Large table Drive System

[mikie]

Hey guys,

I just purchased a table and am looking at the systems to drive it.
The X and Y axis are 2 mtrs.

Out of the following, which do you think would be better.

1. 2" wide belts
2. Acme screw
3. Rack and pinion
4. chain

On my prototype machine, which is small, i ran both belt and screw. I found screw was better as i could drive the machine better, (force and repetition).
I found belt better from a setup and maintenance point, plus backlash is built in.

My thoughts are that a screw on this length is going to be a pain, whipping, and un-even wear across the length of the screw, plus sag across the length will be a problem.

If i use belt, then setup and cost is minimal, with gearing i can get good resolution, in some cases the same as a screw. I am assuming that i would have a screw of 10mm pitch for this size run and if i gear belt to 30:1 with a 100mm drive "sprocket", i would get the same.

Any comments and findings would be valued

Michael

[2muchstuff]

Due to the size of your table, I would defintely say that chain is out of the question because as chain and sprockets wear you would spend too much time adjusting the tension. Spring loaded chain tensioners allow too much play into the system. Acme or ballscrew could be used but at that length you would have to go with a rigid screw and a rotating nut arrangement. If you didn't the shaft would have to be rather large in diameter to prevent whipping and bending. With a large shaft comes the large motor to overcome the moment of inertia. Rack and pinion is good in that it is a positive, rigid setup even though you will need to have a reduction unit on the motor. A 2" wide belt is a good low cost alternative like you said. With a 2 meter axis you are almost obligated to have both sides of your gantry driven to prevent racking or twisting.

[mikie]

Cheers 2 muchstuff, just as i was thinking,
BELT is really the only viable option over this span.

/Mikie

[andy_ck87028]

Mikie

Clearly long threaded rod can have whip problems if spun and so a static threaded rod with spinning nut, as suggested by 2muchstuff is an alternative. Often static threaed rods are presumed to be large in diameter to make them 'more rigid'.

Have you considered placing a static threaded rod immediately adjacent (visualise above as a likely scenario) to a track with a trolley on it with the sole purpose of the trolley to position the rod correctly for the nut. This could be achieved by the trolley being part of the nut asembly.

In theory the rod diameter then becomes largely irrelevant. There would be no whip problem; the nut is always in the right place. If there is sag in the rod when the trolley is at one end... so what.

Has anyone tried this method?

Andy