Hi All - The spacing of the column bearings often comes up in many areas of the forum. Its an area that I have looked at intensely over the last 3 years. There's no guidelines that I know of other than "position them as far apart as possible". So I'd like to engage the forum to share its knowledge on these things. Firstly my history on the subject:
1) 3 years ago I built my first machine Scoot Alpha. Scoots design credo was "minimal function prototype" which meant the design issue was looked at and the minimum solution was implemented. This meant it used one car per axis and one rail for the gantry and Z axis. The bearing design manuals had lots of examples of single rail/single car solutions and the the router worked well within the cars load specified capacity.
2) Shortly into Scoots life it developed a wobble. The cars were zero clearance cars so after settling in actually had some clearance. This brought me to investigate the different preloads and what to use them for. Scoot had been designed so that two rails could be used across the gantry and I ordered a new rail and a high preload car for the Z axis. The Z axis car was then to be used on the gantry second rail
3) I also decided to up the cars from one to two on the gantry columns, so ordered 3 high preload cars... This became Scoot Beta. ScootB was much more stable and I even cut some aluminium composite panel that was impossible with Scoot-A.

So to discuss the bearing arrangement - from this experience its clear that 2 rails and 3 or 4 cars are needed for stiffness. The further apart along the rail they are the better. One guideline is that a group of bearings needs to be square. With square rails this gives very good stiffness in all directions as the bearings themselves support moments. If round rails are used the square pattern is good as well as this removes the rolling freedom a round rail has. But its not practical to have column bearings in a square pattern as they would have to be the table length apart so the gantry would not travel far or even not at all!! . This leads to the bearing group having a huge aspect ratio. So huge in fact that I think the LHS and RHS have to be considered as separate bearing groups. This means the gantry is considered as two cantilevers joined in the middle. Which sort of makes sense as gantries no matter how stiff "walk" so each side is behaving independently, one side catching up with the other. This is a structural issue vs a bearing arrangement issue I think.

Very large machines address this by using two rails each side and having 4 cars each side. So this has a square bearing group with a cantilever and each cantilever meeting in the middle. Cantilever loads have there own set of problems with bushes that is minimised or solved by using rolling element cars. This geometric problem is taken advantage of with F clamps and various other clamping systems allowing the clamp to bind when loaded. Exactly what we don't want to happen...

https://www.linearmotiontips.com/can...levered-loads/

So our typical moving gantry is like a car in which its wheelbase is considerably shorter then its track ie it will steer badly! Some builders expect the drive system to correct this situation. Maybe it can but mostly it can't at a basic level. Using some sort of positional feedback is the general solution.

So my guideline is make the bearings at least one dimension apart prefer more, where the dimension is the along the rail bolt spacing. eg on a std 20mm flanged car this would be 40mm min between the middle bolt holes.

Question - Has anyone used one long car in this application vs two cars close to each other?

That's about it would like to hear anything anyone has to say on this.... Peter