latter are what are used in industrial systems and are equivalent to the aforementioned Y carriages. 4 fixed is what is used in reprap, and there is 1 degree of overguiding. If there was no play, the carriage would be blocked because of that. Our low cost bushings have quite a bit of that so it still works, but because of the overguiding, you are almost sure to get yaw parasite movements (causing waves on walls). However course reversing is clean, and tilting stability is much better.

So 4 fixed is actually inferior to 3, but due to the imperfections of our components easier to handle.

I 'm designing some stuff right now and will use an Y arrangement but with a longer (or double) bushing on the single bearing side, thus reaping some benefits from both approaches.

Thanks guys. I think I'll make a carriage with openbeam and experiment a bit. The idea I had was that the play in 4 bearings was sufficient that having 4 fixed bearings with a driving force centred between them would involve inconsequential overguiding given accurate positioning of the rods and their supports.

Open beam is a square profile. Be very careful if you use 2 of them to not introduce 2 more degrees of overguiding (rotational in that case).
You can probably get away with that as they are not very stiff, but still a cause of yaw.

With the very light loads we have, yaw, roll & play are really the only things we should focus on.

Assembly tip: (Depends on the application. My experience is based on using precision low profile miniature bearings with zero play, and was typically a PITA.)

If you decide to use four bearings (good idea for three bearings as well), place a layer of cellophane tape between the bearing and the table.
You will need to do this to all of them to maintain an equal elevation.
If the bearings are tight fitting, or your linear rails are not perfectly parallel the tape will allow for a very small amount of yaw compensation.

The bearings on the floating guide rail, literally need to float (they are loose).
It's a bit of an art to get it right.
In some applications I would leave the bolts loose on the floating bearing only, and apply Locktite (glue) to the threads so they don't fall out.

You should also locate the drive mechanism on the side of the fixed bearings, or offset from the center towards the fixed bearing side.
This is to help prevent the table from cocking and binding.

If you're going to go with four fixed bearings, you will have to compromise, the fit up will need to be loose.

alj_rprp brings up good points to consider, and again refer to a bearing manufacturers catalog to learn more.