How many trucks to hold Z axis?

[Rhodan]

I'm making a Y-Axis with two IKO 20mm linear rails with LWES20 trucks. At first I was planning on using four trucks (two each top and bottom) but after playing with the trucks for a while they seem really, REALLY steady. I'm thinking I might be able to use just two trucks, one per rail. Is this a bad idea?

The machine its going on uses 272oz/in steppers with a 1/2-10 leadscrew and pushes around a 2.25HP router. Do you think this would be too weak when moving along the Y axis?

[rustamd]

I think it would be more rigid with 4 trucks. The weight of a router I would think would make it bind, maybe not much but at least little bit. If you already have the trucks, might as well use them

[Rhodan]

Actually, here's a quicky rendering of what I mean

[ger21]

You'll probably be limited to around 100ipm with that motor screw combo, so you should be OK. If you had a LOT of force on a long Z axis, you might have problems. But I wouldn't think so with what you're doing. I would suggest 1/2-8 2 start acme, though.

[Rhodan]

Ok thanks!

If one doesn't work out, I can always just drill more holes and use both.

[digits]

Originally Posted by Rhodan I'm making a Y-Axis with two IKO 20mm linear rails with LWES20 trucks. At first I was planning on using four trucks (two each top and bottom) but after playing with the trucks for a while they seem really, REALLY steady. I'm thinking I might be able to use just two trucks, one per rail. Is this a bad idea? The machine its going on uses 272oz/in steppers with a 1/2-10 leadscrew and pushes around a 2.25HP router. Do you think this would be too weak when moving along the Y axis?

Have you had a look at this thread? New Machine Design book from Alexander Slocum (of Precision Machine Design)
The PDF's on machine design are excellent reading

Using 4 trucks, which is what just about everyone does is actually overconstraining the design - 3 trucks is in theory optimal! As you say though, those linear rails are amazingly tough - I would have a look at the data sheets to decide whether or not the trucks can take it. You will probably find you have a massive saftey factor with just two trucks though.

[svenakela]

I have a friend who's routing solid oak with a deWalt router mounted on one THK 25 rail with one truck.

[Rhodan]

Lol, solid oak!

They do seem really solid. Can't get it to wiggle at all in any direction.

I guess what I really have to worry about is the mounting plate deforming due to torque on the rail. I have 4 x 3/8ths aluminium on there now and I can't imagine plunging with enough force to twist that aluminium - I only have 272oz-in motors with kerk nuts. I think I'd stall long before the aluminium deformed.

[digits]

Originally Posted by Rhodan Lol, solid oak! They do seem really solid. Can't get it to wiggle at all in any direction. I guess what I really have to worry about is the mounting plate deforming due to torque on the rail. I have 4 x 3/8ths aluminium on there now and I can't imagine plunging with enough force to twist that aluminium - I only have 272oz-in motors with kerk nuts. I think I'd stall long before the aluminium deformed.

If you're driving with screws, you have to remember to multiply up the torque by roughly 2PI/pitch (which doesn't take into account the screw efficiency) to get the motor force - my motors produce about 3.5Nm peak, but my 5mm pitch ballscrews boost that by 1250x to 4400N of force - enough to lift 440kg/900lb! It can literally bend a 80x80mm (3" square) aluminium extrusion with out any drama on my 35mm HiWin blocks/rails - it also bent a 50mm wide, 20mm thick (2"x3/4") aluminium plate like it wasn't there! Don't worry about your rails, worry about what you bolt them to!

[unterhaus]

I was thinking about this since I got some nice rails with only one truck each for my z axis. I was thinking about offsetting them a little. These are wide rails, so I probably could use one rail.