Ballscrew position in between linear guide rails

[sh2005]

Hello,
In all linear slide designs i have seen where a single ball screw is used in betwen two linear guide rails, the ball screw is located equidistant from the two rails (or so it seems from the pictures). Will placing the screw closer to one rail cause any problem loke racking and/or uneven stress on the rguide blocks?

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[rodw]

I think you need to be "guided" by your torque requirements and the torque handling ability of the carriages you choose.

[peteeng]

Hi 2005 - If the rail and cars are the square type they do not have to be central. If round type maybe. If plain bushings then yes central is best. Square rails and cars support moments and have no stick slip so do not have to be central to eliminate eccentric loads being transmitted to the drive nut. They just move the mass around. If you have some rails and cars on something you can test this just be pushing it around by hand from different places. Central is logically best but sounds like your design needs some space which is fine. Peter

[sh2005]

Hi 2005 - If the rail and cars are the square type they do not have to be central. If round type maybe. If plain bushings then yes central is best. Square rails and cars support moments and have no stick slip so do not have to be central to eliminate eccentric loads being transmitted to the drive nut. They just move the mass around. If you have some rails and cars on something you can test this just be pushing it around by hand from different places. Central is logically best but sounds like your design needs some space which is fine. Peter

Thank you for the response, peteeng. Yes, i am using the square type rails and cars. Thw offset doesnt need to be extreme, just enough to fit some components in the middle.

Does it then imply that i can design wherr the ratio of the “length s width”
Of the carriage (measured distance between the cars) can be less than 1 and not have racking? I have read recommendation that that number should be at least 1.6, but if i can have shorter length for the carriage, i can get more travel.

[peteeng]

Hi 2005 - For square cars there is no recommended geometry ratio. The geometry ratio is important for plain bushes as this can be an issue with stick slip behavior due to the required clearance in the bush. There are many machines where the cars are tight against each other. With any machine you are limited by the required working envelope to the minimal footprint situation that's something you have to figure out. My advice is to make everything as big as possible, you will not be able to make the gantry stiff enough or space the bearings far enough apart to have "zero" racking unless its a fixed column machine. Racking is a function of the gantry size, its column size and the bearing foundation. Even commercial large machine gantries rack slightly by themselves. It's the drive system that keeps it square. Figure out what you think a machine part should be then make it bigger or thicker, no-one has ever complained about a machine that is too stiff. Peter

edit- even one car will do. On my small machines I use one car that is a heavy preload spec and it deals with timber and plastic really well, I have cut aluminium but the 10mm belt is not up to it, its a bit springy... You can get extra long cars as well. Depends on what you want to do? Read the suppliers design manuals they are full of good info. Look up the Hiwin site they publish very good manuals.

[sh2005]

The machine looks inpressive, given how “light” looking it looks, but can cut wood. Is the rail on the top only supported on its ends?

[peteeng]

Hi sh2005 - Yes only on its ends. I sell the mechanical kits for this if interested. PM me if that's interesting. It cuts carbon fibre quite well too. Peter